Abstract:
A mobile station in a wireless communication network. The mobile station including a radio communication unit that communicates with a first base station via a plurality of component carriers, and a control unit that controls the radio communication unit to initiate a handover procedure to a second base station after receiving at least one handover command.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a user equipment, a method for performing a handover, a base station, and a radio communication system. 
       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. 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 (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]    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-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 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]    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]    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 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, 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]    PTL 1: Japanese Unexamined Patent Application Publication No. 2009-232293 
       SUMMARY OF INVENTION  
     Technical Problem 
       [0008]    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, when executing a handover for each of component carriers, a situation occurs where a handover is completed for a part of a plurality of component carriers and a handover is not completed for the rest of the plurality of component carriers. In such a situation, when a cyclic prefix length used by a source base station and a cyclic prefix length used by a target base station are different, for example, there is a possibility that a radio communication unit of a user equipment needs to handle the both of cyclic prefix lengths simultaneously. However, implementing a user equipment which incorporates a radio communication unit that can simultaneously handle different cyclic prefix lengths results in complication of a circuit, an increase in manufacturing cost and an increase in processing load. Further, the necessity of defining a new protocol for aggregating a plurality of signals having different cyclic prefix lengths in an upper layer arises. 
         [0010]    In light of the foregoing, it is desirable to provide a novel and improved user equipment, method for performing a handover, base station and radio communication system that can avoid simultaneous handling of different cyclic prefix lengths during a handover procedure in a radio communication involving the carrier aggregation. 
       Solution to Problem 
       [0011]    According to one embodiment, the present invention is directed to a mobile station in a wireless communication network, the mobile station comprising: a radio communication unit configured to communicate with a first base station via a plurality of component carriers; and a control unit configured to control the radio communication unit to initiate a handover procedure to a second base station after receiving at least one handover command for the plurality of component carriers. 
         [0012]    The radio communication unit may be configured to communicate with the first base station via one of the plurality of component carriers having a first cyclic prefix length, and communicate with the second base station via one of the plurality of component carriers having a second cyclic prefix length. The radio communication may be configured to receive a plurality of handover commands, each corresponding to one of the plurality of component carriers, and the control unit may be configured to control the radio communication unit to initiate a handover procedure to the second base station for each of the plurality of component carriers after receiving the plurality of handover commands. 
         [0013]    According to another embodiment, the present invention is directed to a mobile station in a wireless communication network, the mobile station comprising: a radio communication unit configured to communicate with a first base station via a plurality of component carriers each having a first cyclic prefix length; and a control unit configured to control the radio communication unit to initiate a handover procedure from the first base station to a second base station after receiving a handover command, wherein, after a handover for one of the plurality of component carriers, the control unit is configured to control the radio communication unit to communicate with the second base station via a component carrier allocated to a time slot different from a time slot to which each of the plurality of component carriers are allocated, and having a second cyclic prefix length different from the first cyclic prefix length. The control unit may be configured to control the radio communication unit to release a first one of the plurality of component carrier connections with the first base station and to connect to the second base station using the component carrier having the second cyclic prefix length. The control unit may further be configured to control the radio communication unit to initiate a handover procedure to the second base station for a second one of the plurality of component carriers after the handover procedure for the first one of the plurality of component carriers has been initiated. 
         [0014]    According to another embodiment, the present invention is directed to a base station in a wireless communication network, the base station comprising: a radio communication unit configured to communicate with a mobile station via a plurality of component carriers; and a control unit configured to control the radio communication unit to initiate a handover procedure to a second base station for one of the plurality of component carriers after receiving a report from the mobile station corresponding to the one of the plurality of component carriers. The control unit may be configured to initiate the handover procedure by controlling the radio communication unit to transmit a handover request command to the second base station and transmit a handover command to the mobile station. 
         [0015]    According to another embodiment, the present invention is directed to a base station in a wireless communication network, the base station comprising: a radio communication unit configured to communicate with a mobile station via a plurality of component carriers each having a first cyclic prefix length; and a control unit configured to control the radio communication unit to initiate a handover procedure to a second base station for one of the plurality of component carriers, wherein, after a handover for one of the plurality of component carriers, the mobile station communicates with the second base station via a component carrier allocated to a time slot different from a time slot to which each of the plurality of component carriers is allocated, and having a second cyclic prefix length different from the first cyclic prefix length. 
         [0016]    According to another embodiment, the present invention is directed to a handover method performed by a mobile station in a wireless communication network, the method comprising: communicating, by a radio communication unit of the mobile station, with a first base station via a plurality of component carriers; and initiating, by a control unit of the mobile station, a handover procedure to a second base station after receiving at least one handover command for the plurality of component carriers. 
         [0017]    According to another embodiment, the present invention is directed to a handover method performed by a mobile station in a wireless communication network, the method comprising: communicating, by a radio communication unit of the mobile station, with a first base station via a plurality of component carriers each having a first cyclic prefix length; initiating, by a control unit of the mobile station, a handover procedure from the first base station to the second base station after receiving a handover command; and controlling, by the control unit after a handover for one of the plurality of component carriers, the radio communication unit to communicate with the second base station via a component carrier allocated to a time slot different from a time slot to which each of the plurality of component carriers are allocated, and having a second cyclic prefix length different from the first cyclic prefix length. 
         [0018]    According to another embodiment, the present invention is directed to a handover method performed by a base station in a wireless communication network, the method comprising: communicating, by a radio communication unit of the base station, with a mobile station via a plurality of component carriers; and initiating, by a control unit of the base station, a handover procedure to a second base station for one of the plurality of component carriers after receiving a report from the mobile station corresponding to the one of the plurality of component carriers. 
         [0019]    According to another embodiment, the present invention is directed to a handover method performed by a base station in a wireless communication network, the method comprising: communicating, by a radio communication unit of the base station, with a mobile station via a plurality of component carriers each having a first cyclic prefix length; and initiating, by a control unit of the base station, a handover procedure to a second base station for one of the plurality of component carrier; wherein, after a handover for one of the plurality of component carriers, the mobile station communicates with the second base station via a component carrier allocated to a time slot different from a time slot to which each of the plurality of component carriers is allocated, and having a second cyclic prefix length different from the first cyclic prefix length 
         [0020]    According to another embodiment, the present invention is directed to a non-transitory computer readable medium including computer-program instructions, which when executed by a mobile station in a wireless communication network, cause the mobile station to perform a handover method comprising: communicating with a first base station via a plurality of component carriers; and initiating a handover procedure to a second base station after receiving at least one handover command for the plurality of component carriers. 
         [0021]    According to another embodiment, the present invention is directed to A non-transitory computer readable medium including computer-program instructions, which when executed by a mobile station in a wireless communication network, cause the mobile station to perform a handover method comprising: communicating with a first base station via a plurality of component carriers each having a first cyclic prefix length; initiating a handover procedure from the first base station to the second base station after receiving a handover command; and communicating, after a handover for one of the plurality of component carriers, with the second base station via a component carrier allocated to a time slot different from a time slot to which each of the plurality of component carriers are allocated, and having a second cyclic prefix length different from the first cyclic prefix length. 
         [0022]    According to another embodiment, the present invention is directed to a non-transitory computer readable medium including computer-program instructions, which when executed by a base station in a wireless communication network, cause the base station to perform a handover method comprising: communicating with a mobile station via a plurality of component carriers; and initiating a handover procedure to a second base station for one of the plurality of component carriers after receiving a report from the mobile station corresponding to the one of the plurality of component carriers. 
         [0023]    According to another embodiment, the present invention is directed to a non-transitory computer readable medium including computer-program instructions, which when executed by a base station in a wireless communication network, cause the base station to perform a handover method comprising: communicating with a mobile station via a plurality of component carriers each having a first cyclic prefix length; and initiating a handover procedure to a second base station for one of the plurality of component carriers, wherein, after a handover for one of the plurality of component carriers, the mobile station communicates with the second base station via a component carrier allocated to a time slot different from a time slot to which each of the plurality of component carriers is allocated, and having a second cyclic prefix length different from the first cyclic prefix length. 
         [0024]    According to another embodiment, the present invention is directed to a wireless communication system comprising: a mobile station including a radio communication unit configured to communicate with a first base station via a plurality of component carriers; a first control unit at the first base station configured to initiate a handover procedure to a second base station for one of the plurality of component carriers after receiving a report from the mobile station corresponding to the one of the plurality of component carriers; and a second control unit at the mobile station configured to initiate a handover procedure to the second base station after receiving at least one handover command for the plurality of component carriers from the first base station. 
         [0025]    According to another embodiment, the present invention is directed to A wireless communication system comprising: a first base station configured to communicate with a mobile station via a plurality of component carriers each having a first cyclic prefix length; the mobile station configured to initiate a handover procedure from the first base station to a second base station for one of the plurality of component carriers after receiving a handover command from the first base station; and the second base station configured to communicate with the mobile station via a component carrier allocated to a time slot different from a time slot to which each of the plurality of component carriers is allocated, and having a second cyclic prefix length different from the first cyclic prefix length. 
       Advantageous Effects of Invention 
       [0026]    As described above, the user equipment, the method for performing a handover, the base station and the radio communication system according to the embodiments of the present invention can avoid simultaneous handling of different cyclic prefix lengths during a handover procedure in a radio communication involving the carrier aggregation. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0027]    [ FIG. 1 ]  FIG. 1  is a sequence chart to describe a flow of a typical handover procedure. 
           [0028]    [ FIG. 2 ]  FIG. 2  is an explanatory view to describe an example of a structure of a communication resource. 
           [0029]    [ FIG. 3 ]  FIG. 3  is an explanatory view to describe a cyclic prefix length that can be adopted generally. 
           [0030]    [ FIG. 4 ]  FIG. 4  is a schematic view showing an outline of a radio communication system according to an embodiment. 
           [0031]    [ FIG. 5 ]  FIG. 5  is a block diagram showing an example of a configuration of a user equipment according to a first embodiment. 
           [0032]    [ FIG. 6 ]  FIG. 6  is a block diagram showing an example of a detailed configuration of a radio communication unit according to the first embodiment. 
           [0033]    [ FIG. 7 ]  FIG. 7  is a block diagram showing an example of a configuration of a base station according to the first embodiment. 
           [0034]    [FIG  8 A]  FIG. 8A  is the first half of a sequence chart showing an example of a flow of a handover procedure according to the first embodiment. 
           [0035]    [ FIG. 8B ]  FIG. 8B  is the second half of a sequence chart showing an example of a flow of a handover procedure according to the first embodiment. 
           [0036]    [ FIG. 9 ]  FIG. 9  is a sequence chart showing another example of a flow of a handover procedure according to the first embodiment. 
           [0037]    [ FIG. 10 ]  FIG. 10  is an explanatory view to describe an outline of switching of a cyclic prefix length in time division. 
           [0038]    [ FIG. 11 ]  FIG. 11  is a block diagram showing an example of a configuration of a user equipment according to a second embodiment. 
           [0039]    [ FIG. 12 ]  FIG. 12  is a block diagram showing an example of a detailed configuration of a radio communication unit according to the second embodiment. 
           [0040]    [ FIG. 13 ]  FIG. 13  is a block diagram showing an example of a configuration of a base station according to the second embodiment. 
           [0041]    [ FIG. 14 ]  FIG. 14  is an explanatory view to describe an example of a flow of a handover procedure according to the second embodiment. 
       
    
    
     DESCRIPTION OF EMBODIMENTS  
       [0042]    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. 
         [0043]    Preferred embodiments of the present invention will be described hereinafter in the following order. 
         [0044]    1. Description of Related Art 
         [0045]    1-1. Handover Procedure 
         [0046]    1-2. Structure of Communication Resource 
         [0047]    1-3. Description of Issue 
         [0048]    2. Outline of Radio Communication System 
         [0049]    3. Description of First Embodiment 
         [0050]    3-1. Exemplary Configuration of User Equipment 
         [0051]    3-2. Exemplary Configuration of Base Station 
         [0052]    3-3. Flow of Process 
         [0053]    3-4. Summary of First Embodiment 
         [0054]    4. Description of Second Embodiment 
         [0055]    4-1. Exemplary Configuration of User Equipment 
         [0056]    4-2. Exemplary Configuration of Base Station 
         [0057]    4-3. Flow of Process 
         [0058]    4-4. Summary of Second Embodiment 
       1. Description of Related Art 
       [0059]    (1-1. Handover Procedure) 
         [0060]    A technique related to the present invention is described hereinafter with reference to  FIGS. 1 to 3 .  FIG. 1  shows a flow of a handover procedure in conformity with LTE in a radio communication not involving the carrier aggregation as an example of a typical handover procedure. In this example, a user equipment (UE), a source base station (source eNB), a target base station (target eNB), and a mobility management entity (MME) are involved in the handover procedure. 
         [0061]    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 S 2 ). 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 channel with the source base station by receiving a reference signal contained in a downlink channel from the source base station. 
         [0062]    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, allocates measurement gaps to the user equipment (step S 4 ). 
         [0063]    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 S 12 ). Note that the user equipment can recognize a peripheral base station to search according to a list that is provided in advance from the source base station. 
         [0064]    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 allocation of data communication related to the user equipment so as to avoid occurrence of data transmission by the user equipment. 
         [0065]    Upon completion of the measurement, the user equipment transmits a measurement report containing measurements to the source base station (step S 22 ). 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. 
         [0066]    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 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 S 24 ). 
         [0067]    Receiving the handover request message, the target base station determines whether it is possible to accept the user equipment according to the availability of a communication service offered by itself or the like. When it is possible to accept the user equipment, the target base station transmits a handover request confirm message to the source base station (step S 26 ). 
         [0068]    Receiving the handover request confirm message, the source base station transmits a handover command to the user equipment (step S 28 ). Then, the user equipment acquires synchronization with the downlink channel of the target base station (step S 32 ). 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 S 34 ). During this period, the source base station forwards data addressed to the user equipment to the target base station (step S 36 ). Then, after success in the random access, the user equipment transmits a handover complete message to the target base station (step S 42 ). 
         [0069]    Receiving the handover complete message, the target base station requests the MME to perform route update for the user equipment (step S 44 ). 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 S 46 ). A series of handover procedure thereby ends. 
         [0070]    (1-2. Structure of Communication Resource) 
         [0071]      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 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-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 or an initial access to a target base station in a handover procedure, for example. 
         [0072]    (1-3. Description of Issue) 
         [0073]    An issue related to a handover procedure in a radio communication involving the carrier aggregation is described hereinafter with reference to  FIG. 3 .  FIG. 3  is an explanatory view to describe a cyclic prefix length (which is referred to hereinafter as a CP length) that can be adopted generally in the structure of a communication resource which is described above with reference to  FIG. 2 . 
         [0074]    A cyclic prefix length (CP) is a guard interval which is inserted between data symbols in order to avoid the occurrence of inter-symbol interference due to frequency selective fading caused by multipath delay spread over a communication channel. By inserting the cyclic prefix into data symbols, it is possible to avoid inter-symbol interference caused by delay spread up to the length of the inserted cyclic prefix. 
         [0075]      FIG. 3  shows two types of cyclic prefixes, an extended CP and a normal CP, by way of illustration. In the case of the extended CP, six symbols are included in the 0.5 ms slot which is described earlier with reference to  FIG. 2 . Further, the extended cyclic prefix having a length of 16.7 microsec is inserted between the symbols. On the other hand, in the case of the normal CP, seven symbols are included in the 0.5 ms slot. Further, the normal cyclic prefix having a length of 4.7 microsec is inserted between the symbols. The cyclic prefix typically contains a copy of the last portion of the data symbol. 
         [0076]    Which of the extended CP or the normal CP is to be used is generally determined in each base station. Then, the type of cyclic prefix to be used is notified from a base station to a user equipment through a broadcast channel. In many cases, the extended CP is used in the place where the multipath delay spread is particularly large. However, because the use of the extended CP leads to an increase in overhead, throughput decreases when using the extended CP compared to when using the normal CP. Thus, there is a possibility that the extended CP and the normal CP are used in the base stations which are adjacent to each other, depending on the effect of the multipath delay spread. 
         [0077]    In a handover in a radio communication not involving the carrier aggregation, the user equipment can switch the setting of the CP length of the radio communication unit at the point of synchronization or a random access to the target base station. However, in a handover in a radio communication involving the carrier aggregation, the situation occurs where a handover is completed for only a part of a plurality of component carriers. If, in view of such a situation, a circuit for simultaneously handling different CP lengths is applied in the radio communication unit, disadvantages occur such as complication of the circuit, an increase in manufacturing cost and an increase in processing load. Therefore, it is effective to employ the technique that can avoid simultaneous handling of different CP lengths during a handover procedure as in two embodiments of the present invention which are described in detail below. 
       2. Outline of Radio Communication System 
       [0078]      FIG. 4  is a schematic view showing an outline of a radio communication system  1  according to an embodiment of the present invention. Referring to  FIG. 4 , the radio communication system  1  includes a user equipment  100 , a base station  200   a  and a base station  200   b.  It is assumed that the base station  200   a  is a serving base station for the user equipment  100 . 
         [0079]    The user equipment  100  is located inside a cell  202   a  where a radio communication service is provided by the base station  200   a.  The user equipment  100  can perform a data communication with another user equipment (not shown) via the base station  200   a  over a communication channel formed by aggregating a plurality of component carriers (i.e. by carrier aggregation). However, because the distance between the user equipment  100  and the base station  200   a  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  202   b  where a radio communication service is provided by the base station  200   b.  Therefore, the base station  200   b  can be a candidate for a target base station for a handover of the user equipment  100 . 
         [0080]    Further, it is assumed for example that the base station  200   a  uses the CP length of the normal CP and the base station  200   b  uses the CP length of the extended CP, which are described earlier with reference to  FIG. 3 . Therefore, in order for the user equipment  100  to carry out a handover from the base station  200   a  to the base station  200   b,  it is necessary to change the setting of the CP length in the equipment at some point in time during a handover procedure. 
         [0081]    The base station  200   a  can communicate with the base station  200   b  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  200   a  and the base station  200   b.  Further, the base station  200   a  and the base station  200   b  can communicate with the MME, which is an upper node, through Si interface, for example. 
         [0082]    It should be noted that, when there is no particular need to distinguish between the base station  200   a  and the base station  200   b  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. 
       3. Description of First Embodiment 
       [0083]    A first embodiment of the present invention for avoiding simultaneous handling of different CP lengths during a handover procedure in a radio communication involving the carrier aggregation is described hereinafter with reference to  FIGS. 5 to 9 . 
         [0084]    (3-1. Exemplary Configuration of User Equipment) 
         [0085]      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  110 , a signal processing unit  150 , a buffer  152 , a control unit  160 , and a measurement unit  170 . 
         [0086]    (Radio communication unit) 
         [0087]    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. 
         [0088]      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  122   a  to  122   c,  a plurality of filters  124   a  to  124   c,  a plurality of analogue-to-digital converters (ADCs)  126   a  to  126   c,  a demodulation unit  128 , a modulation unit  130 , a plurality of digital-to-analogue converters (DACs)  132   a  to  132   c,  a plurality of filters  134   a  to  134   c,  a plurality of up-converters  136   a  to  136   c,  a combiner  138 , and a power amplifier (PA)  140 . Further, the demodulation unit  128  includes a CPU removal unit  129 . The modulation unit  130  includes a CPU insertion unit  131 . 
         [0089]    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  amplifies the received signal. The down-converter  122   a  and the filter  124   a  separate a baseband signal of the first component carrier (CC 1 ) from the received signal amplified by the LNA  120 . Then, the separated baseband signal is converted to a digital signal by the ADC  126   a  and output to the demodulation unit  128 . Likewise, the down-converter  122   b  and the filter  124   b  separate a baseband signal of the second component carrier (CC 2 ) from the received signal amplified by the LNA  120 . Then, the separated baseband signal is converted to a digital signal by the ADC  126   b  and output to the demodulation unit  128 . Further, the down-converter  122   c  and the filter  124   c  separate a baseband signal of the third component carrier (CC 3 ) from the received signal amplified by the LNA  120 . Then, the separated baseband signal is converted to a digital signal by the ADC  126   c  and output to the demodulation unit  128 . After that, in the demodulation unit  128 , the CPU removal unit  129  removes a cyclic prefix from the baseband signals of the respective component carriers. The demodulation unit  128  then generates a data signal by demodulating the baseband signals and outputs the data signal to the signal processing unit  150 . 
         [0090]    Further, when a data signal is input from the signal processing unit  150 , the modulation unit  130  modulates the data signal and generates baseband signals of the respective component carriers. Further, in the modulation unit  130 , the CP insertion unit  131  inserts a cyclic prefix to the baseband signals. Among those baseband signals, the baseband signal of the first component carrier (CC 1 ) is converted to an analog signal by the DAC  132   a.  Then, a frequency component corresponding to the first component carrier in a transmission signal is generated from the analog signal by the filter  134   a  and the up-converter  136   a.  Likewise, the baseband signal of the second component carrier (CC 2 ) is converted to an analog signal by the DAC  132   b.  Then, a frequency component corresponding to the second component carrier in the transmission signal is generated from the analog signal by the filter  134   b  and the up-converter  136   b.  Further, the baseband signal of the third component carrier (CC 3 ) is converted to an analog signal by the DAC  132   c.  Then, a frequency component corresponding to the third component carrier in the transmission signal is generated from the analog signal by the filter  134   c  and the up-converter  136   c.  After that, the generated frequency components corresponding to the three component carriers are combined by the combiner  138 , and the transmission signal is formed. The PA  140  amplifiers the transmission signal and outputs the transmission signal to the antenna  112  through the switch  114 . Then, the antenna  112  transmits the transmission signal as a radio signal to the base station  200 . 
         [0091]    Although the case where the radio communication unit  110  handles three component carriers is described in  FIG. 6 , the number of component carriers handled by the radio communication unit  110  may be two, or four or more. 
         [0092]    Further, instead of processing the signals of the respective component carriers in the analog region as in the example of  FIG. 6 , the radio communication unit  110  may process the signals of the respective component carriers in the digital region. In the latter case, at the time of reception, a digital signal converted by one ADC is separated into the signals of the respective component carriers by a digital filter. Further, at the time of transmission, after digital signals of the respective component carriers are frequency-converted and combined, the signal is converted into an analog signal by one DAC. The load of the ADC and the DAC is generally smaller when processing the signals of the respective component carriers in the analog region. On the other hand, when processing the signals of the respective component carriers in the digital region, a sampling frequency for AD/DA conversion is higher, and the load of the ADC and the DAC can thereby increase. 
         [0093]    (Signal processing unit) 
         [0094]    Referring back to  FIG. 5 , an example of a configuration of the user equipment  100  is further described below. 
         [0095]    The signal processing unit  150  performs signal processing such as deinterleaving, decoding or error correction on the demodulated data signal that is input from the radio communication unit  110 . Then, the signal processing unit  150  outputs the processed data signal to an upper layer. Further, the signal processing unit  150  performs signal processing such as encoding or interleaving on the data signal that is input from the upper layer. Then, the signal processing unit  150  outputs the processed data signal to the radio communication unit  110 . 
         [0096]    (Control unit) 
         [0097]    The control unit  160  controls the overall functions of the user equipment  100  by using a processing device such as a central processing unit (CPU) or a digital signal processor (DSP). For example, the control unit  160  controls the timing of data communication by the radio communication unit  110  according to scheduling information that is received from the base station  200  by the radio communication unit  110 . Further, the control unit  160  controls the measurement unit  170  to measure the channel quality by using a reference signal from the base station  200 , which is a serving base station, and transmits the channel quality report to the base station  200  through the radio communication unit  110 . Further, the control unit  160  controls the measurement unit  170  to execute measurement during the periods of measurement gaps which are allocated by the base station  200 . 
         [0098]    Further, in this embodiment, before a handover command from the source base station is received, the control unit  160  acquires the CP length being used by the target base station by receiving system information from the relevant target base station. The system information is delivered through a broadcast channel which is placed in a prescribed position in the radio frame, for example. Then, at the time of a handover, the control unit  160  controls the radio communication unit  110  to initiate a random access to the target base station after all of handover commands for all of the plurality of component carriers are received from the source base station by the radio communication unit  110 . Further, at the initiation of a random access to the target base station, for example, the control unit  160  changes the setting of the CP length in the radio communication unit  110  to the CP length being used by the target base station. 
         [0099]    Note that, when the CP length being used by the source base station and the CP length being used by the target base station are equal, the control unit  160  may control the radio communication unit  110  to initiate a random access to the target base after receiving a handover command for any of the component carriers, without waiting for handover commands for the other component carriers. In this case, the control unit  160  does not need to change the setting of the CP length in the radio communication unit  110 . 
         [0100]    (Measurement unit) 
         [0101]    The measurement unit  170  measures the channel quality for each of the component carriers by using a reference signal from the base station  200  according to control from the control unit  160 , for example. Further, the measurement unit  170  executes measurement for a handover with respect to each of the component carriers by using the measurement gaps which are allocated by the base station  200 . A result of the measurement executed by the measurement unit  170  is converted to a predetermined format for a measurement report by the control unit  160  and transmitted to the base station  200  through the radio communication unit  110 . After that, the base station  200  determines, based on the measurement report, whether a handover should be executed or not for the user equipment  100 . 
         [0102]    (3-2. Exemplary Configuration of Base Station) 
         [0103]      FIG. 7  is a block diagram showing an example of a configuration of the base station  200  according to the embodiment. Referring to  FIG. 7 , the base station  200  includes a radio communication unit  210 , an interface unit  250 , a component carrier (CC) management unit  260 , and a control unit  280 . 
         [0104]    (Radio communication unit) 
         [0105]    A specific configuration of the radio communication unit  210  may be similar to the configuration of the radio communication unit  110  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 the user equipment over a communication channel which is formed by aggregating a plurality of component carriers with use of the carrier aggregation technology. 
         [0106]    (Interface Unit) 
         [0107]    The interface unit  250  mediates a communication between the radio communication unit  210  or the control unit  280  and an upper node through the S 1  interface illustrated in  FIG. 4 , for example. Further, the interface unit  250  mediates a communication between the radio communication unit  210  or the control unit  280  and another base station through the X2 interface illustrated in  FIG. 4 , for example. 
         [0108]    (CC Management Unit) 
         [0109]    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. 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 . 
         [0110]    (Control Unit) 
         [0111]    The control unit  280  controls the overall functions of the base station  200  by using a processing device such as a CPU or a DSP. For example, the control unit  280  allocates communication resources for data communication to the user equipment  100  and other user equipments and then delivers scheduling information over a broadcast channel in a given sub-frame. Further, the control unit  280  delivers other system information over the broadcast channel, for example. The system information contains the set value of the CP length being used by the base station  200 , for example. Further, the control unit  280  controls the base station  200  to operate in the same manner as the source base station or the target base station in the handover procedure which is descried with reference to  FIG. 1 . 
         [0112]    (3-3. Flow of Process) 
         [0113]    A handover procedure according to the embodiment is described hereinafter with reference to  FIGS. 8A and 8B . Note that, in the following scenario, it is assumed that a handover procedure is performed among the user equipment  100 , the base station  200   a  serving as a source base station, and the base station  200   b  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 S 2  to S 14 ) in the typical handover procedure illustrated in  FIG. 1 , explanation is omitted because there is no significant difference. 
         [0114]    Referring to  FIG. 8A , the user equipment  100  first transmits a measurement report for the component carrier CC 1 , for example, to the base station  200   a  (step S 112 ). Receiving the measurement report, the base station  200   a  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  200   b  is better than a channel quality between the user equipment  100  and the base station  200   a  by a predetermined threshold or greater, it can be determined that a handover is necessary. In this case, the base station  200   a  transmits a handover request message for the component carrier CC 1  to the base station  200   b  (step S 114 ). Receiving the handover request message, the base station  200   b  determines whether it is possible to accept the user equipment  100  according to the availability of a communication service offered by itself or the like. When the base station  200   b  determines that it is possible to accept the user equipment  100 , the base station  200   b  transmits a handover request confirm message to the base station  200   a  (step S 116 ). Receiving the handover request confirm message, the base station  200   a  transmits a handover command for the component carrier CC 1  to the user equipment  100  (step S 118 ). 
         [0115]    Up to this point of time, the user equipment  100  acquires the CP length being used by the base station  200   b  by receiving system information from the base station  200   b  (for example, the system information may be received during execution of the measurement). For example, the CP length being used by the base station  200   b  is the CP length of the extended CP. On the other hand, the CP length being used by the base station  200   a  is the CP length of the normal CP. In this case, the user equipment  100  which has received a handover command from the base station  200   a  determines to wait until a handover command for another component carrier (i.e. CC 2 ) is received, without initiating a handover (S 120 ). 
         [0116]    Further, the user equipment  100  transmits a measurement report for the component carrier CC 2  to the base station  200   a  (step S 122 ). Then, the base station  200   a  transmits a handover request message for the component carrier CC 2  to the base station  200   b  (step S 124 ). Receiving the handover request message, the base station  200   b  transmits a handover request confirm message to the base station  200   a  (step S 126 ). Receiving the handover request confirm message, the base station  200   a  transmits a handover command for the component carrier CC 2  to the user equipment  100  (step S 128 ). 
         [0117]    When the step S 128  ends, handover commands for all of the plurality of component carriers constituting the communication channel between the user equipment  100  and the base station  200   a  have been received by the user equipment  100 . As a result, the user equipment  100  determines to initiate a handover (step S 130 ). 
         [0118]    Referring to  FIG. 8B , the user equipment  100  which has determined the initiation of a handover first acquires synchronization with the downlink channel of the component carrier CC 1  of the base station  200   b.  Then, the user equipment  100  makes a random access to the base station  200   b  by using a random access channel in a given time slot of the component carrier CC 1  (step S 134 ). During this period, the base station  200   a  forwards data addressed to the user equipment  100  to the base station  200   b  (step S 136 ). Then, after success in the random access for the component carrier CC 1 , the user equipment  100  transmits a handover complete message to the base station  200   b  (step S 142 ). Receiving the handover complete message, the base station  200   b  requests the MME to perform route update for the component carrier CC 1  of the user equipment  100  (step S 144 ). Upon updating the route of user data by the MME, the user equipment  100  becomes able to communicate with another device through a new base station (i.e. the base station  200   b ). Note that the request for route update may be performed for each of the plurality of component carriers, or performed only once for the plurality of component carriers. Then, the base station  200   b  transmits acknowledgement for the handover complete message to the user equipment  100  (step S 146 ). 
         [0119]    Further, the user equipment  100  acquires synchronization with the downlink channel of the component carrier CC 2  of the base station  200   b.  Then, the user equipment  100  makes a random access to the base station  200   b  by using a random access channel in a given time slot of the component carrier CC 2  (step S 154 ). During this period, the base station  200   a  keeps forwarding data addressed to the user equipment  100  to the base station  200   b  (step S 156 ). Then, after success in the random access for the component carrier CC 2 , the user equipment  100  transmits a handover complete message to the base station  200   b  (step S 162 ). Receiving the handover complete message, the base station  200   b  requests the MME to perform route update for the component carrier CC 2  of the user equipment  100  (step S 164 ). Then, the base station  200   b  transmits acknowledgement for the handover complete message to the user equipment  100  (step S 166 ). 
         [0120]    It should be noted that, except that the user equipment  100  should initiate a random access after receiving handover commands for all of the component carriers, the respective steps in the scenario illustrated in  FIG. 8A  and  FIG. 8B  may be executed in a different sequence.  FIG. 9  is a sequence chart showing another example of a flow of a handover procedure according to the embodiment. 
         [0121]    Referring to  FIG. 9 , the user equipment  100  first transmits a measurement report for the component carrier CC 1  to the base station  200   a  (step S 212 ). Further, the user equipment  100  first transmits a measurement report for the component carrier CC 2  to the base station  200   a  (step S 213 ). Note that those measurement reports may be integrated. 
         [0122]    Receiving the measurement reports, the base station  200   a  determines the necessity of a handover based on the measurement report. When the base station  200   a  determines that a handover is necessary, the base station  200   a  first transmits a handover request message for the component carrier CC 1  to the base station  200   b  (step S 214 ). The base station  200   a  then transmits a handover request message for the component carrier CC 2  to the base station  200   b  (step S 215 ). 
         [0123]    Receiving the handover request messages, the base station  200   b  determines whether it is possible to accept the user equipment  100  according to the availability of a communication service offered by itself or the like. When the base station  200   b  determines that it is possible to accept the user equipment  100 , the base station  200   b  first transmits a handover request confirm message for the component carrier CC 1  to the base station  200   a  (step S 216 ). The base station  200   b  then transmits a handover request confirm message for the component carrier CC 2  to the base station  200   a  (step S 217 ). 
         [0124]    Receiving the handover request confirm messages, the base station  200   a  transmits a handover command for the component carrier CC 1  to the user equipment  100  (step S 218 ). Further, the base station  200   a  transmits a handover command for the component carrier CC 2  to the user equipment  100  (step S 219 ). 
         [0125]    Receiving the handover command for the component carrier CC 1 , the user equipment  100  waits until the handover command for the component carrier CC 2  is received. Then, when the user equipment  100  receives the handover command for the component carrier CC 2 , it determines to initiate a handover (step S 220 ). 
         [0126]    The, the user equipment  100  first acquires synchronization with the downlink channel of the component carrier CC 1  of the base station  200   b.  Further, the user equipment  100  makes a random access to the base station  200   b  by using a random access channel in a given time slot of the component carrier CC 1  (step S 222 ). Then, the user equipment  100  acquires synchronization with the downlink channel of the component carrier CC 2  of the base station  200   b.  Further, the user equipment  100  makes a random access to the base station  200   b  by using a random access channel in a given time slot of the component carrier CC 2  (step S 223 ). During this period, the base station  200   a  forwards data addressed to the user equipment  100  to the base station  200   b  (steps S 224 , S 225 ). 
         [0127]    After that, upon success in the random access for the component carrier CC 1 , the user equipment  100  transmits a handover complete message for the component carrier CC 1  to the base station  200   b  (step S 232 ). Further, upon success in the random access for the component carrier CC 2 , the user equipment  100  transmits a handover complete message for the component carrier CC 2  to the base station  200   b  (step S 233 ). Receiving the handover complete messages, the base station  200   b  requests the MME to perform route update for the component carriers CC 1  and CC 2  of the user equipment  100  (steps S 234 , S 235 ). Then, the base station  200   b  transmits acknowledgements for the respective handover complete messages to the user equipment  100  (steps S 236 , S 237 ). 
         [0128]    (3-4. Summary of First Embodiment) The first embodiment of the present invention is described above with reference to  FIGS. 5 to 9 . According to the embodiment, the control unit  160  of the user equipment  100  initiates a random access to the target base station by the radio communication unit  110  after receiving handover commands for all of the plurality of component carriers constituting the communication channel. In this case, a situation does not occur where the user equipment performs a communication with the source base station in a certain component carrier and also performs a communication with the target base station in another component carrier at the same time. It is thereby possible to avoid the user equipment  100  from simultaneously handing different CP lengths during a handover procedure. 
         [0129]    Further, when the CP length being used by the source base station and the CP length being used by the target base station are equal, the user equipment  100  may initiate a random access soon after receiving a handover command for any of the component carriers, without waiting for a handover command for the other component carrier. It is thereby possible to complete a handover promptly for the component carrier for which a handover request is confirmed when it is not necessary to switch the CP length. 
         [0130]    Furthermore, the user equipment  100  may acquire the CP length being used by the target base station by using the system information which is broadcasted from the relevant target base station. The user equipment  100  may receive the system information which is broadcasted from the target base station at the time of measurement, for example. This configuration eliminates the need for exchanging an additional message for determination about waiting for a random access depending on the CP length, and it is thereby possible to reduce the impact on the existing system. 
       4. Description of Second Embodiment  
       [0131]    A second embodiment of the present invention for avoiding simultaneous handling of different CP lengths during a handover procedure in a radio communication involving the carrier aggregation is described hereinafter with reference to  FIGS. 10 to 14 . 
         [0132]    In the second embodiment of the present invention, communication resources are scheduled in such a way that the user equipment can perform a radio communication with the source base station and the target base station by switching the CP length in time division during a handover procedure. 
         [0133]      FIG. 10  is an explanatory view to describe an outline of switching of a cyclic prefix length in time division. 
         [0134]    Referring to  FIG. 10 , as a result that a handover is executed sequentially for three component carriers CC 1  to CC 3 , a series of handover procedure can be divided into four stages. The first stage is before a handover. At the first stage, all component carriers are connected to the source base station. The second stage is after a handover of the component carrier CC 1 . At the second stage, the component carrier CC 1  is connected to the target base station, and the component carriers CC 2  and CC 3  are connected to the source base station. The third stage is after a handover of the component carriers CC 1  and CC 2 . At the third stage, the component carriers CC 1  and CC 2  are connected to the target base station, and the component carrier CC 3  is connected to the source base station. The fourth stage is after a handover of all the component carriers. At the fourth stage, all component carriers are connected to the target base station. 
         [0135]    Among those stages, at the first stage, because all component carriers are connected to the source base station, the user equipment can use the same CP length (e.g. the normal CP) at any point of time. Thus, the source base station can allocate communication resources for each component carrier without depending on the CP length. Likewise, at the fourth stage, because all component carriers are connected to the target base station, the user equipment can use the same CP length (e.g. the extended CP) at any point of time. Thus, the target base station can allocate communication resources for each component carrier without depending on the CP length. 
         [0136]    On the other hand, at the second stage and the third stage, a part of a plurality of component carriers is connected to the target base station, and the rest is connected to the source base station. In this case, a communication resource is allocated only for any one component carrier with respect to each sub-frame by the source base station and the target base station, for example. For example, in the example of  FIG. 10 , at the second stage, a communication resource of the sub-frame SF 1  is allocated for the component carrier CC 1 , a communication resource of the sub-frame SF 2  is allocated for the component carrier CC 2 , and a communication resource of the sub-frame SF 3  is allocated for the component carrier CC 3 . Further, at the third stage, a communication resource of the sub-frame SF 4  is allocated for the component carrier CC 1 , a communication resource of the sub-frame SF 5  is allocated for the component carrier CC 2 , and a communication resource of the sub-frame SF 6  is allocated for the component carrier CC 3 . Such an allocation of communication resources can be achieved by exchanging scheduling information between the source base station and the target base station through a backhaul link, for example. 
         [0137]    As described above, in this embodiment, a communication resource is allocated only for any one component carrier with respect to each sub-frame. In this case, even when the CP lengths to be used are different between the source base station and the target base station, the user equipment can communicate with the respective base stations in time division by changing the setting of the CP length with respect to each sub-frame. Changing the CP length to use with respect to each subframe during a handover procedure results in avoiding simultaneous handling of different cyclic prefix lengths in one user equipment. 
         [0138]    Note that, in such scheduling, it is preferred to set the ratio (ratio in a given time window) of the amount of communication resource allocated for a part of component carrier for which a handover is completed and the amount of communication resource allocated for the rest of the component carriers to be substantially the same as the radio of the number of component carriers between the former and the latter. In the example of  FIG. 10 , the ratio of the amount of communication resources at the second stage is 1:2 (after:before handover). Further, the ratio of the amount of communication resources at the third stage is 2:1 (after:before handover). According to such scheduling, it is possible to share traffic among a plurality of component carriers in the case of allocating a communication resource only for any one component carrier with respect to each sub-frame. 
         [0139]    The configurations of a user equipment  300  and a base station  400  according to the embodiment for achieving the above-described switching of the SP length in time division during a handover procedure are specifically described hereinbelow. 
         [0140]    (4-1. Exemplary Configuration of User Equipment) 
         [0141]      FIG. 11  is a block diagram showing an example of a configuration of the user equipment  300  according to the embodiment. Referring to  FIG. 11 , the user equipment  300  includes a radio communication unit  310 , a signal processing unit  150 , a control unit  360 , and a measurement unit  170 . 
         [0142]    (Radio Communication Unit) 
         [0143]    The radio communication unit  310  performs a radio communication with the base station  400  over a communication channel formed by aggregating a plurality of component carriers with use of the carrier aggregation technology. 
         [0144]      FIG. 12  is a block diagram showing an example of a detailed configuration of the radio communication unit  310 . Referring to  FIG. 12 , the demodulation unit  128  of the radio communication unit  310  includes a CPU removal unit  329 . Further, the modulation unit  130  includes a CPU insertion unit  331 . 
         [0145]    The CPU removal unit  329  removes a cyclic prefix from the baseband signal of each component carrier which is input to the demodulation unit  128 . The CP length of the cyclic prefix which is removed by the CPU removal unit  329  can be switched with respect to each sub-frame according to a CP length control signal SIG 1  that is input from the control unit  360 . Further, the CPU insertion unit  331  inserts a cyclic prefix into the baseband signal which is generated by the modulation unit  130 . The CP length of the cyclic prefix which is inserted by the CPU insertion unit  331  can be switched with respect to each sub-frame according to a CP length control signal SIG 2  that is input from the control unit  360 . The operation of the radio communication unit  310  other than the switching of the setting of the CP length in the CPU removal unit  329  and the CPU insertion unit  331  may be the same as the operation of the radio communication unit  110  according to the first embodiment described above with reference to  FIG. 5 . 
         [0146]    (Control Unit) 
         [0147]    The control unit  360  controls the overall functions of the user equipment  300 , like the control unit  160  according to the first embodiment. For example, the control unit  360  controls the timing of data communication by the radio communication unit  310  according to scheduling information that is received from the base station  400  by the radio communication unit  310 . Further, in this embodiment, the control unit  360  switches the setting of the CP length of the CPU removal unit  329  and the CPU insertion unit  331  of the radio communication unit  310  with respect to each sub-frame according to the allocation of communication resources by outputting the CP length control signals SIG 1  and SIG 2  to the radio communication unit  310  at the state during a handover procedure (e.g. at the second and third stages described with reference to  FIG. 10 ). 
         [0148]    It should be noted that, the synchronization to a downlink channel from the base station is performed by using both of a primary synchronization signal that is transmitted in the first sub-frame and a secondary synchronization signal that is transmitted in the sixth sub-frame in one radio frame. Therefore, it is preferred in the synchronization processing to switch the base station to be synchronized with respect to each radio frame, not each sub-frame. Alternatively, in the radio communication unit  310 , a synchronous circuit for the normal CP and a synchronous circuit for the extended CP may be placed in parallel in the radio communication unit  310 . In this case, it is not necessary to switch the base station to be synchronized with respect to each radio frame. Because the synchronous circuit is implemented only in the physical layer, an impact on the system is small even when a plurality of synchronous circuits are placed in parallel. 
         [0149]    Further, delivery of system information from the base station is also performed in a particular sub-frame in one radio frame. Therefore, for the reception of system information in the user equipment  100 , like the synchronization processing described above, it is preferred to switch the base station to be synchronized with respect to each radio frame, not each sub-frame. In this case, because the base station  400  delivers the same system information over a plurality of successive radio frames, the user equipment  300  can acquire the system information of the source base station and the target base station without a loss. 
         [0150]    (4-2. Exemplary Configuration of Base Station) 
         [0151]      FIG. 13  is a block diagram showing an example of a configuration of the base station  400  according to the embodiment. Referring to  FIG. 13 , the base station  400  includes a radio communication unit  210 , an interface unit  250 , a CC management unit  260 , and a control unit  480 . 
         [0152]    (Control Unit) 
         [0153]    The control unit  480 , like the control unit  280  according to the first embodiment, controls the overall functions of the base station  400 . For example, the control unit  480  allocates communication resources for data communication to the user equipment  300  and other user equipments and then delivers scheduling information over a broadcast channel in a given sub-frame. Further, the control unit  480  delivers other system information over the broadcast channel, for example. The system information contains the set value of the CP length being used by the base station  400 , for example. 
         [0154]    In this embodiment, at a stage during a handover procedure, the control unit  480  of the source base station allocates a communication resource in a sub-frame, which is different from a sub-frame in which a communication resource is allocated by the target base station for a part of component carriers for which a handover is completed, for the rest of the component carriers. Specifically, the control unit  480  acquires scheduling information for a part of component carriers for which a handover is completed from the target base station through a backhaul link, for example. Then, the control unit  480  allocates a communication resource in a sub-frame in which a communication resource is not allocated in the scheduling information for the rest of the component carriers. 
         [0155]    Further, the control unit  480  adjusts scheduling so that the ratio of the amount of communication resource allocated for a part of component carriers for which a handover is completed and the amount of communication resource allocated for the rest of the component carriers is substantially the same as the radio of the number of component carriers between the former and the latter. Specifically, the control unit  480  of the source base station acquires the ratio between the number of component carriers after handover completion and the number of component carriers before handover completion (which is referred to hereinafter as the ratio of the number of CCs) based on information exchange with the target base station. Then, the control unit  480  allocates communication resources to the user equipment  100  in such a way that the ratio of the amount of communication resources is substantially the same as the ratio of the number of CCs. Alternatively, the control unit  480  may designate the amount of communication resources which the target base station should allocate to the user equipment  100  to the target base station through a backhaul link. 
         [0156]    (4-3. Flow of Process) 
         [0157]      FIG. 14  is an explanatory view to describe an example of a flow of a handover procedure according to the embodiment.  FIG. 14  shows an allocation of communication resources at the four stages related to the handover procedure which is performed in the same manner as shown in  FIG. 10 . Note that, in  FIG. 14 , like  FIG. 10 , the user equipment sequentially executes a handover from the source base station to the target base station for three component carriers CC 1  to CC 3 . 
         [0158]    First, at the first stage, all component carriers are connected to the source base station. Therefore, any sub-frame can be allocated for the component carriers before completion of a handover (using the normal CP). Note that it is not practically necessary to allocate communication resources to all of those sub-frames. Further, the synchronization signal is used for the synchronization with the source base station. 
         [0159]    Next, at the second stage, the ratio of the number of CCs is 1:2 (after:before handover). Therefore, among the six successive sub-frames (excluding the sub-frame for the synchronization signal), for example, two sub-frames can be allocated for the component carrier after completion of a handover (using the extended CP), and four sub-frames can be allocated for the component carrier before completion of a handover. For example, when a handover is completed for the component carrier CC 1 , a first sub-frame (# 1 ) and a second sub-frame (# 2 ) can be allocated for the component carrier CC 1 . Further, after the synchronization signal in one radio frame is used for the synchronization with the source base station, the synchronization signal in the next radio frame can be used for the synchronization with the target base station. 
         [0160]    Then, at the third stage, the ratio of the number of CCs is 2:1 (after:before handover). Therefore, among the six successive sub-frames (excluding the sub-frame for the synchronization signal), for example, four sub-frames can be allocated for the component carrier after completion of a handover (using the extended CP), and two sub-frames can be allocated for the component carrier before completion of a handover. For example, when a handover is completed for the component carrier CC 2  subsequent to the component carrier CC 1 , a third sub-frame (# 3 ) and a fourth sub-frame (# 4 ) can be allocated for the component carrier CC 2 . Further, after the synchronization signal in one radio frame is used for the synchronization with the source base station, the synchronization signal in the next radio frame can be used for the synchronization with the target base station. 
         [0161]    Finally, at the fourth stage, all component carriers are connected to the target base station. Therefore, any sub-frame can be allocated for the component carriers after completion of a handover. Further, the synchronization signal is used for the synchronization with the target base station. 
         [0162]    (4-4. Summary of Second Embodiment) 
         [0163]    The second embodiment of the present invention is described above with reference to  FIGS. 10 to 14 . According to the embodiment, when a handover to the target base station is completed only for a part of a plurality of component carriers, a communication resource in a sub-frame which is different from a sub-frame within which a communication resource is allocated by the target base station for the part of the plurality of component carriers is allocated by the source base station for the rest of the plurality of component carriers. In this configuration, a situation does not occur where the user equipment  300  performs a radio communication with the source base station and performs a radio communication with the target base station at the same time during a handover procedure. Therefore, the user equipment  300  can sequentially perform radio communications with the source base station and the target base station during a handover procedure by switching the CP length in time division manner. It is thereby possible to avoid simultaneous handling of different cyclic prefix lengths during a handover procedure in a radio communication involving the carrier aggregation. 
         [0164]    Further, according to the embodiment, scheduling is adjusted so that the ratio of the amount of communication resource allocated by the target base station for the component carrier with handovers completed and the amount of communication resource allocated by the source base station for the component carrier with handovers not yet completed is substantially the same as the radio of the number of component carriers with handovers completed and the number of component carriers with handovers not yet completed. The amount of traffic is thereby equally shared among the component carriers, and it is thus possible to carry out data transmission during a handover procedure. 
         [0165]    Further, the user equipment  300  may perform the synchronization with the source base station by using the synchronization signal in a radio frame which is different from a radio frame in which the synchronization signal to be used for the synchronization with the target base station is contained. Further, the user equipment  300  can receive system information from the source base station in a radio frame which is different from a radio frame in which system information from the target base station is received. This configuration eliminates the need for using a plurality of synchronous circuits or a plurality of processing units for handling different CP lengths, and it is thereby possible to suppress the manufacturing cost of the device. 
         [0166]    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 appended claims or the equivalents thereof. 
       REFERENCE SIGNS LIST  
       [0167]      1  RADIO COMMUNICATION SYSTEM 
         [0168]      100 ,  300  USER EQUIPMENT 
         [0169]      110 ,  310  RADIO COMMUNICATION UNIT (USER EQUIPMENT) 
         [0170]      160 ,  360  CONTROL UNIT (USER EQUIPMENT) 
         [0171]      200 ,  400  BASE STATION 
         [0172]      210  RADIO COMMUNICATION UNIT (BASE STATION) 
         [0173]      280 ,  480  CONTROL UNIT (BASE STATION)