Patent Publication Number: US-2011065378-A1

Title: Wireless terminal and communication process

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefits of priority from the prior Japanese Patent Application No. 2009-215128, filed on Sep. 17, 2009, the entire contents of which are incorporated herein by reference. 
     FIELD 
     The embodiments discussed herein relate to a wireless terminal and a communication process. 
     BACKGROUND 
     The mobile communication systems include the independent (private or nonpublic) communication systems as well as the public mobile telephone systems and the Internet communication system. (For example, the communication systems used by police or fire organizations for the purpose of rescue or disaster prevention are independent communication systems.) The independent communication systems include the systems in which wireless communication is performed between a wireless terminal and a base station in a similar manner to the cellular communication, and the systems in which wireless communication is performed between wireless terminals which are located outside the coverage areas of base stations (for example, wireless terminals located in a disaster field). 
       FIG. 20  illustrates a system configuration of an independent communication system in which wireless communication is performed between wireless terminals and a base station. In  FIG. 20 , a base station  101  and wireless terminals  111  to  114  are illustrated, where the arrowed solid line indicates uplink transmission, and the arrowed dashed line indicates downlink transmission. As illustrated in  FIG. 20 , the wireless terminals  111  to  114  are located within the coverage area of the base station  101 , and perform communication through the base station  101 . For example, in the case where the wireless terminal  112  makes a broadcast call to the wireless terminals  111 ,  113 , and  114 , data to be transmitted from the wireless terminal  112  is once transmitted to the base station  101 , and is then transmitted to the wireless terminals  111 ,  113 , and  114 . 
       FIG. 21  illustrates a system configuration of an independent communication system in which wireless communication is performed between wireless terminals located outside the coverage areas of base stations. In  FIG. 21 , wireless terminals  121  and  131  to  133  are illustrated, where the arrowed solid line indicates uplink transmission, and the arrowed dashed line indicates downlink transmission. In the case where the wireless terminals  121  and  131  to  133  are located outside the coverage areas of the base stations, wireless communication between the wireless terminals can be secured by making one of the wireless terminals operate as a master (e.g., perform the operations of the base station  101  in  FIG. 20  including scheduling). For example, wireless communication between the wireless terminals  131  to  133  can be secured by making the wireless terminal  121  operate as a master. 
     For example, the above manner of making one of the wireless terminals operate as a master is used by police or fire organizations in disaster fields. The wireless terminal  121  which is used as the master is mounted, for example, on a command car. 
     In a known ad hoc network (as disclosed in Japanese Laid-open Patent Publication No. 2008-109286), in order to prevent exhaustion of resources of spreading codes, spreading codes for the respective terminals are managed by a base station to which the respective terminals belong even when communication to another base station is performed. 
     Unlike the fixedly arranged base station, the master wireless terminal can be mounted on a vehicle or the like. Therefore, efficient battery use is required in the master wireless terminal. Nevertheless, when broadcast transmission is performed, the amount of processing increases, so that power consumption increases. 
     For example, when the wireless terminal  131  performs broadcast transmission to the wireless terminals  132  and  133  in the configuration of  FIG. 21 , the wireless terminal  121  once receives data transmitted from the wireless terminal  131 , and then broadcasts the received data to the wireless terminals  132  and  133 . Therefore, the data transfer through the wireless terminal  121  increases the amount of processing performed by the wireless terminal  121 , and the consumption power increases. 
     SUMMARY 
     According to an aspect of the present invention, a wireless terminal which performs wireless communication with other wireless terminals by using a first time interval for data transmission and a second time interval for data reception, comprising: a receiver which receives from one of the other wireless terminals a request for broadcast communication; and a controller which generates broadcast control information and controls the wireless terminal so as to transmit the broadcast control information to the other wireless terminals in the first time interval after the receiver receives the request, where the broadcast control information instructs the one of the other wireless terminals to transmit broadcast data in the second time interval, and instructs the other ones of the other wireless terminals to receive the broadcast data in the second time interval. 
     The objects and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
     It is to be understood that both the forgoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1A  illustrates an example of a configuration of a wireless communication system according to a first embodiment; 
         FIG. 1B  illustrates time intervals of transmission and reception in a wireless frame in the case where wireless communication is performed between a wireless terminal and a master wireless terminal in the wireless communication system according to the first embodiment; 
         FIG. 2  illustrates an example of a configuration of a wireless communication system according to a second embodiment; 
         FIG. 3  illustrates time intervals of transmission and reception in a wireless frame in the case where wireless communication is performed between a wireless terminal and a master wireless terminal in the wireless communication system according to the second embodiment; 
         FIG. 4  illustrates an example of a structure of a wireless subframe in the downlink interval indicated in  FIG. 3 ; 
         FIG. 5  illustrates time intervals of transmission and reception in a wireless frame in the case where a wireless terminal performs broadcast transmission in the wireless communication system according to the second embodiment; 
         FIG. 6  illustrates an example of a structure of a wireless subframe in the downlink interval indicated in  FIG. 5 ; 
         FIG. 7  indicates a flow of processing performed by wireless terminals and a master wireless terminal for broadcast communication according to the second embodiment; 
         FIG. 8  illustrates timing adjustment in a cellular communication mode according to a third embodiment; 
         FIG. 9  indicates a sequence of operations performed for timing adjustment in the cellular communication mode according to the third embodiment; 
         FIG. 10  illustrates timing adjustment in a broadcast communication mode according to the third embodiment; 
         FIG. 11  indicates a sequence of operations performed for timing adjustment in the broadcast communication mode according to the third embodiment; 
         FIG. 12  indicates a flow of processing for broadcast communication according to a fourth embodiment; 
         FIG. 13  illustrates an example of a data structure of a broadcast reception timing table according to the fourth embodiment; 
         FIG. 14  indicates a flow of processing performed by wireless terminals and a master wireless terminal for broadcast communication according to the fourth embodiment; 
         FIG. 15  illustrates an example of a data structure of a timing-adjustment interval table according to a fifth embodiment; 
         FIG. 16  indicates a flow of processing for periodically performing timing adjustment according to the fifth embodiment; 
         FIG. 17  indicates a flow of processing performed for timing adjustment in the broadcast communication mode according to a sixth embodiment; 
         FIG. 18  is a block diagram illustrating a construction of a master wireless terminal; 
         FIG. 19  is a block diagram illustrating a construction of a wireless terminal; 
         FIG. 20  illustrates an example of a configuration of an independent communication system in which wireless communication is performed between a base station and wireless terminals; and 
         FIG. 21  illustrates an example of a configuration of an independent communication system in which wireless communication is performed between wireless terminals located out of a coverage area of a base station. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     The embodiments will be explained below with reference to the accompanying drawings, wherein like reference numbers refer to like elements throughout. 
     1. First Embodiment 
     The first embodiment is explained in detail below with reference to  FIGS. 1A and 1B .  FIG. 1A  illustrates an example of a configuration of a wireless communication system according to the first embodiment. In  FIG. 1A , a master wireless terminal  1  and wireless terminals  2  to  4  which constitute the wireless communication system according to the first embodiment are illustrated.  FIG. 1B  illustrates time intervals of transmission and reception in a wireless frame in the case where wireless communication is performed between the wireless terminals  2  to  4  and the master wireless terminal  1  in the wireless communication system according to the first embodiment. (In  FIG. 1B , time progresses from the left to the right.) For example, as indicated in  FIG. 1B , in the first time interval, the master wireless terminal  1  transmits data, and the wireless terminals  2  to  4  receives the data. In addition, in the second time interval, the wireless terminals  2  to  4  transmit data, and the master wireless terminal  1  receives the data. Further, in the second time interval, the wireless terminals  3  and  4  switch their operation mode from the data transmission to the data reception. 
     The master wireless terminal  1  comprises a receiver  1   a  and a controller  1   b . Normally, the master wireless terminal  1  performs wireless communication with the wireless terminals  2  to  4  by using the first time interval for data transmission and the second time interval for data reception. The receiver  1   a  receives from one of the wireless terminals  2  to  4  a request for broadcast communication. When the receiver  1   a  receives the request for broadcast communication, the controller  1   b  generates broadcast control information, and controls the master wireless terminal  1  so as to transmit the generated broadcast control information to the wireless terminals  2  to  4  in the first time interval. The broadcast control information instructs the one of the wireless terminals  2  to  4  to transmit the broadcast data in the second time interval, and instructs the other ones of the wireless terminals  2  to  4  to receive the broadcast data in the second time interval. 
     The wireless terminal  2  comprises a receiver  2   a  and a controller  2   b . The wireless terminal  2  performs wireless communication with the master wireless terminal  1  by using the first time interval for data reception and the second time interval for data transmission. Although not illustrated, each of the wireless terminals  3  and  4  also has a receiver and a controller which are respectively similar to the receiver  2   a  and the controller  2   b  in the wireless terminal  2 . 
     The receiver  2   a  receives the broadcast control information from the master wireless terminal  1  in the first time interval. When the receiver  2   a  receives the broadcast control information, the controller  2   b  performs, in the second time interval, broadcast transmission of broadcast data to the other wireless terminals  3  and  4  or reception of broadcast data from one of the other wireless terminals  3  and  4 , based on the broadcast control information. 
     For example, when the wireless terminal  2  transmits to the master wireless terminal  1   a  request for broadcast communication, the controller  1   b  in the master wireless terminal  1  generates broadcast control information, and controls the master wireless terminal  1  so as to transmit the generated broadcast control information to the wireless terminals  2  to  4  in the first time interval. The broadcast control information instructs the wireless terminal  2  to transmit broadcast data in the second time interval, and instructs the wireless terminals  3  and  4  to receive the broadcast data in the second time interval. 
     When the receiver  2   a  in the wireless terminal  2  receives the broadcast control information in the first time interval, the controller  2   b  controls the wireless terminal  2  so as to broadcast the broadcast data to the wireless terminals  3  and  4  in the second time interval on the basis of the received broadcast control information as illustrated in  FIG. 1B . 
     When the receiver in each of the wireless terminals  3  and  4  receives the broadcast control information in the first time interval, the controller in the wireless terminal controls the wireless terminal so as to receive the broadcast data in the second time interval on the basis of the received broadcast control information as illustrated in  FIG. 1B . 
     As explained above, when broadcast communication is performed, the wireless terminals  3  and  4  switch their operation mode in the second time interval from the data transmission mode to the data reception mode. Therefore, transmission and reception of broadcast data can be performed between the wireless terminals  2  to  4  without being relayed by the master wireless terminal  1 . Thus, the amount of processing performed by the master wireless terminal  1  for realizing the broadcast transmission can be reduced, so that the power consumption in the broadcast transmission can be reduced. 
     2. Second Embodiment 
     The second embodiment is explained in detail below with reference to  FIGS. 2 to 7 . 
       FIG. 2  illustrates an example of a configuration of a wireless communication system according to the second embodiment. In  FIG. 2 , wireless terminals  11  and  21  to  23  which constitute the wireless communication system according to the second embodiment are illustrated. For example, the wireless terminal  11  may be mounted on a vehicle of a police or fire organization, and the wireless terminals  21  to  23  may be mobile telephones. 
     The wireless terminals  11  and  21  to  23  perform wireless communication in accordance with Mobile WiMAX. (WiMAX stands for Worldwide Interoperability For Microwave Access.) The wireless terminal  11  can behave as a base station for the wireless terminals  21  to  23 , and performs, for example, scheduling of the wireless terminals  21  to  23 . (Hereinafter, the wireless terminal  11  may be referred to as the master wireless terminal  11 .) The wireless terminals  11  and  21  to  23  are located at such a distance from each other that broadcast communication can be performed between the wireless terminals  11  and  21  to  23 . For example, the wireless terminal  21  can perform broadcast transmission to the wireless terminals  11 ,  22 , and  23 . 
       FIG. 3  illustrates time intervals of transmission and reception in a wireless frame in the case where wireless communication is performed between the master wireless terminal  11  and the wireless terminals  21  to  23  in the wireless communication system according to the second embodiment. (In  FIG. 3 , time progresses from the left to the right.) As illustrated in  FIG. 3 , each of the wireless terminals  11  and  21  to  23  perform transmission and reception of data in accordance with TDD (Time Division Multiplex). That is, data transmission and data reception are performed in each wireless frame in a time shared manner. Specifically, the time interval of each wireless frame is divided into the downlink interval and the uplink interval. Data is transmitted from the master wireless terminal  11  to the wireless terminals  21  to  23  in the downlink interval, and from the wireless terminals  21  to  23  to the master wireless terminal  11  in the uplink interval. 
       FIG. 4  illustrates an example of a structure of a wireless subframe in the downlink interval indicated in  FIG. 3 . (In  FIG. 4 , the abscissa indicates the progression of time, and the ordinate indicates the carrier frequency.) As illustrated in  FIG. 4 , the wireless subframe in the downlink interval contains the time intervals for the preamble (indicated by “Pre” in  FIG. 4 ), control information, and data. 
     In the time interval for the preamble, a preamble signal is transmitted from the master wireless terminal  11  to the wireless terminals  21  to  23  (which are subordinate to the master wireless terminal  11 ), so that the wireless terminals  21  to  23  can receive the preamble signal, and realize synchronization in reception of the wireless subframe in the downlink interval. 
     In the time interval for the control information, the control information is transmitted from the master wireless terminal  11 . For example, the control information includes a DL-MAP (downlink map), a UL-MAP (uplink map), and information for controlling wireless communication performed by the wireless terminals  21  to  23 , where the DL-MAP and the UL-MAP are generated by scheduling performed by the master wireless terminal  11 , and respectively include information on the data mapping in the downlink interval and the uplink interval. 
     Each of the wireless terminals  21  to  23  receives the control information from the master wireless terminal  11  in the interval indicated in  FIG. 4 , and can recognize the location, in the downlink subframe, of data transmitted from the master wireless terminal  11  to the wireless terminal, on the basis of the DL-MAP contained in the received control information. 
     In addition, each of the wireless terminals  21  to  23  can recognize the location, in an uplink subframe, of data to be transmitted from the wireless terminal, on the basis of the UL-MAP contained in the received control information. The master wireless terminal  11  recognizes the location, in the uplink subframe, of the data transmitted from each of the wireless terminals  21  to  23 , on the basis of the UL-MAP generated by the master wireless terminal  11 , and receives the data transmitted from each of the wireless terminals  21  to  23 . 
     In the time intervals for data (for example, as illustrated in  FIG. 4 ), the master wireless terminal  11  transmits data to each of the wireless terminals  21  to  23  in accordance with the DL-MAP generated by the master wireless terminal  11 , and each of the wireless terminals  21  to  23  receives the data transmitted from the master wireless terminal  11  and addressed to the wireless terminal, on the basis of the DL-MAP transmitted from the master wireless terminal  11  as explained before. 
       FIG. 5  illustrates time intervals of transmission and reception in a wireless frame in the case where a wireless terminal performs broadcast transmission in the wireless communication system according to the second embodiment. As illustrated in  FIG. 5 , in the case where one of the wireless terminals  11  and  21  to  23  performs broadcast transmission to other of the wireless terminals  11  and  21  to  23 , transmission and reception of data in each wireless frame are performed in a time-shared manner similar to the case of  FIG. 3 . In the following explanations, it is assumed that the wireless terminal  21  performs broadcast transmission to the wireless terminals  11 ,  22 , and  23 . 
     Similarly to the case of  FIG. 3 , the time interval of each wireless frame is divided into the downlink interval and the uplink interval. In the downlink interval, data is transmitted from the master wireless terminal  11  to the wireless terminals  21  to  23 . Although the uplink interval is originally a time interval for transmitting data from the wireless terminals  21  to  23  to the master wireless terminal  11 , the operation mode of each of the wireless terminals  21  to  23  is switched to the reception mode even in the uplink interval. For example, when the wireless terminals  22  and  23  are instructed to receive broadcast data from the wireless terminal  21 , the operation mode of the wireless terminals  22  and  23  is switched to the reception mode in order to receive the broadcast data from the wireless terminal  21 . 
       FIG. 6  illustrates an example of a structure of a wireless subframe in the downlink interval indicated in  FIG. 5 . (In  FIG. 6 , the abscissa indicates the progression of time, and the ordinate indicates the carrier frequency.) In the following explanations referring to  FIG. 6 , it is assumed that the wireless terminal  21  performs broadcast transmission to the wireless terminals  11 ,  22 , and  23  in the uplink interval indicated in  FIG. 5 . As illustrated in  FIG. 6 , the wireless subframe in the downlink interval contains the time intervals for the preamble (indicated by “Pre” in  FIG. 6 ), control information, and data. 
     In the time interval for the preamble, a preamble signal is transmitted from the master wireless terminal  11  to the wireless terminals  21  to  23  (which are subordinate to the master wireless terminal  11 ). 
     In the time interval for the control information, the control information is transmitted from the master wireless terminal  11  to the wireless terminals  21  to  23  in a similar manner to the case explained with reference to  FIGS. 3 and 4 . At this time, the UL-MAP contained in the control information includes broadcast control information. The broadcast control information instructs one of the wireless terminals  21  to  23  requesting broadcast communication to transmit broadcast data in the uplink interval, and instructs the other ones of the wireless terminals  21  to  23  to receive the broadcast data in the uplink interval. For example, the broadcast control information includes information such as one or more of the identifiers of the wireless terminals  21  to  23  and information indicating that the broadcast communication is to be performed. 
     The wireless terminals  21  to  23  receive the control information in the time interval for the control information in the downlink interval. Then, the wireless terminals  22  and  23  recognize, on the basis of the broadcast control information contained in the UL-MAP in the received control information, that the broadcast communication is to be performed in the uplink interval. The master wireless terminal  11  recognizes, on the basis of the UL-MAP in the control information produced by the master wireless terminal  11 . 
     Assume that the wireless terminal  21  performs broadcast transmission to the wireless terminals  11 ,  22 , and  23  in the uplink interval as mentioned before. In this case, the wireless terminal  21  (which is to perform broadcast transmission) transmits to the master wireless terminal  11  a request for the broadcast communication, in an uplink interval allocated by the master wireless terminal  11  to the wireless terminal  21 . 
     When the master wireless terminal  11  receives from the wireless terminal  21  the request for the broadcast communication, the master wireless terminal  11  generates control information which enables the wireless terminal  21  to perform broadcast transmission in the uplink interval in the next wireless frame. The control information contains a UL-MAP containing broadcast control information as illustrated in  FIG. 6 , and the broadcast control information includes first information on the wireless terminal ( 21 ) which is to perform the broadcast communication, and second information indicating that the broadcast communication is to be performed. The control information generated by the master wireless terminal  11  is transmitted to the wireless terminals  21  to  23 . 
     When the wireless terminal  21  receives the above control information from the master wireless terminal  11 , the wireless terminal  21  recognizes that the wireless terminal  21  can perform broadcast transmission to the wireless terminals  11 ,  22 , and  23  in the uplink interval, on the basis of the above-mentioned first and second information included in the broadcast control information contained in the control information transmitted from the master wireless terminal  11 , where the first information indicates the wireless terminal  21 , and the second information indicates that broadcast communication is to be performed. 
     On the other hand, when the wireless terminals  22  and  23  receive from the master wireless terminal  11  the control information containing the first and second information, the wireless terminals  22  and  23  recognize, on the basis of the first and second information, that the wireless terminals  22  and  23  are to switch the operation mode to the reception mode so that the wireless terminals  22  and  23  can receive broadcast data transmitted from the wireless terminal  21  in the uplink interval. In addition, on the basis of the broadcast control information in the control information generated by the master wireless terminal  11 , the master wireless terminal  11  recognizes that the master wireless terminal  11  is to receive the broadcast data from the wireless terminal  21 . 
     Thus, the wireless terminal  21  can transmit the broadcast data to the wireless terminals  22  and  23  in the uplink interval without relay by the master wireless terminal  11 . Therefore, the power consumption in the master wireless terminal  11  can be reduced since the master wireless terminal  11  does not perform processing for receiving the broadcast data from the wireless terminal  21  and processing for transmitting the broadcast data to the wireless terminals  22  and  23 . 
       FIG. 7  indicates a flow of processing performed by one of the wireless terminals  21  to  23  and the master wireless terminal  11  for broadcast communication according to the second embodiment. In  FIG. 7 , each step bearing the indication “MS” is performed by one of the wireless terminals  21  to  23  which is requested to perform broadcast transmission, each step bearing the indication “MS′” is performed by the other ones of the wireless terminals  21  to  23 , and each step bearing the indication “MMS” is performed by the master wireless terminal  11 . 
     &lt;Step S 1 &gt; The one of the wireless terminals  21  to  23  which is requested to perform the broadcast transmission transmits to the master wireless terminal  11  a request for broadcast communication in an uplink interval which is allocated by the master wireless terminal  11  to the one of the wireless terminals  21  to  23 . In the following explanations, it is assumed that the one of the wireless terminals  21  to  23  which is requested to perform the broadcast transmission is the wireless terminal  21 . 
     &lt;Step S 2 &gt; When the master wireless terminal  11  receives the request for broadcast communication, the master wireless terminal  11  generates broadcast control information. The broadcast control information instructs the wireless terminal  21  to transmit broadcast data in the uplink interval, and instructs the wireless terminals  11 ,  22 , and  23  to receive the broadcast data in the uplink interval. The master wireless terminal  11  includes the generated broadcast control information in a UL-MAP in control information, and transmits the control information to the wireless terminals  21  to  23  in the downlink interval. 
     &lt;Step S 3 &gt; On the basis of the broadcast control information included in the UL-MAP contained in the received control information, each of the wireless terminals  21  to  23  recognizes that broadcast communication is to be performed in the uplink interval, and determines whether or not the wireless terminal is the source in the transmission of the broadcast data. When yes is determined, the operation goes to step S 4 . When no is determined, the operation goes to step S 5 . In this example, the operation of the wireless terminal  21  goes to step S 4  since the wireless terminal  21  is the source in the transmission of the broadcast data. On the other hand, the operation of the wireless terminals  22  and  23  goes to step S 5  since the wireless terminals  22  and  23  are to receive the broadcast data. 
     &lt;Step S 4 &gt; The wireless terminal  21  performs broadcast transmission of the broadcast data (broadcast signal) in the uplink interval. 
     &lt;Step S 5 &gt; The wireless terminals  22  and  23  receive the broadcast data broadcasted from the wireless terminal  21  in the uplink interval. In addition, the master wireless terminal  11  also receives the broadcast data broadcasted from the wireless terminal  21  on the basis of the broadcast control information generated by the master wireless terminal  11 . 
     As explained above, when the master wireless terminal  11  receives a request for broadcast communication from one of the wireless terminals  21  to  23  which is requested to perform the broadcast transmission, the master wireless terminal  11  transmits broadcast control information to the one of the wireless terminals  21  to  23 . Then, the one of the wireless terminals  21  to  23  performs the broadcast transmission in the uplink interval on the basis of the broadcast control information. The other wireless terminals  22  and  23  which are to receive the broadcast data switch the operation mode to the reception mode in the uplink interval. Thus, the broadcast data can be transmitted and received by the wireless terminals  21  to  23  without being relayed by the master wireless terminal  11 , so that the amount of processing performed by the master wireless terminal  11  and the electric power consumed in the master wireless terminal  11  for the broadcast communication can be reduced. 
     3. Third Embodiment 
     The third embodiment is explained in detail below with reference to  FIGS. 8 to 11 . The following explanations are focused on the timing adjustment of the wireless frame. Specifically, first, the timing adjustment in the cellular communication mode (i.e., in a communication mode in which one-to-one communication is performed between wireless terminals through a master wireless terminal) is explained, and then the timing adjustment in the broadcast communication mode (i.e., in a communication mode in which transmission from a wireless terminal to multiple wireless terminals is performed) is explained. In the following explanations on the third embodiment, a system configuration similar to the second embodiment is assumed. 
       FIG. 8  illustrates timing adjustment in the cellular communication mode according to the third embodiment. In  FIG. 8 , wireless frames which are transmitted between the master wireless terminal  11  and the wireless terminal  21  are illustrated. 
     As illustrated in  FIG. 8 , the master wireless terminal  11  transmits to the wireless terminal  21  (which is subordinate to the master wireless terminal  11 ) a preamble signal P 1  and a control signal C 1  in the downlink interval in the wireless frame F 1 . The control signal C 1  contains an instruction to the wireless terminal  21  for transmission of a measurement reference signal. When the wireless terminal  21  receives the preamble signal P 1  and the control signal C 1  which are transmitted from the master wireless terminal  11 , a propagation delay as illustrated in  FIG. 8  occurs in the reception of the preamble signal P 1  and the control signal C 1 , and the wireless terminal  21  transmits a measurement reference signal M 1  to the master wireless terminal  11 . When the master wireless terminal  11  receives the measurement reference signal M 1  transmitted from the wireless terminal  21 , the master wireless terminal  11  calculates an amount of timing adjustment, which corresponds to an offset of the received measurement reference signal M 1  from an expected uplink interval (i.e., the time interval in which the master wireless terminal  11  is expected to receive data from the wireless terminal  21 ). 
     For example, when the measurement reference signal M 1  from the wireless terminal  21  is received x samples in advance of the uplink interval expected by the master wireless terminal  11 , the master wireless terminal  11  calculates the amount of timing adjustment so as to correct the advance of x samples in the transmission timing of the measurement reference signal M 1  from the wireless terminal  21 . 
     In the downlink interval in the next wireless frame F 2 , the master wireless terminal  11  transmits a preamble signal P 2  and a control signal C 2  to the wireless terminal  21  (which is subordinate to the master wireless terminal  11 ). The control signal C 2  contains the calculated amount of timing adjustment. When the wireless terminal  21  receives the preamble signal P 2  and the control signal C 2  from the master wireless terminal  11 , the wireless terminal  21  transmits a data signal D 1  to the master wireless terminal  11 . At this time, the wireless terminal  21  adjusts the timing of the transmission of the data signal D 1  on the basis of the amount of timing adjustment contained in the control signal C 2 . For example, when the amount of timing adjustment is such an amount as to correct the advance of x samples in the measurement reference signal M 1 , the wireless terminal  21  transmits the data signal D 1  at the timing x samples delayed from the timing of the transmission of the measurement reference signal M 1 . Thus, the master wireless terminal  11  can receive the data signal D 1  (transmitted from the wireless terminal  21 ) in an appropriate uplink interval. In addition, the wireless terminals  22  and  23  can also make a timing adjustment similarly to the wireless terminal  21 . 
       FIG. 9  indicates a sequence of operations performed for timing adjustment in the cellular communication mode according to the third embodiment. 
     &lt;Step S 11 &gt; The master wireless terminal  11  transmits a preamble signal to the wireless terminal  21  (which is subordinate to the master wireless terminal  11 ). The wireless terminal  21  performs processing for timing synchronization on the basis of the preamble signal transmitted from the master wireless terminal  11 . For example, the wireless terminal  21  determines the leading edge of the downlink interval. 
     &lt;Step S 12 &gt; After the processing for timing synchronization, the wireless terminal  21  performs processing for network entry. Specifically, processing for authentication and registration is performed by the master wireless terminal  11  and the wireless terminal  21  for enabling the wireless terminal  21  to perform wireless communication in the wireless communication network illustrated in  FIG. 2 . 
     &lt;Step S 13 &gt; The master wireless terminal  11  transmits to the wireless terminal  21  an instruction for transmission of a measurement reference signal. 
     &lt;Step S 14 &gt; When the wireless terminal  21  receives the instruction for transmission of a measurement reference signal, the wireless terminal  21  transmits a measurement reference signal to the master wireless terminal  11 . Then, the master wireless terminal  11  calculates the amount of timing adjustment on the basis of the measurement reference signal transmitted from the wireless terminal  21 . 
     &lt;Step S 15 &gt; The master wireless terminal  11  transmits the calculated amount of timing adjustment to the wireless terminal  21 , and instructs the wireless terminal  21  to adjust the transmission timing. 
     &lt;Step S 16 &gt; The wireless terminal  21  adjusts the timing of transmission of data to the master wireless terminal  11 , on the basis of the amount of timing adjustment transmitted from the master wireless terminal  11  before performing uplink transmission. 
     Next, the timing adjustment in the broadcast communication mode is explained below. 
       FIG. 10  illustrates timing adjustment in the broadcast communication mode according to the third embodiment. In  FIG. 10 , wireless frames which are transmitted and received by the master wireless terminal  11  and the wireless terminals  21  and  22  are illustrated, and it is assumed that the wireless terminal  21  is to transmit broadcast data, and the wireless terminal  22  is to receive the broadcast data from the wireless terminal  21 . 
     As illustrated in  FIG. 10 , the master wireless terminal  11  transmits to the wireless terminals  21  and  22  (which are subordinate to the master wireless terminal  11 ) a preamble signal P 11  and a control signal C 11  in the downlink interval in the wireless frame F 11 . The control signal C 11  contains an instruction to the wireless terminal  21  for transmission of a measurement reference signal. When the wireless terminals  21  and  22  receive the preamble signal P 11  and the control signal C 11  transmitted from the master wireless terminal  11 , a propagation delay as illustrated in  FIG. 10  occurs in the reception of the preamble signal P 11  and the control signal C 11  by the wireless terminals  21  and  22 , and the wireless terminal  21  transmits a measurement reference signal M 11  to the master wireless terminal  11 . At this time, the wireless terminal  22 , as well as the master wireless terminal  11 , receives the measurement reference signal M 11 . A propagation delay as illustrated in  FIG. 10  occurs in the reception of the master wireless terminal  11  by the wireless terminal  22 , and the wireless terminal  22  memorizes the timing of the reception of the measurement reference signal M 11  transmitted from the wireless terminal  21 . For example, the wireless terminal  22  memorizes the timing of the reception of the measurement reference signal M 11  with respect to the leading edge of a downlink interval which is determined on receipt of the preamble signal P 11 , and the timing of the reception is represented, for example, by the number of samples counted from the leading edge of the downlink interval. 
     When the master wireless terminal  11  receives the measurement reference signal M 11  from the wireless terminal  21 , the master wireless terminal  11  calculates the amount of timing adjustment, which corresponds to an offset of the received measurement reference signal M 11  from an expected uplink interval. 
     In the downlink interval in the next wireless frame F 12 , the master wireless terminal  11  transmits a preamble signal P 12  and a control signal C 12  to the wireless terminals  21  and  22  (which are subordinate to the master wireless terminal  11 ). The control signal C 12  contains the calculated amount of timing adjustment. When the wireless terminal  21  receives the preamble signal P 12  and the control signal C 12  from the master wireless terminal  11 , the wireless terminal  21  broadcasts a broadcast signal D 11  to the master wireless terminal  11  and the wireless terminal  22 . At this time, the wireless terminal  21  adjusts the timing of the broadcast transmission of the broadcast signal D 11  on the basis of the amount of timing adjustment contained in the control signal C 12 . 
     On the other hand, the wireless terminal  22  adjusts the timing of reception of the broadcast signal D 11  transmitted from the master wireless terminal  11 , on the basis of the timing of the reception of the measurement reference signal M 11  memorized in the wireless terminal  22  and the amount of timing adjustment contained in the control signal C 12  transmitted from the master wireless terminal  11 . Then, the wireless terminal  22  receives the broadcast signal D 11  at the adjusted timing. 
     For example, in the case where the master wireless terminal  11  calculates that the timing of the transmission of the measurement reference signal M 11  from the wireless terminal  21  is x samples in advance of the uplink interval expected by the master wireless terminal  11 , the wireless terminal  22  adjusts the timing of reception of the broadcast signal D 11  transmitted from the wireless terminal  21 , to the timing which is x samples delayed from the timing of the reception of the measurement reference signal M 11  memorized in the wireless terminal  22 . 
     Thus, the wireless terminal  22  can receive the broadcast signal D 11  transmitted from the wireless terminal  21 , at an appropriate timing. Although the wireless terminal  23  is not mentioned in the above explanations, the wireless terminal  23  can also adjust the timing of reception in a similar manner to the wireless terminal  22 . 
       FIG. 11  indicates a sequence of operations performed by the master wireless terminal  11  and the wireless terminals  21  and  22  for timing adjustment in the broadcast communication mode according to the third embodiment. In the following explanations, it is assumed that the wireless terminal  21  is to transmit broadcast data, and the master wireless terminal  11  and the wireless terminal  22  are to receive the broadcast data from the wireless terminal  21 . 
     &lt;Step S 21 &gt; The master wireless terminal  11  transmits a preamble signal to the wireless terminals  21  and  22  (which are subordinate to the master wireless terminal  11 ). The wireless terminals  21  and  22  perform processing for timing synchronization on the basis of the preamble signal transmitted from the master wireless terminal  11 . For example, the wireless terminals  21  and  22  determine the leading edge of the downlink interval. 
     &lt;Step S 22 &gt; After the processing for timing synchronization, the wireless terminal  21  performs processing for network entry. Specifically, processing for authentication and registration is performed by the master wireless terminal  11  and the wireless terminal  21  for enabling the wireless terminal  21  to perform wireless communication in the wireless communication network illustrated in  FIG. 2 . 
     &lt;Step S 23 &gt; After the processing for timing synchronization, the wireless terminal  22  performs processing for network entry. Specifically, processing for authentication and registration is performed by the master wireless terminal  11  and the wireless terminal  22  for enabling the wireless terminal  22  to perform wireless communication in the wireless communication network illustrated in  FIG. 2 . 
     &lt;Step S 24 &gt; The master wireless terminal  11  transmits to the wireless terminal  21  an instruction for transmission of a measurement reference signal. At this time, both of the wireless terminals  21  and  22  receive the instruction for transmission of a measurement reference signal transmitted from the master wireless terminal  11 . 
     &lt;Step S 25 &gt; When the wireless terminal  21  receives the instruction for transmission of a measurement reference signal, the wireless terminal  21  transmits a measurement reference signal to the master wireless terminal  11 . Then, the master wireless terminal  11  calculates the amount of timing adjustment on the basis of the measurement reference signal transmitted from the wireless terminal  21 . The wireless terminal  22  memorizes the timing of the reception of the measurement reference signal transmitted from the wireless terminal  21 . 
     &lt;Step S 26 &gt; The master wireless terminal  11  transmits the calculated amount of timing adjustment to the wireless terminal  21 , and instructs the wireless terminal  21  to adjust the transmission timing. At this time, the wireless terminal  22  also receives the amount of timing adjustment. 
     The wireless terminal  21  adjusts the timing of transmission of a broadcast (data) signal to the master wireless terminal  11  and the wireless terminal  22 , on the basis of the amount of timing adjustment transmitted from the master wireless terminal  11 , and the wireless terminal  22  adjusts the timing of reception of the broadcast signal to be transmitted from the wireless terminal  21 , on the basis of the memorized timing of reception of the measurement reference signal and the amount of timing adjustment transmitted from the master wireless terminal  11 . 
     &lt;Step S 27 &gt; The wireless terminal  21  transmits the broadcast signal to the master wireless terminal  11  and the wireless terminal  22  at the adjusted timing of transmission, and the wireless terminal  22  receives the broadcast signal transmitted from the wireless terminal  21 , at the adjusted timing of reception. 
     As explained above, the wireless terminal  21  (which is to perform broadcast transmission) performs operations similar to the operations for the timing adjustment performed in cooperation with the master wireless terminal  11  in the cellular communication mode, and the wireless terminal  22  (which is to receive the broadcast signal) adjusts the timing of reception in the uplink interval in the broadcast communication mode. In addition, the wireless terminals  21  and  22  perform the above operations in parallel. Thus, it is possible to realize synchronization processing for broadcast communication between the wireless terminals  21  and  22 . 
     Alternatively, the calculation of the amount of timing adjustment, the adjustment of the timing of transmission, and the adjustment of the timing of reception may be performed on the basis of time, instead of the number of samples. 
     4. Fourth Embodiment 
     The fourth embodiment is explained in detail below with reference to  FIGS. 12 to 14 . According to the fourth embodiment, timing adjustment for broadcast communication is performed in each of the wireless terminals belonging to the wireless communication network of  FIG. 2 , although a similar system configuration to  FIG. 2  is assumed in the following explanations on the fourth embodiment. 
       FIG. 12  indicates a flow of processing for broadcast communication according to the fourth embodiment. In  FIG. 12 , each step bearing the indication “MS” is performed by each of the wireless terminals  21  to  23 , and each step bearing the indication “MMS” is performed by the master wireless terminal  11 . In the following explanations, it is assumed that the processing for network entry of the wireless terminals  21  to  23  is already performed. 
     &lt;Step S 31 &gt; The master wireless terminal  11  determines a broadcast communication group. For example, the broadcast communication group consists of one or more of the wireless terminals  21  to  23  which have a function of performing broadcast transmission and have already performed the processing for network entry. 
     &lt;Step S 32 &gt; The master wireless terminal  11  transmits to one of the wireless terminals belonging to the broadcast communication group an instruction for transmission of a measurement reference signal. 
     &lt;Step S 33 &gt; When the one of the wireless terminals receives from the master wireless terminal  11  the instruction for transmission of a measurement reference signal, the wireless terminal transmits a measurement reference signal to the master wireless terminal  11  and the other ones of the wireless terminals  21  to  23 . (For example, when the wireless terminal  21  receives from the master wireless terminal  11  the instruction for transmission of a measurement reference signal, the wireless terminal  21  transmits a measurement reference signal to the master wireless terminal  11  and the wireless terminals  22  and  23 .) 
     &lt;Step S 34 &gt; When the master wireless terminal  11  receives the measurement reference signal from the one of the wireless terminals belonging to the broadcast communication group, the master wireless terminal  11  calculates an amount of timing adjustment on the basis of the received measurement reference signal. At this time, each of the other ones of the wireless terminals also receives the measurement reference signal, and memorizes the timing of the reception of the measurement reference signal. (For example, when the wireless terminal  21  transmits a measurement reference signal in response to the instruction for transmission of a measurement reference signal, each of the wireless terminals  22  and  23  receives the measurement reference signal, and memorizes the timing of the reception of the measurement reference signal.) 
     &lt;Step S 35 &gt; The master wireless terminal  11  transmits a calculated amount of timing adjustment to the one of the wireless terminals belonging to the broadcast communication group, and instructs the one of the wireless terminals to adjust the transmission timing. Then, the one of the wireless terminals  21  to  23  belonging to the broadcast communication group stores, in a memory, the amount of timing adjustment transmitted from the master wireless terminal  11 . 
     In addition, each of the other ones of the wireless terminals (which are not instructed by the master wireless terminal  11  to transmit a measurement reference signal) calculates the timing of reception of broadcast data to be transmitted from the one of the wireless terminals belonging to the broadcast communication group (which receives the instruction for transmission of a measurement reference signal), on the basis of the amount of timing adjustment transmitted from the master wireless terminal  11  and the timing of the reception of the measurement reference signal memorized in step S 34 . For example, in the case where the wireless terminal  21  is the one of the wireless terminals belonging to the broadcast communication group, the wireless terminal  22  (which are not instructed by the master wireless terminal  11  to transmit a measurement reference signal) calculates the timing of reception of broadcast data to be transmitted from the wireless terminal  21 , on the basis of the amount of timing adjustment transmitted from the master wireless terminal  11  and the timing of the reception of the measurement reference signal memorized in step S 34 . 
     &lt;Step S 36 &gt; Each of the other ones of the wireless terminals (which are not instructed by the master wireless terminal  11  to transmit a measurement reference signal) stores, in a broadcast reception timing table, the calculated timing of reception of broadcast data in correspondence with the one of the wireless terminals belonging to the broadcast communication group (which receives from the master wireless terminal  11  the instruction for transmission of a measurement reference signal). For example, in the case where the wireless terminal  21  is the one of the wireless terminals belonging to the broadcast communication group (which receives from the master wireless terminal  11  the instruction for transmission of a measurement reference signal), each of the wireless terminals  22  and  23  (which are not instructed by the master wireless terminal  11  to transmit a measurement reference signal) stores, in the broadcast reception timing table, the calculated timing of reception of broadcast data in correspondence with the wireless terminal  21 . 
     &lt;Step S 37 &gt; The master wireless terminal  11  determines whether or not the timing of reception of broadcast data is already calculated on the basis of the measurement reference signal transmitted from every one of the wireless terminals belonging to the broadcast communication group. When yes is determined in step S 37 , the processing of  FIG. 12  is completed. When no is determined in step S 37 , the operation goes to step S 32 , and the instruction for transmission of a measurement reference signal is transmitted, in step S 32 , to another of the wireless terminals belonging to the broadcast communication group to which the instruction for transmission of a measurement reference signal is not yet transmitted. 
       FIG. 13  illustrates an example of a data structure of the broadcast reception timing table according to the fourth embodiment. The broadcast reception timing table  31  illustrated in  FIG. 13  is assumed to be arranged in the wireless terminal  21 . (Although not illustrated, each of the wireless terminals  22  and  23  also has a similar broadcast reception timing table.) As illustrated in  FIG. 13 , the broadcast reception timing table  31  has the column “Wireless Terminal” and the column “Timing of Broadcast Reception.” For example, the identifiers of the wireless terminals  22  and  23  are recorded in the column “Wireless Terminal”, and the timing of reception of a broadcast signal to be transmitted from each of the wireless terminals  22  and  23  are recorded in the column “Timing of Broadcast Reception.” For example, in the case where the wireless terminal  22  is to transmit a broadcast signal, the wireless terminal  21  refers to the broadcast reception timing table  31 , and acquires the timing of broadcast reception corresponding to the wireless terminal  22 . Then, the wireless terminal  21  receives a broadcast signal transmitted from the wireless terminal  22 , at the acquired timing of broadcast reception. 
       FIG. 14  indicates a flow of processing performed by wireless terminals and the master wireless terminal for broadcast communication according to the fourth embodiment. In  FIG. 14 , each step bearing the indication “MS” is performed by one of the wireless terminals  21  to  23  which is requested to perform broadcast transmission, each step bearing the indication “MS′” is performed by the other ones of the wireless terminals  21  to  23 , and each step bearing the indication “MMS” is performed by the master wireless terminal  11 . 
     &lt;Step S 41 &gt; One of the wireless terminals  21  to  23  which is requested to perform broadcast transmission transmits to the master wireless terminal  11  a request for broadcast communication in an uplink interval which is allocated by the master wireless terminal  11  to the one of the wireless terminals  21  to  23 . In the following explanations, it is assumed that the one of the wireless terminals  21  to  23  which is requested to perform the broadcast transmission is the wireless terminal  23 . 
     &lt;Step S 42 &gt; When the master wireless terminal  11  receives from the wireless terminal  23  the request for broadcast communication, the master wireless terminal  11  generates broadcast control information. The broadcast control information instructs the wireless terminal  23  to transmit broadcast data in the uplink interval, and instructs the wireless terminals  11 ,  21 , and  22  to receive the broadcast data in the uplink interval. The master wireless terminal  11  includes the generated broadcast control information in a UL-MAP in control information, and transmits the control information to the wireless terminals  21  to  23  in the downlink interval. 
     &lt;Step S 43 &gt; On the basis of the broadcast control information included in the UL-MAP contained in received control information, each of the wireless terminals  21  to  23  recognizes that broadcast communication is to be performed in the uplink interval, and determines whether or not the wireless terminal is the source in transmission of the broadcast data. When yes is determined, the operation goes to step S 44 . When no is determined, the operation goes to step S 45 . In this example, the operation of the wireless terminal  23  goes to step S 44  since the wireless terminal  23  is the source in transmission of the broadcast data. On the other hand, the operation of the wireless terminals  21  and  22  goes to step S 45  since the wireless terminals  21  and  22  are to receive the broadcast data. 
     &lt;Step S 44 &gt; The wireless terminal  23  performs broadcast transmission of the broadcast data (broadcast signal) to the wireless terminals  11 ,  21 , and  22  in the uplink interval. At this time, the wireless terminal  23  adjusts the timing of the transmission of the broadcast data on the basis of the amount of timing adjustment stored in the memory in the wireless terminal  23 . 
     &lt;Step S 45 &gt; The wireless terminals  21  and  22  receive the broadcast data transmitted from the wireless terminal  23  in the uplink interval. At this time, the wireless terminals  21  and  22  acquires the timing of broadcast reception corresponding to the wireless terminal  23  and being stored in the broadcast reception timing table  31 , and receives the broadcast data at the acquired timing. In addition, the master wireless terminal  11  also receives the broadcast data transmitted from the wireless terminal  23  on the basis of the broadcast control information generated by the master wireless terminal  11 . 
     As explained above, since each of the wireless terminals  21  to  23  memorizes the timing of broadcast reception in the wireless terminal, the wireless terminals  21  to  23  can appropriately receive the broadcast data whichever of the wireless terminals  21  to  23  obtains the rights to perform broadcast transmission and transmits the broadcast data. 
     5. Fifth Embodiment 
     The fifth embodiment is explained in detail below with reference to  FIGS. 15 and 16 . The amount of timing adjustment for each terminal and the timing of reception by each terminal vary with the location of the wireless terminal. When the wireless terminal moves, the amount of timing adjustment for and the timing of reception by each terminal vary with time. Further, the movement speed of each wireless terminal is different. According to the fifth embodiment, timing adjustment is performed at predetermined regular intervals. 
       FIG. 15  illustrates an example of a data structure of a timing-adjustment interval table according to the fifth embodiment. The timing-adjustment interval table  32  illustrated in  FIG. 15  is arranged in the master wireless terminal  11 . As illustrated in  FIG. 15 , the timing-adjustment interval table  32  has the column “Wireless Terminal” and the column “Timing-adjustment Interval.” For example, the identifiers of the wireless terminals  21  to  23  are recorded in the column “Wireless Terminal”, and the timing-adjustment interval of each of the wireless terminals  21  to  23  (i.e., the intervals at which the timing adjustment is to be performed by each of the wireless terminals  21  to  23 ) is recorded in the column “Timing Adjustment Interval.” For example, the timing-adjustment interval table  32  of  FIG. 15  indicates that the timing-adjustment interval of the wireless terminal  21  having the identifier “A” is “x”. 
     The master wireless terminal  11  recognizes the intervals at which the timing adjustment is to be performed by the wireless terminals  21  to  23 , by reference to the timing-adjustment interval table  32 . When the time at which each of the wireless terminals  21  to  23  is to perform the timing adjustment comes, the master wireless terminal  11  transmits to the wireless terminal an instruction for transmission of a measurement reference signal. For example, when the master wireless terminal  11  recognizes, by reference to the timing-adjustment interval table  32 , that the time at which the wireless terminal  21  is to perform the timing adjustment comes, the master wireless terminal  11  transmits to the wireless terminal  21  an instruction for transmission of a measurement reference signal. On receipt of the instruction, the wireless terminal  21  transmits a measurement reference signal. Thereafter, the wireless terminal  21  receives an updated amount of timing adjustment from the master wireless terminal  11 . Similarly, the wireless terminals  22  and  23  also receives the updated amount of timing adjustment, and updates the timing of broadcast reception corresponding to the wireless terminal  21 , which is stored in the timing-adjustment interval table  32 . 
     The timing-adjustment interval is set so as to decrease with increase in the movement speed since the variations in the location of each wireless terminal increase with the movement speed. 
     In addition, the master wireless terminal  11  can change the values of the timing-adjustment interval for the wireless terminals  21  to  23 . For example, the master wireless terminal  11  periodically receives the movement speeds of the wireless terminals  21  to  23  from the wireless terminals, and changes the values of the timing-adjustment interval stored in the timing-adjustment interval table  32  on the basis of the received movement speeds. 
       FIG. 16  indicates a flow of processing for periodically performing timing adjustment according to the fifth embodiment. In  FIG. 16 , each step bearing the indication “MS” is performed by each of the wireless terminals  21  to  23 , and each step bearing the indication “MMS” is performed by the master wireless terminal  11 . The processing of  FIG. 16  is performed after the processing of  FIG. 12  is performed once. 
     &lt;Step S 51 &gt; The master wireless terminal  11  determines, by reference to the timing-adjustment interval table  32 , whether or not a wireless terminal of which timing adjustment is to be performed exists. When yes is determined, the operation goes to step S 52 . When no is determined, the processing of  FIG. 16  is completed. 
     &lt;Step S 52 &gt; The master wireless terminal  11  transmits an instruction for transmission of a measurement reference signal, to a wireless terminal of which timing adjustment is to be performed. For example, when the master wireless terminal  11  determines, in step S 51 , that timing adjustment of the wireless terminal  21  is to be performed, the master wireless terminal  11  transmits to the wireless terminal  21  an instruction for transmission of a measurement reference signal. Thereafter, operations similar to the operations in step S 34  to S 36  in  FIG. 12  are performed. 
     As explained above, the timing adjustment of the wireless terminals  21  to  23  is periodically performed according to the fifth embodiment, so that it is possible to cope with variations in the transmission timing in the wireless terminals  21  to  23 , and the broadcast data can be appropriately transmitted and received. 
     6. Sixth Embodiment 
     The sixth embodiment is explained in detail below with reference to  FIG. 17 . According to the sixth embodiment, the timing adjustment is performed when broadcast communication is requested. 
       FIG. 17  indicates a flow of processing performed for timing adjustment in the broadcast communication mode according to the sixth embodiment. In  FIG. 17 , each step bearing the indication “MS” is performed by one of the wireless terminals  21  to  23  which is requested to perform broadcast transmission, each step bearing the indication “MS′” is performed by the other ones of the wireless terminals  21  to  23 , and each step bearing the indication “MMS” is performed by the master wireless terminal  11 . In the following explanations, it is assumed that the broadcast communication group consists of the wireless terminals  11  and  21  to  23 . When the master wireless terminal  11  receives from one of the wireless terminals  21  to  23  a request for broadcast communication, broadcast communication is performed between the wireless terminals  11  and  21  to  23 . 
     &lt;Step S 61 &gt; One of the wireless terminals  21  to  23  which is requested to perform broadcast transmission transmits to the master wireless terminal  11  a request for broadcast communication in an uplink interval which is allocated by the master wireless terminal  11  to the one of the wireless terminals  21  to  23 . In the following explanations, it is assumed that the one of the wireless terminals  21  to  23  which is requested to perform the broadcast transmission is the wireless terminal  21 . 
     &lt;Step S 62 &gt; The master wireless terminal  11  transmits an instruction for transmission of a measurement reference signal, to the wireless terminal  21  (which transmits the request for broadcast communication). Thereafter, operations similar to the operations in step S 34  to S 36  in  FIG. 12  are performed. Thus, the wireless terminal  21  (which is to perform broadcast transmission) performs timing adjustment so that the broadcast data is appropriately received by the master wireless terminal  11 . In addition, the wireless terminals  22  and  23  (which are to receive the broadcast data) adjust the timing of reception so that the wireless terminals  22  and  23  can appropriately receive the broadcast data. 
     &lt;Step S 63 &gt; The master wireless terminal  11  generates broadcast control information. The broadcast control information instructs the wireless terminal  21  to transmit broadcast data in the uplink interval, and instructs the wireless terminals  11 ,  22 , and  23  to receive the broadcast data in the uplink interval. The master wireless terminal  11  includes the generated broadcast control information in a UL-MAP in control information, and transmits the control information to the wireless terminals  21  to  23  in the downlink interval. 
     &lt;Step S 64 &gt; On the basis of the broadcast control information included in the UL-MAP contained in received control information, each of the wireless terminals  21  to  23  recognizes that broadcast communication is to be performed in the uplink interval, and determines whether or not the wireless terminal is the source in transmission of the broadcast data. When yes is determined, the operation goes to step S 65 . When no is determined, the operation goes to step S 66 . In this example, the operation of the wireless terminal  21  goes to step S 65  since the wireless terminal  21  is the source in transmission of the broadcast data. On the other hand, the operation of the wireless terminals  22  and  23  goes to step S 66  since the wireless terminals  22  and  23  are to receive the broadcast data. 
     &lt;Step S 65 &gt; The wireless terminal  21  performs broadcast transmission of the broadcast data (broadcast signal) to the wireless terminals  11 ,  22 , and  23  in the uplink interval. At this time, the wireless terminal  21  adjusts the timing of the transmission of the broadcast data on the basis of the amount of timing adjustment stored in the memory in the memory. 
     &lt;Step S 66 &gt; The wireless terminals  22  and  23  receive the broadcast data transmitted from the wireless terminal  21  in the uplink interval. At this time, the wireless terminals  22  and  23  acquires the timing of broadcast reception corresponding to the wireless terminal  21  and being stored in the broadcast reception timing table  31 , and receives the broadcast data at the acquired timing. In addition, the master wireless terminal  11  also receives the broadcast data transmitted from the wireless terminal  21  on the basis of the broadcast control information generated by the master wireless terminal  11 . 
     As explained above, according to the sixth embodiment, the master wireless terminal  11  performs timing adjustment when the master wireless terminal  11  receives from one of the wireless terminals  21  to  23  a request for broadcast communication. Therefore, it is possible to appropriately transmit and receive broadcast data even when the locations of the wireless terminals change. 
     7. Construction of Wireless Terminals 
     Hereinbelow, the constructions of wireless terminals which realizes the operations according to the second to sixth embodiments are explained below with reference to  FIGS. 18 and 19 . 
       FIG. 18  is a block diagram illustrating a construction of the master wireless terminal. In  FIG. 18 , the antenna duplexer  41  switches the operation mode of the master wireless terminal  11  between the transmission mode and the reception mode according to information on the timings of transmission and reception in each wireless frame. Alternatively, an antenna may be provided for each of transmission and reception. 
     The receiver  42  comprises, for example, a receiver amplifier, a filter, a frequency converter, an analog-to-digital converter, and an orthogonal converter, and converts a received RF (radio frequency) signal into a digital baseband signal. The demodulator  43  analyzes the control information transmitted to the master wireless terminal  11 , in a predetermined manner, and demodulates the received signal on the basis of the result of the analysis. The data decoder  44  decodes the demodulated signal outputted from the demodulator  43 . The user-data extractor  45  extracts user data from the signal decoded by the data decoder  44 . That is, the user-data extractor  45  extracts user data transmitted from the wireless terminals  21  to  23 . The control-information extractor  46  extracts the control information from the demodulated signal outputted from the demodulator  43 , on the basis of a signal outputted from the data decoder  44 . 
     The communication controller  47  generates the information on the timings of transmission and reception on the basis of information including an instruction from an upper layer such as a communication application, the control information extracted from the received signal, and the broadcast control information (which is outputted from the broadcast communication controller  53 ), and controls the operations of the respective portions of the master wireless terminal  11 . 
     The control-information generator  48  generates the control information on the basis of the information outputted from the communication controller  47  and the broadcast communication controller  53 . The user-data generator  49  generates data to be transmitted to a user (e.g., data for voice call or data communication). 
     The data encoder  50  encodes the user data and the control information. The modulator  51  modulates the data outputted from the data encoder  50 , on the basis of the control information outputted from the control-information generator  48 . The transmitter  52  comprises a transmitter amplifier, a filter, a frequency converter, an digital-to-analog converter, and an orthogonal converter, and converts the baseband signal outputted from the modulator  51 , into an RF signal. 
     The broadcast communication controller  53  controls the processing for broadcast communication. Specifically, when the master wireless terminal  11  receives from one of the wireless terminals  21  to  23  a request for broadcast communication, the broadcast communication controller  53  generates the broadcast control information, and controls the respective portions of the master wireless terminal  11  so as to transmit the broadcast control information to the one of the wireless terminals  21  to  23  in the downlink interval. In addition, the broadcast communication controller  53  generates information for transmitting to one of the wireless terminals  21  to  23  an instruction for transmission of a measurement reference signal (for adjustment of transmission timing). The instruction contains control information which instructs the one of the wireless terminals  21  to  23  to transmit a measurement reference signal in a specific region of a wireless frame. 
     Further, the broadcast communication controller  53  calculates the amount of timing adjustment for the one of the wireless terminals  21  to  23  on the basis of information on a measured timing. Furthermore, in order to realize the fifth embodiment, the broadcast communication controller  53  can generate information for transmitting to the wireless terminals  21  to  23  an instruction for transmission of a measurement reference signal at intervals which are different for each of the wireless terminals  21  to  23 . Moreover, in order to realize the sixth embodiment, the broadcast communication controller  53  can control the respective portions of the master wireless terminal  11  so that each of the wireless terminals  21  to  23  performs timing adjustment when the master wireless terminal  11  receives a request for broadcast communication from the wireless terminal. The measurement-reference-signal receiver  54  receives the measurement reference signal, and outputs, as the information on the measured timing, information on the timing of the reception of the measurement reference signal. 
       FIG. 19  is a block diagram illustrating a construction of the wireless terminal  21 . Although not illustrated, each of the wireless terminals  22  and  23  has a similar construction to the wireless terminal  21 . In  FIG. 19 , the antenna duplexer  61  switches the operation mode of the wireless terminal  21  between the transmission mode and the reception mode according to information on the timings of transmission and reception in each wireless frame. Alternatively, an antenna may be provided for each of transmission and reception. 
     The receiver  62  comprises, for example, a receiver amplifier, a filter, a frequency converter, an analog-to-digital converter, and an orthogonal converter, and converts a received RF (radio frequency) signal into a digital baseband signal. The receiver  62  extracts from the preamble signal information on a downlink timing, which is used as a reference timing by the respective portions of the wireless terminal  21 . For example, the information on the downlink timing indicates the leading edge of the downlink interval. 
     The demodulator  63  analyzes, in a predetermined manner, control information transmitted to the wireless terminal  21 , and demodulates the received signal on the basis of the result of the analysis. The data decoder  64  decodes the demodulated signal outputted from the demodulator  63 . The user-data extractor  65  extracts user data from the signal decoded by the data decoder  64 . That is, the user-data extractor  65  extracts user data transmitted from the other ones of the wireless terminals  21  to  23 . The control-information extractor  66  extracts the control information from the demodulated signal outputted from the demodulator  63 , on the basis of a signal outputted from the data decoder  64 . 
     The communication controller  67  generates the information on the timings of transmission and reception on the basis of information including an instruction from an upper layer such as a communication application, the control information extracted from the received signal, and the broadcast control information (which is outputted from the broadcast communication controller  73 ), and controls the operations of the respective portions of the wireless terminal  21 . 
     The control-information generator  68  generates the control information on the basis of the information outputted from the communication controller  67  and the broadcast communication controller  73 . The user-data generator  69  generates data to be transmitted by a user (e.g., data for voice call or data communication). 
     The data encoder  70  encodes the user data and the control information. The modulator  71  modulates the data outputted from the data encoder  70 , on the basis of the control information outputted from the control-information generator  68 . The transmitter  72  comprises a transmitter amplifier, a filter, a frequency converter, an digital-to-analog converter, and an orthogonal converter, and converts the baseband signal outputted from the modulator  71 , into an RF signal. 
     The broadcast communication controller  73  controls the processing for broadcast communication. Specifically, when the wireless terminal  21  receives a request for broadcast communication from an upper layer such as a communication application, the broadcast communication controller  73  controls the respective portions of the wireless terminal  21  so that the wireless terminal  21  transmits to the master wireless terminal  11  a request for broadcast communication. In addition, the broadcast communication controller  73  controls the respective portions of the wireless terminal  21 , on the basis of the broadcast control information transmitted from the master wireless terminal  11 , so that the wireless terminal  21  transmits broadcast data to the wireless terminals  11 ,  22 , and  23  in the uplink interval, or receives broadcast data from one of the wireless terminals  11 ,  22 , and  23 . At this time, on the basis of the broadcast control information included in the control information transmitted from the master wireless terminal  11 , the broadcast communication controller  73  determines whether the wireless terminal  21  is to transmit broadcast data or to receive broadcast data. In addition, when the wireless terminal  21  receives from the master wireless terminal  11  an instruction for transmission of a measurement reference signal, the broadcast communication controller  73  generates broadcast control information containing the measurement reference signal. Further, on the basis of information on the timing of broadcast reception which is outputted from the reception-timing storage  76 , the broadcast communication controller  73  controls the respective portions of the wireless terminal  21  so that the wireless terminal  21  receives broadcast data transmitted from one of the wireless terminals  22  and  23  which transmits to the master wireless terminal  11  a request for broadcast communication. Furthermore, the broadcast communication controller  73  controls the respective portions of the wireless terminal  21  so that the wireless terminal  21  performs timing adjustment by the amount of timing adjustment transmitted from the master wireless terminal  11  before broadcast transmission. 
     The measurement-reference-signal receiver  74  receives a measurement reference signal transmitted from one of the wireless terminals  22  and  23 , measures the timing of reception of the measurement reference signal, and outputs information on the measured timing. The reception-timing calculator  75  calculates the timing of broadcast reception on the basis of the information on the measured timing and the amount of timing adjustment transmitted from the master wireless terminal  11 . The reception-timing calculator  75  outputs the calculated timing of broadcast reception to the reception-timing storage  76 . 
     The reception-timing storage  76  has the broadcast reception timing table  31 . The reception-timing storage  76  stores the timing of broadcast reception (calculated by the reception-timing calculator  75 ) in correspondence with the one of the wireless terminals  22  and  23  which can perform broadcast transmission. The reception-timing storage  76  outputs to the broadcast communication controller  73  the timing of broadcast reception corresponding to one of the wireless terminals  22  and  23  which transmits to the master wireless terminal  11  a request for broadcast communication and is indicated in the broadcast control information. 
     In the case where the timing adjustment is performed when the master wireless terminal  11  receives a request for broadcast communication, the master wireless terminal  11  having the construction of  FIG. 18  and the wireless terminal  21  having the construction of  FIG. 19  operate as explained below. 
     When the wireless terminal  21  receives a request for broadcast transmission from an upper layer such as a communication application, the broadcast communication controller  73  in the wireless terminal  21  transmits to the master wireless terminal  11  a request for broadcast communication in the uplink interval. When the master wireless terminal  11  receives a request for broadcast communication from one of the wireless terminals  21  to  23 , the broadcast communication controller  53  in the master wireless terminal  11  determines a broadcast communication group, and transmits to the one of the wireless terminals  21  to  23  a request for timing adjustment (i.e., an instruction for transmission of a measurement reference signal), where the request for timing adjustment is contained in the control information in the downlink interval. 
     The one of the wireless terminals  21  to  23  which receives the request for timing adjustment and is designated by the control information to transmit a measurement reference signal transmits a measurement reference signal to the master wireless terminal  11  in an allocated time interval in a designated manner. Hereinafter, it is assumed that the wireless terminal  21  receives the request for timing adjustment. 
     When the master wireless terminal  11  receives the measurement reference signal from the wireless terminal  21 , the measurement-reference-signal receiver  54  in the master wireless terminal  11  measures the timing of reception of the measurement reference signal. Then, the broadcast communication controller  53  calculates the amount of timing adjustment for the wireless terminal  21  on the basis of the measured timing of the reception of the measurement reference signal so that data can be transmitted from the wireless terminal  21  in the uplink interval which is set by the master wireless terminal  11 . The amount of timing adjustment is included in the control information, and the master wireless terminal  11  transmits the control information to the wireless terminal  21  in the downlink interval. The broadcast communication controller  53  in the wireless terminal  21  controls the respective portions of the wireless terminal  21  so that the wireless terminal  21  performs timing adjustment by the amount of timing adjustment transmitted from the master wireless terminal  11 , and then performs broadcast transmission. 
     The other wireless terminals  22  and  23  also receive the measurement reference signal transmitted from the wireless terminal  21 , and the measurement-reference-signal receiver in each of the wireless terminals  22  and  23  measures the timing of reception of the measurement reference signal. Then, the reception-timing calculator in each of the wireless terminals  22  and  23  calculates the timing of broadcast reception on the basis of the amount of timing adjustment (transmitted from the master wireless terminal  11 ) and the information on the measured timing (outputted from the measurement-reference-signal receiver), and stores the calculated timing of broadcast reception in the reception-timing storage  76 . 
     When each of the wireless terminals  22  and  23  receives the broadcast control information (indicating an instruction for broadcast communication), the wireless terminal switches the operation mode to the reception mode, sets a timing of reception in correspondence with the wireless terminal  21  (which perform broadcast transmission), and receives broadcast data. 
     Further, the master wireless terminal  11  can perform broadcast transmission in the downlink interval in a similar manner to the conventional broadcasting or multicast transmission. In addition, the wireless terminals illustrated in  FIGS. 18 and 19  can operate to realize any one or combination of the second to sixth embodiments. 
     The above wireless terminal can reduce power consumption in broadcast transmission. 
     8. Additional Matters 
     All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding 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 invention 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.