Abstract:
A mobile terminal includes: a detection unit configured to detect reception timing of radio frames transmitted from first and second base stations; and an adjustment unit configured to adjust transmission timing of a radio frame transmitted from the mobile terminal to the first base station so that predetermined relationship between transmission timing of a radio frame transmitted from the first base station to the mobile terminal and the reception timing of a radio frame received by the first base station is satisfied, wherein the adjustment unit adjusts transmission timing of a radio frame transmitted from the mobile terminal to the second base station based on the adjusted transmission timing to the first base station and a difference between the reception timing of a radio frame transmitted from the first base station and the reception timing of a radio frame transmitted from the second base station detected by the detection unit.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2009-144820, filed on Jun. 18, 2009, the entire contents of which are incorporated herein by reference. 
       FIELD 
       [0002]    The present invention relates to a mobile terminal performing handover processing to switch a radio base station for communication in a mobile communication system. 
       BACKGROUND 
       [0003]    In a mobile communication field, at present, standardization of an LTE (Long Term Evolution) system is in progress. The LTE system is an evolved standard system of W-CDMA (Wideband Code Division Multiple Access) high-speed data communication HSDPA (High Speed Downlink Packet Access). The standardization work of the LTE system is being carried out by 3GPP (the 3rd Generation Partnership Project). 
         [0004]    In the LTE mobile communication system (hereafter simply referred to as LTE system), a radio base station receives uplink signals from a mobile terminal in communication, in synchronization with reference timing repeated with a predetermined period, and transmits downlink signals to the mobile terminal in communication. Accordingly, in consideration of a propagation delay between the radio base station and the mobile terminal, the mobile terminal transmits each uplink signal at earlier timing than the reference timing by a predetermined time so that the radio base station can receive the uplink signal at the reference timing. Information of the above uplink signal transmission timing is sent from the radio base station to the mobile terminal. The propagation delay between the radio base station and the mobile terminal is always changed dependent on a distance between the radio base station and the mobile terminal. Therefore, according to a distance change along the movement of the mobile terminal, the radio base station sends transmission timing to the mobile terminal. 
         [0005]    Further, the radio base station multiplexes and transmits PSC (Primary Synchronization Code) and SSC (Secondary Synchronization Code) in the downlink signal. PSC and SSC are codes to identify the radio base station, and the mobile terminal detects a cell identification signal (cell ID) from PSC and SSC included in the received signal (cell search). When a mobile terminal is to start communication with a base station, the operation of the mobile terminal to detect the cell ID of the base station covering an area the mobile terminal is located in is referred to as initial cell search. Also, the operation of the mobile terminal currently communicating with the base station to detect the cell ID of the base station covering the neighboring area is referred to as neighboring cell search. 
         [0006]    As a result of the movement of the mobile terminal, when the mobile terminal encounters a bad communication condition with the radio base station currently in communication, if a more appropriate radio base station exists, the mobile terminal performs handover processing to switch over a radio base station for communication. For this purpose, the mobile terminal is required to have a detected surrounding base station other than the radio base station currently in communication, and the mobile terminal performs the neighboring cell search accordingly. 
         [0007]    When the mobile terminal moves near the cell edge of the radio base station currently in communication, handover processing is performed to switch over the communication target from the radio base station currently in communication to the adjacent radio base station. In the LTE system, the mobile terminal acquires transmission timing through RACH procedure, based on a propagation delay corresponding to a distance to the radio base station to be handed over (or handover target). The operation of the RACH procedure is specified in 3GPP TS 36.213 version 8.6.0 “6 Random access procedure”. The mobile terminal requests the radio base station of handover target to send transmission timing through an RA (Random access) channel. Then the radio base station of handover target decides the transmission timing, and transmits it to the mobile terminal. 
         [0008]      FIGS. 1A through 1C  illustrate diagrams illustrating the conventional handover processing. A mobile terminal UE (User Apparatus) located in the cell of a radio base station eNB 1  (e-Node B #1) moves toward the direction of the cell of a radio base station eNB 2 , while communicating with the radio base station eNB 1  ( FIG. 1A ). An uplink signal from the mobile terminal UE is transmitted at transmission timing X 1 . In the mobile terminal UE, the information of the above transmission timing X 1  to the radio base station eNB 1  is received from the radio base station eNB 1 . When the mobile terminal UE enters the cell of the radio base station eNB 2  and arrives near the cell edge of the radio base station eNB 1 , the mobile terminal UE starts handover processing to the radio base station eNB 2  ( FIG. 1B ). At the start of the handover processing, the mobile terminal UE disconnects communication with the radio base station eNB 1 , and starts the RACH procedure with the radio base station eNB 2 . Through the RACH procedure, the mobile terminal UE transmits to the radio base station eNB 2  a request signal for transmission timing. The radio base station eNB 2  decides transmission timing X 2  from a time difference between signal reception timing from the mobile terminal UE and reference timing, and sends the transmission timing X 2  to the mobile terminal. On receiving the transmission timing, the mobile terminal UE transmits an uplink signal in synchronization with the above transmission timing X 2 , and starts communication with the radio base station eNB 2  ( FIG. 1C ). 
         [0009]    In Patent document 1 illustrated below, there is disclosed a method for communicating with a radio base station of handover target, by sending transmission timing from a radio base station of handover source. The radio base station of handover target sends transmission timing information, calculated from a phase difference between the reception timing of an uplink signal from the mobile terminal and reference timing, to the radio base station of handover source via an upper-level base station control unit. 
         [0010]    However, while the above-mentioned RACH procedure is being executed, the mobile terminal cannot perform data communication either with the radio base station of handover source, nor with the radio base station of handover target. This may possibly cause degradation of service quality, such as communication speed reduction and voice interruption. In particular, at handover processing, because the mobile terminal UE is located near the cell edge of the radio base station eNB 2  of handover target, receiving conditions at the radio base station eNB 2  is relatively degraded. This makes retransmission processing apt to occur, and also the RACH procedure period may possibly be elongated. 
         [0000]    [Patent documents] 
         [0011]    Japanese Laid Open Patent Hei9-163432 
         [0000]    [Non-patent documents] 
         [0012]    3GGP tTS36.213 Ver. 8.6.06 Random access procedure 
       SUMMARY 
       [0013]    According to an aspect of the invention, a mobile terminal performing radio communication with a base station, includes: a detection unit configured to detect reception timing of each radio frame transmitted from first and second base stations; and an adjustment unit configured to adjust transmission timing in the self-mobile terminal relative to the first base station so that predetermined relationship between the transmission timing and the reception timing of the radio frame relative to the self-mobile terminal is satisfied in the first base station, wherein the adjustment unit adjusts transmission timing relative to the second base station, using the adjusted transmission timing relative to the first base station and a difference between the reception timing relative to the first base station and the reception timing relative to the second base station respectively detected by the detection unit. 
         [0014]    According to an aspect of the invention, A transmission timing adjustment apparatus for adjusting transmission timing in a mobile terminal performing radio communication with a base station, includes: a detection unit configured to detect reception timing of radio frames transmitted from first and second base stations; and an adjustment unit configured to adjust transmission timing of a radio frame transmitted from the mobile terminal to the first base station so that predetermined relationship between transmission timing of a radio frame transmitted from the first base station to the mobile terminal and the reception timing of a radio frame received by the first base station and transmitted from the mobile terminal is satisfied, wherein the adjustment unit adjusts transmission timing of a radio frame transmitted from the mobile terminal to the second base station based on the adjusted transmission timing to the first base station and a difference between the reception timing of a radio frame transmitted from the first base station and the reception timing of a radio frame transmitted from the second base station respectively detected by the detection unit. 
         [0015]    According to an aspect of the invention, a transmission timing adjustment method of a mobile terminal performing radio communication with a base station, includes: detecting reception timing of radio frames transmitted from first and second base stations; adjusting transmission timing of a radio frame transmitted from the mobile terminal to the first base station so that predetermined relationship between transmission timing of a radio frame transmitted from the first base station to the mobile terminal and the reception timing of a radio frame received by the first base station and transmitted from the mobile terminal is satisfied, adjusting transmission timing of a radio frame transmitted from the mobile terminal to the second base station based on the adjusted transmission timing to the first base station and a difference between the reception timing of a radio frame transmitted from the first base station and the reception timing of a radio frame transmitted from the second base station respectively detected by the detection unit. 
         [0016]    Additional objects and advantages of the invention (embodiment) will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims. 
         [0017]    It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0018]      FIGS. 1A-1C  are diagrams illustrating the conventional handover processing. 
           [0019]      FIGS. 2A-2B  are diagrams illustrating a transmission timing decision method in a mobile terminal. 
           [0020]      FIGS. 3A-3C  are diagrams illustrating calculation examples of the transmission timing t 2   s.    
           [0021]      FIG. 4  illustrates an exemplary configuration of the mobile communication system according to the present embodiment. 
           [0022]      FIG. 5  illustrates a diagram illustrating an exemplary configuration of the mobile terminal UE according to the present embodiment. 
           [0023]      FIG. 6  is a flowchart of transmission timing adjustment processing according to the present embodiment. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0024]    Preferred embodiments of the present invention will be explained with reference to accompanying drawings. 
         [0025]    In a mobile terminal according to the present embodiment, at the time of handover processing, transmission timing to a radio base station of handover target is decided in the self-mobile terminal without the execution of RACH procedure. A method for deciding the transmission timing in the mobile terminal will be described below. 
         [0026]      FIGS. 2A and 2B  illustrate diagrams illustrating a transmission timing decision method in a mobile terminal. Let D 1  be a distance between a mobile terminal UE and a radio base station eNB 1  (handover source) currently communicating therewith, and also D 2  be a distance between the mobile terminal UE and a radio base station eNB 2  of handover target ( FIG. 2A ). Also, let t 1   s  be transmission timing at which the mobile terminal UE transmits an uplink signal to the radio base station eNB 1 , and t 1   r  be reception timing at which the mobile terminal UE receives a downlink signal from the radio base station eNB 1 . Further, let t 2   s  be transmission timing at which the mobile terminal UE transmits an uplink signal to the radio base station eNB 2 , and t 2   r  be reception timing at which the mobile terminal UE receives a downlink signal from the radio base station eNB 2  ( FIG. 2B ). 
         [0027]    By letting V be a radio signal propagation speed in the air, the following expressions (1), (2) hold. 
         [0000]      2× D 1=( t 1 r−t 1 s )× V   (1)
 
         [0000]      2 ×D 2=( t 2 r−t 2 s )× V   (2)
 
         [0028]    Also, the reference timing in the radio base station eNB 1  is synchronized with the reference timing in the radio base station eNB 2 . Therefore, from the difference between the reception timing t 1   r  and t 2   r , the following expression (3) holds. 
         [0000]        D 2− D 1=( t 2 r−t 1 r )× V   (3)
 
         [0029]    By substituting expressions (2) and (1) into expression (3), and rearranging with the transmission timing t 2   s , the transmission timing t 2   s  can be obtained by expression (4). 
         [0000]        t 2 s=t 1 s+t 1 r−t 2 r   (4)
 
         [0030]    As such, the transmission timing t 2   s  to the radio base station eNB 2  of handover (HO) target can be calculated using the transmission timing t 1   s  to the radio base station eNB 1  of HO source and the difference (t 1   r −t 2   r ) between the reception timing from the radio base station eNB 1  of HO source and the reception timing from the radio base station eNB 2  of HO target. 
         [0031]      FIGS. 3A through 3C  illustrate diagrams illustrating calculation examples of the transmission timing t 2   s .  FIG. 3A  illustrates the reception timing t 1   r  and the transmission timing t 1   s  of the mobile terminal UE, communicating with the radio base station eNB 1 , before the start of handover processing. The reception timing t 1   r  is timing at which the mobile terminal UE receives a downlink radio frame (DL subframe) from the radio base station eNB 1 , and is detected by the mobile terminal UE. 
         [0032]    The transmission timing t 1   s  is timing at which the mobile terminal UE transmits an uplink radio frame (UL subframe) to the radio base station eNB 1 . The initial value of the transmission timing t 1   s  informed from the radio base station eNB 1  is time difference information relative to the reception timing t 1   r . In  FIG. 3A , the transmission timing t 1   s  is equal to the reception timing t 1   r —333.3 [μs]. When starting communication with the radio base station eNB 1 , the mobile terminal UE transmits a signal (preamble) for requesting transmission timing through the RACH procedure, using an uplink radio frame. The mobile terminal UE transmits the above preamble signal at the timing of the reception timing t 1   r  of the downlink radio frame. The radio base station eNB 1  transmits a time difference between the reception timing of the preamble signal and the reference signal, as transmission timing signal information. Therefore, the initial value of the transmission timing t 1   s  is the time difference from the reception timing t 1   r . Thereafter, when the transmission timing t 1   s  changes because of the movement of the mobile terminal UE, the radio base station eNB 1  sends to the mobile terminal UE the information of a time difference from the current transmission timing t 1   s . Even in the above case, by accumulating difference values of the change of the transmission timing t 1   s , the mobile terminal UE can grasp the transmission timing t 1   s , as time difference information relative to the reception timing t 1   r . As the distance between the mobile terminal UE and the radio base station eNB 1  becomes longer, the transmission timing t 1   s  relative to the reception timing t 1   r  becomes earlier. 
         [0033]      FIG. 3B  illustrates the reception timing t 1   r  of the downlink radio frame from the radio base station eNB 1  of handover source at the time of handover processing, and the reception timing t 2   r  of the downlink radio frame from the radio base station eNB 2  of handover target. The reception timing is different depending on a difference of the distance between the mobile terminal UE and the radio base station eNB 1  from the distance between the mobile terminal UE and the radio base station eNB 2 . In the example illustrated in  FIG. 3B , the reception timing t 2   r  to receive a radio frame from the radio base station eNB 2  is 66.6 [μs] earlier than the reception timing t 1   r  to receive a radio frame from the radio base station eNB 1 . 
         [0034]      FIG. 3C  illustrates the reception timing t 2   r  and the transmission timing t 2   s  of the mobile terminal UE communicating with the radio base station eNB 2  of handover target. The transmission timing t 2   s  can be obtained from the aforementioned expression (4), based on the reception timing t 1   r , transmission timing t 1   s  and the reception timing t 2   r . By the calculation of the transmission timing t 2   s  as time difference information relative to the reception timing t 2   r , from expression (4), the transmission timing t 2   s  can be obtained as follows. 
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         [0035]    As such, the mobile terminal UE can obtain, in the self-mobile terminal, the transmission timing t 2   s  to the radio base station eNB 2  of handover target. Accordingly, at the time of handover processing, the RACH procedure for requesting the radio base station eNB 2  to send the transmission timing t 2   s  becomes unnecessary, and thus a communication disabled period due to the RACH procedure can be eliminated. 
         [0036]      FIG. 4  illustrates an exemplary configuration of the mobile communication system according to the present embodiment. The mobile communication system is an LTE system in which the LTE scheme is adopted. The LTE system includes mobile terminal (UE: User Apparatus), radio base station (eNB: evolved Node B) and a gateway apparatus (SGW: Serving Gateway). There are disposed a plurality of radio base stations, and in  FIG. 4 , the plurality of radio base stations including two eNB (eNB 1 , eNB 2 ) corresponding to the aforementioned radio base stations. Each radio base station is connected to the SGW via a router. Each radio base station eNB can perform radio communication with the plurality of mobile terminals UE located in the self-cell. The gateway apparatus SGW is upper-level apparatus for transferring radio frames to an IP network. 
         [0037]      FIG. 5  illustrates a diagram illustrating an exemplary configuration of the mobile terminal UE according to the present embodiment. The mobile terminal UE includes receiver unit  11 , reception timing detection unit  12 , reception timing memory unit  13 , reception signal demodulation unit  14 , transmission timing adjustment unit  15 , transmitter unit  16  and transmission data generation unit  17 . Receiver unit  11  receives a downlink radio frame from the radio base station eNB. Receiver unit performs frequency conversion of the received analog signal, and further converts to a digital signal through A/D conversion. Reception timing detection unit  12  detects the reception timing of the downlink radio frame from sampling timing, and sends the detected reception timing to transmission timing adjustment unit  15 . Reception timing memory unit  13  is an internal buffer of transmission timing adjustment unit  15 , and stores the reception timing after identifying each radio base station. Reception signal demodulation unit demodulates digital control information and data information. Demodulation unit  14  demodulates the control information, and when transmission timing information is included in the control information, demodulation unit  14  sends the transmission timing information to transmission timing adjustment unit  15 . Also, by demodulating the control information, reception signal demodulation unit  14  sends to transmission timing adjustment unit  15  the start/completion of the handover processing and handover information in regard to the radio base station of handover target. 
         [0038]    During normal communication, transmission timing adjustment unit  15  adjusts the transmission timing of the uplink radio frame according to the transmission timing information sent from reception signal demodulation unit  14 . As described above, the initial value of the transmission timing is counted from the criterion of the reception timing, transmission timing adjustment unit  15  includes reception timing memory unit  13  for storing the reception timing as internal buffer. When the transmission timing is not included in the control information, the most recent transmission timing stored in reception timing memory unit  13  is applied, and the transmission timing is not changed. When the distance between with the mobile terminal UE changes, necessitating a transmission timing change, the radio base station sends the change value of the transmission timing to the mobile terminal UE. 
         [0039]    On deciding the execution of handover processing based on the handover information sent from reception signal demodulation unit  14 , transmission timing adjustment unit  15  calculates the transmission timing to the radio base station of handover target, and adjusts the transmission timing to the calculated value. Transmitter unit  16  performs modulation processing of a transmission signal generated in transmission signal generation unit  17 , and after converting it to an analog signal through D/A conversion, performs conversion to a radio frequency. Transmitter unit  16  then transmits the transmission signal at transmission timing specified by timing adjustment unit  15 . 
         [0040]      FIG. 6  is a flowchart of transmission timing adjustment processing according to the present embodiment. Reception timing detection unit  12  detects the reception timing of a received downlink radio frame (S 100 ). Reception signal demodulation unit  14  demodulates the received signal (S 101 ). If transmission timing information is included in the control signal, reception signal demodulation unit  14  sends the above transmission timing information to transmission timing adjustment unit  15 . Transmission timing adjustment unit  15  decides the presence or absence of transmission timing information at each predetermined decision interval (S 102 ). If the transmission timing information is sent, transmission timing adjustment unit  15  adjusts the transmission timing to a specified value included in the transmission timing information (S 103 ). If the transmission timing information is not sent, transmission timing adjustment unit  15  decides whether the handover processing is in progress, based on the handover information from reception signal demodulation unit  14  (S 104 ). When the handover processing is in progress, transmission timing adjustment unit  15  extracts from reception timing memory unit  13  the reception timing in both the radio base station of handover source and the radio base station of handover target (S 105 ). Then, according to the transmission timing decision method described earlier, transmission timing adjustment unit  15  calculates the transmission timing to the radio base station of handover target, based on the above two data of the reception timing and the transmission timing to the radio base station of handover source which is currently set (S 106 ). Transmission timing adjustment unit  15  adjusts the transmission timing to the calculated value (S 107 ). In step S 104 , if the handover processing is not in progress, the specified value of transmission timing is not sent, and therefore the current set value of the transmission timing is retained without being changed (S 108 ). 
         [0041]    The above-mentioned present embodiment is not limited to the mobile terminal in the LTE mobile communication system, and it is also applicable to mobile terminals in mobile communication systems based on other schemes in which each radio base station sends transmission timing to the mobile terminal at handover processing. 
         [0042]    In the present embodiment, the description has been given to such a case that a radio base station receives an uplink signal from a mobile terminal and transmits a downlink signal to the mobile terminal, at the reference timing of a predetermined period. Here, in regard to the above reference timing, it is not limited to the case that reception timing in the radio base station matches with transmission timing. It is also possible even if there is a time difference between the reference timing for receiving the uplink signal and the reference timing for transmitting the downlink signal. In this case, the period of the reference timing for receiving the uplink signal is identical to the period of the reference timing for transmitting the downlink signal, and the time difference is constant. Further, even when the time difference exists, it is possible to calculate the transmission timing, using the aforementioned expression (4). 
         [0043]    The mobile terminal can decide the transmission timing to the radio base station of handover target by the self-mobile terminal alone. Also, it is possible to eliminate a disabled condition of data communication at handover, accompanying the RACH procedure execution to request the radio base station to send transmission timing. Thus, it is possible to prevent the deterioration of communication service quality, such as communication speed reduction and voice interruption. 
         [0044]    All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the principles of the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiments of the present inventions have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.