The present invention relates to a method for performing a handover, a user
equipment, 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.
[0005]
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
- i".& .
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 [PTL 11
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
cany out a handover procedure in a radio communication involving the carrier
aggregation.
[0009]
In the above-described existing handover procedure, processing such as request for a
handover, confirmation of the request, issue of a handover command or random
5 access to a target base station is performed based on the assumption that one
communication channel is composed of one component carrier. In a radio
communication involving the carrier aggregation also, it is desirable to perform such
processing in the same manner as the existing procedure in order not to cause a
significant impact on a system or a device which is widely used today.
10 [OOlO]
In light of the foregoing, it is desirable to provide a novel and improved method for
performing a handover, user equipment, base station, and radio communication
system that can carry out a handover during a radio communication involving the
carrier aggregation without causing a significant impact on an existing system or
15 device.
[Solution to Problem]
[OOll]
According to some embodiments, a base station includes a radio communication unit
configured to establish communication with a mobile communication terminal using
20 a plurality of component carriers. The base station further includes a control unit
configured to transmit a command to the mobile communication terminal to reduce
the plurality of component carriers to one prior to handover of communication
between the mobile communication terminal and the base station to another base
station.
25 [0012]
According to some embodiments, a mobile communication terminal includes a radio
communication unit configured to establish communication with a base station using
a plurality of component carriers. The mobile communication terminal further
includes a control unit configured to receive a command from the base station to
30 reduce the plurality of component carriers to one prior to handover of
communication between the mobile communication terminal and the base station to
another base station.
[00 131
According to some embodiments, a communication system includes a first base
station and a mobile communication terminal. The first base station includes a
5 radio communication unit configured to establish communication with a mobile
communication terminal using a plurality of component carriers. The first base
station further includes a first control unit configured to transmit a command to the
mobile communication terminal to reduce the plurality of component carriers to one
prior to handover of communication between the mobile communication terminal
10 and the base station to a second base station. The mobile communication terminal
includes a second control unit configured to reduce the plurality of component
carriers to one upon reception of the command from the first base station.
[00 1 41
According to some embodiments, a non-transitory computer readable medium
15 having instructions stored thereon, which when executed by a processor in a base
station causes the processor to establish communication with a mobile
communication terminal using a plurality of component carriers. The instructions
further cause the processor to transmit a command to the mobile communication
terminal to reduce the plurality of component carriers to one prior to handover of
20 communication between the mobile communication terminal and the base station to
another base station.
[Advantageous Effects of Invention]
[00 1 51
As described above, the method for performing a handover, the user equipment, the
25 base station, and the radio communication system according to the embodiments of
the present invention can carry out a handover during a radio communication
involving the carrier aggregation without causing a significant impact on an existing
system or device.
[Brief Description of Drawings]
30 [0016]
[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.
5 [Fig. 31
Fig. 3 is a schematic view showing an outline of a radio communication system
according to an embodiment.
[Fig. 41
Fig. 4 is a block diagram showing an example of a configuration of a user equipment
10 according to a first embodiment.
[Fig. 51
Fig. 5 is a block diagram showing an example of a detailed configuration of a radio
communication unit according to the first embodiment.
[Fig. 61
15 Fig. 6 is a block diagram showing an example of a configuration of a base station
according to the first embodiment.
[Fig. 71
Fig. 7 is a sequence chart showing an example of a flow along a first scenario of a
handover procedure according to the first embodiment.
20 [Fig. 81
Fig. 8 is a sequence chart showing an example of a flow along a second scenario of a
handover procedure according to the first embodiment.
[Fig. 91
Fig. 9 is a block diagram showing an example of a configuration of a user equipment
25 according to a second embodiment.
[Fig. 101
Fig. 10 is a block diagram showing an example of a configuration of a base station
according to the second embodiment.
[Fig. 1 I]
30 Fig. 11 is a sequence chart showing an example of a flow along a first scenario of a
handover procedure according to the second embodiment.
[Fig. 121
Fig. 12 is a sequence chart showing an example of a flow along a second scenario of
a handover procedure according to the second embodiment.
[Fig. 131
5 Fig. 13 is a block diagram showing an example of a configuration of a user
equipment according to a third embodiment.
[Fig. 141
Fig. 14 is a block diagram showing an example of a configuration of a base station
according to the third embodiment.
10 [Fig. 151
Fig. 15 is a sequence chart showing an example of a flow of a handover procedure
according to the third embodiment.
[Description of Embodiments]
[00 171
15 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.
20 [0018]
Preferred embodiments of the present invention will be described hereinafter in the
following order.
1. Description of Related Art
1 - 1. Handover Procedure
25 1-2. Structure of Communication Resource
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
30 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
4-3. Flow of Process
5 4-4. Summary of Second Embodiment
5. Description of Third Embodiment
5- 1. Exemplary Configuration of User Equipment
5-2. Exemplary Configuration of Base Station
5-3. Flow of Process
10 5-4. Summary of Third Embodiment
[00 191
4. Description of Related Art>
(1 - 1. Handover Procedure)
A technique related to the present invention is described hereinafter with reference to
15 Figs. 1 and 2. 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
entity (MME) are involved in the handover procedure.
20 [0020]
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
a regular basis or when the channel quality falls below a predetermined reference
25 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 from the source base station.
[002 11
Then, the source base station determines the needs of measurement based on the
30 quality report received from the user equipment and, if measurement is necessary,
allocates measurement gaps to the user equipment (step S4).
[0022]
Then, the user equipment searches for a downlink channel fiom a peripheral base
station (i.e. performs cell search) during the periods of the allocated measurement
gaps (step S12). Note that the user equipment can recognize a peripheral base
5 station to search according to a list that is provided in advance fiom the source base
station.
[0023]
When the user equipment acquires synchronization with a downlink channel, the user
equipment performs measurement by using a reference signal contained in the
10 downlink channel (step S 14). 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.
[0024]
Upon completion of the measurement, the user equipment transmits a measurement
15 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.
[0025]
20 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
greater, it can be determined that a handover is necessary. In this case, the source
25 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).
[0026]
Receiving the handover request message, the target base station determines whether
30 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).
[0027]
Receiving the handover request confirm message, the source base station transmits a
5 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
this period, the source base station forwards data addressed to the user equipment to
10 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).
[0028]
Receiving the handover complete message, the target base station requests the MME
15 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
series of handover procedure thereby ends.
20 [0029]
(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.
Referring to Fig. 2, the communication resource in LTE is segmented in the time
25 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.
One resource block includes twelve sub-carriers in the fiequency direction.
30 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 Erame with a given frequency band is reserved as a random
access channel. The random access channel can be used for an access to a base
5 station by a user equipment that has changed from an idle mode to an active mode or
an initial access to a target base station in a handover procedure, for example.
[0030]
<2. Outline of Radio Communication System>
Fig. 3 is a schematic view showing an outline of a radio communication system 1
10 according to an embodiment of the present invention. Referring to Fig. 3, 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 100.
[003 11
15 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
20 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.
25 [0032]
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
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
30 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 S1 interface, for example.
[0033]
It is now assumed that the necessity of a handover to the base station 200b arises
while the user equipment 100 performs a radio communication involving the carrier
5 aggregation with the base station 200a. In this case, as a way to carry out a
handover among the user equipment 100, the base station 200a and the base station
200b without largely altering the existing handover procedure described with
reference to Fig. 1, the number of component carriers constituent of a
communication channel may be shrunk temporarily. Shrinking the number of
10 component carriers means reducing the number of component carriers constituent of
one communication channel by the carrier aggregation technology. For example, if
the number of component carriers is temporarily shrunk to one, a handover can be
carrier out according to the same procedure as the existing handover procedure.
However, the reduction of the number of component carriers can cause a temporary
15 decrease in throughput. The decrease in throughput raises the possibility of the
occurrence of buffer under flow at the receiving end of data or buffer over flow at the
transmitting end of data, which can lead to a failure of a communication service such
as contents delivery. Therefore, in the case of temporarily shrinking the number of
component carriers, it is effective to avoid or reduce the above-described possibility
20 due to a decrease in throughput as much as possible as in the first to third
embodiments of the present invention which are described in detail in the following
sections.
[0034]
It should be noted that, when there is no particular need to distinguish between the
25 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
elements.
[0035]
30 <3. Description of First Embodiment>
A first embodiment of the present invention is described hereinafter with reference to
Figs. 4 to 8.
[0036]
(3- 1. Exemplary Configuration of User Equipment)
Fig. 4 is a block diagram showing an example of a configuration of the user
5 equipment 100 according to the embodiment. Referring to Fig. 4, the user
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.
[0037]
(Radio communication unit)
10 The radio communication unit 110 performs a radio communication with the base
station 200 over a communication channel formed by aggregating a plurality of
component carriers with use of the carrier aggregation technology.
[003 81
Fig. 5 is a block diagram showing an example of a more detailed configuration of the
15 radio communication unit 110. Referring to Fig. 5, the radio communication unit
11 0 includes an antenna 112, 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)
20 132a to 132c, a plurality of filters 134a to 134c, a plurality of up-converters 136a to
136c, a combiner 138, and a power amplifier (PA) 140.
[0039]
The antenna 112 receives a radio signal transmitted from the base station 200 and
outputs the received signal to the LNA 120 through the switch 114. The LNA 120
25 amplifies the received signal. The down-converter 122a and the filter 124a separate
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
30 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
converted to a digital signal by the ADC 126c and output to the demodulation unit
5 128. After that, the demodulation unit 128 generates a data signal by demodulating
the baseband signals of the respective component carriers and outputs the data signal
to the signal processing unit 150.
[0040]
Further, when a data signal is input from the signal processing unit 150, the
10 modulation unit 130 modulates the data signal and generates baseband signals of the
respective component carriers. Among those baseband 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
15 up-converter 136a. Likewise, the baseband signal of the second component carrier
(CC2) is converted to an analog signal by the DAC 132b. 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 up-converter 136b.
Further, the baseband signal of the third component carrier (CC3) is converted to an
20 analog signal by the DAC 132c. Then, a frequency component corresponding to the
third component carrier in the transmission signal is generated from the analog signal
by the filter 134c and the up-converter 136c. After that, the generated fiequency
components corresponding to the three component carriers are combined by the
combiner 138, and the transmission signal is formed. The PA 140 amplifiers the
25 transmission signal and outputs the transmission signal to the antenna 112 through
the switch 114. Then, the antenna 11 2 transmits the transmission signal as a radio
signal to the base station 200.
[004 11
Although the case where the radio communication unit 11 0 handles three component
30 carriers is described in Fig. 5, the number of component carriers handled by the radio
communication unit 11 0 may be two, or four or more.
[0042]
Further, instead of processing the signals of the respective component carriers in the
analog region as in the example of Fig. 5, the radio communication unit 110 may
process the signals of the respective component carriers in the digital region. In the
5 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.
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
10 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
in the digital region, a sampling frequency for ADIDA conversion is higher, and the
load of the ADC and the DAC can thereby increase.
[0043]
15 (Signal processing unit)
Referring back to Fig. 4, an example of a configuration of the user equipment 100 is
further described below.
[0044]
The signal processing unit 150 performs signal processing such as deinterleaving,
20 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
processed data signal to an upper layer. Note that the signal processing unit 150
performs buffer control of data signals by using the buffer 152. Specifically, the
signal processing unit 150 stores processed data signals into the buffer 152 once and
25 then outputs the data signal to an upper layer, following the first-in first-out (FIFO)
rule, for example. Thus, when the data signals are input at a rate exceeding a
normal data rate of a communication service, the amount of data accumulated in the
buffer 152 increases. Further, when the input rate of the data signals decreases, the
amount of data accumulated in the buffer 152 decreases. Further, the signal
30 processing unit 150 performs signal processing such as encoding or interleaving on
the data signal that is input from the upper layer. In this case also, the signal
processing unit 150 may perform buffer control of data signals by using the buffer
152. Then, the signal processing unit 150 outputs the processed data signals to the
radio communication unit 1 10.
[0045]
5 (Buffer)
The buffer 152 temporarily accumulates the data signals that are input fiom the
signal processing unit 150 by using a storage medium such as a hard disk or
semiconductor memory. The data signals are read following the first-in first-out
rule, for example, and processed by the signal processing unit 150.
10 [0046]
(Control unit)
According to some embodiments, the control unit is further configured to restart use
of the plurality of component carriers after completion of the handover. In further
embodiments, the control unit is further configured to receive a command fiom the
15 base station to increase data throughput of the communication between the mobile
communication terminal and the base station prior to reception of the command to
reduce the plurality of component carriers to one. In additional embodiments, the
command from the base station to increase the data throughput instructs the control
unit to increase the plurality of component carriers used between the base station and
20 mobile communication terminal.
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
25 information that is received from the base station 200 by the radio communication
unit 110. Further, the control unit 160 increases or decreases the number of
component carriers constituent of the communication channel with the base station
200 according to a command from the base station 200. For example, when an
expansion (increase) command of the number of component carriers is received fiom
30 the base station 200, the control unit 160 increases the number of component carriers.
Further, when a shrink command of the number of component carriers is received
from the base station 200, the control unit 160 decreases the number of component
carriers. Besides, the control unit 160 controls the user equipment 100 to operate in
the same manner as the user equipment in the handover procedure which is descried
with reference to Fig. 1.
5 [0047]
(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
10 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 11 0. After that, the base station
15 200 determines, based on the measurement report, whether a handover should be
executed or not for the user equipment 100.
[0048]
(3-2. Exemplary Configuration of Base Station)
Fig. 6 is a block diagram showing an example of a configuration of the base station
20 200 according to the embodiment. Referring to Fig. 6, 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.
[0049]
(Radio communication unit)
25 A specific configuration of the radio communication unit 210 may be similar to the
configuration of the radio communication unit 11 0 of the user equipment 100 which
is described above with reference to Fig. 5, 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
30 the user equipment over a communication channel which is formed by aggregating a
plurality of component carriers with use of the carrier aggregation technology.
[0050]
(Interface unit)
The interface unit 250 mediates a communication between the radio communication
unit 2 10 or the control unit 280 and an upper node through the S 1 interface illustrated
5 in Fig. 3, 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. 3, for example.
[005 11
(CC management unit)
10 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
station 200 or when the connected user equipment changes its component carriers.
15 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.
[0052]
(Control unit)
According to some embodiments, the control unit is further configured to increase
20 data throughput of the communication between the mobile communication terminal
and the base station prior to transmitting the command to reduce the plurality of
component carriers to one. In further embodiments, the control unit is further
configured to transmit a command to the mobile communication terminal to increase
the plurality of component carriers used between the base station and mobile
25 communication terminal to increase the data throughput. In additional
embodiments, the control unit is further configured to increase one or more resource
blocks to be allocated to the mobile communication terminal in at least one
component carrier from the plurality of component carriers to increase the data
throughput.
30 [0053]
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.
5 [0054]
Further, in this embodiment, in the case where the base station 200 is a source base
station, when a handover request is confirmed by a target base station, the control
unit 280 temporarily increases the amount of communication resources to be
allocated to the user equipment 100. The control unit 280 may increase the above-
10 described amount of communication resources by increasing the number of
component carriers constituent of the communication channel with the user
equipment 100, for example. The number of component carriers can be increased
by transmitting an expansion command of the number of component carriers to the
user equipment 100, for example. Alternatively, the control unit 280 may increase
15 the above-described amount of communication resources by increasing the number
of resource blocks to be allocated to the user equipment 100 in at least one
component carrier. After the control unit 280 transmits/receives data tolfrom the
user equipment 100 at a higher rate than usual by using the communication resources
which are increased in this manner, before transmitting a handover command to the
20 user equipment 100, the control unit 280 transmits a shrink command of the number
of component carriers to the user equipment 100. The number of component
carriers constituent of the communication channel between the user equipment 100
and the base station 200 is thereby shrunk to one. Then, the control unit 280
transmits a handover command for one component carrier after the shrinkage to the
25 user equipment 100.
[0055]
Further, in the case where the base station 200 is a target base station, when a
handover by the user equipment 100 is completed, the control unit 280 restarts the
radio communication involving the carrier aggregation by the user equipment 100 in
30 response to a request from the user equipment 100. Besides, 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.
[0056]
(3-3. Flow of Process)
5 Two scenarios of a handover procedure according to the embodiment are described
hereinbelow. Note that, in the following scenarios, 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 target base station.
Further, for the procedure up to measurement in the user equipment (steps S2 to S 14)
10 in the typical handover procedure illustrated in Fig. 1, explanation is omitted because
there is no significant difference.
[0057]
Fig. 7 is a sequence chart showing an example of a flow along the first scenario of a
handover procedure according to the embodiment. Referring to Fig. 7, the user
15 equipment 100 first transmits a measurement report for a plurality of component
carriers constituent of a communication channel to the base station 200a (step S122).
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
between the user equipment 100 and the base station 200b is better than a channel
20 quality between the user equipment 100 and the base station 200a by a predetermined
threshold or greater in any component carrier, it can be determined that a handover is
necessary. In this case, the base station 200a transmits a handover request message
to the base station 200b (step S124). Receiving the handover request message, the
base station 200b determines whether it is possible to accept the user equipment 100
25 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 S 126).
[005S]
30 Receiving the handover request confirm message, the base station 200a transmits an
expansion command for increasing the number of component carriers to the user
equipment 100 (step S130). In response thereto, the user equipment 100 makes a
random access to the base station 200a for a new component carrier to establish a
communication channel with the base station 200a and thereby increases the number
of component carriers (step S132). After that, the base station 200a transmits data
5 to the user equipment 100 at a rate exceeding a normal data rate over the expanded
communication channel (step S 134). The transmitted data is temporarily
accumulated in the buffer 152 of the user equipment 100.
[0059]
Then, the base station 200a transmits a shrink command for decreasing the number
10 of component carriers to the user equipment 100 (step S136). In response thereto,
the user equipment 100 shrinks the number of component carriers constituent of the
communication channel with the base station 200a to one (step S140). The base
station 200a then transmits a handover command for one component carrier after the
shrinkage to the user equipment 100 (step S 142).
15 [0060]
Receiving the handover command, the user equipment 100 acquires synchronization
with the downlink channel of the base station 200b (step S152). Then, 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 synchronized downlink channel (step S 154).
20 During this period, the base station 200a forwards data addressed to the user
equipment 100 to the base station 200b (step S156). Then, after success in the
random access, the user equipment 100 transmits a handover 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 user equipment
25 100 (step S164). 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). Then, the base station 200b transmits
acknowledgement for the handover complete message to the user equipment 100
(step S 166).
30 [0061]
After that, the user equipment 100 restarts the radio communication involving the
carrier aggregation with the base station 200b, which is a new serving base station
(step S180). Specifically, the user equipment 100 makes a random access to the
base station 200b for an additional component carrier to establish a communication
channel with the base station 200b, for example, and thereby increases the number of
5 component carriers. It is thereby possible in the user equipment 100 to use the
communication service again at substantially the same data rate as that before the
start of the handover procedure.
[0062]
Fig. 8 is a sequence chart showing an example of a flow along the second scenario of
10 a handover procedure according to the embodiment. As is understood from
comparison between Fig. 7 and Fig. 8, in the second scenario, the steps S130 and
S 132 in the first scenario are replaced by step S 13 1.
[0063]
In the second scenario, the base station 200a which has received the handover
15 request confirm message first increases the number of resource blocks to be allocated
to the user equipment 100 in at least one component carrier. The amount of
communication resources available for the user equipment 100 thereby increases
temporarily (step S 13 1). After that, the base station 200a transmits data to the user
equipment 100 at a rate exceeding a normal data rate by using the temporarily
20 increased communication resources (step S 134). The transmitted data is
temporarily accumulated in the buffer 152 of the user equipment 100. Then, in the
same procedure as in the first scenario, after the number of component carriers
constituent of the communication channel between the user equipment 100 and the
base station 200a is shrunk to one, a handover from the base station 200a to the base
25 station 200b is carried out. The radio communication involving the carrier
aggregation is then restarted between the user equipment 100 and the base station
200b.
[0064]
(3-4. Summary of First Embodiment)
30 The first embodiment of the present invention is described above with reference to
Figs. 4 to 8. According to the embodiment, before a handover command is
transmitted fiom the source base station to the user equipment, the number of
component carriers constituent of the communication channel between the user
equipment and the source base station is shrunk to one. It is thereby possible to
carry out the procedure from transmission of a handover command to completion of
5 a handover in the same procedure as the existing handover procedure.
[0065]
Further, according to the embodiment, before the number of component carriers is
shrunk to one, the amount of communication resources allocated to the user
equipment is temporarily increased, and data is transmitted to the user equipment at a
10 rate exceeding a normal data rate by using the communication resources. It is
thereby possible to reduce the possibility of the occurrence of a failure caused by
buffer under flow in the user equipment during the period until the user equipment
restarts carrier aggregation with the target base station. Further, a temporary
increase in the amount of communication resources can be made by an increase in
15 the number of component carriers or an increase in the number of resource blocks.
Because the increase in the number of component carriers and the increase in the
number of resource blocks can be made by applying the existing scheme, the impact
of such processing on the whole system is small. Further, in this embodiment,
because a change 'in the number of component carriers is made under control of the
20 base station, the impact on the user equipment, particularly, can be minimized.
[0066]
Note that, in the step S134 of Figs. 7 and 8, data may be transmitted fiom the user
equipment to the base station, rather than transmitted fiom the base station to the user
equipment at a rate exceeding a normal data rate by using the temporarily increased
25 communication resources. In this case, in the user equipment that transmits
contents to another equipment, a large amount of content data is transmitted in
advance before the shrinkage of the number of component carriers, for example,
thereby avoiding the occurrence of buffer over flow during the handover procedure.
[0067]
30 <4. Description of Second Embodiment>
A second embodiment of the present invention is described hereinafter with reference
to Figs. 9 to 12.
[0068]
(4- 1. Exemplary Configuration of User Equipment)
Fig. 9 is a block diagram showing an example of a configuration of a user equipment
5 300 according to the embodiment. Referring to Fig. 9, the user equipment 300
includes a radio communication unit 110, a signal processing unit 150, a buffer 152,
a control unit 360, and a measurement unit 170.
[0069]
(Control unit)
10 The control unit 160 controls the overall functions of the user equipment 300 by
using a processing device such as a CPU or a DSP. For example, the control unit
360 controls the timing of data communication by the radio communication unit 110
according to scheduling information that is received from a base station 400 by the
radio communication unit 110. Further, as the preliminary step toward a handover,
15 the control unit 360 shrinks the number of component carriers constituent of a
communication channel with the base station 400 to one. The control unit 360 may
shrink the number of component carriers before transmitting a measurement report to
the base station 400 when a result of the measurement by the measurement unit 170
indicates that a handover should be started, for example. In this embodiment, the
20 control unit 360 transmits a shrink request to the base station 400 and, after the
shrink request is confirmed by the base station 400, the control unit 360 shrinks the
number of component carriers to one. Further, before shrinking the number of
component carriers to one, the control unit 360 temporarily increases the amount of
communication resources available for the user equipment 300. The control unit
25 360 may temporarily increase the amount of communication resources available for
the user equipment 300 by increasing the number of component carriers constituent
of the communication channel with the base station 400, for example. Alternatively,
the base station 400 which has received the shrink request may temporarily increase
the number of resource blocks to be allocated to the user equipment 300 in at least
30 one component carrier. After the control unit 360 transmits or receives data at a
higher data rate than usual by using the communication resources which are
increased in this manner and further shrinks the number of component carriers to one,
the control unit 360 transmits a measurement report to the base station 400. After
that, the control unit 360 controls the user equipment 300 to operate in the same
manner as the user equipment in the handover procedure which is descried with
5 reference to Fig. 1.
[0070]
(4-2. Exemplary Configuration of Base Station]
Fig. 10 is a block diagram showing an example of a configuration of the base station
400 according to the embodiment. Referring to Fig. 10, the base station 400
10 includes a radio communication unit 210, an interface unit 250, a CC management
unit 260, and a control unit 480.
[0071]
(Control unit)
The control unit 480 controls the overall functions of the base station 400 by using a
15 processing device such as a CPU or a DSP. For example, the control unit 480
allocates communication resources for data communication to the user equipment
300 and other user equipments and then delivers scheduling information over a
broadcast channel in a given sub-frame.
[0072]
20 Further, in this embodiment, in the case where the base station 400 is a source base
station, the control unit 480 receives the above-described shrink request from the
user equipment 300 through the radio communication unit 210. Further, before or
after receiving the shrink request, the control unit 480 temporarily increases the
amount of communication resources to be allocated to the user equipment 300 in
25 response to a request from the user equipment 300. The control unit 480 may
increase the above-described amount of communication resources by increasing the
number of component carriers constituent of the communication channel with the
user equipment 300, for example. Alternatively, the control unit 480 may increase
the above-described amount of communication resources by increasing the number
30 of resource blocks to be allocated to the user equipment 300 in at least one
component carrier, for example. After the control unit 480 transmitsfreceives data
tolfiom the user equipment 300 at a higher rate than usual by using the
communication resources which are increased in this manner, the control unit 480
confirms the shrink request fiom the user equipment 300. As a result, the number
of component carriers constituent of the communication channel between the user
5 equipment 300 and the base station 400 is shrunk to one. Then, the control unit 480
carries out a handover for one component carrier after the shrinkage.
[0073]
Further, in the case where the base station 400 is a target base station, when a
handover by the user equipment 300 is completed, the control unit 480 restarts the
10 radio communication involving the carrier aggregation by the user equipment 300 in
response to a request from the user equipment 300.
[0074]
(4-3. Flow of Process)
Two scenarios of a handover procedure according to the embodiment are described
15 hereinbelow. Note that, in the following scenarios, it is assumed that a handover
procedure is performed among the user equipment 300, the base station 400a serving
as a source base station, and the base station 400b serving as a target base station.
Further, for the procedure up to measurement in the user equipment (steps S2 to S 14)
in the typical handover procedure illustrated in Fig. 1, explanation is omitted because
20 there is no significant difference.
[0075]
Fig. 11 is a sequence chart showing an example of a flow along the first scenario of a
handover procedure according to the embodiment. Referring to Fig. 11, the user
equipment 300 first makes a random access to the base station 400a for a new
25 component carrier to establish a communication channel with the base station 400a
and thereby increases the number of component carriers (step S212). After that, the
base station 400a transmits data to the user equipment 300 at a rate exceeding a
normal data rate over the expanded communication channel (step S214). The
transmitted data is temporarily accumulated in the buffer 152 of the user equipment
30 300. Note that, in the step S214, data may be received from the user equipment 300
rather than transmitted to the user equipment 300.
[0076]
After the sufficient amount of data to avoid buffer under flow are accumulated in the
buffer 152, for example, the user equipment 300 transmits a shrink request to the
base station 400a (step S216). Then, the base station 400a transmits a confirm
5 message for the shrink request to the user equipment 300 (step S222). The user
equipment 300 then shrinks the number of component carriers constituent of the
communication channel with the base station 400a to one (step S224).
[0077]
Then, the user equipment 300 transmits a measurement report for one component
10 carrier after the shrinkage to the base station 400a (step S232). Receiving the
measurement report, the base station 400a transmits a handover request message to
the base station 400b (step S234). Receiving the handover request message, the
base station 400b determines whether it is possible to accept the user equipment 300
according to the availability of a communication service offered by itself or the like.
15 When the base station 400b determines that it is possible to accept the user
equipment 300, it transmits a handover request confirm message to the base station
400a (step S236). Receiving the handover request confirm message, the base
station 400a transmits a handover command to the user equipment 300 (step S242).
[0078]
20 Receiving the handover command, the user equipment 300 continues the handover
procedure for one component carrier after the shrinkage. Specifically, the user
equipment 300 first acquires synchronization with the downlink channel of the base
station 400b (step S252). Then, the user equipment 300 makes a random access to
the base station 400b by using a random access channel in a given time slot of the
25 synchronized downlink channel (step S254). During this period, the base station
400a forwards data addressed to the user equipment 300 to the base station 400b
(step S256). Then, after success in the random access, the user equipment 300
transmits a handover complete message to the base station 400b (step S262).
Receiving the handover complete message, the base station 400b requests the MME
30 to perform route update for the user equipment 300 (step S264). Then, the base
station 400b transmits acknowledgement for the handover complete message to the

involving the carrier aggregation is then restarted between the user equipment 300
and the base station 400b.
[0082]
(4-4. Summary of Second Embodiment)
5 The second embodiment of the present invention is described above with reference to
Figs. 9 to 12. According to the embodiment, before a measurement report is
transmitted from the user equipment to the source base station, the number of
component carriers constituent of the communication channel between the user
equipment and the source base station is shrunk to one. It is thereby possible to
10 carry out the procedure from transmission of a measurement report to completion of
a handover in the same procedure as the existing handover procedure.
[0083]
Further, in this embodiment also, before the number of component carriers is shrunk
to one, the amount of communication resources allocated to the user equipment is
15 temporarily increased, and data is transmitted and received at a rate exceeding a
normal data rate by using the communication resources. It is thereby possible to
reduce the possibility of the occurrence of a failure caused by buffer under flow or
buffer over flow in the user equipment during the period until the user equipment
restarts carrier aggregation with the target base station. Further, in this embodiment,
20 because a change in the number of component carriers is made in response to a
request from the user equipment, the impact on the base station, particularly, can be
minimized.
[0084]
<5. Description of Third Embodiment>
25 A third embodiment of the present invention is described hereinafter with reference
. toFigs.13to15.
[0085]
(5- 1. Exemplary Configuration of User Equipment)
Fig. 13 is a block diagram showing an example of a configuration of a user
30 equipment 500 according to the embodiment. Referring to Fig. 13, the user
equipment 500 includes a radio communication unit 11 0, a signal processing unit 150,
a buffer 152, a control unit 560, and a measurement unit 170.
[0086]
(Control unit)
The control unit 560 controls the overall functions of the user equipment 500 by
5 using a processing device such as a CPU or a DSP. For example, the control unit
560 controls the timing of data communication by the radio communication unit 110
according to scheduling information that is received fiom a base station 600 by the
radio communication unit 110. Further, as the preliminary step toward a handover,
the control unit 560 shrinks the number of component carriers constituent of a
10 communication channel with a source base station to one. Further, after completing
a handover for one component carrier after the shrinkage, the control unit 560
temporarily receives an allocation of many communication resources from a target
base station and performs data communication at a rate exceeding a normal data rate.
[0087]
15 (5-2. Exemplary Configuration of Base Station)
Fig. 14 is a block diagram showing an example of a configuration of the base station
600 according to the embodiment. Referring to Fig. 14, the base station 600
includes a radio communication unit 210, an interface unit 250, a CC management
unit 260, and a control unit 680.
20 [0088]
(Control unit)
According to some embodiments, the control unit is further configured to adjust a
timing of a handover to compensate for a decrease in data throughput due to the
reduction of the plurality of component carriers to one. In further embodiments, the
25 timing of the hand over is decided based on a likelihood of success in a random
access.
The control unit 680 controls the overall functions of the base station 600 by using a
processing device such as a CPU or a DSP. For example, the control unit 680
allocates communication resources for data communication to the user equipment
30 500 and other user equipments and then delivers scheduling information over a
broadcast channel in a given sub-frame.
[0089]
Further, in this embodiment, in the case where the base station 600 is a source base
station, when a handover request is confirmed by a target base station, the control
unit 680 transmits a shrink command to the user equipment 500 before transmitting a
5 handover command. The number of component carriers constituent of the
communication channel between the user equipment 500 and the base station 600 is
thereby shrunk to one. Then, the control unit 680 transmits a handover command
for one component carrier after the shrinkage to the user equipment 500.
[0090]
10 Further, in this embodiment, in the case where the base station 600 is a target base
station, when a handover request is received from a source base station, the control
unit 680 monitors the availability of communication resources. Then, the control
unit 680 adjusts the timing to confirm the handover request from the source base
station so that a decrease in throughput due to the shrinkage of the number of
1 5 component carriers is suppressed or compensated.
[009 11
Specifically, when a random access fiom the user equipment 500 is success, for
example, the control unit 680 confirms the handover request after waiting until the
timing at which a sufficient amount of communication resources can be allocated to
20 the user equipment 500. The sufficient amount may be the amount of
communication resources which enables an allocation of substantially the same
number of component carries as the number of component carriers before the
handover (shrinkage) to a new communication channel, for example. In this case,
by recovering the number of component carriers promptly after the handover, it is
25 possible to reduce the time during which a throughput decreases due to the handover.
Note that the number of component carriers before the handover can be notified from
the source base station to the target base station by advance negotiation or using a
handover request message or another message, for example.
[0092]
30 Further, the sufficient amount may be the amount of communication resources which
enables a recovery of the amount of data accumulated in the buffer of the user
equipment at the receiving end of data to the amount not causing buffer under flow,
for example. The amount of communication resources may be defined in advance,
or notified fiom a source base station by using a handover request message or the
like. In this case, after a random access from the user equipment 500 succeeds, the
5 amount of communication resources allocated to the user equipment 500 is
temporarily increased, thereby compensating a decrease in throughput that occurs
due to the shrinkage of the number of component carriers.
[0093]
Furthermore, the control unit 680 may confirm the handover request after waiting
10 until the timing at which it is determined that the possibility of success in a random
access fiom the user equipment 500 is high. An example of the timing when the
possibility of success in a random access is high is timing at which there are not a
large number of other user equipments that are trying to make a random access (e.g.
another user equipment that has started a handover, a user equipment in the idle
15 mode etc.). In this case, it is possible to prevent a delay of a handover caused by a
failure and retry of a random access and reduce the time during which a throughput
decreases due to the handover.
[0094]
(5-3. Flow of Process)
20 An example of a flow of a handover procedure according to the embodiment is
described hereinbelow. Note that, in the following scenario, it is assumed that a
handover procedure is performed among the user equipment 500, the base station
600a serving as a source base station, and the base station 600b serving as a target
base station. Further, for the procedure up to measurement in the user equipment
25 (steps S2 to S 14) in the typical handover procedure illustrated in Fig. 1, explanation
is omitted because there is no significant difference.
[0095]
Fig. 15 is a sequence chart showing an example of a flow of a handover procedure
according to the embodiment. Referring to Fig. 15, the user equipment 500 first
30 transmits a measurement report for a plurality of component carriers constituent of a
communication channel to the base station 600a (step S322). The base station 600a
which has determined that a handover is necessary based on the measurement report
transmits a handover request message to the base station 600b (step S324).
Receiving the handover request message, the base station 600b adjusts the timing to
confirm the handover request so that a decrease in throughput due to the shrinkage of
5 the number of component carriers is suppressed or compensated. (step S325). Then,
the base station 600b transmits a handover request confirm message to the base
station 600a at timing (TI) when the subsequent random access is likely to succeed,
timing (T2) when more communication resources can be allocated after completion
of a handover, or timing (T3) when carrier aggregation can be restarted promptly
10 after completion of a handover, for example (step S326).
[0096]
Receiving the handover request confirm message, the base station 600a transmits a
shrink command for decreasing the number of component carriers to the user
equipment 500 (step S336). After that, the user equipment 500 shrinks the number
15 of component carriers constituent of the communication channel with the base station
600a to one (step S340). The base station 600a then transmits a handover command
for one component carrier after the shrinkage to the user equipment 500 (step S342).
[0097]
Receiving the handover command, the user equipment 500 continues the handover
20 procedure for one component carrier after the shrinkage. Specifically, the user
equipment 500 first acquires synchronization with the downlink channel of the base
station 600b (step S352). Then, the user equipment 500 makes a random access to
the base station 600b by using a random access channel in a given time slot of the
synchronized downlink channel (step S354). During this period, the base station
25 600a forwards data addressed to the user equipment 500 to the base station 600b
(step S356). Then, after success in the random access, the user equipment 500
transmits a handover complete message to the base station 600b (step S362).
Receiving the handover complete message, the base station 600b requests the MME
to perform route update for the user equipment 500 (step S364). Then, the base
30 station 600b transmits acknowledgement for the handover complete message to the
user equipment 500 (step S366).
[0098]
After that, the base station 600b temporarily allocates a larger number of resource
blocks to the user equipment 500, for example (step S370). Then, the base station
600b transmits data to the user equipment 500 at a rate exceeding a normal data rate
5 (step S372). The transmitted data recovers the amount of data which has decreased
during the handover procedure in the buffer 152 of the user equipment 500. Note
that, in the step S372, data may be received from the user equipment 500, rather than
transmitted to the user equipment 500. After that, the user equipment 500 restarts
the radio communication involving the carrier aggregation with the base station 600b,
10 which is a new serving base station (step S380).
[0099]
(5-4. Summary of Third Embodiment)
The third embodiment of the present invention is described above with reference to
Figs. 13 to 15. According to the embodiment, before a handover command is
15 transmitted from a source base station to a user equipment, the number of component
carriers constituent of the communication channel between the user equipment and
the source base station is shrunk to one. It is thereby possible to carry out the
procedure from transmission of a handover command to completion of a handover in
the same procedure as the existing handover procedure.
20 [OlOO]
Further, in this embodiment, after success in a random access from a user equipment
to a target base station, the amount of communication resources allocated to the user
equipment is temporarily increased in the target base station. Then, by using the
communication resources, data is transmittedlreceived tolfi-om the user equipment at
25 a rate exceeding a normal data rate. It is thereby possible to compensate a decrease
in throughput that occurs due to the shrinkage of the number of component carriers.
[ O l O l ]
Furthermore, in this embodiment, the timing to confirm the handover request in the
target base station is adjusted so as to suppress or compensate a decrease in
30 throughput due to the shrinkage of the number of component carriers. It is thereby
possible to reduce the possibility that a failure occurs in a communication service
such as contents delivery due to a decrease in throughput.
[O 1 021
Although preferred embodiments of the present invention are described in detail
above with reference to the appended drawings, the present invention is not limited
5 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
appended claims or the equivalents thereof.
[Reference Signs List]
10 [0103]
1 RADIO COMMUNICATION SYSTEM
100,300,500 USER EQUIPMENT
1 10 RADIO COMMUNICATION UNIT (USER EQUIPMENT)
160,360,560 CONTROL UNIT (USER EQUIPMENT)
15 200,400,600 BASE STATION
2 10 RADIO COMMUNICATION UNIT (BASE STATION)
280,480,680 CONTROL UNIT (BASE STATION)
In so far as the embodiments of the invention described above are implemented, at
20 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
provided are envisaged as aspects of the present invention.

Claim
A base station comprising:
a radio communication unit configured to establish communication with a
5 mobile communication terminal using a plurality of component carriers; and
a control unit configured to transmit a command to the mobile
communication terminal to reduce the plurality of component carriers to one prior to
handover of communication between the mobile communication terminal and the
base station to another base station.
10 [Claim 21
The base station according to claim 1, wherein the control unit is further configured
to increase data throughput of the communication between the mobile
communication terminal and the base station prior to transmitting the command to
reduce the plurality of component carriers to one.
15 [Claim 31
The base station according to claim 2, wherein the control unit is further configured
to transmit a command to the mobile communication terminal to increase the
plurality of component carriers used between the base station and mobile
communication terminal to increase the data throughput.
20 [Claim 41
The base station according to claim 2, wherein the control unit is further configured
to increase one or more resource blocks to be allocated to the mobile communication
terminal in at least one component carrier from the plurality of component carriers to
increase the data throughput.
25 [Claim 51
The base station according to claim 1, wherein the control unit is further configured
to adjust a timing of a handover to compensate for a decrease in data throughput due
to the reduction of the plurality of component carriers to one.
[Claim 61
30 The base station according to claim 5, wherein
the timing of the hand over is decided based on a likelihood of success in a
random access.
[Claim 71
A mobile communication terminal comprising:
a radio communication unit configured to establish communication with a
5 base station using a plurality of component carriers; and
a control unit configured to receive a command from the base station to
reduce the plurality of component carriers to one prior to handover of
communication between the mobile communication terminal and the base station to
another base station.
10 [Claim 81
The mobile communication terminal according to claim 7, wherein the control unit is
further configured to restart use of the plurality of component carriers after
completion of the handover.
[Claim 91
15 The mobile communication terminal according to claim 7, wherein the control unit is
further configured to receive a command from the base station to increase data
throughput of the communication between the mobile communication terminal and
the base station prior to reception of the command to reduce the plurality of
component carriers to one.
20 [Claim101
The mobile communication terminal according to claim 9, wherein the command
from the base station to increase the data throughput instructs the control unit to
increase the plurality of component carriers used between the base station and
mobile communication terminal.
25 [Claimll]
The mobile communication terminal according to claim 7, wherein
a timing of a handover is decided based on a possibility of success in a
random access.
[Claim 121
30 A communication system comprising:
a first base station including:
a radio communication unit configured to establish communication
with a mobile communication terminal using a plurality of component carriers, and
a first control unit configured to transmit a command to the mobile
communication terminal to reduce the plurality of component carriers to one prior to
5 handover of communication between the mobile communication terminal and the
base station to a second base station, and
the mobile communication terminal including:
a second control unit configured to reduce the plurality of
component carriers to one upon reception of the command fi-om the first base station.
10 [Claim 131
The communication system according to claim 12, wherein the second control unit is
further configured to restart use of the plurality of component chiers after
completion of the handover.
[Claim 141
15 The communication system according to claim 12, wherein the first control unit is
further configured to increase data throughput of the communication between the
mobile communication terminal and the base station prior to transmitting the
command to reduce the plurality of component carriers to one.
[Claim 151
20 The communication system according to claim 14, wherein the first control unit is
further configured to transmit a command to the mobile communication terminal to
increase the plurality of component carriers used between the base station and mobile
communication terminal to increase the data throughput.
[Claim 161
25 The communication system according to claim 14, wherein the first control unit is
further configured to increase one or more resource blocks to be allocated to the
mobile communication terminal in at least one component carrier from the plurality
of component carriers to increase the data throughput.
[Claim 171
30 The communication system according to claim 13, wherein the first control unit is
further configured to adjust a timing of a handover to compensate for a decrease in
data throughput due to the reduction of the plurality of component carriers to one.
[Claim 181
The communication station according to claim 17, wherein the timing of the
handover is decided based on a likelihood of success in a random access.
5 [Claim 191
A non-transitory computer readable medium having instructions stored thereon,
which when executed by a processor in a base station causes the processor to:
establish communication with a mobile communication terminal using a
plurality of component carriers; and
10 transmit a command to the mobile communication terminal to reduce the
plurality of component carriers to one prior to handover of communication between
the mobile communication terminal and the base station to another base station.
Dated this 08/06/20 12