Abstract:
A radio communication system includes a plurality of base stations and a plurality of mobile stations belonging to the service areas covered by the radio zones respectively formed by the base stations and performs data communication by polling. Each base station includes a transmission output control section for controlling the transmission output of a polling signal to sequentially form a plurality of radio zones having different coverages, a section for receiving responses from the plurality of mobile stations to a plurality of polling signals having different transmission outputs and determining the position of each of the mobile stations in a specific one of the plurality of radio zones.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a radio communication system and, more particularly, to a radio communication system which is comprised of a plurality of base stations and a plurality of mobile stations that can be connected to the base stations through radio channels and controls the transmission outputs of polling signals from the base stations to allow the base stations to transmit a plurality of types of polling signals having different transmission outputs, for example, to each other, at predetermined timings. 
     2. Description of the Prior Art 
     For example, in the conventional radio communication system disclosed in Japanese Unexamined Patent Publication No. 8-107382, each base station controls the transmission output. According to the characteristic feature of this system, when a mobile station connected to a given base station or located in a radio zone formed by a given base station receives a signal from another radio zone, the transmission output of the base station or mobile station as the destination of the signal from another radio zone is reduced through a private communication network. 
     FIG. 1 shows an example of the arrangement described in the above reference. Base stations B i  and B i+1  connected to each other through a private communication network respectively form radio zones Z i  and Z i+1 . Mobile stations T j  and T j+1  are performing radio communication with the base station B i  upon determining radio channels. A mobile station mobile station T j+2  is performing radio communication with the base station B i+1  upon determining a radio channel. 
     Since the radio zone formed by the base station B i+1  is large, the mobile station T j+1  receives a signal from the base station B i+1  as well. At this time, the mobile station T j+1  processes the signal from the base station B i+1  to identify and specify the base station B i+1  as a transmission source, and makes a request to reduce the transmission output of the base station B i+1  through the base station B i  and the private communication network. In response to this request, the private communication network reduces the transmission output of the base station B i +, and reduces the radio zone formed by the base station B i +, as shown in FIG.  1 . 
     With this transmission output control, each base station or mobile station can construct a satisfactory communication environment without being interfered by any signals from other adjacent radio zones. 
     As a radio communication system in which a base station polls a plurality of mobile stations to collect data, for example, the system disclosed in Japanese Unexamined Patent Publication No. 2-15740 or 6-29897 is available. According to Japanese Unexamined Patent Publication No. 2-15740, a base station communicates with all mobile stations to collect data, and sequentially collects data. For example, this system can be applied to collection of data from all mobile stations, e.g., collection of questionnaire. 
     The system disclosed in Japanese Unexamined Patent Publication No. 6-29897 has a characteristic feature in that a base station transmits a polling signal with a polling start number and a time slot count being added thereto. Assume that mobile stations T( 1 ), T( 2 ), . . . , T(M) are present in the radio zone formed by a base station, and the base station transmits a polling signal with a polling start number N (1≦N≦M) and a time slot count n (1≦n≦M−N+1) being added thereto. In this case, only mobile stations A(N) to A(N+n) respond to this polling signal. With this operation, a plurality of mobile stations can be managed in units of small groups. 
     In the scheme disclosed in Japanese Unexamined Patent Publication No. 8-107382, to prevent communication interference between adjacent radio zones, the transmission output of a communication station (base or mobile station) located in one radio zone is reduced. This conventional scheme is effective when a combination of one base station and a plurality of mobile stations belonging thereto is almost fixed, and the mobile stations do not move to other radio zones. If, however, such a mobile station moves between a plurality of base stations, when the mobile station receives a signal from another base station or mobile station, it is not easy for the mobile station to identify the signal as a signal from a radio zone other than the zone in which the mobile station is located. In practice, it is difficult to control the transmission output. 
     Consider a case wherein the base stations B i  and B i+1  form adjacent radio zones Z i  and Z i+1 , and the mobile station T j+1  is located near the periphery of the radio zone Z i+1  and forms a radio zone Z(T j+1 ). The mobile station T j  belonging to the base station B i  receives a signal from the radio zone Z(T j+1 ), and hence makes a request to reduce the radio zone formed by the mobile station T j+1  through the base station B i  and a private communication network. As a result, a transmission signal from the mobile station T j+1  cannot reach the base station B i+1 , and data communication with the base station B i+1  cannot be performed. 
     In the radio communication system disclosed in Japanese Unexamined Patent Publication No. 2-15740, even when data needs to be transmitted to a specific mobile station, all the mobile stations in the system must be polled, resulting in a very long processing time. As an example of the countermeasures against this problem, the radio communication system disclosed in Japanese Unexamined Patent Publication No. 6-29897 is available. In this system, mobile stations are sequentially polled in groups by using idle time for processing other than polling. In this case as well, it takes a very long processing time to poll all the mobile stations. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in consideration of the above situation, and has as its object to provide a radio communication system which can perform efficient data communication between base stations and mobile stations by arranging an apparatus capable of specifying the relative positional relationship between the respective base stations and the respective mobile stations. 
     In order to achieve the above object, according to the first main aspect of the present invention, there is provided a radio communication system which comprises a plurality of base stations and a plurality of mobile stations belonging to service areas covered by radio zones respectively formed by the base stations and performs data communication by polling, each of the plurality of base stations comprising transmission output control means for controlling a transmission output of a polling signal to sequentially form a plurality of radio zones having different coverages, and means for receiving responses from the plurality of mobile stations to a plurality of polling signals having different transmission outputs and determining a position of each of the mobile stations in a specific one of the plurality of radio zones. 
     According to the second main aspect of the present invention, there is provided a radio communication system which comprises a plurality of base stations and a plurality of mobile stations belonging to service areas covered by radio zones respectively formed by the base stations and performs data communication by polling, each of the plurality of base stations comprising transmission output control means for controlling a transmission output of a polling signal to alternately transmit a long-distance polling signal having a strong transmission output and a short-distance polling signal having a weak transmission output, thereby alternately forming large and small radio zones, and means for receiving responses from the plurality of mobile stations to the long- and short-distance polling signals and determining a specific one of the large and small zones in which each of the mobile stations is located. 
     The first and second main aspects have the following auxiliary aspects. 
     Each of the different polling signals has an ID for identifying a base station that has originated the polling signal and a transmission output value. 
     The mobile station transmits to a corresponding base station a response signal with an output value corresponding to the transmission output value of a received polling signal. 
     The mobile station comprises means for determining, from an ID number of the base station and the transmission output which are obtained by receiving the polling signal, a specific one of the plurality of radio zones in which the mobile station is located and a specific one of the plurality of base stations which forms the radio zone in which the mobile station is located. 
     Each of the plurality of base stations continuously transmits only the long-distance polling signal without transmitting the short-distance polling signal when there is no mobile station that responds to the long- and short-distance polling signals. 
     Each of the plurality of mobile stations makes no response when the polling signal received following the short-distance polling signal is the long-distance polling signal. 
     When different data need to be transmitted to the plurality of mobile stations, each of the base stations preferentially transmits the data to a mobile station which is located in the large zone formed by each of the plurality of base stations but is not located in the small zone. 
     The small zones respectively formed by the plurality of base stations are arranged such that the small zones formed by the adjacent base stations do not overlap. 
     According to the radio communication system of the present invention, each base station transmits polling signals while controlling their transmission outputs so as to form two radio zones having different sizes, and can independently monitor the respective zones. For this reason, each base station can roughly discriminate the distribution of mobile stations located around the self-station and the relative distances to the respective mobile stations. Therefore, when an information service network for providing appropriate information for each mobile station is to be constructed, more precise information can be provided. 
     In addition, if the small zones formed by adjacent base stations do not overlap, a mobile station located in a given small zone can communicate with only one base station. The base station can therefore provide information unique to itself for the mobile station inside the small zone formed by the base station itself. If the large zones formed by base stations overlap, an area which cannot be covered by any base station can be eliminated in the radio zone. 
     If there is no mobile station that responds to a long-distance polling signal, i.e., no mobile station is present in the large zone, control can be performed not to transmit any short-distance polling signal. Therefore, any unnecessary short-distance polling signal need not be transmitted. 
     In addition, since each mobile station can be controlled not to respond to any long-distance polling signal that is received after the reception of a short-distance polling signal, the mobile stations in the radio zone formed by the base station can be classified into a group located inside the small zone and a group located outside the small zone. If, for example, many mobile stations are present in a radio zone, this grouping can reduce the possibility of interference. 
     Furthermore, when a base station needs to simultaneously transmit different data to a plurality of mobile stations, since the data are preferentially provided to mobile stations located outside the small zone, the data can be transmitted more reliably to the mobile stations which are located outside the small zone and hence may move outside the radio zone of the base station with higher possibility than mobile stations inside the smaller zone. 
     The above and many other objects, features and advantages of the present invention will become manifest to those skilled in the art upon making reference to the following detailed description and accompanying drawings in which preferred embodiments incorporating the principles of the present invention are shown by way of illustrative examples. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic view showing an example of the arrangement of a conventional radio communication system; 
     FIG. 2 is a view for explaining a problem in the prior art; 
     FIG. 3 is a schematic view showing the arrangement of a radio communication system according to the first embodiment of the present invention; 
     FIG. 4 is a block diagram showing the arrangement of a base station in the first embodiment shown in FIG. 3; 
     FIG. 5 is a block diagram showing the arrangement of a mobile station according to the first embodiment in FIG. 3; 
     FIG. 6 is a timing chart showing how polling signal transmission output control is performed; 
     FIG. 7 is a view showing a plurality of base stations and the large and small zones formed by the base stations in the present invention; 
     FIG. 8 is a view showing an example of the data format of a polling signal; 
     FIG. 9 is a timing chart showing the output timing of a polling response signal; 
     FIG. 10 is a view showing an example of the data format of a polling response signal; 
     FIG. 11 is a timing chart showing how base stations and mobile stations exchange polling signals and polling response signals; 
     FIG. 12 is a block diagram showing another arrangement of a base station in the present invention; 
     FIG. 13 is a block diagram showing another arrangement of a mobile station in the present invention; and 
     FIG. 14 is a table showing the response determination results obtained by a response determination section in the mobile station in FIG.  13 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A few preferred embodiments of the present invention will be described below with reference to the accompanying drawings. 
     A radio communication system of the present invention is comprised of base stations B 1 , B 2 , . . . , B n  which are fixed or semi-fixed in a movable state and mobile stations T 1 , T 2 , . . . , T m  which move inside and outside the radio zones formed by the base stations B 1 , B 2 , . . . , B n.  Each of the base stations B 1 , B 2 , . . . , B n  forms a radio zone by transmitting a polling signal. When a mobile station enters this radio zone and receives a polling signal from the base station, the mobile station makes some response to the base station to notify the base station of the presence of the mobile station. When data to be transmitted to the base station or mobile station is generated, the polling signal is temporarily stopped by corresponding communication control, and data communication is performed. 
     FIG. 3 shows the first embodiment of the radio communication system of the present invention, and more specifically, adjacent base stations B i  and B i+1 , of the base stations B 1 , B 2 , . . . , B n,  and mobile stations T j , T j+1 , T j+2 , and T j+3  located around the base stations. FIG. 3 does not show base stations other than the base stations B i  and B i+1 , and mobile stations other than the mobile stations T j , T j+1 , T j+2 , and T j+3 . 
     FIG. 4 shows an example of the arrangement of the base station B i  (i=1, 2, . . . , n). The base station B i  includes an antenna  10 (B i ) for transmitting a polling signal to mobile stations and transmitting/receiving data, a reception section  12 (B i ) for receiving data, a transmission section  13 (B i ) for transmitting a polling signal and data, and a transmission output control section  131 (B i ) for controlling the transmission output of the transmission section  13 (B i ). 
     The base station B i  further includes a control section  14 (B i ) for controlling the overall base station B i , a mobile station position determination section  15 (B i ) for determining the position of each mobile station in the radio zone in accordance with a response from each mobile station to a polling signal, a polling transmission interval generating timer  17 (B i ) for generating time intervals at which polling signals are transmitted, a switch  11 (B i ) for switching the transmission/reception state of the base station B i  under the control of the control section  14 (B i ), and an interface section  19 (B i ) connected to, for example, a network that connects base stations. 
     FIG. 5 shows an example of the mobile station T j  (j=1, 2, . . . , m). The mobile station T j  includes an antenna  10 (T j ) for receiving a polling signal from the base station B i  and transmitting/receiving data, a reception section  12 (T j ) for receiving a polling signal and data, a transmission section  13 (T j ) for transmitting data, and a pooling response determination section  132 (T j ) for determining whether to respond to a polling signal from the base station B i . 
     The mobile station T j  further includes a control section  14 (T j ) for controlling the overall mobile station T j , a self-position determination section  16 (T j ) for determining the position of the self-station in the radio zone from the transmission output value obtained from a polling signal from the base station B i , a polling response timing generation timer  18 (T j ) for generating the timing of a response to a polling signal, a switch  11 (T j ) for switching the transmission/reception state of the mobile station T j  under the control of the control section  14 (T j ), and an interface section  19 (T j ) connected to a display, input button, or the like. 
     In the base station B i  in FIG. 4, the control section  14 (B i ) controls the polling transmission interval generating timer  17 (B i ) to generate the timing of transmission of a polling signal. In accordance with this timing, the control section  14 (B i ) sends, to the transmission section  13 (B i ), an instruction to transmit a polling signal. 
     The control section  14 (B i ) controls the transmission output control section  131 (B i ) to adjust the transmission output of a polling signal, as shown in FIG.  6 . FIG. 6 shows a case wherein polling signals are output at predetermined intervals (time ti). However, the control section  14 (B i ) may control the polling transmission interval generating timer  17 (B i ) to transmit polling signals irregularly instead of transmitting them at predetermined intervals. 
     The transmission section  13 (B i ) changes the transmission output of a polling signal in accordance with the transmission output value sent from the transmission output control section  131 (B i ), and transmits the polling signal through the switch  11 (B i ) and the antenna  10 (B i ). At this time, the switch  11 (B i ) has already been switched to the transmission section  13 (B i ) side. The switch  11 (B i ) is switched to the transmission side when a polling signal or other data is to be transmitted, but is always switched to the reception side except for such an occasion. 
     As shown in FIG. 6, the base station B i  changes the transmission output of a polling signal to transmit a long-distance polling signal having a large transmission output and a short-distance polling signal having a small transmission output, thereby forming a radio zone constituted by a large zone BZ i  having a large coverage and a small zone SZ i  having a small coverage. At this time, as shown in FIG. 1 or  7 , such radio zones are formed such that the small zones SZ i  and SZ i+1  formed by the adjacent base stations B i  and B i+1  do not overlap. 
     FIG. 8 shows an example of the data format of a polling signal transmitted from the base station B i . The polling signal is made up of flags indicating the start and end of the polling signal, a base station ID for identifying the base station B i  that has originated the polling signal, a transmission output value set when the polling signal is transmitted, and data consisting of other control codes and various information. The control section  14 (B i ) generates a polling signal in accordance with this data format and sends it to the transmission section  13 (B i ). This signal is then transmitted through the switch  11 (B i ) and the antenna  10 (B i ). 
     Upon reception of the polling signal from the base station, the mobile station T j  processes the polling signal through the control section  14 (T j ) to obtain the base station ID and the transmission output value. The control section  14 (T j ) sends the base station ID and the transmission output value to the self-position determination section  16 (T j ). The self-position determination section  16 (T j ) processes the base station ID and the transmission output value to check whether the mobile station T j  is located in the large or small zone in the radio zone formed by the base station. 
     When the mobile station T j  receives the polling signal from the base station, the control section  14 (T j ) instructs the polling response timing generation timer  18 (T j ) to generate the timing of transmission of a polling response signal. As shown in FIG. 9, the polling response timing generation timer  18 (T j ) generates a response timing td by randomly selecting one of random slots. 
     The control section  14 (T j ) generates a polling response signal and sends it to the transmission section  13 (T j ) in accordance with the response timing td. This signal is then transmitted through the switch  11 (T j ) and the antenna  10 (T j ). When a polling response signal or another data is to be transmitted, the switch  11 (T j ) is switched to the transmission side, but is always switched to the reception side except for such an occasion. 
     The transmission timing of a polling response signal is set such that a plurality of slots are generated a predetermined period of time after reception of a polling signal, one slot is randomly selected from the generated slots, and the polling response signal is transmitted in the selected slot. The time required for the generation of random slots, the number of slots, and the time per slot remain constant throughout all the mobile stations. For this reason, polling response signals do not interfere with each other unless mobile stations that are to respond to the same polling signal select the same slot. 
     FIG. 10 shows an example of the data format of a polling response signal. The polling response signal is made up of flags indicating the start and end of the polling response signal, a mobile station ID for identifying the mobile station T j  that has originated the polling response signal, a base station ID for identifying the base station that has received the polling response signal, and data consisting of other control codes and various information. The control section  14 (T j ) generates a polling response signal in accordance with this data format and sends it to the transmission section  13 (T j ). This signal is then transmitted through the switch  11 (T j ) and the antenna  10 (T j ). 
     Upon reception of the polling response signal from the mobile station, the base station B i  processes the polling response signal through the control section  14 (B i  ) to obtain the mobile station ID. The mobile station position determination section  15 (B i  ) checks whether the received polling response signal is a response to a long-distance polling signal or short-distance polling signal, thereby checking whether the mobile station corresponding to the obtained mobile station ID is located inside or outside the small zone SZ i  of the radio zone formed by the base station B i . 
     FIG. 11 shows how polling signals and polling response signals are exchanged in a case wherein base stations and mobile stations are arranged as shown in FIG.  3 . FIG. 11 shows the output levels of the respective signals along the time axis extending to the right. The base stations B i  and B i+1  alternately originate long-distance polling signals and short-distance polling signals at predetermined time intervals t. At this time, synchronization is not necessarily established between the base stations B i  and B i+1 . 
     Since the mobile station T j  is located inside the small zone of the base station B i , the mobile station T j  receives both short- and long-distance polling signals from the base station B i , and hence responds to them. Since the mobile station T j+1  is located in a place where the large zone of the base station B i  overlaps the large zone of the base station B i+1 , the mobile station T j+1  responds to long-distance polling signals from the two base stations. The mobile station T j+2  is located inside the large zone of the base station B i+1 , and hence responds to a long-distance polling signal from the base station B i+1 . The mobile station T j+3  is located outside the radio zones of the two base stations, and hence originates no polling response signal. 
     As described above, since the timing of transmission of a polling response signal is generated by selecting one of random slots through the polling response timing generation timer, the probability that polling response signals responding to the same polling signal interfere with each other is low. 
     Referring to FIG. 11, the output level of each polling response signal is set to be almost equal to that of each long-distance polling signal. However, the output level of a polling response signal may be changed in accordance with the transmission output value contained in a polling signal. When a polling signal with a small transmission output value is received, the power consumed by the mobile station can be saved by returning a polling response signal with an output value similar to the transmission output value. 
     As described above, a polling signal and a polling response signal respectively contain the corresponding base station ID or mobile station ID. For this reason, when these signals are received, specific stations from which the signals originated can be determined. Therefore, the base station can determine whether the mobile station located around the base station is located inside or outside the small zone. The mobile station can also determine, from the received polling signal, the position of itself within a specific zone of a specific base station. 
     Although a given base station may receive a polling signal originated from another base station, since a base station ID is contained in the polling signal, the signal does not interfere with a polling response signal. Although a given mobile station may receive a polling response signal originated from another mobile station, since a mobile station ID is contained in the polling response signal, the signal does not interfere with a polling signal from a base station. 
     Information originated from the base station includes various data as well as a polling signal. When different data need to be transmitted from the base station B i  to a plurality of mobile stations, the base station B i  transmits first the data to mobile stations located outside the small zone in the radio zone formed by the base station B i  on the basis of the determination result obtained by the mobile station position determination section  15 (B i ). After completion of data communication to mobile stations outside the small zone, the base station B i  transmits the data to mobile stations inside the small zone. 
     FIG. 12 shows another arrangement of a base station in the present invention. According to this arrangement, a mobile station presence determination section  20 (B i ) is added to the arrangement of the base station described above and is connected to the control section  14 (B i ). As described above, the base station B i  alternately transmits long- and short-distance polling signals. 
     If a polling response signal is returned in response to a long-distance polling signal, the mobile station presence determination section  20 (B i ) determines that the mobile station is present inside the large zone formed by the base station B i . Otherwise, the mobile station presence determination section  20 (B i ) determines that the mobile station is not present in the large zone. The mobile station presence determination section  20 (B i ) then sends the corresponding information to the control section  14 (B i ). When the control section  14 (B i ) determines, on the basis of the determination result sent from the mobile station presence determination section  20 (B i ), that the mobile station is not present in the large zone formed by the base station B i , the control section  14 (B i ) stops the origination of a short-distance polling signal, and continuously transmits a long-distance polling signal. If it is determined that the mobile station is present in the large zone, long- and short-distance polling signals are alternately originated, as described above. 
     FIG. 13 shows another arrangement of the mobile station in the present invention. According to this arrangement, a response determination section  30 (T j ), a transmission output value buffer  31 (T j ), and a transmission output value determination section  32 (T j ) are added to the arrangement of the above mobile station and are connected to the control section  14 (T j ). 
     Upon reception of a polling signal from a base station, the control section  14 (T j ) processes the polling signal to obtain the transmission output value of the polling signal in the above-described manner. The control section  14 (T j ) sends the transmission output value to the response determination section  30 (T j ). The response determination section  30 (T j ) sends the transmission output value sent from the control section  14 (T j ) to the transmission output value buffer  31 (T j ). 
     The transmission output value buffer  31 (T j ) includes two transmission output value memories  311 (T j ) and  312 (T j ) capable of storing transmission output values. Upon reception of a transmission output value from the response determination section  30 (T j ), the transmission output value buffer  31 (T j ) stores the transmission output value, stored in the transmission output value memory  311 (T j ), in the transmission output value memory  312 (T j ), and stores the transmission output value, sent from the response determination section  30 (T j ), in the transmission output value memory  311 (T j ). 
     If no data is stored in the transmission output value memory  311 (T j ), the transmission output value sent from the response determination section  30 (T j ) may be simply stored in the transmission output value memory  311 (T j ). With this operation, the transmission output value of the currently received polling signal and the transmission output value of the previously received polling signal are respectively stored in the transmission output value memories  311 (T j ) and  312 (T j ). 
     If no transmission output value is sent from the control section  14 (T j ) for a predetermined period of time, the response determination section  30 (T j ) clears the transmission output value memories  311 (T j ) and  312 (T j ) in the transmission output value buffer  31 (T j ). The transmission output value determination section  32 (T j ) refers to/processes the transmission output value in the transmission output value buffer  31 (T j ) to check whether the polling signal is a long- or short-distance polling signal, and sends the determination result to the response determination section  30 (T j ). The response determination section  30 (T j ) generates and outputs a determination result indicating whether to transmit a polling response signal on the basis of the determination result sent form the transmission output value determination section  32 (T j ). 
     FIG. 14 shows the determination results sent from the transmission output value determination section  32 (T j ) and the response determination results obtained by the response determination section  30 (T j ). As shown in FIG. 14, the determination results sent from the transmission output value determination section  32 (T j ) are classified into six patterns. Of the six patterns, patterns  1  to  4  correspond to whether the mobile station is located inside the large or small zone in the radio zone formed by the base station. 
     Pattern  1  corresponds to a case wherein the mobile station is located inside the large zone of the base station. Patterns  2  and  3  correspond to a case wherein the mobile station is located inside the small zone. Pattern  4  corresponds to a case wherein the mobile station is located in the small zone but fails to receive a long-distance polling signal for some reason, and the short-distance polling signal received before last is left in the transmission output value buffer  31 (T j ). Patterns  5  and  6  correspond to a case wherein the transmission output value buffer  31 (T j ) is empty because, for example, the mobile station is located in an area that does not belong to any of the radio zones of the base stations, and no polling signal is received within a predetermined period of time. 
     The response determination section  30 (T j ) determines, on the basis of the determination result obtained by the transmission output value determination section  32 (T j ), a specific pattern, of the six patterns, to which the current and previous polling signals correspond. Assume that the response determination section  30 (T j ) determines pattern  2 , in which the currently received polling signal is a long-distance polling signal and the previously received polling signal is a short-distance polling signal. In this case, a determination result indicating that no response is to be made is output to the control section  14 (T j ). If pattern  1 ,  3 ,  4 ,  5 , or  6  is determined, a determination result indicating that a polling response signal is to be output to the base station in response to the polling signal is output to the control section  14 (T j ). On the basis of this determination result, the control section  14 (T j ) controls transmission/non-transmission of a polling response signal. 
     According to the above description of the embodiment of the present invention, the transmission output of a polling signal is controlled in two steps. However, the transmission output of a polling signal may be controlled in three or more steps. However, since the propagation of radio waves is influenced by environments, when transmission output control is performed in multiple steps, a distinctive radio zone may not be formed in some case. For this reason, the number of steps must be set within the range in which the boundaries between radio zones remain distinct. 
     In addition, according to the description of the embodiment of the present invention, small zones are formed by base stations such that the small zones formed by the respective adjacent base stations do not overlap, as shown in FIG.  7 . However, the present invention is not limited to this arrangement. The sizes of large and small zones can be arbitrarily set in accordance with the system to which the present invention is applied. 
     Although the number of radio channels is not specifically limited in the present invention, the present invention is effective especially for radio communication using a signal frequency.