U.S. patent application number 16/188243 was filed with the patent office on 2020-05-14 for handover techniques in wireless communications.
The applicant listed for this patent is QUALCOMM Incorporated. Invention is credited to Prashanth Hande, Gavin Bernard Horn, Keiichi Kubota, Karthika Paladugu.
Application Number | 20200154330 16/188243 |
Document ID | / |
Family ID | 68766842 |
Filed Date | 2020-05-14 |
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United States Patent
Application |
20200154330 |
Kind Code |
A1 |
Paladugu; Karthika ; et
al. |
May 14, 2020 |
HANDOVER TECHNIQUES IN WIRELESS COMMUNICATIONS
Abstract
Methods, systems, and devices for wireless communications are
described that provide a handover may be performed based on one or
more conditions at a UE and in conjunction with a handover
procedure from a source base station to a target base station. The
one or more conditions at the UE may be associated with successful
establishment or maintenance of the second connection. In some
cases, the one or more conditions at the UE may correspond to one
or more measurements associated with the source base station, the
target base station, one or more neighboring base stations, or any
combinations thereof.
Inventors: |
Paladugu; Karthika; (San
Diego, CA) ; Kubota; Keiichi; (Tokyo, JP) ;
Horn; Gavin Bernard; (La Jolla, CA) ; Hande;
Prashanth; (San Diego, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
QUALCOMM Incorporated |
San Diego |
CA |
US |
|
|
Family ID: |
68766842 |
Appl. No.: |
16/188243 |
Filed: |
November 12, 2018 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H04W 76/15 20180201;
H04W 36/00837 20180801; H04W 36/18 20130101; H04W 74/0833 20130101;
H04W 36/30 20130101; H04W 36/026 20130101; H04W 36/08 20130101;
H04W 36/0069 20180801 |
International
Class: |
H04W 36/18 20060101
H04W036/18; H04W 36/30 20060101 H04W036/30; H04W 74/08 20060101
H04W074/08; H04W 76/15 20060101 H04W076/15; H04W 36/08 20060101
H04W036/08; H04W 36/00 20060101 H04W036/00 |
Claims
1. A method for wireless communication, comprising: establishing,
at a user equipment (UE), a first connection with a first base
station; receiving a first radio resource control (RRC)
reconfiguration message from the first base station comprising an
indication to perform a handover procedure with a second base
station; initiating, at the UE, a connection establishment with the
second base station to establish a second connection responsive to
the first RRC reconfiguration message, wherein the first connection
is maintained during the connection establishment with the second
base station; measuring one or more channel conditions associated
with one or more of the first base station, the second base station
or a third base station; transmitting, responsive to the first RRC
reconfiguration message, a measurement report to at least one of
the first base station or the second base station responsive to
measuring the one or more channel conditions; and receiving,
responsive to the measurement report, a second RRC reconfiguration
message from the second base station.
2. The method of claim 1, further comprising: establishing the
second connection with the second base station, wherein the second
connection is a primary connection and the first connection is
maintained as a secondary connection; receiving, responsive to the
measurement report, the second RRC reconfiguration message from the
second base station to reconfigure the first connection as the
primary connection and the second connection as the secondary
connection; and reconfiguring the first connection and the second
connection responsive to the second RRC reconfiguration
message.
3. The method of claim 2, further comprising: transmitting one or
more additional measurement reports via the primary connection.
4. The method of claim 2, wherein the measurement report is
transmitted responsive to the measured one or more channel
conditions of the secondary connection being better than
corresponding channel conditions of the primary connection.
5. The method of claim 2, wherein the reconfiguring the first
connection and the second connection responsive to the second RRC
reconfiguration message is performed without performing a random
access procedure.
6. The method of claim 2, further comprising: transmitting and
receiving duplicated data via each of the primary connection and
the secondary connection.
7. The method of claim 1, further comprising: releasing the first
connection after the second connection has been a primary
connection for a predetermined time period.
8. The method of claim 1, further comprising: receiving, responsive
to the measurement report, a third RRC reconfiguration message from
the first base station to abort the handover procedure with the
second base station.
9. The method of claim 8, wherein the measurement report is
transmitted responsive to measured channel conditions of the third
base station being better than measured channel conditions of the
second base station.
10. The method of claim 8, wherein the third RRC reconfiguration
message further includes a second handover message from the first
base station to perform a second handover procedure with the third
base station.
11. A method for wireless communication, comprising: establishing,
at a first base station, a first connection with a user equipment
(UE); transmitting a first radio resource control (RRC)
reconfiguration message to the UE comprising an indication to
perform a handover procedure with a second base station; initiating
a handover of the UE to the second base station, wherein the first
connection with the first base station is maintained during the
handover and an establishment of a second connection between the UE
and the second base station; receiving a measurement report
responsive to the initiating the handover; and modifying the
handover of the UE based at least in part on the measurement
report, wherein the modifying the handover of the UE comprises
reconfiguring the first connection to be the primary connection and
the second connection to be the secondary connection based at least
in part on the measurement report.
12. The method of claim 11, further comprising: transmitting a
handover indication to the second base station; and receiving, from
the second base station prior to receiving the measurement report,
a role switch indication that indicates that the second connection
between the UE and the second base station is a primary connection
of the UE and the first connection is a secondary connection.
13. The method of claim 12, further comprising: receiving one or
more additional measurement reports; and further reconfiguring the
primary connection and the secondary connection based at least in
part on the one or more additional measurement reports.
14. The method of claim 12, wherein the measurement report is
transmitted responsive to the measured one or more channel
conditions of the secondary connection being better than
corresponding channel conditions of the primary connection.
15. The method of claim 12, further comprising: exchanging
information with the second base station to establish duplication
of data communicated with the UE via both the first connection and
the second connection; and communicating duplicated data with the
UE via the first connection.
16. The method of claim 11, further comprising: releasing the first
connection after the second connection has been a primary
connection for a predetermined time period.
17. The method of claim 11, wherein the modifying the handover of
the UE comprises: transmitting, responsive to the measurement
report, a second RRC reconfiguration message to the UE to abort the
handover with the second base station.
18. The method of claim 17, wherein the modifying the handover of
the UE further comprises: transmitting an indication to the second
base station that the handover of the UE to the second base station
is aborted.
19. The method of claim 17, wherein the measurement report is
transmitted responsive to measured channel conditions of a third
base station being better than measured channel conditions of the
second base station.
20. The method of claim 17, wherein the second RRC reconfiguration
message further includes a second handover message to the UE to
perform a second handover procedure with a third base station.
21. A method for wireless communication, comprising: receiving, at
a second base station from a first base station, a first radio
resource control (RRC) reconfiguration message to initiate a
handover of a user equipment (UE) from the first base station to
the second base station; establishing a second connection with the
UE responsive to the first RRC reconfiguration message; receiving a
measurement report from the UE that includes one or more channel
measurements associated with the first base station and the second
base station; and modifying the handover of the UE based at least
in part on the measurement report, wherein the modifying the
handover of the UE comprises reconfiguring the first connection to
be the primary connection and the second connection to be the
secondary connection based at least in part on the measurement
report.
22. The method of claim 21, further comprising: transmitting, to
the first base station prior to receiving the measurement report, a
role switch indication that indicates that the second connection
between the UE and the second base station is a primary connection
of the UE and a first connection between the UE and the first base
station is a secondary connection.
23. The method of claim 22, further comprising: receiving one or
more additional measurement reports; and further reconfiguring the
primary connection and the secondary connection based at least in
part on the one or more additional measurement reports.
24. The method of claim 22, wherein the measurement report is
transmitted by the UE responsive to the measured one or more
channel conditions of the secondary connection being better than
corresponding channel conditions of the primary connection.
25. The method of claim 22, further comprising: exchanging
information with the first base station to establish duplication of
data communicated with the UE via both the first connection and the
second connection; and communicating duplicated data with the UE
via the second connection.
26. The method of claim 21, further comprising: releasing the
second connection after a first connection has been a primary
connection for a predetermined time period.
27. An apparatus for wireless communication, comprising: a
processor, memory in electronic communication with the processor;
and instructions stored in the memory and executable by the
processor to cause the apparatus to: establish, at a user equipment
(UE), a first connection with a first base station; receive a first
radio resource control (RRC) reconfiguration message from the first
base station comprising an indication to perform a handover
procedure with a second base station; initiate, at the UE, a
connection establishment with the second base station to establish
a second connection responsive to the first RRC reconfiguration
message, wherein the first connection is maintained during the
connection establishment with the second base station; measure one
or more channel conditions associated with one or more of the first
base station, the second base station or a third base station;
transmit, responsive to the first RRC reconfiguration message, a
measurement report to at least one of the first base station or the
second base station responsive to measuring the one or more channel
conditions; receive, responsive to the measurement report, a second
RRC reconfiguration message from the second base station.
28. The apparatus of claim 27, wherein the instructions are further
executable by the processor to cause the apparatus to: establish
the second connection with the second base station, wherein the
second connection is a primary connection and the first connection
is maintained as a secondary connection; receive, responsive to the
measurement report, the second RRC reconfiguration message from the
second base station to reconfigure the first connection as the
primary connection and the second connection as the secondary
connection; and reconfigure the first connection and the second
connection responsive to the second RRC reconfiguration
message.
29. The apparatus of claim 28, wherein the reconfiguring the first
connection and the second connection responsive to the second RRC
reconfiguration message is performed without performing a random
access procedure.
30. The apparatus of claim 27, wherein the instructions are further
executable by the processor to cause the apparatus to: release the
first connection after the second connection has been a primary
connection for a predetermined time period.
Description
BACKGROUND
[0001] The following relates generally to wireless communications,
and more specifically to handover techniques in wireless
communications.
[0002] Wireless communications systems are widely deployed to
provide various types of communication content such as voice,
video, packet data, messaging, broadcast, and so on. These systems
may be capable of supporting communication with multiple users by
sharing the available system resources (e.g., time, frequency, and
power). Examples of such multiple-access systems include fourth
generation (4G) systems such as Long Term Evolution (LTE) systems,
LTE-Advanced (LTE-A) systems, or LTE-A Pro systems, and fifth
generation (5G) systems which may be referred to as New Radio (NR)
systems. These systems may employ technologies such as code
division multiple access (CDMA), time division multiple access
(TDMA), frequency division multiple access (FDMA), orthogonal
frequency division multiple access (OFDMA), or discrete Fourier
transform spread orthogonal frequency division multiplexing
(DFT-S-OFDM). A wireless multiple-access communications system may
include a number of base stations or network access nodes, each
simultaneously supporting communication for multiple communication
devices, which may be otherwise known as user equipment (UE).
[0003] When operating in a wireless communications system, a UE may
move between coverage areas of multiple different base stations. In
cases where radio signals of a neighboring base station, which may
be referred to as a target base station, will provide an enhanced
connection with a UE relative to a currently serving (or source)
base station, the UE may be handed over from the source base
station to the target base station. Such techniques may be referred
to as handover procedures or mobility procedures, and help to
provide continuous connectivity to a UE as it moves in a wireless
communications system. In some systems, a UE may release an active
connection with the source base station and establish a new
connection with the target base station in response to a handover
communication from the source base station. Enhanced techniques for
performing handover may help to enhance the overall efficiency and
reliability of a wireless communications system.
SUMMARY
[0004] The described techniques relate to improved methods,
systems, devices, and apparatuses for performing handovers in
wireless communications. In various aspects, the described
techniques provide that a handover may be performed based on one or
more conditions at a UE in conjunction with a handover procedure
from a source base station to a target base station. In some cases,
the one or more conditions at the UE may be associated with
successful establishment or maintenance of the second connection.
In some cases, the one or more conditions at the UE may correspond
to one or more measurements associated with the source base
station, the target base station, one or more neighboring base
stations, or any combinations thereof.
[0005] In some cases, for example, the UE may be unable to
establish the second connection, or the second connection may be
established and then fail shortly afterward. In such cases, the UE
may report a radio link failure to the source base station, and
maintain the first connection. In some cases, the source base
station may discontinue the handover and initiate a new handover
procedure at the UE to a different neighboring base station.
[0006] Additionally or alternatively, the UE may perform a
measurement as part of the handover, and the handover may be
modified based at least in part on the measurement. For example,
the measurement may be performed after establishment of the second
connection at the target base station, where the second connection
is a primary connection and the first connection is a secondary
connection. In such cases, the source and target base stations may
perform a role switch based on the measurement (e.g., when the
measurement indicates the first connection has better channel
conditions than the second connection) to change the first
connection to be the primary connection and the second connection
to be the secondary connection. In some cases, two or more role
switches may be performed based on multiple measurements provided
by the UE in accordance with a ping-pong handling procedure, until
one of the connections remains the primary connection for a
predetermined time period.
[0007] In some cases, the UE may perform the measurement as part of
a random access procedure with the target base station and may
measure channel quality of one or more neighboring base stations.
In such cases, the UE may transmit the measurement to the source
base station in the event that a neighboring base station has a
channel quality that exceeds a threshold. The source base station,
based at least in part on the measurement, may cancel the handover
to the second base station and initiate a handover with the
neighboring base station.
[0008] In some cases, the UE may transmit a measurement report to
the source base station that may initiate a handover of the UE from
the source base station to the target base station. In some cases,
a second connection with the target base station may be established
using a dual connectivity (DC) technique, in which the first
connection is changed to a secondary connection and the newly
established second connection is set as a primary connection. In
some cases, the source base station may maintain the first
connection as a primary connection, and switch the first connection
to be a secondary connection responsive to a role switch
communication with the target base station. In some cases, the role
switch communication with the target base station may be combined
with a handover request to the target base station and an
associated acknowledgment from the target base station.
[0009] A method of wireless communication is described. The method
may include establishing, at a UE, a first connection with a first
base station, receiving a handover message from the first base
station to perform a handover procedure with a second base station,
transmitting, responsive to the handover message, a request to the
second base station to establish a second connection with the
second base station, where the first connection with the first base
station is maintained during the handover procedure, determining
that a radio link failure of the first connection or the second
connection has occurred, and transmitting an indication of the
radio link failure to the first base station or the second base
station responsive to the determining.
[0010] An apparatus for wireless communication is described. The
apparatus may include a processor, memory in electronic
communication with the processor, and instructions stored in the
memory. The instructions may be executable by the processor to
cause the apparatus to establish, at a UE, a first connection with
a first base station, receive a handover message from the first
base station to perform a handover procedure with a second base
station, transmit, responsive to the handover message, a request to
the second base station to establish a second connection with the
second base station, where the first connection with the first base
station is maintained during the handover procedure, determine that
a radio link failure of the first connection or the second
connection has occurred, and transmit an indication of the radio
link failure to the first base station or the second base station
responsive to the determining.
[0011] Another apparatus for wireless communication is described.
The apparatus may include means for establishing, at a UE, a first
connection with a first base station, receiving a handover message
from the first base station to perform a handover procedure with a
second base station, transmitting, responsive to the handover
message, a request to the second base station to establish a second
connection with the second base station, where the first connection
with the first base station is maintained during the handover
procedure, determining that a radio link failure of the first
connection or the second connection has occurred, and transmitting
an indication of the radio link failure to the first base station
or the second base station responsive to the determining.
[0012] A non-transitory computer-readable medium storing code for
wireless communication is described. The code may include
instructions executable by a processor to establish, at a UE, a
first connection with a first base station, receive a handover
message from the first base station to perform a handover procedure
with a second base station, transmit, responsive to the handover
message, a request to the second base station to establish a second
connection with the second base station, where the first connection
with the first base station is maintained during the handover
procedure, determine that a radio link failure of the first
connection or the second connection has occurred, and transmit an
indication of the radio link failure to the first base station or
the second base station responsive to the determining.
[0013] In some examples of the method, apparatuses, and
non-transitory computer-readable medium described herein, the
determining may include operations, features, means, or
instructions for determining that establishment of the connection
with the second base station may have failed or the second
connection failed after establishment of the second connection, and
maintaining the first connection with the first base station. In
some examples of the method, apparatuses, and non-transitory
computer-readable medium described herein, the indication of the
radio link failure includes a failure cause associated with the
second connection and a target cell group identification associated
with the second base station. In some examples of the method,
apparatuses, and non-transitory computer-readable medium described
herein, the indication of the radio link failure further indicates
one or more of a timer expiration associated with the second
connection, a random access procedure failure, or a maximum number
of retransmissions is reached for a communication using the second
connection. In some examples of the method, apparatuses, and
non-transitory computer-readable medium described herein, the
indication of the radio link failure further indicates a
measurement of one or more signals received at the UE from one or
more neighboring base stations.
[0014] In some examples of the method, apparatuses, and
non-transitory computer-readable medium described herein, the
determining may include operations, features, means, or
instructions for determining that the first connection with the
first base station may have failed prior to completion of an
establishment of the second connection, and completing the
establishment of the second connection with the second base
station. In some examples of the method, apparatuses, and
non-transitory computer-readable medium described herein, the
completing the establishment of the second connection preempts a
triggering of a reestablishment of the first connection with the
first base station. Some examples of the method, apparatuses, and
non-transitory computer-readable medium described herein may
further include operations, features, means, or instructions for
releasing the first connection and a source stack associated with
the first connection responsive to completing the establishment of
the second connection with the second base station. In some
examples of the method, apparatuses, and non-transitory
computer-readable medium described herein, the indication of the
radio link failure indicates a failure cause associated with the
first connection and a source cell group identification associated
with the first base station.
[0015] A method of wireless communication is described. The method
may include establishing, at a first base station, a first
connection with a UE, initiating a handover procedure to handover
the UE to a second base station, where the first connection with
the first base station is maintained during the handover procedure,
receiving, from the UE, an indication of a failure of the handover
procedure, and discontinuing the handover procedure to handover the
UE to the second base station.
[0016] An apparatus for wireless communication is described. The
apparatus may include a processor, memory in electronic
communication with the processor, and instructions stored in the
memory. The instructions may be executable by the processor to
cause the apparatus to establish, at a first base station, a first
connection with a UE, initiate a handover procedure to handover the
UE to a second base station, where the first connection with the
first base station is maintained during the handover procedure,
receive, from the UE, an indication of a failure of the handover
procedure, and discontinue the handover procedure to handover the
UE to the second base station.
[0017] Another apparatus for wireless communication is described.
The apparatus may include means for establishing, at a first base
station, a first connection with a UE, initiating a handover
procedure to handover the UE to a second base station, where the
first connection with the first base station is maintained during
the handover procedure, receiving, from the UE, an indication of a
failure of the handover procedure, and discontinuing the handover
procedure to handover the UE to the second base station.
[0018] A non-transitory computer-readable medium storing code for
wireless communication is described. The code may include
instructions executable by a processor to establish, at a first
base station, a first connection with a UE, initiate a handover
procedure to handover the UE to a second base station, where the
first connection with the first base station is maintained during
the handover procedure, receive, from the UE, an indication of a
failure of the handover procedure, and discontinue the handover
procedure to handover the UE to the second base station.
[0019] In some examples of the method, apparatuses, and
non-transitory computer-readable medium described herein, the
receiving the indication of the failure of the handover procedure
further may include operations, features, means, or instructions
for receiving a radio link failure message from the UE indicating a
timer expiration associated with a random access procedure to
establish a second connection with the second base station. Some
examples of the method, apparatuses, and non-transitory
computer-readable medium described herein may further include
operations, features, means, or instructions for receiving a radio
link failure message from the UE indicating a second connection
with the second base station was established and then failed. In
some examples of the method, apparatuses, and non-transitory
computer-readable medium described herein, the radio link failure
message indicates a failure cause associated with the second
connection, and where the failure cause indicates one or more or a
timer expiration associated with the second connection, a random
access procedure failure, or a maximum number of retransmissions is
reached for a communication using the second connection. In some
examples of the method, apparatuses, and non-transitory
computer-readable medium described herein, the radio link failure
message further indicates a target cell group identification
associated with the second base station. In some examples of the
method, apparatuses, and non-transitory computer-readable medium
described herein, the indication of the failure of the handover
procedure further indicates a measurement of one or more signals
received at the UE from a neighboring base station.
[0020] Some examples of the method, apparatuses, and non-transitory
computer-readable medium described herein may further include
operations, features, means, or instructions for transmitting, to
the second base station and responsive to the receiving the
indication of the failure of the handover procedure, a message to
the second base station to cancel the handover procedure.
[0021] Some examples of the method, apparatuses, and non-transitory
computer-readable medium described herein may further include
operations, features, means, or instructions for initiating a
second handover procedure to handover the UE to a third base
station, where the first connection with the first base station may
be maintained during the second handover procedure. In some
examples of the method, apparatuses, and non-transitory
computer-readable medium described herein, the initiating the
second handover procedure further may include operations, features,
means, or instructions for transmitting, to the UE, a
reconfiguration message indicating the UE is to perform the second
handover procedure with the third base station.
[0022] A method of wireless communication is described. The method
may include receiving, at a second base station, a handover message
to initiate a handover of a UE from a first base station to the
second base station, initiating, responsive to the handover
message, a connection establishment with the UE to establish a
second connection between the UE and the second base station,
receiving from the UE an indication of a failure of a first
connection between the UE and the first base station, where the
first connection was to be maintained during the handover of the UE
from the first base station to the second base station, and
forwarding the indication of the failure of the first connection to
the first base station.
[0023] An apparatus for wireless communication is described. The
apparatus may include a processor, memory in electronic
communication with the processor, and instructions stored in the
memory. The instructions may be executable by the processor to
cause the apparatus to receive, at a second base station, a
handover message to initiate a handover of a UE from a first base
station to the second base station, initiate, responsive to the
handover message, a connection establishment with the UE to
establish a second connection between the UE and the second base
station, receive from the UE an indication of a failure of a first
connection between the UE and the first base station, where the
first connection was to be maintained during the handover of the UE
from the first base station to the second base station, and forward
the indication of the failure of the first connection to the first
base station.
[0024] Another apparatus for wireless communication is described.
The apparatus may include means for receiving, at a second base
station, a handover message to initiate a handover of a UE from a
first base station to the second base station, initiating,
responsive to the handover message, a connection establishment with
the UE to establish a second connection between the UE and the
second base station, receiving from the UE an indication of a
failure of a first connection between the UE and the first base
station, where the first connection was to be maintained during the
handover of the UE from the first base station to the second base
station, and forwarding the indication of the failure of the first
connection to the first base station.
[0025] A non-transitory computer-readable medium storing code for
wireless communication is described. The code may include
instructions executable by a processor to receive, at a second base
station, a handover message to initiate a handover of a UE from a
first base station to the second base station, initiate, responsive
to the handover message, a connection establishment with the UE to
establish a second connection between the UE and the second base
station, receive from the UE an indication of a failure of a first
connection between the UE and the first base station, where the
first connection was to be maintained during the handover of the UE
from the first base station to the second base station, and forward
the indication of the failure of the first connection to the first
base station.
[0026] In some examples of the method, apparatuses, and
non-transitory computer-readable medium described herein, the
indication of the failure of the first connection indicates that
resources and context associated with the first connection is to be
deleted. In some examples of the method, apparatuses, and
non-transitory computer-readable medium described herein, the
initiating the connection establishment with the UE further may
include operations, features, means, or instructions for performing
a random access procedure with the UE to establish the second
connection between the UE and the second base station, and where
the indication of the failure of the first connection may be
received subsequent to the establishment of the second connection.
In some examples of the method, apparatuses, and non-transitory
computer-readable medium described herein, the indication of the
failure of the first connection includes a failure cause associated
with the first connection and a source cell group identification
associated with the first base station. In some examples of the
method, apparatuses, and non-transitory computer-readable medium
described herein, the indication of the failure of the first
connection further includes a measurement of one or more signals
received at the UE from a neighboring base station.
[0027] A method of wireless communication is described. The method
may include establishing, at a UE, a first connection with a first
base station, receiving a handover message from the first base
station to perform a handover procedure with a second base station,
initiating, at the UE, a connection establishment with the second
base station to establish a second connection responsive to the
handover message, where the first connection is maintained during
the connection establishment with the second base station,
measuring one or more channel conditions associated with one or
more of the first base station, the second base station or a third
base station, and transmitting, responsive to the handover message,
a measurement report to at least one of the first base station or
the second base station responsive to measuring the one or more
channel conditions.
[0028] An apparatus for wireless communication is described. The
apparatus may include a processor, memory in electronic
communication with the processor, and instructions stored in the
memory. The instructions may be executable by the processor to
cause the apparatus to establish, at a UE, a first connection with
a first base station, receive a handover message from the first
base station to perform a handover procedure with a second base
station, initiate, at the UE, a connection establishment with the
second base station to establish a second connection responsive to
the handover message, where the first connection is maintained
during the connection establishment with the second base station,
measure one or more channel conditions associated with one or more
of the first base station, the second base station or a third base
station, and transmit, responsive to the handover message, a
measurement report to at least one of the first base station or the
second base station responsive to measuring the one or more channel
conditions.
[0029] Another apparatus for wireless communication is described.
The apparatus may include means for establishing, at a UE, a first
connection with a first base station, receiving a handover message
from the first base station to perform a handover procedure with a
second base station, initiating, at the UE, a connection
establishment with the second base station to establish a second
connection responsive to the handover message, where the first
connection is maintained during the connection establishment with
the second base station, measuring one or more channel conditions
associated with one or more of the first base station, the second
base station or a third base station, and transmitting, responsive
to the handover message, a measurement report to at least one of
the first base station or the second base station responsive to
measuring the one or more channel conditions.
[0030] A non-transitory computer-readable medium storing code for
wireless communication is described. The code may include
instructions executable by a processor to establish, at a UE, a
first connection with a first base station, receive a handover
message from the first base station to perform a handover procedure
with a second base station, initiate, at the UE, a connection
establishment with the second base station to establish a second
connection responsive to the handover message, where the first
connection is maintained during the connection establishment with
the second base station, measure one or more channel conditions
associated with one or more of the first base station, the second
base station or a third base station, and transmit, responsive to
the handover message, a measurement report to at least one of the
first base station or the second base station responsive to
measuring the one or more channel conditions.
[0031] Some examples of the method, apparatuses, and non-transitory
computer-readable medium described herein may further include
operations, features, means, or instructions for establishing the
second connection with the second base station, where the second
connection is a primary connection and the first connection is
maintained as a secondary connection, receiving, responsive to the
measurement report, a reconfiguration message from the second base
station to reconfigure the first connection as the primary
connection and the second connection as the secondary connection,
and reconfiguring the first connection and the second connection
responsive to the reconfiguration message.
[0032] Some examples of the method, apparatuses, and non-transitory
computer-readable medium described herein may further include
operations, features, means, or instructions for transmitting one
or more additional measurement reports via the primary connection.
In some examples of the method, apparatuses, and non-transitory
computer-readable medium described herein, the measurement report
may be transmitted responsive to the measured one or more channel
conditions of the secondary connection being better than
corresponding channel conditions of the primary connection.
[0033] In some examples of the method, apparatuses, and
non-transitory computer-readable medium described herein, the
reconfiguring the first connection and the second connection
responsive to the reconfiguration message may be performed without
performing a random access procedure. Some examples of the method,
apparatuses, and non-transitory computer-readable medium described
herein may further include operations, features, means, or
instructions for transmitting and receiving duplicated data via
each of the primary connection and the secondary connection.
[0034] Some examples of the method, apparatuses, and non-transitory
computer-readable medium described herein may further include
operations, features, means, or instructions for releasing the
first connection after the second connection may have been a
primary connection for a predetermined time period. Some examples
of the method, apparatuses, and non-transitory computer-readable
medium described herein may further include operations, features,
means, or instructions for receiving, responsive to the measurement
report, a reconfiguration message from the first base station to
abort the handover procedure with the second base station. In some
examples of the method, apparatuses, and non-transitory
computer-readable medium described herein, the measurement report
may be transmitted responsive to measured channel conditions of the
third base station being better than measured channel conditions of
the second base station.
[0035] In some examples of the method, apparatuses, and
non-transitory computer-readable medium described herein, the
reconfiguration message further includes a second handover message
from the first base station to perform a second handover procedure
with the third base station.
[0036] A method of wireless communication is described. The method
may include establishing, at a first base station, a first
connection with a UE, initiating a handover of the UE to a second
base station, where the first connection with the first base
station is maintained during the handover and an establishment of a
second connection between the UE and the second base station,
receiving a measurement report responsive to the initiating the
handover, and modifying the handover of the UE based on the
measurement report.
[0037] An apparatus for wireless communication is described. The
apparatus may include a processor, memory in electronic
communication with the processor, and instructions stored in the
memory. The instructions may be executable by the processor to
cause the apparatus to establish, at a first base station, a first
connection with a UE, initiate a handover of the UE to a second
base station, where the first connection with the first base
station is maintained during the handover and an establishment of a
second connection between the UE and the second base station,
receive a measurement report responsive to the initiating the
handover, and modify the handover of the UE based on the
measurement report.
[0038] Another apparatus for wireless communication is described.
The apparatus may include means for establishing, at a first base
station, a first connection with a UE, initiating a handover of the
UE to a second base station, where the first connection with the
first base station is maintained during the handover and an
establishment of a second connection between the UE and the second
base station, receiving a measurement report responsive to the
initiating the handover, and modifying the handover of the UE based
on the measurement report.
[0039] A non-transitory computer-readable medium storing code for
wireless communication is described. The code may include
instructions executable by a processor to establish, at a first
base station, a first connection with a UE, initiate a handover of
the UE to a second base station, where the first connection with
the first base station is maintained during the handover and an
establishment of a second connection between the UE and the second
base station, receive a measurement report responsive to the
initiating the handover, and modify the handover of the UE based on
the measurement report.
[0040] Some examples of the method, apparatuses, and non-transitory
computer-readable medium described herein may further include
operations, features, means, or instructions for transmitting a
handover indication to the second base station, receiving, from the
second base station prior to receiving the measurement report, a
role switch indication that indicates that a second connection
between the UE and the second base station is a primary connection
of the UE and the first connection is a secondary connection, and
where the modifying the handover of the UE includes reconfiguring
the first connection to be the primary connection and the second
connection to be the secondary connection based on the measurement
report.
[0041] Some examples of the method, apparatuses, and non-transitory
computer-readable medium described herein may further include
operations, features, means, or instructions for receiving one or
more additional measurement reports, and further reconfiguring the
primary connection and the secondary connection based on the one or
more additional measurement reports. In some examples of the
method, apparatuses, and non-transitory computer-readable medium
described herein, the measurement report may be transmitted
responsive to the measured one or more channel conditions of the
secondary connection being better than corresponding channel
conditions of the primary connection.
[0042] Some examples of the method, apparatuses, and non-transitory
computer-readable medium described herein may further include
operations, features, means, or instructions for exchanging
information with the second base station to establish duplication
of data communicated with the UE via both the first connection and
the second connection, and communicating duplicated data with the
UE via the first connection.
[0043] Some examples of the method, apparatuses, and non-transitory
computer-readable medium described herein may further include
operations, features, means, or instructions for releasing the
first connection after the second connection may have been a
primary connection for a predetermined time period. In some
examples of the method, apparatuses, and non-transitory
computer-readable medium described herein, the modifying the
handover of the UE may include operations, features, means, or
instructions for transmitting, responsive to the measurement
report, a reconfiguration message to the UE to abort the handover
procedure with the second base station. In some examples of the
method, apparatuses, and non-transitory computer-readable medium
described herein, the modifying the handover of the UE further may
include operations, features, means, or instructions for
transmitting an indication to the second base station that the
handover of the UE to the second base station may be aborted.
[0044] In some examples of the method, apparatuses, and
non-transitory computer-readable medium described herein, the
measurement report may be transmitted responsive to measured
channel conditions of a third base station being better than
measured channel conditions of the second base station. In some
examples of the method, apparatuses, and non-transitory
computer-readable medium described herein, the reconfiguration
message further includes a second handover message to the UE to
perform a second handover procedure with a third base station.
[0045] A method of wireless communication is described. The method
may include receiving, at a second base station from a first base
station, a handover message to initiate a handover of a UE from the
first base station to the second base station, establishing a
second connection with the UE responsive to the handover message,
receiving a measurement report from the UE that includes one or
more channel measurements associated with the first base station
and the second base station, and modifying the handover of the UE
based on the measurement report.
[0046] An apparatus for wireless communication is described. The
apparatus may include a processor, memory in electronic
communication with the processor, and instructions stored in the
memory. The instructions may be executable by the processor to
cause the apparatus to receive, at a second base station from a
first base station, a handover message to initiate a handover of a
UE from the first base station to the second base station,
establish a second connection with the UE responsive to the
handover message, receive a measurement report from the UE that
includes one or more channel measurements associated with the first
base station and the second base station, and modify the handover
of the UE based on the measurement report.
[0047] Another apparatus for wireless communication is described.
The apparatus may include means for receiving, at a second base
station from a first base station, a handover message to initiate a
handover of a UE from the first base station to the second base
station, establishing a second connection with the UE responsive to
the handover message, receiving a measurement report from the UE
that includes one or more channel measurements associated with the
first base station and the second base station, and modifying the
handover of the UE based on the measurement report.
[0048] A non-transitory computer-readable medium storing code for
wireless communication is described. The code may include
instructions executable by a processor to receive, at a second base
station from a first base station, a handover message to initiate a
handover of a UE from the first base station to the second base
station, establish a second connection with the UE responsive to
the handover message, receive a measurement report from the UE that
includes one or more channel measurements associated with the first
base station and the second base station, and modify the handover
of the UE based on the measurement report.
[0049] Some examples of the method, apparatuses, and non-transitory
computer-readable medium described herein may further include
operations, features, means, or instructions for transmitting, to
the first base station prior to receiving the measurement report, a
role switch indication that indicates that a second connection
between the UE and the second base station is a primary connection
of the UE and a first connection between the IE and the first base
station is a secondary connection, and where the modifying the
handover of the UE includes reconfiguring the first connection to
be the primary connection and the second connection to be the
secondary connection based on the measurement report.
[0050] Some examples of the method, apparatuses, and non-transitory
computer-readable medium described herein may further include
operations, features, means, or instructions for receiving one or
more additional measurement reports, and further reconfiguring the
primary connection and the secondary connection based on the one or
more additional measurement reports. In some examples of the
method, apparatuses, and non-transitory computer-readable medium
described herein, the measurement report may be transmitted by the
UE responsive to the measured one or more channel conditions of the
secondary connection being better than corresponding channel
conditions of the primary connection.
[0051] Some examples of the method, apparatuses, and non-transitory
computer-readable medium described herein may further include
operations, features, means, or instructions for exchanging
information with the first base station to establish duplication of
data communicated with the UE via both the first connection and the
second connection, and communicating duplicated data with the UE
via the second connection.
[0052] Some examples of the method, apparatuses, and non-transitory
computer-readable medium described herein may further include
operations, features, means, or instructions for releasing the
second connection after the first connection has been a primary
connection for a predetermined time period.
[0053] A method of wireless communication is described. The method
may include establishing, at a first base station, a first
connection with a UE, initiating a handover of the UE to a second
base station, where the first connection with the first base
station is maintained during the handover and an establishment of a
second connection between the UE and the second base station, and
receiving, from the second base station, a role switch indication
that indicates that the second connection between the UE and the
second base station is a primary connection of the UE.
[0054] An apparatus for wireless communication is described. The
apparatus may include a processor, memory in electronic
communication with the processor, and instructions stored in the
memory. The instructions may be executable by the processor to
cause the apparatus to establish, at a first base station, a first
connection with a UE, initiate a handover of the UE to a second
base station, where the first connection with the first base
station is maintained during the handover and an establishment of a
second connection between the UE and the second base station, and
receive, from the second base station, a role switch indication
that indicates that the second connection between the UE and the
second base station is a primary connection of the UE.
[0055] Another apparatus for wireless communication is described.
The apparatus may include means for establishing, at a first base
station, a first connection with a UE, initiating a handover of the
UE to a second base station, where the first connection with the
first base station is maintained during the handover and an
establishment of a second connection between the UE and the second
base station, and receiving, from the second base station, a role
switch indication that indicates that the second connection between
the UE and the second base station is a primary connection of the
UE.
[0056] A non-transitory computer-readable medium storing code for
wireless communication is described. The code may include
instructions executable by a processor to establish, at a first
base station, a first connection with a UE, initiate a handover of
the UE to a second base station, where the first connection with
the first base station is maintained during the handover and an
establishment of a second connection between the UE and the second
base station, and receive, from the second base station, a role
switch indication that indicates that the second connection between
the UE and the second base station is a primary connection of the
UE.
[0057] In some examples of the method, apparatuses, and
non-transitory computer-readable medium described herein, the
initiating the handover may include operations, features, means, or
instructions for configuring one or more secondary node (SN)
terminated bearers at the second base station for the second
connection, and transmitting a reconfiguration message to the UE
that indicates the handover to the second base station.
[0058] In some examples of the method, apparatuses, and
non-transitory computer-readable medium described herein, the
initiating the handover further may include operations, features,
means, or instructions for transmitting a role switch request to
the second base station, and receiving an acknowledgment of the
role switch request from the second base station. In some examples
of the method, apparatuses, and non-transitory computer-readable
medium described herein, the role switch request may be transmitted
with a SN addition request to the second base station, and the
acknowledgment of the role switch request may be received with a SN
addition acknowledgment from the second base station. In some
examples of the method, apparatuses, and non-transitory
computer-readable medium described herein, the reconfiguration
message to the UE indicates a secondary carrier group associated
with the second connection and a source cell group identification
associated with the first connection. In some examples of the
method, apparatuses, and non-transitory computer-readable medium
described herein, the role switch indication may be received
subsequent to performance of a random access procedure between the
second base station and the UE. Some examples of the method,
apparatuses, and non-transitory computer-readable medium described
herein may further include operations, features, means, or
instructions for releasing the first connection with the UE
responsive to the role switch indication.
[0059] In some examples of the method, apparatuses, and
non-transitory computer-readable medium described herein, the
releasing the first connection further may include operations,
features, means, or instructions for transmitting a secondary node
(SN) status transfer message to the second base station to indicate
completion of the role switch. Some examples of the method,
apparatuses, and non-transitory computer-readable medium described
herein may further include operations, features, means, or
instructions for receiving data associated with the UE during the
handover, and forwarding the received data to the second base
station.
[0060] Some examples of the method, apparatuses, and non-transitory
computer-readable medium described herein may further include
operations, features, means, or instructions for configuring the UE
to maintain the first connection during the handover and to use the
first connection for transmitting and receiving data while the
second connection is established.
[0061] A method of wireless communication is described. The method
may include receiving, at a second base station from a first base
station, a handover message to initiate a handover of a UE from the
first base station to the second base station, establishing a
second connection with the UE responsive to the handover message,
and transmitting, responsive to the establishing the second
connection, a role switch indication to the first base station that
indicates that the second connection between the UE and the second
base station is a primary connection of the UE.
[0062] An apparatus for wireless communication is described. The
apparatus may include a processor, memory in electronic
communication with the processor, and instructions stored in the
memory. The instructions may be executable by the processor to
cause the apparatus to receive, at a second base station from a
first base station, a handover message to initiate a handover of a
UE from the first base station to the second base station,
establish a second connection with the UE responsive to the
handover message, and transmit, responsive to the establishing the
second connection, a role switch indication to the first base
station that indicates that the second connection between the UE
and the second base station is a primary connection of the UE.
[0063] Another apparatus for wireless communication is described.
The apparatus may include means for receiving, at a second base
station from a first base station, a handover message to initiate a
handover of a UE from the first base station to the second base
station, establishing a second connection with the UE responsive to
the handover message, and transmitting, responsive to the
establishing the second connection, a role switch indication to the
first base station that indicates that the second connection
between the UE and the second base station is a primary connection
of the UE.
[0064] A non-transitory computer-readable medium storing code for
wireless communication is described. The code may include
instructions executable by a processor to receive, at a second base
station from a first base station, a handover message to initiate a
handover of a UE from the first base station to the second base
station, establish a second connection with the UE responsive to
the handover message, and transmit, responsive to the establishing
the second connection, a role switch indication to the first base
station that indicates that the second connection between the UE
and the second base station is a primary connection of the UE.
[0065] In some examples of the method, apparatuses, and
non-transitory computer-readable medium described herein, the
receiving the handover message further may include operations,
features, means, or instructions for receiving an indication from
the first base station to configure one or more secondary node (SN)
terminated bearers at the second base station for the second
connection. In some examples of the method, apparatuses, and
non-transitory computer-readable medium described herein, the
receiving the handover message further may include operations,
features, means, or instructions for receiving a role switch
request from the first base station that indicates the second
connection with the UE is to be the primary connection of the UE,
and transmitting an acknowledgment of the role switch request to
the first base station.
[0066] In some examples of the method, apparatuses, and
non-transitory computer-readable medium described herein, the role
switch request may be received with a SN addition request from the
first base station, and the acknowledgment of the role switch
request may be transmitted with a SN addition acknowledgment to the
first base station.
[0067] In some examples of the method, apparatuses, and
non-transitory computer-readable medium described herein, the
establishing a second connection with the UE may include
operations, features, means, or instructions for performing a
random access procedure with the UE to establish the second
connection. In some examples of the method, apparatuses, and
non-transitory computer-readable medium described herein, the role
switch indication may be transmitted after the performing the
random access procedure.
[0068] Some examples of the method, apparatuses, and non-transitory
computer-readable medium described herein may further include
operations, features, means, or instructions for receiving, from
the first base station responsive to the role switch indication, a
secondary node (SN) status transfer message, and transmitting an
indication to the UE to release a first connection with the first
base station. Some examples of the method, apparatuses, and
non-transitory computer-readable medium described herein may
further include operations, features, means, or instructions for
receiving, from the first base station, data associated with the UE
that is received at the first base station during the handover.
BRIEF DESCRIPTION OF THE DRAWINGS
[0069] FIG. 1 illustrates an example of a system for wireless
communications that supports handover techniques in wireless
communications in accordance with aspects of the present
disclosure.
[0070] FIG. 2 illustrates an example of a wireless communications
system that supports handover techniques in wireless communications
in accordance with aspects of the present disclosure.
[0071] FIG. 3 illustrates an example of a process flow that
supports handover techniques in wireless communications in
accordance with aspects of the present disclosure.
[0072] FIG. 4 illustrates an example of dual connectivity handover
protocol stacks in wireless communications in accordance with
aspects of the present disclosure.
[0073] FIG. 5 illustrates an example of a process flow that
supports handover techniques in wireless communications in
accordance with aspects of the present disclosure.
[0074] FIG. 6 illustrates an example of a process flow that
supports handover techniques in wireless communications in
accordance with aspects of the present disclosure.
[0075] FIG. 7 illustrates an example of a process flow that
supports handover techniques in wireless communications in
accordance with aspects of the present disclosure.
[0076] FIG. 8 illustrates an example of a process flow that
supports handover techniques in wireless communications in
accordance with aspects of the present disclosure.
[0077] FIGS. 9 and 10 show block diagrams of devices that support
handover techniques in wireless communications in accordance with
aspects of the present disclosure.
[0078] FIG. 11 shows a block diagram of a communications manager
that supports handover techniques in wireless communications in
accordance with aspects of the present disclosure.
[0079] FIG. 12 shows a diagram of a system including a device that
supports handover techniques in wireless communications in
accordance with aspects of the present disclosure.
[0080] FIGS. 13 and 14 show block diagrams of devices that support
handover techniques in wireless communications in accordance with
aspects of the present disclosure.
[0081] FIG. 15 shows a block diagram of a communications manager
that supports handover techniques in wireless communications in
accordance with aspects of the present disclosure.
[0082] FIG. 16 shows a diagram of a system including a device that
supports handover techniques in wireless communications in
accordance with aspects of the present disclosure.
[0083] FIGS. 17 through 28 show flowcharts illustrating methods
that support handover techniques in wireless communications in
accordance with aspects of the present disclosure.
DETAILED DESCRIPTION
[0084] Various aspects of the present disclosure provide enhanced
techniques for handover in a wireless communications system. In
some cases, a user equipment (UE) may establish a first connection
with a first base station, which may be referred to as a source
base station. The UE may receive a handover message, such as a
radio resource control (RRC) reconfiguration message to establish a
second connection with a second base station, which may be referred
to as a target base station. In some cases, the handover of the UE
from the source base station to the target base station may be
triggered by a measurement report from the UE that indicates
channel conditions at the source base station and target base
station that meet handover criteria. In some cases, the UE may
maintain the first connection while the second connection is
established, according to a dual-connectivity (DC) handover
technique or a make-before-break handover technique. Such a DC or
make-before-break handover technique may allow the UE to remain in
a RRC connected state with one base station during the handover
procedure, and may support a zero millisecond or almost zero
millisecond handover interruption.
[0085] In various aspects of the disclosure, described techniques
provide that a handover may be performed based on one or more
conditions at a UE in conjunction with the handover procedure. In
some cases, the one or more conditions at the UE may be associated
with successful establishment or maintenance of the second
connection, or one or more measurements made at the UE in
conjunction with the handover. In some cases, for example, the UE
may be unable to establish the second connection, or the second
connection may be established and then fail shortly afterward. In
such cases, the UE may report a radio link failure to the source
base station, and maintain the first connection. In some cases, the
source base station may discontinue the handover and initiate a new
handover procedure at the UE to a different neighboring base
station.
[0086] In some aspects of the disclosure, a UE may perform a
measurement as part of a handover, and the handover may be modified
based at least in part on the measurement. For example, the
measurement may be performed after establishment of the second
connection at the target base station, where the second connection
is a primary connection and the first connection is a secondary
connection. In such cases, the source and target base stations may
perform a role switch based on the measurement (e.g., when the
measurement indicates the first connection has better channel
conditions than the second connection) to change the first
connection to be the primary connection and the second connection
to be the secondary connection. In some cases, two or more role
switches may be performed based on multiple measurements provided
by the UE in accordance with a ping-pong handling procedure, until
one of the connections remains the primary connection for a
predetermined time period.
[0087] In some cases, the UE may perform the measurement as part of
a random access procedure with the target base station and may
measure channel quality of one or more neighboring base stations.
In such cases, the UE may transmit the measurement to the source
base station in the event that a neighboring base station has a
channel quality that exceeds a threshold. The source base station,
based at least in part on the measurement, may cancel the handover
to the second base station and initiate a handover with the
neighboring base station.
[0088] In some cases, a second connection with the target base
station may be established using a dual connectivity (DC)
technique, in which the first connection is changed to a secondary
connection and the newly established second connection is set as a
primary connection. In such cases, the source base station may
switch the first connection to be a secondary connection responsive
to a role switch communication with the target base station. In
some cases, a role switch request may be combined with a handover
request to the target base station and an associated acknowledgment
from the target base station may include a role switch request
acknowledgment, and the target base station may transmit a role
switch setup complete upon establishment of the second connection
that triggers the switch of the first connection to be a secondary
connection.
[0089] Such techniques may provide for enhanced reliability and
efficiency in handovers, and may provide 0 ms or almost 0 ms
interruption handovers. Such techniques may allow for a UE to
remain in an RRC connected state and allow to UE to transmit or
receive communications during a handover procedure. Such
connectivity may allow for enhanced communications that may have
relatively strict latency and/or reliability requirements (e.g.,
ultra-reliable low latency communications (URLLC)) during UE
mobility. Further, in some cases, a primary and secondary
connection at the UE may be switched on one or more occasions
during a handover procedure, which may allow communications on a
relatively reliable connection, which may enhance overall
reliability of communications. Additionally, in some cases a UE may
measure one or more channel conditions that may allow for
relatively fast handover to a more preferable base station, and
thus further enhance network reliability.
[0090] Aspects of the disclosure are initially described in the
context of a wireless communications system. Handover techniques in
accordance with various examples and then discussed. Aspects of the
disclosure are further illustrated by and described with reference
to apparatus diagrams, system diagrams, and flowcharts that relate
to handover techniques in wireless communications.
[0091] FIG. 1 illustrates an example of a wireless communications
system 100 that supports handover techniques in wireless
communications in accordance with aspects of the present
disclosure. The wireless communications system 100 includes base
stations 105, UEs 115, and a core network 130. In some examples,
the wireless communications system 100 may be a Long Term Evolution
(LTE) network, an LTE-Advanced (LTE-A) network, an LTE-A Pro
network, or a New Radio (NR) network. In some cases, wireless
communications system 100 may support enhanced broadband
communications, ultra-reliable (e.g., mission critical)
communications, low latency communications, or communications with
low-cost and low-complexity devices. UEs 115 may move within the
wireless communications system, and may perform handovers between
different base stations 105 in accordance with one or more handover
techniques as discussed herein.
[0092] Base stations 105 may wirelessly communicate with UEs 115
via one or more base station antennas. Base stations 105 described
herein may include or may be referred to by those skilled in the
art as a base transceiver station, a radio base station, an access
point, a radio transceiver, a NodeB, an eNodeB (eNB), a
next-generation NodeB or giga-NodeB (either of which may be
referred to as a gNB), a Home NodeB, a Home eNodeB, or some other
suitable terminology. Wireless communications system 100 may
include base stations 105 of different types (e.g., macro or small
cell base stations). The UEs 115 described herein may be able to
communicate with various types of base stations 105 and network
equipment including macro eNBs, small cell eNBs, gNBs, relay base
stations, and the like.
[0093] Each base station 105 may be associated with a particular
geographic coverage area 110 in which communications with various
UEs 115 is supported. Each base station 105 may provide
communication coverage for a respective geographic coverage area
110 via communication links 125, and communication links 125
between a base station 105 and a UE 115 may utilize one or more
carriers. Communication links 125 shown in wireless communications
system 100 may include uplink transmissions from a UE 115 to a base
station 105, or downlink transmissions from a base station 105 to a
UE 115. Downlink transmissions may also be called forward link
transmissions while uplink transmissions may also be called reverse
link transmissions.
[0094] The geographic coverage area 110 for a base station 105 may
be divided into sectors making up only a portion of the geographic
coverage area 110, and each sector may be associated with a cell.
For example, each base station 105 may provide communication
coverage for a macro cell, a small cell, a hot spot, or other types
of cells, or various combinations thereof. In some examples, a base
station 105 may be movable and therefore provide communication
coverage for a moving geographic coverage area 110. In some
examples, different geographic coverage areas 110 associated with
different technologies may overlap, and overlapping geographic
coverage areas 110 associated with different technologies may be
supported by the same base station 105 or by different base
stations 105. The wireless communications system 100 may include,
for example, a heterogeneous LTE/LTE-A/LTE-A Pro or NR network in
which different types of base stations 105 provide coverage for
various geographic coverage areas 110.
[0095] The term "cell" refers to a logical communication entity
used for communication with a base station 105 (e.g., over a
carrier), and may be associated with an identifier for
distinguishing neighboring cells (e.g., a physical cell identifier
(PCID), a virtual cell identifier (VCID)) operating via the same or
a different carrier. In some examples, a carrier may support
multiple cells, and different cells may be configured according to
different protocol types (e.g., machine-type communication (MTC),
narrowband Internet-of-Things (NB-IoT), enhanced mobile broadband
(eMBB), or others) that may provide access for different types of
devices. In some cases, the term "cell" may refer to a portion of a
geographic coverage area 110 (e.g., a sector) over which the
logical entity operates, or may refer to a radio head or
distributed unit (DU) and a base station 105 may control one or
more cells.
[0096] UEs 115 may be dispersed throughout the wireless
communications system 100, and each UE 115 may be stationary or
mobile. A UE 115 may also be referred to as a mobile device, a
wireless device, a remote device, a handheld device, or a
subscriber device, or some other suitable terminology, where the
"device" may also be referred to as a unit, a station, a terminal,
or a client. A UE 115 may also be a personal electronic device such
as a cellular phone, a personal digital assistant (PDA), a tablet
computer, a laptop computer, or a personal computer. In some
examples, a UE 115 may also refer to a wireless local loop (WLL)
station, an Internet of Things (IoT) device, an Internet of
Everything (IoE) device, or an MTC device, or the like, which may
be implemented in various articles such as appliances, vehicles,
meters, or the like.
[0097] Some UEs 115, such as MTC or IoT devices, may be low cost or
low complexity devices, and may provide for automated communication
between machines (e.g., via Machine-to-Machine (M2M)
communication). M2M communication or MTC may refer to data
communication technologies that allow devices to communicate with
one another or a base station 105 without human intervention. In
some examples, M2M communication or MTC may include communications
from devices that integrate sensors or meters to measure or capture
information and relay that information to a central server or
application program that can make use of the information or present
the information to humans interacting with the program or
application. Some UEs 115 may be designed to collect information or
enable automated behavior of machines. Examples of applications for
MTC devices include smart metering, inventory monitoring, water
level monitoring, equipment monitoring, healthcare monitoring,
wildlife monitoring, weather and geological event monitoring, fleet
management and tracking, remote security sensing, physical access
control, and transaction-based business charging.
[0098] In some cases, a UE 115 may also be able to communicate
directly with other UEs 115 (e.g., using a peer-to-peer (P2P) or
device-to-device (D2D) protocol). One or more of a group of UEs 115
utilizing D2D communications may be within the geographic coverage
area 110 of a base station 105. Other UEs 115 in such a group may
be outside the geographic coverage area 110 of a base station 105,
or be otherwise unable to receive transmissions from a base station
105. In some cases, groups of UEs 115 communicating via D2D
communications may utilize a one-to-many (1:M) system in which each
UE 115 transmits to every other UE 115 in the group. In some cases,
a base station 105 facilitates the scheduling of resources for D2D
communications. In other cases, D2D communications are carried out
between UEs 115 without the involvement of a base station 105.
[0099] Base stations 105 may communicate with the core network 130
and with one another. For example, base stations 105 may interface
with the core network 130 through backhaul links 132 (e.g., via an
S1, N2, N3, or other interface). Base stations 105 may communicate
with one another over backhaul links 134 (e.g., via an X2, Xn, or
other interface) either directly (e.g., directly between base
stations 105) or indirectly (e.g., via core network 130).
[0100] The core network 130 may provide user authentication, access
authorization, tracking, Internet Protocol (IP) connectivity, and
other access, routing, or mobility functions. The core network 130
may be an evolved packet core (EPC), which may include at least one
mobility management entity (MME), at least one serving gateway
(S-GW), and at least one Packet Data Network (PDN) gateway (P-GW).
The MME may manage non-access stratum (e.g., control plane)
functions such as mobility, authentication, and bearer management
for UEs 115 served by base stations 105 associated with the EPC.
User IP packets may be transferred through the S-GW, which itself
may be connected to the P-GW. The P-GW may provide IP address
allocation as well as other functions. The P-GW may be connected to
the network operators IP services. The operators IP services may
include access to the Internet, Intranet(s), an IP Multimedia
Subsystem (IMS), or a Packet-Switched (PS) Streaming Service.
[0101] At least some of the network devices, such as a base station
105, may include subcomponents such as an access network entity,
which may be an example of an access node controller (ANC) or
centralized unit (CU). Each access network entity may communicate
with UEs 115 through a number of other access network transmission
entities, which may be referred to as a radio head, a smart radio
head, a transmission/reception point (TRP), or a distributed unit
(DU). In some configurations, various functions of each access
network entity or base station 105 may be distributed across
various network devices (e.g., radio heads and access network
controllers) or consolidated into a single network device (e.g., a
base station 105). For example, a CU may control two or more DUs,
which may each be associated with a different cell.
[0102] Wireless communications system 100 may operate using one or
more frequency bands, typically in the range of 300 megahertz (MHz)
to 300 gigahertz (GHz). Generally, the region from 300 MHz to 3 GHz
is known as the ultra-high frequency (UHF) region or decimeter
band, since the wavelengths range from approximately one decimeter
to one meter in length. UHF waves may be blocked or redirected by
buildings and environmental features. However, the waves may
penetrate structures sufficiently for a macro cell to provide
service to UEs 115 located indoors. Transmission of UHF waves may
be associated with smaller antennas and shorter range (e.g., less
than 100 km) compared to transmission using the smaller frequencies
and longer waves of the high frequency (HF) or very high frequency
(VHF) portion of the spectrum below 300 MHz.
[0103] Wireless communications system 100 may also operate in a
super high frequency (SHF) region using frequency bands from 3 GHz
to 30 GHz, also known as the centimeter band. The SHF region
includes bands such as the 5 GHz industrial, scientific, and
medical (ISM) bands, which may be used opportunistically by devices
that may be capable of tolerating interference from other
users.
[0104] Wireless communications system 100 may also operate in an
extremely high frequency (EHF) region of the spectrum (e.g., from
30 GHz to 300 GHz), also known as the millimeter band. In some
examples, wireless communications system 100 may support millimeter
wave (mmW) communications between UEs 115 and base stations 105,
and EHF antennas of the respective devices may be even smaller and
more closely spaced than UHF antennas. In some cases, this may
facilitate use of antenna arrays within a UE 115. However, the
propagation of EHF transmissions may be subject to even greater
atmospheric attenuation and shorter range than SHF or UHF
transmissions. Techniques disclosed herein may be employed across
transmissions that use one or more different frequency regions, and
designated use of bands across these frequency regions may differ
by country or regulating body.
[0105] In some cases, wireless communications system 100 may
utilize both licensed and unlicensed radio frequency spectrum
bands. For example, wireless communications system 100 may employ
License Assisted Access (LAA), LTE-Unlicensed (LTE-U) radio access
technology, or NR technology in an unlicensed band such as the 5
GHz ISM band. When operating in unlicensed radio frequency spectrum
bands, wireless devices such as base stations 105 and UEs 115 may
employ listen-before-talk (LBT) procedures to ensure a frequency
channel is clear before transmitting data. In some cases,
operations in unlicensed bands may be based on a carrier
aggregation configuration in conjunction with component carriers
operating in a licensed band (e.g., LAA). Operations in unlicensed
spectrum may include downlink transmissions, uplink transmissions,
peer-to-peer transmissions, or a combination of these. Duplexing in
unlicensed spectrum may be based on frequency division duplexing
(FDD), time division duplexing (TDD), or a combination of both.
[0106] In some examples, base station 105 or UE 115 may be equipped
with multiple antennas, which may be used to employ techniques such
as transmit diversity, receive diversity, multiple-input
multiple-output (MIMO) communications, or beamforming. For example,
wireless communications system 100 may use a transmission scheme
between a transmitting device (e.g., a base station 105) and a
receiving device (e.g., a UE 115), where the transmitting device is
equipped with multiple antennas and the receiving device is
equipped with one or more antennas. MIMO communications may employ
multipath signal propagation to increase the spectral efficiency by
transmitting or receiving multiple signals via different spatial
layers, which may be referred to as spatial multiplexing. The
multiple signals may, for example, be transmitted by the
transmitting device via different antennas or different
combinations of antennas. Likewise, the multiple signals may be
received by the receiving device via different antennas or
different combinations of antennas. Each of the multiple signals
may be referred to as a separate spatial stream, and may carry bits
associated with the same data stream (e.g., the same codeword) or
different data streams. Different spatial layers may be associated
with different antenna ports used for channel measurement and
reporting. MIMO techniques include single-user MIMO (SU-MIMO) where
multiple spatial layers are transmitted to the same receiving
device, and multiple-user MIMO (MU-MIMO) where multiple spatial
layers are transmitted to multiple devices.
[0107] Beamforming, which may also be referred to as spatial
filtering, directional transmission, or directional reception, is a
signal processing technique that may be used at a transmitting
device or a receiving device (e.g., a base station 105 or a UE 115)
to shape or steer an antenna beam (e.g., a transmit beam or receive
beam) along a spatial path between the transmitting device and the
receiving device. Beamforming may be achieved by combining the
signals communicated via antenna elements of an antenna array such
that signals propagating at particular orientations with respect to
an antenna array experience constructive interference while others
experience destructive interference. The adjustment of signals
communicated via the antenna elements may include a transmitting
device or a receiving device applying certain amplitude and phase
offsets to signals carried via each of the antenna elements
associated with the device. The adjustments associated with each of
the antenna elements may be defined by a beamforming weight set
associated with a particular orientation (e.g., with respect to the
antenna array of the transmitting device or receiving device, or
with respect to some other orientation).
[0108] In one example, a base station 105 may use multiple antennas
or antenna arrays to conduct beamforming operations for directional
communications with a UE 115. For instance, some signals (e.g.,
synchronization signals, reference signals, beam selection signals,
or other control signals) may be transmitted by a base station 105
multiple times in different directions, which may include a signal
being transmitted according to different beamforming weight sets
associated with different directions of transmission. Transmissions
in different beam directions may be used to identify (e.g., by the
base station 105 or a receiving device, such as a UE 115) a beam
direction for subsequent transmission and/or reception by the base
station 105.
[0109] Some signals, such as data signals associated with a
particular receiving device, may be transmitted by a base station
105 in a single beam direction (e.g., a direction associated with
the receiving device, such as a UE 115). In some examples, the beam
direction associated with transmissions along a single beam
direction may be determined based at least in in part on a signal
that was transmitted in different beam directions. For example, a
UE 115 may receive one or more of the signals transmitted by the
base station 105 in different directions, and the UE 115 may report
to the base station 105 an indication of the signal it received
with a highest signal quality, or an otherwise acceptable signal
quality. Although these techniques are described with reference to
signals transmitted in one or more directions by a base station
105, a UE 115 may employ similar techniques for transmitting
signals multiple times in different directions (e.g., for
identifying a beam direction for subsequent transmission or
reception by the UE 115), or transmitting a signal in a single
direction (e.g., for transmitting data to a receiving device).
[0110] A receiving device (e.g., a UE 115, which may be an example
of a mmW receiving device) may try multiple receive beams when
receiving various signals from the base station 105, such as
synchronization signals, reference signals, beam selection signals,
or other control signals. For example, a receiving device may try
multiple receive directions by receiving via different antenna
subarrays, by processing received signals according to different
antenna subarrays, by receiving according to different receive
beamforming weight sets applied to signals received at a plurality
of antenna elements of an antenna array, or by processing received
signals according to different receive beamforming weight sets
applied to signals received at a plurality of antenna elements of
an antenna array, any of which may be referred to as "listening"
according to different receive beams or receive directions. In some
examples a receiving device may use a single receive beam to
receive along a single beam direction (e.g., when receiving a data
signal). The single receive beam may be aligned in a beam direction
determined based at least in part on listening according to
different receive beam directions (e.g., a beam direction
determined to have a highest signal strength, highest
signal-to-noise ratio, or otherwise acceptable signal quality based
at least in part on listening according to multiple beam
directions).
[0111] In some cases, the antennas of a base station 105 or UE 115
may be located within one or more antenna arrays, which may support
MIMO operations, or transmit or receive beamforming. For example,
one or more base station antennas or antenna arrays may be
co-located at an antenna assembly, such as an antenna tower. In
some cases, antennas or antenna arrays associated with a base
station 105 may be located in diverse geographic locations. A base
station 105 may have an antenna array with a number of rows and
columns of antenna ports that the base station 105 may use to
support beamforming of communications with a UE 115. Likewise, a UE
115 may have one or more antenna arrays that may support various
MIMO or beamforming operations.
[0112] In some cases, wireless communications system 100 may be a
packet-based network that operate according to a layered protocol
stack. In the user plane, communications at the bearer or Packet
Data Convergence Protocol (PDCP) layer may be IP-based. A Radio
Link Control (RLC) layer may perform packet segmentation and
reassembly to communicate over logical channels. A Medium Access
Control (MAC) layer may perform priority handling and multiplexing
of logical channels into transport channels. The MAC layer may also
use hybrid automatic repeat request (HARQ) to provide
retransmission at the MAC layer to improve link efficiency. In the
control plane, the Radio Resource Control (RRC) protocol layer may
provide establishment, configuration, and maintenance of an RRC
connection between a UE 115 and a base station 105 or core network
130 supporting radio bearers for user plane data. At the Physical
layer, transport channels may be mapped to physical channels.
[0113] In some cases, UEs 115 and base stations 105 may support
retransmissions of data to increase the likelihood that data is
received successfully. HARQ feedback is one technique of increasing
the likelihood that data is received correctly over a communication
link 125. HARQ may include a combination of error detection (e.g.,
using a cyclic redundancy check (CRC)), forward error correction
(FEC), and retransmission (e.g., automatic repeat request (ARQ)).
HARQ may improve throughput at the MAC layer in poor radio
conditions (e.g., signal-to-noise conditions). In some cases, a
wireless device may support same-slot HARQ feedback, where the
device may provide HARQ feedback in a specific slot for data
received in a previous symbol in the slot. In other cases, the
device may provide HARQ feedback in a subsequent slot, or according
to some other time interval.
[0114] The term "carrier" refers to a set of radio frequency
spectrum resources having a defined physical layer structure for
supporting communications over a communication link 125. For
example, a carrier of a communication link 125 may include a
portion of a radio frequency spectrum band that is operated
according to physical layer channels for a given radio access
technology. Each physical layer channel may carry user data,
control information, or other signaling. A carrier may be
associated with a pre-defined frequency channel (e.g., an evolved
universal mobile telecommunication system terrestrial radio access
(E-UTRA) absolute radio frequency channel number (EARFCN)), and may
be positioned according to a channel raster for discovery by UEs
115. Carriers may be downlink or uplink (e.g., in an FDD mode), or
be configured to carry downlink and uplink communications (e.g., in
a TDD mode). In some examples, signal waveforms transmitted over a
carrier may be made up of multiple sub-carriers (e.g., using
multi-carrier modulation (MCM) techniques such as orthogonal
frequency division multiplexing (OFDM) or discrete Fourier
transform spread OFDM (DFT-S-OFDM)).
[0115] The organizational structure of the carriers may be
different for different radio access technologies (e.g., LTE,
LTE-A, LTE-A Pro, NR). For example, communications over a carrier
may be organized according to TTIs or slots, each of which may
include user data as well as control information or signaling to
support decoding the user data. A carrier may also include
dedicated acquisition signaling (e.g., synchronization signals or
system information, etc.) and control signaling that coordinates
operation for the carrier. In some examples (e.g., in a carrier
aggregation configuration), a carrier may also have acquisition
signaling or control signaling that coordinates operations for
other carriers.
[0116] Physical channels may be multiplexed on a carrier according
to various techniques. A physical control channel and a physical
data channel may be multiplexed on a downlink carrier, for example,
using time division multiplexing (TDM) techniques, frequency
division multiplexing (FDM) techniques, or hybrid TDM-FDM
techniques. In some examples, control information transmitted in a
physical control channel may be distributed between different
control regions in a cascaded manner (e.g., between a common
control region or common search space and one or more UE-specific
control regions or UE-specific search spaces).
[0117] Wireless communications system 100 may support communication
with a UE 115 on multiple cells or carriers, a feature which may be
referred to as carrier aggregation or multi-carrier operation. A UE
115 may be configured with multiple downlink component carriers and
one or more uplink component carriers according to a carrier
aggregation configuration. Carrier aggregation may be used with
both FDD and TDD component carriers.
[0118] In some cases, a UE 115 may perform a handover procedure
from a source base station 105 to a target base station 105 when
moving within the wireless communications system. In some cases,
such a handover may be performed in accordance with one or more
handover techniques provided herein that may provide a 0 ms or
almost 0 ms interruption handover at the UE 115.
[0119] FIG. 2 illustrates an example of a wireless communications
system 200 that supports handover techniques in wireless
communications in accordance with aspects of the present
disclosure. In some examples, wireless communications system 200
may implement aspects of wireless communications system 100. In
some examples, wireless communications system 200 may implement
aspects of wireless communications system 100. The wireless
communications system 200 may include a first base station 105-a, a
second base station 105-b, and UE 115-a, which may be examples of a
base station 105 and a UE 115, as described with reference to FIG.
1.
[0120] First base station 105-a may be a source base station 105
and the second base station 105-b may be a target base station 105
in a handover 215 of the UE 115-a from the first base station 105-a
to the second base station 105-b. First base station 105-a and
second base station 105-b may be in communication with each other,
such as via backhaul link 134-a (e.g., via an X2, Xn, or other
interface), which may be a wired or wireless interface. While the
example of FIG. 2 shows the first base station 105-a in direct
communication with the second base station 105-b, in other cases
the communication may be indirect, such as via a core network
(e.g., core network 130 or FIG. 1). In this example, the UE 115-a
and the first base station 105-a may establish a first connection
205. In the event that a handover is triggered, the UE 115-a may
establish a second connection 210 with the second base station
105-b. Various techniques as discussed herein provide for efficient
handovers that may provide zero ms or almost 0 ms interruptions in
the handover 215 of the UE 115-a.
[0121] In some traditional systems, when performing a handover 215
from the first base station 105-a to the second base station 105-b,
the UE 115-a may release the first connection 205 and perform a
connection establishment procedure (e.g., a random access
procedure) to establish the second connection 210. In such cases,
in the event that data is to be communicated with the UE 115-a,
there may be an interruption in connectivity with the UE 115-a
after the first connection 205 is released and until the second
connection 210 is established. When performing a 0 ms or almost 0
ms handover, such an interruption is eliminated or almost
eliminated, and thus connectivity with the UE 115-a is
enhanced.
[0122] In some cases, the handover 215 of the UE 115-a from the
first base station 105-a to the second base station 105-b may be a
make-before-break handover in which the UE 115-a may have a
capability to simultaneously transmit and receive communications,
and the UE 115-a may maintain the first connection 205 while the
second connection 210 is being established. In other cases, the
handover 215 of the UE 115-a may be a dual-connectivity (DC)
handover, in which the UE 115-a has a capability of maintaining
multiple connections with multiple cells and the second connection
210 is established according to a DC procedure before the first
connection 205 is released. In some cases, a serving base station
105 (e.g., first base station 105-a) may identify a capability of
the UE 115-a for maintaining multiple connections during a
handover, such as, for example, based on a capability indication
transmitted by the UE 115-a, or a UE category indicated by the UE
115-a, during a connection establishment procedure to establish the
first connection 205. Based on the UE 115-a capability, the first
base station 105-a may configure the UE 115-a to perform a
make-before-break handover or a DC handover (e.g., via radio
resource control (RRC) signaling).
[0123] In some cases, the first base station 105-a may determine
that the UE 115-a is to be handed over to the second base station
105-b (e.g., based on a measurement report provided by the UE
115-a). Based on such a determination, the first base station 105-a
may transmit a RRC reconfiguration to the UE 115-a that may
indicate that UE 115-a is to perform a make-before-break handover
procedure (or DC handover procedure) with the second base station
105-b using a simultaneous transmit and receive capability of UE
115-a. In such a case, UE 115-a may determine that the first
connection 205 is to be maintained with the first base station
105-a while establishing the second connection 210 with the second
base station 105-b. For example, UE 115-a may perform a random
access procedure with the second base station 105-b (e.g., to
initiate and establish a connection with the second base station
105-b). In some aspects, UE 115-a can continue to exchange user
data (e.g., uplink/downlink user data) via the first base station
105-a during and after the random access procedure. In some cases,
after establishment of the second connection 210, the first
connection 205 may be released. Various techniques for such
handovers that provide for enhanced connectivity and efficiency are
described herein.
[0124] FIG. 3 illustrates an example of a process flow 300 that
supports handover techniques in wireless communications in
accordance with aspects of the present disclosure. The process flow
300 may include a first base station 105-c, a second base station
105-d, and a UE 115-b, which may be examples of the corresponding
devices described with reference to FIGS. 1 and 2. In some
examples, the process flow 300 may implement aspects of the
wireless communications system 100 and 200. For example, the first
base station 105-c, the second base station 105-d, and the UE
115-b, may support 0 ms or almost 0 ms interruption handovers.
[0125] In the following description of the process flow 300, the
operations between the first base station 105-c, second base
station 105-d, and the UE 115-b may be transmitted in a different
order than the exemplary order shown, or the operations performed
by the base stations 105 and the UE 115-b may be performed in
different orders or at different times. Certain operations may also
be left out of the process flow 300, or other operations may be
added to the process flow 300.
[0126] In this example a handover between the first base station
105-c and the second base station 105-d (which may be referred to
as a source and a target, respectively) is an inter-CU handover,
using a DC handover, in which both the first base station 105-c and
the second base station 105-d are associated with a different
CU.
[0127] In some examples, the process flow 300 may commence at 305
with the first base station 105-c establishing a first connection
with the UE 115-b (e.g., performing a cell acquisition procedure, a
random access procedure, an RRC connection procedure, an RRC
configuration procedure, etc.).
[0128] At 310, an event trigger may occur that may cause the UE
115-b to perform a measurement procedure. Such a measurement
procedure may include, for example, signal measurements of
neighboring base stations by the UE 115-b to identify one or more
neighboring base stations that may be candidates for the handover
procedure. In some cases, the event trigger may be a signal
measurement (e.g., a reference signal received power (RSRP)) of the
first base station 105-c dropping below a threshold value, may be a
time-based periodic event, or combinations thereof. At 315, the UE
115-c may transmit measurement report to the first base station
105-c, and the measurement report may indicate that the second base
station 105-d is a candidate base station for a handover.
[0129] At 320, the first base station may make a handover decision
to handover the UE 115-b from the first base station 105-c to the
second base station 105-d. In some cases, the first base station
105-c may make the handover decision based on the measurement
report provided by the UE 115-b, one or more threshold values for
initiating a handover, a differential in one or more signal
measurements from one or more prior measurement reports, one or
more measurement reports from other UEs, or any combinations
thereof. In the example of FIG. 3, the first base station 105-c may
determine that the UE 115-b is to be handed over to the second base
station 105-d. Further, the first base station 105-c may determine
that the UE 115-b has a DC capability and may make a decision to
perform a DC based handover of the UE 115-b. For example, the first
base station 105-c (or an associated CU) may select from possible
handover procedures that may be performed by UE 115-b based at
least in part on the indicated capability of UE 115-b. In the
example of FIG. 3, the first base station 105-c may select an DC
handover procedure for UE 115-b based at least in part on UE 115-b
indicating a DC capability. In other examples, the first base
station 105-c may select a make-before-break handover procedure or
a different handover procedure (e.g., a legacy handover procedure),
and different operations associated with the selected handover
procedure may be performed.
[0130] At 325, the first base station 105-c may transmit a
secondary node (SN) addition request to the second base station
105-d. The SN addition request may be provided based on the DC
handover decision, in order to establish a DC connection between
both the first base station 105-c and the second base station 105-d
and the UE 115-b. The initial connection between the first base
station 105-c and the UE 115-b in such cases is a master node (MN)
connection and the second base station 105-d may be added initially
as a SN based on the SN addition request. At 330, the second base
station 105-d may transmit a SN addition acknowledgment to the
first base station 105-c to acknowledge that the second base
station 105-d received, and that may indicate an ability to serve
UE 115-b after the handover procedure.
[0131] In some examples, at 335, the first base station 105-c may
transmit a role switch request to the second base station 105-d.
The role switch request may indicate that the second base station
105-d is to switch to be the master node in the DC connection with
the UE 115-b once the connection between the UE 115-b and the
second base station is established, and the first base station
105-c is to switch to be the SN connection. The second base station
105-d may receive the role switch request and identify that the UE
115-b is to be handed over to the second base station 105-d, and
transmit a role switch acknowledgment at 340.
[0132] In some cases, the role switch request may be combined with
the SN addition request, and the role switch acknowledgment may be
combined with the SN addition acknowledgment. In such cases,
signaling between the first base station 105-c and the second base
station 105-d may be reduced. Thus, in the example of FIG. 3, the
role switch request at 335 and the role switch acknowledgment at
340 are shown as optional operations, and may be skipped in cases
where the SN addition request at 325 also includes the role switch
request, and the SN addition acknowledgment at 330 also includes
the role switch acknowledgment.
[0133] At 345, the first base station 105-c may transmit a RRC
reconfiguration message to the UE 115-b. The RRC reconfiguration
message may indicate to the UE 115-b that a connection with the
second base station 105-d is to be established according to a DC
connection establishment procedure. For example, the RRC
reconfiguration may indicate that UE 115-b is to perform DC
handover procedure with second base station 105-d using the DC
capability of UE 115-b. In some aspects, the RRC reconfiguration
can include information identifying second base station 105-d,
information identifying a handover configuration, identifying a DC
configuration, or combinations thereof. For example, in some cases,
the RRC reconfiguration message may provide an indication of a
secondary cell group associated with the second base station 105-d
that is to be configured as part of the DC based handover, and also
indicate the source cell group identification of the first base
station 105-c. In some cases, the operations at 310 through 345 may
be referred to as a phase-I handover preparation portion of a
handover.
[0134] Following the transmission of the RRC reconfiguration at 345
and until a second connection with the second base station 105-d is
established, downlink data that is to be transmitted to the UE
115-b may be provided to the first base station 105-c (e.g., from a
user plane function (UPF) at the core network), and the first base
station 105-c may transmit the downlink data to the UE 115-b via
the first connection. Similarly, uplink data transmitted from the
UE 115-b may be transmitted to the first base station 105-c via the
first connection. In some cases, the first base station 105-c may
forward downlink user data to the second base station 105-d.
[0135] In some cases, at 346, the UE 115-b may configure a SN based
on the RRC reconfiguration based on a DC connection procedure, and
may transmit an RRC reconfiguration complete indication to the
first base station 105-c to indicate that the second connection is
being established. During the establishment of the second
connection with the second base station 105-d, the UE 115-b may
maintain the first connection with the first base station 105-c,
and thus have an active established connection during the handover,
as indicated at 397. As part of the DC connection procedure, at
347, the first base station 105-c may transmit a SN reconfiguration
complete indication to the second base station 105-d that may
indicate that the UE 115-b is configuring the second
connection.
[0136] At 350, the UE 115-b and the second base station 105-d may
perform a random access procedure to establish the second
connection with the second base station 105-d. The random access
procedure may be performed according to established random access
procedures (e.g., random access channel (RACH) procedures that are
established in LTE or NR). Upon completion of the random access
procedure and establishment of the second connection, the UE 115-b
may, at 355, transmit a RRC reconfiguration complete indication to
the second base station 105-d. At this point, the second base
station 105-d may be the master node in the DC connection with both
the first base station 105-c and second base station 105-d, and
thus the second connection is the primary link as indicated at 360.
In some cases, the UE 115-b may transmit a PDCP status transfer and
uplink user data to the second base station 105-d via the second
connection. In some aspects, after the RRC reconfiguration is
complete, UE 115-b may perform uplink user/control plane
duplication with first base station 105-c. For example, control
plane data may be duplicated and shared between the first base
station 105-c and the second base station 105-d. Accordingly, UE
115-b may achieve improved reliability when receiving the data on
the downlink. In some cases, the operations at 350 through 360 may
be referred to as a phase-II handover execution portion of a
handover.
[0137] At 365, the second base station 105-d may transmit a role
switch complete indication to the first base station 105-c. In some
cases, the role switch complete indication may provide an
indication to the first base station 105-c (e.g., via an Xn
backhaul link) that the second connection has been established with
the UE 115-b, and that signaling radio bearers (SRBs) and data
radio bearers (DRBs) have been established in the second
connection.
[0138] At 370, the first base station 105-c may transmit a SN
status transfer to the second base station 105-d. The SN status
transfer may provide the most up to date PDCP status and downlink
sequence number to use, for example. The SN status transfer may
also indicate to the second base station 105-d that the first
connection with the first base station 105-c may be released in
accordance with the DC handover procedure.
[0139] At 375, the second base station 105-d may make a handover
complete decision and determine that the source first connection
with the first base station 105-c may be released. At 380, the
second base station 105-d, responsive to determining that the
handover is complete, may transmit a RRC reconfiguration to the UE
115-b to release the first connection with the first base station
105-c. In some cases, the second base station 105-d may provide a
handover completion indication to a UPF in the core network.
Additionally or alternatively, the second base station 105-d may
transmit a PDCP status transfer to the UE 115-b.
[0140] At 385, the UE 115-b may release the first connection with
the first base station 105-c. The UE 115-b may release the first
connection, for example, by removing DRBs and SRBs associated with
the first base station 105-c, and PDCP status and sequence numbers
associated with the first connection. At 390, the UE 115-b may
transmit a RRC reconfiguration complete indication to the second
base station 105-d to indicate that the first connection has been
released.
[0141] At 395, responsive to the indication that the first
connection is released handover completion may be performed. In
some cases, as part of the handover completion, the second base
station 105-d may transmit a path switch request to an access and
mobility function (AMF) of the core network, and the AMF and UPF
may perform a path switch related to core network internal
signaling to provide the actual downlink path used to the UPF for
subsequent downlink data transmissions. Further, in some cases, in
the event that the first base station 105-c received fresh downlink
data for the UE 115-b prior to the path switch at the UPF, the
first base station 105-c may transmit the fresh downlink data to
the second base station 105-d for downlink transmission to the UE
115-b. In some cases, the AMF may provide an end marker to the
first base station 105-c to indicate that the downlink path has
been changed to the second base station 105-d, and the first base
station 105-c may provide an indication to the second base station
105-d that the end marker has been received. In some cases, the AMF
may provide a path switch request acknowledgment to the second base
station 105-d. Following the path switch acknowledgment, the second
base station 105-d may transmit a UE context release command to the
first base station 105-c, and the first base station 105-c may
release UE context for the first connection and transmit a UE
context release complete back to the second base station 105-d. In
some cases, the operations at 365 through 395 may be referred to as
a phase-III handover completion portion of a handover.
[0142] In some aspects, UE 115-b may use a dual protocol stack,
which includes a source protocol stack for communicating with first
base station 105-c and a target protocol stack for communicating
with second base station 105-d. Each of these protocol stacks may
include a packet data convergence protocol (PDCP) layer, a radio
link control (RLC) layer, a medium access control (MAC) layer,
and/or a physical (PHY) layer. In some aspects, the source protocol
stack and the target protocol stack may share one or more layers,
such as a PDCP layer. In some aspects, the target protocol stack
may be used for uplink data transmissions. An example of protocol
stacks used at UEs 115 and base stations 105 in various aspects of
the disclosure is discussed with respect to FIG. 4.
[0143] FIG. 4 illustrates an example of dual connectivity handover
protocol stacks 400 that supports handover techniques in wireless
communications in accordance with aspects of the present
disclosure. In some examples, dual connectivity handover 400 may
implement aspects of wireless communications system 100 or 200. The
example DC handover protocol stacks 400 show network protocol stack
and data flow for a DC handover procedure (e.g., similar to the
handover procedure described with respect to process flow 300 of
FIG. 3).
[0144] In this example, a UE 415 may perform a DC handover between
a source DU 445 and a target DU 475. The UE 415 may be an example
of UEs 115 described herein, DUs 445 and 475 may be examples of DUs
or base stations 105 described herein. In an initial setup phase
405-a in this example, a target gNB may include a target CU 490 and
a target DU 475, and a source gNB may include a source CU 460 and
source DU 445. The initial setup phase 405-a, which may correspond
to the phase-I handover preparation phase, may provide setup at the
target DU 475 and the target CU 490.
[0145] In this example, an inter-gNB-CU handover is illustrated,
and during the initial setup phase 405-a the UE 415-a may use a
source protocol stack for a first connection, which may include a
PDCP layer 420-a, a radio link control (RLC) layer 425-a, and a
medium access control layer 430-a, which the UE 415-a may use to
communicate with a core network, such as a 5GC 470-a via a source
CU 460-a having PDCP layer 465-a and source DU 445-a having MAC
layer 450-a and RLC layer 455-a. The target gNB may configure the
target DU 475-a with MAC layer 480-a and RLC layer 485-a, and may
configure the target CU 490-a with PDCH layer 490-a, in preparation
for the DC handover. The source DU 445-a may transmit a RRC
reconfiguration to the UE 415-a (e.g., via a SRB), and the UE 415-a
may configure a DC RLC layer 435-a and MAC layer 440-a in
preparation for random access channel (RACH) transmissions as part
of a random access procedure to establish a second connection with
the target DU 475-a. In some cases, and the source CU 460-a may
instruct the target CU 490-a (e.g., via an Xn backhaul interface
497-a) to configure the target protocol stack at the target CU
490-a and the target DU 475-a.
[0146] During a second SN status transfer phase 405-b, which may be
an example of a phase-II handover execution phase, the target
protocol stacks may be used for communications with UE 415-b. As
illustrated in FIG. 4, the target DU 475-b may communicate uplink
data with the UE 415-b via PDCP layer 420-b, RLC layer 435-b, and
MAC layer 440-b. The target DU 475-b may have an active and
established MAC layer 480-b and RLC layer 485-b, which may be used
for communication with the UE 415-b and target CU 490-b that may
have PDCP layer 495-b and may communicate with 5GC 470-b. In this
example, the UE 415-b may maintain the protocol stack associated
with the source DU 445-b, which may include PDCP layer 420-b, RLC
layer 425-b, and MAC layer 430-b. Likewise, the source gNB may
maintain MAC layer 450-b and RLC layer 455-b at source DU 445-b, as
well as PDCP layer 465-b at source CU 460-b. In this example,
uplink data may be transmitted using the target protocol stack, and
downlink data may be forwarded via Xn backhaul interface 497-b from
the source CU 460-b to the target CU 490-b for transmission to the
UE 415-b. In some aspects, the UE 415-b, source CU 460-b, target CU
490-b, and/or 5GC 470-b may use different security keys
corresponding to a same PDCP entity and/or may have different PDCP
entities with a common reordering entity. In such cases, the UE
415-b may decide on a security key (e.g., one or more security keys
from source CU 460-b or one or more security keys from a target CU
490-b) to use based on the RLC/MAC/PHY stack from which data is
received on the downlink or based on an explicit indication in the
PDCP PDU header that indicates which security key to use.
[0147] During a third source release and path switch phase 405-c,
which may be an example of a phase-III handover completion phase,
the target protocol stacks may be used for communications with UE
415-c. As illustrated in FIG. 4, the target DU 475-c may
communicate uplink/downlink data with the UE 415-c via PDCP layer
420-c, RLC layer 435-c, and MAC layer 440-c. The target DU 475-c
may have an active and established MAC layer 480-c and RLC layer
485-c, which may be used for communication with the UE 415-c and
target CU 490-c that may have PDCP layer 495-c and may communicate
with 5GC 470-c. In this example, the UE 415-c, the source DU 445-b
and the source CU 460-b may release the corresponding source
protocol stacks. The remaining protocol stacks would then become
source protocol stacks in a subsequent handover procedure. It is
noted that FIG. 4 is provided as an example, and that other example
protocol stacks are possible and may differ from what was described
with respect to FIG. 4.
[0148] FIG. 5 illustrates an example of a process flow 500 that
supports handover techniques in wireless communications in
accordance with aspects of the present disclosure. In some
examples, process flow 500 may implement aspects of wireless
communications system 100 or 200. As discussed herein, in some
cases a UE or base station may experience radio link issues during
a handover procedure. For example, a UE may experience fast fading
of a signal from a source base station or target base station that
may result in the associated signal being unusable by the UE. In
such cases, if the UE can establish or maintain a connection with
the other base station, an indication of the link failure may be
provided as part of the handover procedure.
[0149] The process flow 500 may include a source gNB DU 105-e, a
target gNB DU 105-f, a CU 505, and a UE 115-c. Source gNB DU 105-e
and target gNB DU 105-f may be examples of DU, a TRP, or a
generally a base station 105 as described herein. UE 115-c may be
an example of corresponding UE devices described herein, and CU 505
may be an example of a CU, an ANC, or generally a base station 105
as described herein. In some examples, the process flow 500 may
implement aspects of the wireless communications system 100 and
200. For example, the source gNB DU 105-e, the target gNB DU 105-f,
CU 505, and the UE 115-c, may support 0 ms or almost 0 ms
interruption handovers.
[0150] In the following description of the process flow 500, the
operations between the source gNB DU 105-e, target gNB DU 105-f, CU
505, and the UE 115-c may be transmitted in a different order than
the exemplary order shown, or the operations performed by the DUs
105 and the UE 115-c may be performed in different orders or at
different times. Certain operations may also be left out of the
process flow 500, or other operations may be added to the process
flow 500.
[0151] In this example a handover between the source gNB DU 105-e
and the target gNB DU 105-f is an enhanced make-before-break
intra-CU handover in which CU 505 is associated with both the
source gNB DU 105-e and the target gNB DU 105-f. In some examples,
the process flow 500 may commence at 510 with the source gNB DU
105-e establishing a first connection with the UE 115-c (e.g.,
performing a cell acquisition procedure, a random access procedure,
an RRC connection procedure, an RRC configuration procedure,
etc.).
[0152] At 515, an event trigger may occur that may cause the UE
115-c to perform a measurement procedure. Such a measurement
procedure may include, for example, signal measurements of
neighboring base stations by the UE 115-c to identify one or more
neighboring base stations that may be candidates for the handover
procedure. In some cases, the event trigger may be a signal
measurement of the source gNB DU 105-e dropping below a threshold
value, may be a time-based periodic event, or combinations thereof.
At 520, the UE 115-c may transmit measurement report to the source
gNB DU 105-e, and the measurement report may indicate that the
target gNB DU 105-f is a candidate for a handover.
[0153] At 525, the source gNB DU 150-e may transmit a uplink RRC
transfer message to the CU 505. In some cases, the uplink RRC
transfer message may include the measurement report from the UE
115-c, and may indicate that the target gNB DU 105-f is a candidate
for a handover.
[0154] At 530, the CU 515 may make a handover decision to handover
the UE 115-c from the source gNB DU 105-e to the target gNB DU
105-f In some cases, the CU 505 may make the handover decision
based on the measurement report provided by the UE 115-c, one or
more threshold values for initiating a handover, a differential in
one or more signal measurements from one or more prior measurement
reports, one or more measurement reports from other UEs, or any
combinations thereof. In the example of FIG. 5, the CU 505 may
determine that the UE 115-c is to be handed over to the target gNB
DU 105-f. Further, the CU 505 may determine that the UE 115-c has a
capability of simultaneous transmissions and receptions and may
make a decision to perform an enhanced make-before-break based
handover of the UE 115-c. In some cases, the CU 505 determines a
handover configuration that is to be used for the handover
procedure for UE 115-c. For example, CU 505 may select from
possible handover procedures that may be performed by UE 115-c
based at least in part on the indicated capability of UE 115-c. In
the example of FIG. 5, CU 505 may select an enhanced
make-before-break handover procedure for UE 115-c based at least in
part on UE 115-c indicating a capability for simultaneous
transmissions and receptions that may be used in an enhanced
make-before-break handover. In other examples, the CU 505 may
select a DC handover procedure or a different handover procedure
(e.g., a legacy handover procedure), and different operations
associated with the selected handover procedure may be
performed.
[0155] At 535, the CU and the target gNB DU 105-f may perform
handover preparation. In some cases, handover preparation may
include a UE context setup request that is transmitted from the CU
505 to the target gNB DU 105-f, and an associated UE context setup
response from the target gNB DU 105-f when the UE context has been
configured.
[0156] At 540, the CU 505 may transmit a downlink RRC transfer to
the source gNB DU 105-e, that may indicate an RRC reconfiguration.
In some cases, the downlink RRC transfer may indicate that an
enhanced make-before-break handover is to be performed, and may
provide a cell ID of the target gNB DU 105-f.
[0157] At 545, the source gNB DU 105-e may transmit a RRC
reconfiguration message to the UE 115-c. The RRC reconfiguration
message may indicate to the UE 115-c that a connection with the
target gNB DU 105-f is to be established according to an enhanced
make-before-break handover procedure. For example, the RRC
reconfiguration may indicate that UE 115-c is to perform the
make-before-break handover procedure with target gNB DU 105-f using
the simultaneous transmission/reception capability of UE 115-c. In
some aspects, the RRC reconfiguration can include information
identifying target gNB DU 105-f, information identifying a handover
configuration, an indication of the enhanced make-before-break
handover procedure, or combinations thereof. In some cases, the
operations at 510 through 545 may be referred to as a phase-I
handover preparation portion of a handover.
[0158] Following the transmission of the RRC reconfiguration at 545
and until a second connection with the target gNB DU 105-f is
established, downlink data that is to be transmitted to the UE
115-c may be provided to the source gNB DU 105-e (e.g., from a user
plane function (UPF) at the core network), and the source gNB DU
105-e may transmit the downlink data to the UE 115-c via the first
connection. Similarly, uplink data transmitted from the UE 115-c
may be transmitted to the source gNB DU 105-e via the first
connection.
[0159] During the establishment of the second connection with the
target gNB DU 105-f, the UE 115-c may maintain the first connection
with the source gNB DU 105-e, and thus have an active established
connection during the handover, as indicated at 575.
[0160] At 550, the UE 115-c and the target gNB DU 105-f may perform
a random access procedure to establish the second connection with
the target gNB DU 105-f The random access procedure may be
performed according to established random access procedures (e.g.,
random access channel (RACH) procedures that are established in LTE
or NR).
[0161] At 555, it may be determined that the UE 115-c failed to
connect to the target gNB DU 105-f, or that the second connection
failed after being established. In some cases, the failure may
occur prior to completion of the random access procedure, or
following completion of the random access procedure. In cases where
the failure is prior to completing the random access procedure, for
example, the UE 115-c may determine that a timer (e.g., a T304
timer in LTE or NR) associated with receipt of a random access
response message from the target gNB DU 105-f may expire, and thus
the UE 115-c may determine that a handover failure has occurred. In
cases where the failure is identified after completion of the
random access procedure, the failure may be determined based on a
number of retransmissions of a message to the target gNB DU 105-f,
or based on a timer (e.g., a T310 timer in LTE or NR) associated
with a RLC layer at the UE 115-c.
[0162] At 560, the UE 115-c may transmit a radio link failure (RLF)
indication to the source gNB DU 105-e, that may indicate a cause of
the RLF as being a handover failure (e.g., a T304 expiry). In some
cases, the RLF indication may preempt the UE 115-c from declaring
RLF, and thus the UE 115-c does not trigger an RRC reestablishment,
due to the first connection remaining active. In cases where the UE
115-c detects RLF on the target gNB DU 105-f after successfully
connected and while still connected to source gNB DU 105-e as part
of the handover procedure, UE 115-c does not declare RLF and will
not trigger RRC reestablishment as the first connection is active.
In such cases where the second connection fails after the
establishment of the connection, the RLF indication at 560 may
indicate a failure cause (e.g., T310 expiry, RandomAccessProblem,
or RLCMaxReTx), the target cell group ID, or both. In some cases,
the UE 115-c may also include one or more measurements (e.g., in a
MeasResultHO-Failure indication) available for the target gNB DU
105-e.
[0163] At 565, the source gNB DU 105-e may transmit a uplink RRC
transfer message to the CU 505. In some cases, the uplink RRC
transfer message may include a RLF information or cause indication
to the CU 505. Further, in cases where the UE 115-c provides one or
more measurements associated with the target gNB DU 105-e, the
source gNB DU 105-e may include them with the uplink RRC transfer
message.
[0164] At 570, the UE 115-c and the source gNB DU 105-e may
continue uplink/downlink transmissions using the first connection.
While the example of FIG. 5 illustrates a make-before-break
handover, similar techniques may be used in a DC handover to report
a failure of a connection at the source or target. Further, similar
techniques of reporting a connection failure may be used for both
inter-gNB-CU handovers and intra-gNB-CU handovers. Such techniques
may allow the UE 115-c to maintain the first connection even in the
event that the second connection fails or is not able to be
established. Thus, rather than reporting a radio link failure and
RRC reestablishment, the UE 115-c may simply continue with the
first connection, and thus a service interruption is avoided. In
some cases, the source gNB-DU 105-e may repeat operations starting
at 525 based on the RLF indication.
[0165] In some cases, the CU 505 may repeat operations starting at
530 based on the uplink RRC transfer message. For example, the CU
505 use the RLF information to identify the handover failure and
may release the UE resources/context on the target gNB DU 105-f
(e.g., a connection release at the target gNB in cases of
intra-gNB-CU handover, or a handover context cancel with the target
gNB-CU in cases of inter-gNB-CU handover). In some cases, the CU
505 may use this information to trigger an immediate handover to
another target cell by sending a new downlink RRC transfer (e.g., a
new RRC reconfiguration message) with the new target cell
information.
[0166] In some cases, the UE 115-c may initiate the establishment
of the second connection with the target gNB DU 105-f, and the
first connection with the source gNB DU 105-e may experience a RLF.
In such cases, if the UE 115-c detects RLF on source gNB DU 105-e
while performing the random access procedure at the target gNB DU
105-f, or after successfully connected to target gNB DU 105-f, the
UE 115-c may continue the random access procedure without
triggering a RRC reestablishment. The UE 115-c may then release the
source stack and the source connection implicitly, and ignores any
RRC reconfiguration to release the first connection. Additionally
or alternatively, the UE 115-c in such cases, may provide a RLF
indication (e.g., that indicates RLF on source gNB DU 105-e and the
source cell group ID) to the target gNB DU 105-f, which may be
provided to the CU 505. The CU 505 may use this information to
release the UE 115-c resources/context on the source gNB DU 105-e
(or source DU in case of Intra-gNB-CU HO and with the source
gNB-CU/source gNB-DU in case of Inter-gNB-CU HO). In cases where
the second connection fails after the first connection is released,
the UE 115-c may trigger RRC reestablishment.
[0167] FIG. 6 illustrates an example of a process flow 600 that
supports handover techniques in wireless communications in
accordance with aspects of the present disclosure. In some
examples, process flow 600 may implement aspects of wireless
communications system 100 or 200. Similarly as discussed with
respect to FIG. 5, in this example a UE or base station may
experience radio link issues during a handover procedure (e.g., a
UE may experience fast fading of a signal from a source base
station or target base station that may result in the associated
signal being unusable by the UE). In this example, an example of an
inter-gNB-CU enhanced make-before-break handover is
illustrated.
[0168] The process flow 600 may include a source base station
105-g, a target base station 105-h, and a UE 115-d. Source base
station 105-g and target base station 105-h may be examples of a
base station 105 as described herein, and in some cases may include
one or more associated DUs or TRPs, and a CU or ANC. UE 115-d may
be an example of corresponding UE devices described herein. In some
examples, the process flow 600 may implement aspects of the
wireless communications system 100 and 200. For example, the source
base station 105-g, the target base station 105-h, and the UE
115-d, may support 0 ms or almost 0 ms interruption handovers.
[0169] In the following description of the process flow 600, the
operations between the source base station 105-g, target base
station 105-h, and the UE 115-d may be transmitted in a different
order than the exemplary order shown, or the operations performed
by the CUs 105 and the UE 115-d may be performed in different
orders or at different times. Certain operations may also be left
out of the process flow 600, or other operations may be added to
the process flow 600.
[0170] In this example a handover between the source base station
105-g and the target base station 105-h, as indicated above, is an
enhanced make-before-break inter-CU handover in which both the
source base station 105-g and the target base station 105-h have
different associated CUs. In some examples, the process flow 600
may commence at 610 with the source base station 105-g establishing
a first connection with the UE 115-d (e.g., performing a cell
acquisition procedure, a random access procedure, an RRC connection
procedure, an RRC configuration procedure, etc.).
[0171] At 615, an event trigger may occur that may cause the UE
115-d to perform a measurement procedure. Such a measurement
procedure may include, for example, signal measurements of
neighboring base stations by the UE 115-d to identify one or more
neighboring base stations that may be candidates for the handover
procedure. In some cases, the event trigger may be a signal
measurement of the source base station 105-g dropping below a
threshold value, may be a time-based periodic event, or
combinations thereof. At 620, the UE 115-d may transmit measurement
report to the source base station 105-g, and the measurement report
may indicate that the target base station 105-h is a candidate for
a handover.
[0172] At 625, the source base station 105-g (e.g., a CU associated
with the source base station 105-g) may make a handover decision to
handover the UE 115-d from the source base station 105-g to the
target base station 105-h. In some cases, the source base station
105-g may make the handover decision based on the measurement
report provided by the UE 115-d, one or more threshold values for
initiating a handover, a differential in one or more signal
measurements from one or more prior measurement reports, one or
more measurement reports from other UEs, or any combinations
thereof. In the example of FIG. 6, the source base station 105-g
may determine that the UE 115-d is to be handed over to the target
base station 105-h. Further, the source base station 105-g may
determine that the UE 115-d has a capability of simultaneous
transmissions and receptions and may make a decision to perform an
enhanced make-before-break based handover of the UE 115-d. In some
cases, the source base station 105-g determines a handover
configuration that is to be used for the handover procedure for UE
115-d. For example, source base station 105-g may select from
possible handover procedures that may be performed by UE 115-d
based at least in part on the indicated capability of UE 115-d. In
the example of FIG. 6, source base station 105-g may select an
enhanced make-before-break handover procedure for UE 115-d based at
least in part on UE 115-d indicating a capability for simultaneous
transmissions and receptions that may be used in an enhanced
make-before-break handover. In other examples, the source base
station 105-g may select a DC handover procedure or a different
handover procedure (e.g., a legacy handover procedure), and
different operations associated with the selected handover
procedure may be performed.
[0173] At 630, the source base station 105-g may transmit a
handover request to the target base station 105-h. In some cases,
the handover request may indicate that the target base station
105-h is a selected for a handover of UE 115-d. In some cases, the
handover request may also include the measurement report from the
UE 115-d.
[0174] At 635, the target base station 105-h (e.g., a CU associated
with the target base station 105-h) may perform admission control,
and may determine that the second base station 105-h can
accommodate the handover. In some cases, admission control may
include a UE context setup. At 640, the target base station 105-h
may transmit a handover request acknowledgment to the source base
station 105-g.
[0175] At 645, the source base station 105-g may transmit a RRC
reconfiguration message to the UE 115-d. The RRC reconfiguration
message may indicate to the UE 115-d that a connection with the
target base station 105-h is to be established according to an
enhanced make-before-break handover procedure. For example, the RRC
reconfiguration may indicate that UE 115-d is to perform the
make-before-break handover procedure with target base station 105-h
using the simultaneous transmission/reception capability of UE
115-d. In some aspects, the RRC reconfiguration can include
information identifying target base station 105-h, information
identifying a handover configuration, an indication of the enhanced
make-before-break handover procedure, or combinations thereof. In
some cases, the operations at 610 through 645 may be referred to as
a phase-I handover preparation portion of a handover.
[0176] Following the transmission of the RRC reconfiguration at 645
and until a second connection with the target base station 105-h is
established, downlink data that is to be transmitted to the UE
115-d may be provided to the source base station 105-g (e.g., from
a user plane function (UPF) at the core network), and the source
base station 105-g may transmit the downlink data to the UE 115-d
via the first connection. Similarly, uplink data transmitted from
the UE 115-d may be transmitted to the source base station 105-g
via the first connection.
[0177] During the establishment of the second connection with the
target base station 105-h, the UE 115-d may maintain the first
connection with the source base station 105-g, and thus have an
active established connection during the handover, as indicated at
675.
[0178] At 650, the UE 115-d and the target base station 105-h may
perform a random access procedure to establish the second
connection with the target base station 105-h. The random access
procedure may be performed according to established random access
procedures (e.g., random access channel (RACH) procedures that are
established in LTE or NR).
[0179] At 655, it may be determined that the UE 115-d failed to
connect to the target base station 105-h, or that the second
connection failed after being established. In some cases, the
failure may occur prior to completion of the random access
procedure, or following completion of the random access procedure.
In cases where the failure is prior to completing the random access
procedure, for example, the UE 115-d may determine that a timer
(e.g., a T304 timer in LTE or NR) associated with receipt of a
random access response message from the target base station 105-h
may expire, and thus the UE 115-d may determine that a handover
failure has occurred. In cases where the failure is identified
after completion of the random access procedure, the failure may be
determined based on a number of retransmissions of a message to the
target base station 105-h, or based on a timer (e.g., a T310 timer
in LTE or NR) associated with a RLC layer at the UE 115-d.
[0180] At 660, the UE 115-d may transmit a radio link failure (RLF)
indication to the source base station 105-g, that may indicate a
cause of the RLF as being a handover failure (e.g., a T304 expiry).
In some cases, the RLF indication may preempt the UE 115-d from
declaring RLF, and thus the UE 115-d does not trigger an RRC
reestablishment, due to the first connection remaining active. In
cases where the UE 115-d detects RLF on the target base station
105-h after successfully connected and while still connected to
source base station 105-g as part of the handover procedure, UE
115-d does not declare RLF and will not trigger RRC reestablishment
as the first connection is active. In such cases where the second
connection fails after the establishment of the connection, the RLF
indication at 660 may indicate a failure cause (e.g., T310 expiry,
RandomAccessProblem, or RLCMaxReTx), the target cell group ID, or
both. In some cases, the UE 115-d may also include one or more
measurements (e.g., in a MeasResultHO-Failure indication) available
for the target base station 105-g. At 665, the source base station
105-g may transmit a handover cancel message to the target base
station 105-h. In some cases, the handover cancel message may
include RLF information or cause indication.
[0181] At 670, the UE 115-d and the source base station 105-g may
continue uplink/downlink transmissions using the first connection.
While the example of FIG. 6 illustrates a make-before-break
handover, similar techniques may be used in an inter-gNB-CU DC
handover to report a failure of a connection at the source or
target, and may perform further actions such as discussed above
with respect to FIG. 5.
[0182] FIG. 7 illustrates an example of a process flow 700 that
supports handover techniques in wireless communications in
accordance with aspects of the present disclosure. In some
examples, process flow 700 may implement aspects of wireless
communications system 100 or 200. In some cases, during a handover
of a UE, channel conditions of the source and the target may be
relatively close to one another (e.g., due to the UE being
approximately a same distance away from a cell edge of both the
source and the target). In some cases, a hysteresis may be applied
to channel measurements for initiating a handover to prevent
excessive ping-pong handovers between source and target base
stations. However, such a hysteresis may result in a UE that is not
taking advantage of a base station that may have better channel
conditions and thus better efficiency and reliability. In the
example of FIG. 7, an example of ping-pong handling is discussed in
which a UE may maintain connections with both the source and target
and utilize a connection with better channel conditions, and may
thus provide enhanced reliability and throughput.
[0183] The process flow 700 may include a source base station
105-i, a target base station 105-j, and a UE 115-e. Source base
station 105-i and target base station 105-j may be examples of a
base station 105 as described herein, and in some cases may include
one or more associated DUs or TRPs, and a CU or ANC. UE 115-e may
be an example of corresponding UE devices described herein. In some
examples, the process flow 700 may implement aspects of the
wireless communications system 100 and 200. For example, the source
base station 105-i, the target base station 105-j, and the UE
115-e, may support 0 ms or almost 0 ms interruption handovers.
[0184] In the following description of the process flow 700, the
operations between the source base station 105-i, target base
station 105-j, and the UE 115-e may be transmitted in a different
order than the exemplary order shown, or the operations performed
by the CUs 105 and the UE 115-e may be performed in different
orders or at different times. Certain operations may also be left
out of the process flow 700, or other operations may be added to
the process flow 700.
[0185] In this example a handover between the source base station
105-i and the target base station 105-j, as indicated above, is an
enhanced make-before-break inter-CU handover in which both the
source base station 105-i and the target base station 105-j have
different associated CUs. In some examples, the process flow 700
may commence at 710 with the source base station 105-i establishing
a first connection with the UE 115-e (e.g., performing a cell
acquisition procedure, a random access procedure, an RRC connection
procedure, an RRC configuration procedure, etc.).
[0186] At 715, an event trigger may occur that may cause the UE
115-e to perform a measurement procedure. Such a measurement
procedure may include, for example, signal measurements of
neighboring base stations by the UE 115-e to identify one or more
neighboring base stations that may be candidates for the handover
procedure. In some cases, the event trigger may be a signal
measurement of the source base station 105-i dropping below a
threshold value, may be a time-based periodic event, or
combinations thereof. At 720, the UE 115-e may transmit measurement
report to the source base station 105-i, and the measurement report
may indicate that the target base station 105-j is a candidate for
a handover.
[0187] At 725, the source base station 105-i (e.g., a CU associated
with the source base station 105-i), the target base station 105-j
(e.g., a CU associated with the target base station 105-j), and the
UE 115-e may perform an enhanced make-before-break handover
procedure, such as discussed above, and may establish a second
connection between the UE 115-e and the target base station 105-j.
Once established, the second connection may be the primary
connection for the UE 115-e, and the first connection may be the
secondary connection of the UE 115-e. In some cases, the UE 115-e
may maintain both the first connection and the second connection,
as indicated at 770, until one of the connections is relatively
stable and provides a better channel for a certain period of
time.
[0188] In this example, the UE 115-e, source base station 105-i,
and target base station 105-j, may perform ping-pong handling 730,
in which a primary connection of the UE 115-e may be switched based
on which of the first connection or second connection has more
favorable channel conditions.
[0189] At 735, while the UE 115-e is connected to both the source
base station 105-i and the target base station 105-j, an event
trigger may occur that may cause the UE 115-e to perform a
measurement procedure. Such a measurement procedure may include,
for example, signal measurements of the source base station 105-i,
the target base station 105-j, and/or one or more other neighboring
base stations. In some cases, the event trigger may be a signal
measurement of the source base station 105-i. In some cases, the
event trigger may be when the secondary connection has a signal
strength that exceeds the primary connection signal strength by a
threshold value.
[0190] At 740, the UE 115-e may transmit measurement report to the
target base station 105-j (i.e., to the base station 105 that is
currently providing the primary connection), and the measurement
report may indicate that the source base station 105-i has better
channel conditions than the target base station 105-j.
[0191] At 745, based on the measurement report, the target base
station 105-j may transmit a handover switch request to the source
base station 105-i, which requests to switch the primary connection
to the source base station 105-i, and the secondary connection to
be the connection with the target base station 105-j. At 750, the
source base station 105-i may transmit a handover switch
acknowledgment that indicates that the source base station 105-i is
prepared to provide the primary connection with the UE 115-e.
[0192] At 755, the target base station 105-j may transmit a RRC
reconfiguration to the UE 115-e to switch the primary and secondary
connections. The UE 115-e, based on the RRC reconfiguration, may
reconfigure the primary and secondary connections to swap the
connections, and may, at 760, transmit a RRC reconfiguration
complete indication to the source base station 105-i. In some
cases, the indication to switch the primary and secondary
connections may be provided to the UE 115-e via a MAC CE.
[0193] The operations described at 735 through 760 may be repeated
with the swapped primary and secondary connections until, at 765,
it is determined that one of the first connection or second
connection has served as the primary connection for a duration of a
ping-pong timer. In some cases, a duration of the ping-pong timer
may be determined based on channel conditions at the UE 115-e, a
predetermined duration, a history of ping-pong procedure durations,
or any combinations thereof. At 775, the secondary connection may
be released, and communication may continue using the primary
connection until a subsequent handover procedure is initiated.
[0194] Such ping-pong handling 730 may be useful in certain
deployments, such as deployments that use mmW or in inter-frequency
handover scenarios, where the source base station 105-i may decide
to initiate the handover to the target base station 105-j much
earlier than the actual need for handover to reduce the possibility
for handover command failure. In such cases, once the target base
station 105-j link is established, it is possible that the target
link becomes weaker than source and ping-pong happens. Thus, such
techniques may be beneficial to maintain the source base station
105-i connection for some time until the target base station 105-j
link is good enough to maintain the primary connection. By
maintaining the source base station 105-i connection, the target
base station 105-j may handle ping-pong efficiently through a RRC
Reconfiguration without the need to perform a random access
procedure or other backend procedures on the source base station
105-j again. While the techniques of FIG. 7 show enhanced
make-or-break handover, such techniques may also be used for DC
based handover.
[0195] In some cases, SRB/DRB data can be transmitted/received by
the UE 115-e only on the primary link, or can be duplicated over
both the links during the ping-pong handling procedure at 730. In
cases where duplication of data is provided, which may enhance
reliability of communications, additional configuration information
may be exchanged over a backhaul link (e.g., an Xn link) between
the source base station 105-i and the target base station 105-jm to
establish backhaul duplication and the tunnel information. Further,
uplink duplicated data sent by the UE 115-e may be forwarded over
the backhaul link to the primary connection for forwarding to the
UPF. In such cases, downlink data is still received by the original
source base station 105-i (due to the core network path switch not
happening until a stable link is decided), and forwarded to the
other base station via backhaul link for duplication of downlink
data on the target base station 105-j link. In some cases, the UE
115-e may store both base station keys and use the key based on the
transmission/reception connection. Such techniques may be used for
both intra-gNB-CU and inter-gNB-CU handover. In the case of
intra-gNB-CU handover, the switch may be controlled by the gNB-CU
to move between the cells under different DUs.
[0196] FIG. 8 illustrates an example of a process flow 800 that
supports handover techniques in wireless communications in
accordance with aspects of the present disclosure. In some
examples, process flow 800 may implement aspects of wireless
communications system 100 or 200. In some cases, a UE may perform a
measurement that may indicate that a neighboring base station has a
better channel condition than a source or target base station. In
some cases, the UE may provide a measurement report that indicates
the neighboring base station, and may trigger a handover abort of a
handover to the target base station in favor of the neighboring
base station with better channel conditions. Such handover abort
may allow the UE to establish a connection with a more favorable
base station and thereby enhance reliability and communications in
a wireless communications system. In this example, an example of an
inter-gNB-CU enhanced make-before-break handover is illustrated,
although such techniques may be used for DC handover, intra-gNB-CU
handovers, or any combinations thereof.
[0197] The process flow 800 may include a source base station
105-k, a first target base station 105-l, a second target base
station 105-m, and a UE 115-f. Source base station 105-k, first
target base station 105-l, and second target base station 105-m may
be examples of a base station 105 as described herein, and in some
cases may include one or more associated DUs or TRPs, and a CU or
ANC. UE 115-f may be an example of corresponding UE devices
described herein. In some examples, the process flow 800 may
implement aspects of the wireless communications system 100 and
200. For example, the source base station 105-k, the target base
stations 105, and the UE 115-f, may support 0 ms or almost 0 ms
interruption handovers.
[0198] In the following description of the process flow 800, the
operations between the source base station 105-k, first target base
station 105-l, second target base station 105-m and the UE 115-f
may be transmitted in a different order than the exemplary order
shown, or the operations performed by the base stations 105 and the
UE 115-f may be performed in different orders or at different
times. Certain operations may also be left out of the process flow
800, or other operations may be added to the process flow 800.
[0199] In this example a handover between the source base station
105-k and the target base station 105-l, as indicated above, is an
enhanced make-before-break inter-CU handover in which both the
source base station 105-k and the target base station 105-l have
different associated CUs. In some examples, the process flow 800
may commence at 810 with the source base station 105-k establishing
a first connection with the UE 115-f (e.g., performing a cell
acquisition procedure, a random access procedure, an RRC connection
procedure, an RRC configuration procedure, etc.).
[0200] At 815, an event trigger may occur that may cause the UE
115-f to perform a measurement procedure. Such a measurement
procedure may include, for example, signal measurements of
neighboring base stations by the UE 115-f to identify one or more
neighboring base stations that may be candidates for the handover
procedure. In some cases, the event trigger may be a signal
measurement of the source base station 105-k dropping below a
threshold value, may be a time-based periodic event, or
combinations thereof. At 820, the UE 115-f may transmit measurement
report to the source base station 105-k, and the measurement report
may indicate that the first target base station 105-l is a
candidate for a handover.
[0201] At 825, the source base station 105-k (e.g., a CU associated
with the source base station 105-k) may make a handover decision to
handover the UE 115-f from the source base station 105-k to the
first target base station 105-l. In some cases, the source base
station 105-k may make the handover decision based on the
measurement report provided by the UE 115-f, one or more threshold
values for initiating a handover, a differential in one or more
signal measurements from one or more prior measurement reports, one
or more measurement reports from other UEs, or any combinations
thereof. In the example of FIG. 8, the source base station 105-k
may determine that the UE 115-f is to be handed over to the first
target base station 105-l. Further, the source base station 105-k
may determine that the UE 115-f has a capability of simultaneous
transmissions and receptions and may make a decision to perform an
enhanced make-before-break based handover of the UE 115-f. In some
cases, the source base station 105-k determines a handover
configuration that is to be used for the handover procedure for UE
115-f. For example, source base station 105-k may select from
possible handover procedures that may be performed by UE 115-f
based at least in part on the indicated capability of UE 115-f In
the example of FIG. 8, source base station 105-k may select an
enhanced make-before-break handover procedure for UE 115-f based at
least in part on UE 115-f indicating a capability for simultaneous
transmissions and receptions that may be used in an enhanced
make-before-break handover. In other examples, the source base
station 105-k may select a DC handover procedure or a different
handover procedure (e.g., a legacy handover procedure), and
different operations associated with the selected handover
procedure may be performed.
[0202] At 830, the source base station 105-k may transmit a
handover request to the first target base station 105-l. In some
cases, the handover request may indicate that the first target base
station 105-l is a selected for a handover of UE 115-f. In some
cases, the handover request may also include the measurement report
from the UE 115-f.
[0203] At 835, the first target base station 105-l (e.g., a CU
associated with the target base station 105-l) may perform
admission control, and may determine that the first target base
station 105-l can accommodate the handover. In some cases,
admission control may include a UE context setup. At 840, the first
target base station 105-l may transmit a handover request
acknowledgment to the source base station 105-k.
[0204] At 845, the source base station 105-k may transmit a RRC
reconfiguration message to the UE 115-f The RRC reconfiguration
message may indicate to the UE 115-f that a connection with the
first target base station 105-l is to be established according to
an enhanced make-before-break handover procedure. For example, the
RRC reconfiguration may indicate that UE 115-f is to perform the
make-before-break handover procedure with first target base station
105-l using the simultaneous transmission/reception capability of
UE 115-f In some aspects, the RRC reconfiguration can include
information identifying first target base station 105-l,
information identifying a handover configuration, an indication of
the enhanced make-before-break handover procedure, or combinations
thereof.
[0205] Following the transmission of the RRC reconfiguration at 845
and until a second connection with the first target base station
105-l is established, downlink data that is to be transmitted to
the UE 115-f may be provided to the source base station 105-k
(e.g., from a user plane function (UPF) at the core network), and
the source base station 105-k may transmit the downlink data to the
UE 115-f via the first connection. Similarly, uplink data
transmitted from the UE 115-f may be transmitted to the source base
station 105-k via the first connection.
[0206] During the establishment of the second connection with the
first target base station 105-l, the UE 115-f may maintain the
first connection with the source base station 105-k, and thus have
an active established connection during the handover, as indicated
at 875.
[0207] At 850, the UE 115-f and the first target base station 105-l
may perform a random access procedure to establish the second
connection with the first target base station 105-l. The random
access procedure may be performed according to established random
access procedures (e.g., random access channel (RACH) procedures
that are established in LTE or NR). In the example of FIG. 8, the
UE 115-f may continue to perform neighbor cell measurements while
acquiring the first target base station 105-k.
[0208] Such neighbor cell measurements may be initiated at 860,
where an event trigger may occur that may cause the UE 115-f to
perform a measurement procedure for neighbor cells, which may
include a measurement of the second target base station 105-m. Such
a measurement procedure may include, for example, signal
measurements of neighboring base stations by the UE 115-f to
identify one or more neighboring base stations that may be better
candidates for handover than the first target base station 105-l.
In some cases, the event trigger may be a signal measurement of the
first target base station 105-l dropping below a threshold value,
may be a time-based periodic event, or combinations thereof. At
865, the UE 115-f may transmit measurement report to the source
base station 105-k, and the measurement report may indicate that
the second target base station 105-m has better channel conditions
than the first target base station 105-l.
[0209] At 870, the source base station 105-k (e.g., a CU associated
with the source base station 105-k) may make a handover decision to
handover the UE 115-f from the source base station 105-k to the
second target base station 105-m. In some cases, the source base
station 105-k may make the handover decision based on the
measurement report provided by the UE 115-f, one or more threshold
values for aborting the handover to the first target base station
105-l, a differential in one or more signal measurements from one
or more prior measurement reports, one or more measurement reports
from other UEs, or any combinations thereof. In the example of FIG.
8, the source base station 105-k may determine that the handover to
the first target base station 105-l is to be aborted in favor of a
handover to the second target base station 105-m.
[0210] At 870, the source base station 105-k may transmit a
handover request to the second target base station 105-m. In some
cases, the handover request may indicate that the second target
base station 105-m is a selected for a handover of UE 115-f. In
some cases, the handover request may also include the measurement
report from the UE 115-f.
[0211] At 875, the second target base station 105-m (e.g., a CU
associated with the target base station 105-l) may perform
admission control, and may determine that the second target base
station 105-m can accommodate the handover. In some cases,
admission control may include a UE context setup. At 880, the
second target base station 105-m may transmit a handover request
acknowledgment to the source base station 105-k.
[0212] At 885, the source base station 105-k may transmit a RRC
reconfiguration message to the UE 115-f The RRC reconfiguration
message may indicate to the UE 115-f that the handover with the
first target base station 105-l is to be aborted in favor of a
handover to the second target base station 105-m. Following the
transmission of the RRC reconfiguration at 885, the source base
station 105-k may transmit a handover cancel indication to the
first target base station 105-l, at indicated at 890.
[0213] At 895, the UE 115-f and the second target base station
105-m may perform a random access procedure to establish a second
connection with the second target base station 105-m. The UE 115-m,
the first target base station 105-k, and the second target base
station 105-m may follow enhanced make-or-break handover procedures
such as discussed herein (e.g., connection establishment, radio
link failures associated with a connection, handover abort,
etc.).
[0214] In some cases, the UE 115-f may treat the source base
station 105-k as a Pcell and the first target base station 105-l as
the Scell, and may send measurement reports (e.g., an A3 and A6
measurement report in an LTE or NR system). Based on the
measurement reports, the source base station 105-k or associated CU
may consider that another better neighbor cell is detected if the
A3 and A6 measurement reports are received within certain time from
the UE. In other cases, a new measurement event (e.g., an A36
measurement event) may be defined, which is to report when a
neighbor cell is detected to be stronger than the source base
station 105-k and the first target base station 105-l.
[0215] FIG. 9 shows a block diagram 900 of a device 905 that
supports handover techniques in wireless communications in
accordance with aspects of the present disclosure. The device 905
may be an example of aspects of a UE 115 as described herein. The
device 905 may include a receiver 910, a communications manager
915, and a transmitter 920. The device 905 may also include a
processor. Each of these components may be in communication with
one another (e.g., via one or more buses).
[0216] The receiver 910 may receive information such as packets,
user data, or control information associated with various
information channels (e.g., control channels, data channels, and
information related to handover techniques in wireless
communications, etc.). Information may be passed on to other
components of the device 905. The receiver 910 may be an example of
aspects of the transceiver 1220 described with reference to FIG.
12. The receiver 910 may utilize a single antenna or a set of
antennas.
[0217] In some cases, the communications manager 915 may establish
a first connection with a first base station, receive a handover
message from the first base station to perform a handover procedure
with a second base station, transmit, responsive to the handover
message, a request to the second base station to establish a second
connection with the second base station, where the first connection
with the first base station is maintained during the handover
procedure, determine that a radio link failure of the first
connection or the second connection has occurred, and transmit an
indication of the radio link failure to the first base station or
the second base station responsive to the determining.
[0218] In some cases, the communications manager 915 may also
establish a first connection with a first base station, receive a
handover message from the first base station to perform a handover
procedure with a second base station, initiate a connection
establishment with the second base station to establish a second
connection responsive to the handover message, where the first
connection is maintained during the connection establishment with
the second base station, measure one or more channel conditions
associated with one or more of the first base station, the second
base station or a third base station, and transmit, responsive to
the handover message, a measurement report to at least one of the
first base station or the second base station responsive to
measuring the one or more channel conditions. The communications
manager 915 may be an example of aspects of the communications
manager 1210 described herein.
[0219] The communications manager 915, or its sub-components, may
be implemented in hardware, code (e.g., software or firmware)
executed by a processor, or any combination thereof. If implemented
in code executed by a processor, the functions of the
communications manager 915, or its sub-components may be executed
by a general-purpose processor, a DSP, an application-specific
integrated circuit (ASIC), a FPGA or other programmable logic
device, discrete gate or transistor logic, discrete hardware
components, or any combination thereof designed to perform the
functions described in the present disclosure.
[0220] The communications manager 915, or its sub-components, may
be physically located at various positions, including being
distributed such that portions of functions are implemented at
different physical locations by one or more physical components. In
some examples, the communications manager 915, or its
sub-components, may be a separate and distinct component in
accordance with various aspects of the present disclosure. In some
examples, the communications manager 915, or its sub-components,
may be combined with one or more other hardware components,
including but not limited to an input/output (I/O) component, a
transceiver, a network server, another computing device, one or
more other components described in the present disclosure, or a
combination thereof in accordance with various aspects of the
present disclosure.
[0221] The transmitter 920 may transmit signals generated by other
components of the device 905. In some examples, the transmitter 920
may be collocated with a receiver 910 in a transceiver module. For
example, the transmitter 920 may be an example of aspects of the
transceiver 1220 described with reference to FIG. 12. The
transmitter 920 may utilize a single antenna or a set of
antennas.
[0222] FIG. 10 shows a block diagram 1000 of a device 1005 that
supports handover techniques in wireless communications in
accordance with aspects of the present disclosure. The device 1005
may be an example of aspects of a device 905, or a UE 115 as
described herein. The device 1005 may include a receiver 1010, a
communications manager 1015, and a transmitter 1045. The device
1005 may also include a processor. Each of these components may be
in communication with one another (e.g., via one or more
buses).
[0223] The receiver 1010 may receive information such as packets,
user data, or control information associated with various
information channels (e.g., control channels, data channels, and
information related to handover techniques in wireless
communications, etc.). Information may be passed on to other
components of the device 1005. The receiver 1010 may be an example
of aspects of the transceiver 1220 described with reference to FIG.
12. The receiver 1010 may utilize a single antenna or a set of
antennas.
[0224] The communications manager 1015 may be an example of aspects
of the communications manager 915 as described herein. The
communications manager 1015 may include a connection establishment
manager 1020, a handover manager 1025, a random access manager
1030, a RLF manager 1035, and a measurement component 1040. The
communications manager 1015 may be an example of aspects of the
communications manager 1210 described herein.
[0225] The connection establishment manager 1020 may establish a
first connection with a first base station.
[0226] The handover manager 1025 may receive a handover message
from the first base station to perform a handover procedure with a
second base station.
[0227] The random access manager 1030 may transmit, responsive to
the handover message, a request to the second base station to
establish a second connection with the second base station, where
the first connection with the first base station is maintained
during the handover procedure.
[0228] The RLF manager 1035 may determine that a radio link failure
of the first connection or the second connection has occurred and
transmit an indication of the radio link failure to the first base
station or the second base station responsive to the
determining.
[0229] The measurement component 1040 may measure one or more
channel conditions associated with one or more of the first base
station, the second base station or a third base station and
transmit, responsive to the handover message, a measurement report
to at least one of the first base station or the second base
station responsive to measuring the one or more channel
conditions.
[0230] The transmitter 1045 may transmit signals generated by other
components of the device 1005. In some examples, the transmitter
1045 may be collocated with a receiver 1010 in a transceiver
module. For example, the transmitter 1045 may be an example of
aspects of the transceiver 1220 described with reference to FIG.
12. The transmitter 1045 may utilize a single antenna or a set of
antennas.
[0231] FIG. 11 shows a block diagram 1100 of a communications
manager 1105 that supports handover techniques in wireless
communications in accordance with aspects of the present
disclosure. The communications manager 1105 may be an example of
aspects of a communications manager 915, a communications manager
1015, or a communications manager 1210 described herein. The
communications manager 1105 may include a connection establishment
manager 1110, a handover manager 1115, a random access manager
1120, a RLF manager 1125, a measurement component 1130, and a DC
manager 1135. Each of these modules may communicate, directly or
indirectly, with one another (e.g., via one or more buses).
[0232] The connection establishment manager 1110 may establish a
first connection with a first base station. In some examples, the
connection establishment manager 1110 may determine that the first
connection with the first base station has failed prior to
completion of an establishment of the second connection.
[0233] In some examples, the connection establishment manager 1110
may complete the establishment of the second connection with the
second base station. In some examples, the connection establishment
manager 1110 may release the first connection and a source stack
associated with the first connection responsive to completing the
establishment of the second connection with the second base
station. In some cases, the completing the establishment of the
second connection preempts a triggering of a reestablishment of the
first connection with the first base station in the event of a RLF
at the second connection.
[0234] The handover manager 1115 may receive a handover message
from the first base station to perform a handover procedure with a
second base station. In some examples, the handover manager 1115
may release the first connection after the second connection has
been a primary connection for a predetermined time period. In some
cases, a reconfiguration message includes a second handover message
from the first base station to perform a second handover procedure
with the third base station.
[0235] The random access manager 1120 may transmit, responsive to
the handover message, a request to the second base station to
establish a second connection with the second base station, where
the first connection with the first base station is maintained
during the handover procedure.
[0236] The RLF manager 1125 may determine that a radio link failure
of the first connection or the second connection has occurred. In
some examples, the RLF manager 1125 may transmit an indication of
the radio link failure to the first base station or the second base
station responsive to the determining. In some examples, the RLF
manager 1125 may determine that establishment of the connection
with the second base station has failed or the second connection
failed after establishment of the second connection.
[0237] In some cases, the indication of the radio link failure
includes a failure cause associated with the second connection and
a target cell group identification associated with the second base
station. In some cases, the indication of the radio link failure
further indicates one or more of a timer expiration associated with
the second connection, a random access procedure failure, or a
maximum number of retransmissions is reached for a communication
using the second connection. In some cases, the indication of the
radio link failure indicates a failure cause associated with the
first connection and a source cell group identification associated
with the first base station.
[0238] The measurement component 1130 may measure one or more
channel conditions associated with one or more of the first base
station, the second base station or a third base station. In some
examples, the measurement component 1130 may transmit, responsive
to the handover message, a measurement report to at least one of
the first base station or the second base station responsive to
measuring the one or more channel conditions. In some examples, the
measurement component 1130 may transmit one or more additional
measurement reports via the primary connection. In some cases, the
indication of the radio link failure further indicates a
measurement of one or more signals received at the UE from one or
more neighboring base stations.
[0239] In some cases, the measurement report is transmitted
responsive to the measured one or more channel conditions of the
secondary connection being better than corresponding channel
conditions of the primary connection. In some cases, the
measurement report is transmitted responsive to measured channel
conditions of the third base station being better than measured
channel conditions of the second base station.
[0240] The DC manager 1135 may establish the second connection with
the second base station, where the second connection is a primary
connection and the first connection is maintained as a secondary
connection.
[0241] In some examples, the DC manager 1135 may receive,
responsive to the measurement report, a reconfiguration message
from the second base station to reconfigure the first connection as
the primary connection and the second connection as the secondary
connection. In some examples, the DC manager 1135 may reconfigure
the first connection and the second connection responsive to the
reconfiguration message. In some examples, the DC manager 1135 may
transmit and receiving duplicated data via each of the primary
connection and the secondary connection. In some cases, the
reconfiguring the first connection and the second connection
responsive to the reconfiguration message is performed without
performing a random access procedure.
[0242] In some examples, the DC manager 1135 may receive,
responsive to the measurement report, a reconfiguration message
from the first base station to abort the handover procedure with
the second base station.
[0243] FIG. 12 shows a diagram of a system 1200 including a device
1205 that supports handover techniques in wireless communications
in accordance with aspects of the present disclosure. The device
1205 may be an example of or include the components of device 905,
device 1005, or a UE 115 as described herein. The device 1205 may
include components for bi-directional voice and data communications
including components for transmitting and receiving communications,
including a communications manager 1210, an I/O controller 1215, a
transceiver 1220, an antenna 1225, memory 1230, and a processor
1240. These components may be in electronic communication via one
or more buses (e.g., bus 1245).
[0244] The communications manager 1210 may establish, at a UE, a
first connection with a first base station, receive a handover
message from the first base station to perform a handover procedure
with a second base station, transmit, responsive to the handover
message, a request to the second base station to establish a second
connection with the second base station, where the first connection
with the first base station is maintained during the handover
procedure, determine that a radio link failure of the first
connection or the second connection has occurred, and transmit an
indication of the radio link failure to the first base station or
the second base station responsive to the determining. The
communications manager 1210 may also establish, at a UE, a first
connection with a first base station, receive a handover message
from the first base station to perform a handover procedure with a
second base station, initiate, at the UE, a connection
establishment with the second base station to establish a second
connection responsive to the handover message, where the first
connection is maintained during the connection establishment with
the second base station, measure one or more channel conditions
associated with one or more of the first base station, the second
base station or a third base station, and transmit, responsive to
the handover message, a measurement report to at least one of the
first base station or the second base station responsive to
measuring the one or more channel conditions.
[0245] The I/O controller 1215 may manage input and output signals
for the device 1205. The I/O controller 1215 may also manage
peripherals not integrated into the device 1205. In some cases, the
I/O controller 1215 may represent a physical connection or port to
an external peripheral. In some cases, the I/O controller 1215 may
utilize an operating system such as iOS.RTM., ANDROID.RTM.,
MS-DOS.RTM., MS-WINDOWS.RTM., OS/2.RTM., UNIX.RTM., LINUX.RTM., or
another known operating system. In other cases, the I/O controller
1215 may represent or interact with a modem, a keyboard, a mouse, a
touchscreen, or a similar device. In some cases, the I/O controller
1215 may be implemented as part of a processor. In some cases, a
user may interact with the device 1205 via the I/O controller 1215
or via hardware components controlled by the I/O controller
1215.
[0246] The transceiver 1220 may communicate bi-directionally, via
one or more antennas, wired, or wireless links as described above.
For example, the transceiver 1220 may represent a wireless
transceiver and may communicate bi-directionally with another
wireless transceiver. The transceiver 1220 may also include a modem
to modulate the packets and provide the modulated packets to the
antennas for transmission, and to demodulate packets received from
the antennas.
[0247] In some cases, the wireless device may include a single
antenna 1225. However, in some cases the device may have more than
one antenna 1225, which may be capable of concurrently transmitting
or receiving multiple wireless transmissions.
[0248] The memory 1230 may include RAM and ROM. The memory 1230 may
store computer-readable, computer-executable code 1235 including
instructions that, when executed, cause the processor to perform
various functions described herein. In some cases, the memory 1230
may contain, among other things, a BIOS which may control basic
hardware or software operation such as the interaction with
peripheral components or devices.
[0249] The processor 1240 may include an intelligent hardware
device, (e.g., a general-purpose processor, a DSP, a CPU, a
microcontroller, an ASIC, an FPGA, a programmable logic device, a
discrete gate or transistor logic component, a discrete hardware
component, or any combination thereof). In some cases, the
processor 1240 may be configured to operate a memory array using a
memory controller. In other cases, a memory controller may be
integrated into the processor 1240. The processor 1240 may be
configured to execute computer-readable instructions stored in a
memory (e.g., the memory 1230) to cause the device 1205 to perform
various functions (e.g., functions or tasks supporting handover
techniques in wireless communications).
[0250] The code 1235 may include instructions to implement aspects
of the present disclosure, including instructions to support
wireless communications. The code 1235 may be stored in a
non-transitory computer-readable medium such as system memory or
other type of memory. In some cases, the code 1235 may not be
directly executable by the processor 1240 but may cause a computer
(e.g., when compiled and executed) to perform functions described
herein.
[0251] FIG. 13 shows a block diagram 1300 of a device 1305 that
supports handover techniques in wireless communications in
accordance with aspects of the present disclosure. The device 1305
may be an example of aspects of a base station 105 as described
herein. The device 1305 may include a receiver 1310, a
communications manager 1315, and a transmitter 1320. The device
1305 may also include a processor. Each of these components may be
in communication with one another (e.g., via one or more
buses).
[0252] The receiver 1310 may receive information such as packets,
user data, or control information associated with various
information channels (e.g., control channels, data channels, and
information related to handover techniques in wireless
communications, etc.). Information may be passed on to other
components of the device 1305. The receiver 1310 may be an example
of aspects of the transceiver 1620 described with reference to FIG.
16. The receiver 1310 may utilize a single antenna or a set of
antennas.
[0253] In some cases, the base station may be a source base station
and the communications manager 1315 may establish a first
connection with a UE, initiate a handover procedure to handover the
UE to a second base station, where the first connection with the
first base station is maintained during the handover procedure. In
some cases, the base station may discontinue the handover procedure
to handover the UE to the second base station, based on an
indication of a failure of the handover procedure.
[0254] In some cases, the base station may be a target base station
and the communications manager 1315 may also receive a handover
message to initiate a handover of a UE from a first or source base
station to the target base station, and initiate, responsive to the
handover message, a connection establishment with the UE to
establish a second connection between the UE and the target base
station. In some cases, the target base station may receive from
the UE an indication of a failure of a first connection between the
UE and the first base station, where the first connection was to be
maintained during the handover of the UE from the first base
station to the target base station, and forward the indication of
the failure of the first connection to the first base station.
[0255] In some cases, when the base station is a source base
station, the communications manager 1315 may also establish a first
connection with a UE, initiate a handover of the UE to a second
base station, where the first connection with the first base
station is maintained during the handover and an establishment of a
second connection between the UE and the second base station, and
may modify the handover of the UE based on a measurement report
from the UE.
[0256] In some cases, when the base station is a target base
station, the communications manager 1315 may also receive a
handover message to initiate a handover of a UE from the first base
station to the target base station, modify the handover of the UE
based on a measurement report, establish a second connection with
the UE responsive to the handover message, and receive a
measurement report from the UE that includes one or more channel
measurements associated with the first base station and the target
base station.
[0257] In some cases, the base station may be a source base
station, and may receive from a target base station a role switch
indication that indicates that the second connection between the UE
and the target base station is a primary connection of the UE. The
communications manager 1315 may also transmit, when the base
station is a target base station, responsive to the establishing
the second connection, a role switch indication to the source base
station that indicates that the second connection between the UE
and the target base station is a primary connection of the UE. The
communications manager 1315 may be an example of aspects of the
communications manager 1610 described herein.
[0258] The communications manager 1315, or its sub-components, may
be implemented in hardware, code (e.g., software or firmware)
executed by a processor, or any combination thereof. If implemented
in code executed by a processor, the functions of the
communications manager 1315, or its sub-components may be executed
by a general-purpose processor, a DSP, an application-specific
integrated circuit (ASIC), a FPGA or other programmable logic
device, discrete gate or transistor logic, discrete hardware
components, or any combination thereof designed to perform the
functions described in the present disclosure.
[0259] The communications manager 1315, or its sub-components, may
be physically located at various positions, including being
distributed such that portions of functions are implemented at
different physical locations by one or more physical components. In
some examples, the communications manager 1315, or its
sub-components, may be a separate and distinct component in
accordance with various aspects of the present disclosure. In some
examples, the communications manager 1315, or its sub-components,
may be combined with one or more other hardware components,
including but not limited to an input/output (I/O) component, a
transceiver, a network server, another computing device, one or
more other components described in the present disclosure, or a
combination thereof in accordance with various aspects of the
present disclosure.
[0260] The transmitter 1320 may transmit signals generated by other
components of the device 1305. In some examples, the transmitter
1320 may be collocated with a receiver 1310 in a transceiver
module. For example, the transmitter 1320 may be an example of
aspects of the transceiver 1620 described with reference to FIG.
16. The transmitter 1320 may utilize a single antenna or a set of
antennas.
[0261] FIG. 14 shows a block diagram 1400 of a device 1405 that
supports handover techniques in wireless communications in
accordance with aspects of the present disclosure. The device 1405
may be an example of aspects of a device 1305, or a base station
105 as described herein. The device 1405 may include a receiver
1410, a communications manager 1415, and a transmitter 1445. The
device 1405 may also include a processor. Each of these components
may be in communication with one another (e.g., via one or more
buses).
[0262] The receiver 1410 may receive information such as packets,
user data, or control information associated with various
information channels (e.g., control channels, data channels, and
information related to handover techniques in wireless
communications, etc.). Information may be passed on to other
components of the device 1405. The receiver 1410 may be an example
of aspects of the transceiver 1620 described with reference to FIG.
16. The receiver 1410 may utilize a single antenna or a set of
antennas.
[0263] The communications manager 1415 may be an example of aspects
of the communications manager 1315 as described herein. The
communications manager 1415 may include a connection establishment
manager 1420, a handover manager 1425, a RLF manager 1430, a random
access manager 1435, and a measurement component 1440. The
communications manager 1415 may be an example of aspects of the
communications manager 1610 described herein.
[0264] The connection establishment manager 1420 may establish, at
a first base station, a first connection with a UE.
[0265] The handover manager 1425 may initiate a handover procedure
to handover the UE to a second base station, where the first
connection with the first base station is maintained during the
handover procedure, and discontinue the handover procedure to
handover the UE to the second base station in some cases. In some
cases, the handover manager 1425 may modify the handover of the UE
based on a measurement report. In some cases, the handover manager
1425 receive a role switch indication that indicates a primary
connection and secondary connection are to be switched.
[0266] The RLF manager 1430 may receive, from the UE, an indication
of a failure of the handover procedure. In some cases, the RLF
manager 1430 may be associated with a target second base station,
and may receive from the UE an indication of a failure of a first
connection between the UE and the first base station, where the
first connection was to be maintained during the handover of the UE
from the first base station to the second base station and forward
the indication of the failure of the first connection to the first
base station.
[0267] The random access manager 1435 may initiate, responsive to
the handover message, a connection establishment with the UE to
establish a second connection between the UE and the second base
station.
[0268] The measurement component 1440 may receive a measurement
report responsive to the initiating the handover. In some cases,
the measurement report from the UE may include one or more channel
measurements associated with the first base station and the second
base station.
[0269] The transmitter 1445 may transmit signals generated by other
components of the device 1405. In some examples, the transmitter
1445 may be collocated with a receiver 1410 in a transceiver
module. For example, the transmitter 1445 may be an example of
aspects of the transceiver 1620 described with reference to FIG.
16. The transmitter 1445 may utilize a single antenna or a set of
antennas.
[0270] FIG. 15 shows a block diagram 1500 of a communications
manager 1505 that supports handover techniques in wireless
communications in accordance with aspects of the present
disclosure. The communications manager 1505 may be an example of
aspects of a communications manager 1315, a communications manager
1415, or a communications manager 1610 described herein. The
communications manager 1505 may include a connection establishment
manager 1510, a handover manager 1515, a RLF manager 1520, a
measurement component 1525, a random access manager 1530, and a DC
manager 1535. Each of these modules may communicate, directly or
indirectly, with one another (e.g., via one or more buses).
[0271] The connection establishment manager 1510 may establish, at
a first base station, a first connection with a UE. In some
examples, the connection establishment manager 1510 may be
associated with a second base station and may establish a second
connection with the UE responsive to the handover message.
[0272] The handover manager 1515 may initiate a handover procedure
to handover the UE to a second base station, where the first
connection with the first base station is maintained during the
handover procedure. In some examples, the handover manager 1515 may
discontinue the handover procedure to handover the UE to the second
base station. In some examples, the handover manager 1515 may
receive, at a second base station, a handover message to initiate a
handover of a UE from a first base station to the second base
station.
[0273] In some examples, the handover manager 1515 may initiate a
handover of the UE to a second base station, where the first
connection with the first base station is maintained during the
handover and an establishment of a second connection between the UE
and the second base station. In some examples, the handover manager
1515 may modify the handover of the UE based on the measurement
report.
[0274] In some examples, the handover manager 1515 may receive,
from the second base station, a role switch indication that
indicates that the second connection between the UE and the second
base station is a primary connection of the UE. In some examples,
the handover manager 1515 may be associated with the second base
station, and may transmit, responsive to the establishing the
second connection, a role switch indication to the first base
station that indicates that the second connection between the UE
and the second base station is a primary connection of the UE.
[0275] In some examples, the handover manager 1515 may be
associated with a first base station and may transmit, to the
second base station and responsive to the receiving the indication
of the failure of the handover procedure, a message to the second
base station to cancel the handover procedure. In some examples,
the handover manager 1515 may initiate a second handover procedure
to handover the UE to a third base station, where the first
connection with the first base station is maintained during the
second handover procedure. In some examples, the handover manager
1515 may transmit, to the UE, a reconfiguration message indicating
the UE is to perform the second handover procedure with the third
base station.
[0276] In some examples, the handover manager 1515 may transmit an
indication to the second base station that the handover of the UE
to the second base station is aborted. In some examples, the
handover manager 1515 may release the second connection after the
first connection has been a primary connection for a predetermined
time period. In some examples, the handover manager 1515 may
release the first connection with the UE responsive to a role
switch indication.
[0277] The RLF manager 1520 may receive, from the UE, an indication
of a failure of the handover procedure. In some examples, the first
connection was to be maintained during the handover of the UE from
the first base station to the second base station. In some
examples, the RLF manager 1520 may forward the indication of the
failure of the first connection to the first base station. In some
examples, the radio link failure message from the UE may indicate a
timer expiration associated with a random access procedure to
establish a second connection with the second base station.
[0278] In some examples, the RLF manager 1520 may receive a radio
link failure message from the UE indicating a second connection
with the second base station was established and then failed. In
some cases, the radio link failure message indicates a failure
cause associated with the second connection, and where the failure
cause indicates one or more or a timer expiration associated with
the second connection, a random access procedure failure, or a
maximum number of retransmissions is reached for a communication
using the second connection. In some cases, the radio link failure
message further indicates a target cell group identification
associated with the second base station. In some cases, the
indication of the failure of the first connection indicates that
resources and context associated with the first connection are to
be deleted. In some cases, the indication of the failure of the
first connection includes a failure cause associated with the first
connection and a source cell group identification associated with
the first base station.
[0279] The measurement component 1525 may receive a measurement
report responsive to the initiating the handover. In some examples,
the measurement component 1525 may receive a measurement report
from the UE that includes one or more channel measurements
associated with the first base station and the second base station.
In some examples, the measurement component 1525 may receive one or
more additional measurement reports.
[0280] In some cases, the indication of the failure of the handover
procedure further indicates a measurement of one or more signals
received at the UE from a neighboring base station. In some cases,
the indication of the failure of the first connection further
includes a measurement of one or more signals received at the UE
from a neighboring base station. In some cases, the measurement
report is transmitted responsive to the measured one or more
channel conditions of the secondary connection being better than
corresponding channel conditions of the primary connection. In some
cases, the measurement report is transmitted responsive to measured
channel conditions of a third base station being better than
measured channel conditions of the second base station.
[0281] The random access manager 1530 may initiate, responsive to
the handover message, a connection establishment with the UE to
establish a second connection between the UE and the second base
station.
[0282] The DC manager 1535 may receive, from the second base
station prior to receiving the measurement report, a role switch
indication that indicates that a second connection between the UE
and the second base station is a primary connection of the UE and
the first connection is a secondary connection. In some examples,
the modifying the handover of the UE includes reconfiguring the
first connection to be the primary connection and the second
connection to be the secondary connection based on the measurement
report. In some examples, the DC manager 1535 may further
reconfigure the primary connection and the secondary connection
based on the one or more additional measurement reports.
[0283] In some examples, the DC manager 1535 may exchange
information with the second base station to establish duplication
of data communicated with the UE via both the first connection and
the second connection. In some examples, the DC manager 1535 may
communicate duplicated data with the UE via the first connection.
In some examples, the DC manager 1535 may transmit, responsive to
the measurement report, a reconfiguration message to the UE to
abort the handover procedure with the second base station.
[0284] In some examples, the DC manager 1535 may transmit, to the
first base station prior to receiving the measurement report, a
role switch indication that indicates that a second connection
between the UE and the second base station is a primary connection
of the UE and a first connection between the UE and the first base
station is a secondary connection. In some examples, the modifying
the handover of the UE includes reconfiguring the first connection
to be the primary connection and the second connection to be the
secondary connection based on the measurement report.
[0285] In some examples, the DC manager 1535 may configure one or
more secondary node (SN) terminated bearers at the second base
station for the second connection. In some examples, the DC manager
1535 may transmit a reconfiguration message to the UE that
indicates the handover to the second base station. In some
examples, the DC manager 1535 may transmit a role switch request to
the second base station. In some examples, the DC manager 1535 may
receive an acknowledgment of the role switch request from the
second base station.
[0286] In some cases, the role switch request is transmitted with a
SN addition request to the second base station, and the
acknowledgment of the role switch request is received with a SN
addition acknowledgment from the second base station. In some
cases, the reconfiguration message to the UE indicates a secondary
carrier group associated with the second connection and a source
cell group identification associated with the first connection. In
some cases, the role switch indication is received subsequent to
performance of a random access procedure between the second base
station and the UE.
[0287] FIG. 16 shows a diagram of a system 1600 including a device
1605 that supports handover techniques in wireless communications
in accordance with aspects of the present disclosure. The device
1605 may be an example of or include the components of device 1305,
device 1405, or a base station 105 as described herein. The device
1605 may include components for bi-directional voice and data
communications including components for transmitting and receiving
communications, including a communications manager 1610, a network
communications manager 1615, a transceiver 1620, an antenna 1625,
memory 1630, a processor 1640, and an inter-station communications
manager 1645. These components may be in electronic communication
via one or more buses (e.g., bus 1650).
[0288] The communications manager 1610 may establish, at a first
base station, a first connection with a UE, initiate a handover
procedure to handover the UE to a second base station, where the
first connection with the first base station is maintained during
the handover procedure, discontinue the handover procedure to
handover the UE to the second base station, and receive, from the
UE, an indication of a failure of the handover procedure. The
communications manager 1610 may also receive, at a second base
station, a handover message to initiate a handover of a UE from a
first base station to the second base station, initiate, responsive
to the handover message, a connection establishment with the UE to
establish a second connection between the UE and the second base
station, receive from the UE an indication of a failure of a first
connection between the UE and the first base station, where the
first connection was to be maintained during the handover of the UE
from the first base station to the second base station, and forward
the indication of the failure of the first connection to the first
base station. The communic