U.S. patent application number 14/710530 was filed with the patent office on 2015-08-27 for handover robustness in cellular radio communications.
The applicant listed for this patent is Telefonaktiebolaget L M Ericsson (publ). Invention is credited to Angelo Centonza, Konstantinos Dimou, Walter Muller, Oumer Teyeb.
Application Number | 20150245261 14/710530 |
Document ID | / |
Family ID | 45063199 |
Filed Date | 2015-08-27 |
United States Patent
Application |
20150245261 |
Kind Code |
A1 |
Teyeb; Oumer ; et
al. |
August 27, 2015 |
HANDOVER ROBUSTNESS IN CELLULAR RADIO COMMUNICATIONS
Abstract
An indication of the speed of movement of a UE in a radio
communications system is received at a network node of the radio
communications system. The parameters of the radio communications
system are then analyzed using the speed of movement indication.
The system parameters are then adjusted using the analysis.
Inventors: |
Teyeb; Oumer; (Stockholm,
SE) ; Centonza; Angelo; (Winchester, GB) ;
Dimou; Konstantinos; (Stockholm, SE) ; Muller;
Walter; (Upplands Vasby, SE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Telefonaktiebolaget L M Ericsson (publ) |
Stockholm |
|
SE |
|
|
Family ID: |
45063199 |
Appl. No.: |
14/710530 |
Filed: |
May 12, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13376379 |
Jan 29, 2013 |
9031564 |
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PCT/SE2011/051271 |
Oct 26, 2011 |
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14710530 |
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61515225 |
Aug 4, 2011 |
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Current U.S.
Class: |
455/437 ;
455/561 |
Current CPC
Class: |
Y02D 30/70 20200801;
H04W 88/08 20130101; H04W 76/28 20180201; H04W 36/22 20130101; H04W
52/0212 20130101; H04W 36/08 20130101; H04W 36/0083 20130101; H04W
36/0085 20180801; H04W 36/32 20130101 |
International
Class: |
H04W 36/00 20060101
H04W036/00; H04W 52/02 20060101 H04W052/02; H04W 36/32 20060101
H04W036/32 |
Claims
1. A method implemented in a network node of a cellular radio
communications system to improve system functionality using speed
information regarding a user equipment (UE), the method comprising:
receiving an indication of a speed of movement of the UE; analyzing
system parameters using the speed of movement indication; and
adjusting the system parameters using the analysis.
2. The method of claim 1: wherein receiving an indication comprises
receiving a report, for example an HO Report or an RLF INDICATION
message, from another network node indicating a signal strength
measured by the UE during an attempted handover from the network
node to the other cell, and a time elapsed between a connection
request by the UE and a failure of the connection together with the
indication of a speed of movement of the UE; wherein the system
parameters comprise mobility thresholds for the UE; wherein
analyzing comprises analyzing the handover report to determine
whether the handover failure is due to UE speed of movement; and
wherein adjusting comprises adjusting mobility thresholds for the
UE with respect to neighbor cells.
3. The method of claim 2, wherein receiving a report comprises
receiving a handover failure report, for example an HO Report
including a UE RLF Report Container IE, the report including an
indication of the size of the other cell, and wherein analyzing
includes analyzing using the size of the other cell.
4. The method of claim 2, further comprising sending a mapping
table to the UE indicating priorities for cells of the cellular
radio communications system, the mapping table optionally including
scaling factors for analyzing received signal strength for handover
to each network node of the mapping table or times to trigger for
determining when to request handover to each network node of the
mapping table.
5. The method of claim 1, further comprising prioritizing target
cells for the UE depending on the indication of the speed of
movement of the UE for example by sending a per UE per target CIO
to fast moving UEs.
6. The method of claim 1, wherein the system parameters include a
list of candidate UEs to move from the network node to another
network node as a load balancing move and wherein receiving an
indication of speed comprises receiving speed information regarding
speed of movement of a move candidate EU, for example in UE history
information or an RLF Report from another node, the method further
comprising: selecting move candidate UEs; determining whether a
selected move candidate UE is a high mobility UE using the received
speed information; and excluding the selected move candidate UE
from a load balancing move if the selected move candidate UE is
determined to be a high mobility UE.
7. The method of claim 6, wherein determining whether the selected
move candidate EU is a high mobility EU comprises comparing the
received speed information to a threshold speed.
8. The method of claim 6, wherein receiving speed information
comprises receiving speed information as an information element of
location information regarding the selected move candidate or as an
information element of a failed handover report, for example a UE
RLF Report Container IE in an HO Report, and wherein receiving
speed information optionally comprises receiving speed information
from the move candidate UE.
9. The method of claim 1, wherein the system parameters comprise a
list of UEs that have been commanded to operate in an idle mode,
the method comprising: determining that a UE has a high speed of
movement and a low traffic data rate; commanding the UE to move to
an idle mode after a UE data transmission; and receiving a
connection request from the UE when the UE has additional data to
transmit, the UE moving from the idle mode to send the connection
request; and sending information to another network node of the
radio communications system that the UE has been commanded to move
to an idle mode upon determining that the UE has moved near the
other cell.
10. The method of claim 8, wherein the UE returns to the idle mode
after transmitting the additional data.
11. The method of claim 9, further comprising waking the UE from
the idle mode to receive transmitted data.
12. The method of claim 9, wherein the connection request is a
request to connect to another cell and the connection request, for
example an RRC Connection Setup Request or RRC Connection Setup
Complete message, and wherein the request includes sending the
information within the request as an indication that the UE has
been commanded to move to an idle mode by the first cell.
13. The method of claim 9, wherein sending information comprises
the UE sending information to the other network node as part of the
connection request.
14. The method of claim 9, wherein sending information comprises
sending information as an information element of UE history
information.
15. A network node of a cellular radio communications system
configured to improve system functionality using speed information
regarding a user equipment (UE), the network node comprising: a
receiver configured to receive an indication of a speed of movement
of the UE; a controller configured to analyze system parameters
using the speed of movement indication and to adjust system
parameters; and a transmitter configured to transmit system
parameter adjustments to other nodes.
16. The network node of claim 15, wherein: receiving an indication
further comprises receiving a handover report from another network
node indicating a signal strength measured by the UE during an
attempted handover and a time elapsed between a connection request
by the UE and a failure of the connection; analyzing system
parameters further comprises analyzing the handover report to
determine whether the handover failure is due to UE speed of
movement; and adjusting system parameters further comprises
adjusting mobility thresholds for the UE with respect to neighbor
network nodes.
17. The network node of claim 15, wherein: the controller is
further configured to determine that a UE has a high speed of
movement and a low traffic data rate; the transmitter is further
configured to transmit system parameter adjustments by commanding
the UE to move to an idle mode after a UE data transmission; the
receiver is further configured to receive a connection request from
the UE when the UE has additional data to transmit; and the network
node further comprising a system interface configured to send
information to another cell of the radio communications system that
the UE has been commanded to move to an idle mode upon the
controller determining that the UE has moved near the other
cell.
18. The network node of claim 15, wherein: the controller is
further configured to determine that a UE has a high speed of
movement and a low traffic data rate; the transmitter is further
configured to transmit system parameter adjustments by commanding
the UE to move to an idle mode after a UE data transmission; the
receiver is further configured to receive a connection request from
the UE when the UE has additional data to transmit; and the network
node further comprising a system interface configured to send
information to another cell of the radio communications system that
the UE has been commanded to move to an idle mode upon the
controller determining that the UE has moved near the other
cell.
19. The network node of claim 15, wherein the transmitter transmits
system parameter adjustment as in information element of UE history
information.
20. A network node of a cellular radio communications system
configured to improve system functionality using speed information
regarding a user equipment (UE), the network node comprising: means
for receiving an indication of a speed of movement of the UE; means
for analyzing system parameters using the speed of movement
indication and to adjust system parameters; and means for
transmitting system parameter adjustments to other nodes.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 13/376,379, filed Jan. 29, 2013, which is a
National stage of International Application No. PCT/SE2011/051271,
filed Oct. 26, 2011, which claims priority to U.S. Provisional
Patent Application Ser. No. 61/515,225, filed Aug. 4, 2011, which
are incorporated by reference herein, the priorities of which are
hereby claimed.
TECHNICAL FIELD
[0002] The present disclosure relates to the field of adjusting
parameters in cellular radio communications and, in particular, to
adjusting operational system parameters using mobility information
regarding mobile devices.
BACKGROUND
[0003] Handover is one of the important aspects of any mobile
communication system, in which the system tries to assure service
continuity for the User Equipment (UE) by transferring the
connection of the UE from one cell to another referred to as a
Handover (HO). HO decisions usually depend on several factors such
as signal strength, load conditions, service requirements, etc. The
provision of efficient and effective handovers (minimum number of
unnecessary handovers, minimum number of handover failures, minimum
handover delay, etc.), may affect not only the Quality of Service
(QoS) of the end user but also the overall mobile network capacity
and performance.
[0004] In Long Term Evolution (LTE), UE-assisted, network
controlled handover is used (3GPP TS 36.300, Third Generation
Partnership Project Technical Specification No. 36.300). The
network configures the UE to send measurement reports and, based on
these reports, the UE can be handed over, to the most appropriate
cell. A UE measurement report configuration consists of the
reporting criteria (whether it is periodic or event triggered) as
well as the measurement information that the UE is required to
report.
[0005] In Evolved Universal Terrestrial Radio Access Networks
(E-UTRAN), the decision to handover from the current serving
evolved Node B (eNB) to a target eNB is made within the serving eNB
and is made on the basis of measurements on the downlink (DL).
These measurements are performed by the UE that measures the DL
signals it receives from the different eNBs that it can
receive.
[0006] The following event-triggered criteria are specified for
intra-RAT (Radio Access Technologies) measurement reporting in LTE
(3GPP TS 36.331):
[0007] Event A1: Serving cell becomes better than absolute
threshold;
[0008] Event A2: Serving cell becomes worse than absolute
threshold;
[0009] Event A3: Neighbor cell becomes better than an offset
relative to the serving cell;
[0010] Event A4: Neighbor cell becomes better than absolute
threshold;
[0011] Event A5: Serving cell becomes worse than one absolute
threshold and neighbor cell becomes better than another absolute
threshold.
[0012] The most important measurement report triggering event
related to handover is A3, and its usage is illustrated in FIG. 1.
The triggering conditions for event A3 can be formulated as the
Equation below:
N>S+(H.sub.S+CIO.sub.S,N) (Equation)
where N and S are the signal strengths of the neighbor and serving
cells, respectively, H.sub.S is the hysteresis parameter that the
serving cell applies for event A3, and CIO.sub.S,N is the Cell
Individual Offset (CIO) set by the serving cell for that specific
neighbor cell. If this condition is satisfied and it remains valid
for a certain duration known as Time To Trigger (TTT), then the UE
sends a measurement report to the serving eNB (e-UTRAN, evolved
UMTS Radio Access, Node B). FIG. 1 shows signal strength on the
vertical axis and time on the horizontal axis. A first solid line
curve, shown high at time 0 is the signal strength of the signal
received from the serving cell S. This is shown as declining in
strength over time as the UE moves away from S and toward the
neighboring cell N. The signal strength at N is low at time 0 and
increases as the UE moves toward N.
[0013] The value for S+(H.sub.S-CIO.sub.S,N) is indicated by the
dotted line in FIG. 1. Event A3 is satisfied at point A and a
measurement report is sent at point B in time. Point A and Point B
are separated by the TTT. When it gets the measurement report, the
serving eNB makes a decision whether to initiate a handover toward
the neighbor.
[0014] Handover in LTE is performed via an X2 connection, whenever
available, and if not, using S1 (i.e. involving the Core Network
(CN)). The X2 Handover process is shown in FIG. 2. The handover
procedure can be sub-divided into three stages: preparation
(initiation); execution; and completion. During the preparation
stage (steps 1 to 3 of FIG. 2), based on the measurement results
(step 2), the source eNB is getting from the UE, the source eNB
decides whether to handover the connection to another eNB or not
(step 3). Then a handover execution stage (steps 4 to 7 of FIG. 2)
is entered and the decision to handover is communicated to the
target eNB (step 4), and if the target eNB is able to admit the UE
(step 5), a message is sent to the UE (steps 6 and 7) to initiate
the handover. DL (Downlink) data arriving at the source eNB for the
UE are then forwarded to the new target eNB (step 8).
[0015] The handover completion stage is entered once the target eNB
and the UE are synchronized (steps 9 and 10) and a handover confirm
message (step 11 of FIG. 2) is received by the target eNB. After a
proper setup of the connection with the target eNB is performed
(steps 12 and 13) (which include the switching of the DL path in
the serving gateway, step 14, 15, 16), the old connection is
released (step 17) and any remaining data in the source eNB that is
destined for the UE is forwarded to the target eNB. Then normal
packet flow can ensue through the target eNB.
[0016] As explained above, handover in LTE is controlled via
several parameters. Incorrect parameter settings can lead to
several problems such as (see e.g. 3GPP TS 36.902, 3GPP TS 36.300)
radio link failure, ping pong handover, handover failure, etc.
[0017] A Radio Link Failure (RLF), is a failure that occurs when a
radio link connection is lost for a predetermined time duration.
For RLF, if the parameters are set in such a way that the UE does
not report handover measurements on time, the UE might lose the
connection with the original cell before handover is initiated.
This is one example, of what is known as Too Late HO in which the
UE tries to re-establish the connection with another cell after the
RLF detection timers have expired. On the other hand, if the
parameters are set to trigger handover very early, RLF might occur
shortly after handover in the target cell. This is known as Too
Early HO in which the UE tries to re-establish the connection with
the source cell after the RLF detection timers have expired. Even
if the handover is triggered at the right time, incorrect settings
of the CIO can make the UE handover to the wrong cell, which is
followed by a RLF and a re-establishment request in a cell other
than the target cell or the source cell. This is known as HO to a
wrong cell
[0018] In a ping pong handover, improper handover parameter
settings can make the UE handover back and forth between two
neighboring cells. An example of this is a setting that makes the
triggering conditions for the handover events (A3) valid between
the source and neighbor cells at the same time.
[0019] When the UE receives a certain number of (N310) consecutive
"out of sync" indications from the physical layer, it assumes a
physical layer problem is ensuing, and a timer (T310) is started.
If the UE does not receive a certain number of (N311) consecutive
"in sync" indications from the lower layer before T310 expires, RLF
is detected. RLF is also detected when a random access problem is
indicated from the MAC (Media Access Control) layer or upon
indication that the maximum number of RLC (Radio Link Control)
layer retransmissions has been reached.
[0020] Another type of failure is a HO failure in which the radio
link between the UE and network was functioning correctly, but
handover signaling messages failed to be exchanged. This might be
due to congestion or because a maximum number of RLC (Radio Link
Control) retransmissions was reached. When the UE receives an HO
command (i.e. RRC Connection Reconfiguration Request with mobility
Control Info, as shown in FIG. 2), it starts a timer (T304), and if
this timer expires before the HO is completed (i.e. RRC Connection
Reconfiguration Complete message is sent by the UE), an HO failure
is detected.
[0021] When a RLF is detected by the UE, the UE starts a timer
(T311) and tries to re-establish the connection to the best
available cell (e.g. the source cell, another cell belonging to the
same source eNB or a neighbor cell belonging to another eNB). When
sending the re-establishment request (RRC Connection
Reestablishment Request), the UE includes the following information
(3GPP TS 36.331):
[0022] Global Cell ID (GCID) of the last cell the UE was connected
to before RLF;
[0023] UE Identity: the Cell Radio Network Temporary Identifier
(C-RNTI) as well as MAC ID for context lookup;
[0024] Re-establishment cause: whether the request is due to
handover failure, reconfiguration failure, or other causes.
[0025] If the UE context is found in the cell (if it is the source
cell or if it was a cell prepared for handover, (i.e. handover was
ongoing when the RLF happened and the cell where the UE re-appeared
already has the UE context, which was communicated to it from the
source cell during Handover Request message exchange), the
connection is re-established. Otherwise (if UE context is not
available, or re-establishment did not succeed before T311
expires), then the UE has to go to idle mode and has to tear down
all the active bearers, if any, and might restart the bearer
setups, if required.
[0026] The eNB to which the UE is reconnecting, either through a
successful RRC Re-establishment or via RRC Connection Setup after
idle mode, can ask for more detailed information about the failure
after the connection is completed via the UE Information Request
procedure, where the eNB can ask for the RLF report. The UE
responds by sending a UE Information Response message with a
detailed RLF report which can include information such as (3GPP TS
36.331):
[0027] Measurement result of the last served cell before RLF;
[0028] Measurement result of the neighbor cells performed before
RLF;
[0029] Location info, which can include last co-ordinates as well
as velocity of the UE when RLF was detected;
[0030] CGI (and if that is not available Physical Cell ID (PCI)) of
the cell where RLF occurred;
[0031] and, if the RLF occurred after the reception of a HO command
(i.e. RRC Connection Reconfiguration message including the mobility
Control Info), then also;
[0032] The CGI where this message was received;
[0033] The elapsed time since the reception of this message;
and
[0034] The RLF type: i.e. whether it is a normal radio link failure
or a handover failure.
[0035] Configuring all the HO parameters manually is expensive and
can be very challenging. As such, Mobility Robustness Optimization
(MRO) has been introduced in 3GPP to automate the dynamic
configuration of handover parameters. Mobility Robustness
Optimization (MRO) tries to gather statistics on the occurrence of
Too Late HOs, Too Early HOs and HO to the wrong cell, and these
statistics are used to adjust the handover parameters such as
Hysteresis, CIO and TTT.
[0036] For MRO, the different HO problems discussed above are
communicated between neighboring cells in different ways (3GPP TS
36.300, 3GPP TS 36.423, and 3GPP TS 36.331).
[0037] For Too Late Handovers, an RLF INDICATION message is sent
via X2 from the eNB to which the UE tries to re-establish a
connection to the eNB where the RLF occurred. The RLF INDICATION
message contains:
[0038] Physical Cell Identifier (PCI) of the cell in which the UE
was connected prior to RLF (known as failure cell);
[0039] E-UTRAN Cell Global Identifier (ECGI) of the cell where RRC
re-establishment attempt was made;
[0040] UE Identity: C-RNTI and MAC ID of the UE in the failure
cell; and
[0041] RLF report (in a UE RLF Report Container Information Element
(IE)).
[0042] If an eNB receives an RLF INDICATION message from a neighbor
eNB, and if it finds out that it has sent a UE CONTEXT RELEASE
message towards that neighbor eNB within the last Tstore_UE_cntxt
seconds (i.e. it means that very recently the concerned UE was
handed over properly to it from the same eNB), the eNB responds by
sending a HANDOVER REPORT message that indicates Too Early
Handover.
[0043] If an eNB receives an RLF INDICATION message from a neighbor
eNB, and if it finds out that it has sent a UE CONTEXT RELEASE
message towards another neighbor eNB within the last
Tstore_UE_cntxt seconds (i.e. it means that very recently the
concerned UE was handed over properly to it from another eNB), the
eNB responds by sending a HANDOVER REPORT message that indicates
Handover to the Wrong Cell.
[0044] The HANDOVER REPORT message contains:
[0045] Type of detected handover problem (Too Early Handover,
Handover to Wrong Cell);
[0046] ECGI of source and target cells in the handover;
[0047] ECGI of the re-establishment cell (in the case of Handover
to Wrong Cell); and
[0048] Handover cause (signaled by the source during handover
preparation).
[0049] Thus, by analyzing the received RLF INDICATION and HANDOVER
REPORT messages within a certain duration, eNBs can configure the
optimal HO parameters to be used with their neighbors.
[0050] As mentioned above, current mechanisms such as MRO try to
optimize mobility by fine tuning mobility thresholds such as CIOs
with the objective of preventing further failures to occur.
[0051] Failures can also occur when a UE is moving at a high speed.
LTE provides for speed dependent scaling of measurement-related
parameters. UE speed information may be used to adjust cell
reselection (cell reselection thresholds) and handover parameters
(TTT). The UE can currently estimate its speed (high, medium,
normal, known as the mobility state of the UE) based on a Mobility
State Parameters configuration received from the eNB. The number of
handovers in a given time is used to determine the mobility state
as high, if there are many handovers, medium, if there are a medium
level of handovers, and low, if there are few handovers in the
given time. Thus, in the case of handover, the UE calculates its
mobility state and the TTT may be adjusted accordingly by
multiplying the TTT with a scaling factor associated with each
mobility state.
SUMMARY
[0052] It is an object to improve the operation of a cellular radio
communications system using mobility information. This information
may be useful in analyzing handover failures, in balancing loads
between different node of the system and in controlling operational
modes of UE in the system. The mobility information may be received
from the network or from UEs. It may be generated by observing UE
operation or by UEs directly.
[0053] In one example an indication of the speed of movement of a
UE in a radio communications system is received at a network node
of the radio communications system. The parameters of the radio
communications system are then analyzed using the speed of movement
indication. The system parameters are then adjusted using the
analysis.
[0054] In another example, a network node of a cellular radio
communications system is configured to improve system functionality
using speed information regarding a user equipment. The network
node includes a receiver configured to receive an indication of a
speed of movement of the UE, a controller configured to analyze
system parameters using the speed of movement indication and to
adjust system parameters, and a transmitter configured to transmit
system parameter adjustments to other nodes.
[0055] In another example, the network node includes means for
receiving an indication of a speed of movement of the UE, means for
analyzing system parameters using the speed of movement indication
and to adjust system parameters, and means for transmitting system
parameter adjustments to other nodes
[0056] Also disclosed herein is a method performed in a network
node of a cellular radio communications system to hand a user
equipment (UE) connected to the network node over to be connected
to another network node of the cellular radio communications
system. The method includes receiving a report from the other
network node indicating a signal strength of the other network node
measured by the UE during an attempted handover from the network
node to the other network node, an indication of the speed of
movement of the UE, and a time elapsed between a connection request
by the UE and a failure of the connection, analyzing the handover
report to determine whether the handover failure is due to UE
speed, and adjusting mobility thresholds for the UE toward neighbor
network nodes.
[0057] The disclosed method may also include receiving a handover
failure report, that includes an indication of the size of the
other network node, and wherein analyzing includes analyzing using
the size of the other network node. The method may include sending
a mapping table to the UE indicating priorities for network nodes
of the cellular radio communications system. The mapping table may
include scaling factors for analyzing received signal strength for
handover to each network node of the mapping table. The mapping
table may include times to trigger between a time of a reduction in
signal strength of one network node and a time of an increase in
signal strength of another network node for determining when to
handover to each network node of the mapping table. The disclosed
method may also include prioritizing target network nodes for the
UE depending on UE mobility.
[0058] Also disclosed herein is a network node of a cellular radio
communications system that hands a user equipment (UE) connected to
the network node over to be connected to another network node of
the cellular radio communications system. The network node includes
a receiver to receive a report from the other network node
indicating a signal strength of the other network node measured by
the UE during an attempted handover from the network node to the
other network node, an indication of the speed of movement of the
UE, and a time elapsed between a connection request by the UE and a
failure of the connection, and a controller to analyze the handover
report to determine whether the handover failure is due to UE
speed, and to adjust mobility thresholds for the UE towards
neighbor network nodes.
[0059] Also disclosed herein is a network node that includes means
for receiving a report from the other network node indicating a
signal strength of the other network node measured by the UE during
an attempted handover from the network node to the other network
node, an indication of the speed of movement of the UE, and a time
elapsed between a connection request by the UE and a failure of the
connection, means for analyzing the handover report to determine
whether the handover failure is due to UE speed, and means for
adjusting mobility thresholds for the UE towards neighbor network
nodes.
[0060] Also disclosed herein is a method performed in a network
node of a cellular radio communications system to balance
distribution of user equipments (UEs) over multiple network nodes
of the cellular radio communications system. The method includes
selecting candidate UEs to move from the network node to another
network node as a load balancing move, receiving speed of movement
information for a candidate EU, determining whether a selected
candidate UE has a high speed, and excluding the selected candidate
UE from a load balancing move if the selected candidate UE is
determined to have a high speed.
[0061] The disclosed method may also include comparing the received
speed of movement information to a threshold speed. Receiving speed
of movement information may include receiving speed information as
an information element of location information regarding the
selected move candidate. Receiving speed information may include
receiving speed information as an information element of a failed
handover report.
[0062] Also disclosed herein is a method performed in a network
node of a cellular radio communications system to increase use of
an idle mode by a UE in the cellular radio communications system.
The method includes determining that a UE has high speed and low
traffic, commanding the UE to move to an idle mode after
transmission, receiving a connection request from the UE when the
UE has additional data to transmit, and sending information to
another network node of the radio communications system that the UE
has been commanded to move to an idle mode upon determining that
the UE has moved near the other network node.
[0063] The disclosed method may also include the UE returning to
the idle mode after transmitting the additional data, and waking
from the idle mode to receive transmitted data. The connection
request may be a request to connect to the other network node.
Information may be sent to the other network node as part of the
connection request, or as an information element of UE history
information. In addition, the idle mode may be an idle mode forced
by a network node.
[0064] The techniques described above allow UEs to be handled
differently in different situations. In particular, high mobility
UEs, as determined by UE velocity relative to the size of the cells
are treated differently to provide less overhead and more robust
handover. Current approaches provide one configuration for all
types of UEs (high speed and low speed) and for all types of target
cells (very small, small, medium and large).
[0065] By differentiating on the basis of UE speed and target cell
size, UE performance is improved by avoiding mobility failures that
result in Key Performance Indicator (KPI) and Quality of Service
(QoS) degradations. Load distribution in the radio network is made
more dynamic and robust by selecting a more reliable cell for
traffic offloading. UE power consumption may also be reduced by
minimizing the number of cells for which the UE needs to collect
and report measurements and by reducing the amount of mobility
signaling generated by the UE. Network performance is also improved
by reducing handover failures and radio link failures. Finally,
network scalability is improved by reducing the amount of handover
signaling towards the core network.
[0066] The operations of the flow and signaling diagrams are
described with reference to exemplary embodiments. However, it
should be understood that the operations of the flow diagrams can
be performed by variations other than those discussed with
reference to these other diagrams, and the variations discussed
with reference to these other diagrams can perform operations
different than those discussed with reference to the flow
diagrams.
BRIEF DESCRIPTION OF THE DRAWINGS
[0067] The present disclosure may best be understood by referring
to the following description and accompanying drawings. In the
drawings:
[0068] FIG. 1 is a diagram of a graph illustrating the
determination of a time for handover according to standards for
LTE.
[0069] FIG. 2 is a signaling diagram illustrating signaling and
equipment operations performed for a handover according to
standards for LTE.
[0070] FIG. 3 is a simplified diagram of three heterogeneous radio
cells traversed by a UE connected according to standards for
UTRAN.
[0071] FIG. 4 is a simplified diagram of two heterogeneous radio
cells traversed by a UE.
[0072] FIG. 5 is a signaling diagram of a failed handover with
additional information exchange.
[0073] FIG. 6 is a signaling diagram of a failed handover with
additional information exchange.
[0074] FIG. 7 is a simplified hardware block diagram of a radio
terminal.
[0075] FIG. 8 is process flow diagram of improved handover
robustness.
[0076] FIG. 9 is a process flow diagram of improved mobility load
balancing.
[0077] FIG. 10 is a process flow diagram of reducing signaling
overhead using forced idle mode.
DETAILED DESCRIPTION
[0078] In the following description, numerous specific details such
as logic implementations, opcodes, means to specify operands,
resource partitioning/sharing/duplication implementations, types
and interrelationships of system components, and logic
partitioning/integration choices are set forth. It will be
appreciated, however, by one skilled in the art that the different
implementations may be practiced without such specific details. In
other instances, control structures, gate level circuits and full
software instruction sequences have not been shown in detail in
order not to obscure the description.
[0079] References in the specification to "one embodiment," "an
embodiment," "an example embodiment," etc., indicate that the
embodiment described may include a particular feature, structure,
or characteristic, but every embodiment may not necessarily include
the particular feature, structure, or characteristic. Moreover,
such phrases are not necessarily referring to the same embodiment.
Further, when a particular feature, structure, or characteristic is
described in connection with an embodiment, it is submitted that it
is within the knowledge of one skilled in the art to effect such
feature, structure, or characteristic in connection with other
embodiments whether or not explicitly described.
[0080] In the following description and claims, the terms "coupled"
and "connected," along with their derivatives, may be used. It
should be understood that these terms are not intended as synonyms
for each other. "Coupled" is used to indicate that two or more
elements, which may or may not be in direct physical or electrical
contact with each other, co-operate or interact with each other.
"Connected" is used to indicate the establishment of communication
between two or more elements that are coupled with each other.
[0081] A user may communicate using user equipment (UE) via a
communications system and send and receive data to other UEs in the
system or outside the system. Access to the communications system
may be provided by a fixed line or wireless communication
interface, or a combination of these. Examples of wireless access
systems providing mobility for UEs include cellular access
networks, various wireless local area networks (WLANs), wireless
personal area networks (WPANs), satellite based communication
systems and various combinations of these. A communication system
typically operates in accordance with a standard and/or a set of
specifications and protocols which set out what the various
elements of the system are permitted to do and how that should be
achieved. For example, it is typically defined if the user, or more
precisely user device, is provided with a circuit switched or a
packet switched communications, or both. Also, the manner in which
communication should be implemented between the user device and the
various elements of the communication and their functions and
responsibilities are typically defined by a predefined
communication protocol. Various functions and features are
typically arranged in a hierarchical or layered structure, so
called protocol stack, wherein the higher level layers may
influence the operation of the lower level functions. In cellular
systems a network entity in the form of a base station provides a
node for communication with mobile devices in one or more cells or
sectors. In certain systems a base station is called `Node B`.
Typically the operation of a base station apparatus and other
apparatus of an access system required for the communication is
controlled by a particular control entity, such as a base station
controller, mobile switching center, or packet data support
node.
[0082] The present disclosure is described in the context of the
third generation (3G) mobile communications systems of the
universal mobile telecommunications system (UMTS) and, in
particular, long term evolution (LTE). A particular example of LTE
is the Evolved Universal Terrestrial Radio Access (E-UTRA). An
Evolved Universal Terrestrial Radio Access Network (E-UTRAN)
includes E-UTRAN Node Bs (eNBs) which are configured to provide
base station and control functionalities. However, the invention is
not so limited.
[0083] In the following description and claims, the terms "UE" and
"User Equipment" are used to refer to remote terminals, mobile
devices or radios, subscriber equipment and any other type of
mobile device that may connect to more than cell and experience a
handover. The term "handover" also includes "handoff." The term
"eNB" or "cell" is used generally to refer to a base station, an
access point, a fixed terminal and similar devices and to the area
of radio coverage of a base station, a cell, or a sector. The
description is presented in the context of LTE for illustration
purposes, but the invention is not so limited.
[0084] In LTE, mobility is used to refer to handover and handover
protocols in general, as in mobility robustness optimization (MRO).
It is also used to refer to an information element that indicates
the speed with which a UE moves through cells. The information
element may have one of three values, high, medium, and low. This
mobility is related to the speed with which the UE moves across the
ground and to an LTE parameter referred to as horizontal velocity.
In the following description and claims, mobility is used to refer
to both handover in a general sense and the rate at which a UE
travels. The rate at which a UE sends and receives data will be
referred to in other terms, such as data rate, high or low traffic,
etc.
[0085] Though the standardized MRO mechanisms described above may
be very useful, they have limitations, specifically in
Heterogeneous Network (HetNet) scenarios. HetNet is an environment
in which the cells have different sizes, i.e. the land area covered
by different eNBs is different. The cells involved in mobility are
a mixture of large coverage layer cells and small hot spot coverage
cells aimed at increasing capacity in a very localized way.
[0086] The success of UE mobility in a HetNet scenario strongly
depends on the time the UE is expected to spend on a given cell
(i.e. how big the cell is and how fast the UE is moving). If this
is not considered, UEs may experience unnecessary handovers (e.g. a
fast moving UE may be handed over to a small sized neighbor cell
and then very quickly handed over to another neighbor) or even
failures (e.g. after the handover of a high speed UE to a small
sized neighbor is initiated, the UE may move so far away from the
small cell that the target cell signal may become very weak by the
time handover is completed and failure may occur).
[0087] While the current use of UE mobility state parameters help
to improve HO robustness, the drawbacks of this way of handover
parameter adjustment, especially in a HetNet scenario include:
[0088] The speed calculation based on a number of handovers is not
accurate even in a homogenous network, and it becomes even worse in
a HetNet;
[0089] All neighbors are treated equally. For example, the UE
adjusts the TTT to the same value whether the concerned target is a
big macro cell or a very small pico cell; and
[0090] The TTT is optionally adjusted in a speed dependent manner
but no other parameters are adjusted at all, and the same CIO is
used regardless of the UE's speed, which may greatly undermine the
benefit of adjusting the TTT.
[0091] Overview
[0092] In one context described herein, UEs moving at high speeds
are not handed over to small area cells. This is because the UE
will remain physically in the cell for a very short time. It may
also be because of a high probability of mobility failures due to
the small area of the target cell. The cell's signal may become
very weak by the time the handover is completed. Therefore, for UEs
moving at high speed, the handover target cell may be selected not
only on the basis of the strongest received signal but also on the
basis of the risk of handover failure or on the basis of the target
cell being small compared to the speed of the UE. For these UEs,
the best target may be a wide coverage cell.
[0093] However, if source cell mobility parameters such as TTT are
adjusted to allow fast moving UE's to jump small target cells and
to select larger coverage cells, then this same adjustment in a
slow moving UE may degrade the mobility performance. Slow moving
UEs, by contrast, may be better handed over to small coverage cells
because they might be able to camp on such cells for a long time
while moving at slow speed. Current mobility solutions do not allow
handover target selection to differentiate on the basis of the UE
speed and target cell size.
[0094] In a first area of the disclosure, UEs in high speed
mobility are treated differently as a consequence of mobility
failures. In another area of the disclosure, high speed UEs are
treated differently for Mobility Load Balancing (MLB) between
neighbor cells. MLB allows a cell in overload to offload some of
its UEs by triggering mobility to a less loaded cell. However, if a
UE is moving at relatively high speed, it may not be advisable to
trigger mobility for the purpose of off-loading the serving cell.
This is because this UE will most likely exit the cell in a
relatively short time (therefore naturally offloading the source
cell). Moreover, a UE moving at high speed is already subject to
the risk of mobility failures. If the normal course of mobility
procedures for such a UE is also disturbed by mobility due to load
balancing reasons, the risk of mobility failure may become even
higher. Hence, handover may be improved by differentiating between
high speed UEs and low speed UEs for the purpose of mobility load
balancing. Current solutions do not take into account UE mobility
when offloading UEs to neighbor cells.
[0095] In yet another area of the disclosure, when analyzing UEs
moving at high speed it is plausible to think that these UEs will
handover frequently from cell to cell. If such UEs were subject to
low traffic exchange, it may be beneficial to move the UEs to idle
mode for as long as possible. This is because a UE in idle mode
moving from cell to cell may not trigger handover signaling and may
not be subject to handover failures. Recent advances in LTE
technology allow UEs to be kept in a so called "always on" state.
In this state, the UE is kept in RRC Connected even if there is no
scheduled traffic. The "always on" state allows signaling to be
saved when the UE moves from RRC idle to RRC Connected state and
because the UE is already connected, it reduces end to end delays
during traffic transmission.
[0096] However, for UEs with little or no traffic moving at high
speed, the "always on" state has the drawback of generating high
numbers of HOs, with the risk of handover failures and with issues
deriving from high HO signaling levels. Therefore, in systems
configured in a way to keep all UEs in the "always on" state, there
might be high signaling loads and mobility performance degradation
for all nearby UEs caused by a high speed UE. This issue may be
solved by allowing UEs moving at high speed to be differentiated
and put into idle mode as much as possible. The idle mode reduces
handover failures, handover signaling, and UE measurement
reporting.
[0097] The idle mode is made more effective if, in a HO, the target
cell is informed that a UE has been deliberately kept in idle mode
due to its high speed mobility. This is because the target may
immediately adopt this configuration for the UE moving into its
coverage. The target cell may move the UE to idle mode as soon as
possible.
[0098] Three Areas
[0099] The present disclosure presents, inter alia, one or more of
three areas of HetNet mobility. The first area is mobility
optimization for high speed UEs in HetNet deployments. A cell size
information element (IE) indicating the size of the cell
originating the message may be added to the X2: RLF INDICATION
message (radio link failure indication message). This, combined
with UE speed information already contained in the UE RLF Report
Container IE, allows the receiving node to adjust its target
selection criteria based on UE mobility and neighbor cell size.
[0100] Further, an indication of cell size may be included for the
cells involved in mobility using the UE RLF Report Container IE in
the X2: HANDOVER REPORT message sent from a target eNB to a source
eNB. The source eNB thus becomes aware of UE measurements taken at
the time of failure and the size of the neighboring cells. This
information allows the source to adjust its mobility thresholds
towards neighbor cells and to prioritize certain targets depending
on UE mobility.
[0101] Two mechanisms for differentiated target selection for high
speed UEs may be used:
[0102] 1) a per-UE, per target cell, CIO (Cell Individual
Offset);
[0103] 2) target prioritization at the source eNB based on
previously monitored failure events.
[0104] The second area is addressed by including UE mobility
information in the UE History IE, which is passed to the target eNB
as part of handover messages. The mobility information may be used
to evaluate whether it is appropriate to select the UE as a
candidate for offloading to a neighbor cell, as it may not be
beneficial to handover fast moving UEs for the sake of mobility
load balancing.
[0105] The third area is addressed by keeping a UE that has a high
mobility state in idle mode as long as possible. This is to avoid
frequent handovers (HOs) and instead force the UE to perform cell
reselection. A flag or IE is communicated to the new target cell
that the UE has been kept in idle mode due to its high mobility.
Such communication may be achieved either via a new IE transmitted
during handover signaling as part of the UE History Information IE
or via RRC level signaling from the UE to the new target cell
[0106] First Area
[0107] Returning to the first area, HO performance for fast moving
UEs is improved in HetNets. FIG. 3 and FIG. 4 show two examples of
conditions in which UEs moving at relatively high speed may be
subject to failure conditions during mobility in HetNet
environments. FIG. 3 shows an example of HO failure in HetNet, with
re-establishment in a third cell. In FIG. 3, a UE is quickly moving
from Cell A to Cell B and then to Cell C as indicated by the arrows
pointing to the right in the figure. Cell B is much smaller than
Cell A and Cell C. In an attempted HO from Cell A to Cell B, due to
the high speed mobility of UE, the connection to Cell B may fail.
For example, Random Access Channel (RACH) access may fail or RRC
Connection Reconfiguration Complete messaging may fail because the
UE is not in Cell B long enough to successfully complete the HO.
The UE may then successfully re-establish connection to Cell C
after dropping the connection while passing through Cell B.
Alternatively, the UE might succeed in connecting to Cell B but
shortly afterwards be out of range of Cell B and be subject to RLF
(Radio Link Failure). Again the UE may then re-establish the
connection through Cell C.
[0108] FIG. 3 also shows a base station, such as an eNB, or similar
structure near the center of each of the three cells. While the
base stations are shown as being near the center of each cell, they
may alternatively define the sides of each cell using sectorized
antennas or be in any other desired configuration. In E-UTRAN,
different user equipment terminals (UEs) are wirelessly connected
to radio base stations (usually referred to as evolved NodeB (eNB))
and are handed over to different eNBs as they move from cell to
cell, as shown. In E-UTRAN the radio base stations are directly
connected to a Radio Network Controller (RNC) which controls the
eNBs connected to it. The eNBs are connected to a core network (CN)
via an S1 interface. The eNBs are also connected to each other via
an X2 interface. An Operation and Support System (OSS) is logically
connected to all the radio base stations as well as to the CN, via
an OSS interface.
[0109] FIG. 4 shows an example of mobility failure in HetNet with
re-establishment in the source cell. The base stations and their
connections are not shown as in FIG. 3, in order to simplify the
drawing, but these base stations and connections may still be
present. In the example of FIG. 4, Cell B is inside of and much
smaller than Cell A. In FIG. 4, after preparation to HO from Cell A
to Cell B as the UE enters Cell B, the connection may fail due to
the high speed mobility of UE. For example, the connection to Cell
B may fail due to a failed RACH access or failed RRC Connection
Reconfiguration Complete. The UE might then re-establish connection
to Cell A after it has passed through Cell B. Alternatively, the UE
may succeed in connecting to Cell B but shortly after be subject to
RLF and re-establish connection to Cell A.
[0110] The mobility robustness optimization (MRO) solution
currently standardized would not attribute the cause of mobility
failures to the fact that the UE is moving above a certain speed
and in a HetNet environment. It is likely that standardized MRO
algorithms would not react to the failure conditions in an
appropriate way. In current MRO procedure descriptions, if the UE
re-establishes or re-connects to a target cell or to a third cell,
then HO too early is excluded as the cause. The system will select
HO too late or HO to the wrong cell depending on measurements and
on the sequence of events (e.g. if the UE does not receive the HO
Command, then the standard deems the failure to be HO too late,
even if the actual case may be something else).
[0111] FIG. 3 and FIG. 4 correspond to the "Handover to Wrong Cell"
and the "Too Early Handover" cases discussed above, respectively.
The source cell A may try to reduce its cell individual offset
(CIO) towards Cell B. However, this may have a negative impact on
low mobility UEs which may be safely handed over to cell B, and
also cell B might still be taken as a potential candidate for high
mobility UEs if the right signal conditions are met. For the high
mobility UEs, Cell B will most likely generate mobility failures,
regardless of the measured signal quality for handover purposes.
Hence there is risk in taking Cell B into consideration as a target
at all for high mobility UEs, provided that other coverage cells
are available.
[0112] In a first implementation, the X2: RLF INDICATION message is
extended to include an optional Cell Type IE (information element)
of the cell from which the message is originated. The Cell Type IE
is not currently a part of the X2: RLF INDICATION message. The Cell
Type IE contains the Cell Size IE and is defined in section 9.2.42
of 3GPP TS 36.423. The Cell Size IE may be extended to include more
categories of cell sizes beside the currently standardized Very
Small, Small, Medium and Large. The cell size is an indication of
the geographic area of the cell, so that a cell with a smaller size
takes less time for a UE to traverse than a cell with a larger
size.
[0113] In a second implementation, the X2: HANDOVER REPORT message
is extended to include an optional UE RLF Report Container IE (as
defined in TS 36.423), an optional RRC Connection Setup Indicator
IE (as defined in TS 36.423) and an optional Cell Type IE relative
to the cell from which the X2: HANDOVER REPORT message has been
originated. The X2: HANDOVER REPORT message does not currently
include these IEs. This extension is more suitable for the case of
"Handover to the Wrong cell".
[0114] The main aim of including the additional IEs in the RLF
INDICATION and HANDOVER REPORT messages is to allow the source eNB
to avoid selection of small cells for high speed UEs in the future.
By using existing IEs the cost and complexity of implementation in
E-UTRAN is reduced. However, instead of existing IEs, new IEs may
be developed that provide more accurate and more relevant
information. In other systems, which do not have the IEs similar to
the additional IEs mentioned above, new or different messaging may
be used to convey similar information at a similar time in the
protocol. The use of the additional IEs is described in the context
of FIG. 5 and FIG. 6.
[0115] FIG. 5 shows enhanced signaling supporting the resolution of
HO to the wrong cell. FIG. 5 shows, in time sequence from top to
bottom, a Measurement Control message from Cell A to UE. This is
answered by a Measurement Report from UE to Cell A. Cell A then
sends a Handover Request to Cell B. Cell B answers with Handover
Request Acknowledgment. Cell A then sends RRC Connection
Reconfiguration to UE. If UE is high mobility as in FIGS. 3 and 4,
the handover may fail and UE will try to connect to the nearest
Cell, such as Cell C in FIG. 3 or Cell A in FIG. 4. In FIG. 5, UE
has moved to Cell C and sends RRC Connection Re-establishment
Request or RRC Connection Setup Request to Cell C. If a connection
is available, then Cell C sends RRC Connection Re-establishment
complete in response to the RRC Connection Re-establishment message
or RRC Connection Setup Complete in response to the RRC Connection
Setup Message. Both messages may include an RLF-Info-Available
indicator.
[0116] With the connection complete, Cell C can then send a UE
Information Request with an RLF-Report-Req. The UE may then reply
by providing the RLF report in the form of a UE Information
Response with the RLF-Report. Cell C may then send the RLF
Indication including the received RLF Report and a cell size IE as
described above to Cell B. Cell B sends an HO Report including the
Cell B and Cell C size IE described above to Cell A together with
the UE RLF Report Container IE that is contained in the RLF
Indication message received from Cell C.
[0117] Thanks to the cell size information and to the UE speed
information contained in the UE RLF Report Container IE (already
present in the RLF INDICATION message), Cell A and Cell-B may
deduce that the mobility failure was due to a high speed UE
attempting to handover to a small cell (Cell B). Cell A may
therefore exclude Cell B as target cell for high speed moving UE
and prioritize Cell C.
[0118] In the scenario of Handover to Wrong Cell described in FIG.
5, the standardized solution for MRO foresees sending the RLF
INDICATION message from Cell C to Cell B and thereafter sending the
HANDOVER REPORT from Cell B to Cell A. However, due to the absence
of cell size information from any of these messages and due to the
absence of the UE RLF Report Container IE (containing UE
measurements, UE speed information, failure cell details and more)
from the HANDOVER REPORT, it may be difficult using the
standardized MRO for the eNB serving Cell A to determine that the
failure was not due to erroneous mobility settings but due to a
high speed UE attempting to handover to a very small cell.
[0119] The currently standardized MRO solution specifies that if
the UE re-establishes or re-connects (from idle) to the prepared
target cell (Cell B) or to a third cell (Cell C), then the case of
HO too early is excluded (due to the UE not going back to source
cell). Whether the failure is due to HO Too Late or HO to Wrong
Cell, determination of the cause of the failure depends on: 1) the
interpretation of the measurements collected by the UE and embedded
in the UE RLF Report Container IE and 2) the sequence of events
(e.g. if the UE does not receive the HO Command, the failure is
likely to be HO Too Late). Therefore, propagating the measurements
in the UE RLF Report Container IE also via the HO REPORT makes it
easier to accurately determine the cause of the failure.
[0120] FIG. 6 shows an example of enhanced signaling supporting the
determination that a HO failed because it was too early. In FIG. 6,
Cell A sends a Measurement Control message to UE. UE responds with
Measurement Report. Cell A then sends a Handover Request message to
Cell B and Cell B responds with Handover Request Acknowledgment.
Cell A then sends an RRC Connection Reconfiguration message to UE.
The handover then fails due to the high mobility of UE. Through the
failure of the handover, the signaling is the same as in the
example of FIG. 5. However, in this case, corresponding to a
scenario such as that of FIG. 4, UE does not reconnect and move on
to Cell C but instead reconnects to Cell A. This is shown as Cell A
sending RRC Connection Re-establishment Request or RRC Connection
Setup Request to UE. If UE is able to connect, then it sends RRC
Connection Re-establishment Complete or RRC Connection Setup
Complete, depending on the type of request that it received.
[0121] Cell A having connected sends UE Information Request with
and RLF-Report-Req to UE. UE responds by providing the requested
report as a UE Information Response including the RLF-Report.
Having received this information from the UE, Cell A, then sends
RLF indication including the received RLF report to Cell B and Cell
B responds with a Handover Report. The Handover Report may include
have the too early HO flag set and include the IE for the size of
cell B as described above.
[0122] Thanks to the cell size information and to the UE
measurements collected prior to the failure (and included in RLF
INDICATION), Cell A may deduce that the mobility failure was due to
a high mobility UE attempting to handover to a small cell (Cell B)
and that coverage on Cell A was still sufficiently good prior to
handover. Cell A may therefore exclude Cell B as a target cell for
a high mobility UE and allow the UE to stay on Cell A. It has been
assumed above that Cell A keeps the contents of the UE RLF report
that it has sent to Cell B for a certain duration, so that when the
HANDOVER REPORT is received later on, Cell A can correlate to it
and use the information in the HANDOVER REPORT such as the UE
speed. However it is not necessary that Cell A saves the UE RLF.
Alternatively, Cell C may include the UE RLF report in the HANDOVER
REPORT messages, as in the case of Handover to the wrong cell
discussed above.
[0123] In the scenario of Too Early Handover described in FIG. 6,
the standardized MRO solution sends an RLF INDICATION to Cell B,
which triggers the sending of a HANDOVER REPORT towards Cell A.
Cell A is already aware of the Too Early HO. This is because when
the UE re-connects to the source it will report, among various
other measurements included in the RRC RLF Report, the time elapsed
from reception of HO COMMAND to connection failure. It is therefore
possible to understand at the source if the failure is for HO too
early. In addition UE mobility information is contained in the RRC
RLF Report received by the source Cell (Cell A in this case).
[0124] The RLF INDICATION message is sent to the target in order to
allow the target to adjust its mobility parameters. However, due to
the lack of cell size information in the HO Report message, Cell A
may not understand that the mobility failure is due to a high
mobility UE attempting to handover to a small cell and that such
small cell is still within the coverage of Cell A.
[0125] The robustness of handover may be improved still further by
providing additional parameters, such as per-target handover
parameters and per-UE handover parameters. There are at least two
different ways of achieving this. One way is to use per-UE,
per-target CIOs. Namely, when eNB-A (Cell A, above) identifies a UE
moving at relatively high speed it may assign, via signaling over
UE dedicated channels, a specific CIO for Cell B (and possibly for
Cell C). Such an offset will not be broadcast in Cell A system
information blocks (SIBs) and it will be used only by the UE in
high speed. Such a CIO may allow the UE to ignore Cell B and to
either report Cell C as the best target (as shown in FIG. 3) or to
remain on Cell A (as shown in FIG. 4).
[0126] An enhancement of this concept is to communicate a mapping
table to the UE with a scaling factor known to the UE and
applicable to different UE speeds (e.g. for speeds less than 10
Km/h apply scale factor of 1, for 10 Km/h-50 Km/h, apply a scale
factor of 1.5, etc.). This may reduce the amount signaling for
cases where per UE, per target CIOs are transmitted each time a
speed transition is recorded (low to medium, medium to high, high
to very high, etc.). This mapping may either be communicated to
each UE during RRC connection setup via RRC Reconfiguration, or
broadcasted over SIBs (very infrequently). UEs may then, based on
their speed, pick the corresponding scaling factor and multiply the
CIO by it, and use the result, for example, in the A3 event
triggering calculations.
[0127] Per-UE, per-target TTTs may be sent similarly to the per-UE,
per-target CIOs above, but in this case, the cell size of the
candidate target cells and UE speed are employed to adjust the
times to trigger (TTTs). That is, both the UE speed and the target
cell size are employed to scale the TTT. Current standards only
provide for scaling down the TTT for high speed UEs. For example,
the scaling may be based on the expected time the UE will spend on
the target, i.e. target cell size/UE speed.
[0128] eNB-A (Cell A) may receive measurement reports from the UE
in high speed mobility, including several candidate cells, among
which Cell B and Cell C. eNB-A will force selection of the handover
target in a way to avoid handovers to Cell B. Namely eNB-A may
choose Cell C (in the case of FIG. 3) or it will not initiate any
handovers (in the case of FIG. 4) despite the fact that Cell B, as
reported by the UE, is the best candidate for mobility.
[0129] FIG. 7 is an example hardware diagram of a device
architecture suitable for the UE and for a eNB. The hardware 700
includes one or more antenna elements 701. There may be separate
transmit and receive arrays, sectorized or diversity antennas or a
single omnidirectional antenna element. For transmission, data is
collected in a transmit queue 703 from which it is transferred to a
baseband modulator 705 for conversion to symbols, modulation and
upconversion. A transmitter 707 further modulates and amplifies the
signal for transmission through the antenna.
[0130] On the receive side, received symbols are demodulated and
downconverted to baseband in a receive chain 709. The baseband
system extracts a bit sequence from the received signal and
generates any error detection codes that may be needed. The bit
stream is stored in a receive buffer or queue 713 for use by the
system.
[0131] A controller 715 controls the operation of the receive and
transmit chains, applies data to the outbound queue and receives
data from the inbound queue. It also generates messages to support
the wireless and wired protocols over which it communicates. The
controller is coupled to one or more memory systems 717 which may
contain software, intermediate cached values, configuration
parameters, user and system data. The controller may also include
internal memory in which any one or more of these types of
information and data may be stored instead of or in addition to
being stored in the external memory system. The controller is
coupled to a system input/output interface 719 which allows for
communication with external devices and a user input/output
interface 721 to allow for user control, consumption,
administration and operation of the system.
[0132] In the case of an eNB, the system interface 719 may provide
access over the S1, OSS and X2 interfaces to the rest of the
network equipment to send and receive data, messages, and
administrative data. However, one or more of these interfaces may
also use the radio interface 701 or another interface (not shown).
In the case of a UE, the system interface may connect to other
components on the device, such as sensors, microphones, and
cameras, as well as to other devices, such as personal computers or
other types of wireless networks, through wireless or wired
interfaces.
[0133] FIG. 8 is a flow diagram of an example of a process that may
be performed by the cell to enhance handovers for a high mobility
UE. The process is performed by a cell, here exemplified by a eNB.
However, the technique may be adopted for use by any of a variety
of base stations or access points using different wireless
protocols and standards. The technique may be performed at the
station that communicates with the UE or aspects of the process may
be performed at a central location, such as a base station
controller, or network administration node.
[0134] At 801, an eNB or other wireless radio station in a cellular
radio communications system receives a report from another node,
such as a neighbor eNB about a particular UE. This report may
contain many different parameters and measures regarding the UE.
The measurements may be made by the UE or by another eNB, or other
type of base station or access point. In the described examples,
the report includes any one or more of the signal strength measured
by the UE and/or by the reporting base station during an attempted
handover, the speed of the UE around the time of the handover, the
time elapsed between the connection request from the UE and the
failure of the connection and other information. The report may
also identify the particular UE to which the report refers.
[0135] The report may include additional information such as the
current location of the UE, physical parameters or characteristics
of the UE, timing information including the time at which a failed
handover was attempted, etc. In addition, the report may contain
information about the reporting cell, in particular the size of the
reporting cell, however, other information may also be included. In
LTE, an RLF Indication or a Handover Report may be used. For other
standards or protocols other reports may be used.
[0136] At 803, the received report is analyzed to determine causes
of the failed handover. In the present example, the cell is
differentiating between high speed or high mobility UEs and slower
speed or lower mobility UEs. The speed of the UE may be inferred
from information in the report or the speed may be measured by the
UE or by the base stations and one or more speed measurements may
be included in the report. For example, given a time elapsed
between the connection request and the failure of the connection,
the speed may be inferred. The accuracy of this speed may be
improved using the signal strength of the signal from the UE. The
information on speed may be used to determine whether the speed of
the UE is a cause of the handover failure, as described above.
[0137] As mentioned above, the size of the target cell may also be
included in the report or obtained from a lookup table stored
locally or remotely. If the UE speed is high and the target cell is
small, then it may often be inferred that the handover failed
because of the speed of the UE. All of this information may be used
to adjust mobility thresholds for the UE. The cell may store
locally or at a central location a table of neighboring cells and
mobility thresholds for each one. The mobility thresholds may be
used to determine if a particular UE may be handed over to each
respective neighboring cell. In a simple example, the mobility
thresholds are signal strength thresholds. The signal strength
thresholds may be the strength of the UE signal as measured by one
or more of the cells, or the strength of cell signals as measured
by the UE. Alternatively, timing offsets or propagation delay may
be used to determine when a UE is able to be handed off to another
cell. At 805, the thresholds are adjusted to accommodate the speed
of the UE and sizes of the neighboring cells.
[0138] At 807, the cell may send a mapping table to the UE for the
UE to use in requesting a handover or in requesting a connection.
The table may include a list of neighboring cells with priorities,
mobility thresholds, scaling factors, and other information for the
UE to use. In a simple example, the mapping table is a list of the
neighboring cells that, for a high speed UE, places a higher
priority on larger cells. This will reduce the number of
unnecessary or short duration handovers to small cells and allow
slower UEs to make better use of smaller cells.
[0139] The thresholds may be signal strength thresholds, so that
the UE will not request a connection or handover to a particular
cell unless the signal received from that cell exceeds the
threshold. The table may also include time offsets for how long a
cell's signal must exceed the threshold before the UE requests a
connection or handover. Alternatively, the time may be a TTT.
[0140] The operations of FIG. 8 may be repeated when a certain
number of reports are received or periodically after a certain time
interval has elapsed. These operation can occur as new reports
arrive and as new UEs arrive. If a particular UE leaves the cell,
then the operations will not be repeated for that UE unless it
returns.
[0141] Second Area
[0142] Turning to the second area, differentiated treatment of UEs
moving in high speed may be done for the purpose of Mobility Load
Balancing (MLB). As mentioned above, if a UE moving at high speed
is selected as a mobility load balancing candidate, failures may
occur. This is because high mobility UEs undergo challenging radio
conditions and are likely to exit the serving cell in a relatively
short time. Forcing the UE to handover to a neighbor cell when
radio conditions do not necessarily require, and when the UE
permanence in the serving cell is likely to be short, may not be
efficient.
[0143] In this second area, the latest version of the locationInfo
IE (as defined in TS36.331) monitored by the serving cell is added
to the UE History Information IE (as defined in TS36.423). The
locationInfo IE may be modified to include information about the UE
horizontal velocity (as defined in TS36.355) in addition to
location coordinates. Horizontal velocity is a measure of the speed
at which the UE travels. It provides more accuracy than UE mobility
state, which may be used as an alternative. Therefore, during
handover signaling the target eNB may be informed about the UE
velocity at the last time it was recorded in the source cell. If
the velocity is above a pre-determined threshold, the target eNB
may decide not to consider such a UE as a candidate for mobility
load balancing procedures.
[0144] Even without the communication of additional information,
already available velocity and other information may be used to
evaluate whether the UE shall be considered as a Mobility Load
Balancing (MLB) candidate. There are cases in which the UE attempts
to handover to a target cell but the attempt will fail and the UE
will try an RRC Connection Re-establishment with a third cell (as
in the case described in FIG. 3). In these cases, the eNB serving
the third cell involved in the mobility procedure will not receive
the UE History Information IE due to the fact that this IE is only
included in the X2: HANDOVER REQUEST message.
[0145] In order to allow the eNB serving the third cell to receive
UE velocity information, the RRC Connection Re-Establishment
message provides an IE named RLF-InfoAvailable IE, which indicates
availability of an RLF report from the UE. This report may also
contain UE velocity information. The eNB serving the cell in which
the UE attempts re-establishment may make use of this velocity
information to determine whether to consider the UE as a candidate
for MLB procedures.
[0146] FIG. 9 is an example process flow diagram implementing
techniques described above to support the MLB process. FIG. 9
represents a process performed by a node in the wireless system,
such as an eNB or similar device. As in the example of FIG. 8, some
of the operations may be performed locally and others in a central
location. Instructions, tables, and parameters may also be stored
locally or centrally. At 901, the node or a related control entity
determines to perform a load balancing process. The MLB process
will include handing some UEs over to other cells, to distribute
the load between cells more evenly.
[0147] At 903, the cell or a related entity in the system selects
candidate UEs for load balancing moves. At 905, velocity
information is obtained or derived for at least some of the
candidate UEs. Examples of obtaining this information are provided
above and in the context of FIG. 8.
[0148] At 907, the velocity information is correlated with the UE
candidates for MLB. This allows all or some of the candidate UEs to
be identified as high or low mobility. The UEs may be grouped into
two or more different classes based on mobility and on other
factors. At 909, the groupings or identifications may be used to
determine whether to exclude some of the UEs from the load
balancing operations. As mentioned above, high mobility UEs often
move quickly out of a cell. Therefore, if a high mobility UE is
moved to another cell through handover, it may very soon require
another handover in the same or a different direction. On the other
hand if a high mobility UE is not moved, it may very soon request a
handover, or be handed over nevertheless as it move to another
cell.
[0149] Third Area
[0150] Turning to the third area the frequent handover signaling
generated by UEs in high mobility is addressed. UEs in high
mobility are sometimes only transmitting a small amount of data
traffic. Therefore, by keeping such UEs in idle mode as much as
possible, failures are minimized and mobility signaling due to
Active Mode handovers is also minimized. This solution is contrary
to the standardized approach of keeping the UEs in "always on"
mode. Such a UE is in RRC Connected mode even when it has no
traffic to send or to receive. It will require handovers as it
moves from cell to cell even if it does not send or receive any
data.
[0151] By keeping a high mobility UE in idle mode for as long as
possible it is possible to reduce the number of handovers and to
force the UE to carry out cell reselection instead. Except for
Tracking Area Updates done independently of connection
establishment, cell reselection does not require any signaling with
an eNB. Once the UE is placed into a forced idle mode status, there
are at least two ways for a new cell on which the UE has camped to
know that the UE has been kept in forced idle due to its high
mobility and low data traffic exchange. In both examples, below, a
forced idle information element is sent on a signaling bearer.
However, other messages may be used instead, including various
types of flags.
[0152] In one example, information about the forced idle status may
be sent by the UE as UE History Information. At some point, after
the UE is put into the forced idle, it will likely need to transmit
to the cell station. This normally occurs by moving into an RRC
Connected state in order to exchange data traffic. If the UE moves
from a serving cell to a target cell during this time window, the
information about the forced idle UE status may be transferred to
the target cell as part of a UE History Information IE. The UE
History Information IE may be extended to also include an idle mode
IE. The idle mode IE specifies that the UE has been kept in forced
idle in the previous serving cell.
[0153] As an alternative, the idle mode IE may be sent by the UE as
part of Connection Re-establishment message. As explained above,
active mode mobility may be subject to failure and the UE may try
to re-establish a connection on another cell. The RRC Connection
Re-establishment message may be extended to also include an idle
mode IE that indicates that the UE has been kept in forced idle
mode in the previously serving cell.
[0154] The addition of the new IE, regardless of how and when it is
transmitted, gives the new serving eNB the opportunity to
immediately move the UE back into forced idle mode and to check
more frequently if the UE mobility and data transfer remain such as
to justify the forced idle status.
[0155] Under the standards, if the UE moves to a different cell
while in idle mode, there will be no handover signaling but purely
cell reselection. In another alternative, in order to quickly
inform the newly reselected cell about the forced idle mode, an
idle mode IE may be added to the RRC Connection Setup message that
the UE sends as soon as it needs to send or receive data. Using the
idle mode IE with a connection setup messages allows the UE to send
data shortly after it is necessary. Accordingly, the UE can
transfer the idle mode information independent of any communication
between eNBs. It is not necessary for the network to provide a UE
History Information or Connection Re-establishment message. As a
result, the UE in idle mode is not prevented from sending data.
Instead, as soon as there is data to transmit, the UE sets up a
connection. As soon as the data has been transmitted, the UE moves
back to idle mode in the quickest way. This is more efficient and
requires far fewer handovers that keeping the UE always on, where
the UE will always be in RRC Connected mode even during long
discontinuous reception (DRX) and discontinuous transmission (DTX)
periods.
[0156] FIG. 10 shows an example process flow for using an idle mode
with a UE to reduce handover overhead and failed handovers. At
1001, a cell receives information about one or more UEs that are
registered with or connected to it. The information may come from
direct observation, from the UEs directly, or from the information
elements and reports described above. Any combination of these
including local, neighboring, and central information sources may
be used.
[0157] At 1003, the cell determines that a UE has high mobility and
low traffic. The following operations are particularly useful when
the low traffic is delay sensitive. This determination may be done
in whole, or in part, by the cell using local or central data and
processing resources. As with all of the techniques described
herein, the particular speed required to qualify a UE as high
mobility or high velocity and the particular data rate required to
qualify a UE as low data traffic may be adapted to suit any
particular implementation. In the example of failed handovers, a
high mobility UE may be defined as a UE that fails to handover to a
particular small cell. Alternatively, in that and other situations,
the standards of high and low may be based on the amount of time
required to service the UE with redundant requests, or a certain
percentage of UEs may be assigned to be high and low. As a further
alternative, the UE may determine a mobility or velocity value
using techniques defined in the standards or in other ways such as
using satellite or terrestrial location information, and measuring
data communication rates over time.
[0158] In the example of FIG. 10, the high mobility, low traffic UE
is ideally one that will create less signaling overhead if forced
to idle without any noticeable distraction or irritation of the
user. The particular thresholds to achieve this ideal will depend
on the particular implementation as well as the particular UE and
its geographic circumstances.
[0159] At 1005 the high mobility, low traffic UE is commanded to
move to an idle state. This is also referred to herein as forced
idle. Typically, such a command will be sent or executed after the
UE is finished transmitting and receiving. Typically, the cell and
the UE have transmit buffers. After all of the data in the transmit
buffers on both sides has been transmitted and received, the UE may
be idled with no adverse effect on data transmission. In one
example, there is a delay time before idling, so that new data does
not immediately arrive and occasion a new transmission immediately
after the UE idles. In another example, a time threshold can be
specified in the go to idle mode command and the UE can stay in
that mode for that period, unless the speed becomes lower than a
certain threshold or there is a data that needs to be transmitted.
The UE may then stay in the cell or move to another cell before the
condition is fulfilled to return to connected mode (i.e. before the
UE has data to transmit or receive, a timer expiration, a speed
change, etc.).
[0160] At some later point in time, (e.g. microseconds or minutes),
at 1007, the UE has new uplink data to send and requests a
connection, or the closest cell has identified the idling UE and
establishes a connection with the UE to send new downlink data. The
UE and the cell exchange data as necessary and then at 1009, the UE
returns to idle.
[0161] For faster, more efficient operation, at 1011 the cell that
determined that the UE should be forced to idle may send this
information to another cell of the system. Even in idle mode, the
cell, or a combination of cells, may be able to track the UE and
detect when it has moved into another cell. By informing the other
cell that the UE has been forced to idle, the other cell may
continue to force the UE to idle. In addition, the signaling
protocols between the UE and the cell may be more efficient if the
cell is already aware of the status of the UE.
CONCLUSION
[0162] The techniques described above allow UEs to be handled
differently in different situations. In particular, high mobility
UEs, as determined by UE velocity relative to the size of the cells
are treated differently to provide less overhead and more robust
handover. Current approaches provide one configuration for all
types of UEs (high speed and low speed) and for all types of target
cells (very small, small, medium and large).
[0163] By differentiating on the basis of UE speed and target cell
size, UE performance is improved by avoiding mobility failures that
result in Key Performance Indicator (KPI) and Quality of Service
(QoS) degradations. Load distribution in the radio network is made
more dynamic and robust by selecting a more reliable cell for
traffic offloading. UE power consumption may also be reduced by
minimizing the number of cells for which the UE needs to collect
and report measurements and by reducing the amount of mobility
signaling generated by the UE. Network performance is also improved
by reducing handover failures and radio link failures. Finally,
network scalability is improved by reducing the amount of handover
signaling towards the core network.
[0164] The operations of the flow and signaling diagrams are
described with reference to exemplary embodiments. However, it
should be understood that the operations of the flow diagrams can
be performed by variations other than those discussed with
reference to these other diagrams, and the variations discussed
with reference to these other diagrams can perform operations
different than those discussed with reference to the flow
diagrams.
[0165] As described herein, instructions may refer to specific
configurations of hardware such as application specific integrated
circuits (ASICs) configured to perform certain operations or having
a predetermined functionality or software instructions stored in
memory embodied in a non-transitory computer readable medium. Thus,
the techniques shown in the figures can be implemented using code
and data stored and executed on one or more electronic devices
(e.g., a UE, an eNB, etc.). Such electronic devices store and
communicate (internally and/or with other electronic devices over a
network) code and data using machine-readable media, such as
non-transitory machine-readable storage media (e.g., magnetic
disks; optical disks; random access memory; read only memory; flash
memory devices; phase-change memory) and transitory
machine-readable communication media (e.g., electrical, optical,
acoustical or other form of propagated signals--such as carrier
waves, infrared signals, digital signals, etc.). In addition, such
electronic devices typically include a set of one or more
processors coupled to one or more other components, such as one or
more storage devices (non-transitory machine-readable storage
media), user input/output devices (e.g., a keyboard, a touchscreen,
and/or a display), and network connections. The coupling of the set
of processors and other components is typically through one or more
busses and bridges (also termed as bus controllers). Thus, the
storage device of a given electronic device typically stores code
and/or data for execution on the set of one or more processors of
that electronic device. Of course, one or more parts of an
embodiment of the invention may be implemented using different
combinations of software, firmware, and/or hardware.
[0166] The operations of the flow diagrams are described with
reference to the exemplary embodiments of the other diagrams.
However, it should be understood that the operations of the flow
diagrams can be performed by embodiments of the invention other
than those discussed with reference to these other diagrams, and
the embodiments of the invention discussed with reference to these
other diagrams can perform operations different than those
discussed with reference to the flow diagrams.
[0167] While the flow diagrams in the figures show a particular
order of operations performed by certain embodiments of the
invention, it should be understood that such order is exemplary
(e.g., alternative embodiments may perform the operations in a
different order, combine certain operations, overlap certain
operations, etc.).
[0168] While the invention has been described in terms of several
embodiments, those skilled in the art will recognize that the
invention is not limited to the embodiments described, can be
practiced with modification and alteration. The description is thus
to be regarded as illustrative instead of limiting.
* * * * *