[Technical Field]
[OOO 11
The present invention relates to a user equipment, a method for performing a
handover, a base station, and a radio communication system.
5 [Background Art]
[0002]
In Long Term Evolution-Advanced (LTE-A), which is the next-generation cellular
communication standard that is discussed in Third Generation Partnership Project
(3GPP), introduction of technology called carrier aggregation (CA) has been studied.
10 The carrier aggregation is technology that forms a communication channel between a
user equipment (UE) and a base station (BS, or evolved Node B (eNB)) by
aggregating a plurality of frequency bands that are supported in LTE, for example,
and thereby improves communication throughput. Each frequency band included in
one communication channel by the carrier aggregation is called a component carrier
15 (CC). The bandwidths of frequency bands that are available in LTE are 1.4 MHz,
3.0 MHz, 5.0 MHz, 10 MHz, 15 MHz, and 20 MHz. Accordingly, if five bands of
20 MHz are aggregated as component carriers, a communication channel of 100
MHz in total can be formed.
[0003]
20 Component carriers that are included in one communication channel in the carrier
aggregation are not necessarily contiguous to one another in the frequency direction.
The mode in which component carriers are arranged contiguous to one another in the
frequency direction is called a contiguous mode. On the other hand, the mode in
which component carriers are arranged not contiguous to one another is called a non-
25 contiguous mode.
[0004]
Further, in the carrier aggregation, the number of component carriers in an uplink
and the number of component carriers in a downlink are not necessarily equal. The
mode in which the number of component carriers in an uplink and the number of
30 component carriers in a downlink are equal is called a symmetric mode. On the
other hand, the mode in which the number of component carriers in an uplink and the
number of component carriers in a downlink are not equal is called an asymmetric
mode. For example, in the case of using two component carriers in an uplink and
three component carriers in a downlink, it is asymmetric carrier aggregation.
1 [OOOS]
5 Further, in LTE, any one of frequency division duplex (FDD) and time division
duplex (TDD) can be used as duplex operation. Because the direction of a link
(uplink or downlink) of each component carrier does not change in time in FDD,
FDD is better suited to the carrier aggregation compared to TDD.
[0006]
10 A handover, which is a basic technique for achieving the mobility of a user
equipment in the cellular communication standard, is one of important subjects in
LTE-A. In LTE, a user equipment measures a communication quality over a
channel with a serving base station (a currently connected base station) and
communication qualities with peripheral base stations and transmits a measurement
15 report containing measurements to the serving base station. Receiving the
measurement report, the serving base station determines whether to execute a
handover based on the measurements contained in the report. Then, if it is
determined that a handover is to be executed, a handover is carried out among a
source base station (the serving base station before a handover), the user equipment,
20 and a target base station (a serving base station after a handover) in accordance with
' a prescribed procedure (e.g. cf. Patent Literature 1 below)
[Citation List]
[Patent Literature]
[0007]
25 [PTLl]
Japanese Unexamined Patent Application Publication No. 2009-232293
[Summary of Invention]
[Technical Problem]
[OOOS]
30 However, no case has been reported where active consideration is given to how to
carry out a handover procedure in a radio communication involving the carrier
3
aggregation.
[0009]
For example, when executing a handover for each of component carriers, a situation
occurs where a handover is completed for a part of a plurality of component carriers
5 and a handover is not completed for the rest of the plurality of component carriers.
In such a situation, when a cyclic prefix length used by a source base station and a
cyclic prefix length used by a target base station are different, for example, there is a
possibility that a radio communication unit of a user equipment needs to handle the
both of cyclic prefix lengths simultaneously. However, implementing a user
10 equipment which incorporates a radio communication unit that can simultaneously
handle different cyclic prefix lengths results in complication of a circuit, an increase
in manufacturing cost and an increase in processing load. Further, the necessity of
defining a new protocol for aggregating a plurality of signals having different cyclic
prefix lengths in an upper layer arises.
15 [OOlO]
In light of the foregoing, it is desirable to provide a novel and improved user
equipment, method for performing a handover, base station and radio communication
system that can avoid simultaneous handling of different cyclic prefix lengths during
a handover procedure in a radio communication involving the carrier aggregation.
20 [Solution to Problem]
[OO 1 11
According to one embodiment, the present invention is directed to a mobile station in
a wireless communication network, the mobile station comprising: a radio
communication unit configured to communicate with a first base station via a
25 plurality of component carriers; and a control unit configured to control the radio
communication unit to initiate a handover procedure to a second base station after
receiving at least one handover command for the plurality of component carriers.
[OO 121
The radio communication unit may be configured to communicate with the first base
30 station via one of the plurality of component carriers having a first cyclic prefix
length, and communicate with the second base station via one of the plurality of
component carriers having a second cyclic prefix length. The radio communication
may be configured to receive a plurality of handover commands, each corresponding
to one of the plurality of component carriers, and the control unit may be configured
to control the radio communication unit to initiate a handover procedure to the
5 second base station for each of the plurality of component carriers after receiving the
plurality of handover commands.
[OO 1 31
According to another embodiment, the present invention is directed to a mobile
station in a wireless communication network, the mobile station comprising: a radio
10 communication unit configured to communicate with a first base station via a
plurality of component carriers each having a first cyclic prefix length; and a control
unit configured to control the radio communication unit to initiate a handover
procedure from the first base station to a second base station after receiving a
handover command, wherein, after a handover for one of the plurality of component
15 carriers, the control unit is configured to control the radio communication unit to
communicate with the second base station via a component carrier allocated to a time
slot different from a time slot to which each of the plurality of component carriers are
allocated, and having a second cyclic prefix length different from the first cyclic
prefix length. The control unit may be configured to control the radio
20 communication unit to release a first one of the plurality of component carrier
connections with the first base station and to connect to the second base station using
the component carrier having the second cyclic prefix length. The control unit may
M e r be configured to control the radio communication unit to initiate a handover
procedure to the second base station for a second one of the plurality of component
25 carriers after the handover procedure for the first one of the plurality of component
carriers has been initiated.
[OO 141
According to another embodiment, the present invention is directed to a base station
in a wireless communication network, the base station comprising: a radio
30 communication unit configured to communicate with a mobile station via a plurality
of component carriers; and a control unit configured to control the radio
communication unit to initiate a handover procedure to a second base station for one
of the plurality of component carriers after receiving a report fiom the mobile station
corresponding to the one of the plurality of component carriers. The control unit
may be configured to initiate the handover procedure by controlling the radio
5 communication unit to transmit a handover request command to the second base
station and transmit a handover command to the mobile station.
[00 1 51
According to another embodiment, the present invention is directed to a base station
in a wireless communication network, the base station comprising: a radio
10 communication unit configured to communicate with a mobile station via a plurality
of component carriers each having a first cyclic prefix length; and a control unit
configured to control the radio communication unit to initiate a handover procedure
to a second base station for one of the plurality of component carriers, wherein, after
a handover for one of the plurality of component carriers, the mobile station
15 communicates with the second base station via a component carrier allocated to a
time slot different from a time slot to which each of the plurality of component
carriers is allocated, and having a second cyclic prefix length different from the first
cyclic prefix length.
[00 1 61
20 According to another embodiment, the present invention is directed to a handover
method performed by a mobile station in a wireless communication network, the
method comprising: communicating, by a radio communication unit of the mobile
station, with a first base station via a plurality of component carriers; and initiating,
by a control unit of the mobile station, a handover procedure to a second base station
25 after receiving at least one handover command for the plurality of component
carriers.
[00 171
According to another embodiment, the present invention is directed to a handover
method performed by a mobile station in a wireless communication network, the
30 method comprising: communicating, by a radio communication unit of the mobile
station, with a first base station via a plurality of component carriers each having a
first cyclic prefix length; initiating, by a control unit of the mobile station, a
handover procedure from the first base station to the second base station after
receiving a handover command; and controlling, by the control unit after a handover
for one of the plurality of component carriers, the radio communication unit to
communicate with the second base station via a component carrier allocated to a time
5 slot different from a time slot to which each of the plurality of component carriers are
allocated, and having a second cyclic prefix length different from the first cyclic
prefix length.
[OO 1 81
According to another embodiment, the present invention is directed to a handover
10 method performed by a base station in a wireless communication network, the
method comprising: communicating, by a radio communication unit of the base
station, with a mobile station via a plurality of component carriers; and initiating, by
a control unit of the base station, a handover procedure to a second base station for
one of the plurality of component carriers after receiving a report from the mobile
15 station corresponding to the one of the plurality of component carriers.
[00 191
According to another embodiment, the present invention is directed to a handover
method performed by a base station in a wireless communication network, the
method comprising: communicating, by a radio communication unit of the base
20 station, with a mobile station via a plurality of component carriers each having a first
cyclic prefix length; and initiating, by a control unit of the base station, a handover
procedure to a second base station for one of the plurality of component carrier;
wherein, after a handover for one of the plurality of component carriers, the mobile
station communicates with the second base station via a component carrier allocated
25 to a time slot different from a time slot to which each of the plurality of component
carriers is allocated, and having a second cyclic prefix length different from the first
cyclic prefix length
[0020]
According to another embodiment, the present invention is directed to a non-
30 transitory computer readable medium including computer-program instructions,
which when executed by a mobile station in a wireless communication network,
cause the mobile station to perform a handover method comprising: communicating
with a first base station via a plurality of component carriers; and initiating a
handover procedure to a second base station after receiving at least one handover
command for the plurality of component carriers.
[002 11
5 According to another embodiment, the present invention is directed to A nontransitory
computer readable medium including computer-program instructions,
which when executed by a mobile station in a wireless communication network,
cause the mobile station to perform a handover method comprising: communicating
with a first base station via a plurality of component carriers each having a first
10 cyclic prefur length; initiating a handover procedure from the first base station to the
second base station after receiving a handover command; and communicating, after a
handover for one of the plurality of component carriers, with the second base station
via a component carrier allocated to a time slot different from a time slot to which
each of the plurality of component carriers are allocated, and having a second cyclic
15 prefix length different fkom the first cyclic prefix length.
[0022]
According to another embodiment, the present invention is directed to a nontransitory
computer readable medium including computer-program instructions,
which when executed by a base station in a wireless communication network, cause
20 the base station to perform a handover method comprising: communicating with a
mobile station via a plurality of component carriers; and initiating a handover
procedure to a second base station for one of the plurality of component carriers after
receiving a report from the mobile station corresponding to the one of the plurality of
component carriers.
25 [0023]
According to another embodiment, the present invention is directed to a nontransitory
computer readable medium including computer-program instructions,
which when executed by a base station in a wireless communication network, cause
the base station to perform a handover method comprising: communicating with a
30 mobile station via a plurality of component carriers each having a first cyclic prefix
length; and initiating a handover procedure to a second base station for one of the
plurality of component carriers, wherein, after a handover for one of the plurality of
component carriers, the mobile station communicates with the second base station
via a component carrier allocated to a time slot different fiom a time slot to which
each of the plurality of component carriers is allocated, and having a second cyclic
prefix length different fiom the first cyclic prefix length.
5 [0024]
According to another embodiment, the present invention is directed to a wireless
communication system comprising: a mobile station including a radio
communication unit configured to communicate with a first base station via a
plurality of component carriers; a first control unit sat the first base station configured
10 to initiate a handover procedure to a second base station for one of the plurality of
component carriers after receiving a report fiom the mobile station corresponding to
the one of the plurality of component carriers; and a second control unit at the mobile
station configured to initiate a handover procedure to the second base station after
receiving at least one handover command for the plurality of component carriers
15 fiom the first base station.
[0025]
According to another embodiment, the present invention is directed to A wireless
communication system comprising: a first base station configured to communicate
with a mobile station via a plurality of component carriers each having a first cyclic
20 prefix length ; the mobile station configured to initiate a handover procedure from
the first base station to a second base station for one of the plurality of component
carriers after receiving a handover command fiom the first base station; and the
second base station configured to communicate with the mobile station via a
component carrier allocated to a time slot different fiom a time slot to which each of
25 the plurality of component carriers is allocated, and having a second cyclic prefix
length different from the first cyclic prefix length.
[Advantageous Effects of Invention]
[0026]
As described above, the user equipment, the method for performing a handover, the
30 base station and the radio communication system according to the embodiments of
the present invention can avoid simultaneous handling of different cyclic prefix
lengths during a handover procedure in a radio communication involving the carrier
aggregation.
[Brief Description of Drawings]
[0027]
5 [Fig. 11
Fig. 1 is a sequence chart to describe a flow of a typical handover procedure.
[Fig. 21
Fig. 2 is an explanatory view to describe an example of a structure of a
communication resource.
10 [Fig. 31
Fig. 3 is an explanatory view to describe a cyclic prefix length that can be adopted
generally.
[Fig. 41
Fig. 4 is a schematic view showing an outline of a radio communication system
15 according to an embodiment.
[Fig. 51
Fig. 5 is a block diagram showing an example of a configuration of a user equipment
according to a first embodiment.
[Fig. 61
20 Fig. 6 is a block diagram showing an example of a detailed configuration of a radio
communication unit according to the first embodiment.
[Fig. 71
Fig. 7 is a block diagram showing an example of a configuration of a base station
according to the first embodiment.
25 [Fig. 8A]
Fig. 8A is the first half of a sequence chart showing an example of a flow of a
handover procedure according to the first embodiment.
[Fig. 8B]
Fig. 8B is the second half of a sequence chart showing an example of a flow of a
30 handover procedure according to the first embodiment.
[Fig. 91
Fig. 9 is a sequence chart showing another example of a flow of a handover
procedure according to the first embodiment.
[Fig. 101
Fig. 10 is an explanatory view to describe an outline of switching of a cyclic prefix
5 length in time division.
[Fig. 111
Fig. 11 is a block diagram showing an example of a configuration of a user
equipment according to a second embodiment.
[Fig. 121
10 Fig. 12 is a block diagram showing an example of a detailed configuration of a radio
communication unit according to the second embodiment.
[Fig. 131
Fig. 13 is a block diagram showing an example of a configuration of a base station
according to the second embodiment.
15 [Fig. 141
Fig. 14 is an explanatory view to describe an example of a flow of a handover
procedure according to the second embodiment.
[Description of Embodiments]
[0028]
20 Hereinafter, preferred embodiments of the present invention will be described in
detail with reference to the appended drawings. Note that, in this specification and
the appended drawings, structural elements that have substantially the same function
and structure are denoted with the same reference numerals, and repeated
explanation of these structural elements is omitted.
25 [0029]
Preferred embodiments of the present invention will be described hereinafter in the
following order.
1. Description of Related Art
1 - 1. Handover Procedure
30 1-2. Structure of Communication Resource
1-3. Description of Issue
2. Outline of Radio Communication System
3. Description of First Embodiment
3- 1. Exemplary Configuration of User Equipment
3-2. Exemplary Configuration of Base Station
5 3-3. Flow of Process
3-4. Summary of First Embodiment
4. Description of Second Embodiment
4- 1. Exemplary Configuration of User Equipment
4-2. Exemplary Configuration of Base Station
1 0 4-3. Flow of Process
4-4. Summary of Second Embodiment
[0030]

(1 - 1. Handover Procedure)
15 A technique related to the present invention is described hereinafter with reference to
Figs. 1 to 3. Fig. 1 shows a flow of a handover procedure in conformity with LTE
in a radio communication not involving the carrier aggregation as an example of a
typical handover procedure. In this example, a user equipment (UE), a source base
station (source eNB), a target base station (target eNB), and a mobility management
20 entity (MME) are involv.ed in the handover procedure.
[003 11
As a preliminary step toward a handover, the user equipment first reports the channel
quality of a communication channel between the user equipment and the source base
station to the source base station (step S2). The channel quality may be reported on
25 a regular basis or when the channel quality falls below a predetermined reference
value. The user equipment can measure the channel quality of the communication
channel with the source base station by receiving a reference signal contained in a
downlink channel fiom the source base station.
[0032]
30 Then, the source base station determines the needs of measurement based on the
quality report received fiom the user equipment and, if measurement is necessary,
allocates measurement gaps to the user equipment (step S4).
[0033]
Then, the user equipment searches for a downlink channel from a peripheral base
station (i.e. performs cell search) during the periods of the allocated measurement
5 gaps (step S12). Note that the user equipment can recognize a peripheral base
station to search according to a list that is provided in advance from the source base
station.
[0034]
When the user equipment acquires synchronization with a downlink channel, the user
10 equipment performs measurement by using a reference signal contained in the
downlink channel (step S14). During this period, the source base station restricts an
allocation of data communication related to the user equipment so as to avoid
occurrence of data transmission by the user equipment.
[0035]
15 Upon completion of the measurement, the user equipment transmits a measurement
report containing measurements to the source base station (step S22). The
measurements contained in the measurement report may be the average value or the
central value of measured values over a plurality of times of measurement or the like.
Further, the measurements may contain data about a plurality of frequency bands.
20 [0036]
Receiving the measurement report, the source base station determines whether or not
to execute a handover based on the contents of the measurement report. For
example, when the channel quality of another base station in the periphery is higher
than the channel quality of the source base station by a predetermined threshold or
25 greater, it can be determined that a handover is necessary. In this case, the source
base station determines to carry out a handover procedure with the relevant another
base station as a target base station, and transmits a handover request message to the
target base station (step S24).
[0037]
30 Receiving the handover request message, the target base station determines whether
it is possible to accept the user equipment according to the availability of a
communication service offered by itself or the like. When it is possible to accept
the user equipment, the target base station transmits a handover request confirm
message to the source base station (step S26).
[003 81
5 Receiving the handover request confirm message, the source base station transmits a
handover command to the user equipment (step S28). Then, the user equipment
acquires synchronization with the downlink channel of the target base station (step
S32). After that, the user equipment makes a random access to the target base
station by using a random access channel in a given time slot (step S34). During
10 this period, the source base station forwards data addressed to the user equipment to
the target base station (step S36). Then, after success in the random access, the user
equipment transmits a handover complete message to the target base station (step
S42).
[0039]
15 Receiving the handover complete message, the target base station requests the MME
to perform route update for the user equipment (step S44). Upon updating the route
of user data by the MME, the user equipment becomes able to communicate with
another device through a new base station (i.e. the target base station). Then, the
target base station transmits acknowledgement to the user equipment (step S46). A
20 series of handover procedure thereby ends.
[0040]
(1 -2. Structure of Communication Resource)
Fig. 2 shows a structure of a communication resource in LTE as an example of a
structure of a communication resource to which the present invention is applicable.
25 Referring to Fig. 2, the communication resource in LTE is segmented in the time
direction into radio frames each having a length of 10 msec. One radio frame
includes ten sub-frames, and one sub-frame is made up of two 0.5 msec slots. In
LTE, the sub-frame is one unit of an allocation of a communication resource to each
user equipment in the time direction. Such one unit is called a resource block.
30 One resource block includes twelve sub-carriers in the frequency direction.
Specifically, one resource block has a size of 1 msec with 12 sub-carriers in the timefrequency
domain. Throughput of data communication increases as a larger
number of resource blocks are allocated for data communication on condition of the
same bandwidth and time length. Further, in such a structure of a communication
resource, a part of radio frame with a given frequency band is reserved as a random
5 access channel. The random access channel can be used for an access to a base
station by a user equipment that has changed fkom an idle mode to an active mode or
an initial access to a target base station in a handover procedure, for example.
[0041]
(1 -3. Description of Issue)
10 An issue related to a handover procedure in a radio communication involving the
carrier aggregation is described hereinafter with reference to Fig. 3. Fig. 3 is an
explanatory view to describe a cyclic prefix length (which is referred to hereinafter
as a CP length) that can be adopted generally in the structure of a communication
resource which is described above with reference to Fig. 2.
15 [0042]
A cyclic prefix length (CP) is a guard interval which is inserted between data
symbols in order to avoid the occurrence of inter-symbol interference due to
frequency selective fading caused by multipath delay spread over a communication
channel. By inserting the cyclic prefix into data symbols, it is possible to avoid
20 inter-symbol interference caused by delay spread up to the length of the inserted
cyclic prefix.
[0043]
Fig. 3 shows two types of cyclic prefixes, an extended CP and a normal CP, by way
of illustration. In the case of the extended CP, six symbols are included in the
25 0.5ms slot which is described earlier with reference to Fig. 2. Further, the extended
cyclic prefix having a length of 16.7microsec is inserted between the symbols. On
the other hand, in the case of the normal CP, seven symbols are included in the 0.5ms
slot. Further, the normal cyclic prefix having a length of 4.7microsec is inserted
between the symbols. The cyclic prefix typically contains a copy of the last portion
30 of the data symbol.
[0044]
Which of the extended CP or the normal CP is to be used is generally determined in
each base station. Then, the type of cyclic prefix to be used is notified fiom a base
station to a user equipment through a broadcast channel. In many cases, the
extended CP is used in the place where the multipath delay spread is particularly
5 large. However, because the use of the extended CP leads to an increase in
overhead, throughput decreases when using the extended CP compared to when
using the normal CP. Thus, there is a possibility that the extended CP and the
normal CP are used in the base stations which are adjacent to each other, depending
on the effect of the multipath delay spread.
10 [0045]
In a handover in a radio communication not involving the carrier aggregation, the
user equipment can switch the setting of the CP length of the radio communication
unit at the point of synchronization or a random access to the target base station.
However, in a handover in a radio communication involving the carrier aggregation,
15 the situation occurs where a handover is completed for only a part of a plurality of
component carriers. If, in view of such a situation, a circuit for simultaneously
handling different CP lengths is applied in the radio communication unit,
disadvantages occur such as complication of the circuit, an increase in manufacturing
cost and an increase in processing load. Therefore, it is effective to employ the
20 technique that can avoid simultaneous handling of different CP lengths during a
handover procedure as in two embodiments of the present invention which are
described in detail below.
[0046]
<2. Outline of Radio Communication System>
25 Fig. 4 is a schematic view showing an outline of a radio communication system 1
according to an embodiment of the present invention. Referring to Fig. 4, the radio
communication system 1 includes a user equipment 100, a base station 200a and a
base station 200b. It is assumed that the base station 200a is a serving base station
for the user equipment 1 00.
30 [0047]
The user equipment 100 is located inside a cell 202a where a radio communication
service is provided by the base station 200a. The user equipment 100 can perform a
data communication with another user equipment (not shown) via the base station
200a over a communication channel formed by aggregating a plurality of component
carriers (i.e. by carrier aggregation). However, because the distance between the
5 user equipment 100 and the base station 200a is not short, there is a possibility that a
handover is required for the user equipment 100. Further, the user equipment 100 is
located inside a cell 202b where a radio communication service is provided by the
base station 200b. Therefore, the base station 200b can be a candidate for a target
base station for a handover of the user equipment 100.
10 [0048]
Further, it is assumed for example that the base station 200a uses the CP length of the
normal CP and the base station 200b uses the CP length of the extended CP, which
are described earlier with reference to Fig. 3. Therefore, in order for the user
equipment 100 to carry out a handover fiom the base station 200a to the base station
15 200b, it is necessary to change the setting of the CP length in the equipment at some
point in time during a handover procedure.
[0049]
The base station 200a can communicate with the base station 200b through a
backhaul link (e.g. X2 interface). Various kinds of messages in the handover
20 procedure as described with reference to Fig. 1, scheduling information related to the
user equipment belonging to each cell or the like, for example, can be transmitted
and received between the base station 200a and the base station 200b. Further, the
base station 200a and the base station 200b can communicate with the MME, which
is an upper node, through S 1 interface, for example.
25 [0050]
It should be noted that, when there is no particular need to distinguish between the
base station 200a and the base station 200b in the following description of the
specification, they are collectively referred to as a base station 200 by omitting the
alphabetical letter at the end of the reference symbol. The same applies to the other
30 elements.
[005 11
<3. Description of First Embodiment>
A first embodiment of the present invention for avoiding simultaneous handling of
different CP lengths during a handover procedure in a radio communication
involving the carrier aggregation is described hereinafter with reference to Figs. 5 to
5 9.
[0052]
(3- 1. Exemplary Configuration of User Equipment)
Fig. 5 is a block diagram showing an example of a configuration of the user
equipment 100 according to the embodiment. Referring to Fig. 5, the user
10 equipment 100 includes a radio communication unit 11 0, a signal processing unit 150,
a buffer 152, a control unit 160, and a measurement unit 170.
[0053]
(Radio communication unit)
The radio communication unit 110 performs a radio communication with the base
15 station 200 over a communication channel formed by aggregating a plurality of
component carriers with use of the carrier aggregation technology.
[0054]
Fig. 6 is a block diagram showing an example of a more detailed configuration of the
radio communication unit 110. Referring to Fig. 6, the radio communication unit
20 1 10 includes an antenna 1 12, a switch 114, a low noise amplifier (LNA) 120, a
plurality of down-converters 122a to 122c, a plurality of filters 124a to 124c, a
plurality of analogue-to-digital converters (ADCs) 126a to 126c, a demodulation unit
128, a modulation unit 130, a plurality of digital-to-analogue converters (DACs)
132a to 132c, a plurality of filters 134a to 134c, a plurality of up-converters 136a to
25 136c, a combiner 138, and a power amplifier (PA) 140. Further, the demodulation
unit 128 includes a CPU removal unit 129. The modulation unit 130 includes a
CPU insertion unit 13 1.
[0055]
I The antenna 112 receives a radio signal transmitted from the base station 200 and
30 outputs the received signal to the LNA 120 through the switch 114. The LNA 120
amplifies the received signal. The down-converter 122a and the filter 124a separate
18
a baseband signal of the first component carrier (CCl) from the received signal
amplified by the LNA 120. Then, the separated baseband signal is converted to a
digital signal by the ADC 126a and output to the demodulation unit 128. Likewise,
the down-converter 122b and the filter 124b separate a baseband signal of the second
5 component carrier (CC2) from the received signal amplified by the LNA 120. Then,
the separated baseband signal is converted to a digital signal by the ADC 126b and
output to the demodulation unit 128. Further, the down-converter 122c and the
filter 124c separate a baseband signal of the third component carrier (CC3) from the
received signal amplified by the LNA 120. Then, the separated baseband signal is
10 converted to a digital signal by the ADC 126c and output to the demodulation unit
128. After that, in the demodulation unit 128, the CPU removal unit 129 removes a
cyclic prefix from the baseband signals of the respective component carriers. The
demodulation unit 128 then generates a data signal by demodulating the baseband
signals and outputs the data signal to the signal processing unit 150.
15 [0056]
Further, when a data signal is input from the signal processing unit 150, the
modulation unit 130 modulates the data signal and generates baseband signals of the
respective component carriers. Further, in the modulation unit 130, the CP insertion
unit 131 inserts a cyclic prefix to the baseband signals. Among those baseband
20 signals, the baseband signal of the first component carrier (CC1) is converted to an
analog signal by the DAC 132a. Then, a frequency component corresponding to the
first component carrier in a transmission signal is generated from the analog signal
by the filter 134a and the up-converter 136a. Likewise, the baseband signal of the
second component carrier (CC2) is converted to an analog signal by the DAC 132b.
25 Then, a frequency component corresponding to the second component carrier in the
transmission signal is generated from the analog signal by the filter 134b and the upconverter
136b. Further, the baseband signal of the third component carrier (CC3)
is converted to an analog signal by the DAC 132c. Then, a frequency component
corresponding to the third component carrier in the transmission signal is generated
30 from the analog signal by the filter 134c and the up-converter 136c. After that, the
generated frequency components corresponding to the three component carriers are
combined by the combiner 138, and the transmission signal is formed. The PA 140
amplifiers the transmission signal and outputs the transmission signal to the antenna
112 through the switch 114. Then, the antenna 112 transmits the transmission
signal as a radio signal to the base station 200.
5 [0057]
Although the case where the radio communication unit 11 0 handles three component
carriers is described in Fig. 6, the number of component carriers handled by the radio
communication unit 110 may be two, or four or more.
[0058]
10 Further, instead of processing the signals of the respective component carriers in the
analog region as in the example of Fig. 6, the radio communication unit 110 may
process the signals of the respective component carriers in the digital region. In the
latter case, at the time of reception, a digital signal converted by one ADC is
separated into the signals of the respective component carriers by a digital filter.
15 Further, at the time of transmission, after digital signals of the respective component
carriers are frequency-converted and combined, the signal is converted into an
analog signal by one DAC. The load of the ADC and the DAC is generally smaller
when processing the signals of the respective component carriers in the analog region.
On the other hand, when processing the signals of the respective component carriers
20 in the digital region, a sampling frequency for AD/DA conversion is higher, and the
load of the ADC and the DAC can thereby increase.
[0059]
(Signal processing unit)
Referring back to Fig. 5, an example of a configuration of the user equipment 100 is
25 further described below.
[0060]
The signal processing unit 150 performs signal processing such as deinterleaving,
decoding or error correction on the demodulated data signal that is input from the
radio communication unit 110. Then, the signal processing unit 150 outputs the
30 processed data signal to an upper layer. Further, the signal processing unit 150
performs signal processing such as encoding or interleaving on the data signal that is
input from the upper layer. Then, the signal processing unit 150 outputs the
processed data signal to the radio communication unit 1 10.
[006 11
(Control unit)
5 The control unit 160 controls the overall functions of the user equipment 100 by
using a processing device such as a central processing unit (CPU) or a digital signal
processor (DSP). For example, the control unit 160 controls the timing of data
communication by the radio communication unit 110 according to scheduling
information that is received from the base station 200 by the radio communication
10 unit 110. Further, the control unit 160 controls the measurement unit 170 to
measure the channel quality by using a reference signal from the base station 200,
which is a serving base station, and transmits the channel quality report to the base
station 200 through the radio communication unit 1 10. Further, the control unit 160
controls the measurement unit 170 to execute measurement during the periods of
15 measurement gaps which are allocated by the base station 200.
[0062]
Further, in this embodiment, before a handover command from the source base
station is received, the control unit 160 acquires the CP length being used by the
target base station by receiving system information from the relevant target base
20 station. The system information is delivered through a broadcast channel which is
placed in a prescribed position in the radio h e , for example. Then, at the time of
a handover, the control unit 160 controls the radio communication unit 1 10 to initiate
a random access to the target base station after all of handover commands for all of
the plurality of component carriers are received from the source base station by the
25 radio communication unit 110. Further, at the initiation of a random access to the
target base station, for example, the control unit 160 changes the setting of the CP
length in the radio communication unit 11 0 to the CP length being used by the target
base station.
[0063]
30 Note that, when the CP length being used by the source base station and the CP
length being used by the target base station are equal, the control unit 160 may
control the radio communication unit 110 to initiate a random access to the target
base after receiving a handover command for any of the component carriers, without
waiting for handover commands for the other component carriers. In this case, the
control unit 160 does not need to change the setting of the CP length in the radio
communication unit 1 10.
[0064]
(Measurement unit)
The measurement unit 170 measures the channel quality for each of the component
carriers by using a reference signal from the base station 200 according to control
from the control unit 160, for example. Further, the measurement unit 170 executes
measurement for a handover with respect to each of the component carriers by using
the measurement gaps which are allocated by the base station 200. A result of the
measurement executed by the measurement unit 170 is converted to a predetermined
format for a measurement report by the control unit 160 and transmitted to the base
station 200 through the radio communication unit 110. After that, the base station
200 determines, based on the measurement report, whether a handover should be
executed or not for the user equipment 100.
[0065]
(3-2. Exemplary Configuration of Base Station)
Fig. 7 is a block diagram showing an example of a configuration of the base station
200 according to the embodiment. Referring to Fig. 7, the base station 200 includes
a radio communication unit 210, an interface unit 250, a component carrier (CC)
management unit 260, and a control unit 280.
[0066]
(Radio communication unit)
A specific configuration of the radio communication unit 210 may be similar to the
configuration of the radio communication unit 1 10 of the user equipment 100 which
is described above with reference to Fig. 6, although the number of component
carriers to be supported, the requirements of processing performance or the like are
different. The radio communication unit 2 10 performs a radio communication with
the user equipment over a communication channel which is formed by aggregating a
plurality of component carriers with use of the carrier aggregation technology.
[0067]
(Interface unit)
The interface unit 250 mediates a communication between the radio communication
5 unit 2 10 or the control unit 280 and an upper node through the S 1 interface illustrated
in Fig. 4, for example. Further, the interface unit 250 mediates a communication
between the radio communication unit 210 or the control unit 280 and another base
station through the X2 interface illustrated in Fig. 4, for example.
[0068]
10 (CC management unit)
The CC management unit 260 holds data that indicates which component carrier
each user equipment is using for communication with respect to each of the user
equipments belonging to the cell of the base station 200. Such data can be updated
by the control unit 280 when an additional user equipment joins the cell of the base
15 station 200 or when the existing user equipment changes its component carriers.
Thus, the control unit 280 can recognize which component carrier the user equipment
100 is using by referring to the data held by the CC management unit 260.
[0069]
(Control unit)
20 The control unit 280 controls the overall functions of the base station 200 by using a
processing device such as a CPU or a DSP. For example, the control unit 280
allocates communication resources for data communication to the user equipment
100 and other user equipments and then delivers scheduling information over a
broadcast channel in a given sub-frame. Further, the control unit 280 delivers other
25 system information over the broadcast channel, for example. The system
information contains the set value of the CP length being used by the base station
200, for example. Further, the control unit 280 controls the base station 200 to
operate in the same manner as the source base station or the target base station in the
handover procedure which is descried with reference to Fig. 1.
30 [0070]
(3-3. Flow of Process)
A handover procedure according to the embodiment is described hereinafter with
reference to Figs. 8A and 8B. Note that, in the following scenario, it is assumed
that a handover procedure is performed among the user equipment 100, the base
station 200a serving as a source base station, and the base station 200b serving as a
5 target base station. Further, for simplification of description, it is assumed in this
scenario that the user equipment 100 performs a radio communication by using two
component carriers. Furthermore, for the procedure up to measurement in the user
equipment (steps S2 to S14) in the typical handover procedure illustrated in Fig. 1,
explanation is omitted because there is no significant difference.
10 [0071]
Referring to Fig. 8A, the user equipment 100 first transmits a measurement report for
the component carrier CCl, for example, to the base station 200a (step S112).
Receiving the measurement report, the base station 200a determines the necessity of
a handover based on the measurement report. For example, when a channel quality
15 between the user equipment 100 and the base station 200b is better than a channel
quality between the user equipment 100 and the base station 200a by a predetermined
threshold or greater, it can be determined that a handover is necessary. In this case,
the base station 200a transmits a handover request message for the component carrier
CC1 to the base station 200b (step S114). Receiving the handover request message,
20 the base station 200b determines whether it is possible to accept the user equipment
100 according to the availability of a communication service offered by itself or the
like. When the base station 200b determines that it is possible to accept the user
equipment 100, the base station 200b transmits a handover request confirm message
to the base station 200a (step S116). Receiving the handover request confirm
25 message, the base station 200a transmits a handover command for the component
carrier CC 1 to the user equipment 100 (step S 1 1 8).
[0072]
Up to this point of time, the user equipment 100 acquires the CP length being used
by the base station 200b by receiving system information from the base station 200b
30 (for example, the system information may be received during execution of the
measurement). For example, the CP length being used by the base station 200b is
the CP length of the extended CP. On the other hand, the CP length being used by
the base station 200a is the CP length of the normal CP. In this case, the user
equipment 100 which has received a handover command from the base station 200a
determines to wait until a handover command for another component carrier (i.e.
5 CC2) is received, without initiating a handover (S 120).
[0073]
Further, the user equipment 100 transmits a measurement report for the component
carrier CC2 to the base station 200a (step S122). Then, the base station 200a
transmits a handover request message for the component carrier CC2 to the base
10 station 200b (step S124). Receiving the handover request message, the base station
200b transmits a handover request confirm message to the base station 200a (step
S126). Receiving the handover request confirm message, the base station 200a
transmits a handover command for the component carrier CC2 to the user equipment
100 (step S128).
15 [0074]
When the step S128 ends, handover comrnmds for all of the plurality of component
carriers constituting the communication channel between the user equipment 100 and
the base station 200a have been received by the user equipment 100. As a result,
the user equipment 100 determines to initiate a handover (step S 130).
20 [0075]
Referring to Fig. 8B, the user equipment 100 which has determined the initiation of a
handover first acquires synchronization with the downlink channel of the component
carrier CC1 of the base station 200b. Then, the user equipment 100 makes a
random access to the base station 200b by using a random access channel in a given
25 time slot of the component carrier CCl (step S134). During this period, the base
station 200a forwards data addressed to the user equipment 100 to the base station
200b (step S136). Then, after success in the random access for the component
carrier CC1, the user equipment 100 transmits a handover complete message to the
base station 200b (step S142). Receiving the handover complete message, the base
30 station 200b requests the MME to perform route update for the component carrier
CC 1 of the user equipment 100 (step S 144). Upon updating the route of user data
by the MME, the user equipment 100 becomes able to communicate with another
device through a new base station (i.e. the base station 200b). Note that the request
for route update may be performed for each of the plurality of component carriers, or
performed only once for the plurality of component carriers. Then, the base station
5 200b transmits acknowledgement for the handover complete message to the user
equipment 100 (step S 146).
[0076]
Further, the user equipment 100 acquires synchronization with the downlink channel
of the component carrier CC2 of the base station 200b. Then, the user equipment
10 100 makes a random access to the base station 200b by using a random access
channel in a given time slot of the component carrier CC2 (step S 154). During this
period, the base station 200a keeps forwarding data addressed to the user equipment
,
100 to the base station 200b (step S 156). Then, after success in the random access
for the component carrier CC2, the user equipment 100 transmits a handover
15 complete message to the base station 200b (step S162). Receiving the handover
complete message, the base station 200b requests the MME to perform route update
for the component carrier CC2 of the user equipment 100 (step S164). Then, the
base station 200b transmits acknowledgement for the handover complete message to
the user equipment 100 (step S 166).
20 [0077]
It should be noted that, except that the user equipment 100 should initiate a random
access after receiving handover commands for all of the component carriers, the
respective steps in the scenario illustrated in Fig. 8A and Fig. 8B may be executed in
a different sequence. Fig. 9 is a sequence chart showing another example of a flow
25 of a handover procedure according to the embodiment.
[0078]
Referring to Fig. 9, the user equipment 100 first transmits a measurement report for
the component carrier CC 1 to the base station 200a (step S212). Further, the user
equipment 100 first transmits a measurement report for the component carrier CC2 to
30 the base station 200a (step S213). Note that those measurement reports may be
integrated.
[0079]
Receiving the measurement reports, the base station 200a determines the necessity of
a handover based on the measurement report. When the base station 200a
determines that a handover is necessary, the base station 200a first transmits a
5 handover request message for the component carrier CC1 to the base station 200b
(step S214). The base station 200a then transmits a handover request message for
the component carrier CC2 to the base station 200b (step S215).
[OOSO]
Receiving the handover request messages, the base station 200b determines whether
10 it is possible to accept the user equipment 100 according to the availability of a
communication service offered by itself or the like. When the base station 200b
determines that it is possible to accept the user equipment 100, the base station 200b
first transmits a handover request confirm message for the component carrier CC1 to
the base station 200a (step S2 16). The base station 200b then transmits a handover
15 request confirm message for the component carrier CC2 to the base station 200a
(step S217).
[008 11
Receiving the handover request confirm messages, the base station 200a transmits a
handover command for the component carrier CC1 to the user equipment 100 (step
20 S218). Further, the base station 200a transmits a handover command for the
component carrier CC2 to the user equipment 100 (step S2 19).
[0082]
Receiving the handover command for the component carrier CC1, the user
equipment 100 waits until the handover command for the component carrier CC2 is
25 received. Then, when the user equipment 100 receives the handover command for
the component carrier CC2, it determines to initiate a handover (step S220).
[OOS3]
The, the user equipment 100 first acquires synchronization with the downlink
channel of the component carrier CC1 of the base station 200b. Further, the user
30 equipment 100 makes a random access to the base station 200b by using a random
access channel in a given time slot of the component carrier CC1 (step S222). Then,
the user equipment 100 acquires synchronization with the downlink channel of the
component carrier CC2 of the base station 200b. Further, the user equipment 100
makes a random access to the base station 200b by using a random access channel in
a given time slot of the component carrier CC2 (step S223). During this period, the
5 base station 200a forwards data addressed to the user equipment 100 to the base
station 200b (steps S224, S225).
[0084]
After that, upon success in the random access for the component carrier CC1, the
user equipment 100 transmits a handover complete message for the component
10 carrier CC1 to the base station 200b (step S232). Further, upon success in the
random access for the component carrier CC2, the user equipment 100 transmits a
handover complete message for the component carrier CC2 to the base station 200b
(step S233). Receiving the handover complete messages, the base station 200b
requests the MME to perform route update for the component carriers CC1 and CC2
15 of the user equipment 100 (steps S234, S235). Then, the base station 200b
transmits acknowledgements for the respective handover complete messages to the
user equipment 100 (steps S236, S237).
[0085]
(3-4. Summary of First Embodiment)
20 The first embodiment of the present invention is described above with reference to
Figs. 5 to 9. According to the embodiment, the control unit 160 of the user
equipment 100 initiates a random access to the target base station by the radio
communication unit 110 after receiving handover commands for all of the plurality
of component carriers constituting the communication channel. In this case, a
25 situation does not occur where the user equipment performs a communication with
the source base station in a certain component carrier and also performs a
communication with the target base station in another component carrier at the same
time. It is thereby possible to avoid the user equipment 100 from simultaneously
handing different CP lengths during a handover procedure.
30 [0086]
Further, when the CP length being used by the source base station and the CP length
being used by the target base station are equal, the user equipment 100 may initiate a
random access soon after receiving a handover command for any of the component
carriers, without waiting for a handover command for the other component carrier.
It is thereby possible to complete a handover promptly for the component carrier for
5 which a handover request is confirmed when it is not necessary to switch the CP
length.
[0087]
Furthermore, the user equipment 100 may acquire the CP length being used by the
target base station by using the system information which is broadcasted from the
10 relevant target base station. The user equipment 100 may receive the system
information which is broadcasted from the target base station at the time of
measurement, for example. This configuration eliminates the need for exchanging
an additional message for determination about waiting for a random access
depending on the CP length, and it is thereby possible to reduce the impact on the
15 existing system.
[OOSS]
<4. Description of Second Embodiment>
A second embodiment of the present invention for avoiding simultaneous handling of
different CP lengths during a handover procedure in a radio communication
20 involving the carrier aggregation is described hereinafter with reference to Figs. 10 to
14.
[0089]
In the second embodiment of the present invention, communication resources are
scheduled in such a way that the user equipment can perform a radio communication
25 with the source base station and the target base station by switching the CP length in
time division during a handover procedure.
[0090]
Fig. 10 is an explanatory view to describe an outline of switching of a cyclic prefix
length in time division.
30 [0091]
Referring to Fig. 10, as a result that a handover is executed sequentially for three
component carriers CCl to CC3, a series of handover procedure can be divided into
four stages. The first stage is before a handover. At the first stage, all component
carriers are connected to the source base station. The second stage is after a
handover of the component carrier CC1. At the second stage, the component carrier
CC1 is connected to the target base station, and the component carriers CC2 and CC3
are connected to the source base station. The third stage is after a handover of the
component carriers CCl and CC2. At the third stage, the component carriers CC1
and CC2 are connected to the target base station, and the component carrier CC3 is
connected to the source base station. The fourth stage is after a handover of all the
component carriers. At the fourth stage, all component carriers are connected to the
target base station.
[0092]
Among those stages, at the first stage, because all component carriers are connected
to the source base station, the user equipment can use the same CP length (e.g. the
normal CP) at any point of time. Thus, the source base station can allocate
communication resources for each component carrier without depending on the CP
length. Likewise, at the fourth stage, because all component carriers are connected
to the target base station, the user equipment can use the same CP length (e.g. the
extended CP) at any point of time. Thus, the target base station can allocate
communication resources for each component carrier without depending on the CP
length.
[0093]
On the other hand, at the second stage and the third stage, a part of a plurality of
component carriers is connected to the target base station, and the rest is connected to
the source base station. In this case, a communication resource is allocated only for
any one component carrier with respect to each sub-frame by the source base station
and the target base station, for example. For example, in the example of Fig. 10, at
the second stage, a communication resource of the sub-frame SF1 is allocated for the
component carrier CC1, a communication resource of the sub-fiame SF2 is allocated
for the component carrier CC2, and a communication resource of the sub-frame SF3
is allocated for the component carrier CC3. Further, at the third stage, a
communication resource of the sub-frame SF4 is allocated for the component carrier
CC1, a communication resource of the s u b - h e SF5 is allocated for the component
carrier CC2, and a communication resource of the sub-frame SF6 is allocated for the
component carrier CC3. Such an allocation of communication resources can be
5 achieved by exchanging scheduling information between the source base station and
the target base station through a backhaul link, for example,
[0094]
As described above, in this embodiment, a communication resource is allocated only
for any one component carrier with respect to each sub-frame. In this case, even
10 when the CP lengths to be used are different between the source base station and the
target base station, the user equipment can communicate with the respective base
stations in time division by changing the setting of the CP length with respect to each
sub-frame. Changing the CP length to use with respect to each subframe during a
handover procedure results in avoiding simultaneous handling of different cyclic
15 prefix lengths in one user equipment.
[0095]
Note that, in such scheduling, it is preferred to set the ratio (ratio in a given time
window) of the amount of communication resource allocated for a part of component
carrier for which a handover is completed and the amount of communication
20 resource allocated for the rest of the component carriers to be substantially the same
as the radio of the number of component carriers between the former and the latter.
In the example of Fig. 10, the ratio of the amount of communication resources at the
second stage is 1:2 (after:before handover). Further, the ratio of the amount of
communication resources at the third stage is 2:l (after:before handover).
25 According to such scheduling, it is possible to share traffic among a plurality of
component caniers in the case of allocating a communication resource only for any
one component carrier with respect to each sub-frame.
[0096]
The configurations of a user equipment 300 and a base station 400 according to the
30 embodiment for achieving the above-described switching of the SP length in time
division during a handover procedure are specifically described hereinbelow.
[0097]
(4- 1. Exemplary Configuration of User Equipment)
Fig. 11 is a block diagram showing an example of a configuration of the user
equipment 300 according to the embodiment. Referring to Fig. 11, the user
5 equipment 300 includes a radio communication unit 3 10, a signal processing unit 150,
a control unit 360, and a measurement unit 170.
[0098]
(Radio communication unit)
The radio communication unit 310 performs a radio communication with the base
10 station 400 over a communicati,on channel formed by aggregating a plurality of
component carriers with use of the carrier aggregation technology.
[0099]
Fig. 12 is a block diagram showing an example of a detailed configuration of the
radio communication unit 3 10. Referring to Fig. 12, the demodulation unit 128 of
15 the radio communication unit 310 includes a CPU removal unit 329. Further, the
modulation unit 130 includes a CPU insertion unit 33 1.
[O 1 001
The CPU removal unit 329 removes a cyclic prefix from the baseband signal of each
component carrier which is input to the demodulation unit 128. The CP length of
20 the cyclic prefix which is removed by the CPU removal unit 329 can be switched
with respect to each sub-frame according to a CP length control signal SIGl that is
input from the control unit 360. Further, the CPU insertion unit 33 1 inserts a cyclic
prefix into the baseband signal which is generated by the modulation unit 130. The
CP length of the cyclic prefix which is inserted by the CPU insertion unit 33 1 can be
25 switched with respect to each sub-fiame according to a CP length control signal
SIG2 that is input from the control unit 360. The operation of the radio
communication unit 3 10 other than the switching of the setting of the CP length in
the CPU removal unit 329 and the CPU insertion unit 331 may be the same as the
operation of the radio communication unit 110 according to the first embodiment
30 described above with reference to Fig. 5.
[OlOl]
(Control unit)
The control unit 360 controls the overall hctions of the user equipment 300, like
the control unit 160 according to the first embodiment. For example, the control
unit 360 controls the timing of data communication by the radio communication unit
5 3 10 according to scheduling information that is received from the base station 400 by
the radio communication unit 310. Further, in this embodiment, the control unit
360 switches the setting of the CP length of the CPU removal unit 329 and the CPU
insertion unit 33 1 of the radio communication unit 3 10 with respect to each subframe
according to the allocation of communication resources by outputting the CP
10 length control signals SIGl and SIG2 to the radio communication unit 310 at the
state during a handover procedure (e.g. at the second and third stages described with
reference to Fig. 10).
[O 1 021
It should be noted that, the synchronization to a downlink channel from the base
15 station is performed by using both of a primary synchronization signal that is
transmitted in the first sub-frame and a secondary synchronization signal that is
transmitted in the sixth sub-frame in one radio frame. Therefore, it is preferred in
the synchronization processing to switch the base station to be synchronized with
respect to each radio h e , not each sub-frame. Alternatively, in the radio
20 communication unit 3 10, a synchronous circuit for the normal CP and a synchronous
circuit for the extended CP may be placed in parallel in the radio communication unit
310. In this case, it is not necessary to switch the base station to be synchronized
with respect to each radio frame. Because the synchronous circuit is implemented
only in the physical layer, an impact on the system is small even when a plurality of
25 synchronous circuits are placed in parallel.
[0 1031
Further, delivery of system information from the base station is also performed in a
particular sub-frame in one radio frame. Therefore, for the reception of system
information in the user equipment 100, like the synchronization processing described
30 above, it is preferred to switch the base station to be synchronized with respect to
each radio frame, not each sub-frame. In this case, because the base station 400
delivers the same system information over a plurality of successive radio frames, the
user equipment 300 can acquire the system information of the source base station and
the target base station without a loss.
[O 1041
5 (4-2. Exemplary Configuration of Base Station)
Fig. 13 is a block diagram showing an example of a configuration of the base station
400 according to the embodiment. Referring to Fig. 13, the base station 400
includes a radio communication unit 210, an interface unit 250, a CC management
unit 260, and a control unit 480.
10 [0105]
(Control unit)
The control unit 480, like the control unit 280 according to the first embodiment,
controls the overall functions of the base station 400. For example, the control unit
480 allocates communication resources for data communication to the user
15 equipment 300 and other user equipments and then delivers scheduling information
over a broadcast channel in a given sub-frame. Further, the control unit 480
delivers other system information over the broadcast channel, for example. The
system information contains the set value of the CP length being used by the base
station 400, for example.
20 [0106]
In this embodiment, at a stage during a handover procedure, the control unit 480 of
the source base station allocates a communication resource in a sub-frame, which is
different from a s u b - h e in which a communication resource is allocated by the
target base station for a part of component carriers for which a handover is
25 completed, for the rest of the component carriers. Specifically, the control unit 480
acquires scheduling information for a part of component carriers for which a
handover is completed from the target base station through a backhaul link, for
example. Then, the control unit 480 allocates a communication resource in a subframe
in which a communication resource is not allocated in the scheduling
30 information for the rest of the component carriers.
[0 1071
Further, the control unit 480 adjusts scheduling so that the ratio of the amount of
communication resource allocated for a part of component carriers for which a
handover is completed and the amount of communication resource allocated for the
rest of the component carriers is substantially the same as the radio of the number of
5 component carriers between the former and the latter. Specifically, the control unit
480 of the source base station acquires the ratio between the number of component
carriers after handover completion and the number of component carriers before
handover completion (which is referred to hereinafter as the ratio of the number of
CCs) based on information exchange with the target base station. Then, the control
10 unit 480 allocates communication resources to the user equipment 100 in such a way
that the ratio of the amount of communication resources is substantially the same as
the ratio of the number of CCs. Alternatively, the control unit 480 may designate
the amount of communication resources which the target base station should allocate
to the user equipment 100 to the target base station through a backhaul link.
15 [0108]
(4-3. Flow of Process)
Fig. 14 is an explanatory view to describe an example of a flow of a handover
procedure according to the embodiment. Fig. 14 shows an allocation of
communication resources at the four stages related to the handover procedure which
20 is performed in the same manner as shown in Fig. 10. Note that, in Fig. 14, like Fig.
10, the user equipment sequentially executes a handover from the source base station
to the target base station for three component carriers CCl to CC3.
[0 1 091
First, at the first stage, all component carriers are connected to the source base station.
25 Therefore, any s u b - h e can be allocated for the component carriers before
completion of a handover (using the normal CP). Note that it is not practically
necessary to allocate communication resources to all of those sub-frames. Further,
the synchronization signal is used for the synchronization with the source base
station.
30 [OllO]
Next, at the second stage, the ratio of the number of CCs is 1:2 (after:before
handover). Therefore, among the six successive sub-frames (excluding the subframe
for the synchronization signal), for example, two sub-frames can be allocated
for the component carrier after completion of a handover (using the extended CP),
and four sub-frames can be allocated for the component carrier before completion of
5 a handover. For example, when a handover is completed for the component carrier
CCl, a first sub-fiame (#I) and a second sub-fiame (#2) can be allocated for the
component carrier CC1. Further, after the synchronization signal in one radio frame
is used for the synchronization with the source base station, the synchronization
signal in the next radio frame can be used for the synchronization with the target base
10 station.
[Olll]
Then, at the third stage, the ratio of the number of CCs is 2: 1 (after:before handover).
Therefore, among the six successive sub-frames (excluding the s u b - h e for the
synchronization signal), for example, four sub-frames can be allocated for the
15 component carrier after completion of a handover (using the extended CP), and two
sub-frames can be allocated for the component carrier before completion of a
handover. For example, when a handover is completed for the component carrier
CC2 subsequent to the component carrier CCl, a third sub-frame (#3) and a fourth
sub-frame (#4) can be allocated for the component carrier CC2. Further, after the
20 synchronization signal in one radio frame is used for the synchronization with the
source base station, the synchronization signal in the next radio b e can be used for
the synchronization with the target base station.
[0112]
Finally, at the fourth stage, all component carriers are connected to the target base
25 station. Therefore, any sub-frame can be allocated for the component carriers after
completion of a handover. Further, the synchronization signal is used for the
synchronization with the target base station.
[0113]
(4-4. Summary of Second Embodiment)
30 The second embodiment of the present invention is described above with reference to
Figs. 10 to 14. According to the embodiment, when a handover to the target base
station is completed only for a part of a plurality of component carriers, a
communication resource in a sub-Me which is different fiom a s u b - b e within
which a communication resource is allocated by the target base station for the part of
the plurality of component carriers is allocated by the source base station for the rest
5 of the plurality of component carriers. In this configuration, a situation does not
occur where the user equipment 300 performs a radio communication with the source
base station and performs a radio communication with the target base station at the
same time during a handover procedure. Therefore, the user equipment 300 can
sequentially perform radio communications with the source base station and the
10 target base station during a handover procedure by switching the CP length in time
division manner. It is thereby possible to avoid simultaneous handling of different
cyclic prefix lengths during a handover procedure in a radio communication
involving the carrier aggregation.
[0114]
15 Further, according to the embodiment, scheduling is adjusted so that the ratio of the
amount of communication resource allocated by the target base station for the
component carrier with handovers completed and the amount of communication
resource allocated by the source base station for the component carrier with
handovers not yet completed is substantially the same as the radio of the number of
20 component carriers with handovers completed and the number of component carriers
with handovers not yet completed. The amount of traffic is thereby equally shared
among the component carriers, and it is thus possible to carry out data transmission
during a handover procedure.
[0115]
25 Further, the user equipment 300 may perform the synchronization with the source
base station by using the synchronization signal in a radio frame which is different
fiom a radio frame in which the synchronization signal to be used for the
synchronization with the target base station is contained. Further, the user
equipment 300 can receive system information from the source base station in a radio
30 fiame which is different from a radio frame in which system information from the
target base station is received. This configuration eliminates the need for using a
plurality of synchronous circuits or a plurality of processing units for handling
different CP lengths, and it is thereby possible to suppress the manufacturing cost of
the device.
[0116]
5 Although preferred embodiments of the present invention are described in detail
above with reference to the appended drawings, the present invention is not limited
thereto. It should be understood by those skilled in the art that various
modifications, combinations, sub-combinations and alterations may occur depending
on design requirements and other factors insofar as they are within the scope of the
10 appended claims or the equivalents thereof.
[Reference Signs List]
[0117]
1 RADIO COMMUNICATION SYSTEM
100,300 USER EQUIPMENT
1 5 1 10,3 10 RADIO COMMUNICATION UNIT (USER EQUIPMENT)
160,360 CONTROL UNIT (USER EQUIPMENT)
200,400 BASE STATION
2 10 RADIO COMMUNICATION UNIT (BASE STATION)
280,480 CONTROL UNIT (BASE STATION)
20
In so far as the embodiments of the invention described above are implemented, at
least in part, using software-controlled data processing apparatus, it will be
appreciated that a computer program providing such software control and a
transmission, storage or other medium by which such a computer program is
25 provided are envisaged as aspects of the present invention.

CLAIMS
A mobile station in a wireless communication network, the mobile station
comprising:
5 a radio communication unit configured to communicate with a first base
station via a plurality of component carriers; and
a control unit configured to control the radio communication unit to initiate
a handover procedure to a second base station after receiving at least one handover
command for the plurality of component carriers.
10 [Claim 21
The mobile station of claim 1, wherein
the radio communication unit is configured to communicate with the first
base station via one of the plurality of component carriers having a first cyclic prefix
length, and communicate with the second base station via one of the plurality of
15 component carriers having a second cyclic prefix length.
[Claim 31
The mobile station of claim 1, wherein
the radio communication unit is configured to receive a plurality of
handover commands, each corresponding to one of the plurality of component
20 carriers, and
the control unit is configured to control the radio communication unit to
initiate a handover procedure to the second base station for each of the plurality of
component carriers after receiving the plurality of handover commands.
[Claim 41
25 A mobile station in a wireless communication network, the mobile station
comprising:
a radio communication unit configured to communicate with a first base
station via a plurality of component carriers each having a first cyclic prefix length;
and
3 0 a control unit configured to control the radio communication unit to initiate
a handover procedure from the first base station to a second base station after
receiving a handover command,
wherein, after a handover for one of the plurality of component carriers, the
control unit is configured to control the radio communication unit to communicate
with the second base station via a component carrier allocated to a time slot different
5 from a time slot to which each of the plurality of component carriers are allocated,
and having a second cyclic prefix length different from the first cyclic prefix length.
[Claim 51
The mobile station of claim 4, wherein
the control unit is configured to control the radio communication unit to
10 release a first one of the plurality of component carrier connections with the first
base station and to connect to the second base station using the component carrier
having the second cyclic prefix length.
[Claim 61
The mobile station of claim 5, wherein
15 the control unit is configured to control the radio communication unit to
initiate a handover procedure to the second base station for a second one of the
plurality of component carriers after the handover procedure for the first one of the
plurality of component carriers has been initiated.
[Claim 71
20 A base station in a wireless communication network, the base station comprising:
a radio communication unit configured to communicate with a mobile
station via a plurality of component carriers; and
a control unit configured to control the radio communication unit to initiate
a handover procedure to a second base station for one of the plurality of component
25 carriers after receiving a report from the mobile station corresponding to the one of
the plurality of component carriers.
[Claim 81
The base station of claim 7, wherein
the control unit is configured to initiate the handover procedure by
30 controlling the radio communication unit to transmit a handover request command to
the second base station and transmit a handover command to the mobile station.
[Claim 91
A base station in a wireless communication network, the base station comprising:
a radio communication unit configured to communicate with a mobile
station via a plurality of component carriers each having a first cyclic prefix length;
5 and
a control unit configured to control the radio communication unit to initiate
a handover procedure to a second base station for one of the plurality of component
carriers,
wherein, after a handover for one of the plurality of component carriers, the
10 mobile station communicates with the second base station via a component carrier
allocated to a time slot different from a time slot to which each of the plurality of
component carriers is allocated, and having a second cyclic prefix length different
from the first cyclic p r e f ~le ngth.
[Claim 101
15 The base station of claim 9, wherein
the control unit is configured to initiate the handover procedure by
controlling the radio communication unit to transmit a handover request command to
the second base station and transmit a handover command to the mobile station.
[Claim 111
20 A handover method performed by a mobile station in a wireless communication
network, the method comprising:
communicating, by a radio communication unit of the mobile station, with a
first base station via a plurality of component carriers; and
initiating, by a control unit of the mobile station, a handover procedure to a
25 second base station after receiving at least one handover command for the plurality
of component carriers.
[Claim 121
A handover method performed by a mobile station in a wireless communication
network, the method comprising:
3 0 communicating, by a radio communication unit of the mobile station, with a
first base station via a plurality of component carriers each having a first cyclic
prefix length;
initiating, by a control unit of the mobile station, a handover procedure from
the first base station to the second base station after receiving a handover command;
and
5 controlling, by the control unit after a handover for one of the plurality of
component carriers, the radio communication unit to communicate with the second
base station via a component carrier allocated to a time slot different from a time slot
to which each of the plurality of component carriers are allocated, and having a
second cyclic prefix length different fkom the first cyclic prefix length.
10 [Claim 131
A handover method performed by a base station in a wireless communication
network, the method comprising:
communicating, by a radio communication unit of the base station, with a
mobile station via a plurality of component carriers; and
15 initiating, by a control unit of the base station, a handover procedure to a
second base station for one of the plurality of component carriers after receiving a
report from the mobile station corresponding to the one of the plurality of component
carriers.
[Claim 141
20 A handover method performed by a base station in a wireless communication
network, the method comprising:
communicating, by a radio communication unit of the base station, with a
mobile station via a plurality of component carriers each having a first cyclic prefix
length; and
25 initiating, by a control unit of the base station, a handover procedure to a
second base station for one of the plurality of component carrier;
wherein, after a handover for one of the plurality of component carriers, the
mobile station communicates with the second base station via a component carrier
allocated to a time slot different from a time slot to which each of the plurality of
30 component carriers is allocated, and having a second cyclic prefix length different
from the first cyclic prefix length.
[Claim 151
A non-transitory computer readable medium including computer-program
instructions, which when executed by a mobile station in a wireless communication
network, cause the mobile station to perform a handover method comprising:
5 communicating with a first base station via a plurality of component
carriers; and
initiating a handover procedure to a second base station after receiving at
least one handover command for the plurality of component carriers.
[Claim 161
10 A non-transitory computer readable medium including computer-program
instructions, which when executed by a mobile station in a wireless communication
network, cause the mobile station to perform a handover method comprising:
communicating with a first base station via a plurality of component carriers
each having a first cyclic prefix length;
15 initiating a handover procedure from the first base station to the second base
station after receiving a handover command; and
communicating, after a handover for one of the plurality of component
carriers, with the second base station via a component carrier allocated to a time slot
different from a time slot to which each of the plurality of component carriers are
20 allocated, and having a second cyclic prefix length different from the first cyclic
prefix length.
[Claim 171
A non-transitory computer readable medium including computer-program
instructions, which when executed by a base station in a wireless communication
25 network, cause the base station to perform a handover method comprising:
communicating with a mobile station via a plurality of component carriers;
and
initiating a handover procedure to a second base station for one of the
plurality of component carriers after receiving a report from the mobile station
30 corresponding to the one of the plurality of component carriers.
[Claim 181
A non-transitory computer readable medium including computer-program
instructions, which when executed by a base station in a wireless communication
network, cause the base station to perform a handover method comprising:
communicating with a mobile station via a plurality of component carriers
5 each having a first cyclic prefix length; and
initiating a handover procedure to a second base station for one of the
plurality of component carriers,
wherein, after a handover for one of the plurality of component carriers, the
mobile station communicates with the second base station via a component carrier
10 allocated to a time slot different fiom a time slot to which each of the plurality of
component carriers is allocated, and having a second cyclic prefix length different
fiom the first cyclic prefix length.
[Claim 191
A wireless communication system comprising:
15 a mobile station including a radio communication unit configured to
communicate with a first base station via a plurality of component carriers;
a first control unit at the first base station configured to initiate a handover
procedure to a second base station for one of the plurality of component carriers after
receiving a report fiom the mobile station corresponding to the one of the plurality of
20 component carriers; and
a second control unit at the mobile station configured to initiate a handover
procedure to the second base station after receiving at least one handover command
for the plurality of component carriers fiom the first base station.
[Claim 201
25 A wireless communication system comprising:
a first base station configured to communicate with a mobile station via a
plurality of component carriers each having a first cyclic prefix length;
the mobile station configured to initiate a handover procedure fiom the first
base station to a second base station for one of the plurality of component carriers
30 after receiving a handover command fiom the first base station; and
the second base station configured to communicate with the mobile station
via a component carrier allocated to a time slot different fiom a time slot to which
each of the plurality of component carriers is allocated, and having a second cyclic
prefix length different fiom the first cyclic prefix length.