U.S. patent application number 11/199178 was filed with the patent office on 2008-02-28 for system and method for handoff between base stations.
This patent application is currently assigned to Nextel Communications, Inc.. Invention is credited to Kamran Etemad, Masoud Olfat.
Application Number | 20080051086 11/199178 |
Document ID | / |
Family ID | 35908096 |
Filed Date | 2008-02-28 |
United States Patent
Application |
20080051086 |
Kind Code |
A2 |
Etemad; Kamran ; et
al. |
February 28, 2008 |
System and Method for Handoff Between Base Stations
Abstract
A system and method for handoff are provided. A mobile station
performs a make-before-break handoff of a control channel between a
serving and target base station and a break-before-make handoff of
a traffic channel between the serving and target base stations. The
traffic channel handoff is performed after the control channel
handoff has completed.
Inventors: |
Etemad; Kamran; (Potomac,
MD) ; Olfat; Masoud; (Clarksville, MD) |
Correspondence
Address: |
CROWELL & MORING LLP;INTELLECTUAL PROPERTY GROUP
P.O. BOX 14300
WASHINGTON
DC
20044-4300
UNITED STATES
202-624-2500
202-628-8844
|
Assignee: |
Nextel Communications, Inc.
2001 Edmund Halley Drive
Reston
VA
20191
|
Prior
Publication: |
|
Document Identifier |
Publication Date |
|
US 20060035639 A1 |
February 16, 2006 |
|
|
Family ID: |
35908096 |
Appl. No.: |
11/199178 |
Filed: |
August 9, 2005 |
Current U.S.
Class: |
455/436 |
Current CPC
Class: |
H04W 36/18 20130101;
H04W 36/0072 20130101 |
Class at
Publication: |
455/436 |
International
Class: |
H04Q 7/20 20060101
H04Q007/20 |
Claims
1. A method for handoff of a mobile station from a serving base
station to a target base station, the method comprising the acts
of: receiving data from a serving base station during a first and
second time period; transmitting control information to the target
base station during a time period between the first and second time
periods; receiving data from the target base station during a time
period subsequent to the second time period, wherein the mobile
station receives data from only one base station during any
particular time period.
2. The method of claim 1, wherein the control information comprises
ranging information.
3. The method of claim 1, further comprising the act of: providing
a base station signal measurement report to the serving base
station.
4. The method of claim 3, wherein a signal of the target base
station is included in the base station signal measurement
report.
5. The method of claim 1, further comprising the act of: receiving,
from the serving base station, a message instructing the mobile
station to attempt to handoff to the target base station.
6. The method of claim 5, wherein the message identifies a reserved
ranging channel of the target base station and a connection
identification to be used for communicating with the target base
station.
7. The method of claim 6, wherein the message also identifies the
time period between the first and second time periods.
8. The method of claim 1, wherein the mobile station transmits a
handoff complete indication to the serving base station during the
second time period.
9. The method of claim 1, wherein the mobile station receives a
traffic channel allocation from the target base station during the
time period subsequent to the second time period.
10. A method for handoff, comprising the acts of: informing a
mobile station of a first set of time periods for communicating
with a target base station over a second control channel;
scheduling information for transmission to the mobile station
during time periods other than the first set of time periods; and
transmitting the scheduled information to the mobile station over a
first traffic channel during periods of time other than the first
set of time periods.
11. The method of claim 10, further comprising the act of:
identifying the target base station based on signal measurement
reports.
12. The method of claim lo, further comprising the act of:
informing the target base station of information related to the
mobile station.
13. The method of claim 12, wherein the information related to the
mobile station includes context information.
14. The method of claim 10, further comprising the act of:
informing the mobile station of a reserved ranging channel of the
target base station and a connection identification to be used for
communicating with the target base station.
15. The method of claim 10, further comprising the acts of:
receiving a handoff complete message; and terminating
communications with the mobile station in response to the handoff
complete message.
16. The method of claim 10, wherein the information for
transmission to the mobile station includes unicast and multicast
data.
17. The method of claim 10, wherein the first set of time periods
are every other n frames.
18. The method of claim 10, wherein the handoff is an
intra-frequency handoff.
19. The method of claim 10, wherein the handoff is an
inter-frequency handoff.
20. A method for a mobile station to handoff from a serving base
station to target base station, the method comprising the acts of:
communicating with the serving base station over a first traffic
channel and a first control channel during a first time period;
communicating with the target base station over a second control
channel during a second time period; communicating with the serving
base station over the first traffic and control channels during a
third time period, which is subsequent to the second time period;
and communicating with the target base station of the second
control channel and a second traffic channel during a fourth time
period.
21. The method of claim 20, wherein the communication with the
target base station during the second period of time comprises
ranging information.
22. The method of claim 20, further comprising the act of:
providing a base station signal measurement report to the serving
base station.
23. The method of claim 22, wherein a signal of the target base
station is included in the base station signal measurement
report.
24. The method of claim 20, further comprising the act of:
receiving, from the serving base station, a message instructing the
mobile station to attempt to handoff to the target base
station.
25. The method of claim 24, wherein the message identifies a
reserved ranging channel of the target base station and a
connection identification to be used for communicating with the
target base station.
26. The method of claim 25, wherein the message also identifies the
second time period.
27. The method of claim 20, wherein the mobile station transmits a
handoff complete indication to the serving base station during the
third time period.
28. The method of claim 20, wherein the mobile station receives a
traffic channel allocation from the target base station during the
fourth time period.
29. The method of claim 20, wherein the handoff is an
intra-frequency handoff.
30. The method of claim 20, wherein the handoff is an
inter-frequency handoff.
31. A method for a mobile station handoff from a serving base
station to a target base station, the method comprising the acts
of: identifying a base station as the target base station;
exchanging control channel messages with the target base station;
and communicating over a traffic channel with the serving base
while exchanging the control channel messages with the target base
station.
32. The method of claim 31, further comprising the acts of:
receiving a feedback channel allocation from the serving base
station; and transmitting an identification of the target base
station to the serving base station over the feedback channel.
33. The method of claim 32, further comprising the act of:
receiving, from the serving base station, a signaling channel
allocation for the target base station, wherein at least one of the
control channel messages is a ranging message which is transmitted
over the allocated signaling channel.
34. The method of claim 31, further comprising the acts of:
receiving a connection identification from the target base station;
and communicating over an allocated traffic channel with the target
base station.
35. A method for a serving base station supporting a handoff of a
mobile station to a target base station, the method comprising the
acts of: receiving, from the mobile station, an identification of
the target base station; informing the target base station of a
handoff attempt by the mobile station; communicating over a traffic
channel with the mobile station, while the mobile station exchanges
control channel messages with the mobile station.
36. The method of claim 35, further comprising the act of:
receiving, from the target base station, a control channel
allocation for the mobile station; and transmitting an indication
of the control channel allocation to the mobile station.
37. The method of claim 35, further comprising the acts of:
receiving, from the mobile station, an indication of a traffic
channel handoff to the target base station; and transmitting an
indication of the traffic channel handoff to the target base
station.
38. The method of claim 37, further comprising the act of:
terminating a traffic channel to the mobile station in response to
the indication of the traffic channel handoff.
39. A method for a mobile station handoff from a serving base
station to a target base station, the method comprising the acts
of: performing a make-before-break handoff of a control channel
between the serving and target base stations; and performing a
break-before-make handoff of a traffic channel between the serving
and target base stations.
40. The method of claim 39, wherein the mobile station performs the
handoff of traffic channels only after the mobile station performs
the act of: indicating completion of the signaling channel
handoff.
41. The method of claim 40, wherein the completion of the control
channel handoff is performed over a handoff switching channel.
Description
[0001] The present application claims priority under 35 U.S.C.
.sctn. 119 to U.S. Provisional Application No. 60/599,916, filed on
Aug. 10, 2004, the entire disclosure of which is herein expressly
incorporated by reference.
BACKGROUND OF THE INVENTION
[0002] In wireless communication systems, mobility between
different geographical areas is provided by allowing mobile
stations to handoff their communications between base stations. The
base station from which a mobile station is handing off from is
typically referred to as a serving base station, and the base
station to which the mobile station is to hand off to is typically
referred to as a target base station. Mobile stations can include
cellular telephones, personal digital assistants (PDAs), computers
equipped with wireless transceivers (such as integrated
transceivers or transceivers on PCMCIA cards), pagers, and the
like.
[0003] There are two general types of handoff, hard handoff and
soft handoff. Hard handoff is performed in a break-before-make
manner, in which a mobile station ceases all traffic and control
channel communications with the serving base station and then
attempts to establish traffic and control channel communications
with the target base station. This approach is used mostly to
preserve air-link and network resources utilized by mobile stations
in the handoff areas, and might be the preferred mode for
high-speed packet data channels. It also simplifies the handling of
data packets at the network infrastructure. However, hard handoff
does not provide full diversity gain associated with soft handoff,
thereby increasing the possibility of temporary session
interruption. Such a temporary interruption would impact Quality of
Service (QoS) for real-time applications, such as voice.
[0004] Using a soft handoff technique, a mobile station will
simultaneously communicate with the serving and target base
stations over both base stations traffic and control channels. This
type of handoff provides diversity, as the mobile station can
receive the same information from both base stations. Due to low
chance of session interruption, soft handoff is typically preferred
where real-time applications such as voice are involved. However,
soft handoff requires synchronous resource allocation and
scheduling, and coordinated packet handling on the network side.
This has a direct impact on throughput efficiency and network
complexity. Soft handoff also increases the required backhaul
capacity, especially for high rate traffic channels. Additionally,
it is only feasible when frequency reuse is 1:1 and strict traffic
synchronization is required among all bases stations involved in
the soft handoff. Moreover, soft handoff complicates packet
scheduling and automatic retransmission request (ARQ) messages.
[0005] While many wireless communication technologies provide
either soft or hard handoff, broadband communication technologies
including cdma2000, 1xEV-DO, WCDMA, Flarion's Flash OFDM.RTM. and
IEEE80216e provide both hard and soft handoff. Specifically, soft
handoff is typically employed for communications that are sensitive
to interruptions or errors, while hard handoff is employed for all
other communications.
SUMMARY OF THE INVENTION
[0006] Exemplary embodiments of the present invention provide a
hybrid handoff scheme in which control channel communications are
handed off from a serving base station to a target base station,
while the mobile station continues to perform data communications
with the serving base station. Once the handoff of the control
channel communications are completed, the data communications are
handed off from the serving base station to the target base
station.
[0007] In accordance with one embodiment of the present invention,
the serving base station informs the mobile station of time
periods, corresponding to frames, during which the mobile station
can perform the signaling with the target base station necessary
for handing off the control channel communications from the serving
base station to the target base station. The serving base station
then adjusts its scheduler in such a way that the serving base
station transmits unicast and multicast messages to the mobile
station during time periods other than those during which the
mobile station is performing signaling with the target base
station.
[0008] In accordance with another embodiment of the present
invention, a fast cell switch feedback channel is allocated to the
mobile station when there is a possibility or a need for a handoff.
The mobile station uses the fast cell switch feedback channel to
perform the signaling necessary for handing off the control channel
communications from the serving base station to the target base
station. The mobile station then can handoff the traffic channel
communications from the serving base station to the target base
station.
[0009] Other objects, advantages and novel features of the present
invention will become apparent from the following detailed
description of the invention when considered in conjunction with
the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] FIG. 1 is a signaling diagram illustrating an exemplary
method for handoff of a mobile station between a serving base
station and a target base station in accordance with one embodiment
of the present invention; and
[0011] FIG. 2 is a signaling diagram illustrating an exemplary
method for handoff of a mobile station between a serving base
station and a target base station in accordance with another
embodiment of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0012] FIG. 1 is a signaling diagram illustrating an exemplary
method for handoff of a mobile station between a serving base
station and a target base station in accordance with one embodiment
of the present invention. During the time period of Frame 1, the
mobile station receives a neighbor advertisement (NBR-ADV) message
broadcast by the serving base station (BS1) identifying base
stations for which the mobile station is to measure their signals.
The mobile station can also receive the downlink MAP (DL_MAP) and
uplink MAP (UP_MAP) during Frame 1. The mobile station then
measures the received carrier-to-interference plus noise ratio
(CINR) of all neighboring base stations advertised in the NBR-ADV
message and transmits a mobile station handoff request (MSSHO-REQ)
message during the time period of Frame 2. The MSSHO-REQ message
identifies the measured base stations and their corresponding
CINRs.
[0013] Base stations with CINR's above a predetermined threshold
are considered as "candidate" base stations for handoff. Based on
the received measurements, and other logical considerations, the
serving base station, or the handoff controller entity in the
network, selects one of the bases stations among the top candidate
neighbor base stations as the target base station for handoff. The
serving base station, which also serves as an "anchor" base station
during the cell switching process, notifies the target base station
(BS2) that the mobile station will attempt a handoff and transfer
the mobile stations context to the target base station using, for
example, HO Notification and Context Transfer messages.
[0014] The target base station reserves a ranging channel and basic
connection identifier (CID) for the mobile station, and forwards
this information to the serving base station. The serving base
station, during the time period of Frame 3, transmits a message to
the mobile station instructing it to attempt a handoff to the
target base station. The message includes the target base station's
identification, the reserved ranging channel and the basic CID. The
reserved ranging channel can be a physical channel or a logical
channel defined by a code. This message can be a HO Preparation
Order Message, or can be an extension to a base station handoff
response (BSHO-RSP) message. The message also identifies upcoming
time periods, or frames, in which the mobile station is allowed to
be unreachable, herein referred to as "T-Frames". The serving base
station's scheduler is adjusted to prevent scheduling any unicast
or multicast messages for transmission to the mobile station during
the T-Frames. In FIG. 1 time periods, or frames, in which the
mobile station is to communicate with the serving base station are
herein referred to as "S-Frames". Although FIG. 1 illustrates the
T-Frames occurring following every other S-Frame, other schedules
of the T-Frames are possible.
[0015] Upon the occurrence of the first T-Frame, i.e., the time
period of Frame 4, the mobile station tunes to the control channel
of the target base station to receive broadcast control messages,
such as the frame control header (FCH), DL_MAP and UL_MAP, and the
like. The mobile station then sends an initial ranging message on
the reserved ranging channel to the target base station. Although
not illustrated, during the T-Frames, the mobile station monitors
broadcast information from the target base station, such as the
DL_MAP, UL_MAP, downlink channel descriptor (DCD), uplink channel
descriptor (UCD), or any other signaling information necessary for
handoff. Additionally, the serving base station, as part of the
HO-Preparation message, can identify the current DCD and UCD of the
target base station, if they are different from that of the serving
base station. Otherwise, the mobile station will have to monitor
the target base station's downlink message until it receives the
DCD and UCD. Knowing the DCD and UCD is necessary, as the mobile
station cannot start the initial ranging until it knows the DCD and
UCD of the target base station.
[0016] The mobile station then tunes again to the serving base
station during the next S-Frame, i.e., during the time period of
Frame 5, in order to receive any data from the serving base
station. During the next T-Frame, i.e., the time period of Frame 6,
the mobile station again tunes to the control channel of the target
base station to receive the response to the ranging message
(RNG-RSP), as well as a CID allocation. Although not illustrated,
the mobile station and target base station can exchange other Layer
2 and Layer 3 messages before the mobile station's traffic channel
is switched to the target base station. These messages may be
related to security, system parameter updates, or other information
needed by the mobile station or the base station to carry the
session seamlessly.
[0017] At the next S-Frame, i.e., the time period of Frame 7, the
mobile station tunes to the serving base station and transmits a
message indicating that the mobile station has completed the
handoff of the control channel to the target base station
(HO-Completion Indication). The serving base station forwards an
indication of the completed handoff to the target base station, at
which point traffic information is switched from the serving to
target base station. The target base station, during the time
period of Frame 8, transmits a traffic channel allocation to the
mobile station and the target base station becomes the serving base
station for all traffic and control channel communications for the
mobile station.
[0018] When the method of FIG. 1 is performed between base stations
that operate over the same frequency or frequencies, some message
exchanges between the target base station and the mobile station
can occur during the S-Frames. If the mobile station reselects the
serving base station before handoff to the target base station is
completed, the serving base station can terminate the handoff
process by notifying the target base station of the early
termination. In this case, the serving base station will remain as
the serving base station, traffic channel frame exchanges will not
be affected, and the mobile station will ignore the T-Frame
allocation.
[0019] FIG. 2 is a signaling diagram illustrating an exemplary
method for handoff of a mobile station between a serving base
station and a target base station in accordance with another
embodiment of the present invention. In accordance with this
embodiment of the present invention, a fast cell switch feedback
channel is allocated to the mobile station when there is a
possibility or the need for handoff. This channel allows timely and
efficient notification of cell switching status, before, during and
upon completion of the process.
[0020] As illustrated in FIG. 2, the mobile station receives
NBR-ADV and DL_MAP and UL_MAP messages from the serving base
station. The mobile station then measures the received CINR of all
of neighboring base stations advertised in the broadcast NBR-ADV
message. The mobile station then transmits a MSSHO-REQ message
identifying the measured base stations and their corresponding
CINRS.
[0021] All reported neighbor base stations with CINR's above a
predetermined threshold are considered as "candidate" base stations
for handoff. Based on the received measurements, and other logical
considerations, the base station specifies a subset of the
candidate neighbors as recommended base stations for handoff,
namely the Active Set. The serving base station, which also serves
as an "anchor" base station during the cell switching process,
allocates a dedicated channel named "Fast Cell Switch" or FCS to
the mobile station. The Active Set and the FCS channel allocation
can be specified through the BSHO-RSP message. The serving base
station also allocates a short, e.g. 3 bit, temporary ID for each
base station in the Active Set. If the system defines an uplink
dedicated control channel for every active user, the FOS may also
be defined as a subchannel or field within uplink dedicated control
channel.
[0022] The FCS channel is used by the mobile station to frequently
indicate, e.g. every 5 msec or once a frame, one of the base
stations in the Active Set with highest CINR to be the next target
base station. The mobile station may consider other factors in
selecting the target base station from those in the Active Set.
[0023] Right after FCS allocation the mobile station may specify
the current anchor base station as the target base station for a
period of time until one of the Active Set base station's CINR
become stronger than the serving base station. The system also
defines a configurable timer for the FCS to allow the network to
perform necessary signaling with the target base station, and for
the mobile station to establish dual signaling with the serving
base station and target base station in a make-before-break manner.
The mobile station continues exchanging traffic frames with the
serving base station until that timer expires, or when it
successfully completes the ranging with the target base
station.
[0024] If the mobile station reselects the serving base station
again, before handoff to the target base station is competed, the
serving base station can stop the cell switching process by
notifying the target base station of the early termination of
handoff. Accordingly, the serving base station does not change and
traffic channel frame exchange will not be impacted. During the
handoff process the mobile station continues measuring the signals
from neighbor base stations and updates the recommended changes to,
i.e., additions to and deletions from, the current Active Set
through MSSHO-REQ message.
[0025] New neighbor base stations will be added to Active Set if
their CINR's are above a predetermined threshold, and current
Active Set members may be deleted if their CINR's are below another
predetermined threshold. The FCS will be automatically released by
the mobile station once the serving base station becomes the only
member of the Active Set.
[0026] When the mobile station notifies the serving base station of
its desire to switch to the target base station, the serving base
station transmits a message to the target base station informing
the target base station of the handoff in process and to transfer
the mobile station's context information. The serving base station
may also send a message (not illustrated) to the target base
station identifying the FCS channel location, so that the target
base station can optionally listen to this channel during the cell
switching process.
[0027] Following the notification, the target base station
allocates a basic CID and a reserved initial ranging CID for the
mobile station, and transmits this information to the serving base
station. The serving base station transmits this information to the
mobile station. The mobile station monitors the broadcast
information from target base station, such as DL/UL_MAP, downlink
channel descriptor (DCD), uplink channel descriptor (UCD), or any
other signaling information needed to switch the traffic channel to
target base station. To expedite the process of switching, the
mobile station can start monitoring some of broadcast messages from
target base station as early as the time it sends the FCS=BS2
message to the serving base station. However, some of information
which requires a specific CID needs to be read once the early
target base station allocation, e.g. basic CID, are forwarded to
mobile station from serving base station.
[0028] Moreover, the mobile station transmits a message to the
target base station on a reserved and contention-free ranging
channel allowing the target base station to perform initial
ranging. Once the mobile station receives the ranging response
along with a CID allocation, it can indicate its handoff completion
to the old serving base station by sending a predefined code word
on the FCS channel. The mobile station then receives a traffic
channel (TCH) allocation from the target base station and the
handoff process is completed.
[0029] As the base stations in the Active Set are given a temporary
ID, e.g. 3 bit TEMP_BS_ID, the mobile station can indicate its
selected base station on the FCS channel using this TEMP_BS_ID. One
of TEMP_BS_ID's, e.g. 000, or a separate additional bit can be
reserved to indicate the handoff completion.
[0030] During the context transfer and ranging process with target
base station, the traffic communication is still performed between
the mobile station and serving base station, as long as the FCS
timer is not expired. After the interaction of necessary signaling
information, and performing initial ranging, and the allocation of
necessary primary and traffic CIDs for mobile station in target
base station, the mobile station indicates the handoff completion
through the reserved ID on the FCS. The mobile station then
releases that channel. This is the moment when the traffic
communication is switched from the serving base station to target
base station.
[0031] Although the present invention has been described in
connection with a handoff between two different base stations, the
present invention can also be employed for handoffs between two
sectors of the same base station. It will be recognized that
intra-base station communication and channel establishments with
the target sector can be performed faster than the inter-base
station processes.
[0032] While the present invention is described in the context of
an OFDMA system, and in IEEE 802.16 specifically, the present
invention is equally applicable to other access technologies and
standards.
[0033] The methods described above in connection with FIGS. 1 and 2
can be used in, for example, high-speed packet data communications
in the control/signaling channels to preserve the link integrity
throughout the handoff process, while the hard handoff can be
performed for the traffic channels. However, if desired, for
real-time and low-rate applications, such as voice, soft handoff
can be performed by both traffic and signaling channels.
[0034] The methods described above in connection with FIGS. 1 and 2
can be used for both handoff between two base stations with the
same frequency, i.e., intra-frequency handoff, and between base
stations of different frequencies, i.e., inter-frequency handoff.
In both cases the described method allows continuous active
communications with the network throughout the handoff process,
even for real-time and constant bit rate services, possibly at
lower data rates, but with negligible handoff latency.
[0035] The foregoing disclosure has been set forth merely to
illustrate the invention and is not intended to be limiting. Since
modifications of the disclosed embodiments incorporating the spirit
and substance of the invention may occur to persons skilled in the
art, the invention should be construed to include everything within
the scope of the appended claims and equivalents thereof.
* * * * *