COMMUNICATION SETTING METHOD, WIRELESS BASE STATION, AND MOBILE
STATION
FIELD
[0001] The present invention relates to a technique for communicating to mobile
stations by multiple wireless base stations while coordinating among the wireless base
stations in a mobile communication system.
BACKGROUND
[0002] The technique for cellar-type mobile communication systems has been shifted
from the Universal mobile telecommunication system (UMTS) to the long term evolution
(LTE). The LTE adopts the orthogonal frequency division multiplexing (OFDM) and single
carrier-frequency division multiple access (SC-FDMA) techniques for downlink and uplink
wireless accesses, respectively, achieving high-speed wireless packet communications
with a downlink peak transmission rate of 100 Mb/s or higher and an uplink peak
transmission rate of 50 Mb/s or higher. The 3rd Generation Partnership Project (3GPP), an
international standardization organization, has initiated a study on the mobile
communication system LTE-A (LTE-Advanced) based on the LTE, for achieving further
faster communications. The LTE-A is aiming at a downlink peak transmission rate of 1
Gb/s and an uplink peak transmission rate of 500 Mb/s, and a various new techniques have
been reviewed for a radio access scheme, a network architecture, and the like (Non-patent
Reference 1).
[0003] The 3GPP is currently studying the coordinated multi-point (CoMP)
communication scheme for the LTE-A. For downlink CoMP communications, techniques,
such as a joint transmission, a coordinated beamforming, a coordinated scheduling, and a
fast cell selection, have been considered.
Hereinafter, the basic concepts of the joint transmission, the coordinated
beamforming, and the coordinated scheduling will be described with reference to FIG. 1. In
the following description, the term "cell" is used to mean each geological range where a
wireless base station provides a wireless service, and it may also be used to mean a part of
the communication function managed by the wireless base station for communicating with
user equipments in the each geological range. In FIG. 1, a serving cell is a sell that
exchanges control information with a mobile station UE. A coordinated cell is a cell that
communicates with a mobile station UE, together with the serving cell, using the CoMP
communication technique (i.e., provides coordinated communications). Although
coordinated transmissions are provided by the two cells in the example in FIG. 1, generally
there may be three or more coordinated cells.
[0004] FIG. 1 (a) schematically depicts the joint transmission by two cells, as one
example.
In the joint transmission, identical data is sent to a single user equipment UE
simultaneously from multiple cells using the same radio resource. In the example in FIG. 1
(a), the mobile station UE measures the characteristics of the downlink propagation paths
from the serving and coordinated cells, and reports the measurement results to the serving
cell. The serving cell and the coordinated cell execute precoding and scheduling based on
information that is fed back to them, and cooperatively send the identical data to the mobile
station UE. Since the mobile station UE receives a synthetic signal from signals from the
serving and coordinated cells, the reception SINR is improved as compared to the case
where the signal is received only from the serving cell. The transmission data, propagation
path information, scheduling information, precoding setting information, and the like are
shared among the cells that cooperate with each other. As an interface among base
stations for sharing the information, the LTE stipulates the X2 interface (Non-patent
Reference 2).
[0005] FIG. 1 (b) schematically depicts the coordinated beamforming and the
coordinated scheduling by two cells, as one example.
In the coordinated beamforming and the coordinated scheduling, data is sent to
different mobile stations (user equipments UE1 and UE2 in the example in FIG. 1 (b)) in
coordinated cells coordinated, and precoding or scheduling is cooperatively controlled.
Precoding is predominantly cooperatively controlled in cooperating cells in the coordinated
beamforming, while scheduling is predominantly cooperatively controlled in cooperating
cells in the coordinated scheduling. Unlike the joint transmission, transmission data may
not be shared among cooperating cells in the coordinated beamforming and the
coordinated scheduling. In the example in FIG. 1 (b), a serving cell sends data to the user
equipment UE1, and the coordinated cell sends the data to the user equipment UE2. Here,
precoding or scheduling is cooperatively controlled among the cells such that the
interference waves from the serving cell to the user equipment UE2 and the interference
waves from the coordinated cell to the user equipment UE1 are reduced. Hence,
communications in the coordinated beamforming and the coordinated scheduling is
improved compared to the cases where each cell communicates with the user equipment
without any cooperation. In the coordinated beamforming and the coordinated scheduling,
propagation path information, scheduling information, and the like are shared among
cooperating cells.
PRIOR ART REFERENCES
NON-PATENT REFERENCE
[0006] Non-patent Reference 1: 3GPP TR 36.912 v9.1.0
Non-patent Reference 2: 3GPP TS 36.423 v9.1.0
DISCLOSURE OF THE INVENTION
Problems to be Solved by the Invention
[0007] In the mean times, when a user equipment under coordinated communications
with multiple cells undergoes a handover, different cells may provide coordinated
communications to the user equipment before and after the handover and the coordinated
communication is temporarily suspended before the handover. In this case, in order to
suppress any reduction in the communication quality or throughput, an earlier resume of
the coordinated communication is more desirable.
[0008] Accordingly, an object of an aspect of the invention is to provide a
communication setting method, a wireless base station, and a mobile station that enable to
resume a coordinated communication after a handover of a mobile station, when a
coordinated communication is made by multiple wireless base stations to the mobile
station.
SUMMARY
[0009] In the first aspect, a method of communication setting for providing coordinated
communications by a plurality of wireless base stations to a mobile station, the plurality of
wireless base stations providing wireless services, is provided.
This method of communication setting includes:
(A) measuring, by a first mobile station connected to a first wireless base station, a
reception quality of a reference signal from the plurality of wireless base stations including
the first wireless base station and a second wireless base station adjacent to the first
wireless base station, and notifying the first wireless base station of the reception quality;
and
(B) notifying the second wireless base station by the first wireless base station, of
the notified reception quality, or information about a candidate wireless base station that is
a candidate of a wireless base station for the coordinated communications identified based
on the notified reception quality, as first information for providing the coordinated
communications to the first mobile station by the plurality of wireless base stations including
the second wireless base station.
[0010] In a second aspect, a wireless base station as a first wireless base station for
providing coordinated communications with other wireless base station as a second
wireless base station to a mobile station is provided.
This wireless base station includes:
(C) a first transceiver that connects to the mobile station and sends and receives a
signal; and
(D) a second transceiver that sends and receives a signal to and from the other
wireless base station.
The first transceiver receives, from a first mobile station connected to the first
wireless base station, a notification of measurement results of reception quality of reference
signals from a plurality of wireless base stations including the first wireless base station and
the second wireless base station adjacent to the first wireless base station, in the first
mobile station. The second transceiver notifies the second wireless base station, of the
notified reception quality, or information about a candidate wireless base station that is a
candidate of a wireless base station for the coordinated communications identified based
on the notified reception quality, as first information for providing the coordinated
communications to the first mobile station by the plurality of wireless base stations including
the second wireless base station.
[0011] In a third aspect, a wireless base station a second wireless base station for
providing wireless services and coordinated communications with other wireless base
station as a first wireless base station to a mobile station is provided.
The second wireless base station includes:
(E) a first transceiver that connects to the mobile station and sends and receives a
signal; and
(F) a second transceiver that sends and receives a signal to and from the first
wireless base station.
The second transceiver receives from the first base station, as first information for
executing coordinated communications among a plurality of wireless base stations
including the second wireless base station for a first mobile station connected to the first
wireless base station adjacent to the second wireless base station, a reception quality of
reference signals from the plurality of wireless base stations including the second wireless
base station and the first wireless base station in the first mobile station, or a notification of
information about a candidate wireless base station that is a wireless base station for a
candidate of the coordinated communications.
[0012] In a fourth aspect, a mobile station that is provided with coordinated
communications by a plurality of wireless base stations providing wireless services is
provided.
The mobile station includes:
(G) a third transceiver that sends and receives a signal to and from a wireless base
station; and
(H) a quality measurement section that measures a reception quality of reference
signals from the plurality of wireless base stations including a first wireless base station and
a second wireless base station adjacent to the first wireless base station.
The third transceiver notifies the first wireless base station of the measured
reception quality. Thereby enabling the first wireless base station to notify of the second
wireless base station, as first information for executing coordinated communications among
the plurality of wireless base stations including the second wireless base station to the
mobile station, the notified reception quality, or information about a candidate wireless base
station that is a candidate of a wireless base station for the candidate coordinated
communications identified by the first wireless base station based on the notified reception
quality.
Effect of the Invention
[0013] In accordance with the communication setting method, the wireless base station,
and the mobile station disclosed, a coordinated communication after a handover of a
mobile station can be resumed earlier, when a coordinated communication is made by
multiple wireless base stations to the mobile station.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] FIG. 1 is a diagram illustrating the basic concept of the CoMP communication
scheme;
FIG. 2 is a diagram illustrating a handover of user equipments in a mobile
communication system where multiple cells are selected as coordinated cell candidates for
a user equipment;
FIG. 3 is a diagram illustrating a handover of a mobile station in a mobile
communication system where multiple cells are selected as coordinated cell candidates for
the mobile station;
FIG. 4 is a diagram illustrating an example of a series of sequences of a CoMP
setting procedure before and after a handover for a mobile station;
FIG. 5 is a diagram illustrating an example of a series of sequences of a CoMP
setting procedure before and after a handover for a mobile station in a first embodiment;
FIG. 6 is a block diagram illustrating of a schematic configuration of an evolutional
Node B in the first embodiment;
FIG. 7 is a block diagram illustrating of a schematic configuration of a mobile station
in the first embodiment;
FIG. 8 is a diagram illustrating an example of a series of sequences of a CoMP
setting procedure before and after a handover for a mobile station in a second embodiment;
and
FIG. 9 is a diagram illustrating an example of a series of sequences of a CoMP
setting procedure before and after a handover for a mobile station in a third embodiment.
DESCRIPTION OF EMBODIMENTS
[0015] In the following description, evolutional Node Bs as wireless base stations may
be abbreviated as "eNBs", while user equipments as mobile stations may be abbreviated
as "UEs", where necessary. One evolutional Node B manages one or more cells. The
term "cell" is used to mean each geological range where a wireless evolutional Node B
provides a wireless service, and it may also be used to mean a part of the communication
function managed by the wireless evolutional Node B for communicating with user
equipments in the each geological range. The term "coordinated cell" is used to mean a'
cell where communications are made to user equipments by cooperating with the serving
cell of the user equipments, and the term "cooperating eNB" is used to mean a wireless
evolutional Node B that manages a coordinated cell and makes coordinated
communications with user equipments by cooperating with a wireless evolutional Node B
managing a serving cell.
[0016] Here, before describing embodiments, the method of selecting multiple cells for
coordinated communications in the cells for user equipments, for facilitating understanding
of the embodiments.
More specifically, coordinated communications for all cells in a mobile
communication system are not realistic since the processing for measuring the
characteristics of the downlink propagation path in a user equipment and the amount of
data for feed backing the measured values to the wireless evolutional Node B become tob
extensive. Hence, relatively a small number of cells are selected as cell candidates
(hereinafter, referred to as "coordinated cell candidates") for coordinated communications.
In the following, techniques of selecting coordinated cell candidates will be discussed,
where a user equipment uses the reference signal received powers (RSRPs) in
surrounding cells for reporting to a wireless evolutional Node B, for example.
[0017] In a first technique of selecting coordinated cell candidates, when the RSRP
value Pj, as a reception quality of a cell / of surrounding cells of a user equipment satisfies
the following Eq. (1), the cell / is selected as a coordinated cell candidate. Alternatively, a
certain number of cells in the descending order of Pj / Po may be selected among
surrounding cells that satisfy Eq. (1). At this time, a user equipment that measures the
reception SINR of a downlink signal in that user equipment smaller than a predetermined
value is selected as target for coordinated communications, for example. Such a user
equipment is typically a user equipment located in a periphery of a cell. This technique is
based on the idea that other cells sending signals that is received by a user equipment at a
higher level relative to the level of received signals from the serving cell may become
interference sources with higher level when the other cells do not participate in the
coordinated communications. In other words, selecting these cells (cells other than the
serving cells) as coordinated cells can significantly reduce the interference.
[0018] Eq. (1)

[0019] In Eq. (1):
Po is the RSRP value relative to the reference signal from the serving cell,
Pi is the RSRP value relative to the reference signal from a surrounding cell /, and
Y is a predetermined threshold.
[0020] In a second technique of selecting coordinated cell candidates, when the
estimation of reception SINR as a reception quality in a user equipment when a certain
surrounding cell / is added to coordinated cells becomes better, by predetermined value,
than the reception SINR in the user equipment when the cell / is not added to the
coordinated cells, as indicated in Eq. (2), the cell i is selected as the coordinated cells of
coordinated communications for the user equipment. SINRi.comp and SINRi.non-comp in Eq.
(2) are expressed in Eqs. (3) and (4), respectively. In Eqs. (3) and (4), N is the reception
noise power in the user equipment.
[0021] Eq. (2)

[0022] Next, with reference to FIGS. 2 to 4, a scenario will be discussed where a user
equipment undergoes a handover in a mobile communication system multiple cells are
selected as coordinated cell candidates for the user equipment.
[0023] FIG. 2 depicts an example where a common coordinated communication mode
(one of the CoMP communication technique described above) is used before and after the
handover of the user equipment UE. FIG. 2 (a) depicts a situation where a serving cell
Cell_1 provides coordinated transmissions, with joint transmissions, to a user equipment
UE1 connected to the serving cell CelM, by using a cell Cell_2 selected as a coordinated
cell, among surrounding coordinated cell candidates Cell_2, Cell_6, and Cell_9 before a
handover. FIG. 2 (b) depicts a situation where the serving cell CelM provides coordinated
transmissions, with joint transmissions, to the user equipment UE1 connected to the
serving cell Cell_6, by using a cell Cell_5 selected as a coordinated cell, among
surrounding coordinated cell candidates CelM, Cell_5, and Cell_7 after the handover. In
FIG. 2, the areas of the coordinated cell candidates are enclosed with thick lines.
[0024] FIG. 3 depicts an example where different coordinated communication modes
(one of the CoMP communication technique described above) are used before and after
the handover of the user equipment UE. FIG. 3 (a) depicts a situation where a serving cell
CelM provides coordinated transmissions, with joint transmissions, to a user equipment
UE1 connected to the serving cell CelM, by using a cell Cell_2 selected as a coordinated
cell, among surrounding coordinated cell candidates Cell_2, Cell_6, and Cell_9 before a
handover. FIG. 3 (b) depicts a situation where cells Cell_6 and CelM provide coordinated
transmissions, with the coordinated beamforming or the coordinated scheduling, to user
equipments UE1 and UE2, respectively, after the handover of the user equipments UE1.
Also in FIG. 3, the areas of the coordinated cell candidates are enclosed with thick lines.
[0025] In order to resume coordinated transmissions for a user equipment after the
handover of the user equipment, setting processing that may include the following
processing (i) to (vi) (hereinafter, referred to as "CoMP setting procedure") may be
executed.
(i) a wireless evolutional Node B (wireless evolutional Node B that manages the
serving cell after the handover) requests signal measurements (e.g., the RSRPs) in
surrounding cells of the user equipment, to that user equipment;
(ii) the user equipment notifies the wireless evolutional Node B of the signal
measurement results or coordinated cell candidates;
(iii) when the user equipment notifies no coordinated cell candidate in (ii), the
wireless evolutional Node B determines coordinated cell candidates and notifies the user
equipment of them;
(iv) the wireless evolutional Node B requests the user equipment to measure
propagation path information (e.g., the channel status information (CSI)) between the user
equipment and each of the coordinated cell candidates;
(v) the user equipment measures the propagation path information with the
coordinated cell candidates, and notifies the wireless evolutional Node B of the
measurement results; and
(vi) the wireless evolutional Node B determines a coordinated cell, and exchanges,
with coordinated cell, information for executing coordinated transmissions to the user
equipment.
[0026] Here, an example of a series of sequences of the CoMP setting procedure
before and after the handover (HO) of the user equipment UE is depicted in FIG. 4. In the
sequence depicted in FIG. 4, as one example, a user equipment UE undergoes a handover
when data is sent to that user equipment UE in the CoMP communication technique by
means of the joint transmission. More specifically, before the handover of the user
equipment UE, a source evolutional Node B (source eNB) and a cooperating evolutional
Node B (cooperating eNB) sends data to the user equipment UE; after the handover, a
target evolutional Node B (target eNB, the wireless evolutional Node B that manages a new
serving cell) and the cooperating evolutional Node B (the cooperating eNB) send data to "
the user equipment UE. Here, the term "cooperating evolutional Node B" (cooperating
eNB) means a wireless evolutional Node B that manages a coordinated cell, and provides
coordinated communications among cells to the user equipment while cooperating with a
wireless evolutional Node B that manages the serving cell.
[0027] In this sequence, before the handover (HO), after executing the CoMP setting
procedure (Steps S 10 to S 16), the source evolutional Node B (source eNB) and its
cooperating evolutional Node B (cooperating eNB) executes coordinated communications
(coordinated transmissions in this case) to the user equipment UE (Step S 18). In
response to the handover of the user equipment UE being detected (Step S 20), a
coordinated communication that is being executed is terminated (Step S 21) and a series of
handover procedures is executed (Steps S 20 to S 28). Once the handover procedure is
completed, data is sent to the user equipment UE via the target evolutional Node B (target
eNB)(Step S 30). Thereafter, the target eNB executes the CoMP setting procedure from
the beginning (Step S 40 and the subsequent steps).
In the sequence depicted in FIG. 4, no CoMP setting procedure is executed during
the handover procedure, the target eNB executes the CoMP setting procedure from the
beginning, after the handover. That is, no CoMP setting procedure is executed during the
handover procedure, and accordingly, start of coordinated communication is delayed after
the handover, which may cause reduced communication qualities or throughput.
[0028] (1) First Embodiment
Hereinafter, a first embodiment will be described.
[0029] (1-1) Summary of CoMP Setting Procedure in Present Embodiment
First, a summary of the CoMP setting procedure in a mobile communication system
of the present embodiment will be described.
In the mobile communication system of the present embodiment, as depicted in
FIG. 1, communications are made for a user equipment while multiple cells cooperate with
each other. For example, coordinated transmissions are executed where multiple wireless
evolutional Node Bs (hereinafter, abbreviated as "eNBs") cooperate with each other for
transmissions to a user equipment. When a user equipment undergoes a handover while a
coordinated transmission is being executed to user equipment, the cell coordinating to the
user equipment may be updated and the coordinated transmission before the handover is
suspended. When the CoMP setting procedure is executed from its beginning after
completion of the handover, however, resume of the coordinated transmission may be
delayed. Thus, in the present embodiment, the CoMP setting procedure is executed
during the handover procedure in a duplicated manner. In other words, in the present
embodiment, when a handover of a user equipment is detected during a coordinated
transmission to that user equipment, at least a part of the CoMP setting procedure that may
include the processing (i) to (vi) described above is executed before completion of the
handover, in order to resume the coordinated transmission for the user equipment earlier
after the handover.
[0030] Before the handover, a user equipment of the present embodiment periodically
receives reference signals from surrounding evolutional Node Bs to obtain the results of
signal measurements (e.g., the RSRPs), and notifies the evolutional Node B (i.e., the
source evolutional Node B (source eNB) to be handed over) of the results. In response to
detecting a handover of the user equipment, the source evolutional Node B (first wireless
base station) immediately notifies a target evolutional Node B (second wireless base
station) of first information for executing coordinated communications, through a
communication link between the evolutional Node Bs. The first information includes results
of signal measurements from the surrounding cells which is notified from user equipment
when the handover is detected, or coordinated cell candidates identified based on the
results. The techniques described above, i.e., the techniques indicated using Eq. (1) or Eq.
(2), may be applied as a technique of identifying coordinated cell candidates, for example.
[0031] In the sequence depicted in FIG. 4, in order to execute the CoMP setting
procedure after completion of the handover procedure, coordinated cell candidates are
identified by an evolutional Node B that is a target of the handover. In contrast, in the
present embodiment, before completion of the handover procedure, the serving cell of user
equipment after the handover (hereinafter, referred to as "target cell") and its coordinated'
cell may be identified in the source evolutional Node B. In this identification technique, if
the technique represented by Eq. (1) described above, for example, Po may be calculated
as the RSRP value relative to the reference signal from the target cell. More specifically, at
the time when a handover of the user equipment is detected, the target cell is included in
the surrounding cells, of which signal measurement results are notified from the user
equipment to the source evolutional Node B. Hence, cell candidates of a coordinated cell
of a target cell after the handover (hereinafter, referred to as "coordinated cell candidates")
can be identified by the source evolutional Node B.
[0032] Further, in the present embodiment, preferably, the processing (iii) described
above is executed during the CoMP setting procedure, before completion of the handover
procedure of the user equipment. More preferably, the processing (iii) and (iv) described
above is executed during the CoMP setting procedure, before completion of the handover
procedure of the user equipment. Further preferably, the processing (iii) to (v) described
above is executed during the CoMP setting procedure, before completion of the handover
procedure of the user equipment.
[0033] (1-2) Specific CoMP Setting Procedure
Next, the CoMP setting procedure before and after a handover of a user equipment
in the mobile communication system of a first embodiment will be described with reference
to FIG. 5. FIG. 5 is a diagram illustrating an example of a series of sequences of the CoMP
setting procedures (communication setting method) before and after a handover of a user
equipment in the first embodiment. Hereinafter, an example where the technique
represented by Eq. (1) described above is used as the technique of identifying coordinated
cell candidates will be described, as one example.
[0034] In FIG. 5, Steps S 10 to S 18 represent the CoMP setting procedure in a user
equipment UE before a handover. Firstly, the source evolutional Node B (source eNB)
sends a surrounding cell measurement request message to a user equipment UE. In
response to that request, the user equipment UE measures the RSRPs of reference signal
from surrounding cells, and notifies the source evolutional Node B of the measurement
results (Step S 10). Note that the measurement results are also used for determination by
source evolutional Node B as of whether the user equipment UE has been handed over
(i.e., detection of the handover).
[0035] The source evolutional Node B identifies the serving cell of the user equipment
UE and coordinated cell candidates, based on the notified measurement result, with the
technique represented by Eq. (1) described above. The source evolutional Node B sends
a coordinated communication request message to the cooperating evolutional Node Bs
(cooperating eNBs) cooperating with the source evolutional Node B, and receives
responses for the request from the respective evolutional Node Bs (Step S 12).
Thereafter, the source evolutional Node B sends a CSI request message requesting to
measure the CSI, as propagation path information between each of the coordinated cell
candidates and the user equipment UE, to the user equipment UE, and receives
measurement results from the user equipment UE in a CSI report message (Step S 14).
The source evolutional Node B can identify a serving cell and a coordinated cell among the
coordinated cell candidates, based on the CSI measurement results with the respective
coordinated cell candidates, which are notified from the user equipment UE.
[0036] One technique of identifying a coordinated cell from coordinated cell candidates
is described, for example, in the following Non-patent Reference, namely, "3GPP TSG RAN
WG1 Meeting #57, R1-092160, San Francisco, USA 4-8 May 2009, Title: DL non-coherent
multi-user MIMO joint transmission (MU-MIMO JT) scheme and system performance
evaluations in TDD systems" which is incorporated herein by a reference. Hereinafter, the
technique described in the reference is summarized.
As one example, when Hn, H12, and H13 are the CSIs of downlink signals from the
serving cell and two coordinated cell candidate to the user equipment UE, respectively, and
W11, W12, and W13 are the precoding matrixes set in accordance with Hn, Hi2, and H13, the
CSI under coordinated communications among the three cells, H0, is represented by the
following Eq. (5). Here, when the condition is satisfied in that the communication capacity
calculated based on H0 is three or more times higher than the communication capacity
calculated by the CSI of the serving cell (Hn), it is preferable to provide coordinated
transmissions by the three cells. Hence, for example, it is possible to identify two cells that
satisfy the above condition from three or more coordinated cell candidate.
[0037] Eq. (5)
H0=H11W11+H12W12+H13W13 ...(5)
[0038] Once the coordinated cell is identified, the source evolutional Node B sends
CoMP information to the cooperating evolutional Node B through a communication link
between the evolutional Node B, such as the one embodied by the X2 interface, for
example, and receives an acknowledgement signal for that transmission (Steps 16). The
contents of the CoMP information may vary depending on the particular CoMP
communication technique. In the case where the CoMP communication technique is the
joint transmissions, for example, transmission data, propagation path information,
scheduling information, precoding setting information, and the like are included.
Thereafter, the source evolutional Node B and its cooperating evolutional Node B executes
coordinated transmissions of data signals (data) to the user equipment UE (Step S 18).
[0039] Then, in response to detecting a handover based on measurement results of the
RSRPs of the reference signals from surrounding cells by the user equipment UE, the
source evolutional Node B sends an HO request message to the evolutional Node B that is
target of the handover (target eNB), and receives an acknowledgement signal (HO request
Ack) for the request (Step S 20). Here, the HO request message is generated so as to
include, as first information, the surrounding cell measurement results from the user
equipment UE obtained by the source evolutional Node B in Step S 10 (the latest results at
the time of the detection of the handover), or information about coordinated cell candidates
identified using the surrounding cell measurement results. This helps to accelerates
processing of the CoMP setting procedure in the target evolutional Node B.
[0040] The source evolutional Node B also requests (CSI request) the user equipment
UE to measures the CSI as propagation path information between the respective
coordinated cell candidates and the user equipment UE, when surrounding cells of the user
equipment UE or coordinated cell candidates have been already identified. This request
is included in a Radio Resource Control (RRC) reconfiguration message (RRC reconfig)
sent in Step S 22. Note that, in the present embodiment, the Information Element (IE)
included in the RRC reconfiguration message is extended from the standards of the LTE,
for example, with addition of the CSI request. In this manner, by sending a CSI request by
the source evolutional Node B to the user equipment UE, in place of the target evolutional
Node B, communication processing between the source evolutional Node B and the target
evolutional Node B can be simplified and the target evolutional Node B can obtain CSI
measurement results earlier from the user equipment UE. This helps to accelerate the
CoMP setting procedure.
[0041] In Step S 24, the user equipment UE establishes communications with the target
evolutional Node B using an uplink random access channel (RACH). For example, in
response to the HO request message, the target evolutional Node B allocates an access
slot of a dedicated RACH to the user equipment UE, and notifies the user equipment UE of
the allocated access slot. When the link with the source evolutional Node B is
disconnected, the user equipment UE sends the dedicated RACH using the allocated
access slot, and executes processing for establishing a communication link with the target
evolutional Node B
[0042] Once a communication with the target evolutional Node B is established, the
user equipment UE sends an RRC reconfiguration complete message ("RRC reconfig
complete") to the target evolutional Node B (Step S 26). At this time, the user equipment
UE has measured the CSI with the coordinated cell candidates in response to the CSI
request received in Step S 22, the user equipment UE includes the measurement results
(CSI report) in the RRC reconfiguration complete message. Note that, in the present
embodiment, the IE included in the RRC reconfiguration complete message is extended
from the standards of the LTE, for example, with addition of the CSI report. In the present
embodiment, the target evolutional Node B is able to obtain the CSI report earlier, and
accordingly, the target evolutional Node B can identify a coordinated cell swiftly.
Thereafter, a UE context release message (UE Context Release) is sent from the target
evolutional Node B to the source evolutional Node B (Step S 28), and the handover
procedure is completed.
[0043] At the time when the handover procedure is completed, a coordinated cell has
been already identified by the target evolutional Node B, in the mobile communication
system of the present embodiment. Hence, the target evolutional Node B can immediately
send CoMP information to a cooperating evolutional Node B, and obtain acknowledgement
signal for the transmission (Step S 30). Thereafter, the source evolutional Node B and the
cooperating evolutional Node B executes coordinated transmissions of data signals (data)
to the user equipment UE (Step S 32). After coordinated transmissions for the handover
are started, in the similar manner to Step S 10, for further handover of the user equipment
UE and/or update of coordinated cells, the source evolutional Node B periodically sends a
surrounding cell measurement request message to a user equipment UE (Step S 40).
[0044] (1-3) Configurations of Evolutional Node B and User Equipment
Next, examples of the configurations of the evolutional Node B and the user
equipment for achieving the CoMP setting procedure depicted in FIG. 5 will be described
with reference to FIGS. 6 and 7. The configurations depicted in FIGS. 6 and 7 are
compliant with the LTE communication standards, more specifically, downlink
communications adopt the OFDM while uplink communications adopt the SC-FDMA.
[0045] Firstly, with reference to FIG. 6, an evolutional Node B eNB of the present
embodiment includes a receiver 11 as a first transceiver, an FFT unit 12, a demodulator 13,
a decoder 14, a L2 processor 15, an X2 interface 16 as a second transceiver, an L2
processor 17, an encoder 18, a modulator 19, a precoder20, a multiplexer 21, an IFFT unit
22, a transmitter 23 as the first transceiver, and a radio resource controller 24.
[0046] The L2 processor 17 executes protocol processing (signal conversion
processing) in Layer 2 (L2) on packets to be sent from upper layers. More specifically, the
L2 processor 17 executes processing in two steps, namely the sublayers of the Radio Link
Control (RLC) and the Medium Access Control (MAC). In the processing on the RLC
sublayer, the packets to be sent are divided or combined to form a processing length
suitable for retransmission control and order control processing, and a RLC-protocol data
unit (PDU) is generated where information required for processing such as flow control
function, protocol error detection and recovery, and the like. In the processing on the MAC
sublayer, multiplexing or demultiplexing a logical channel, relations between the logical
channel and a transport channel, priority control, and scheduling processing are executed.
At this time, a MAC-PDU that may include an MAC header, and one or more MAC-service
data units (SDU) and MAC control elements is generated.
[0047] In the signal processing from the encoder 18 to the transmitter 23, processing on
the PHY layer (physical layer) is executed. In the encoder 18, for a signal supplied from
the L2 processor 17, encoding processing for error correction is executed, such as with
turbo encoding, for example, and the modulator 19 executes modulation processing on the
encoded signal. Preferably, modulation and encoding techniques in the encoder 18 and
the modulator 19 are selected adaptively based on downlink quality information fedback
from the user equipment.
[0048] The precoder 20 executes encoding on modulated signal obtained in the
modulator 19, based on the Precoding matrix, as precoding setting information provided
from the radio resource controller 24. Thereby, respective weights for transmission
antennas from each layer are identified, when multilayer communications are executed by
the multi input multi output (MIMO).
[0049] The multiplexer 21 multiplexes the signal from the precoder 20, a pilot signal as
a reference signal, notification information, and control information for each user equipment
UE. Te IFFT unit 22 converts multiplexed signal obtained in the multiplexer 21 to a time
domain signal (baseband signal) to each subcarrier signal by executing an inverse fast
Fourier transform (IFFT) processing on the multiplexed signal.
The transmitter 23 includes a digital to analog (D/A) converter, a local frequency
oscillator, a mixer, a power amplifier, a filter, and the like. The transmitter 23 upconverts
and the like the baseband signal from the IFFT unit 22 from a baseband frequency to a
radio frequency, and transmits it to space from the transmission antenna.
[0050] The receiver 11 converts the RF signal received from the user equipment via the
reception antenna to a digital baseband signal. The receiver 11 includes a bandwidth
shaping filter, a low noise amplifier (LNA), a local frequency oscillator, an orthogonal
demodulator, an automatic gain control (AGC) amplifier, an analog to digital (A/D), and the
like.
The FFT unit 12 executes FFT processing on the received baseband signal using a
predetermined fast Fourier transform (FFT) window to generate an encoded symbol series
(frequency domain signal) for each subcarrier. The frequency domain signal demodulated
and decoded by the demodulator 13 and the decoder 14, respectively. The L2 processor
15 executes protocol processing in Layer 2 (L2) on the decoded signal to obtain a reception
packet.
[0051] The radio resource controller 24 primarily executes controls for coordinated
transmissions, including the CoMP setting procedure depicted in FIG. 5, and controls for a
handover of the user equipment UE.
[0052] The radio resource controller 24 controls the L2 processor 17 such that a
surrounding cell measurement request is sent to each user equipment UE in the CoMP
setting procedure. The radio resource controller 24 further identifies coordinated cell
candidates in accordance with the technique represented by Eq. (1) described above, for
example, based on a notification of measurement results of surrounding cells (results of an
RSRP measurement) from the user equipment UE, which is included in a control signal
from the user equipment UE obtained by the L2 processor 15.
The radio resource controller 24 sends a coordinated communication request to
evolutional Node Bs of identified coordinated cell candidates, and receives response
signals for the coordinated communication request from the respective evolutional Node Bs,
through the X2 interface 16, in the CoMP setting procedure.
[0053] The radio resource controller 24 controls the L2 processor 17 to send a CSI
request for requesting the user equipment UE that is a target of coordinated transmissions,
to measures the CSIs with the coordinated cell candidates, in the CoMP setting procedure.
The radio resource controller 24 further identifies coordinated cells based on a CSI report
from the user equipment UE, included in the control signal from the user equipment UE
obtained by the L2 processor 15, and notifies the L2 processor 17 and the precoder 20 of
the precoding matrix for executing coordinated transmissions. Note that, the precoding
matrix based on the CSI may be selected based on the codebook scheme, for example.
[0054] In the present embodiment, the CSI request is made included in an IE in an RRC
reconfiguration message (RRC reconfig) while a handover procedure is being executed. In
the present embodiment, a CSI report from the user equipment UE for a CSI request is
made included in an IE in an RRC reconfiguration complete message ("RRC reconfig
complete").
[0055] The radio resource controller 24 sends CoMP information to a cooperating
evolutional Node B which cooperates with the evolutional Node B, via the interface 16, in
the CoMP setting procedure. The CoMP information includes transmission data to the user
equipment UE, propagation path information (CSI report from the user equipment UE),
scheduling information, precoding setting information (precoding matrix to be set in the
serving and coordinated cells), and the like, for example. The scheduling information
includes the location of a resource element in a resource block to which transmission data
(PDSCH) to the user equipment UE is allocated, for example.
[0056] The radio resource controller 24 detects a handover based on the measurement
results of the RSRPs of the reference signals from surrounding cells by the user equipment
UE, and communicates with the target evolutional Node B thorugh the X2 interface 16
when executing controls for the handover of the user equipment UE.
For example, in response to detecting a handover, the radio resource controller 24
sends an HO request signal to the target evolutional Node B thorugh the X2 interface 16,
and receives an acknowledgement signal for the request. Here, the HO request signal is'
generated so as to include the surrounding cell measurement results of the user equipment
UE possessed by the source evolutional Node B (the latest results at the time of the
detection of the handover), or coordinated cell candidates identified using the surrounding
cell measurement results.
[0057] Next, with reference to FIG. 7, a user equipment of the present embodiment UE
includes a receiver 31 as a third transceiver, an FFT unit 32, a demodulator 33, a decoder
34, a control channel demodulator 35, a radio resource controller 36, a control information
processor 37, a multiplexer 38, a symbol mapper 39, a multiplexer 40, an FFT unit 41, a
frequency mapper 42, an IFFT unit 43, a transmitter 44 as the third transceiver, and an L2
processor 45.
[0058] The receiver 31 converts an RF signal received at the reception antenna to a "
digital baseband signal, and the FFT unit 32 executes FFT processing on the digital
baseband signal to generate a frequency domain signal.
The control channel demodulator 35 demodulates a control channel signal in the
frequency domain signal obtained from the FFT unit 32, and provides the demodulator 33
and the decoder 34 with information of a resource allocation included in that signal. The
control channel demodulator 35 also provides the demodulator 33 and the decoder 34 with
the demodulator 33 and the decoder 34 of the radio resource allocated to the user
equipment included in the demodulated control information. The demodulated control
information is also provided to the control information processor 37.
The control information processor 37, as an estimator and a quality measurement
section, measures the RSRPs and the CSIs with coordinated cell candidates, based on
reference signals, such as pilot signals (from cell connecting to the user equipment and
from surrounding cells signal) obtained by the FFT unit 32. These measurement results
are included in dedicated control information, for example. The control information
processor 37 may be embodied using a digital signal processor (DSP) or a part of a DSP,
for example.
[0059] The L2 processor 45 executes protocol processing in Layer 2 (L2) on decoded
signal. The radio resource controller 36 primarily executes controls for coordinated
transmissions, including the CoMP setting procedure depicted in FIG. 5, and controls for a
handover of the user equipment.
The radio resource controller 36, in response to a surrounding cell measurement
request from an evolutional Node B in a message included in the control information
obtained by the L2 processor 45, and an CSI request, controls the control information
processor 37 to measures the surrounding cells and the CSI. The radio resource controller
36 also controls such that these measurement results (surrounding cell measurement result
notifications and CSI report) are included in a message sent to the evolutional Node B.
The radio resource controller 36 establishes a communication with the target
evolutional Node B, using an uplink channel RACH during a handover procedure. Once
communications with a target evolutional Node B are established, the radio resource
controller 36 controls such that an RRC reconfiguration complete message ("RRC reconfig
complete") including a CSI report is sent to the target evolutional Node B, in response to a
CSI request from a source evolutional Node B.
[0060] The multiplexer 38 multiplexes dedicated control information and transmission
data for an evolutional Node B provided from the control information processor 37. The
symbol mapper 39 allocates the multiplexed signal to multiple symbols in a unit of
transmission time interval (TTl). The multiplexer 40 executes further multiplexing by
allocating a pilot signal as a reference signal in the unit of TTl as a reference symbol. The
FFT unit 41 converts a time domain signal output from the multiplexer 40 to a frequency
domain signal by executing FFT processing in the size the same as the size of an input
symbol. The frequency domain signal is mapped to continuous frequencies allocated to
the user equipment by the frequency mapper 42 (frequencies which do not overlap with
frequencies for other user equipments UE), and then is converted to a time domain signal
in the IFFT unit 43. The time domain signal is upconverted by the transmitter 44 and is
sent to the evolutional Node B.
[0061] As set forth above, in the present embodiment, when a handover of a user
equipment which is a target of coordinated communications among multiple wireless
evolutional Node Bs is detected, at least a part of setting procedure for the coordinated
communications (CoMP setting procedure described above) is executed before completion
of the handover. Accordingly, the coordinated transmission for the user equipment can be
resumed earlier after the handover, and any reduction in the communication quality or
throughput caused by the resume of the coordinated communication after the handover of
the user equipment can be prevented.
[0062] (2) Second Embodiment
Hereinafter, the CoMP setting procedure before and after a handover of a user
equipment in the mobile communication system of a second embodiment will be described
with reference to FIG. 8. FIG. 8 is a diagram illustrating an example of a series of
sequences of the CoMP setting procedures (communication setting method) before and
after a handover of a user equipment in the second embodiment.
[0063] For eliminating any duplicated descriptions, processing in the sequence of FIG.
8 different from that of FIG. 5 in relation to the first embodiment will be described. In the
sequence of the present embodiment, a CSI request to a user equipment UE is not
included in an RRC reconfiguration message (RRC reconfig) from a source evolutional
Node B, and a CSI request is notified from a target evolutional Node B to the evolutional
Node B eNB (Step S 24) during a establishment of communications between the user
equipment UE and the target evolutional Node B. In this case, the user equipment UE
notifies a CSI request from the target evolutional Node B in a down link shared channel
(DL-SCH).
Note that a the target evolutional Node B receives a CSI report in an RRC
reconfiguration complete message ("RRC reconfig complete") after communications are
established between the user equipment UE and the target evolutional Node B, in the
manner similar to the first embodiment.
[0064] Similarly to the first embodiment, in the mobile communication system of the
present embodiment, the target evolutional Node B also an receive a CSI report before
completion of the handover procedure, which helps to accelerates processing of the CoMP
setting procedure in the target evolutional Node B. In other words, similarly to the first
embodiment, the coordinated transmission for the user equipment can be resumed earlier
after the handover, and any reduction in the communication quality or throughput caused
by the resume of the coordinated communication after the handover of the user equipment
can be prevented.
[0065] (3) Third Embodiment
Hereinafter, the CoMP setting procedure before and after a handover of a user
equipment in the mobile communication system of a third embodiment will be described
with reference to FIG. 9. FIG. 9 is a diagram illustrating an example of a series of
sequences of the CoMP setting procedures (communication setting method) before and
after a handover of a user equipment in the third embodiment.
[0066] For eliminating any duplicated descriptions, processing in the sequence of FIG.
9 different from that of FIG. 5 in relation to the first embodiment will be described. In the
sequence of the present embodiment, after a UE context release message (UE Context
Release) is sent to a user equipment (Step S 28), a CSI request to the user equipment
UE is sent from a target evolutional Node B to the user equipment UE (Step S 29).
Thereafter, the target evolutional Node B identifies a coordinated cell, and sends CoMP
information to a cooperating evolutional Node B that manages that coordinated cell (Step S
30).
[0067] In the mobile communication system of the present embodiment, a CSI request
is made from the target evolutional Node B to a user equipment UE, in a later timing
compared to the first and second embodiments. However, a notification of surrounding
cell measurement results by the user equipment UE is made at earlier stage of the
handover procedure (i.e., in Step S 20). Hence, the target evolutional Node B does not
receive notifications of cell measurement results by the user equipment UE after the
handover, and the coordinated communication processing by the target evolutional Node B
is accelerated. In other words, similarly to the first embodiment, the coordinated
transmission for the user equipment can be resumed earlier after the handover, and any
reduction in the communication quality or throughput caused by the resume of the
coordinated communication after the handover of the user equipment can be prevented.
[0068] While the embodiments of the present invention have been described in detail, a
communication setting method, a wireless base station, and a mobile station of the present
invention are not limited to the embodiments discussed above. It is noted that various
modifications and variations may be practiced without departing from the spirit of the
invention.
[0069] The above-described embodiment has been described where at least a part of
the CoMP setting procedure is executed when a handover of a user equipment is detected
and the at least a part of the CoMP setting procedure is executed before completion of the
handover. However, processing of the CoMP setting procedure is not limited to the timing
when a handover is detected or the completion of the handover. For example, although a
HO request message that includes surrounding cell measurement or information about
coordinated cell candidates identified using the surrounding cell measurement results is
sent from a source evolutional Node B to a target evolutional Node B in Step S 20 in FIG. 5,
in the first embodiment, this is not limiting. The timing when the measurement results or
the information is sent to the target evolutional Node B is not limited to timing of the
handover and may be sent anytime based on the surrounding cell measurement results.
Description of Reference Characters
[0070]
eNB evolutional Node B
11 receiver
12 FFTunit
13 demodulator
14 decoder
15 L2 processor
16 X2 interface
17 L2 processor
18 encoder
19 modulator
20 precoder
21 multiplexer
22 FFT unit
23 transmitter
24 radio resource controller
UE user equipment
31 receiver
32 FFT unit
33 demodulator
34 decoder
35 control channel demodulator
36 radio resource controller
37 control information processor
38 multiplexer
39 symbol mapper
40 multiplexer
41 FFT unit
42 frequency mapper
43 IFFT unit
44 transmitter
45 L2 processor
We Claim:
1. A method of communication setting for providing coordinated communications by a
plurality of wireless base stations to a mobile station, the plurality of wireless base stations
providing wireless services, the method comprising:
measuring, by a first mobile station connected to a first wireless base station, a
reception quality of a reference signal from the plurality of wireless base stations including a
first wireless base station and a second wireless base station adjacent to the first wireless
base station, and notifying the first wireless base station of the reception quality; and
notifying the second wireless base station by the first wireless base station, of the
notified reception quality, or information about a candidate wireless base station that is a
candidate of a wireless base station for the coordinated communications identified based
on the notified reception quality, as first information for providing the coordinated
communications to the first mobile station by the plurality of wireless base stations including
the second wireless base station.
2. The method for communication setting according to claim 1, wherein the notification
to the second wireless base station by the first wireless base station is made after detection
of the handover of the first mobile station from the first wireless base station to the second
wireless base station based on the reception quality notified from the first mobile station,
and before the handover is completed.
3. The method for communication setting according to claim 2, further comprising:
requesting, by the first wireless base station, after notifying the second wireless
base station of the first information, the first mobile station to notify propagation path
information between the first mobile station and the candidate wireless base station before
the completion of the handover; and
notifying, by the first mobile station, the second wireless base station of the
propagation path information.
4. The method for communication setting according to claim 2, further comprising:
requesting, by the second wireless base station, in response to the notification of
the first information, the first mobile station to notify propagation path information between
the first mobile station and the candidate wireless base station, before or after the
completion of the handover; and
notifying, by the first mobile station, the second wireless base station of the
propagation path information.
5. The method for communication setting according to any one of claims 2-4, wherein
notifying the second wireless base station by the first wireless base station comprises
sending a hand over request signal so as to include the first information
6. The method for communication setting according to claim 3, wherein the requesting
by the first wireless base station to notify the propagation path information comprising
sending a signal related to a radio resource control to the first mobile station so as to
include the request.
7. The method for communication setting according to claim 4, wherein the requesting
by the second wireless base station to notify the propagation path information comprising
sending a signal related to an access control to the first mobile station so as to include the
request.
8. The method for communication setting according to claim 3 or 4, wherein the
notifying the propagation path information by the first mobile station to the second wireless
base station comprises sending a response signal to a signal related to a radio resource
control so as to include the propagation path information.
9. A wireless base station as a first wireless base station for providing coordinated
communications with a second wireless base station to a mobile station, the first wireless
base station comprising:
a first transceiver that connects to the mobile station and sends and receives a
signal; and
a second transceiver that sends and receives a signal to and from the other
wireless base station,
wherein the first transceiver receives, from a first mobile station connected to the
wireless base station, a notification of measurement results of reception quality of reference
signals from a plurality of wireless base stations including the first wireless base station and
the second wireless base station adjacent to the first wireless base station, in the first
mobile station, and
the second transceiver notifies the second wireless base station, of the notified
reception quality, or information about a candidate wireless base station that is a candidate
of a wireless base station for the coordinated communications identified based on the
notified reception quality, as first information for providing the coordinated communications
to the first mobile station by the plurality of wireless base stations including the second
wireless base station.
10. The wireless base station according to claim 9, wherein.
the second transceiver performs the notification of the first information after
detection of the handover of the first mobile station from the first wireless base station to the
second wireless base station based on the reception quality notified from the first mobile
station, and before the handover is completed.
11. The wireless base station according to claim 10, wherein the first transceiver
requests, after notifying the second wireless base station of the first information by the
second transceiver, the first mobile station to notify propagation path information between
the first mobile station and the candidate wireless base station before the completion of the
handover.
12. A wireless base station as a second wireless base station for providing wireless
services and coordinated communications with a first wireless base station to a mobile
station, the second wireless base station comprising:
a first transceiver that connects to the mobile station and sends and receives a
signal; and
a second transceiver that sends and receives a signal to and from the first wireless
base station,
wherein the second transceiver receives from the first wireless base station, as first
information for executing coordinated communications among a plurality of wireless base
stations including the second wireless base station for a first mobile station connected to
the first wireless base station adjacent to the second wireless base station, a reception
quality of reference signals from the plurality of wireless base stations including the second
wireless base station and the first wireless base station in the first mobile station, or a
notification of information about a candidate wireless base station that is a wireless base
station for a candidate of the coordinated communications.
13. The wireless base station according to claim 12, wherein.
the second transceiver receives the notification of the first information, after a
handover of the first mobile station from the first wireless base station to the second
wireless base station is requested from the first wireless base station and before the
handover is completed.
14. The wireless base station according to claim 13, wherein
the first transceiver requests, in response to the notification of the first information,
the first mobile station to notify propagation path information between the first mobile
station and the candidate wireless base station, before or after the completion of the
handover.
15. A mobile station that is provided with coordinated communications by a plurality of
wireless base stations providing wireless services, the mobile station comprising:
a third transceiver that sends and receives a signal to and from a wireless base
station; and
a quality measurement section that measures a reception quality of reference
signals from the plurality of wireless base stations including a first wireless base station and
a second wireless base station adjacent to the first wireless base station,
wherein the third transceiver notifies the first wireless base station of the measured
reception quality, thereby enabling the first wireless base station to notify of the second
wireless base station, as first information for executing coordinated communications among
the plurality of wireless base stations including the second wireless base station to the
mobile station, the notified reception quality, or information about a candidate wireless base
station that is a candidate of a wireless base station for the candidate coordinated
communications identified by the first wireless base station based on the notified reception
quality.
16. The mobile station according to claim 15, further comprising an estimator that
estimates propagation path information between the mobile station and the wireless base
stations,
wherein the third transceiver, before a handover of the mobile station from the first
wireless base station to the second wireless base station is completed, receives a request
to notify the propagation path information between the mobile station and the wireless base
stations from the first wireless base station, and notifies to the second wireless base station
of the propagation path information obtained by the estimator.
17. The mobile station according to claim 15, further comprising an estimator that
estimates propagation path information between the mobile station and the wireless base
stations,
wherein the third transceiver, before a handover of the mobile station from the first
wireless base station to the second wireless base station is completed or after the
completion of the handover, receives a request to notify the propagation path information
between the mobile station and the wireless base stations from the second wireless base
station, and notifies to the second wireless base station of the propagation path information
obtained by the estimator.