Patent Publication Number: US-2011069683-A1

Title: Allocation method and base station apparatus using the same

Description:
TECHNICAL FIELD 
     The present invention relates to an allocation technique of a time slot, in particular, to a method of allocating a time slot to a terminal device in a moving object and a base station apparatus using the method. 
     BACKGROUND ART 
     Using a wireless communication system in a moving object, such as a train, which moves at a high speed, has been widely available. In a wireless communication system, a base station apparatus is provided in each wireless zone, and a terminal device performs communication with the base station apparatus of the wireless zone. Accordingly, if a moving object traverses a wireless zone, a terminal device in the moving object is disconnected from the communication with the base station apparatus. In order to enable communication even in this situation, if the moving object moves into another wireless zone, the terminal device in the moving object perform handover to switch the connection from the base station apparatus in the wireless zone of the movement origin to a base station apparatus in a wireless zone of a movement destination (for example, refer to JP-A-2000-229571). 
     DISCLOSURE OF THE INVENTION 
     Problems to be Solved by the Invention 
     In handover, it is required to stably switch connection so as to avoid disconnection of communication between a base station and a terminal device. In a recent wireless communication system, in order to realize large amount of data transmission, a plurality of terminal devices are frequency multiplexed. Therefore, time required for ranging process in handover could increase depending on an allocation method of time slots, which would cause deterioration in communication quality and unstable communication state at a connection switching. 
     The present invention has been made in view of the above circumstances, and one object is to provide an allocation method of a time slot and a base station apparatus using the method which can perform handover while maintaining a stable communication state. 
     Means to Solve the Problem 
     An aspect of the present invention provides a base station apparatus. The base station apparatus includes a detection unit which detects a terminal device existing in a moving object, and an allocation unit which, if the detection unit detects a plurality of terminal devices in the moving object, allocates a sub-channel being frequency-multiplexed in a certain time slot, to the plurality of the terminal devices. 
     It is preferable that the allocation unit specifies a time slot for each of a plurality of moving objects, and allocates, to a plurality of terminal devices existing in a same moving object, sub-channels included in the time slot specified for the moving object. 
     It is preferable that the base station apparatus further includes a search unit which searches a use status of time slots in another base station apparatus, and a performing unit which performs a handover process with a plurality of terminal devices existing in a same moving object, and when allocating sub-channels to the plurality of terminal devices in the handover process, the performing unit controls the allocation unit to specify a time slot, which is less used in a handover originating base station apparatus, for the plurality of terminal devices, based on the use status searched by the search unit. 
     It is preferable that in order to detect a terminal device existing in a moving object, the detection unit uses a relative distance to the terminal device, a movement speed of the terminal device, and a movement direction of the terminal device, as determination factors. 
     It is preferable that the detection unit sets a priority to the relative distance to the terminal device, the movement speed of the terminal device, and the movement direction of the terminal device, in this order, and detects the terminal device with reference to the determination factors based on the priority. 
     It is preferable that if the movement speed of the terminal device is smaller than a predetermined threshold value, the detection unit does not perform the detection. 
     It is preferable that in order to detect a terminal device existing in a moving object, the detection unit uses a location of the terminal device as a determination factor. 
     Another aspect of the present invention provides a base station apparatus. The base station apparatus includes an acquisition unit which acquires information, the information being based on locations of a plurality of terminal devices, a group forming unit which forms a moving object group including a plurality of terminal devices based on the acquired information, and an allocation unit which allocates a plurality of sub-channel being frequency multiplexed in a certain time slot to the plurality of terminal devices belonging to the moving object group. 
     The base station apparatus may further include a storing unit which stores a table for associating a moving object group formed in the group forming unit and a time slot, and the allocation unit may allocate a plurality of sub-channels in the time slot associated with the moving object group in the table. 
     The group forming unit may form a plurality of moving object groups, and the table may associate different time slots to the plurality of moving object groups, respectively. 
     The moving object group may be associated with a property value related to a terminal device belonging to the moving object group, and if the acquisition unit acquires information based on a location of a terminal device, the group forming unit may compare the acquired information with the property value to determine whether the corresponding terminal device is to belong to the moving object group. 
     The property value may be obtained based on the information acquired by the acquisition unit, the information being based on locations of the plurality of terminal devices belonging to the moving object group. 
     The information based on the locations may include a relative distance of the terminal device to the base station apparatus, a movement speed of the terminal device, and a movement direction of the terminal device. 
     EFFECTS OF THE INVENTION 
     According to the present invention, deterioration of communication quality can be reduced, and handover can be performed while maintaining a stable communication state. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a concept view showing the configuration of a moving object communication system according to an embodiment of the present invention. 
         FIG. 2  is a concept view showing the configuration of a TDMA frame. 
         FIG. 3  is a concept view showing time slots which are frequency multiplexed in accordance with OFDMA. 
         FIG. 4  is a concept view showing the configuration of a sub-channel block. 
         FIG. 5  is a concept view showing the configuration of a base station apparatus according to an embodiment of the present invention. 
         FIG. 6  is a concept view showing a data structure of a table for associating a moving object group and a time slot. 
         FIG. 7  is a sequence view showing a handover procedure. 
         FIG. 8  is a sequence view showing a connection procedure to establish a communication channel. 
         FIG. 9  is a flow chart showing the procedure of classification into moving object groups. 
         FIG. 10  is a flow chart showing the procedure of specifying a time slot. 
     
    
    
     DESCRIPTION OF REFERENCE NUMERALS 
     
         
           101 : Wireless unit 
           102 : Transmitting and receiving unit 
           103 : Modulation and demodulation unit 
           104 : IF unit 
           105 : Control unit 
           1050 : Terminal detection unit 
           1051 : Time slot specifying unit 
           1052 : Channel allocation unit 
           1053 : Handover processing unit 
           1054 : Recording unit 
       
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     The concept of the present invention is described below prior to the detailed description thereof. An embodiment of the present invention relates to a moving object communication system, which is configured by PAC (Paging Area Controller), a plurality of base station apparatuses, and a terminal device, like a second-generation codeless telephone system. One of the PAC is connected to the plurality of base station apparatuses, and the other end thereof is connected to a network. Each of the plurality of base station apparatuses configures a wireless zone and is connected to a terminal device in the wireless zone. If the terminal device moves from one wireless zone to another wireless zone, the terminal device performs handover from the base station apparatus of the movement origin to a base station apparatus of a movement destination. 
     In the handover, the terminal device disconnects the connection to the base station apparatus of the movement original and performs process to establish connection to a base station apparatus of a movement destination. In addition, the terminal device also performs a ranging process to adjust transmission power of a signal or the like between the terminal device and the base station apparatus of the movement destination. In the ranging process, the base station apparatus detects a differential amount from an ideal value of receiving power for a message transmitted from the terminal device. And, based on the differential amount, the base station apparatus calculates a correction amount to control transmission power in the terminal device and transmits a message, which includes the correction amount, to the terminal device. Upon receiving the message from the base station apparatus, the terminal device adjusts the transmission power based on the correction amount contained in the message, and then transmits a message to the base station apparatus. The terminal device and the base station apparatus repeatedly perform transmitting and receiving messages until a differential amount from an ideal value of transmission timing in the terminal device falls within a predetermined range. 
     The base station apparatus frequency-multiplexes a plurality of terminal devices in a time slot. In order to allocate a terminal device, which is an object for handover, to a time slot, to which a terminal device has been allocated, it is necessary to adjust transmission power, and etc., without damaging the communication quality of the existing terminal device. As such, the degree of freedom for adjustment may be reduced, and performing the ranging process within a short time period is difficult. As a result, the communication quality of the existing terminal device, as well as the terminal device, which is an object for handover, may be deteriorated. Especially, this problem occurs easily when a plurality of terminal devices on a moving object, which traverses a wireless zone, are objects for handover all together. Meanwhile, a plurality of terminal devices on a moving object may be placed under a similar propagation environment, such as a relative distance from a base station apparatus, and if the terminal devices are objects for handover, performing ranging process within a short time period is possible. 
     Accordingly, in the base station apparatus according to an embodiment of the present invention, sub-channels included in a certain time slot are preferentially allocated to terminal devices existing in a same moving object. 
     As a result, when handover is performed, it is possible to perform a ranging process for terminal devices placed under a similar propagation environment, in a certain time slot, so that time required for ranging process is reduced. Accordingly, deterioration of communication quality can be prevented, and handover can be performed while maintaining a stable communication state. 
       FIG. 1  is a concept view showing the configuration of a moving object communication system  1  according to an embodiment of the present invention. The moving object communication system  1  includes a first base station apparatus  10   a  and a second base station apparatus  10   b , which are collectively referred to as a base station apparatus  10 , a terminal device  20 , a PAC  30 , a network  40 , and a moving object  50 . 
     The base station apparatus  10  configures a wireless zone (indicated by a dash-dot line in the drawing) and performs communication with the terminal device  20  by using a predetermined wireless communication method. Herein, for the wireless communication method, a method of TDMA/TDD (Time Division Multiple Access/Time Division Duplex) system applied with OFDMA (Orthogonal Frequency Division Multiple Access) system is considered. 
     One end of the base station apparatus  10  is connected to the terminal device  20  through wireless communication, and the other end thereof is connected to the PAC  30  through wired communication. Here, it is assumed that the first base station apparatus  10   a  is a base station apparatus of a movement origin, and the second base station apparatus  10   b  is a base station apparatus of a movement destination. 
     The terminal device  20  disconnects the connection with the first base station apparatus  10   a  in response to a start command from either one of the terminal device  20  itself or the first base station apparatus  10   a , and perform handover to establish connection with the second base station apparatus  10   b.    
     The PAC  30  controls the base station apparatus  10  and realizes transmitting and receiving data between the terminal device  20  and a communication device, not illustrated herein, through the network  40 . For example, the PAC  30  transmits data received from the network  40  and addressed to the terminal device  20 , to the base station apparatus  10 . Therefore, when the terminal device  20  is connected to the base station apparatus  10 , the PAC  30  performs a location registration with the terminal device  20 . When the terminal device  20  performs handover from the first base station apparatus  10   a  to the second base station apparatus  10   b , the PAC  30  performs terminal connection change with the base station apparatus  10 . 
     One end of the network  40  is connected to the PAC  30 , and the other end thereof is connected to a communication device, not illustrated herein. For the network  40 , for example, a network through a TCP/IP (Transmission Control Protocol/Internet Protocol) method is considered. 
     The moving object  50  transports the terminal device  20 . For the moving object  50 , for example, a train is considered. Hereinafter, for explanation convenience, if the terminal device  20  exists in one train, it is assumed that the terminal device  20  exist in one moving object. 
       FIG. 2  is a concept view showing the frame configuration of TDMA/TDD system in a wireless communication method of the moving object communication system  1 . As illustrated in  FIG. 2 , a TDMA frame (hereinafter “frame”) is configured by four time slots for uplink communication (from the terminal device  20  to the base station apparatus  10 ) and four time slots for downlink communication (from the base station apparatus  10  to the terminal device  20 ). In addition, frames are continuously arranged. In an embodiment of the present invention, since uplink communication and downlink communication are symmetric, hereinafter, for explanation convenience, only uplink communication or downlink communication will be described. However, the description is identically applied to the other one. 
     In a wireless communication method of the moving object communication system  1 , as illustrated in  FIG. 3 , OFDMA also is applied, and a plurality of terminal devices  20  are allocated to one time slot. In  FIG. 3 , the horizontal axis direction shows arrangement of time slots on a time axis, and the vertical axis direction shows arrangement of sub-channels on a frequency axis. In other words, the multiplexing in the horizontal axis corresponds to TDMA, and the multiplexing in the vertical axis corresponds to OFDMA. In  FIG. 3 , one frame includes a first time slot (“T 1 ” in the drawing) to a fourth time slot (“T 4 ” in the drawing). In addition, in  FIG. 3 , each time slot includes a first sub-channel (“SC 1 ” in the drawing) to an eighteenth sub-channel (“SC 18 ” in the drawing). In  FIG. 3 , “terminal device A” is allocated to the second sub-channel of the first time slot, “terminal device B” is allocated to the second sub-channel to the fourth sub-channel of the second time slot, “terminal device C” is allocated to the sixteenth sub-channel of the third time slot, and “terminal device D” is allocated to the thirteenth sub-channel to the fifteenth sub-channel of the fourth time slot. It is noted that in  FIG. 3 , the first sub-channel is secured as a sub-channel only for a control channel. In the drawing, the first base station apparatus  10   a  allocates “control channel BS 1 ” to the first sub-channel of the first time slot, and the second base station apparatus  10   b  allocates “control channel BS 2 ” to the first sub-channel of the second time slot. 
       FIG. 4  is a concept view showing the configuration of a sub-channel block specified by one time slot and one sub-channel of  FIG. 3 . In  FIG. 3 , the horizontal direction shows a time axis, and the vertical direction shows a frequency axis. The numerals, “1” to “24” are numbers of sub-carriers. That is, a sub-channel is configured by OFDM multi-carrier signals. In the drawing, “TS” is a training symbol, which includes known signals, such as a symbol for synchronization detection and a symbol for estimation of a characteristic of a transmission channel. “GS” is a guard symbol, in which no effective signal is provided. “PS” is a pilot symbol, which is configured by known signals. “DS” is a data symbol, namely, refers to data to be transmitted. 
     The base station apparatus  10  and the terminal device  20  form a control channel by using a sub-channel block and establish a communication channel for transmitting and receiving data by transmitting and receiving a predetermined control message. For example, the base station apparatus  10  forms a logical control channel (“LCCH”), which is configured by a notification channel (hereinafter “BCCH”) for notifying start of system information, an incoming call information channel (hereinafter “PCH”) for notifying an incoming call in the terminal device  20 , and a channel allocation control channel (hereinafter “SCCH”) for notifying channels allocated to the terminal device  20 , and occasionally transmits it to the terminal device  20  by means of an exclusive sub-channel block. The terminal device  20  receives a control message of “BCCH” occasionally transmitted by the base station apparatus  10   b  to perceive the base station apparatus  10   b.    
       FIG. 5  is a concept view showing the configuration of the base station apparatus  10 . In  FIG. 5 , an antenna  100  transmits and receives a signal of wireless frequency. 
     As a transmitting operation, the wireless unit  101  frequency-converts a baseband signal from the transmitting and receiving unit  102  to obtain a multi-carrier signal of wireless frequency. In addition, as a receiving performance, the wireless unit  101  frequency-converts the multi-carrier signal of wireless frequency received in the antenna  100  to obtain a baseband signal and outputs it to the transmitting and receiving unit  102 . The baseband signal is formed with an in-phase component and a quadrature-phase component. However, for simplification of explanation,  FIG. 5  illustrates only one signal line. The wireless unit  101  includes an AGC (Auto Gain Control) or an A/D (Analog/Digital) conversion unit. 
     As a transmitting operation, the transmitting and receiving unit  102  forms a multi-carrier signal of a frequency domain by allocating a frequency domain signal transmitted from the modulation and demodulation unit  103  to a plurality of sub-channels, and converts it into a time domain signal to output it to the wireless unit  101 . In addition, for the conversion from the frequency domain signal into the time domain signal, IFFT (Inversed Fast Fourier Transform) is employed. 
     As a receiving operation, the transmitting and receiving unit  102  converts the time domain signal transmitted from the wireless unit  101  into a multi-carrier signal of a frequency domain, and divides it into a frequency domain signal per sub-carrier to output it to the modulation and demodulation unit  103 . In addition, for the conversion from the time domain signal into the multi-carrier signal of the frequency domain, FFT (Fast Fourier Transform) is employed. 
     As a transmitting operation, the modulation and demodulation unit  103  modulates a signal inputted from the IF unit  104 . For the modulation method, BPSK (Binary Phase Shift Keying), QPSK (Quadrature Phase Shift Keying), 16QAM (Quadrature Amplitude Modulation), 64QAM, and 256QAM, etc., are employed. As a receiving operation, the modulation and demodulation unit  103  demodulates the signal of the frequency domain transmitted from the transmitting and receiving unit  102  and outputs it to the IF unit  104 . 
     As a receiving operation, the IF unit  104  outputs the signal demodulated in the modulation and demodulation unit  103  to the PAC  30 . As a transmitting operation, the IF unit  104  outputs the signal inputted from the PAC  30  to the modulation and demodulation unit  103 . 
     The control unit  105  performs control of timing of the entire base station apparatus  10 . The control unit  105  includes a terminal detection unit  1050 , a time slot specifying unit  1051 , a channel allocation unit  1052 , a handover processing unit  1053 , and a recording unit  1054 , and performs handover with the terminal device  20 . 
     The terminal detection unit  1050  acquires location information of the terminal device  20  and calculates relative distance from the base station apparatus  10  to the terminal device  20 . In order to calculate relative distance to the terminal device  20 , for example, the terminal detection unit  1050  records its location information in the recording unit  1054  and obtains a difference from acquired location information of the terminal device  20 . 
     The terminal detection unit  1050  acquires location information of the terminal device  20  at every predetermined time and calculates a movement direction and a movement distance of the terminal device  20  based on the variation. 
     The location information of the terminal device  20  may be acquired directly from the terminal device  20  if the terminal device  20  has GPS (Global Positioning System). Alternatively, the location information of the terminal device  20  may be estimated by prerecording a table for defining the correspondence between propagation distance and propagation loss in the recording unit  1054 , acquiring information about transmission power of a known signal from the terminal device  20 , measuring receiving power when the known signal is received, and referencing the table. 
     The terminal detection unit  1050  detects a terminal device  20  existing in the moving object  50  by using relative distance to the terminal device  20 , a movement speed of the terminal device  20 , and a movement direction of the terminal device  20  as determination factors. 
     The terminal detection unit  1050  may detect the terminal device  20  existing in the moving object  50  by using location of the terminal device  20  as a determination factor. And, the terminal detection unit  1050  forms a set (hereinafter a “moving object group”) including a plurality of terminal devices  20  which are determined to exist in one moving object. In order to form moving object groups, the terminal detection unit  1050  performs group classification of the terminal device  20  based on the determination factor. 
     Accordingly, the terminal detection unit  1050  first confirms whether the terminal device  20  can be classified into an existing moving object group. Here, each moving object group is associated with three values (hereinafter “property values”) including a relative distance to the terminal device  20  belonging to the group, a movement speed of the terminal device  20  belonging to the group, and a movement direction of the terminal device  20  belonging to the group. 
     The terminal detection unit  1050  compares values of the determination factors of a terminal device  20  and property values of the existing moving object group. If differences between values of all the determination factors and property values of the existing moving object group are within certain ranges, respectively, the terminal detection unit  1050  classifies the terminal device  20  into the existing moving object group. 
     If the terminal devices  20  can not be classified into any existing moving object group, the terminal detection unit  1050  forms a new moving object group and classifies the terminal device  20  therein. In this case, the values of the determination factors of the classified terminal device  20  are kept in association with property values of the new moving object group. 
     By performing these process, the terminal detection unit  1050  forms a moving object group, which is a set of a plurality of terminal devices  20  existing in one moving object. 
     Comparison of values of the determination factors may be performed by setting a priority to a relative distance to the terminal device  20 , a movement speed of the terminal device  20 , and a movement direction of the terminal devices  20 , in this order, such that the comparison is performed first for the determination factor having the highest priority. This is because group classification of the terminal device  20  can be effectively performed. 
     In addition, property values of a moving object group may be updated by measuring a relative distance, a movement speed, and a movement direction of each terminal device  20  belonging to the moving object group at a predetermined cycle and averaging them, respectively. This is because, even if the movement state of a moving object changes from the time when the moving object group has been formed, it is possible to classify the terminal device  20  existing in the moving object into an appropriate moving object group. 
     Furthermore, the process of classifying the terminal device  20  into a moving object group may not be performed if the movement speed of the terminal device  20  is smaller than a predetermined threshold value. This is because the terminal device  20 , which does not exist in the moving object, are not objects for the process. As a result, process efficiency is improved. 
     The time slot specifying unit  1051  specifies a time slot to allocate a sub-channel block to the terminal device  20 . Especially, a time slot is specified such that a certain time slot is preferentially allocated to the terminal device  20  existing in one moving object. Accordingly, the time slot specifying unit  1051  records a table for associating a moving object group and a time slot in the recording unit  1054 . With reference to the table, for a terminal device  20  which has been classified into a moving object group by the terminal detection unit  1050 , a time slot corresponding to the moving object group is specified. Accordingly, for the terminal device  20  existing in one moving object, a time slot corresponding to the moving object can be preferentially allocated. 
     It is noted that association between a moving object group and a time slot is performed for a predetermined maximum number of slots (for example, “2”) and is not performed for slots exceeding the number. Here, the maximum number of slots is preferably less than half of the number of time slots configuring a frame. This is because securing a time slot to associate a moving object group thereto becomes easy. In addition, different time slots are associated with a plurality of moving object groups, respectively. 
     The time slot specifying unit  1051  associates a moving object group and a time slot in the second base station apparatus  10   b , based on the use status of time slots in the first base station apparatus  10   a . As a result, it is easy to collectively perform handover a plurality of terminal devices  20  in a moving object group, i.e., the terminal device  20  existing in one moving object, in the unit of time slot. In addition, overlapping use of time slots can be reduced, and deterioration of communication quality can be prevented. 
     For the above, the time slot specifying unit  1051  acquires information about the use status of time slots from the first base station apparatus  10   a  through the handover processing unit  1053 . And, with reference to the information about the use status of time slots, the time slot specifying unit  1051  associates a moving object group with a time slot in the second base station apparatus  10   b  without overlapping with a time slot which the first base station apparatus  10   a  has associated with a moving object group. 
     The time slot specifying unit  1051  specifies a time slot, which is not associated to a moving object group and has an empty sub-channel block, for another terminal device  20 , i.e., a terminal device  20  which does not belong to any moving object group, or a terminal device  20  which belongs to a moving object group, which is not associated with any time slot. 
     The channel allocation unit  1052  allocates an empty sub-channel block included in the time slot specified by the time slot specifying unit  1051  to the terminal device  20 . The allocation of the sub-channel block is performed in response to a new or additional wireless resource acquisition request, or the like, from the terminal device  20 . Here, a new wireless resource acquisition request is generated, for example, in the case where the terminal device  20  starts communication or the terminal device  20  hands over from the first base station apparatus  10   a  to the second base station apparatus  10   b  during communication. 
     The handover processing unit  1053  performs handover process with the terminal device  20 . That is, the handover processing unit  1053  monitors communication state between the first base station apparatus  10   a  and the terminal device  20 . If it is determined that communication state is deteriorated, the handover processing unit  1053  determines performing handover. For example, if RSSI (Received Signal Strength Indication) of a signal received from the terminal device  20  is smaller than a predetermined threshold value, or the number of errors detected through CRC (Cyclic Redundancy Check) is larger than a predetermined threshold value, the handover processing unit  1053  determines that communication state is deteriorated. 
     The handover processing unit  1053  performs disconnection process between the first base station apparatus  10   a  and the terminal device  20 . As the disconnection process, for example, a command is sent to the channel allocation unit  1052  to release a sub-channel block allocated to the terminal device  20 . 
     The handover processing unit  1053  performs connection process between the second base station apparatus  10   b  and the terminal device  20 . As the connection process, for example, a command is sent to a ranging processing unit, not illustrated herein, to perform adjustment of transmission power, and etc., with the terminal device  20 . Furthermore, a command is sent to the time slot specifying unit  1051  and the channel allocation unit  1052  to allocate a sub-channel block to the terminal device  20  which has requested handover. 
     In this case, as described above, the handover processing unit  1053  controls the time slot specifying unit  1051  to prevent the first base station apparatus  10   a  and the second base station apparatus  10   b  from associating a moving object group to an overlapping time slot. 
     That is, the handover processing unit  1053  acquires information about the use status of time slots from the first base station apparatus  10   a . Information about the use state of time slots may be acquired from the first base station apparatus  10   a , for example, through the PAC  30  or may be transmitted from the first base station apparatus  10   a  to the terminal device  20 , and then acquired from the terminal device  20  when performing handover. 
     The recording unit  1054  records a table, and etc., which is referenced by the time slot specifying unit  1051  to specify time slots.  FIG. 6  is a concept view showing a data structure of the table for associating a moving object group and a time slot, which is recorded by the recording unit  1054 . As illustrated, the table includes a time slot column and a use status column. In the time slot column, identification information of time slots configuring a frame is stored. In the use status column, identification information of moving object groups associated to time slots is stored. In  FIG. 6 , “Moving Object Group  1 ” is associated with a “First Time Slot,” and “Moving Object Group  2 ” is associated with a “Second Time Slot.” As illustrated in  FIG. 2 , since the number of time slots configuring one frame is “4,” the maximum number of slots is “2.” Accordingly, even if “Moving Object Group  3 ” exists, the time slot specifying unit  1051  does not associate it to a “Third Time Slot” or a “Fourth Time Slot.” 
     The operation of the moving object communication system  1  according to the above configuration will be described.  FIG. 7  is a sequence view showing the handover procedure when the terminal device  20  hands over from the first base station apparatus  10   a  to the second base station apparatus  10   b . During communication with the first base station apparatus  10   a  (S 100 ), if RSSI of a received signal becomes lower than a predetermined threshold value, the terminal device  20  decides handover (S 101 ). The terminal device  20  receives a control message of “BCCH” notified from the second base station apparatus  10   b  to perceive the second base station apparatus  10   b . The terminal device  20  perform handover from the first base station apparatus  10   a  to the second base station apparatus  10   b  (S 103 ), and establish a communication channel with the second base station apparatus  10   b  (S 104 ). 
       FIG. 8  is a sequence view showing the connection procedure to establish a communication channel with the second base station apparatus  10   b . The terminal device  20  requests allocation of a wireless channel in the process of handover to the second base station apparatus  10   b  (S 200 ). The second base station apparatus  10   b  calculates a relative distance to the terminal device  20 , a movement speed of the terminal device  20 , and movement direction of the terminal device  20  (S 201 ). Based on these determination factors, the terminal device  20  is classified into a moving object group, to which the terminal device  30  should belong (S 202 ). Based on information about the use status of time slots acquired from the first base station apparatus  10   a , the second base station apparatus  10   b  specifies a time slot to allocate a wireless channel to the terminal device  20  (S 203 ). And, an empty sub-channel block included in the specified time slot is allocated to the terminal device  20  (S 204 ), and allocation of a communication channel is responded to the terminal device  20  (S 205 ). 
       FIG. 9  is a flow chart showing the procedure of classification of the terminal device  20  into the moving object groups. The terminal detection unit  1050  acquires property values of an existing moving object group (S 300 ). The property values are compared with values of the determination factors of the terminal device  20 , which is an object, and it is confirmed whether differences in the values are within certain ranges, respectively (S 301 ). If there are property values having differences from determination factors within certain ranges, respectively (Y of S 301 ), the terminal device  20  is classified into a moving object group, which is associated with the property values (S 302 ). If there are no property values having differences from determination factors within certain ranges (N of S 302 ), the total number of existing moving object groups is compared with the maximum number of slots (S 303 ). If the total number of existing moving object groups is smaller than the maximum number of slots (Y of S 303 ), a new moving object group having values of the determination factors as property values is formed (S 304 ), and the terminal devices ( 20 ) are classified thereinto (S 305 ). If the total number of existing moving object groups is larger than the maximum number of slots (N of S 303 ), classification into moving object groups is not performed. 
       FIG. 10  is a flow chart showing the procedure of specifying a time slot. The time slot specifying unit  1051  confirms whether the terminal device  20 , which is an object, belongs to any moving object group (S 400 ). If the terminal device  20  belongs to a moving object group (Y of S 400 ), the time slot specifying unit  1051  confirms whether the moving object group is associated with a time slot, with reference to the table illustrated in  FIG. 6  (S 401 ). If the moving object group is associated with a time slot (Y of S 401 ), it is confirmed whether there is an empty sub-channel block in the associated time slot (S 402 ). If there is an empty sub-channel block (Y of S 402 ), the time slot is specified as a time slot to allocate a wireless channel to the terminal device  20 , which is an object (S 403 ). If the terminal device  20 , which is an object, dose not belong to any moving object group (N of S 400 ), the moving object group is not associated with a time slot (N of S 401 ), or there is no empty space in the associated time slot (N of S 402 ), a time slot, which is not associated with any moving object group and which has an empty sub-channel block, is specified (S 404 ). 
     According to an embodiment of the present invention, the advantageous effects set forth below can be achieved. 
     (1) The terminal detection unit  1050  classifies the terminal device  20  into a predetermined moving object group based on determination factors. The time slot specifying unit  1051  associates the moving object group to a time slot and specifies the associated time slot for the terminal device  20  of the moving object group. The channel allocation unit  1052  allocates a sub-channel block included in the time slot specified by the time slot specifying unit  1051  to the terminal device  20  of the moving object group. As a result, the terminal device  20  existing in one moving object is allocated to a specified time slot, so that a difference of transmission power or transmission timing, which should be adjusted, between a plurality of terminal devices  20  allocated to the time slot can be reduced, and thereby reducing time required for ranging process. In addition, deterioration of communication quality can be prevented, and handover can be performed while maintaining a stable communication state. 
     (2) In addition, the handover processing unit  1053  acquires information about the use status of time slots from the first base station apparatus  10   a , and controls the time slot specifying unit  1051  such that the second base station  10   b  associates a moving object group and a time slot without overlapping with a time slot, to which the first base station apparatus  10   a  associates a moving object group. As a result, it is easy to collectively perform handover a plurality of terminal devices  20  existing in one moving object in the unit of time slots. In addition, overlapping use of time slots can be reduced, and deterioration of communication quality can be prevented. 
     (3) In addition, the terminal detection unit  1050  performs classification into a moving object group by using a relative distance to the terminal device  20 , a movement speed of the terminal device  20 , and a movement direction of the terminal device  20  as determination factors. Accordingly, it is possible to surely classify the terminal device  20  existing in one moving object into the corresponding moving object group. 
     (4) In addition, the terminal detection unit  1050  sets a priority to the relative distance to the terminal device  20 , the movement speed of the terminal device  20 , and the movement direction of the terminal devices  20  in this order, such that comparison is performed from the determination factor having the highest priority with property values of an existing moving group. As a result, effective classification is possible. 
     The best mode to carry out the present invention has been described. However, the present invention is not limited to the configuration of the above embodiment, and various modifications, which are within the application scope of the present invention defined in the claims and can achieve the functions according to the configuration in the embodiment that has been described, are possible. 
     An embodiment of the present invention describes, for example, that if the terminal device  20  exist in one train, it is assumed that the terminal device  20  exist in one moving object. However, the present invention is not limited thereto, and whether the terminal device  20  exists in one moving object may be determined based on each train car constituting of a train, or if a plurality of satellite stations are mounted in a train, whether the terminal device  20  exist in one moving object may be determined based on each satellite station. 
     The present application is based on the Japanese Patent Application No. 2008-084904 filed on Mar. 27, 2008, the disclosures of which are hereby incorporated by reference.