The present invention relates to a method for performing a handover, a user
equipment, 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
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 Unex-ed 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
aggregation.
[0009]
For example, an initial access from a user equipment to a target base station during a
handover procedure is performed as a random access over a random access channel
5 which is preset at a prescribed position of communication resources. However, a
random access inherently has a possibility of a delay due to signal collision, retry or
the like. Further, adjustment of transmission timing based on timing of a random
access is the overhead of a communication. Thus, in a radio communication
involving the carrier aggregation, simply repeating random accesses the same
10 number of times as the number of component carriers raises the possibility of
degradation of service quality such as an accumulation of delays due to a failure of a
random access or an increase in overhead.
[OO lo]
In light of the foregoing, it is desirable to provide a novel and improved method for
15 performing a handover, user equipment, and radio communication system that can
minimize the degradation of service quality due to a random access during a
handover procedure in a radio communication involving the carrier aggregation.
[Solution to Problem]
[OO 1 11
20 One exemplary embodiment includes a mobile station in a wireless communication
network. The mobile station including a radio communication unit configured to
transmit an access request message to a base station via a first communication
resource, and receive a timing adjustment in response to the access request message
from the base station; an adjustment value storage unit configured to store the timing
25 adjustment; and a control unit configured to adjust access timing corresponding to a
second communication resource based on the timing adjustment value stored in the
adjustment value storage unit, wherein the radio communication unit is configured to
communicate with the base station via the first communication resource and the
second communication resource.
30 [0012]
The first communication resource may be a first component carrier and the second
communication resource may be a second component carrier. The second
component carrier may be in close proximity to the first component carrier. The
second component carrier may be adjacent to the first component carrier. The
second component carrier may be within a predetermined threshold of the first
5 component carrier.
[00 131
The control unit may be configured to acquire synchronization with the base station
via the first communication resource, and the radio communication unit may be
configured to tiansmit the access request message based on the synchronization.
10 [0014]
The timing adjustment may be based on a signal propagation characteristic between
the mobile station and the base station, and the timing adjustment may depend on a
frequency of the first communication resource.
[00 151
15 Another exemplary embodiment includes a base station in a wireless communication
network. The base station including a radio communication unit configured to
receive an access request message from a mobile station via a first communication
resource, and transmit a timing adjustment in response to the access request message
to the mobile station; a control unit configured to assign a second communication
20 resource for communications with the mobile station, the second communication
resource being assigned based on a predetermined relationship with the first
communication resource, wherein the radio communication unit is configured to
communicate with the mobile station via the first communication resource and the
second communication resource.
25 [0016]
The first communication resource may be a first component carrier and the second
communication resource may be a second component carrier. The control unit may
be configured to assign a component carrier in close proximity to the first component
carrier as the second component carrier, assign a component carrier adjacent to the
30 first component carrier as the second component carrier, andor assign a component
carrier which is within a predetermined threshold of the first component carrier as the
second component carrier. The control unit may also be configured to assign a
component carrier, which is one of a plurality of available component carriers closest
in proximity to the first component carrier, as the second component carrier.
[00 1 71
5 Another exemplary embodiment is directed to a wireless communication network
that includes a mobile station configured to transmit an access request message to a
base station via a first communication resource; the base station configured to
receive the access request message and transmit a timing adjustment in response to
the access request message to the mobile station; an adjustment value storage unit, at
10 the mobile station, configured to store the timing adjustment; a first control unit, at
the base station, configured to assign a second communication resource for
communications with the mobile station, the second communication resource being
assigned based on a predetermined relationship with the first communication
resource; a second control unit, at the mobile station, configured to adjust access
15 timing corresponding to the second communication resource based on the timing
adjustment value stored in the adjustment value storage unit, wherein the mobile
station and the base station are configured to communicate via the first
communication resource and the second communication resource.
[OO 1 81
20 Another exemplary embodiment is directed to a computer-readable medium
including computer program instruction, which when executed by a mobile station in
a wireless communication network, cause the mobile station to perform a method
comprising: transmitting an access request message to a base station via a first
communication resource; receiving a timing adjustment in response to the access
request message from the target base station; storing the timing adjustment; adjusting
an access timing corresponding to a second communication resource based on the
stored timing adjustment value; and communicating with the base station via the first
communication resource and the second communication resource.
[00 1 91
30 Another exemplary embodiment is directed to a computer-readable medium
including computer program instruction, which when executed by a base station in a
wireless communication network, cause the base station to perform a method
comprising: receiving an access request message from a mobile station via a first
communication resource; transmitting a timing adjustment in response to the access
request message to the mobile station; assigning a second communication resource
5 for communications with the mobile station, the second communication resource
being assigned based on a predetermined relationship with the first communication
resource; and communicating with the mobile station via the first communication
resource and the second communication resource.
[0020]
10 Another exemplary embodiment is directed to a handoff method performed by a
mobile communication network, the handoff method comprising: transmitting, from
a mobile station to a base station, an access request message via a first
communication resource; receiving, at the base station, the access request message;
transmitting, from the base station to the mobile station, a timing adjustment in
15 response to the access request message; storing, at an adjustment value storage unit
at the mobile station, the timing adjustment; assigning, by the base station, a second
communication resource for communications with the mobile station, the second
communication resource being assigned based on a predetermined relationship with
the first communication resource; adjusting, by the mobile station, access timing
20 corresponding to the second communication resource based on the timing adjustment
value stored in the adjustment value storage unit; and performing communication
between the base station and the mobile station via the first communication resource
and the second communication resource.
[Advantageous Effects of Invention]
25 [0021]
As described above, the method for performing a handover, the user equipment, and
the radio communication system according to the embodiments of the present
invention can minimize the degradation of service quality due to a random access
during a handover procedure in a radio communication involving the carrier
3 0 aggregation.
[Brief Description of Drawings]
100221
[Fig. 11
Fig. 1 is a sequence chart to describe a flow of a typical handover procedure.
[Fig. 21
5 Fig. 2 is an explanatory view to describe an example of a structure of a
communication resource.
[Fig. 3A]
Fig. 3A is a sequence chart to describe a procedure of a contention-based random
access.
10 [Fig. 3B]
Fig. 3B is a sequence chart to describe a procedure of a contention-free random
access.
[Fig. 41
Fig. 4 is a schematic view showing an outline of a radio communication system
1 5 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. 81
Fig. 8 is an explanatory view to describe an allocation of component carriers by a
target base station according to the first embodiment.
[Fig. 9A]
Fig. 9A is the first half of a sequence chart showing an example of a flow of a
30 handover procedure according to the first embodiment.
[Fig. 9B]
Fig. 9B is the second half of a sequence chart showing an example of a flow of a
handover procedure according to the first embodiment.
[Fig. 101
Fig. 10 is a block diagram showing an example of a configuration of a user
5 equipment according to a second embodiment.
[Fig. 111
Fig. 11 is a block diagram showing an example of a configuration of a base station
according to the second embodiment.
[Fig. 12A]
10 Fig. 12A is the first half of a sequence chart showing an example of a flow of a
handover procedure according to the second embodiment.
[Fig. 12B]
Fig. 12B is the second half of a sequence chart showing an example of a flow of a
handover procedure according to the second embodiment.
1 5 [Description of Embodiments]
[0023]
Hereinafter, preferred embodiments of the present invention will be described in
detail with reference to the appended drawings throughout which like parts are
referred to by like references. Note that, in this specification and the appended
20 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.
[0024]
Preferred embodiments of the present invention will be described hereinafter in the
25 following order.
1. Description of Related Art
1 - 1. Handover Procedure
1-2. Structure of Communication Resource
1-3. Description of Issue Related to Random Access
30 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
3-3. Flow of Process
3-4. Summary of First Embodiment
5 4. Description of Second Embodiment
4- 1. Exemplary Configuration of User Equipment
4-2. Exemplary Configuration of Base Station
4-3. Flow of Process
4-4. Summary of Second Embodiment
10 [0025]
4. Description of Related Art>
(1 - 1. Handover Procedure)
A technique related to the present invention is described hereinafter with reference to
Figs. 1 to 3B. Fig. 1 shows a flow of a handover procedure in conformity with LTE
15 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.
[0026]
20 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
value. The user equipment can measure the channel quality of the communication
25 channel with the source base station by receiving a reference signal contained in a
downlink channel from the source base station.
[0027]
Then, the source base station determines the needs of measurement based on the
quality report received from the user equipment and, if measurement is necessary,
30 allocates measurement gaps to the user equipment (step S4).
[0028]
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
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
5 station.
[0029]
When the user equipment acquires synchronization with a downlink channel, the user
equipment performs measurement by using a reference signal contained in the
downlink channel (step S 14). During this period, the source base station restricts an
10 allocation of data communication related to the user equipment so as to avoid
occurrence of data transmission by the user equipment.
[0030]
Upon completion of the measurement, the user equipment transmits a measurement
report containing measurements to the source base station (step S22). The
15 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.
1003 11
Receiving the measurement report, the source base station determines whether or not
20 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
base station determines to carry out a handover procedure with the relevant another
25 base station as a target base station, and transmits a handover request message to the
target base station (step S24).
[0032]
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
30 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).
[0033]
Receiving the handover request confirm message, the source base station transmits a
handover command to the user equipment (step S28). Then, the user equipment
5 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
the target base station (step S36). Then, after success in the random access, the user
10 equipment transmits a handover complete message to the target base station (step
S42).
[0034]
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
15 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.
[003 51
20 (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
direction into radio frames each having a length of 10 msec. One radio frame
25 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 frequency direction.
Specifically, one resource block has a size of 1 msec with 12 sub-carriers in the time-
30 frequency 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
access channel. The random access channel can be used for an access to a base
station by a user equipment that has changed from an idle mode to an active mode,
5 for example, in addition to for an initial access to a target base station in a handover
procedure as described above.
[0036]
(1-3. Description of Issue Related to Random Access)
Figs. 3A and 3B are sequence charts to further describe general random access
10 procedures. Fig. 3A shows a procedure of a contention-based random access.
[0037]
Referring to Fig. 3A, the user equipment first transmits a random access preamble to
the target base station (step S52). The random access preamble is selected fiom 64
different sequences. Because the 64 sequences are orthogonal to one another, even
15 when the target base station receives different sequences at the same timing with the
same frequency, it can separate those sequences. However, when the same
sequence is transmitted from different user equipments at the same timing with the
same frequency, there is a possibility of collision of random access preambles, which
causes a random access to fail.
20 [0038]
Next, the target base station which has received the random access preamble
determines a timing adjustment value for each user equipment according to the
reception timing (step S54). For example, the random access preamble of a user
equipment which is located farther fiom the target base station arrives at the target
25 base station at the later timing than the random access preamble of a user equipment
which is located closer to the target base station. In this case, the target base station
allocates a timing adjustment value indicating that a signal should be transmitted
earlier to the former (i.e. far) user equipment. Further, the target base station
performs scheduling for a connection request of the user equipment. Note that,
30 when a communication resource for a connection request cannot be allocated due to
the lack of available communication resources, a random access fails.
[0039]
Then, the target base station transmits a random access response to the user
equipment (step S56). The random access response contains the timing adjustment
value determined by the target base station and scheduling information for a
5 connection request by the user equipment. Receiving the random access response,
the user equipment adjusts the transmission timing of a signal to the target base
station by using the timing adjustment value contained in the random access response
(step S58).
[0040]
10 Then, the user equipment transmits a connection request to the target base station by
using the communication resource which is specified by the scheduling information
contained in the random access response (step S60). The connection request
contains identification information of the user equipment or the like. Note that, for
example, there is a possibility that the same random access preamble is transmitted
15 from two or more user equipments in the step S52, and the random access preamble
is received normally by the target base station. In such a case, the two or more user
equipments which have received a random access response transmitted from the
target base station can simultaneously transmit connection requests in the step S60.
In this case also, a collision occurs between the connection requests, and a random
20 access can thereby fail.
[0041]
The target base station which has received the connection request from the user
equipment then transmits acknowledgement to the user equipment (step S62). Note
that, when a communication resource for the acknowledgement cannot be allocated
25 due to the lack of available communication resources, there is a possibility that the
acknowledgement is not transmitted. When the user equipment receives the
acknowledgement in the step S62, it recognizes that the random access has
succeeded, and a series of random access procedure ends. On the other hand, when
the user equipment does not receive the random access response or the
30 acknowledgement, it determines that the random access has failed and retries the
random access after the lapse of a random backoff period.
[0042]
Fig. 3B shows a procedure of a contention-free random access. Referring to Fig.
3B, the target base station first allocates a random access preamble to the user
equipment (step S72). The random access preamble for the contention-free random
5 access is selected by the target base station from a predetermined number (e.g.
reserved 10) of sequences among 64 orthogonal sequences, for example.
[0043]
Next, the user equipment transmits the random access preamble to the target base
station (step S74). In this case, because there is no case where the same random
10 access preamble is used by different user equipments at the same timing with the
same frequency, a collision of random access preambles does not occur.
[0044]
Then, the target base station which has received the random access preamble
determines a timing adjustment value for each user equipment according to the
15 reception timing (step S76). Further, the target base station performs scheduling for
a connection request of the user equipment. Note that, when a communication
resource for a connection request cannot be allocated due to the lack of available
communication resources, a random access fails.
[0045]
20 Then, the target base station transmits a random access response to the user
equipment (step S78). The random access response contains the timing adjustment
value determined by the target base station and scheduling information for a
connection request by the user equipment. Receiving the random access response,
the user equipment adjusts the transmission timing of a signal to the target base
25 station by using the timing adjustment value contained in the random access response
(step S80). After that, the user equipment establishes a connection with the target
base station in the same manner as the contention-based random access procedure.
[0046]
As is understood from the above description, in the random access procedure, a
30 random access can fail due to the collision of signals or the shortage of
communication resources. Then, a random access is retried after the lapse of a
random backoff period. During this period, a communication by the user
equipment delays. Further, the time needed for calculation of a timing adjustment
value and adjustment of transmission timing is the overhead of a communication.
The use of the contention-free random access procedure reduces the possibility of
5 signal collision. However, because the number of random access preambles which
are reserved for the contention-free random access is limited, the contention-free
random access procedure is not always available in a handover procedure.
Therefore, for the purpose of preventing the degradation of service quality, it is
effective to perform a random access during a handover procedure in a radio
10 communication involving the carrier aggregation more efficiency as in two
embodiments of the present invention which are described in detail below.
[0047]
<2. Outline of Radio Communication System>
Fig. 4 is a schematic view showing an outline of a radio communication system 1
15 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.
[0048]
20 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
25 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.
30 [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
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
5 base station 200a and the base station 200b can communicate with the MME, which
is an upper node, through S1 interface, for example.
[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
10 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.
[005 11
<3. Description of First Embodiment>
15 A first embodiment of the present invention for making a more efficient random
access during a handover procedure in a radio communication involving the carrier
aggregation is described hereinafter with reference to Figs. 5 to 9B.
[0052]
(3- 1. Exemplary Configuration of User Equipment)
20 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
equipment 100 includes a radio communication unit 1 10, a signal processing unit 150,
a control unit 160, an adjustment value storage unit 162, and a measurement unit 170.
[0053]
25 (Radio communication unit)
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.
[0054]
30 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
110 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)
5 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.
[0055]
The antenna 112 receives a radio signal transmitted fiom the base station 200 and
outputs the received signal to the LNA 120 through the switch 114. The LNA 120
10 amplifies the received signal. The down-converter 122a and the filter 124a separate
a baseband signal of the first component carrier (CC1) 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
15 component carrier (CC2) fiom 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
20 converted to a digital signal by the ADC 126c and output to the demodulation unit
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.
[0056]
25 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. 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
30 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. 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
5 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 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
10 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.
[0057]
Although the case where the radio communication unit 110 handles three component
15 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]
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
20 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.
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
25 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
in the digital region, a sampling frequency for ADJDA conversion is higher, and the
load of the ADC and the DAC can thereby increase.
30 [0059]
(Signal processing unit)
Referring back to Fig. 5, an example of a configuration of the user equipment 100 is
further described below.
[0060]
The signal processing unit 150 performs signal processing such as deinterleaving,
5 decoding or error correction on the demodulated data signal that is input fiom the
radio communication unit 11 0. Then, the signal processing unit 150 outputs the
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
10 processed data signal to the radio communication unit 11 0.
[0061]
(Control unit)
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
15 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
unit 110. At this time, the control unit 160 adjusts the timing of data transmission
from the radio communication unit 110 by using a timing adjustment value which is
20 notified from the base station 200 in a random access procedure. 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
25 unit 170 to execute measurement during the periods of measurement gaps which are
allocated by the base station 200.
[0062]
Further, in this embodiment, when two or more component carriers are allocated by
the target base station during a handover procedure, the control unit 160 determines
30 whether a distance in the frequency direction between the two or more component
carriers is smaller than a specific threshold. When the distance in the frequency
direction is smaller than a specific threshold, the control unit 160 skips a random
access to the target base station for at least one of the two or more component
carriers. Specifically, when first and second component carriers exist in close
proximity in the frequency direction, the control unit 160 makes a random access for
5 the first component carrier and skips a random access for the second component
carrier. The specific threshold may be the maximum value of a difference between
two frequencies (e.g. the center frequencies of the respective component carriers)
whose transmission timing can be adjusted appropriately based on one timing
adjustment value notified from the target base station in a random access procedure.
10 Thus, even when a random access is skipped for any of the component carriers, the
user equipment 100 can carry out a handover by appropriately adjusting the
transmission timing for the component carrier. A transmission timing difference
depends on a distance between each user equipment and a base station. Thus, a
timing adjustment is typically used between user equipment and a base station for
15 purposes of synchronization. When the user equipment handles a plurality of
component carriers, it is also typical for the user equipment to adjust transmission
timing for each component carrier because the timing difference depends on
frequency. Generally, propagation characteristics include, but not limited to,
characteristics of reflection, diffraction, scattering and propagation. These
20 characteristics are known to depend on frequency of the component
carrier. Therefore, if one frequency is close another frequency, the characteristic
such as propagation delay is similar to each other between two different frequencies.
These propagation characteristics affect the propagation delay amount of the
signals. Therefore, in the case where the a distance in the frequency direction
25 between the two component carriers is smaller than a specific threshold, propagation
delay of the signals over the two component carriers is close to each other. Therefore,
it becomes easier to adjust transmission timing of one component carrier based on
the transmission timing of another component carrier.
100631
30 Further, in the case where a new communication channel with a target base station is
composed of three or more component carriers, when a distance in the frequency
direction between certain component carriers among them is smaller than a specific
threshold, the control unit 160 may skip a random access for at least one of the
component carriers. For example, it is assumed that a distance in the frequency
direction between the component carrier CCl and the component carrier CC2 among
5 the component carriers CCl to CC3 is smaller than a specific threshold, and a
distance between the component carrier CC1, CC2 and the component carrier CC3 is
larger than the specific threshold. In this case, the control unit 160 may make a
random access to the target base station for the component carriers CC1 and CC2 in
one time and further make a random access to the target base station for the
10 component carrier CC3.
[0064]
In the case of skipping a random access for any of the component carriers, the
control unit 160 first executes a random access to the target base station for another
component carrier which exists in close proximity to the relevant component carrier
15 in the frequency direction. Then, the control unit 160 stores the timing adjustment
value which is notified from the target base station in the wake of the random access
into the adjustment value storage unit 162. After that, the control unit 160 adjusts
the transmission timing for the component carrier for which a random access is
skipped by using the timing adjustment value stored in the adjustment value storage
unit 162. Then, the control unit 160 makes an access to the target base station over
the relevant component carrier at the adjusted transmission timing (the access in this
case is not a random access).
[0065]
(Adjustment value storage unit)
The adjustment value storage unit 162 stores a timing adjustment value for one or
more component carriers which is input from the control unit 160 by using a storage
medium such as a hard disk or semiconductor memory. Then, in response to a
command from the control unit 160, the adjustment value storage unit 162 outputs
the stored timing adjustment value for use in adjustment of the transmission timing
30 for another component carrier.
[0066]
(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
5 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
10 200 determines, based on the measurement report, whether a handover should be
executed or not for the user equipment 100.
[0067]
(3-2. Exemplary Configuration of Base Station)
Fig. 7 is a block diagram showing an example of a configuration of the base station
15 200 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.
[0068]
(Radio communication unit)
20 A specific configuration of the radio communication unit 210 may be similar to the
configuration of the radio communication unit 1 1 0 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 210 performs a radio communication with
25 the user equipment over a communication channel which is formed by aggregating a
plurality of component carriers with use of the carrier aggregation technology.
[0069]
(Interface unit)
The interface unit 250 mediates a communication between the radio communication
30 unit 210 or the control unit 280 and an upper node through the S1 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.
[0070]
(CC management unit)
5 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 existing user equipment changes its component carriers.
10 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.
[007 11
(Control unit)
The control unit 280 controls the overall functions of the base station 200 by using a
15 processing device such as a CPU or a DSP. For example, the control unit 280
executes an allocation of communication resources for data communication, i.e.
scheduling, for the user equipment 100 and other user equipments. Then, the
control unit 280 delivers scheduling information over a broadcast channel in a given
sub-frame. Further, when the base station 200 is a target base station of a handover
20 of a radio communication involving the carrier aggregation, the control unit 280
allocates two or more component carriers to a new communication channel. At this
time, when two or more component carriers which are adjacent in the frequency
direction are allocable, the control unit 280 allocates the two or more component
carriers to the new communication channel. Further, when two or more component
25 carriers whose distance in the frequency direction is smaller than the above-described
specific threshold are allocable, the control unit 280 allocates the two or more
component carriers to the new communication channel.
[0072]
Fig. 8 is an explanatory view to describe an allocation of component carriers by a
30 target base station according to the embodiment. Fig. 8 shows a step-by-step
handover including four stages in the case where a communication channel is
composed of three component carriers CCl to CC3. The first stage is before a
handover. The second stage is after a handover of the component carrier CC1.
The third stage is after a handover of the component carriers CC1 and CC2. The
fourth stage is after a handover of all the component carriers.
5 [0073]
First, in the first stage, the component carriers CC1 to CC3 are connected with a
source base station. The positions of the component carriers CC1 to CC3 in the
frequency direction may be any positions.
[0074]
10 Next, when a handover is completed for the component carrier CC1, the handover
procedure proceeds to the second stage. The frequency band to which the
component carrier CC1 is allocated in the target base station may be the same
frequency band as the frequency band in the source base station or a different
frequency band from the frequency band in the source base station.
15 [0075]
Then, the control unit 280 of the target base station which has received a handover
request for the component carrier CC2 determines whether the component carriers
CC1 and CC2 can be allocated so that they are adjacent in the frequency direction.
It is assumed in the example of Fig. 8 that the component carriers CC1 and CC2 can
20 be allocated so that they are adjacent in the frequency direction. Thus, the control
unit 28 allocates the component carriers CC1 and CC2 so that they are adjacent in the
frequency direction (third stage).
[0076]
Further, the control unit 280 of the target base station which has received a handover
25 request for the component carrier CC3 determines whether the component carrier
CC1 or CC2 and the component carrier CC3 can be allocated so that they are
adjacent in the frequency direction. It is assumed in the example of Fig. 8 that
those component carriers are not allocable to be adjacent in the frequency direction.
Then, the control unit 28 determines whether the component carrier CC3 can be
30 allocated to a new communication channel so that a distance from the component
carrier CC1 or CC2 in the frequency direction is smaller than a threshold FTH. The
threshold FTH indicates the above-described specified threshold which can
correspond to the maximum value of a difference between two frequencies whose
transmission timing can be adjusted appropriately based on one timing adjustment
value. It is assumed in the example of Fig. 8 that the component carrier CC3 can be
5 allocated so that the distances from the component carriers CC1 and CC2 are smaller
than the threshold FTH. Thus, the control unit 280 allocates the component carrier
CC3 to the position where the distances fkom the component carriers CC1 and CC2
are smaller than the threshold FTH (fourth stage).
[0077]
10 It should be noted that, Fig. 8 shows the case where, after a handover for one
component carrier is completed, another component carrier is allocated to the new
communication channel. However, the allocation of another component carrier to a
new communication channel may be performed before completion of a handover for
one component carrier. Further, in the example of Fig. 8, it is described that, for a
15 certain component carrier, the frequency band of the component carrier is changed at
the time of an access to the target base station by the user equipment 100. However,
for a certain component carrier, a base station to which the component carrier is
connected may be changed from the source base station to the target base station
after the frequency band has changed in the source base station according to the
20 above-described conditions related to the position in the frequency direction.
[0078]
In addition to such an allocation of component carriers, 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.
25 1.
[0079]
(3-3. Flow of Process)
A flow of a handover procedure according to the embodiment is described
hereinafter with reference to Figs. 9A and 9B. Note that, in the following scenario,
30 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 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
5 procedure illustrated in Fig. 1, explanation is omitted because there is no significant
difference.
[oaso]
Referring to Fig. 9A, the user e&ipment 100 first transmits a measurement report for
the component carrier CC1, for example, to the base station 200a (step S122).
10 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
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,
15 the base station 200a transmits a handover request message for the component carrier
CC1 to the base station 200b (step S124). Receiving the handover request message,
the base station 200b allocates the component carrier CC1 to any frequency band for
a new communication channel with the user equipment 100 according to the
availability of a communication service. Then, the base station 200b transmits a
20 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 CCl to the user equipment 100 (step S 128).
[008 11
Receiving the handover command, the user equipment 100 first acquires
25 synchronization with the downlink channel of the component carrier CC1 of the base
station 200b (step S132). Then, the user equipment 100 makes a random access to
the base station 200b by using a random access channel of the component carrier
CC1 (step S134). During this period, the control unit 160 of the user equipment
100 stores the timing adjustment value for the component carrier CC1 which is
30 notified from the base station 200b into the adjustment value storage unit 162.
During the downlink synchronization and the random access, the base station 200a
27
forwards data addressed to the user equipment 100 to the base station 200b (step
S136).
[0082]
Then, after success in the random access for the component carrier CC1, the user
5 equipment 100 transmits a handover complete message for the component carrier
CC1 to the base station 200b (step S142). Receiving the handover complete
message, the base 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
10 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 with respect to
each component carrier, or performed only once through a plurality of component
carriers. Then, the base station 200b transmits acknowledgement for the handover
complete message to the user equipment 100 (step S146).
15 [0083]
Further, referring to Fig. 9B, the user equipment 100 transmits a measurement report
for the component carrier CC2 to the base station 200a (step S152). Receiving the
measurement report, the base station 200a transmits a handover request message for
the component carrier CC2 to the base station 200b (step S 154).
20 [0084]
Receiving the handover request message, the base station 200b allocates the
component carrier CC2 to any frequency band for the new communication channel
with the user equipment 100 according to the availability of a communication service.
In this scenario, it is assumed that the frequency band to which the component carrier
25 CC2 is allocated is the frequency band which is adjacent to the component carrier
CCl or the frequency band in which the distance from the component carrier CC1 in
the frequency direction is smaller than the threshold FTH. After that, the base
station 200b transmits a handover request confirm message to the base station 200a
(step S156). Receiving the handover request confirm message, the base station
30 200a transmits a handover command for the component carrier CC2 to the user
equipment 100 (step S 1 58).
[0085]
Then, in the user equipment 100 which has received the handover command, the
control unit 160 reads the timing adjustment value for the component carrier CC1
which is stored in the adjustment value storage unit 162. The control unit 160 then
5 adjusts the access timing to the base station 200b for the component carrier CC2 by
using the timing adjustment value which is read from the adjustment value storage
unit 162 (step S164). During this period, the base station 200a forwards data
addressed to the user equipment 100 to the base station 200b (step S166).
[0086]
10 The user equipment 100 then transmits a handover complete message for the
component carrier CC2 to the base station 200b (step S172). 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 S174).
Then, the base station 200b transmits acknowledgement for the handover complete
1 5 message to the user equipment 100 (step S 176).
[0087]
(3-4. Summary of First Embodiment)
The first embodiment of the present invention is described above with reference to
Figs. 5 to 9B. According to the embodiment, in a radio communication involving
20 the carrier aggregation, when a distance in the frequency direction between two or
more component carriers which are allocated to a communication channel with a
target base station is smaller than a specific threshold, the user equipment 100 skips a
random access to the target base station for at least one of the two or more
component carriers. Therefore, an accumulation of delays corresponding to the
25 number of component carriers due to a failure of a random access does not occur, and
the degradation of service quality caused by a random access during a handover
procedure is suppressed. Further, the user equipment 100 adjusts the transmission
timing for the component carrier for which a random access has been skipped by
using the timing adjustment value notified from the target base station for the
30 component carrier for which a random access has been made. A communication
can be thereby performed at appropriate timing also in the component carrier for
which a random access has been skipped. Further, because it is not necessary in the
target base station to redundantly determine the timing adjustment value for the
component carrier, the overhead of a communication is reduced.
[OOSS]
5 Further; according to the embodiment, when two or more component carriers which
are adjacent in the frequency direction are allocable, the base station 200 serving as a
target base station allocates the two or more component carriers to a new
communication channel with the user equipment 100. Further, when two or more
component carriers whose distance in the frequency direction is smaller than a
10 specific threshold are allocable, the base station 200 allocates the two or more
component carriers to a new communication channel. The opportunity of skipping
a random access by the user equipment 100 during a handover procedure thereby
increases, and it is possible to more effectively obtain the above-described
advantages such as prevention of an accumulation of delays due to a failure of a
15 random access or reduction of overhead. Note that the positions in the frequency
direction of those component carriers may be varied after the end of a series of
handover procedure as a matter of course.
[0089]
<4. Description of Second Embodiment>
20 A second embodiment of the present invention for making a more efficient random
access during a handover procedure in a radio communication involving the carrier
aggregation is described hereinafter with reference to Figs. 10 to 12B.
[0090]
(4- 1. Exemplary Configuration of User Equipment)
25 Fig. 10 is a block diagram showing an example of a configuration of the user
equipment 300 according to the embodiment. Referring to Fig. 10, the user
equipment 300 includes a radio communication unit 11 0, a signal processing unit 150,
a control unit 360, and a measurement unit 170.
[009 11
30 (Controlunit)
The control unit 360 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 the base station 400 by the
radio communication unit 110. Further, the control unit 360 controls the
5 measurement unit 170 to measure the channel quality by using a reference signal
from the base station 400, which is a serving base station, and transmits the channel
quality report to the base station 400 through the radio communication unit 110.
Further, the control unit 360 controls the measurement unit 170 to execute
measurement during the periods of measurement gaps which are allocated by the
10 base station 400.
[0092]
Further, in this embodiment, when one or more component carriers are allocated to a
new communication channel by the base station 400 during a handover procedure,
the control unit 360 controls the radio communication unit 110 to make a random
15 access to the base station 400. By a random access for the one or more component
carriers, a new communication channel with the base station 400 is established.
Then, the control unit 360 controls the radio communication unit 110 to notifl the
base station 400 of the timing of a random access for one or more other (remaining)
component carriers to constitute the communication channel over the established
20 new communication channel. The base station 400 as a target base station can
thereby reserve communication resources necessary in the random access procedure
by the user equipment 300 in advance based on the notified timing.
[0093]
(4-2. Exemplary C ~ ~ g u r a t iofn B ase Station]
25 Fig. 11 is a block diagram showing an example of a configuration of the base station
400 according to the embodiment. Referring to Fig. 11, the base station 400
includes a radio communication unit 210, an interface unit 250, a CC management
unit 260, and a control unit 480.
[0094]
30 (Control unit)
-
The control unit 480 controls the overall functions of the base station 400 by using a
processing device such as a CPU or a DSP. For example, the control unit 480
executes an allocation of communication resources for data communication for the
user equipment 300 and other user equipments. Then, the control unit 480 delivers
scheduling information over a broadcast channel in a given s u b - h e . Further,
5 when the base station 400 is a target base station of a handover by the user
equipment 300, the control unit 480 sequentially allocates two or more component
carriers to a new communication channel with the user equipment 300. At this time,
over the new communication channel which is established by a random access for
one or more component carriers, the timing of a random access for one or more other
10 component carriers to constitute the communication channel can be notified fiom the
user equipment 300 as described above. Receiving such a notification, the control
unit 480 reserves communication resources for the user equipment 300 according to
the notified timing so that the subsequent random access for the one or more other
component carriers succeeds. The communication resources reserved thereby
15 include resources of an uplink for a connection request fiom the user equipment 300,
resources of a downlink for acknowledgement to the connection request or the like.
[0095]
In addition to such a reservation of communication resources, the control unit 480
controls the base station 400 to operate in the same manner as the source base station
20 or the target base station in the handover procedure which is descried with reference
to Fig. 1.
[0096]
(4-3. Flow of Process)
A flow of a handover procedure according to the embodiment is described
25 hereinafter with reference to Figs. 12A and 12B. Note that, in the following
scenario, 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, like Figs. 9A and 9B, for
simplification of description, it is assumed in this scenario that the user equipment
30 300 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.
[0097]
Referring to Fig. 12A, the user equipment 300 first transmits a measurement report
5 for the component carrier CC1, for example, to the base station 400a (step S222).
Receiving the measurement report, when the base station 400a determines that a
handover is necessary, the base station 400a transmits a handover request message
for the component carrier CC1 to the base station 400b (step S224). Receiving the
handover request message, the base station 400b allocates the component carrier
10 CC1 to any frequency band for a new communication channel with the user
equipment 300 according to the availability of a communication service. Then, the
base station 400b transmits a handover request confirm message to the base station
400a (step S226). Receiving the handover request confirm message, the base
station 400a transmits a handover command for the component carrier CC1 to the
15 user equipment 300 (step S228).
[0098]
Receiving the handover command, the user equipment 300 first acquires
synchronization with the downlink channel of the component carrier CC1 of the base
station 400b (step S232). Then, the user equipment 300 makes a random access to
20 the base station 400b by using a random access channel of the component carrier
CCl (step S234). When the random access is success, a new communication
channel between the user equipment 300 and the base station 400b is established.
During the downlink synchronization and the random access, the base station 400a
forwards data addressed to the user equipment 300 to the base station 400b (step
25 S236).
[0099]
Then, after success in the random access for the component carrier CC1, the user
equipment 300 transmits a handover complete message for the component carrier
CC1 to the base station 400b (step S242). Receiving the handover complete
30 message, the base station 400b requests the MME to perform route update for the
component carrier CC1 of the user equipment 300 (step S244). Then, the base
station 400b transmits acknowledgement for the handover complete message to the
user equipment 300 (step S246).
[O 1 001
Further, referring to Fig. 12B, the user equipment 300 transmits a measurement
5 report for the component carrier CC2 to the base station 400a (step S252).
Receiving the measurement report, the base station 400a transmits a handover
request message for the component carrier CC2 to the base station 400b (step S254).
[OlOl]
Receiving the handover request message, the base station 400b allocates the
10 component carrier CC2 to any frequency band for the new communication channel
with the user equipment 300 according to the availability of a communication service.
The frequency band to which the component carrier CC2 is allocated may be any
frequency band. After that, the base station 400b transmits a handover request
confirm message to the base station 400a (step S256). Receiving the handover
15 request confirm message, the base station 400a transmits a handover command for
the component carrier CC2 to the user equipment 300 (step S258).
[O 1 021
Then, the user equipment 300 notifies the timing of a random access (RA) for the
component carrier CC2 to the base station 400b over the established new
20 communication channel (which has the component carrier CCl) (step S260). The
base station 400b then reserves communication resources according to the notified
timing so that a random access for the component carrier CC2 by the user equipment
300 does not fail due to the shortage of communication resources (step S261).
[0 1031
25 After that, the user equipment 300 acquires synchronization with the downlink
channel of the component carrier CC2 of the base station 400b (step S262). Then,
the user equipment 300 makes a random access to the base station 400b by using a
random access channel of the component carrier CC2 at the timing notified to the
base station 400b in the step S260 (step S264). Such a random access is likely to
30 succeed unless signal collision occurs because communication resources for a
connection request from the user equipment 300 and acknowledgement for the
request are reserved by the base station 400b. During the downlink synchronization
and the random access, the base station 400a forwards data addressed to the user
equipment 300 to the base station 400b (step S266).
[0 1041
5 The user equipment 300 then transmits a handover complete message for the
component carrier CC2 to the base station 400b (step S272). Receiving the
handover complete message, the base station 400b requests the MME to perform
route update for the component carrier CC2 of the user equipment 300 (step S274).
Then, the base station 400b transmits acknowledgement for the handover complete
10 message to the user equipment 300 (step S276).
[0105]
(4-4. Summary of Second Embodiment)
The second embodiment of the present invention is described above with reference to
Figs. 10 to 12B. According to the embodiment, at the time of a handover in a radio
15 communication involving the carrier aggregation, the timing of a random access for a
component carrier for which a handover has not been completed is notified from the
user equipment 300 to the base station 400 over a component carrier for which a
handover has been completed. The base station 400 can thereby reserve
communication resources so that the subsequent random access for another
20 component carrier from the user equipment 300 does not fail due to the shortage of
communication resources. As a result, the possibility of a failure of a random
access is reduced, and the degradation of service quality due to an accumulation of
delays is suppressed.
[0 1 061
25 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
30 appended claims or the equivalents thereof.
[Reference Signs List]
[0 1071
1 RADIO COMMUNICATION SYSTEM
100,300 USER EQUIPMENT
1 10 RADIO COMMUNICATION UNIT (USER EQUIPMENT)
5 160,360 CONTROL UNIT (USER EQUIPMENT)
200,400 BASE STATION
21 0 RADIO COMMUNICATION UNIT (BASE STATION)
280,480 CONTROL UNIT (BASE STATION)
10 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
provided are envisaged as aspects of the present invention.
15

Claim
A mobile station in a wireless communication network, comprising:
a radio communication unit configured to transmit an access request message to a
5 base station via a first communication resource, and receive a timing adjustment in
response to the access request message from the base station;
an adjustment value storage unit configured to store the timing adjustment;
and
a control unit configured to adjust access timing corresponding to a second
10 communication resource based on the timing adjustment value stored in the
adjustment value storage unit, wherein
the radio communication unit is configured to communicate with the base
station via the first communication resource and the second communication resource.
[Claim 21
15 The mobile station of claim 1, wherein
the first communication resource is a first component carrier and the second
communication resource is a second component carrier.
[Claim 31
The mobile station of claim 1, wherein
20 the control unit is configured to acquire synchronization with the base
station via the first communication resource, and
the radio communication unit is configured to transmit the access request
message based on the synchronization.
[Claim 41
25 The mobile station of claim 1, wherein
the timing adjustment is based on a signal propagation characteristic
between the mobile station and the base station.
[Claim 51
The mobile station of claim 1, wherein
3 0 the timing adjustment depends on a frequency of the first communication
resource.
[Claim 61
The mobile station of claim 2, wherein
the second component carrier is in close proximity to the first component
carrier.
5 [Claim 71
The mobile station of claim 2, wherein
the second component carrier is adjacent to the first component carrier.
[Claim 81
The mobile station of claim 2, wherein
10 the second component carrier is within a predetermined threshold of the first
component carrier.
[Claim 93
A base station in a wireless communication network, the base station comprising:
a radio communication unit configured to receive an access request message
15 from a mobile station via a first communication resource, and transmit a timing
adjustment in response to the access request message to the mobile station;
a control unit configured to assign a second communication resource for
communications with the mobile station, the second communication resource being
assigned based on a predetermined relationship with the first communication
20 resource, wherein
the radio communication unit is configured to communicate with the mobile
station via the first communication resource and the second communication resource.
[Claim 101
The base station of claim 9, wherein
25 the first communication resource is a first component carrier and the second
communication resource is a second component carrier.
[Claim 111
The base station of claim 10, wherein
the control unit is configured to assign a component carrier in close
30 proximity to the first component carrier as the second component carrier.
[Claim 121
The base station of claim 10, wherein
the control unit is configured to assign a component carrier adjacent to the
first component carrier as the second component carrier.
[Claim 131
5 The base station of claim 10, wherein
the control unit is configured to assign a component carrier which is within
a predetermined threshold of the first component carrier as the second component
carrier.
[Claim 141
10 The base station of Claim 10, wherein
the control unit is configured to assign a component carrier, which is one of
a plurality of available component carriers closest in proximity to the first component
carrier, as the second component carrier.
[Claim 151
15 A wireless communication network comprising:
a mobile station configured to transmit an access request message to a base
station via a first communication resource;
the base station configured to receive the access request message and
transmit a timing adjustment in response to the access request message to the mobile
20 station;
an adjustment value storage unit, at the mobile station, configured to store
the timing adjustment;
a first control unit, at the base station, configured to assign a second
communication resource for communications with the mobile station, the second
25 communication resource being assigned based on a predetermined relationship with
the first communication resource;
a second control unit, at the mobile station, configured to adjust access
timing corresponding to the second communication resource based on the timing
adjustment value stored in the adjustment value storage unit, wherein
3 0 the mobile station and the base station are configured to communicate via
the first communication resource and the second communication resource.
[Claim 161
A computer-readable medium including computer program instruction, which when
executed by a mobile station in a wireless communication network, cause the mobile
station to perform a method comprising:
5 transmitting an access request message to a base station via a first
communication resource;
receiving a timing adjustment in response to the access request message
fkom the target base station;
storing the timing adjustment;
10 adjusting an access timing corresponding to a second communication
resource based on the stored timing adjustment value; and
communicating with the base station via the first communication resource
and the second communication resource.
[Claim 171
15 A computer-readable medium including computer program instruction, which when
executed by a base station in a wireless communication network, cause the base
station to perform a method comprising:
receiving an access request message from a mobile station via a first
communication resource;
20 transmitting a timing adjustment in response to the access request message
to the mobile station;
assigning a second communication resource for communications with the
mobile station, the second communication resource being assigned based on a
predetermined relationship with the first communication resource; and
25 communicating with the mobile station via the first communication resource
and the second communication resource.
[Claim 181
A handoff method performed by a mobile communication network, the handoff
method comprising:
3 0 transmitting, from a mobile station to a base station, an access request
message via a first communication resource;
receiving, at the base station, the access request message;
transmitting, from the base station to the mobile station, a timing adjustment
in response to the access request message;
storing, at an adjustment value storage unit at the mobile station, the timing
5 adjustment;
assigning, by the base station, a second communication resource for
communications with the mobile station, the second communication resource being
assigned based on a predetermined relationship with the first communication
resource;
10 adjusting, by the mobile station, access timing corresponding to the second
communication resource based on the timing adjustment value stored in the
adjustment value storage unit; and
performing communication between the base station and the mobile station
via the first communication resource and the second communication resource.
15
Dated this 08/06/2012