Abstract:
A communication terminal device capable of increasing the success rate of reconnection while suppressing signal volume. A communication terminal ( 100 ) connects to and communicates with any cell from among a plurality of cells under the control of a base station. A neighboring cell information decoding unit ( 105 ) acquires neighboring cell information from a cell that is currently connected. A neighboring cell searching unit ( 106 ), when reconnection with a cell becomes necessary, initiates the search for a cell, while detecting from among the cells in the neighboring cell information the cell coinciding with a cell detected by the search as the cell to be reconnected. A reconnection request information generating unit ( 107 ) generates a reconnection request message for the cell to be reconnected, which is detected by the neighboring cell searching unit ( 106 ).

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to communication terminal apparatus and a method of reestablishing connection to a cell, and more particularly relates to a communication terminal apparatus configured to reestablish connection to a cell and to a method of reestablishing connection to a cell particularly in a radio communication system. 
       BACKGROUND ART 
       [0002]    In a radio communication system, the propagation environment between a communication terminal apparatus and a base station changes, momentarily. As compared with a wire communication system, a radio link in a radio communication system is unstable. Such an unstable radio link is sometimes disconnected due to deterioration of the propagation environment or the like. 
         [0003]    Meanwhile, connection reestablishment techniques for promptly recovering a disconnected radio link have been developed. For example, connection reestablishment processing is defined in a Long Term Evolution (LTE)/System Architecture Evolution (SAE) system, which is under standardization by 3rd Generation Partnership Project (3GPP) Standardization body (see, e.g., Non-Patent Literature (hereinafter, abbreviated as “NPL”) 1). The connection reestablishment processing can recover the connection of disconnected radio link using simpler processing than processing to retry the connection procedure. 
         [0004]    In the radio communication system of NPL 1, a base station (eNode B) provides and serves a plurality of cells. Meanwhile, a communication terminal apparatus (UE) in the radio communication system of NPL 1 transmits a connection reestablishment request message for requesting connection reestablishment to an incidentally-detected cell to the base station at the occurrence of a trigger for connection reestablishment. Examples of the trigger for connection reestablishment herein include an out-of-synchronization state between a base station and a communication terminal apparatus, and a handover failure. 
         [0005]    The base station that has received the connection reestablishment request message checks a terminal connection reestablishment ID included in the connection reestablishment request message received from the communication terminal apparatus. The terminal connection reestablishment ID includes a cell radio network temporary identifier (C-RNTI), physical cell ID (PCI) and a shortMAC-I, for example. When a trigger for connection reestablishment due to disconnection of a radio link occurs, the base station transmits a connection reestablishment message for allowing connection reestablishment if the cell in which the communication terminal apparatus had been performing communication until the disconnection and the cell for which the communication terminal apparatus is requesting connection reestablishment are served by the same base station. On the other hand, if the cell in which the communication terminal apparatus had been performing communication until the disconnection, and the cell for which the communication terminal apparatus requesting connection reestablishment are not served by the same base station, the base station transmits a connection reestablishment reject message for rejecting connection reestablishment, and does not allow connection reestablishment. In general, the radio link of a communication terminal apparatus located at a cell edge apart from the base station is more easily disconnected, and a trigger for connection reestablishment is likely to occur, accordingly. 
         [0006]    When a trigger for connection reestablishment due to a handover failure occurs, the base station allows connection reestablishment when the handover source cell and the handover target cell are served by the same base station, and the base station rejects connection reestablishment to a cell served by a different base station. 
         [0007]    Next, a description will be provided, with reference to  FIG. 1 , regarding connection reestablishment processing when disconnection of a radio link is detected as a trigger for connection reestablishment in the radio communication system illustrated in NPL 1.  FIG. 1  is a diagram illustrating connection reestablishment processing performed when disconnection of a radio link is detected as a trigger for connection reestablishment processing in the related art. In this case, the communication terminal apparatus can reestablish connection only to a cell served by the same base station. In  FIG. 1 , base station  20 - 1  provides and serves cells # 1  to # 3  and base station  20 - 2  provides and serves cells # 4  to # 6 . 
         [0008]    First, a case where connection reestablishment processing succeeds will be described with reference to  FIG. 1 . 
         [0009]    In  FIG. 1 , communication terminal apparatus  10 - 1  detects disconnection of a radio link with base station  20 - 1  ( 1 - 1 ) and transmits a connection reestablishment request message for requesting connection reestablishment to cell # 3  to base station  20 - 1  ( 1 - 2 ). 
         [0010]    Next, base station  20 - 1  transmits a connection reestablishment message for allowing connection reestablishment to communication terminal apparatus  10 - 1  ( 1 - 3 ) since cell # 1  to which communication terminal apparatus  10 - 1  had been connected until disconnection and cell # 3  to which communication terminal apparatus  10 - 1  has made the request for connection reestablishment are both served by base station  20 - 1 . 
         [0011]    Accordingly, communication terminal apparatus  10 - 1  reestablishes connection to cell # 3  and resumes communication. 
         [0012]    Next, a case where connection reestablishment processing fails will be described with reference to  FIG. 1 . 
         [0013]    In  FIG. 1 , communication terminal apparatus  10 - 2  detects disconnection of a radio link to base station  20 - 1  in cell # 2  ( 2 - 1 ) and performs a cell search. 
         [0014]    In addition, communication terminal apparatus  10 - 2  transmits a connection reestablishment request message for requesting connection reestablishment to base station  20 - 2 , which provides and serves cell # 4  found by the cell search ( 2 - 2 ). 
         [0015]    Base station  20 - 2 , however, transmits a connection reestablishment reject message for rejecting connection reestablishment to base station  20 - 2  ( 2 - 3 ) since cell # 2  to which communication terminal apparatus  10 - 2  had been connected until disconnection and cell # 4  to which communication terminal apparatus  10 - 2  has made the request for connection reestablishment are served by different base stations. 
         [0016]    Accordingly, communication terminal apparatus  10 - 2  cannot reconnect to cell # 4 . 
         [0017]    Next, a description will be provided, with reference to  FIG. 2 , regarding connection reestablishment processing used when a trigger for connection reestablishment due to a handover failure occurs in the radio communication system illustrated in NPL 1.  FIG. 2  is a diagram illustrating connection reestablishment processing in the related art used when a trigger for connection reestablishment due to a hand over failure occurs. In  FIG. 2 , a communication terminal apparatus can reestablish connection only to a cell served by the base station serving the handover source cell and to a cell served by the base station serving the handover target cell, in  FIG. 2 , base station  20 - 1  serves cells # 1  to # 3 , base station  20 - 2  serves cells # 4  to # 6 , and base station  20 - 3  serves cells # 7  to # 9 . 
         [0018]    First, a case where connection reestablishment processing succeeds will be described with reference to  FIG. 2 . 
         [0019]    In  FIG. 2 , when failing in handover from base station  20 - 1  to base station  20 - 2  ( 3 - 1 ), communication terminal apparatus  10 - 3  transmits a connection reestablishment request message for requesting connection reestablishment to cell # 2  served by base station  20 - 1 , which is the handover source ( 3 - 2 ). Alternatively, when failing in handover from base station  20 - 1  to base station  20 - 2  ( 3 - 1 ), communication terminal apparatus  10 - 3  transmits a connection reestablishment request message for requesting connection reestablishment to cell # 4  served by base station  20 - 2 , which is the handover target ( 3 - 3 ). 
         [0020]    Next, base station  20 - 1 , which is the handover source, or base station  20 - 2 , which is the handover target, transmits a connection reestablishment message for allowing connection reestablishment to communication terminal apparatus  10 - 3  ( 3 - 4  or  3 - 5 ). 
         [0021]    Accordingly, communication terminal apparatus  10 - 3  reestablishes connection to cell # 2  or # 4  and resumes communication. 
         [0022]    Next, a case where connection reestablishment processing fails will be described with reference to  FIG. 2 . 
         [0023]    In  FIG. 2 , when failing in handover from base station  20 - 1  to base station  20 - 2  ( 3 - 1 ), communication terminal apparatus  10 - 3  performs a cell search. 
         [0024]    In addition, communication terminal apparatus  10 - 3  transmits a connection reestablishment request message for requesting connection reestablishment to cell  49  found by cell search ( 3 - 6 ). 
         [0025]    However, base station  20 - 3  transmits a connection reestablishment reject message for rejecting connection reestablishment to communication terminal apparatus  10 - 3  ( 3 - 7 ) since cell # 9  is not served by the handover source base station or handover target base station. 
         [0026]    Accordingly, base station  10 - 3  cannot reconnect to cell # 9 . 
         [0027]    Next, a description will be provided in more detail with reference to  FIG. 3  regarding connection reestablishment processing when connection reestablishment in the related art succeeds.  FIG. 3  is a sequence diagram illustrating connection reestablishment processing when the connection reestablishment in the related art succeeds. 
         [0028]    In  FIG. 3 , when a trigger for connection reestablishment processing occurs (step ST 1 ), a communication terminal apparatus performs a cell search (step ST 2 ) and detects a predetermined cell. 
         [0029]    In addition, the communication terminal apparatus transmits a connection reestablishment request message to a base station serving cell  41  in order to reestablish connection to cell # 1  detected by cell search (step ST 3 ). 
         [0030]    Next, the communication terminal apparatus receives a connection reestablishment message from the base station (step ST 4 ) and then completes the connection reestablishment by transmitting a connection reestablishment complete message (step ST 5 ). Thus, the communication terminal apparatus can reconnect with cell # 1  and starts communication (step ST 6 ). 
         [0031]    In  FIG. 3 , cell # 0  and cell # 1  are served by the same base station, or the base station serving cell # 0  is the handover source base station, and the base station serving cell # 1  is the handover source base station. 
         [0032]    Next, a description will be provided in more detail with reference to  FIG. 4  regarding processing when connection reestablishment in the related art fails.  FIG. 4  is a sequence diagram illustrating processing when the connection reestablishment in the related art fails. 
         [0033]    In  FIG. 4 , when a trigger for connection reestablishment processing occurs (step ST 11 ), a communication terminal apparatus performs a cell search (step ST 12 ) and detects a cell. 
         [0034]    In addition, the communication terminal apparatus transmits a connection reestablishment request message to a base station serving cell # 1  in order to reestablish connection to cell # 1  detected by cell search (step ST 13 ). 
         [0035]    Next, the communication terminal apparatus receives a connection reestablishment reject message (step ST 14 ). 
         [0036]    The communication terminal apparatus performs cell re-selection processing in order to resume communication (step ST 15 ). 
         [0037]    The communication terminal apparatus transmits a connection establishment request message (step ST 16 ) and receives a connection setting message (step ST 17 ). 
         [0038]    The communication terminal apparatus transmits a connection establishment complete/location registration update request message (step ST 18 ) and receives a security mode command message (step ST 19 ). 
         [0039]    Next, the communication terminal apparatus transmits a security mode complete message (step ST 20 ) and receives a terminal performance confirmation message (step ST 21 ). 
         [0040]    The communication terminal apparatus transmits a terminal performance information message (step ST 22 ) and receives a location registration approval message (step ST 23 ). 
         [0041]    Next, the communication terminal apparatus transmits a location registration update complete message (step ST 24 ). 
         [0042]    Accordingly, the communication terminal apparatus can reconnect to cell # 2  and starts communication (step ST 25 ). 
         [0043]    In  FIG. 4 , cell # 0  and cell # 1  are served by different base stations, or cell # 1  is not served by the handover source base station or handover target base station either. 
         [0044]    As described above, when a connection reestablishment request is rejected, the communication terminal apparatus needs to perform the location registration and connection establishment procedure again. In addition, when not being able to detect a connection reestablishment candidate cell in a predetermined period of time during cell search processing, the communication terminal apparatus needs to perform the connection establishment procedure again as a connection reestablishment failure. 
       CITATION LIST 
     Non-Patent Literature 
       [0000]    
       
         NPL1 
         3GPP TS 36.331 V8.8.0 (2009-12) 
       
     
       SUMMARY OF INVENTION 
     Technical Problem 
       [0047]    In the related art system, however, since communication terminal apparatus cannot identify a cell allowing connection reestablishment, the communication terminal apparatus attempts to reestablish connection to an incidentally-detected cell. Accordingly, a communication terminal apparatus located at a cell edge is likely to detect a plurality of different cells and also a cell other than a cell allowing connection reestablishment, compared with a communication terminal apparatus located near the center of the cell. In the related art system, there arises a problem in that unnecessary signaling used for a connection reestablishment request message and a connection reestablishment reject message increases, resulting in an increase in the amount of signaling in this ease. Moreover, another problem in the related art system is that performing the connection establishment processing after connection reestablishment is rejected results in an increase in the amount of signaling compared to resuming communication after connection reestablishment succeeds. 
         [0048]    It is an object of the present invention to provide a communication terminal apparatus and a method of reestablishing connection to a cell each of which can increase the success rate in connection reestablishment and also limit an increase in the amount of signaling. 
       Solution to Problem 
       [0049]    A communication terminal apparatus according to an aspect of the present invention is a communication terminal apparatus that connects to any of a plurality of cells served by a base station, to perform communication, the apparatus including: an acquiring section that acquires, from a cell to which the apparatus is currently connected, neighboring cell information indicating a neighboring cell located near the currently connected cell; a detection section that performs a cell search when a need for connection reestablishment to a cell arises, and that detects, as a connection reestablishment target cell, a cell that matches a cell detected by the search from among cells indicated by the neighboring cell information; and a connection reestablishment section that re-establishes connection to the connection reestablishment target cell detected by the detection section. 
         [0050]    A method of reestablishing connection to a cell according to an aspect of the present invention is a method for a communication terminal apparatus that connects to any of a plurality of cells served by a base station, to perform communication, the method including: acquiring, from a cell to which the apparatus is currently connected, neighboring cell information indicating a neighboring cell located neat the currently connected cell; performing a cell search when a need for connection reestablishment to a cell arises, and detecting, as a connection reestablishment target cell, a cell that matches a cell detected by the search from among cells indicated by the neighboring cell information; and reestablishing connection to the detected connection reestablishment target cell. 
       Advantageous Effects of invention 
       [0051]    According to the present invention, it is possible to increase the success rate in connection reestablishment and also to limit an increase in the amount of signaling. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0052]      FIG. 1  is a diagram illustrating connection reestablishment processing in the related art when disconnection of a radio link is detected as a trigger for the connection reestablishment processing; 
           [0053]      FIG. 2  is a diagram illustrating connection reestablishment processing in the related at when a handover failure occurs and triggers the connection reestablishment processing; 
           [0054]      FIG. 3  is a sequence diagram illustrating an example of the connection reestablishment processing in the related art when connection reestablishment succeeds; 
           [0055]      FIG. 4  is a sequence diagram illustrating processing in tire related art when connection reestablishment fails; 
           [0056]      FIG. 5  is a block diagram illustrating a configuration of a communication terminal apparatus according to Embodiment 1 of the present invention; 
           [0057]      FIG. 6  is a flowchart illustrating operation of the communication terminal apparatus according to Embodiment 1 of the present invention; 
           [0058]      FIG. 7  is a sequence diagram illustrating operation of a radio communication system according to Embodiment 1 of the present invention; 
           [0059]      FIG. 8  is a block diagram illustrating a configuration of a base station according to Embodiment 2 of the present invention; 
           [0060]      FIG. 9  is a block diagram illustrating a configuration of a base station according to Embodiment 3 of the present invention; and 
           [0061]      FIG. 10  is a flowchart illustrating operation of the communication terminal apparatus when lowering the priority for a cell other than a cell indicated by neighboring cell information, as a variation of Embodiments 1 to 3. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0062]    Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. 
       Embodiment 1 
       [0063]      FIG. 5  is a block diagram illustrating a configuration of communication terminal apparatus  100  according to Embodiment 1 of the present invention. 
         [0064]    Communication terminal apparatus  100  mainly includes antenna  101 , radio receiving section  102 , demodulation section  103 , connection reestablishment determining section  104 , neighboring cell information decoding section  105 , neighboring cell search section  106 , connection reestablishment request information generating section  107 , multiplexing/demodulation section  108 , radio transmitting section  109 , and antenna  110 . 
         [0065]    Antenna  101  receives a signal and also outputs the received signal to radio receiving section  102 . 
         [0066]    Radio receiving section  102  converts the signal received as input from antenna  101  from a radio signal to a baseband signal and outputs the baseband signal to demodulation section  103 . 
         [0067]    Demodulation section  103  demodulates the baseband signal received as input from radio receiving section  102  to acquire predetermined information and outputs the acquired information to connection reestablishment determining section  104  and neighboring cell information decoding section  105 . Specifically, demodulation section  103  acquires synchronization information, security information, reconfiguration information or handover information through demodulation and outputs the acquired information to connection reestablishment determining section  104 . In addition, demodulation section  103  acquires neighboring cell information through demodulation and outputs the acquired neighboring cell information to neighboring cell information decoding section  105 . The neighboring cell information is transmitted from a base station via a currently connected cell using a broadcast information message (System Information), a connection setting message (RRC Connection Setup), or a connection reconfiguration message (RRC Connection Reconfiguration). 
         [0068]    Connection reestablishment determining section  104  determines whether or not to perform connection reestablishment on the basis of the synchronization information, security information, reconfiguration information handover information received as input from demodulation section  103 . Specifically, connection reestablishment determining section  104  determines to perform connection reestablishment when detecting information that triggers connection reestablishment from the synchronization information, security information, reconfiguration information or handover information received as input from demodulation section  103 . Connection reestablishment determining section  104  determines to perform connection reestablishment when detecting disconnection of a radio link (radio link failure) from the synchronization information, for example. Connection reestablishment determining section  104  determines to perform connection reestablishment when detecting an integrity check failure from the security information. Connection reestablishment determining section  104  determines to perform connection reestablishment when detecting an RRC connection reconfiguration failure from the reconfiguration information. Moreover, connection reestablishment determining section  104  determines to perform connection reestablishment when detecting a handover failure from the handover information. When determining to perform connection reestablishment, connection reestablishment determining section  104  outputs a neighboring cell search instruction that indicates a neighboring cell search to neighboring cell search section  106 . 
         [0069]    Neighboring cell information decoding section  105  decodes neighboring cell information received as input from demodulation section  103 . Moreover, neighboring cell information decoding section  105  outputs the decoded neighboring cell information to neighboring cell search section  106 . The neighboring cell information to be decoded by neighboring cell information decoding section  105  herein is information on another cell served by the base station that serves a cell to which communication terminal apparatus  100  had been connected until disconnection of the radio link. Alternatively, the neighboring cell information to be decoded herein is information on another cell served by the base station that also serves a cell to which communication terminal apparatus  100  is connected, and a cell served by a base station located near the base station. More specifically, the neighboring cell information in this case is the information on cells served by a handover source base station and a handover target base station, respectively. In addition, the neighboring cell information is cell ID information on the neighboring cell (e.g., PCI of neighboring cell), information on a frequency used in the neighboring cell (e.g., frequency list of neighboring cell), or information on a radio connection system (Radio Access Technology (RAT)) used by the neighboring cell. The radio access technology is information indicating a connection system used in radio communication systems, such as GSM, UTRAN or E-UTRAN. 
         [0070]    Upon receipt of a neighboring cell search instruction from connection reestablishment determining section  104 , neighboring cell search section  106  performs a neighboring cell search and detects a cell allowing connection reestablishment, on the basis of the search result and decoded neighboring cell information received as input from neighboring cell information decoding section  105 . Neighboring cell search section  106  performs, as a neighboring cell search, processing to determine whether or not the cell detected by cell search matches a cell indicated by the cell ID information received as input from neighboring cell information decoding section  105 . In addition, neighboring cell search section  106  performs, as a neighboring cell search, processing to determine whether or not the frequency information used in the cell detected by cell search matches the frequency information received as input from neighboring cell information decoding section  105 . Moreover, neighboring cell search section  106  performs, as a neighboring cell search, processing to determine whether or not the radio connection system information used in the cell detected by cell search matches the radio connection system information received as input from neighboring cell information decoding section  105 . 
         [0071]    When detecting a cell allowing connection reestablishment, neighboring cell search section  106  outputs information, on the cell allowing connection reestablishment to reestablishment request information generating section  107 . Neighboring cell search section  106  detects, as a cell allowing connection reestablishment, a cell that matches the cell detected by cell search, from among cells indicated by the cell ID information received as input from neighboring cell information decoding section  105 , for example. Alternatively, neighboring cell search section  106  detects, as a cell allowing connection reestablishment, a cell that matches the cell detected by cell search, from among cells using the frequency information received as input from neighboring cell information decoding section  105 . Alternatively, neighboring cell search section  106  detects, as a cell allowing connection reestablishment, a cell that matches the cell detected by cell search, from among cells using the radio connection system information received as input from neighboring cell information decoding section  105 . 
         [0072]    Upon receipt of information on a cell allowing connection reestablishment from neighboring cell search section  106 , connection reestablishment request information generating section  107  generates a connection reestablishment request to message intended for the cell allowing connection reestablishment, with reference to the information on this particular cell and outputs the connection reestablishment request message to multiplexing/modulation section  108 . Meanwhile, connection reestablishment request information generating section  107  generates no connection reestablishment request message when not receiving information on a cell allowing connection reestablishment from neighboring cell search section  106 . Accordingly, communication terminal apparatus  100  attempts connection reestablishment to a cell allowing connection reestablishment. The PCI of the cell in which Communication terminal apparatus  100  has been communicating is included in this connection reestablishment request message. 
         [0073]    Multiplexing/modulation section  108  multiplexes the connection reestablishment request message received as input from connection reestablishment request information generating section  107  with other information to generate multiplexed data. Multiplexing/modulation section  108  modulates the generated multiplexed data to generate a modulation signal and outputs the modulation signal to radio transmitting section  109 . 
         [0074]    Radio transmitting section  109  converts the modulation signal received as input from multiplexing/modulation section  108  from a baseband signal to a radio signal and outputs the radio signal to antenna  110 . 
         [0075]    Antenna  110  transmits the signal received as input from radio transmitting section  109 . 
         [0076]    Next, a description will be provided regarding operation of communication terminal apparatus  100  with reference to  FIG. 6 .  FIG. 6  is a flowchart illustrating the operation of communication terminal apparatus  100 . 
         [0077]    First, connection reestablishment determining section  104  detects the occurrence of a trigger for connection reestablishment (step ST 201 ). 
         [0078]    Next, neighboring cell search section  106  performs a neighboring cell search (step ST 202 ). 
         [0079]    Next, neighboring cell search section  106  determines whether or not a cell allowing connection reestablishment has been detected (step ST 203 ). 
         [0080]    When no cell allowing connection reestablishment has been detected (NO: step ST 203 ), the processing of communication terminal apparatus  100  is returned to step ST 202 . 
         [0081]    Meanwhile, when detecting a cell allowing connection reestablishment (Yes: step ST 203 ), connection reestablishment request information generating section  107  generates a connection reestablishment request message and starts connection reestablishment processing with respect to the cell allowing connection reestablishment (step ST 204 ). 
         [0082]    Next, communication terminal apparatus  100  starts data communication with the connection reestablishment target cell (step ST 205 ). 
         [0083]    Next, a description will be provided regarding operation to of a radio communication system including communication terminal apparatus  100  in Embodiment 1 with reference to  FIG. 7 .  FIG. 7  is a sequence diagram illustrating the operation of the radio communication system in Embodiment 1. In  FIG. 7 , cells # 00  and # 01  are served by base station # 0 , while cell # 10  is served by base station # 1 . 
         [0084]    Referring to  FIG. 7 , when a trigger for connection reestablishment processing occurs while communication terminal apparatus  100  is connected to cell # 00  (step ST 301 ), communication terminal apparatus  100  performs a neighboring cell search (step ST 302 ) and detects cell # 10  allowing connection reestablishment. 
         [0085]    Communication terminal apparatus  100  transmits a connection reestablishment request message to detected cell # 10  (step ST 303 ). 
         [0086]    Next, cell # 10  transmits a connection reestablishment information request message to cell # 00  to acquire connection reestablishment information required for reestablishing connection to communication terminal apparatus  100  (step ST 304 ). 
         [0087]    Next, cell # 00  transmits the connection reestablishment information to cell # 10  as a connection reestablishment information response message (step ST 305 ). 
         [0088]    Next, cell # 10  transmits a connection reestablishment message to communication terminal apparatus  100  (step ST 306 ). 
         [0089]    Next, communication terminal apparatus  100  completes connection reestablishment by transmitting a connection reestablishment complete message to cell # 10  (step ST 307 ). 
         [0090]    Accordingly, communication terminal apparatus  100  re-establishes connection to connection reestablishment target cell # 10  and starts communication (step ST 308 ). As described above, communication terminal apparatus  100  detects cell # 10  which allows connection reestablishment, using the neighboring cell information acquired in step ST 302  and thus can surely reconnect to detected cell # 10 . 
         [0091]    Incidentally, handover and re-direction may be used as the processing to move a communication terminal apparatus to a different cell before the occurrence of a trigger for connection reestablishment. However, when a downlink message does not arrive at the communication terminal apparatus because the communication terminal apparatus is at a cell edge, or when a large number of communication terminal apparatuses simultaneously move by train or the like, the command for executing the abovementioned processing may not arrive at an appropriate timing. In such cases, it is effective to perform the connection reestablishment processing of Embodiment 1. 
         [0092]    As described above, according to Embodiment 1, an increase in the amount of signaling can be limited since a connection reestablishment request is made to a cell which allows connection reestablishment and which has been detected using neighboring cell information. 
         [0093]    Moreover, according to Embodiment 1, since a connection reestablishment request is made to the cell which allows connection reestablishment and which has been detected using neighboring cell information, the success rate for connection reestablishment can be increased, which in turn makes it possible to limit an increase in unnecessary traffic. 
         [0094]    In Embodiment 1, the connection reestablishment information required for connection reestablishment to a communication terminal apparatus is directly received or transmitted between base stations, but the present invention is not limited to this case, and such connection reestablishment information may be transmitted and received between base stations via a mobile management entity (MME). 
       Embodiment 2 
       [0095]      FIG. 8  is a block diagram illustrating a configuration of base station  400  according to Embodiment 2 of the present invention. The configuration of a communication terminal apparatus in Embodiment 2 is the same as the configuration illustrated in  FIG. 5 . Accordingly, any duplicate description of the configuration will be omitted in Embodiment 2. 
         [0096]    Base station  400  mainly includes user data generating section  401 , connection-reestablishment allowing-cell indication determining section  402 , connection-reestablishment allowing-cell information generating section  403 , multiplexing/modulation section  404 , radio transmitting section  405 , and antenna  406 . 
         [0097]    User data generating section  401  generates user data for be transmitted to communication terminal apparatus  100  and outputs the generated user data to multiplexing/modulation section  404 . In addition, user data generating section  401  outputs the size of data generated user data for each communication terminal apparatus  100  connection-reestablishment allowing-cell indication determining section  402 . 
         [0098]    Connection-reestablishment allowing-cell indication determining section  402  selects communication terminal apparatus  100  to which a cell allowing connection reestablishment is indicated as neighboring cell information, on the basis of the sizes of data of respective communication terminal apparatuses  100  received as input from user data generating section  401 . Connection-reestablishment allowing-cell indication determining section  402  selects communication terminal apparatus  100  having the size of data not less than a threshold, for example. Connection-reestablishment allowing-cell indication determining section  402  instructs connection-reestablishment allowing-cell information generating section  403  to indicate neighboring cell information on a cell allowing connection reestablishment to selected communication terminal apparatus  100 . 
         [0099]    Connection-reestablishment allowing-cell information generating section  403  generates neighboring cell information on the cell allowing connection reestablishment, to be transmitted to communication terminal apparatus  100 , according to the instruction from connection-reestablishment allowing-cell indication determining section  402 , and outputs the neighboring cell information to multiplexing/modulation section  404 . 
         [0100]    Multiplexing/modulation section  404  multiplexes the user data received as input from user data generating section  401  with the information on the cell allowing connection reestablishment, received as input from connection-reestablishment allowing-cell information generating section  403 , generate multiplexed data. In addition, multiplexing/modulation section  404  modulates the generated multiplexed data to generate a Modulation signal and outputs the generated modulation signal to radio transmitting section  405 . 
         [0101]    Radio transmitting section  405  converts the modulation signal received as input from multiplexing/modulation section  404  from a baseband signal to a radio signal and outputs the radio signal to antenna  406 . 
         [0102]    Antenna  406  transmits the signal received as input from radio transmitting section  405 . 
         [0103]    In Embodiment 2, the operation of communication terminal apparatus  100  is the same as the operation illustrated in  FIG. 6 , and the operation of the radio communication system is also the same as the operation illustrated in  FIG. 7 . Accordingly, a duplicate description of the operation is omitted herein. 
         [0104]    As described above, in Embodiment 2, neighboring cell information is transmitted to a communication terminal apparatus involving a larger loss from a connection reestablishment failure, or a higher probability of occurrence of connection reestablishment. Thus, according to Embodiment 2, it is possible to obtain, in addition to the effect obtained in Embodiment 1, the effect of preventing a reduction in the downlink transmission efficiency and of reducing the amount of signaling since neighboring cell information is transmitted only to a communication terminal apparatus in need of neighboring cell information. 
       Embodiment 3 
       [0105]      FIG. 9  is a block diagram illustrating a configuration of base station  500  according to Embodiment 3 of the present invention. The configuration of a communication terminal apparatus in Embodiment 3 is the same as the configuration illustrated in  FIG. 5 . Thus, any duplicate description of the configuration will be omitted in Embodiment 3. 
         [0106]    Base station  500  mainly includes antenna  501 , radio receiving section  502 , demodulation section  503 , connection-reestablishment allowing-cell indication determining section  504 , connection-reestablishment allowing-cell information generating section  505 , user data generating section  506 , multiplexing/modulation section  507 , radio transmitting section  508 , and antenna  509 . 
         [0107]    Antenna  501  receives a signal and outputs the received signal to radio receiving section  502 . 
         [0108]    Radio receiving section  502  converts the signal received from antenna  501  from a radio signal to a baseband signal and outputs the baseband signal to demodulation section  503 . 
         [0109]    Demodulation section  503  demodulates the baseband signal received as input from radio receiving section  502  to acquire communication terminal apparatus information and outputs the acquired communication terminal apparatus information to connection-reestablishment indication determining section  504 . Demodulation section  503  acquires the electric field strength of the received signal for each communication terminal apparatus  100  and outputs the received electric field strength acquired for each communication terminal apparatus  100  to connection-reestablishment allowing-cell indication determining section  504  as the communication terminal apparatus information. In addition, demodulation section  503  acquires a moving speed of each communication terminal apparatus  100  based on the received signal and outputs the acquired moving speed of each communication terminal apparatus  100  to connection-reestablishment allowing-cell indication determining section  504 . 
         [0110]    Connection-reestablishment allowing-cell indication determining section  504  selects communication terminal apparatus  100  to which a cell allowing connection reestablishment is to be indicated, on the basis of the communication terminal apparatus information received as input from demodulation section  503 . Moreover, connection-reestablishment allowing-cell indication determining section  504  instructs connection-reestablishment allowing-cell information generating section  505  to indicate neighboring cell information on a cell allowing connection reestablishment to selected communication terminal apparatus  100 . Connection-reestablishment allowing-cell indication determining section  504  instructs connection-reestablishment allowing-cell information generating section  505  to indicate neighboring cell information on a cell allowing connection reestablishment only to communication terminal apparatus  100  having an electric field strength not greater than a threshold, for example. In addition, connection-reestablishment allowing-cell indication determining section  504  instructs connection-reestablishment allowing-cell information generating section  505  to indicate neighboring cell information on a cell allowing connection reestablishment only to communication terminal apparatus  100  having a moving speed not less than a threshold 
         [0111]    Connection-reestablishment allowing-cell information generating section  505  generates neighboring cell information on a cell allowing connection reestablishment to be transmitted to communication terminal apparatus  100 , according to the instruction from connection-reestablishment allowing-cell indication determining section  504  and outputs the neighboring cell information to multiplexing/modulation section  507 . 
         [0112]    User data generating section  506  generates user data to be transmitted to communication terminal apparatus  100  and outputs the user data to multiplexing/modulation section  507 . 
         [0113]    Multiplexing/modulation section  507  multiplexes the user data received as input from user data generating section  506  with the neighboring cell information on a cell allowing connection reestablishment received as input from connection-reestablishment allowing-cell information generating section  505  to generate multiplexed data. In addition, multiplexing/modulation section  507  modulates the generated multiplexed data to generate a modulation signal and outputs the generated modulation signal to radio transmitting section  508 . 
         [0114]    Radio transmitting section  508  converts the modulation signal received as input from multiplexing/modulation section  507  from a baseband signal to a radio signal and outputs the radio signal to antenna  509 . 
         [0115]    Antenna  509  transmits a signal received from radio transmitting section  508 . 
         [0116]    The operation of communication terminal apparatus  100  in Embodiment 3 is the same as the operation illustrated in  FIG. 6 , and the operation of the radio communication system in Embodiment 3 is the same as the operation illustrated in  FIG. 7 . Thus, any duplicate description of the operation will be omitted herein. 
         [0117]    As described above, in Embodiment 3, neighboring cell information can be transmitted to a communication terminal apparatus involving a larger loss from a connection reestablishment failure, or a higher probability of occurrence of connection reestablishment. Thus, according to Embodiment 3, it is possible to obtain, in addition to the effect obtained in Embodiment 1, the effect of preventing a reduction in the downlink transmission efficiency and of reducing the amount of signaling since neighboring cell information is transmitted only to a communication terminal apparatus in need of neighboring cell information. 
         [0118]    It should be noted that, although the configuration in which a cell other than the cell indicated by neighboring cell information is not selected as a connection reestablishment target cell is employed in Embodiments 1 to 3, the present invention is not limited to this configuration, and it is possible to employ a configuration in which the priority for a cell other than the cell indicated by neighboring cell information is lowered during detection of a connection reestablishment target cell. 
         [0119]      FIG. 10  is a flowchart illustrating, as a variation of Embodiments 1 to 3, the operation of a communication terminal apparatus when the priority for a cell other than the cell indicated by neighboring cell information is lowered. In  FIG. 10 , the same parts of the operation as those in  FIG. 6  will be assigned the same reference numerals and any duplicate description thereof will be omitted. 
         [0120]    Referring to  FIG. 10 , when detecting a cell allowing connection reestablishment in step ST 203  (Yes in step ST 203 ), connection reestablishment request information generating section  107  starts connection reestablishment processing with a cell allowing connection reestablishment, by generating a connection reestablishment request message (step ST 204 ). 
         [0121]    Meanwhile, when no cell allowing connection reestablishment has been detected in step ST 203  (No in step ST 203 ), communication terminal apparatus  100  starts connection reestablishment processing with the cell found by neighboring cell search in step ST 202  (step ST 1001 ). 
         [0122]    Communication terminal apparatus  100  starts data communication with the connection reestablishment target cell after the connection reestablishment processing in step ST 204  or the connection reestablishment processing in step ST 1001  (step ST 205 ). 
         [0123]    According to the operation in  FIG. 10 , it is possible to prevent processing load required for connection reestablishment from being generated and also delay in connection reestablishment due to repetition of the processing performed in steps ST 202  and ST 203  until detection of a cell allowing connection reestablishment. 
         [0124]    The disclosure of the specification, the drawings, and the abstract included in Japanese Patent Application No. 2010-262410, filed on Nov. 25, 2010, is incorporated herein by reference in its entirety. 
       INDUSTRIAL APPLICABILITY 
       [0125]    The communication terminal apparatus and the method of reestablishing connection to a cell according to the present invention are particularly suitable for performing connection reestablishment to a cell in a radio communication system. 
       REFERENCE SIGNS LIST 
       [0000]    
       
           100  Communication terminal apparatus 
           101 ,  110  Antenna 
           102  Radio receiving section 
           103  Demodulation section 
           104  Connection reestablishment determining section 
           105  Neighboring cell information decoding section 
           106  Neighboring cell search section 
           107  Connection reestablishment request information generating section 
           108  Multiplexing/modulation section 
           109  Radio transmitting section