U.S. patent application number 14/709480 was filed with the patent office on 2015-08-27 for method of enhanced connection recovery and loss-less data recovery.
The applicant listed for this patent is MEDIATEK INC.. Invention is credited to Yih-Shen Chen, Per Johan Mikael Johansson, Tze-Ping Low, William Plumb.
Application Number | 20150245406 14/709480 |
Document ID | / |
Family ID | 48779879 |
Filed Date | 2015-08-27 |
United States Patent
Application |
20150245406 |
Kind Code |
A1 |
Johansson; Per Johan Mikael ;
et al. |
August 27, 2015 |
Method of Enhanced Connection Recovery and Loss-Less DATA
Recovery
Abstract
An enhanced connection recovery upon lost RRC connection due to
radio link failure (RLF) or handover failure (HOF) is proposed. A
UE first establishes an RRC connection in a source cell in a mobile
communication network. Later on, the UE detects a failure event and
starts an RRC reestablishment procedure in a target cell to restore
the RRC connection. In a first novel aspect, a fast NAS recovery
process is applied to reduce the outage time in the target cell. In
a second novel aspect, context fetching is used to reduce the
outage time in the target cell. In a third novel aspect, a
loss-less reestablishment procedure is proposed to reduce data loss
during the connection recovery.
Inventors: |
Johansson; Per Johan Mikael;
(Kungsangen, SE) ; Chen; Yih-Shen; (Hsinchu City,
TW) ; Low; Tze-Ping; (Lexington, MA) ; Plumb;
William; (Charlestown, MA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
MEDIATEK INC. |
Hsinchu |
|
TW |
|
|
Family ID: |
48779879 |
Appl. No.: |
14/709480 |
Filed: |
May 12, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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13743644 |
Jan 17, 2013 |
9055560 |
|
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14709480 |
|
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61587979 |
Jan 18, 2012 |
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Current U.S.
Class: |
370/228 ;
370/216 |
Current CPC
Class: |
H04W 36/0072 20130101;
H04W 76/18 20180201; H04W 76/19 20180201; H04W 48/17 20130101; H04W
36/0061 20130101; H04W 36/24 20130101; H04W 36/0033 20130101; H04W
40/36 20130101; H04W 36/08 20130101; H04W 36/0055 20130101 |
International
Class: |
H04W 76/02 20060101
H04W076/02; H04W 40/36 20060101 H04W040/36; H04W 36/00 20060101
H04W036/00 |
Claims
1. A method for enhanced connection recovery, the method
comprising: receiving a radio resource control (RRC)
reestablishment request message from a user equipment (UE) by a
target base station in a mobile wireless communication network;
transmitting an X2 radio link failure (RLF) indication to a serving
base station, wherein the X2 RLF indication comprises a
data-forwarding request; receiving an X2 message transfer
comprising of packet data convergence protocol (PDCP) serial number
(SN) status from the serving base station; and transmitting an RRC
reestablishment response message to the UE.
2. The method of claim 1, further comprising: receiving User-plane
data-forwarding via an X2 interface from the serving base
station.
3. The method of claim 1, wherein the RLF indication further
comprises a UE context request.
4. The method of claim 3, wherein UE context information comprises
UE capability information and non-access stratum (NAS)
configuration information.
5. The method of claim 3, wherein the RLF indication is carried via
a new information element that requires mandatory response from the
serving base station.
6. The method of claim 1, further comprising: performing an RRC
connection reconfiguration with the UE after transmitting the
response message of RRC reestablishment to the UE.
7. The method of claim 1, further comprising: transmitting an S1
path switch request to a mobility management entity (MME); and
receiving an S1 patch switch acknowledgement from the MME.
8. A base station, comprising: a receiver that receives a radio
resource control (RRC) reestablishment request message from a user
equipment (UE) by a target base station in a mobile wireless
communication network; an X2 interface that transmits an X2 radio
link failure (RLF) indication to a serving base station, wherein
the X2 RLF indication comprises a data-forwarding request; wherein
the X2 interface also receives an X2 message transfer comprising of
packet data convergence protocol (PDCP) serial number (SN) status
from the serving base station; and a transmitter that transmits an
RRC reestablishment response message to the UE.
9. The base station of claim 8, wherein the UE receives User-plane
data-forwarding via the X2 interface from the serving base
station.
10. The base station of claim 8, wherein the RLF indication further
comprises a UE context request.
11. The base station of claim 10, wherein UE context information
comprises UE capability information and non-access stratum (NAS)
configuration information.
12. The base station of claim 10, wherein the RLF indication is
carried via a new information element that requires mandatory
response from the serving base station.
13. The base station of claim 8, further comprising: an RRC
connection management module that performs an RRC connection
reconfiguration with the UE after transmitting the response message
of RRC reestablishment to the UE.
14. The base station of claim 8, further comprising: an S1
interface that transmits an S1 path switch request to a mobility
management entity (MME), wherein the S1 interface also receives an
S1 patch switch acknowledgement from the MME.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation, and claims priority
under 35 U.S.C. .sctn.120 from nonprovisional U.S. patent
application Ser. No. 13/743,644, entitled "Method of Enhanced
Connection Recovery and Loss-less Data Recovery," filed on Jan. 17,
2013, the subject matter of which is incorporated herein by
reference. Application Ser. No. 13/743,644, in turn, claims
priority under 35 U.S.C. .sctn.119 from U.S. Provisional
Application No. 61/587,979, entitled "Method of Fast
Re-Establishment," filed on Jan. 18, 2012, the subject matter of
which is incorporated herein by reference.
TECHNICAL FIELD
[0002] The disclosed embodiments relate generally to enhanced
connection recovery in mobile communication network, and, more
particularly, to enhanced connection recovery upon lost RRC
connection due to radio link failure (RLF) or handover failure
(HOF).
BACKGROUND
[0003] In 3GPP Long-Term Evolution (LTE) networks, an evolved
universal terrestrial radio access network (E-UTRAN) includes a
plurality of base stations, e.g., evolved Node-Bs (eNBs)
communicating with a plurality of mobile stations referred as user
equipments (UEs) over established radio resource control (RRC)
connections. Radio link monitoring (RLM) is a mechanism for a UE to
monitor the quality of a downlink (DL) channel of its serving cell
for determining if the radio link is good enough to continue
transmission. For example, the UE measures cell-specific reference
signal (CRS) to detect the downlink radio link quality for the
serving cell. The UE also compares the estimated DL quality to
thresholds (e.g., Q.sub.OUT and Q.sub.IN) for determining if the
link between the serving cell and the UE is good enough or not. In
addition to RLM, the UE declares radio link failure (RLF) upon the
occurrences of physical layer problems based on N310/N311/T310
mechanism, random access problem indication from MAC layer, and
indication from RLC layer that the maximum number of retransmission
has been reached. Once RLF is detected, the UE gathers and stores
RLF information and attempts to restore the RRC connection by
performing an RRC reestablishment procedure.
[0004] For mobility management in LTE systems, each UE needs to
periodically measure the received reference signal power and the
qualities of the serving cell and neighbor cells and reports
measurement results to its serving eNB for potential handover or
cell reselection. Measurements, such as Reference signal received
power (RSRP) and/or Reference signal received quality (RSRQ) of an
LTE cell, are used to to rank among the different cells for the
purpose of mobility management. Properly managed handover can
prevent loss of connection. In practice, however, handover failure
(HOF) often occurs due to various reasons such as UE signaling
problems and UE measurement configuration problems. Typically, a
radio link failure or handover failure indicates too early
handover, too late handover, or handover to a wrong cell. After the
RLF/HOF event, the UE will attempt an RRC reestablishment procedure
to restore the RRC connection.
[0005] When performing RRC reestablishment, the UE releases current
RRC configuration and performs cell selection. The prerequisite of
a successful RRC reestablishment procedure is that the selected
cell for RRC reestablishment has UE context. If the UE fails to
restore the RRC connection, then the UE enters RRC idle mode and
tries to camp on a cell via a non-access Stratum (NAS) recovery
procedure. The UE may indicate the availability of the RLF report
to eNB and report the RLF/HOF information to eNB upon request after
successful RRC connection reestablishment or RRC connection setup.
Based on the RLF report, possible corrective action may be applied
by the network to prevent future connection failures.
[0006] An LTE-Advanced (LTE-A) system improves spectrum efficiency
by utilizing a diverse set of base stations deployed in a
heterogeneous network (HetNet) fashion. Using a mixture of macro,
pico, femto and relay base stations, heterogeneous networks enable
flexible and low-cost deployments and provide a uniform broadband
user experience. In a heterogeneous network, smarter resource
coordination among base stations, better base station selection
strategies and more advance techniques for efficient interference
management can provide substantial gains in throughput and user
experience as compared to a conventional homogeneous network.
[0007] In HetNet scenario (e.g., macro-pico deployment), however,
it is expected that HOF/RLF rate would increase. For example,
HOF/RLF may occur due to imprecise pico cell measurement or not
enough time for pico-macro handover. It is thus desirable to
improve the connection recovery procedure to reduce outrage time
and to reduce data loss during the connection recovery.
SUMMARY
[0008] An enhanced connection recovery upon lost RRC connection due
to radio link failure (RLF) or handover failure (HOF) is proposed.
A UE first establishes an RRC connection in a source cell in a
mobile communication network. Later on, the UE detects a failure
event and starts an RRC reestablishment procedure in a target cell
to restore the RRC connection. The enhance connection recovery may
be performed from UE/radio access perspective or from network
perspective. From UE/radio access perspective, the enhanced
connection recovery may be applied to reduce the outage time in the
source cell (e.g., via fast RLF) or to reduce the outage time in
the target cell (e.g., via enhance cell selection and multi-RAT
registration). From network perspective, the enhanced connection
recovery may be applied to reduce the outage time in the target
cell (e.g., via fast NAS recovery and context fetch), or to reduce
data loss during the recovery (e.g., via loss-less data
recovery).
[0009] In a first novel aspect, a fast RLF process is applied to
reduce the outage time in the serving cell. In one embodiment, in
addition to legacy T310 timer, a new timer (e.g., T310a) is started
when the UE sends a measurement report to the eNB. RRC
reestablishment is performed when the new timer expires. In another
embodiment, the UE initiates RRC reestablishment before T310 timer
expires if a candidate cell (e.g., a neighbor cell with better
radio link quality than the serving cell) is identified by the UE
itself.
[0010] In a second novel aspect, an enhanced cell selection
mechanism based on cell prioritization information is applied to
reduce the outage time in the target cell. The priority for cell
selection is based on frequency layers with good mobility coverage
or based on intra-frequency cells with good mobility coverage. The
cell prioritization information may be carried by broadcasting or
unicasting and PCI ranging mechanism may be used to identify the
mobility cells. In one embodiment, multi-RAT registration is
applied to steer cell selection. In another embodiment, the cell
prioritization is selectively applied to UEs with high mobility
state.
[0011] In a third novel aspect, a fast NAS recovery process is
applied to reduce the outage time in the target cell. In one
embodiment, a NAS service request is triggered by the RRC
reestablishment request. The target base station inquires UE
context from an MME via S1 interface upon receiving the NAS service
request. Because the NAS service request is triggered earlier, the
target eNB can obtain the UE context information quicker and thus
reduce the outage time in the target cell.
[0012] In a fourth novel aspect, context fetching is used to reduce
the outage time in the target cell. In one embodiment, the target
eNB sends an RLF indication to the source eNB via X2 interface, and
the RLF indication comprises a UE context request. In response to
the RLF indication, the source eNB sends the UE context information
to the target eNB. As a result, RRC reestablishment is successfully
completed and the outage time in the target cell is reduced.
[0013] In a fifth novel aspect, a loss-less reestablishment
procedure is proposed to reduce data loss during the connection
recovery. In one embodiment, the target eNB sends an RLF indication
to the source eNB, and the RLF indication comprises data-forwarding
request. In response to the RLF indication, the source eNB sends
PDCP SN status and U-plane data to the target eNB. The PDCP
operation is thus resumed without data loss.
[0014] Other embodiments and advantages are described in the
detailed description below. This summary does not purport to define
the invention. The invention is defined by the claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] The accompanying drawings, where like numerals indicate like
components, illustrate embodiments of the invention.
[0016] FIG. 1 illustrates a mobile communication network with
enhanced connection recovery in accordance with one novel
aspect.
[0017] FIG. 2 is a diagram that illustrates different embodiments
of enhanced connection recovery in accordance with one novel
aspect.
[0018] FIG. 3 is a simplified block diagram of a UE and an eNodeB
in accordance with one novel aspect.
[0019] FIG. 4 illustrates enhanced connection recovery from radio
access network perspective.
[0020] FIG. 5 illustrates a fast RLF procedure in accordance with
one novel aspect.
[0021] FIG. 6 illustrates one embodiment of fast RLF involving a
new T310a timer.
[0022] FIG. 7 illustrates one embodiment of enhanced cell selection
based on cell prioritization.
[0023] FIG. 8 illustrates one embodiment of enhanced cell selection
with multi-RAT registration.
[0024] FIG. 9 is a flow chart of a method of fast radio link
failure procedure.
[0025] FIG. 10 is a flow chart of a method of enhanced cell
selection with prioritization.
[0026] FIG. 11 illustrates a first embodiment of RRC
reestablishment procedure with fast NAS recovery.
[0027] FIG. 12 illustrates a second embodiment of RRC
reestablishment procedure with fast NAS recovery.
[0028] FIG. 13 illustrates one embodiment of RRC reestablishment
procedure with context fetching.
[0029] FIG. 14 illustrates one embodiment of loss-less RRC
reestablishment procedure.
[0030] FIG. 15 is a flow chart of a method of fast NAS recovery in
target cell from UE perspective.
[0031] FIG. 16 is a flow chart of a method of fast NAS recovery in
target cell from BS perspective.
[0032] FIG. 17 is a flow chart of a method of RRC reestablishment
procedure with context fetching.
[0033] FIG. 18 is a flow chart of a method of loss-less RRC
reestablishment procedure.
DETAILED DESCRIPTION
[0034] Reference will now be made in detail to some embodiments of
the invention, examples of which are illustrated in the
accompanying drawings.
[0035] FIG. 1 illustrates a mobile communication network 100 with
enhanced connection recovery in accordance with one novel aspect.
Mobile communication network 100 comprises a user equipment UE 101,
a radio access network (RAN) 108 having a first base station eNB
102, a second base station eNB 103, a packet core network (CN) 109
having a mobility management entity MME 104, a serving gateway SGW
105, and a packet data network (PDN) gateway PGW 106, and Internet
107. The base stations communicate with each other via the X2
interface, and each base station communicates with MME 104 via the
S1 interface. UE 101 may access Internet 107 via the radio access
network RAN 108 and the packet core network CN 109.
[0036] UE 101 first establishes a radio resource control (RRC)
connection with its serving base station eNB 102 as a source cell.
Later on, UE 101 detects a failure event such as a radio link
failure (RLF) or a handover failure (HOF). UE 101 then performs an
RRC reestablishment procedure to restore the RRC connection. For
example, UE 101 may select a target cell with the target base
station eNB 103 and performs the RRC reestablishment. If the RRC
reestablishment fails, then UE 101 may go to RRC idle mode and
starts non-access stratum (NAS) recovery to establish a new RRC
connection. During the connection recovery process, the RRC
connection is interrupted. The interruption time is referred to as
connection outage time. In addition, certain amount of data may be
lost during the recovery. In one novel aspect, an enhanced
connection recovery process 111 (e.g., that involves UE, RAN, X2
interface and S1 interface) is applied to reduce the outage time in
the source/target cell and to reduce the data loss during the
connection recovery.
[0037] FIG. 2 is a diagram that illustrates different embodiments
of enhanced connection recovery in accordance with one novel
aspect. As illustrated in FIG. 2, an enhanced connection recovery
process 201 may be performed from UE/radio access perspective (as
depicted by box 211) or from network perspective (as depicted by
box 221). From UE/radio access perspective, the enhanced connection
recovery may be applied to reduce the outage time in the source
cell (as depicted by box 231), or to reduce the outage time in the
target cell (as depicted by box 232). To reduce outage time in the
source cell, a fast RLF procedure 241 may be used. On the other
hand, to reduce outage time in the target cell, enhanced cell
selection 242 may be used. For inter radio access technology (IRAT)
scenario, IRAT multi registration 243 may be applied to reduce
outage time in the target cell.
[0038] From network perspective, the enhanced connection recovery
may be applied to reduce the outage time in the target cell (as
depicted by box 232), or to reduce data loss (as depicted by box
233). In order to reduce outage time in the target cell, a novel
fast NAS recovery 244 may be used. Alternatively, a novel context
fetching 245 may be used. On the other hand, in order to reduce
data loss, a loss-less recovery mechanism 246 may be applied via
the X2 interface. The different embodiments of the enhanced
connection recovery proves are illustrated below with additional
details.
[0039] FIG. 3 is a simplified block diagram of a user equipment UE
301 and a base station eNodeB 302 in accordance with one novel
aspect. User equipment UE 301 comprises memory 311 having program
codes 314, a processor 312, a transceiver 313 coupled to an antenna
module 319. User equipment UE 301 also comprises various function
modules including a measurement module 315 that performs various
measurements based on measurement configurations, an RLM/RLF module
316 that performs radio link monitoring, radio link failure
detection and handling, a cell selection module 317 that performs
cell selection for connection establishment and recovery, and an
RRC connection management module 318 that performs RRC connection
setup procedures and maintains RRC connection. Similarly, base
station eNodeB 302 comprises memory 321 having program codes 324, a
processor 322, a transceiver 323 coupled to an antenna module 329.
Base station eNodeB 302 also comprises various function modules
including a configuration module 325 that provides various
configuration to UE 301, an S1 interface module 326 that manages
communication with an MME in the core network, an X2 interface
module 327 that manages communication with other base stations, and
an RRC connection management module 328 that performs RRC
connection setup procedures and maintains RRC connection.
[0040] The different modules are function modules that can be
implemented by software, firmware, hardware, or any combination
thereof. The function modules, when executed by the processors
(e.g., via executing program codes 314 and 324), allow UE 301 and
eNB 302 to perform enhanced connection recovery upon detecting a
connection failure event. In a first example, RLM/RLF module 316
detects a radio link problem and invokes a fast RLF mechanism to
reduce the outage time in the source cell. In a second example,
cell selection module 317 performs enhanced cell selection upon a
failure event to reduce the outage time in the target cell. In a
third example, context fetching or fast NAS recovery is performed
by eNB 302 via the X2/S1 interface module 326/327 to reduce the
outage time in the target cell. Finally, in a fourth example,
loss-less RRC reestablishment is performed by eNB 302 via the X2
interface module 327 to reduce data loss during the connection
recovery.
Fast RLF and Enhanced Cell Selection
[0041] FIG. 4 illustrates enhanced connection recovery from
UE/radio access network perspective in a mobile communication
network 400. Mobile communication network 400 comprises a UE 401, a
serving eNB 402 (for connection in a source cell), and a target eNB
403 (for connection in a target cell). From UE/radio access
perspective, connection recovery may be enhanced by a fast RLF
procedure to reduce outage time in the source cell and by an
enhanced cell selection mechanism to reduce outage time in the
target cell.
[0042] In step 411, UE 401 establishes an RRC connection with eNB
402 in the source cell. Later on, UE 401 detects a possible radio
link problem, e.g., link quality is lower than a threshold
Q.sub.OUT. For legacy RLF procedure, a T310 timer is then started
and RLF is detected when T310 timer expires. UE 401 then selects a
cell and tries to restore the RRC connection. This is an
eNB-controlled mechanism and the timer value is configured by the
network. However, the legacy RLF procedure with T310 timer may not
provide enough flexibility for better performance, especially with
respect to small cells. For example, a small cell may not provide
good coverage of the coverage layer and long RLF detection time may
not be suitable. Under those cases, the legacy RLF solution would
lead to frequent RRC reestablishments.
[0043] In one novel aspect, UE 401 initiates a fast RLF procedure
in step 412, during which the evaluation of RLF is not only
dependent on the source cell timer and filter, but also dependent
on the signal strength and/or quality of the selected
reestablishment cell. For example, if there is an identified
reestablishment cell (e.g., implicitly or explicitly), and the
reestablishment cell is radio-wise "good enough", then RLF
evaluation is shortened and the UE goes to the reestablishment
cell. In step 413, UE 401 performs RRC reestablishment with eNB 403
in the target cell. The RRC reestablishment may fail if the target
eNB 403 does not have UE context information. In step 414, UE 401
goes to RRC idle mode and initiates NAS recovery when RRC
reestablishment fails. After cell selection, in step 415, UE 401
performs RRC connection setup with eNB 403 in the selected target
cell.
[0044] The cell selection in step 414 is performed when RRC
reestablishment failure occurs. In current LTE systems, UE follows
legacy cell selection method to select a suitable cell. However, in
HetNet deployment (e.g., macro-pico), there might be frequent RRC
reestablishment if there is no differentiation among cells and
frequency layers with respect to cell selection. For example, a
high-mobility UE may move out of the coverage of a pico cell
easily. In such case, if UE can reestablish an RRC connection with
a macro cell, then it is expected to have less frequent RRC
connection reestablishment.
[0045] In one novel aspect, UE 401 applies an enhanced cell
selection method in step 414. In one embodiment, after RRC
reestablishment fails, UE 401 selects the target cell with
priority. The priority may be assigned based on frequency layers or
based on intra-frequency cells. In multi-frequency HetNet
deployment, one frequency layer is configured as a mobility layer.
UE selects a cell in the mobility layer with priority when
reestablishment fails. In single-frequency HetNet deployment, cells
with larger coverage such as macro cells are configured as mobility
cells. UE selects one of the macro cells with priority when
reestablishment fails. In another embodiment of enhanced cell
selection, UE with high moving speed should re-connect to the
coverage layer, which is indicated by eNB via broadcasting or
unicasting. For example, if UE mobility state is higher than a
threshold, then UE only re-establishes to the coverage layer.
[0046] FIG. 5 illustrates a fast RLF procedure in accordance with
one novel aspect in a mobile communication network 500. Mobile
communication network 500 comprises a UE 501, a serving eNB 502
(for connection in a source cell), and a target eNB 503 (for
connection in a target cell). In step 511, UE 501 establishes an
RRC connection with eNB 502 in the source cell. In step 512, UE 501
performs radio link monitoring and measures radio signal
strength/quality. In step 513, UE 501 detects a possible radio link
problem, e.g., link quality is lower than a threshold Q.sub.OUT.
Based on the legacy RLF procedure, UE 501 starts T310 timer.
[0047] In addition to the legacy RLF that relies on
network-controlled source cell timer and filter, UE 501 also
initiates a fast RLF procedure, which is a UE-controlled mechanism
as a backup option of the legacy RLF. In one embodiment, the fast
RLF only kicks off when prepared HO is likely to fail. For example,
UE 501 sends a measurement report to eNB 502 in step 514, and eNB
502 makes certain mobility decision in step 515. Based on the
mobile decision, eNB 502 sends an HO command to UE 501 in step 516.
UE 501, however, fails to receive the HO command (e.g., due to poor
radio link quality). Instead of waiting for T310 timer to expire,
UE 501 triggers fast RLF in step 517 before T310 timer expires.
Upon fast RLF, UE 501 performs cell selection in step 518 followed
by an RRC reestablishment procedure in step 519. Without the fast
RLF mechanism, UE 501 is likely to wait too long before T310 timer
expires. There are many ways to shorten the RLF evaluation, and one
of them is to use a new T310a timer (e.g., starts timer t310a when
UE 501 sends measurement report in step 514).
[0048] FIG. 6 illustrates one embodiment of fast RLF involving the
new T310a timer. As illustrated in FIG. 6, each UE starts two
independent processes: an HO process and an RLF process. At time
t1, the HO process detects an event triggering condition of the
source/serving cell. In one example, the triggering event may be
that the channel quality of the serving cell is worse than the
channel quality of a neighbor cell by a non-negative threshold.
Note that the non-negative threshold is used to mitigate ping-pong
effect. In another example, the triggering event may be that the
channel quality of the serving cell is lower than a first threshold
and the channel quality of the neighbor cell is higher than a
second threshold. On the other hand, at time t2, the RLF process
detects a bad radio link condition, and starts T310 timer. Going
back to the HO process, after a time-to-trigger (TTT) period from
time t1, at time t3, the UE is triggered to send out a measurement
report to the network. In addition, at time t3, the RLF process
also starts a new T310a timer at the same time (t3) when the UE
sends out the measurement report. After HO preparation time, at
time t4, the UE fails to receive an HO command from the network. At
time t5, the new T310a timer expires and fast RLF is triggered.
Without the new T310a timer, the legacy RLF would be triggered when
T310 timer expires at time t6, which is much later than time t5.
The fast RLF mechanism thus reduces the outage time in the source
cell.
[0049] In another embodiment of fast RLF, the UE initiates RRC
reestablishment procedure before T310 timer expires if a candidate
cell is identified. In general, the candidate cell is a neighbor
cell with good quality (e.g., based on RSRP/RSRQ measurements). If
the frequency priority or the PCI range of the candidate cell is
assigned by the network, then the UE can select neighbor cells from
the preferred frequency layer. The network can broadcast or unicast
the criteria for candidate cell assignment. In one example, UE can
reuse the same parameters for suitable cell selection as defined in
legacy cell selection procedure (e.g., 3GPP TS36.304). In another
example, eNB can broadcast another set of parameters for cell
selection defined for HetNet deployment. Specifically, a set of PCI
ranging can be attached to the configuration so that UE can
differentiate pico cells from macro cells. Note that, PCI range
refers to a list of cells. This fast RLF mechanism also relates to
enhanced target cell selection illustrated below.
[0050] FIG. 7 illustrates one embodiment of enhanced cell selection
based on cell prioritization information in a mobile communication
network 700. Mobile communication network 700 comprises a UE 701, a
serving eNB 702 (for connection in a source cell), and a target eNB
703 (for connection in a target cell). In step 711, UE 701
establishes an RRC connection with eNB 702 in the source cell. In
step 712, UE 701 receives cell prioritization information from eNB
702. The priority could be based on frequency layers (one frequency
layer is configured as mobility layer in multi-frequency scenario)
or based on intra-frequency cell coverage (cells with larger
coverage are configured as mobility cells in single-frequency
scenario). The cell prioritization information may be carried by a
broadcasting channel (BCH) or by a unicasting RRC message. Note
that the priority list in System Information Block (SIB) is mainly
for cell reselection, which may be inappropriate for cell selection
after RRC reestablishment. Therefore, a specific priority list is
needed for cell selection. In one example, PCI-range method can be
applied in both multi-frequency and single-frequency scenarios
(e.g., a set of mobility cells are identified by PCI ranging).
[0051] In step 713, UE 701 detects RLF and performs RRC
reestablishment procedure with eNB 703 in the selected target cell
(step 714). If the RRC reestablishment fails, then UE 701 goes to
RRC idle mode and starts NAS recovery in step 715. UE 701 again
applies enhanced cell selection based on the cell prioritization
information (step 716). Finally, in step 717, UE 701 performs RRC
connection setup procedure with eNB 703.
[0052] In one embodiment of enhanced cell selection, a UE
selectively applies cell prioritization only when UE has high
moving speed or high mobility. In general, a UE with high moving
speed should only re-connect to a coverage layer, which can be
indicated by eNB via broadcasting or unicasting. In one example, if
UE mobility state is higher than a threshold, then UE only
re-establishes to the coverage layer. The threshold can be signaled
by broadcasting or unicasting method. In another example, if UE
mobility state is high, then UE only re-establishes to the coverage
layer. This could be hard coded in specification where only cells
in certain frequency layers can be used as the cell selection
candidates.
[0053] For some deployments with different radio access
technologies (RATs), it is expected that a UE may move between RATs
frequently. For example, if LTE is deployed spotty, then it is
highly possible that the UE moves out the coverage of LTE and into
the vicinity of UTRA/GERAN networks. In one novel aspect, the UE
registers in both LTE and other RATs (UTRA, GERAN, or CDMA2000)
that provide mobility coverage. In addition, the UE receives
priority indication of the frequency layers or RATs to steer cell
selection for connection recovery.
[0054] FIG. 8 illustrates one embodiment of enhanced cell selection
with multi-RAT registration in a mobile communication network 800.
Mobile communication network 800 comprises UE 801, a first RAT1
802, a second RAT2 803, and a mobility management entity MME 804.
In step 811, UE 801 registers for RAT1 in NAS layer. In step 813,
UE 801 registers for RAT 2 in NAS layer. After registration, NAS
layer negotiation is completed (i.e., UE is attached), and the MME
information is stored in the eNB for both RATs (step 812 and step
814). In step 821, UE 801 establishes an RRC connection with RAT1.
Later on, in step 822, UE 801 detects RLF and performs RRC
reestablishment procedure with RAT2 (step 823). Because UE 801
already registered with RAT2, no additional NAS registration is
needed. The benefit is to avoid the long delay associated with
security setup from a home subscription server (HSS) when UE comes
from a detached state. If the RRC reestablishment fails, then UE
801 goes to RRC idle mode and starts NAS recovery in step 824. UE
801 again applies enhanced cell selection based on the cell
prioritization information (e.g., prioritized frequency layers
and/or RATs) (step 825). Instead of randomly selecting a cell, the
cell selection is steered based on the cell prioritization
information. Finally, in step 831, UE 801 performs RRC connection
setup procedure with RAT2.
[0055] FIG. 9 is a flow chart of a method of fast radio link
failure procedure in accordance with one novel aspect. In step 901,
a UE establishes an RRC connection with a serving eNB in a mobile
communication network. In step 902, the UE detects a radio link
problem of the RRC connection. The UE then starts a first timer
(e.g., T310). In step 903, the UE detects RLF and initiates an RRC
reestablishment procedure with a target cell before the first timer
expires. In step 904, the UE performs RRC reestablishment procedure
with the selected target cell. In one embodiment, a second timer
(e.g., T310a) is started when the UE sends a measurement report to
the eNB. RRC reestablishment is performed when the second timer
expires. In another embodiment, the UE initiates RRC
reestablishment before the first timer expires if a candidate cell
(e.g., a neighbor cell with better radio link quality than the
serving cell) is identified by the network.
[0056] FIG. 10 is a flow chart of a method of enhanced cell
selection with prioritization in accordance with one novel aspect.
In step 1001, a UE establishes an RRC connection with a serving eNB
in a mobile communication network. In step 1002, the UE selects a
target cell based on cell prioritization information transmitted
from the eNB. In step 1003, the UE initiates an RRC reestablishment
procedure upon detecting a radio link failure event. In step 1004,
the UE performs RRC connection setup with the selected target cell.
The priority for cell selection could be based on frequency layers
with good mobility coverage or based on intra-frequency cells with
good mobility coverage. In one embodiment, multi-RAT registration
is applied to steer cell selection. In another embodiment, the cell
prioritization is selectively applied to UEs with high mobility
state.
Enhanced Connection Recovery and Reduced Data Loss
[0057] When a target eNB receives RRC reestablishment message from
UE, the target eNB needs UE context information to restore the RRC
connection. UE context contains information, such as, UE capability
information and NAS configuration information. If the UE context
has been forwarded from the source eNB to the target eNB already,
then the RRC reestablishment is likely to be successful. Otherwise,
if the UE context is unavailable to the target eNB, then the legacy
RRC reestablishment will fail. Several methods are proposed to
enhance the legacy RRC reestablishment procedure from the network
perspective. In a first method, the target eNB tries to obtain the
UE context as quick as possible via a novel fast NAS recovery
procedure. In a second method of context fetching, the target eNB
tries to fetch/obtain the UE context from the source eNB via X2
interface. In a third method, a loss-less reestablishment procedure
is applied to reduce the data loss during recovery. Each method is
now described below with details.
[0058] FIG. 11 illustrates a first embodiment of an RRC
reestablishment procedure with fast NAS recovery in a mobile
communication network. The mobile communication network comprises a
UE, a source eNB1, a target eNB2, an MME, and an SGW. In step 1101,
the UE is connected to source eNB1. In step 1102, the UE detects
RLF and performs cell selection in step 1103. In step 1104, the UE
transmits an RRC reestablishment request message to target eNB2.
The RRC reestablishment may fail because target eNB2 may not have
UE context information. Accordingly, a "NAS service request" is
triggered by the RRC reestablishment request. Once eNB2 receives
the NAS service request, eNB2 inquires the UE context information
by forwarding the NAS service request to the MME via S1 interface
(step 1105). In step 1106, target eNB2 sends an RLF indication to
source eNB1. In step 1107, eNB2 sends an RRC reestablishment reject
message to the UE because eNB2 has not received the UE context
information yet. In step 1108, the UE goes to RRC idle mode and
starts NAS recovery. In step 1109, the UE performs RRC connection
setup with target eNB2. In step 1110, the UE exchanges RRC security
command with target eNB2. In step 1111, SRB1 and security is setup.
In step 1112, the UE performs RRC connection reconfiguration with
target eNB2, which sends an S1 RAB setup message to the MME to
establish EPS bearer via S1 interface. In step 1114, SRB2 and DRBS
are resumed. In steps 1115-1117, the MME and the SGW setup the UP
path.
[0059] In a legacy NAS recovery, the NAS service request is
triggered during the RRC connection setup procedure (e.g., step
1109), which is after the UE goes to RRC idle mode. In the
above-illustrated fast NAS recovery, the NAS service request is
triggered by the RRC reestablishment request (e.g., step 1104). In
this way, the target eNB can try to obtain the UE context as quick
as possible. Typically, it takes some time (e.g., several seconds)
for the target eNB to obtain the UE context from the MME via S1
interface. Therefore, by triggering the NAS service request
earlier, the UE outage time in the target cell is reduced. In an
optimized NAS recovery, if the RRC reestablishment is successful,
then the UE may not need to go to RRC idle mode.
[0060] FIG. 12 illustrates a second embodiment of an RRC
reestablishment procedure with fast NAS recovery in a mobile
communication network. The mobile communication network comprises a
UE, a source eNB1, a target eNB2, an MME, and an SGW. The RRC
reestablishment procedure (steps 1201 to 1217) are substantially
the same as the RRC reestablishment procedure (steps 1101 to 1117)
illustrated in FIG. 11. This second embodiment of fast NAS recovery
provides a small improvement to the first embodiment. In step 1207,
when eNB2 sends an RRC reestablishment reject message to the UE,
eNB2 autonomously provides uplink resource (e.g., an UL grant) to
the UE for the subsequent RRC connection setup procedure (step
1209). Typically, when the UE performs RRC connection setup, the UE
needs to initiate a radon access procedure via a random access
channel (RACH) for uplink resource. Note that, some additional time
is needed for contention resolution in RACH procedure. In the
example of FIG. 12, however, because the UE already receives the UL
grant contained in the RRC reestablishment reject message, the UE
does not need to initiate RACH for RRC connection setup with the
target eNB. As a result, the UE outage time in the target cell is
reduced.
[0061] FIG. 13 illustrates one embodiment of an RRC reestablishment
procedure with context fetching in a mobile communication network.
The mobile communication network comprises a UE, a source eNB1, a
target eNB2, an MME, and an SGW. In step 1301, the UE is connected
to source eNB1. In step 1302, the UE detects RLF and performs cell
selection in step 1303. In step 1304, the UE transmits an RRC
reestablishment request message to target eNB2. The RRC
reestablishment request message indicates to eNB2 that the UE is
from eNB1, and the UE would like to connect to eNB2 due to poor
radio link quality. However, the RRC reestablishment may fail
because eNB2 may not have UE context information. Accordingly, in
step 1305, eNB2 uses an RLF indication as a request for the UE
context from eNB1 via X2 interface. Typically, the RLF indication
only indicates to eNB1 for the purpose of self-organization network
(SON) feature. In one novel aspect, the RLF indication also
comprises a request for the UE context. For example, a new
information element (IE) may be used, which requires mandatory
response. Once eNB1 receives the RLF indication with UE context
request, eNB1 sends the UE context information back to eNB2 during
subsequent HO request and response exchanged in step 1306.
[0062] At the core network side, in step 1311, eNB2 sends a path
switch request to the MME via S1 interface, and the MME sends a
modify bearer request to the SGW in step 1312. In step 1313, the
SGW switches the DL path. In step 1314, the SGW sends a modify
bearer response back to the MME, and the MME sends a path switch
acknowledgment back to eNB2 via S1 interface in step 1315. At the
radio access side, in step 1321, eNB2 sends an RRC reestablishment
response message to the UE after successfully receiving the UE
context information from eNB1. In step 1322, SRB1 and security is
resumed. In step 1323, RRC connection reconfiguration is performed
between the UE and target eNB2. In step 1324, SRB2 and DRBS are
resumed. In the above-illustrated RRC reestablishment procedure,
because target eNB2 tries to obtain the UE context from source eNB1
via X2 interface (instead of obtaining the UE context from the MME
via S1 interface), and because X2 interface is typically much
faster than S1 interface, target eNB2 is able to successfully
complete the RRC reestablishment procedure. As a result, the UE
outage time in the target cell is reduced.
[0063] In addition to reducing outage time, reducing data loss is
also important for the connection recovery process. Typically, to
avoid packet data loss, packet data convergence protocol (PDCP)
operation needs to be reestablished and resumed without
interruption. For HO operation as defined in current LTE
specification, the source eNB forwards the PDCP serial number (SN)
report and data to the target eNB. The PDCP operation can be
resumed when handover procedure is done. For RRC reestablishment,
PDCP layer can be reestablished after successful RRC
reestablishment. However, if the target cell for reestablishment
has no prior PDCP status report, then PDCP operation cannot be
resumed without interruption, i.e., data loss occurs during
recovery. This scenario could happen when RLF occurs abruptly and
UE selects to a cell other than the previous serving cell.
[0064] FIG. 14 illustrates one embodiment of a loss-less RRC
reestablishment procedure in a mobile communication network. The
mobile communication network comprises a UE, a source eNB1, a
target eNB2, an MME, and an SGW. In step 1411, the UE sends a
measurement report to source eNB1 triggered by a certain event. In
step 1412, eNB1 makes mobility decision based on the measurement
report. For example, eNB1 may decide to handover the UE to target
eNB2. In step 1413, eNB1 and eNB2 exchange HO request and response
with UE context information. In step 1414, the UE detects RLF and
performs cell selection in step 1415. In step 1416, the UE sends an
RRC reestablishment request to target eNB2. The RRC reestablishment
will be successful because eNB2 already receives the UE context
information. In step 1421, eNB2 sends an RLF indication to eNB1 via
X2 interface. To prevent data loss, the RLF indication comprises a
data-forwarding request for PDCP status and for U-plane data. In
step 1422, eNB1 sends a PDCP SN status transfer to eNB2.
Optionally, in step 1423, eNB1 also sends U-plane data forwarding
to eNB2.
[0065] At the core network side, in step 1431, eNB2 sends a path
switch request to the MME via S1 interface, and the MME sends a
modify bearer request to the SGW in step 1432. In step 1433, the
SGW switches the DL path. In step 1434, the SGW sends a modify
bearer response back to the MME, and the MME sends a path switch
acknowledgment back to eNB2 via S1 interface in step 1435. At the
radio access side, in step 1424, eNB2 sends an RRC reestablishment
response message to the UE. In step 1425, SRB1 and security is
resumed. In step 1426, RRC connection reconfiguration is performed
between the UE and target eNB2. In step 1427, SRB2 and DRBS are
resumed.
[0066] In the above-illustrated example, X2 interface is used to
trigger PDCP status transfer and data forwarding between the old
and the new serving cell. The proposed PDCP/data forwarding can be
combined with context fetching as illustrated in FIG. 13. For
example, when source eNB1 receives the RLF indication from target
eNB2, the RLF indication may comprises both a UE context request
and a data-forwarding request. Upon receiving the RLF indication,
eNB1 sends a response message that includes the following
information: UE context information, PDCP SN status, and forwarded
U-plane data. Note that, the above-mentioned enhancements by using
RLF indication over X2 interface is one of the possible
embodiments. In other embodiments, a new defined X2 message for UE
context request or PDCP SN request can be used.
[0067] FIG. 15 is a flow chart of a method of fast NAS recovery in
target cell from UE perspective. In step 1501, a UE establishes an
RRC connection with a serving base station in a mobile
communication network. In step 1502, the UE detects RLF of the RRC
connection. In step 1503, the UE transmits an RRC reestablishment
request message to a target base station. The RRC reestablishment
request indicates to the target base station that the UE is from a
serving base station, and the UE would like to connect to the
target base station due to poor radio link quality in the serving
cell. In addition, the RRC reestablishment request message
comprises a NAS service request. The target base station inquires
UE context from an MME upon receiving the NAS service request. In
step 1504, the UE performs RRC connection reconfiguration with the
target base station. Because the NAS service request is triggered
by the RRC reestablishment, the target base station can obtain UE
context quicker. Such fast NAS recovery thus reduces outage time in
the target cell.
[0068] FIG. 16 is a flow chart of a method of fast NAS recovery in
target cell from eNB perspective. In step 1601, a target base
station receives an RRC reestablishment request message from a UE
in a mobile communication network. The RRC reestablishment request
indicates to the target base station that the UE is from a serving
base station, and the UE would like to connect to the target base
station due to poor radio link quality in the serving cell. The RRC
reestablishment request message also comprises a NAS service
request. In step 1602, the target base station inquires UE context
from an MME via S1 interface upon receiving the NAS service
request. In step 1603, the target base station transmits an RLF
indication to the serving base station via X2 interface. Finally,
in step 1604, the target base station performs an RRC connection
reconfiguration with the UE and S1 RAB setup with the MME. Because
the NAS service request is triggered by the RRC reestablishment,
the target base station can obtain UE context quicker. Such fast
NAS recovery thus reduces outage time in the target cell.
[0069] FIG. 17 is a flow chart of a method of RRC reestablishment
procedure with context fetching. In step 1701, a target base
station receives an RRC reestablishment request message from a UE
in a mobile communication network. The RRC reestablishment request
indicates to the target base station that the UE is from a serving
base station, and the UE would like to connect to the target base
station. In step 1702, the target base station transmits an RLF
indication to the serving base station. The RLF indication forwards
the reestablishment request, and contains a UE context request. In
step 1703, the target base station receives an X2 message (e.g., HO
request message) from the serving base station. The HO request
message contains the UE context information. In step 1704, the
target base station transmits an RRC reestablishment response to
the UE. The RRC reestablishment is successful because the target
base station already has UE context. Such context fetching method
by the target base station thus reduces outage time in the target
cell.
[0070] FIG. 18 is a flow chart of a method of performing a
loss-less RRC reestablishment procedure. In step 1801, a target
base station receives an RRC reestablishment request message from a
UE in a mobile communication network. The RRC reestablishment
request indicates to the target base station that the UE is from a
serving base station, and the UE would like to connect to the
target base station. In step 1802, the target base station
transmits an RLF indication to the serving base station via X2
interface. The RLF indication comprises a data-forwarding request.
In step 1803, the target base station receives a PDCP SN status
transfer from the serving base station. The target base station may
also receive U-plane data forwarded from the serving base station.
In step 1804, the target base station transmits an RRC
reestablishment response to the UE and successfully completes the
connection recovery. Because the target base station has prior PDCP
status from the serving base station, the PDCP operation can be
resumed without data loss.
[0071] Although the present invention has been described in
connection with certain specific embodiments for instructional
purposes, the present invention is not limited thereto.
Accordingly, various modifications, adaptations, and combinations
of various features of the described embodiments can be practiced
without departing from the scope of the invention as set forth in
the claims.
* * * * *