Patent Publication Number: US-2007117513-A1

Title: Wireless communication system, weight control apparatus, and weight vector generation method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-333202, filed Oct. 31, 2000, the entire contents of which are incorporated herein by reference.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a wireless communication system for making space-division multiple access, a weight control apparatus, and a weight vector generation method.  
      2. Description of the Related Art  
      In an FWA (Fixed Wireless Access) system, a base station and fixed terminal station make wireless, high-speed communications. In a point-to-multipoint (P-MP) system, a base station communicates with a plurality of terminal stations. In the P-MP system, space-division multiplex access (SDMA) is known as means for increasing the subscriber capacity. As shown in  FIG. 16 , an adaptive array  1002  arranged in a base station  1001  forms orthogonal beams that do not interfere with each other. These beams allow simultaneous communications of a plurality of terminals  1003 .  
       FIG. 17  shows an arrangement of the adaptive array  1002  when the multiplexing degree=2. In beam forming circuits  1051  and  2051 , appropriate weight vectors are set in weighting devices  1501  to  1504  and  2501  to  2504 . These weighting devices and combiners  1512  and  2512  weight and combine the outputs from antenna elements  1011  to  1014 , thereby forming orthogonal beams having maximum directionality toward one terminal, and null directionality toward the other terminal.  
      The same applies to a case wherein the multiplexing degree is more than 3. That is, orthogonal beams are formed to have maximum directionality toward the objective terminal, and null directionality toward a plurality of remaining terminals.  
      In the conventional system, weights for forming orthogonal beams used to attain space-division multiplex access are individually calculated and held in correspondence with all combinations of terminals while considering the multiplexing degree for a given number of base stations. For example, when the multiplexing degree=2 and the total number of terminals=n, the number of combinations of weights to be calculated and held as combinations of terminals is n×(n−1). Therefore, the number of combinations of weights to be held becomes huge with increasing number of registered terminals which must undergo space-division multiplex access.  
      A wireless communication system that makes packet communications by CSMA/CA is known.  
       FIG. 22  shows the arrangement of an IEEE802.11 wireless LAN system using CSMA/CA. A base station  900  senses carriers before transmission of packets to a terminal  911 . Upon receiving packets containing channel reserve information from a terminal  913 , the base station  900  postpones packet transmission during that reserved period. After that, the base station  900  waits for a random time period (back-off process) calculated by a controller  901 , and transmits packets addressed to a target terminal  911 . If the received data is correct, the target terminal  911  transmits an acknowledge response packet (ACK) to the base station after an elapse of a predetermined period of time. If the base station  900  fails to receive ACK from the target terminal  911  after an elapse of a predetermined period of time, it executes the back-off process using the controller  901  and transmits data again.  
      On the other hand, an adaptive array can improve communication quality by forming beams that reduce interference from a neighboring cell, as shown in  FIG. 23 .  
      The adaptive array generally makes beam control on the basis of a received signal. For example, a direction-constrained power minimization method suppresses all received signals as interfering signals while holding the gain in a specific direction. Therefore, if a situation in which only an interference wave from a neighboring cell arrives is formed, and the direction-constrained power minimization method is used in this situation, beams that can cover a specific area in the self cell and can remove the interference wave can be formed.  
      However, in order to suppress all co-channel interference waves from a neighboring cell, the number of antenna elements must be increased, resulting in a large apparatus scale.  
      Since the IEEE802.11 wireless LAN system using CSMA/CA does not make integrated control of packet transmission, as described above, it is difficult to form a situation in which only terminals or base stations to be suppressed transmit packets. As a result, a terminal of the self cell transmits packets in place of the terminal or base station to be suppressed, and unwanted beams that suppress such packets are formed.  
     BRIEF SUMMARY OF THE INVENTION  
      The present invention has been made in consideration of the above situation, and has as its object to provide a wireless communication system, weight control apparatus, and weight vector generation method, which can prevent the number of weights to be held from increasing abruptly even when the number of terminals increases, since orthogonal beams are not prepared in correspondence with combinations of terminals upon space-division multiplex access.  
      It is another object of the present invention to reduce the apparatus scale by limiting the number of interference waves to be suppressed in a wireless communication system that makes packet communications using CSMA/CA, and to form a situation in which only interference waves to be suppressed are present so as to form beams which remove these interference waves.  
      According to one aspect of the present invention, there is provided a wireless communication system which allows simultaneous communications with a plurality of terminals by space-division multiplex access, comprising: a plurality of antenna elements that receive a signal transmitted from the terminals; a plurality of beam forming units configured to output reception signals corresponding to beams having predetermined directionality patterns by weighting and combining reception signals received by the plurality of antenna elements; and a controller configured to set weight vectors used to control weighting and combining in the plurality of beam forming units, wherein when the controller receives a registration request from an unregistered terminal, the controller calculates and stores at least one of the weight vectors used to form a beam having null directionality toward the unregistered terminal and maximum directionality toward a specific area other than an area where the unregistered terminal is located of areas obtained by dividing a cover area covered by the wireless communication system. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING  
       FIG. 1  is a view for explaining an FWA system according to the first embodiment of the present invention;  
       FIG. 2  is a block diagram showing an example of the arrangement of an adaptive array according to the first embodiment;  
       FIG. 3  shows a cover area of a base station according to the first embodiment, which is divided into a plurality of areas;  
       FIG. 4  is a block diagram showing an example of the arrangement of a terminal classification unit of the adaptive array according to the first embodiment;  
       FIG. 5  shows beams formed by initial weights according to the first embodiment;  
       FIG. 6  is a flow chart showing an example of a registration sequence of a terminal according to the first embodiment;  
       FIG. 7  shows beams formed by initial weights according to the first embodiment;  
       FIG. 8  shows beams having null directionality toward terminals according to the first embodiment;  
       FIG. 9  shows beams having null directionality toward terminals according to the first embodiment;  
       FIG. 10  is a table showing an example of the storage contents of a correspondence storage unit according to the first embodiment;  
       FIG. 11  is a flow chart showing an example of a space-division multiplex access sequence for two terminals according to the first embodiment;  
       FIG. 12  shows beams upon space-division multiplex access for two terminals according to the first embodiment;  
       FIG. 13  is a block diagram showing an example of an adaptive array according to the second embodiment of the present invention;  
       FIG. 14  is a flow chart showing an example of a registration sequence of a terminal according to the second embodiment;  
       FIG. 15  is a table showing an example of the storage contents of a correspondence storage unit according to the second embodiment;  
       FIG. 16  is a view for explaining a conventional point-to-multipoint system;  
       FIG. 17  is a block diagram showing an example of the arrangement of a conventional adaptive array when the multiplexing degree is 2;  
       FIG. 18  is a diagram for explaining a wireless communication system according to the third embodiment of the present invention;  
       FIG. 19  is a flow chart showing a control method of an adaptive array for a wireless base station according to the third embodiment;  
       FIG. 20  is a diagram for explaining the internal arrangement of a controller of the adaptive array according to the third embodiment of the present invention;  
       FIG. 21  is a diagram for explaining a wireless communication system according to the fourth embodiment of the present invention;  
       FIG. 22  is a diagram for explaining a conventional IEEE802.11 wireless LAN system using CSMA/CA; and  
       FIG. 23  is a diagram for explaining conventional adaptive array antennas. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      Embodiments of the present invention will be described hereinafter with reference to the accompanying drawings.  
     First Embodiment  
       FIG. 1  shows an example of an FWA (Fixed Wireless Access) system including a base station  1  to which the first embodiment of the present invention is applied.  
      A base station  1  comprises an adaptive array (wireless communication system)  2 , and can make simultaneous communications with a plurality of terminals  3  via a single channel by avoiding interference between terminals using the adaptive array  2 . Note that  FIG. 1  illustrates a 90° sector, but the present invention is not limited to this.  
      Note that this embodiment will exemplify space-division multiplex access (SDMA) of two terminals using an adaptive array consisting of four antenna elements.  
       FIG. 2  shows an example of the arrangement of the adaptive array arranged in the base station according to this embodiment.  
      As shown in  FIG. 2 , the adaptive array of this embodiment comprises m (m is a plural number; m=4 in  FIG. 2 ) antenna elements  11  to  14 , m amplifiers (low noise amplifiers)  21  to  24 , m frequency converters  31  to  34 , m distributors  41  to  44 , n (n is a plural number; n=2 in  FIG. 2 ) beam forming circuits  51  and  52 , n receivers  61  and  62 , and a weight controller  7 .  
      The beam forming circuit  51  includes m weighting devices  5111  to  5114 , and a combiner  512 . Likewise, the beam forming circuit  52  includes m weighting devices  5211  to  5214  and a combiner  522 .  
      Furthermore, the weight controller  7  includes a terminal classification unit  71 , terminal storage unit  72 , weight calculation unit  73 , weight storage unit  74 , and weight selection unit  75 .  
      Note that the base station may comprise a plurality of adaptive arrays shown in  FIG. 2 . For example, when the adaptive array shown in  FIG. 2  covers a 90° sector, the base station can cover 360° full directions if it comprises four adaptive arrays.  
      In the adaptive array shown in  FIG. 2 , reception signals received by the antenna elements  11  to  14  are respectively input to the corresponding amplifiers  21  to  24 . The amplifiers  21  to  24  respectively amplify the reception signals.  
      The reception signals amplified by the amplifiers  21  to  24  are respectively input to the corresponding frequency converters  31  to  34 . The frequency converters  31  to  34  respectively convert the frequency bands of the reception signals from radio frequency (RF) to intermediate frequency (IF) or baseband (BB).  
      The reception signals frequency-converted by the frequency converters  31  to  34  are respectively input to the corresponding distributors  41  to  44 . The distributors  41  to  44  distribute the reception signals and output them to the beam forming circuits  51  and  52 .  
      In this way, the beam forming circuit  51  receives the reception signals received by the antenna elements  11  to  14  from the distributors  41  to  44 . The same applies to the beam forming circuit  52 .  
      The reception signals input to the beam forming circuit  51  are respectively weighted by the weighting devices  5111  to  5114 , and are then combined by the combiner  512 . The combined signal is sent to the receiver  61 . The same applies to the beam forming circuit  52 .  
      The weights in the weighting devices  5111  to  5114  and  5211  to  5214  are appropriately set by the weight controller  7 .  
      In the above description, the reception signals are analog signals. Also, using A/D converters and the like, weighting can be done in a digital signal domain.  
      The weight controller  7  will be described in detail below.  
      Note that this embodiment will examine a case wherein the cover area (by the adaptive array in  FIG. 2 ) of the base station is divided into a plurality of areas in accordance with direction viewed from the base station, as shown in  FIG. 3 , and a given terminal belongs to one of the plurality of areas.  FIG. 3  exemplifies a case wherein when the cover area of the base station is divided into seven areas A 1  to A 7 , a terminal T 1  belongs to the area A 2 , and a terminal T 2  belongs to the area A 5 . The area configuration shown in  FIG. 3  will be taken as an example in the following description. Of course, the present invention may be applied even when the cover area is divided into another number of areas, or when the cover area is divided unequally.  
      The terminal classification unit  71  in the weight controller  7  estimates one of the plurality of areas obtained by dividing the cover area of the base station, where the target terminal  3  is located (one of the areas A 1  to A 7  in  FIG. 3 ) on the basis of the reception signal from the terminal  3 .  
      After the area to which the terminal  3  belongs is estimated, a correspondence between identification information for identifying the terminal  3 , and identification information for identifying the area to which the terminal  3  belongs, and which is estimated by the terminal classification unit  71  is stored as terminal-area correspondence information in a correspondence storage unit  72  (see  FIG. 10 ;  FIG. 10  shows an example of the contents after the terminals T 1  and T 2  have been registered, as will be described later).  
      The weight calculation unit  73  calculates a weight for forming a beam having null directionality toward the terminal  3  and maximum directionality toward the range of a specific area other than the area to which the terminal  3  belongs, on the basis of the contents of the terminal-area correspondence information stored in the correspondence storage unit  72  using the reception signal from the terminal  3  (null directionality points to that terminal, while maximum directionality points to (one of directions) the range of the corresponding area). This weight is calculated for a case wherein each of all areas other than the area to which the terminal  3  belongs or each of areas that satisfy a predetermined condition other than the area to which the terminal  3  belongs is assumed to be the specific area. In the example of the arrangement shown in  FIG. 2 , the weight is a four-dimensional vector.  
      The weights calculated by the weight calculation unit  73  are stored in the weight storage unit  74  so that the corresponding terminal (i.e., a terminal to which null directionality is pointed) and the corresponding area (an area to which maximum directionality is pointed) can be specified.  
      Upon making space-division multiplex access (SDMA) for two terminals, when the weight used to form a beam having null directionality toward the second terminal and maximum directionality toward the range of the area to which the first terminal belongs, and the weight used to form a beam having null directionality toward the first terminal and maximum directionality toward the range of the area to which the second terminal belongs, are stored in the weight storage unit  74 , the weight selection unit  75  sets these two weights respectively in the weighting devices  5111  to  5114  of the beam forming circuit  51  and the weighting devices  5211  to  5214  of the beam forming circuit  52 . In this fashion, even when an identical channel is used, the beam forming circuit  51  can supply the reception output from the first terminal to the receiver  61 , and the beam forming circuit  52  can supply the reception output from the second terminal to the receiver  62  (of course, correspondence between the terminals and the beam forming circuits/receivers may be reversed). In the example of  FIG. 3 , these two weights correspond to a weight used to form a beam having null directionality toward the terminal T 2  and maximum directionality toward the range of the area A 2  to which the terminal T 1  belongs, and a beam having null directionality toward the terminal T 2  and maximum directionality toward the range of the area A 5  to which the terminal T 2  belongs.  
      The arrangement of the terminal classification unit  71  will be described in more detail below.  
       FIG. 4  shows an example of the internal arrangement of the terminal classification unit  71 .  
      An initial weight storage device  711  stores a weight used to form a beam having maximum directionality toward the corresponding area (in a direction of, e.g., its central portion), as an initial weight for each of the plurality of areas obtained by dividing the cover area of the base station. For example, in case of  FIG. 3 , seven different initial weights, i.e., an initial weight used to form a beam having maximum directionality toward the area A 1 , . . . , an initial weight used to form a beam having maximum directionality toward the area A 7 , are stored. Note that the initial weights do not particularly consider the directions of null directionality.  
      B 1 , B 2 , B 3 , . . . , B 7  respectively represent beams having maximum directionality toward the areas A 1 , A 2 , A 3 , . . . , A 7 , and W 1 , W 2 , W 3 , . . . , W 7  represent weights used to form these beams B 1 , B 2 , B 3 , . . . , B 7 .  
       FIG. 5  exemplifies the states of the beams B 1 , B 2 , B 3 , . . . , B 7  formed using the weights W 1 , W 2 , W 3 , . . . , W 7 .  
      A reception state observation device  712  observes a signal from the terminal  3  when upon setting the initial weights stored in the initial weight storage device  711  in the weighting devices  5111  to  5114  of the beam forming circuit  51 , and determines an area to which maximum directionality of a beam that can assure the best reception state of the signal from the terminal  3  (i.e., an area pointed by maximum directionality of a beam formed by the initial weight used in that state) is pointed as an area to which the terminal  3  belongs. The reception state observation device  712  outputs, as the observation result, identification information of the terminal, and that of the area to which that terminal belongs, to the correspondence storage unit  72 .  
      Note that the “best reception state” evaluated by the reception state observation device  712  is a reception state with maximum reception power. Alternatively, for example, the best reception state may be defined as a reception state with a maximum signal to noise power ratio.  
      In the above description, the beam forming circuit  51  is used in observation. Of course, the beam forming circuit  52  may be used. Also, one of the beam forming circuits  51  and  52  may be fixedly used, or may be selected by a predetermined method in each observation. Furthermore, different initial weights may be set in the two beam forming circuits to observe signals parallel to each other. In this way, the time required for evaluation can be shortened.  
      With the aforementioned arrangement, an area in which the terminal is currently located can be estimated using only the output from the beam forming circuit  51  or  52 .  
      The weight calculation unit  73  will be explained below.  
      The weight calculation unit  73  makes predetermined repetitive calculations based on the reception signal from the terminal  3  using the initial weight (i.e., an initial weight used to form a beam having maximum directionality toward a given area) stored in the initial weight storage device  711  as an initial value, thus calculating a weight used to form a beam (having maximum directionality toward a given area and) having null directionality toward the terminal  3  (i.e., the unit  73  repeats calculations of a weight value to converge the direction of null directionality to that of the terminal  3 ).  
      In a practical process for this purpose, a weight vector used to form a beam having a directionality gain in a desired direction is set as an initial value, and null directionality is set in an incoming wave direction while this gain is maintained. Required information is only output power of the beam forming circuit.  
      The operation sequence for making space-division multiple access (SDMA) for two terminals by the base station of this embodiment will be explained below.  
      A case will be examined below wherein the two terminals newly subscribe to a service in the order of terminal T 1 →terminal T 2 . As shown in  FIG. 3 , the terminal T 1  is located within the area A 2 , and the terminal T 2  is located within the area A 5 .  
      A registration process when a new terminal subscribes will be explained first, and a process upon making space-division multiple access (SDMA) for two terminals will be explained.  
       FIG. 6  shows an example of the registration sequence of a new terminal.  
      The registration sequence when the terminal T 1  subscribes as a new terminal will be explained.  
      The terminal T 1  sends a registration request signal to the base station (step S 1 ).  
      The base station receives the signal from the terminal T 1 . When the base station recognizes that the received signal is a registration request, it assigns an empty channel to the terminal T 1 , and instructs the terminal T 1  to send a signal with predetermined power in that assigned channel (step S 2 ). The predetermined power value must be appropriately set to eliminate the influence of overreach interference from a neighboring cell.  
      The terminal T 1  sends a signal using the assigned channel and instructed transmission power (step S 3 ).  
      During this interval, in the base station, the reception state observation device  712  checks the reception power from the terminal T 1  upon setting the initial weights W 1 , W 2 , W 3 , . . . , W 7  stored in the initial weight storage device  711  in the weighting devices  5111  to  5114  of the beam forming circuit  51 , respectively, and stores in the corresponding storage unit  72  the area A 2  ( FIG. 7 ) to which maximum directionality of the beam B 2  corresponding to the maximum reception power is pointed as an area to which the terminal T 1  belongs (step S 4 ).  
      After that, the weight calculation unit  73  calculates a weight used to form a beam having null directionality toward the terminal T 1 . At this time, in this example, as for an area to which maximum directionality is pointed, a condition that areas except for the area A 2  to which the terminal T 1  belongs must be separated a predetermined angle or more from the area to which the terminal T 1  belongs is imposed. Also, as another condition, such area is separated one or more areas from the area to which the terminal T 1  belongs. Under these conditions, areas to which maximum directionality are pointed are four areas A 4 , A 5 , A 6 , and A 7 .  
      More specifically, as shown in  FIG. 8 , weights W_A 4 _T 1  , W_A 5 _T 1  , W_A 6 _T 1  , and W_A 7 _T 1  used to form beams B_A 4 _T 1 , B_A 5 _T 1 , B_A 6 _T 1 , and B_A 7 _T 1  having null directionality toward the terminal T 1  and maximum directionality toward the areas A 4 , A 5 , A 6 , and A 7 , which are separated a predetermined angle or more from the area A 2  to which the terminal T 1  belongs, of those except for the area A 2  are calculated (steps S 5  and S 6 ).  
      In this case, upon calculating the weight W_A 4 _T 1 , a weight used to form a beam having null directionality toward the terminal T 1  is calculated by repetitive calculations based on the reception signal from the terminal T 1  using the initial weight W 4  stored in the initial weight storage device  711  as an initial value. Likewise, the weights W_A 5 _T 1 , W_A 6 _T 1 , and W_A 7 _T 1  are calculated using the initial weights W 5 , W 6 , and W 7  as initial values.  
      These weights W_A 4 _T 1 , W_A 5 _T 1 , W_A 6 _T 1 , and W_A 7 _T 1  calculated by the weight calculation unit  73  are stored in the weight storage unit  74  (steps S 5  and S 6 ).  
      When maximum directionality is separated a predetermined angle or more from null, as described above, the gain difference between maximum directionality and null can be set to be a predetermined value or more.  
      With the aforementioned processes, since registration of the terminal T 1  is complete, the base station instructs the terminal T 1  to end signal transmission (step S 7 ).  
      Subsequently, assume that the terminal T 2  newly subscribes to this service.  
      In this case as well, the same sequence as in  FIG. 6  is executed. That is, the area A 5  to which maximum directionality of the beam B 5  corresponding to the maximum reception power from the terminal T 2  is pointed is stored as an area to which that terminal belongs in the correspondence storage unit  72 . After that, as shown in  FIG. 9 , weights W_A 1 _T 2 , W_A 2 _T 2 , W_A 3 _T 2 , and W_A 7 _T 2  used to form beams B_A 1 _T 2 , B_A 2 _T 2 , B_A 3 _T 2 , and B_A 7 _T 2  having null directionality toward the terminal T 2  and maximum directionality toward the areas A 1 , A 2 , A 3 , and A 7 , which are separated a predetermined angle or more from the area A 5  to which the terminal T 2  belongs, of those except for the area A 5  are calculated. In this case, the initial weights W 1 , W 2 , W 3 , and W 7  are used as initial values. The weights calculated by the weight calculation unit  73  are stored in the weight storage unit  74 . With the aforementioned process, since registration of the terminal T 2  is complete, the base station instructs the terminal T 2  to end signal transmission.  
       FIG. 10  shows an example of the storage contents of the correspondence storage device  72  upon completion of registration of the terminals T 1  and T 2 .  
      In this embodiment, the registration sequence for other terminals T 3 , T 4 , . . . when these terminals T 3 , T 4 , . . . newly subscribe to the service after the terminals T 1  and T 2  is executed in the same manner as described above.  
      A process upon making space-division multiplex access (SDMA) for the two terminals T 1  and T 2  will be explained below.  
       FIG. 11  shows an example of the processing sequence in this case.  
      The terminals T 1  and T 2  send communication request signals to the base station (step S 8 ).  
      The base station receives the signals from the terminals T 1  and T 2 . When the base station recognizes that these signals are communication requests, since the weight W_A 5 _T 1  used to form the beam B_A 5 _T 1  having. null directionality toward the terminal T 1  and maximum directionality toward the area A 5  to which the terminal T 2  belongs, and the weight W_A 2 _T 2  used to form the beam B_A 2 _T 2  having null directionality toward the terminal T 2  and maximum directionality toward the area A 5  to which the terminal T 1  belongs are stored in the weight storage device  74 , the weight selection unit  75  sets, e.g., the weight W_A 5 _T 1  in the weighting devices  5111  to  5114  of the beam forming circuit  51 , and the weight W_A 2 _T 2  in the weighting devices  5211  to  5214  of the beam forming circuit  52  (step S 9 ). Of course, the correspondence between the weights and beam forming circuits may be different from (reversed to) that described above.  
       FIG. 12  exemplifies the states of the beams at that time.  
      After that, an identical channel is assigned to the terminals T 1  and T 2  (step S 10 ).  
      In this way, signals from the terminals T 1  and T 2  can be respectively received by the receivers  62  and  61  without interfering with each other (of course, when the correspondence between the weights and beam forming circuits is reversed to that described above, signals from the terminals T 1  and T 2  are respectively received by the receivers  61  and  62 ).  
      When a communication is made with only one terminal (e.g., T 1 ) after the weights that consider null directionality are calculated, the communication may be made using the initial weight (e.g., W 2 ) used to form a beam having maximum directionality toward the area A 2  to which the terminal T 1  belongs, or using one of weights (e.g., W_A 2 _T 2 ) used to form a beam having maximum directionality toward the area A 2  to which the terminal T 1  belongs and null directionality toward a terminal in another area.  
     Second Embodiment  
      Since this embodiment is basically the same as the first embodiment, differences between this embodiment and the first embodiment will be mainly explained below.  
       FIG. 13  shows an example of the arrangement of an adaptive array arranged in a base station according to this embodiment.  
      As shown in the example of the arrangement of the adaptive array in  FIG. 13 , this embodiment is different from the first embodiment in that an initial value selection unit  76  is added to the example of the arrangement of the adaptive array of the first embodiment shown in  FIG. 2 .  
      This initial value selection unit  76  selects the weight stored in the initial weight storage device  711  of the terminal classification unit  71  or that (used to form a beam having null directionality toward a given terminal and maximum directionality toward the range of a given area) stored in the weight storage unit  74  on the basis of the registration record (terminal-area correspondence information) stored in the correspondence storage unit  72 , and inputs the selected weight as an initial value to the weight calculation unit  73 .  
      When the weight storage unit  74  stores a plurality of weights that can be selected, one of these weights can be selected based on a predetermined criterion (e.g., random or appropriate order, a weight generated first, a weight generated latest, or the like).  
      A process executed when a terminal T 3  which belongs to the same area as the terminal T 2  newly subscribes to a service after registration of the terminals T 1  and T 2  will be explained below.  
      The process at that time is basically the same as that in the registration sequence of the terminals T 1  and T 2  mentioned above, except that the initial value selection unit  76  selects an initial value of a weight in weight calculations as needed.  
       FIG. 14  shows an example of the processing sequence in such case (which has basically the same flow of sequence as in  FIG. 6 ).  
      The terminal T 3  sends a registration request signal to the base station (step S 11 ).  
      The base station receives the signal from the terminal T 3 . When the base station recognizes that the received signal is a registration request, it assigns an empty channel to the terminal T 3 , and instructs the terminal T 3  to send a signal with predetermined power (step S 12 ).  
      The terminal T 3  sends a signal with the designated power in the assigned channel (step S 13 ).  
      The base station stores the area A 5  to which maximum directionality of the beam B 5  corresponding to the maximum reception power from the terminal T 3  in the correspondence storage unit  72  is pointed as an area to which that terminal belongs (step S 14 ).  
       FIG. 15  shows an example of the storage contents of the correspondence storage unit  72  upon completion of registration of the terminal T 3 .  
      After that, the initial value selection unit  76  selects an initial value of a weight to be input to the weight calculation unit  73 . Initially, the initial value selection unit  76  looks up the contents of the correspondence storage unit  72 . When the unit  76  detects that the terminal T 2  has already been registered in the area A 5  to which the terminal T 3  belongs, it inputs weights W_A 1 _T 2 , W_A 2 _T 2 , W_A 3 _T 2 , and W_A 7 _T 2 , which are stored in the weight storage unit  74  and have null directionality toward the terminal T 2 , to the weight calculation unit  73  as initial values of weights. If a plurality of terminals have already been registered in the area A 5  to which the terminal T 3  belongs, a weight corresponding to a minimum reception signal from the terminal T 3  may be used as an initial value.  
      If the terminal T 3  belongs to an area different from those of the already registered terminals T 1  and T 2 , it is detected that no terminal has been registered yet in the area to which the terminal T 3  belongs. In such case, the initial weights W 1 , W 2 , W 3 , and W 7  stored in the initial weight storage device  711  of the terminal classification unit  71  are input to the weight calculation unit  73  as initial values of weights (in this case, the same calculations as in the first embodiment are consequently made).  
      After that, the weight calculation unit  73  calculates weights W_A 1 _T 3 , W_A 2 _T 3 , W_A 3 _T 3 , and W_A 7 _T 3  used to form beams B_A 1 _T 3 , B_A 2 _T 3 , B_A 3 _T 3 , and B_A 7 _T 3  having null directionality toward the terminal T 3  and maximum directionality toward the areas A 1 , A 2 , A 3 , and A 7 , which are separated a predetermined angle or more from the area A 5  to which the terminal T 3  belongs, of those except for the area A 5  (steps S 15  and S 16 ).  
      The weights calculated by the weight calculation unit  73  are stored in the weight storage unit  74  (steps S 15  and S 16 ).  
      With the aforementioned processes, since registration of the terminal T 3  is complete, the base station instructs the terminal T 3  to end signal transmission (step S 17 ).  
      In this manner, when another terminal has already been registered in an identical area, the weights calculated for that already registered terminal can be used as initial values. In this case, since beams having null directionality toward the vicinities of the direction of the terminal T 3 , have already been formed while the initial values of the weights are set, the number of repetitive calculations required to calculate weights used to form beams having null directionality toward the terminal T 3  and maximum directionality toward the areas A 1 , A 2 , A 3 , and A 7 , can be reduced. Therefore, channels to be assigned to the terminal T 3  for null control can be reduced.  
      Note that the operation sequence upon making space-division multiplex access (SDMA) for two terminals by the base station of this embodiment is the same as that in the first embodiment.  
      For example, upon making space-division multiplex access (SDMA) for the two terminals T 1  and T 3 , the weight W_A 5 _T 1  used to form the beam B_A 5 _T 1  having null directionality toward the terminal T 1  and maximum directionality toward the area A 5  to which the terminal T 2  belongs, and the weight W_A 2 _T 3  used to form the beam B_A 2 _T 3  having null directionality toward the terminal T 3  and maximum directionality toward the area A 5  to which the terminal T 1  belongs, are used.  
      In the examples described in the first and second embodiments, the multiplexing degree is 2. Of course, the present invention can be applied to a case wherein the multiplexing degree is 3 or more. Even in such case, the number of weights to be held can be smaller than that in the prior art.  
      For example, when the multiplexing degree is 3, assuming that terminals T 1  to T 3  simultaneously make communications using a single channel, and respectively belong to areas A 1 , A 3 , and A 5 ,  
      for the terminal T 1 , a weight W_A 1 _T 2 _T 3  used to form a beam having null directionality toward the terminals T 2  and T 3  and maximum directionality toward the area A 1  to which the terminal T 1  belongs,  
      for the terminal T 2 , a weight W_A 3 _T 1 _T 3  used to form a beam having null directionality toward the terminals T 1  and T 3  and maximum directionality toward the area A 3  to which the terminal T 2  belongs, and  
      for the terminal T 3 , a weight W_A 5 _T 1 _T 2  used to form a beam having null directionality toward the terminals T 1  and T 2  and maximum directionality toward the area A 5  to which the terminal T 3  belongs,  
      can be respectively set in three beam forming circuits.  
      Even when the multiplexing degree are 3 or more, the same effect can be obtained when the initial value selection unit  76  of the second embodiment selects initial values for weight calculations.  
      As described above, according to this embodiment, an area to which a given terminal belongs of a plurality areas obtained by dividing the cover area of the base station in accordance with direction is estimated in place of preparing for orthogonal beams in correspondence with combinations of terminals, and weights used to form a beam having null directionality toward a terminal and maximum directionality toward a given area are held, thus reducing the number of weights to be held.  
      Also, according to this embodiment, weights used to form beams having maximum directionality toward a plurality of areas obtained by dividing the cover area of the base station are pre-stored, and an area to which maximum directionality of a beam which allows reception of a signal from a terminal with the highest reception power or signal to noise ratio upon setting these weights in the weighting devices is pointed is determined as an area to which the terminal belongs. Hence, an area to which the terminal belongs can be estimated using only the output from the beam forming means.  
      Furthermore, according to this embodiment, upon calculating weights used to form beams having null directionality toward a terminal and maximum directionality toward a given area, since weights used to form beams having maximum directionality toward the given area are set as initial values, the number of required repetitive calculations can be reduced.  
      Moreover, according to this embodiment, upon calculating weights used to form beams having null directionality toward a terminal and maximum directionality toward a given area, since weights used to form beams having null directionality toward the vicinities of the direction of the terminal are set as initial values, the number of required repetitive calculations can be reduced.  
      In addition, according to this embodiment, since weights used to form beams having null directionality toward a terminal and maximum directionality toward areas which are separated by the area to which the terminal belongs of those except for the area to which the terminal belongs are calculated, maximum directionality is separated a predetermined angle or more from null, thus setting the gain difference between maximum directionality and null to be a predetermined value or more.  
     Third Embodiment  
      In the third embodiment, the present invention is applied to an IEEE802.11 wireless LAN system using CSMA/CA. Since the conventional IEEE802.11 wireless LAN system using CSMA/CA does not make integrated control of packet transmission, as described above, it is difficult to obtain a situation in which only a terminal to be suppressed or base station transmits packets. As a result, a terminal of the self cell transmits packets in place of the terminal or base station to be suppressed, and unwanted beams that suppress such packets are formed. This embodiment solves this problem.  
       FIG. 18  shows an example of a wireless communication system according to the third embodiment of the present invention.  
      Base stations (AP)  111  to  114 ,  211  to  214 , and  311  to  314  respectively comprise transmission adaptive arrays (transmission SA)  131  to  134 ,  231  to  234 , and  331  to  334  and reception adaptive arrays (reception SA)  141  to  144 ,  241  to  244 , and  341  to  344 , which share array antennas  101  to  104 ,  201  to  204 , and  301  to  304 .  
      In  FIG. 18 , the transmission and reception adaptive arrays share the array antennas. Alternatively, the transmission and reception adaptive arrays may use independent array antennas.  
      An arbitrary location in a cell is covered by transmission and reception beams formed by the adaptive arrays of at least one AP. With this arrangement, terminals (STA)  121  to  124 ,  221  to  224 , and  321  to  324  located at arbitrary locations in cells can communicate with corresponding APs.  
      This embodiment will exemplify a case wherein interference waves from the APs  312  and  212 , which become interference sources for the AP  112  that uses Ch7, are reduced lower than the carrier sense level by the reception SA  142 .  
      The reason why objects to be suppressed are limited to APs, and the suppression level is limited to the carrier sense level is to reduce the required number of antenna elements, and to reduce the apparatus scale. Although the objects to be suppressed are limited, if the AP traffic is heavier than the STA, as packet transmission delay of the AP due to carrier sense of packets sent from a neighboring AP mainly deteriorates the throughput of the entire system, the effect of improving the throughput according to the present invention is great.  
       FIG. 19  shows a beam control method of the SA  142  in the third embodiment of the present invention.  
      A transmission beam of the transmission SA  132 , a reception beam of the reception SA  142 , a transmission beam of the transmission SA  131 , and a reception beam of the reception SA  141  are set to cover a single area.  
      Also, a transmission beam of the transmission SA  232 , a reception beam of the reception SA  242 , a transmission beam of the transmission SA  231 , and a reception beam of the reception SA  241  are set to cover a single area.  
      Furthermore, a transmission beam of the transmission SA  332 , a reception beam of the reception SA  342 , a transmission beam of the transmission SA  331 , and a reception beam of the reception SA  341  are set to cover a single area (step  101 ).  
      With this arrangement, roaming of the STAs  122 ,  222 , and  322  to the APs  111 ,  211 , and  311  can be made.  
      Subsequently, packet transmission from the transmission SAs  132 ,  232 , and  332  is suspended for time T (step  102 ). In this way, since the STAs  122 ,  222 , and  322  cannot communicate with the APs  112 ,  212 , and  312 , they roam to the APs  112 ,  212 , and  312  after a predetermined period of time.  
      It is checked if the STAs  122 ,  222 , and  322  respectively roam to the APs  112 ,  212 , and  312  (step  103 ). This operation can be made if the MAC layer can be monitored.  
      After confirmation of roaming to the STAs  122 ,  222 , and  322 , packet transmission from the transmission SA  132  is suspended (step  104 ). In this manner, since the AP  112  whose beam control is underway sends no beacon, roaming from the STAs to the AP  112  can be prevented.  
      The reception SAs  242  and  342  are set not to receive any packets (step  105 ). More specifically, the reception SAs  242  and  342  may be disconnected from the APs  212  and  312  by switches. Alternatively, the reception gains of the reception SAs  242  and  342  may be lowered. In this way, since the back-off process of the APs  212  and  312  due to carrier sense can be avoided, a problem of packet transmission delay can be solved.  
      The transmission SAs  232  and  332  send multi-cast packets (step  106 ). In this manner, since the multi-cast packets do not require any ACK reception from the STAs, and the back-off process due to ACK non-reception can be avoided, a problem of packet transmission delay can be solved. Especially, this embodiment is effective when the reception SAs  242  and  342  are set not to receive any packets.  
      The gain of the reception beam of the reception SA  142  in the main lobe direction is constrained (step  107 ).  
      Beam control is made based on the direction-constrained power minimization method, so that the reception power after combining of the reception SA  142  becomes equal to or lower than the carrier sense level (step  108 ).  
      This embodiment uses the direction-constrained power minimization method as a beam control algorithm. This algorithm suppresses all received signals as interference while maintaining a gain in a specific direction. Therefore, when a situation in which only interference waves from only the APs  212  and  312  arrive is formed, and the direction-constrained power minimization method is used, a beam that covers a specific area in the self cell and removes interference waves from the APs  212  and  312  can be formed.  
      With this control method, the number of interference waves as objects to be suppressed can be limited to reduce the apparatus scale, a situation in which only interference waves as objects to be suppressed are present can be set, and a beam that removes these interference waves can be formed.  
      Since the object to be suppressed is limited to a neighboring AP, this embodiment is particularly effective when the AP traffic is heavier than the STA. Since the suppression level is set to be less than the carrier sense-level, packet transmission delay of the AP due to carrier sense of packets sent from a neighboring AP can be avoided, thus improving the throughput of the entire system.  
      In this embodiment, setups of the APs and transmission and reception SAs may be locally directly controlled or may be controlled by a controller that makes integrated control. Upon making integrated control, either wired or wireless control may be used.  
      In this embodiment, a pair of transmission and reception SAs are provided to each AP. Alternatively, a plurality of APs may share a pair of transmission and reception SAs.  
      The internal arrangement of a controller of the adaptive array will be described below using  FIG. 20 .  
      Reception signals received by an array antenna  400  are respectively weighted by a reception beam forming circuit  404  to form a beam. The output from the reception beam forming circuit  404  is input to a reception switch  405 . When the reception switch is ON, the output from the reception beam forming circuit  404  is input to an AP  401 .  
      A packet sent from the AP  401  is input to a transmission switch  402 . When the transmission switch  402  is ON, a signal sent from the AP  401  is input to a transmission beam forming circuit  403 . After the input signal is weighted, that signal is transmitted from the array antenna, thus forming a beam.  
      An interference generation unit  407  comprises a reception disconnector  408  and multi-cast packet transmission controller  409 . The reception disconnector  408  sets the reception switch  405  OFF. The multi-cast packet transmission controller  409  sends a command to the AP  401  to send a multi-cast packet.  
      With this arrangement, when another AP controls weights to have the AP  401  as an interference source, since the AP  401  does not execute a back-off process due to carrier sense or ACK non-reception, a problem of packet transmission delay can be solved.  
      An interference suppression unit  410  comprises a transmission disconnector  411  and weight controller  412 . The transmission disconnector  411  sets the transmission switch  402  OFF. The weight controller  412  calculates and sets weights in the reception beam forming circuit  404  on the basis of the output from the reception beam forming circuit  404 .  
      With this arrangement, when another AP controls weights to have the AP  401  as an interference source, since no beacon is sent to a terminal, the terminal can be prevented from subscribing the AP  401  whose beam control is underway by roaming. Therefore, a situation in which no interference from the terminal is present can be formed. Also, the transmission signal itself from the AP  401  can be prevented from being received by the array antenna  400  to adversely influence the beam control.  
      In this embodiment, the AP  401  is directly controlled by only the multi-cast packet transmission controller, but may be controlled via the backbone  406 . In such case, the AP  401  does not require any additional functions to implement this embodiment, and an existing AP can be used. Also, the interference generation unit  407  and interference suppression unit  410  can be controlled via the backbone  406 .  
     Fourth Embodiment  
      Since this embodiment is basically the same as the third embodiment, only a difference between this embodiment and the third embodiment will be mainly explained below.  
       FIG. 21  shows an example of a wireless communication system according to the fourth embodiment of the present invention.  
      As shown in  FIG. 21 , this embodiment is different from the third embodiment in that neighboring cells use different radio channels, and an arbitrary location in a cell is covered by transmission and reception beams formed by adaptive arrays of at least three APs.  
      Since neighboring cells use different radio channels, the interference level from the neighboring AP lowers, resulting in a small interference suppression amount and a small apparatus scale.  
      Since each cell is covered by three beams, the STA can roam to two APs upon beam control.  
      A case will be exemplified below wherein interference waves from APs  211 ,  212 ,  311 , and  312 , which become interference sources for an AP  412  that uses Ch 5 , are reduced to less than the carrier sense level. STAs  422 ,  221 ,  222 ,  321 , and  322 , which are associated with APs  412 ,  211 ,  212 ,  311 , and  312 , can roam to APs  421  or  413 ,  214 ,  213 ,  314 , and  313 .  
      According to the third and fourth embodiments of the present invention, since the object to be suppressed is limited to a neighboring AP, and the suppression level is limited under the carrier sense level, the number of antenna elements can be reduced, and the apparatus scale can become small. Although the objects to be suppressed are limited, if the AP traffic is heavier than the STA, as packet transmission delay of the AP due to carrier sense of packets sent from a neighboring AP mainly deteriorates the throughput of the entire system, the effect of improving the throughput according to the present invention is great.  
      Since an arbitrary location in a cell is covered by transmission and reception beams of at least two APs, and the direction-constrained power minimization method is executed after roaming of the STA, a beam that suppresses a packet to be sent from the STA can be prevented from being formed. Also, the STA can communicate with another AP even during beam control.  
      Since an AP whose beam control is underway does not send any beacon, roaming of the STA can be prevented.  
      Since the reception SA of an interfering AP is set not to receive any packet, the interfering AP can be prevented from executing a back-off process due to carrier sense, thus solving a problem of packet transmission delay.  
      Since an interfering AP sends a multi-cast packet, which does not require ACK reception from the STA, and a back-off process due to ACK non-reception can be avoided, a problem of packet transmission delay can be solved.  
      Since neighboring cells use different radio channels, the interference level from a neighboring AP lowers, resulting in a small interference suppression amount and a small apparatus scale.  
      Since an arbitrary location in a cell is covered by three beams, the STA can roam to two APs during beam control.  
      Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.