Abstract:
Since the OFDM communication method does not select whether plural sectors use a same terminal as a transmission destination, or always use it as a transmission destination, giving a preference to system throughput deteriorates channel quality of a terminal in a sector boundary, while increasing channel quality in sector boundaries greatly deteriorates system throughput. In a base station, when a sector transmits to a terminal in the front of a beam, only the sector performs the transmission, and when transmission is made to a terminal in a sector boundary off the direction of the beam, a different sector transmits to the same terminal using a same hopping pattern. Thereby, tradeoff between the channel quality of the terminal in a sector boundary and deterioration in system throughput can be minimized.

Description:
CLAIM OF PRIORITY 
       [0001]    The present application Claims priority from Japanese application JP 2006-338626 filed on Dec. 15, 2006, the content of which is hereby incorporated by reference into this application. 
       BACKGROUND OF THE INVENTION 
       [0002]    (1) Field of the Invention 
         [0003]    The present invention relates to cellular radio communication technology that adopts orthogonal frequency division multiplex (OFDM) in radio communication. 
         [0004]    (2) Description of the Related Art 
         [0005]    Research and development is underway on radio communication systems that adopt OFDM (Orthogonal Frequency Division Multiplex). OFDM produces data to be transmitted in a frequency domain, converts it into a signal of a time domain by IFFT (Inverse Fast Fourier Transform), and transmits it as a radio signal. A receiving side converts the signal of the time domain into a signal of the frequency domain by FFT (Fast Fourier Transform) to extract its original information. 
         [0006]    An OFDM cellular radio communication system generally includes plural base station apparatuses and plural terminals, as shown in  FIG. 1 . A base station apparatus  101  is connected to a network  102  over a wire line. Terminals  103 ,  104 ,  105 , and  106  are wirelessly connected with the base station apparatus  101  to be communicatable with the network  102 . Effective communication with a base station require radio channel conditions of a given level or higher, and are generally governed by a distance from the base station. A range communicatable with a certain base station is referred to as a cell, which has a circular shape like  107  when no shielding matter exists. Terminals perform communication with a base station having the best radio channel condition. Therefore, in the example of  FIG. 1 , the terminals  103 ,  104 ,  105 , and  106  that exist in a cell within the base station  101  communicate with the base station  101  as a communication destination. 
         [0007]    When the number of communication terminals per base station is many like cellular radio, the base station communicates with plural terminals at the same time using directional beams  201 ,  202 , and  203  having different directions, as shown in  FIG. 2 . In this case, one cell is logically split by the number of directional beams; the logically split unit is referred to as a sector. 
         [0008]      FIG. 2  shows an example of CDMA (Code Division Multiple Access) 2000 1xEV-DO (Evolution Data Only) system. The number of sectors is 3, and the terminals  103  and  106  use a beam  201  and the terminals  104  and  105  use beams  202  and  203 , to communicate with the base station  101 . Hereinafter, sectors corresponding to the beams  201 ,  202 , and  203  are defined as sectors  1 ,  2 , and  3 . 
         [0009]    In  FIG. 1 , when a terminal in a cell boundary receives data transmission from a base station, since interference power (power originating in a base station other than a communication destination) is stronger than signal power of the base station of a communication destination, channel quality deteriorates. This is also true for terminals in sector boundaries in  FIG. 2 . As means for reducing the influence, for example, as shown in  FIG. 3 , frequency hopping patterns of OFDM can be used.  FIG. 3  shows different patterns for different sectors. To perform communication with a given user, the sector  1  uses time and frequency such as a pattern  301 , and the sector  2  uses a pattern  302  likewise. In an identical sector, patterns with offset of a frequency direction appended are used to avoid the overlap of time and frequency resources of individual users. Use of such patterns  301  and  302  reduces the rate of the overlapping of time and frequencies with users of other sectors such as  303 . Since the terminals demodulate corresponding frequencies every hour, the hopping helps to suppress interference power. As shown in  FIG. 4 , by allocating mutually different hopping patterns  401 ,  402 , and  403  to the sectors  1 ,  2 , and  3 , each sector can communicate with different terminals at the same time with interference suppressed. 
         [0010]    The standardization group IEEE802.20 proposes a radio system based on OFDM, and defines an interference suppression method by the above-described hopping patterns in Section 9.3 of IEEE C802.20-06/04. 
         [0011]    The standardization group 3GPP proposes a radio system based on OFDM as LTE (Long Term Evolution), and defines an interference suppression method by the above-described hopping patterns in Section 7.1.2.6 of 3GPP TR 25.814 V7.0.0 (2006-06). 
         [0012]    Furthermore, the standardization group 3GPP2 proposes a radio system based on OFDM as LBC (Loosely Backwards Compatible), and defines an interference suppression method by the above-described hopping patterns in Section 1.1 of 33GPP2 C30-20060731-040R4. 
         [0013]    In the related art, during communication with different terminals, channel quality greatly changes depending on terminal positions. For example, as shown in  FIG. 5 , a terminal existing in the front direction of the beam  201  such as the terminal  106  can have high channel quality, while a terminal off the direction of the beam  201  such as the terminal  103  can obtain only low channel quality. Since channel quality information corresponds to data rates, in such a case, the terminal  103  may not obtain a desired data rate. 
         [0014]    As one of measures against such a problem, a method of plural sectors operating as substantially one sector by using a same hopping pattern is proposed in IEEE802.20. An example of matching the hopping pattern  402  of the sector  2  to the hopping pattern  401  of the sector  1  is shown in  FIG. 6 . Since the terminal  103  enables synthesis in signal level by receiving the beams  201  and  202  at the same time, channel quality increases in comparison with reception of the beam  201  alone. 
         [0015]    On the other hand, as a method for backing up terminals in cell boundaries, for example, as disclosed in Japanese Patent Application Laid-Open Publication No. H05-110499, for communication with terminals near a base station, hopping patterns permitting concurrent use of other cells and frequencies are used, while for communication with terminals in cell boundaries, patterns not permitting concurrent use is used. This method enables an improvement in channel quality of terminals in cell boundaries while suppressing deterioration of the number of frequency repetitions by switching of hopping patterns. 
       BRIEF SUMMARY OF THE INVENTION 
       [0016]    However, in the proposal of the IEEE802.20, whether to match hopping patterns cannot be changed during operation, and two sectors always operate as one sector. The degree of improving channel quality as a result of synthesizing the beams  201  and  202  is large with terminals near a sector boundary such as the terminal  103 . However, with terminals in the front direction of the beam  201  such as the terminal  106 , since their channel quality is originally high, the degree of improvement is small. Since constant operation as one sector halves throughput in comparison with individual operations as two sectors, the throughput of the entire system resultantly drops. 
         [0017]    On the other hand, as conventional methods for backing up terminals in cell boundaries, patterns resistant to interference of other cells are used to improve channel quality of terminals in cell boundaries, and the idea of backing up terminals in boundaries in cooperation among plural cells is not disclosed. 
         [0018]    An object of the present invention is to provide an OFDM cellular radio communication method, a radio communication system, and base station apparatuses that can increase channel quality of sector boundaries while maintaining the throughput of the entire system. 
         [0019]    To achieve the above-described object, the present invention provides an OFDM cellular radio communication method by which a cell formed by a base station to communicate with a terminal is split into plural sectors corresponding to the number of directional beams. According to this method, when channel quality to the terminal from the base station is lower than a predetermined threshold, a hopping pattern, that is, a pattern of time and frequency resources, used in a downstream line from the base station to the terminal is used as a pattern of time and frequency resources of a downstream line of a sector different from a sector to which the terminal belongs, for transmission to the terminal in cooperation among plural directional beams. 
         [0020]    The present invention switches between a mode in which each sector communicates with different terminals, and a mode in which plural sectors have a same terminal as a transmission destination. Specifically, a base station performs control so that when a sector transmits to a terminal in the front of a beam, only the sector performs transmission, while when the sector transmits to a terminal near a sector boundary off a beam direction, another sector transmits to the same terminal by using the same hopping pattern. 
         [0021]    In the present invention, in an OFDM cellular radio communication method by which a cell formed by a base station to communicate with a terminal is split into plural sectors corresponding to the number of directional beams, when the priority of communication of the terminal is higher than a predetermined threshold, a hopping pattern, that is, a pattern of time and frequency resources, used in a downstream line from the base station to the terminal is used as a pattern of time and frequency resources of a downstream line of a sector different from a sector to which the terminal belongs, for transmission to the terminal in cooperation among plural directional beams. 
         [0022]    According to the present invention, only when a terminal in a sector boundary is a transmission destination, or when a terminal requiring high priority is a transmission destination, plural sectors cooperate for transmission. Thereby, deterioration in system throughput can be minimized, and reduction in terminal throughput due to position and the like can be prevented. 
         [0023]    According to the present invention, in cellular communication based on OFDMA (Orthogonal Frequency Division Multiple Access), deterioration in system throughput can be minimized, and channel quality of terminals near a sector boundary, or terminals requiring high priority can be increased, and bottleneck in QoS assurance service can be eliminated. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]    These and other features, objects and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings wherein: 
           [0025]      FIG. 1  is a drawing showing a rough construction of an OFDM cellular system; 
           [0026]      FIG. 2  is a drawing for explaining the concept of logically splitting a communication range of a base station by directional beams; 
           [0027]      FIG. 3  is a drawing for explaining interference reduction effects between sectors by frequency hopping; 
           [0028]      FIG. 4  is a drawing showing an example of performing communication with different terminals on a sector basis by different frequency hopping; 
           [0029]      FIG. 5  is a drawing for explaining that differences occur in communication quality for each of terminals, depending on angles formed with respect to a beam direction; 
           [0030]      FIG. 6  is a drawing showing the concept of improving communication quality of sector boundaries by use of same hopping patterns over plural sectors; 
           [0031]      FIG. 7  is a sequence diagram of downstream communication (from base station to terminal) of cellular communication; 
           [0032]      FIG. 8  is a flowchart of transmission target update operation in cellular communication; 
           [0033]      FIG. 9  is a sequence diagram in a first embodiment of the present invention; 
           [0034]      FIG. 10  is a flowchart of inter-sector cooperation operation in a first embodiment; 
           [0035]      FIG. 11  is a conceptual diagram of system operation when no inter-sector cooperation occurs, in a first embodiment; 
           [0036]      FIG. 12  is a conceptual diagram of system operation when inter-sector cooperation occurs, in a first embodiment; 
           [0037]      FIG. 13  is a drawing showing a concrete configuration example of a base station for implementing a first embodiment; 
           [0038]      FIG. 14  is a flowchart of inter-sector cooperation operation in a second embodiment of the present invention; 
           [0039]      FIG. 15  is a flowchart of inter-sector cooperation operation in a third embodiment of the present invention; 
           [0040]      FIG. 16  is a flowchart of inter-sector cooperation operation in a fourth embodiment of the present invention; 
           [0041]      FIG. 17  is a drawing for explaining an active set management method in a cellular system; 
           [0042]      FIG. 18  is a drawing for explaining resource use status of a cooperation destination sector before inter-sector cooperation in a first embodiment; and 
           [0043]      FIG. 19  is a drawing for explaining resource use status of a cooperation destination sector after inter-sector cooperation in a first embodiment. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0044]    Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. Prior to it, cellular radio communication prerequisite to the present invention will be briefly described. 
         [0045]    General cellular radio communication are performed according to a procedure as shown in  FIG. 7 . A terminal sends downstream channel quality information  701  to a base station, based on the strength of a transmission signal from the base station. When retransmission control is performed, downstream ACK/NAK (Acknowledge/Negative Acknowledge) information  702  indicating the success or failure of data reception from the base station is also sent. The base station collectively manages data of terminals belonging to subordinate sectors, and on receiving data  703  for belonging terminals from the network, performs buffering processing  704 . The base station performs transmission target update operation  705  (retransmission of data unsuccessfully received, and decision of new transmission destination and transmission rate) at a cycle corresponding to the flaming of wireless link. After the transmission target update operation  705 , the base station transmits resource allocation notification  706  and transmission data  707  to the updated destination terminal. The terminal, from the resource allocation notification  706 , determines whether to receive the data, time eligible for reception, and a hopping pattern of frequency resource, before receiving the data  707 . 
         [0046]      FIG. 8  details an operation  705  to decide a data transmission target in  FIG. 7 . The base station determines, for all subordinate sectors, determines the presence of transmission data to terminals belonging to the sectors and whether to transmit new data ( 801 ). Whether to transmit new data can be determined from whether the retransmission control information  702  needs to be retransmitted, or whether the retransmission control information  702  exists or not. When transmission data exists, and it is determined that new data can be transmitted, the base station decides a concrete destination terminal and a transmission data rate ( 802 ). When there are plural terminals that desire new data transmission, the base station performs a scheduling operation that decides a destination terminal, based on information such as downstream channel quality information  701 . The data rate can be decided using the downstream channel quality information  701 . When a destination terminal is decided, the base station generates allocation information of time and a frequency resource used for communicate with the terminal ( 803 ), and makes notification to the terminal by the resource allocation notification  706 . Thereby, cellular radio communication is enabled between the base station and the terminal. 
       First Embodiment 
       [0047]    A first embodiment of the present invention will be described using  FIGS. 9 and 10 . In the first embodiment, the base station tries cooperation between sectors when the channel quality information of the destination terminal is below a first threshold, and performs cooperation when a cooperation destination sector does not overlap with resources to be used; when the resources overlap, the base station gives a preference to the cooperation destination sector. 
         [0048]      FIG. 9  shows a sequence diagram of the first embodiment. Operations of  701  to  707  are the same as those in  FIG. 7 , except that the base station performs cooperation processing  901  between sectors after the data transmission target decision operation  705 . 
         [0049]      FIG. 10  details processing operations of the inter-sector cooperation processing  901 . The base station determines whether the sectors newly start data transmission ( 1001 ), and compares, for sectors found to be true, channel quality information of a destination terminal with a predetermined threshold (first threshold) ( 1002 ). This operation  1002 , for example, can be realized by referring to downstream channel quality information  701  sent by the terminal and a decided transmission data rate. When the channel quality information is below the first threshold, the base station searches for a subordinate sector that it can cooperate with ( 1003 ). A sector of a cooperation destination can be decided by referring to an active set of the terminal and a terminal managed by the base station. Next, the base station checks the use status of time and frequency resources by existing communication of the destination sector ( 1004 ), and determines the presence of time and frequency resources that might duplicate during communication by use of hopping patterns of a cooperation source ( 1005 ). When they do not duplicate, inter-sector cooperation is performed. When a duplication exists, the resources of the cooperation destination take precedence and are not overwritten ( 1006 ) to avoid inter-sector communication. 
         [0050]    Reference to an active set of terminals performed in deciding a cooperation destination sector will be described. Since channel conditions change due to migration and the like, normally, terminals manage a set of sectors having excellent downstream channel quality information including communication destination base stations, and the set is referred to as an active set. The concept of active set management is described using  FIG. 17 .  FIG. 17  shows an example of updating an active set  1703  held by a terminal  1702  with a base station  1701  as a communication destination base station when the terminal  1702  migrates within a range of a sector  1 - 1 . In a position before migration, downstream channel quality information for the terminal  1702  is poorer in the order of sectors  1 - 1 ,  1 - 2 , and  3 - 3 , and the terminal  1702  registers the three in an active set  1703 . 
         [0051]    By communication between the terminal  1702  and the base station  1701 , the base station  1701  shares the information, and manages information  1704  of active sets of all belonging terminals. When downstream channel quality information for the terminal  1702  became the order of sectors  1 - 1 ,  1 - 3 , and  3 - 3  in a place to which it migrates, the terminal  1702  shares the information with the base station  1701 , and the base station  1701  reports that it updates the active set ( 1705 ). Thereby, the active sets of the both are updated to the latest condition. The base station  1701  can decide a cooperation destination sector by thus referring to information  1704  of an active set of managed terminals. Here, since it is recognized by referring to the active set  1704  that the downstream channel quality information for the terminal  1702  has become the order of sectors  1 - 1 ,  1 - 3 , and  3 - 3 , when a cooperation destination sector is required, the base station  1701  decides the sector  1 - 3  as a cooperation destination sector. A concrete configuration of the base station  1701  will be described later. 
         [0052]    The standardization group 3GPP2 proposes the management of such active sets, and a method of managing active sets as described above is described in Section 8.7.6 of 3GPP2 C.S0024-A V3.0 (2006-09). In this embodiment, active sets are managed based on the management method. 
         [0053]    The following describes the case of cooperating communication to a terminal of the sector  2  with the sector  1  with reference to  FIGS. 18 and 19 , as a concrete example of steps  1004  to  1006  shown in  FIG. 10  in this embodiment.  FIG. 18  shows the use status of time and frequency resources of the cooperation destination sector  1 , and communication using resources such as patterns  1801  to  1803  are scheduled for three terminals of the sector  1 .  FIG. 19  shows changes in the resource use status in the sector  1  by cooperation in this embodiment. The base station, to communicate with a terminal belonging to the sector  2 , tries to allocate resources based on a hopping pattern  1901  of the sector  2  even in the sector  1 . 
         [0054]    As shown  303  in  FIG. 3 , since the sectors  1  and  2  are different in hopping pattern, as  1902  of  FIG. 19 , resources indicated by a pattern  1901  of time and frequency of a downstream line allocated to the terminal of the sector  2  may overlap with a pattern ( 1801  of  FIG. 19 ) scheduled to be used in the sector  1 . In this embodiment, as described above, existing communication of the sector  1  take precedence, and cooperation is not performed with the resource  1902  and existing communication shown in  FIG. 18  are performed. 
         [0055]    An example of operation of this embodiment by such control is described by flowcharts  11  and  12  of inter-sector cooperation operations.  FIG. 11  shows an example of transmission of the sector  1  corresponding to a beam  201  to a terminal  106 . Since the terminal  106  is in the front direction of the beam, and channel quality is good, the sector  1  performs transmission alone. Therefore, the sector  2  corresponding to a beam  202  can communicate with other terminals independently, and for example, can communicate with a terminal  104  at the same time. On the other hand,  FIG. 12  shows an example of transmission of the sector  1  to a terminal  103 . As described using  FIG. 10 , in this case, since the terminal  103  is poor in channel quality, the sector  1  requires cooperation with other sectors for transmission, and the sector  2  close to the terminal  103  is selected as a cooperation target by using the active set described previously. The base station  101  transmits data to the terminal  103  with time and frequency resources corresponding to a hopping pattern  401  of the sector  1  by using a directional beam  202  (beam  2 ). As a result, the terminal  103  can obtain higher channel quality than without cooperation without special awareness of inter-sector cooperation. 
         [0056]    An example of a concrete configuration of the base station apparatus for implementing the above-described first embodiment is shown in  FIG. 13 . An antenna  1301  captures a radio signal and converts it into an electrical signal. An RF (Radio Frequency) unit  1302 , during reception, down-converts a signal of an RF frequency received by an antenna  1301  into a signal of a baseband frequency, and converts an analog signal into a digital signal. The converted digital signal is sent to a baseband (BB) unit  1303 . During transmission, the RF unit converts the digital signal sent from the baseband unit  1303  into an analog signal, and up-converts the analog signal of the baseband frequency to an RF signal. The up-converted signal is transmitted from the antennal  1301  after being amplified to a proper transmission power. 
         [0057]    The baseband unit  1303 , which performs almost all of OFDM signal processings, performs processings such as CP insertion/removal, FFT/IFFT processing, mapping/demapping, channel estimation, modulation/demodulation, and channel coding/decoding. The baseband unit  1303 , according to commands of a DSP (Digital Signal Processor)  1304 , performs processing of specified channel blocks and modulation/demodulation processing of control channels. A digital signal demodulated by the baseband unit  1303  is passed via the DSP  1304  or directly to a network interface unit  1305  (NW interface) though not shown, and reception information is sent to the network (NW). Information sent from the network is received in the network interface unit  1305 , and is passed via the DSP  1304  or directly to the baseband unit  1303  though not shown. The information is mapped to channel blocks specified by the DSP  1304 , based on a modulation system specified by the DSP  1304  in the baseband unit  1303  before being converted into baseband. 
         [0058]    The MPU (Micro Processing Unit)  1306  manages the status and information of the entire radio equipment, and connects with the individual units to perform control such as the collection of management information and the settings of parameters. The MPU  1306 , a general-purpose microprocessor, includes a processing unit and a storage unit internal or external to it. The storing unit stores programs executed by the processing unit, and is used as a work area. 
         [0059]    The flow of  FIG. 10  detailed previously describes a cooperation method between sectors in this embodiment. However, in  FIG. 13 , the MPU  1306  is a main unit that executes the flow, that is, a program. The MPU  1306  acquire various control information from the baseband unit  1303 , the RF unit  1302 , the DSP  1304 , and the network interface unit  1305 . On determining from the acquired information that inter-sector cooperation is required, the MPU  1300  changes assign information of time and frequency resources, stores the changed assign information in the storage unit, and passes the produced assign information of time and frequency resources to the baseband unit  1303 . 
         [0060]    Control information transmitted from a terminal as downstream channel quality information is sent to the MPU  1306  to be used to determine whether channel quality is below the first threshold ( 1002  of  FIG. 10 ). Control information about an active set sent from the terminal is sent to the MPU  1306  via the DSP  1304 , and the MPU  1306  forms an active set  1704  in the storage unit by using the control information, and can search for a cooperation destination sector ( 1003  of  FIG. 10 ). 
         [0061]    According to the first embodiment detailed previously, inter-sector cooperation is performed only during communication with terminals having bad channel quality, and when overlapping time and frequency resources exist during the cooperation, since the resources are used so that existing communication of a cooperation destination take precedence, no influence is exerted on the existing communication of the cooperation destination sector. As a result, without badly affecting the existing communication, channel quality of terminals having bad channel quality in a single sector can be increased. 
       SECOND EMBODIMENT 
       [0062]    A second embodiment will be described using  FIG. 14 . In the second embodiment, inter-sector cooperation is tried when priority (the degree of needing QoS (Quality of Service) control by service such as VoIP (Voice over IP)) of a transmission destination terminal is equal to or greater than a second threshold. When a cooperation destination sector overlaps with a resource to be used, the cooperation destination sector takes precedence. For example, in 3GPP2 C.R1001-E V1. 0 (2005-10), possible main services are defined in the form of Flow Profile ID, and a base station, when receiving the ID on services that the transmission destination terminal receives, can determine priority by associating the ID with the threshold (second threshold). 
         [0063]    A sequence diagram of the second embodiment is the same as that of the first embodiment (see  FIG. 9 ).  FIG. 14  details inter-sector cooperation operation of the second embodiment.  1001  and  1003 - 1006  are the same as those of  FIG. 10 , except that implementation timing  1002  of inter-sector cooperation operation is decided according to the priority of a destination terminal ( 1401 ). Like the first embodiment, the MPU  1306  in a base station, according to the program processing, performs cooperation when the above-described ID received from the terminal is higher than a predetermined priority (second threshold). 
         [0064]    According to this embodiment, inter-sector cooperation is performed only during communication with terminals having high priority, and the cooperation exerts no influence on existing communication of a cooperation destination sector. As a result, channel quality of terminals having high priority can be increased without badly affecting existing communication, and QoS requirements can be probably satisfied. 
       THIRD EMBODIMENT 
       [0065]    A third embodiment will be described using  FIG. 15 . In the third embodiment, inter-sector cooperation is tried when channel quality of transmission destination terminal is below the first threshold, and when a cooperation destination sector overlaps with a resource to be used, resources are overwritten with a cooperation source sector taking precedence. 
         [0066]    A sequence diagram of the third embodiment is the same as that of the first embodiment (see  FIG. 9 ).  FIG. 15  details inter-sector cooperation of the third embodiment.  1001  to  1005  are the same as those of  FIG. 10 , except that resources to be used are overwritten when they overlap with existing communication of a cooperation destination sector ( 1501 ). That is, with reference to  FIG. 19 , also for a resource  1902  being an overlapping pattern, cooperation operation is performed. It goes without saying that these processing are performed by program processing of the MPU  1306  in the base station, like the first embodiment. That is, the MPU  1306  overwrites any possible locations of overlapping with time and frequency resources to be used by a cooperation destination sector to change them, stores the changed resources in the storage unit, and passes assign information of the produced time and frequency resources to the baseband unit  1303 . 
         [0067]    According to this embodiment, inter-sector cooperation is performed only during communication with terminals having bad channel quality, and time and frequency resources of plural sectors are allocated for the terminals without fail. As a result, channel quality of target terminals can be certainly increased although existing communication may be somewhat badly affected. This embodiment may be used at the same time as the second embodiment. 
       FOURTH EMBODIMENT 
       [0068]    A fourth embodiment will be described using  FIG. 16 . In the fourth embodiment, although inter-sector cooperation is tried when channel quality of a transmission destination terminal is below a threshold, cooperation is abandoned when a resource use ratio of a cooperation destination sector, that is, a congestion state is equal to or greater than a third threshold. Here, a resource use ratio indicating a congestion state can be defined, for example, in the example of  FIG. 18 , by counting resources to be used and calculating a ratio to the whole. 
         [0069]    A sequence diagram of the fourth embodiment is the same as that of the first embodiment (see  FIG. 9 ).  FIG. 16  details inter-sector cooperation operation by the fourth embodiment.  1001  to  1006  are the same as those of  FIG. 10 , except that when a congestion degree is equal to or greater than a predetermined threshold (third threshold) as a result of checking the status of existing communication of a cooperation destination sector, the congestion is abandoned ( 1601 ). A congestion degree, that is, a congestion state can be determined based on the use ratio of time and frequency resources of the sector. It goes without saying that the MPU  1306  of the base station can calculate the resource use ratio by a program while referring to time and frequency resource information stored in the storage unit. 
         [0070]    According to this embodiment, inter-sector cooperation is performed only during communication with terminals having bad channel quality, with the result that a bad influence on existing communication of a cooperation destination sector can be avoided without fail. As a result, while deterioration amounts of existing communication are suppressed below a target value, channel quality of terminals having bad channel quality in a single sector can be increased. Although this embodiment is described as a variant of the first embodiment, it goes without saying that use of the resource use ratio of a cooperation destination sector can be used at the same time as the second and third embodiments. 
         [0071]    As has been detailed above, in cellular communication based on OFDMA (Orthogonal Frequency Division Multiple Access), while deterioration in system throughput is suppressed to a minimum level, channel quality of terminals near the boundaries of a sector can be increased, and bottleneck in QoS assurance service can be eliminated.