Abstract:
A reception unit receives signals from a plurality of radio terminals. A transmission unit transmits signals to the radio terminals. A direction determination unit determines the directions of the radio terminals viewed from the transmission unit on the basis of the signals received by the reception unit. A group selection unit distributes, by using a threshold, the transmission powers or downlink data transmission rates of radio terminals which transmit signals in a time-overlapping manner, thereby classifying the radio terminals into two groups. Then, the group selection unit selects a group having a smaller total number of radio terminals. A directivity pattern control unit controls a directivity pattern on the basis of the directions of the radio terminals determined by the direction determination unit.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to a directivity control apparatus for an adaptive antenna used in a radio station such as a radio base station.  
           [0002]    A radio station such as a radio base station which communicates with a radio terminal such as a portable telephone uses an antenna for radiating radio waves. In general, radio waves radiated by the antenna propagate over the range of cells assigned to respective radio base stations. Radiating radio waves in a direction other than the direction of a radio terminal consumes power supplied to the antenna in transmitting radio waves.  
           [0003]    Radio waves are generally radiated strongly in a direction in which a radio terminal exists, and weakly in a direction in which no radio terminal exists. An example of an antenna capable of adjusting the directivity of radio waves is an adaptive antenna having a directivity control apparatus.  
           [0004]    There has conventionally been proposed a technique of installing an adaptive antenna in a radio base station and adjusting the transmission directivity of a signal transmitted from the adaptive antenna in accordance with the position of a moving radio terminal. In Japanese Patent Laid-Open No. 11-298400 (reference 1), a predetermined signal is transmitted to a radio terminal while the directivity of radio waves is changed in a radio base station. The directivity at which the reception power of a predetermined signal received by the radio terminal is strongest is used as the transmission directivity. A similar arrangement is also disclosed in Japanese Patent Laid-Open No.  09-200115  (reference 2).  
           [0005]    In Japanese Patent Laid-Open No. 10-070502 (reference 3), the arrival direction of a reception signal which has arrived at a radio base station is estimated by calculation. The transmission directivity is so controlled as to radiate a transmission signal strongly in the estimated direction.  
           [0006]    When a plurality of radio terminals exist within the cells of a radio base station, the radio base station simultaneously radiates radio waves to the radio terminals. Particularly if there are a plurality of radio terminals which perform communication by signals of the same or neighboring frequencies, radiated radio waves may interfere with each other, failing to ensure proper communication. As a technique of suppressing interference between radio waves, the adaptive antenna also receives a great deal of attention.  
           [0007]    A radio base station using the adaptive antenna comprises a transmission directivity control apparatus for uniquely weighting outputs to a plurality of antenna elements which constitute an antenna, and adjusting the transmission directivity. To generate the transmission directivity pattern of a radio terminal, a main beam in a direction in which radio waves most strongly reach the radio terminal is directed. In addition, a null in a direction in which radio waves weakly reach other radio terminals is directed. The beam and null of the transmission directivity pattern can be directed to desired directions by adjusting antenna weights representing the weighting coefficients of outputs to respective antenna elements. Desired transmission directivity patterns are generated for respective radio terminals. This can suppress interference with radio waves propagating to other radio terminals to an extent that the nulls of respective transmission directivity patterns are directed even if radio waves are simultaneously radiated in transmission.  
           [0008]    [0008]FIG. 18 shows a conventional transmission directivity control apparatus in a radio base station using an adaptive antenna. In the prior art, channels are assigned to first to Mth radio terminals (not shown) for the same time interval in the same frequency band by using CDMA (Code Division Multiple Access). In FIG. 17, a transmission directivity control apparatus  11  comprises first to Nth antenna elements  12   1  to  12   N  which are regularly aligned at an equal interval and transmit/receive radio waves. The antenna elements  12   1  to  12   N  are respectively connected to first to Nth transmission/reception demultiplexing circuits  13   1  to  13   N  for demultiplexing reception and transmission signals of radio waves. The transmission/reception demultiplexing circuits  13   1  to  13   N  are connected to a reception unit  14  for receiving reception signals, and a transmission unit  15  for transmitting transmission signals.  
           [0009]    The reception unit  14  is constituted by first to Nth receivers  16   1  to  16   N  for amplifying and detecting reception signals output from the transmission/reception demultiplexing circuits  13   1  to  13   N , and first to Nth A/D (analog-to-digital) converters  17   1  to  17   N  for converting amplified/detected reception signals into digital signals. The transmission unit  15  is constituted by first to Nth D/A (digital-to-analog) converters  18   1  to  18   N  for converting transmission signals into analog signals, and first to Nth transmitters  19   1  to  19   N  for modulating and amplifying transmission signals converted into analog signals, and outputting the modulated/amplified signals to the transmission/reception demultiplexing circuits  13   1  to  13   N . The reception unit  14  and transmission unit  15  are connected to first to Mth transmission directivity generation units  20   1  to  20   M  for generating transmission directivities assigned to respective first to Mth radio terminals by the radio base station.  
           [0010]    The first transmission directivity generation unit  20   1  has a reception directivity pattern control unit  21   1  for receiving reception signals converted into digital signals by the A/D converters  17   1  to  17   N  and generating a reception directivity pattern. The output of the reception directivity pattern control unit  21   1  is connected to an antenna weight correction unit  22   1  for correcting various errors when each antenna weight calculated in generating a reception directivity pattern is used to generate a transmission directivity pattern. The output of the antenna weight correction unit  22   1  is connected to a transmission directivity pattern control unit  23   1  for receiving each antenna weight obtained by correcting various errors, and generating a transmission directivity pattern. The output of the transmission directivity pattern control unit  23   1  is connected to the D/A converters  18   1  to  18   N  for receiving transmission signals weighted by antenna weights calculated in generating a transmission directivity pattern.  
           [0011]    Similar to the first transmission directivity generation unit  20   1 , the second to Mth transmission directivity generation units  20   2  to  20   M  comprise reception directivity pattern control units  21   2  to  21   M , antenna weight correction units  22   2  to  22   M , and transmission directivity pattern control units  23   2  to  23   M . The same arrangement as that of the transmission directivity control apparatus is also disclosed in Japanese Patent Laid-Open No. 2000-209017 (reference 4). The operations of the transmission directivity generation units  20   1  to  20   M  are the same, and the operation of the transmission directivity generation unit  20   1  will be representatively explained.  
           [0012]    The reception directivity pattern control unit  21   1  receives the reception signals of the antenna elements  12   1  to  12   N  which have simultaneously received radio waves from the first to Mth radio terminals. The reception directivity pattern control unit  21   1  executes despreading of multiplying the reception signals by a spreading code multiplied in the first radio terminal, and separates the reception signals of the first radio terminal from the remaining spread reception signals. The reception directivity pattern control unit  21   1  calculates antenna weights corresponding to the reception signals from the first radio terminal. The reception directivity pattern control unit  21   1  weights the reception signals by the antenna weights to generate a reception directivity pattern for the first radio terminal.  
           [0013]    The antenna weight correction unit  22   1  receives the respective antenna weights calculated by the reception directivity pattern control unit  21   1 . When the frequencies of reception and transmission signals are different from each other, the antenna weight correction unit  22   1  corrects an antenna weight error caused by the frequency difference between the reception and transmission signals. At the same time, the antenna weight correction unit  22   1  corrects antenna weight errors caused by amplitude and phase deviations generated in the reception unit  14  and transmission unit  15 .  
           [0014]    The transmission directivity pattern control unit  23   1  weights a transmission signal  24   1  from a transmission signal generation unit (not shown) by each corrected antenna weight to generate a transmission directivity pattern. The transmission directivity pattern control unit  23   1  performs spreading of multiplying the transmission signal by a spreading code. The spread transmission signal is input to the D/A converters  18   1  to  18   N .  
           [0015]    The transmission directivity pattern generated by the transmission directivity control apparatus  11  is generated using substantially the same antenna weight as each antenna weight calculated in generating a reception directivity pattern. The transmission directivity pattern is substantially the same as the reception directivity pattern. For example, when the reception directivity pattern control units  21   1  to  21   M  perform MMSE (Minimum Mean Square Error) adaptive control as a method of calculating each antenna weight, each antennal weight which directs a null to the direction of a large-reception-power signal is generated. A transmission directivity pattern generated in the transmission directivity control apparatus  11  directs a null to the direction of the large-reception-power signal. Since interference with radio waves from another radio terminal is small in the direction to which the null is directed, necessary transmission power to a radio terminal in this direction is decreased. As a result, interference with another radio terminal can be reduced.  
           [0016]    In recent years, various contents obtained by downloading various data such as image data on the Internet from a radio terminal such as a portable telephone have increasingly being used. In this data communication, a downlink signal tends to have a larger capacity of data than an uplink signal, and higher data transmission rate is being required more and more. In general, the transmission power of radio waves becomes larger for higher data transmission rate. If the data transmission rates of uplink and downlink signals are different, the power distributions of the uplink and downlink signals are also different.  
           [0017]    The conventional radio base station uses a reception directivity pattern optimized for the power distribution of the uplink signal as a transmission directivity pattern on the assumption that the transmission rates of the uplink and downlink signals are the same. Hence, the adaptive antenna effects cannot be fully enhanced to suppress interference.  
           [0018]    To solve this problem, there is proposed a technique of making the direction to which the null is directed correspond to the power distribution of the downlink signal. In Japanese Patent Laid-Open No. 2000-224097 (reference 5), the null is directed to the large-transmission-power direction of a downlink signal. This reduces necessary transmission power to a radio terminal in this direction, and decreases interference in the small-transmission-power direction of another downlink signal.  
           [0019]    The number of nulls which can be adjusted by the transmission directivity control apparatus is restricted by the number of antenna elements. When a plurality of radio terminals exist in addition to a radio terminal to which the main beam is directed, nulls may not be able to be directed toward all the remaining radio terminals. In some cases, the number of nulls exceeds the restricted number of nulls due to an increase in radio terminals which require data communication. In this case, nulls cannot be directed to radio terminals whose downlink signal transmission powers are large. In this case, large transmission power to radio terminals in directions to which no null is directed cannot be reduced. Radio interference with other small-transmission-power radio terminals cannot be suppressed.  
           [0020]    The relationship between the main beam and the null has been described. Similar problems also occur in another case. When the directivity pattern is adjusted such that the radio intensity increases for target transmission radio terminals and decreases for other time-overlapping radio terminals, a plurality of radio terminals cannot be selected as other radio terminals to be selected.  
         SUMMARY OF THE INVENTION  
         [0021]    It is an object of the present invention to provide a directivity control apparatus capable of adjusting a directivity pattern so as to selectively decrease the radio intensity for a plurality of time-overlapping radio terminals in adjusting the directivity pattern.  
           [0022]    To achieve the above objects, according to the present invention, there is provided a directivity control apparatus comprising reception means for receiving signals from a plurality of radio terminals, transmission means for transmitting signals to the radio terminals, direction determination means for determining directions of the radio terminals viewed from the transmission means on the basis of the signals received by the reception means, group selection means for distributing, by using a threshold, transmission powers or downlink data transmission rates of radio terminals which transmit signals in a time-overlapping manner, thereby classifying the radio terminals into two groups, and then selecting a group having a smaller total number of radio terminals, and directivity pattern control means for controlling a directivity pattern on the basis of the directions of the radio terminals determined by the direction determination means so as to increase intensity of radio waves to a target transmission radio terminal and decrease the intensity of radio waves to other radio terminals which transmit signals in the time-overlapping manner and belong to a group selected by the group selection means. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]    [0023]FIG. 1 is a block diagram showing a transmission directivity control apparatus in a radio base station using CDMA according to the first embodiment of the present invention;  
         [0024]    [0024]FIG. 2 is a flow chart showing data processing in a database shown in FIG. 1;  
         [0025]    [0025]FIG. 3 is a table showing an example of a user distribution table processed in the database shown in FIG. 1;  
         [0026]    [0026]FIGS. 4A and 4B are tables showing an example when the user distribution table shown in FIG. 3 is divided into A and B tables;  
         [0027]    [0027]FIG. 5 is a flow chart showing antenna weight calculation processing in a transmission directivity pattern control unit shown in FIG. 1;  
         [0028]    [0028]FIG. 6 is a view showing the transmission directivity pattern of one user generated in the transmission directivity pattern control unit shown in FIG. 1;  
         [0029]    [0029]FIG. 7 is a view showing the transmission directivity patterns of a plurality of users generated in the transmission directivity pattern control unit shown in FIG. 1;  
         [0030]    [0030]FIG. 8 is a table showing an example of a user distribution table processed in a database according to a modification of the first embodiment;  
         [0031]    [0031]FIGS. 9A and 9B are tables showing an example when the user distribution table shown in FIG. 8 is divided into A and B tables;  
         [0032]    [0032]FIG. 10 is a block diagram showing a transmission directivity control apparatus in a radio base station using CDMA according to the second embodiment of the present invention;  
         [0033]    [0033]FIG. 11 is a flow chart showing data processing in a database shown in FIG. 10;  
         [0034]    [0034]FIG. 12 is a table showing an example of a user distribution table processed in the database of the second embodiment;  
         [0035]    [0035]FIGS. 13A and 13B are tables showing an example when the user distribution table shown in FIG. 12 is divided into A and B tables;  
         [0036]    [0036]FIG. 14 is a table showing an example of a user distribution table processed in a database according to a modification of the second embodiment;  
         [0037]    [0037]FIGS. 15A and 15B are tables showing an example when the user distribution table shown in FIG. 14 is divided into A and B tables;  
         [0038]    [0038]FIG. 16 is a block diagram showing a transmission directivity control apparatus in a radio base station using TDMA/FDMA according to the third embodiment of the present invention;  
         [0039]    [0039]FIG. 17A is a functional block diagram showing a CPU which constitutes the database shown in FIG. 1;  
         [0040]    [0040]FIG. 17B is a functional block diagram showing a DSP which constitutes the transmission directivity pattern control unit shown in FIG. 1; and  
         [0041]    [0041]FIG. 18 is a block diagram showing a conventional transmission directivity control apparatus in a radio base station using an adaptive antenna. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0042]    The present invention will be described in detail below with reference to the accompanying drawings.  
         [0043]    &lt;First Embodiment&gt; 
         [0044]    [0044]FIG. 1 shows a transmission directivity control apparatus in a radio base station using CDMA according to the first embodiment of the present invention. In FIG. 1, a transmission directivity control apparatus  101  comprises first to Nth antenna elements  102   1  to  102   N  which are regularly aligned at an equal interval and transmit/receive radio waves. The antenna elements  102   1  to  102   N  are respectively connected to first to Nth transmission/reception demultiplexing circuits  103   1  to  103   N  for demultiplexing reception and transmission signals of radio waves. The transmission/reception demultiplexing circuits  103   1  to  103   N  are connected to a reception unit  104  for receiving reception signals, and a transmission unit  105  for transmitting transmission signals.  
         [0045]    The reception unit  104  is constituted by first to Nth receivers  106   1  to  106   N  for amplifying and detecting reception signals output from the transmission/reception demultiplexing circuits  103   1  to  103   N  and first to Nth A/D (analog-to-digital) converters  107   1  to  107   N  for converting amplified/detected reception signals into digital signals. The transmission unit  105  is constituted by first to Nth D/A (digital-to-analog) converters  108   1  to  108   N  for converting transmission signals into analog signals, and first to Nth transmitters  109   1  to  109   N  for modulating and amplifying transmission signals converted into analog signals, and outputting the modulated/amplified signals to the transmission/reception demultiplexing circuits  103   1  to  103   N . The reception unit  104  and transmission unit  105  are connected to first to Mth transmission directivity generation units  110   1  to  110   M  for generating transmission directivities assigned to M radio terminals (to be referred to as users hereinafter) within the cells of the radio base station.  
         [0046]    The first transmission directivity generation unit  110   1  is made up of an arrival direction estimation unit  111   1  for receiving reception signals from the A/D converters  107   1  to  107   N  and estimating the arrival direction of radio waves from the first user, and a transmission directivity pattern control unit  112   1  for controlling the transmission directivity pattern of the first user. The output of the arrival direction estimation unit  111   1  is connected to the transmission directivity pattern control unit  112   1  and a database  113  such as a magnetic disk which stores data such as the arrival directions of radio waves of the first to Mth users.  
         [0047]    Similar to the transmission directivity generation unit  110   1 , the second to Mth transmission directivity generation units  110   2  to  110   M  are constituted by second to Mth arrival direction estimation units  111   2  to  111   M  for receiving reception signals from the A/D converters  107   1  to  107   N  and estimating the arrival directions of radio waves from the second to Mth users, and second to Mth transmission directivity pattern control units  112   2  to  112   M  for controlling the transmission directivity patterns of the second to Mth users. The database  113  is connected to a channel control unit  114  for assigning a channel to a user, and the transmission directivity pattern control units  112   1  to  112   M .  
         [0048]    The operation of the transmission directivity control apparatus  101  having this arrangement will be explained.  
         [0049]    Reception signals received by the antenna elements  102   1  to  102   N  are input to the reception unit  104  via the transmission/reception demultiplexing circuits  103   1  to  103   N . In the reception unit  104 , the reception signals are amplified and detected by the receivers  106   1  to  106   N , and converted into digital signals by the A/D converters  107   1  to  107   N . Each of first to Nth reception signals  115   1  to  115   N  converted into digital signals is branched by a user count M assigned by the channel control unit  114 , and input to the arrival direction estimation units  111   1  to  111   M .  
         [0050]    The arrival direction estimation units  111   1  to  111   M  are formed from DSPs (Digital Signal Processors) each having a CPU (Central Processing Unit), a storage medium such as a ROM (Read Only Memory) which stores a control program, and a RAM (Random Access Memory) serving as a work memory. The CPU performs various control operations in accordance with programs stored in the ROM.  
         [0051]    The arrival direction estimation unit  111   1  executes despreading in order to separate the reception signal of the first user from the reception signals  115   1  to  115   N . The despread reception signal is assigned a unique identification number so as to identify the user of the signal. Identification numbers are assigned for convenience by the transmission directivity control apparatus  101  to the first to Mth users assigned channels by the channel control unit  114  at the start of communication. As for the reception signal assigned the identification number, the arrival direction estimation unit  111   1  estimates a direction from which the signal has come.  
         [0052]    In estimating the arrival direction, the arrival direction estimation unit  111   1  adopts a conventional beam former method of scanning the beam of a reception directivity pattern and detecting a direction in which reception power maximizes. First data  116   1  representing an arrival direction θ 1  estimated by the arrival direction estimation unit  111   1  and a set user identification number U 1  is stored in the database  113 , and at the same time input to the transmission directivity pattern control unit  112   1 .  
         [0053]    When M users exist within the cells of the radio base station of the transmission directivity control apparatus  101 , the M arrival direction estimation units  111   1  to  111   M  operate to perform the same processing. As the arrival direction estimation method, many known arrival direction estimation methods such as a MUSIC (MUltiple SIgnal Classification) algorithm can be employed.  
         [0054]    The channel control unit  114  assigns downlink channels to the first to M users in accordance with a downlink data transmission rate required by each user. The channel control unit  114  is comprised of a CPU, a storage medium such as a ROM which stores a control program, and a RAM serving as a work memory, and executes predetermined channel control by a stored program method.  
         [0055]    On a downlink channel, a transmittable data transmission rate is determined in advance in accordance with the data capacity of a signal transmitted to each user. That is, when the user starts communication, a downlink channel is assigned to the user on the basis of the data transmission rate so as to adjust the data capacity to a channel capacity which can be stored in the radio base station. In assigning a channel, a data transmission rate required by each user is set. Data  117  representing data transmission rates R 1  to R M  and user identification numbers U 1  to U M  in the downlink channels of the first to Mth users assigned by the channel control unit  114  are stored in the database  113 .  
         [0056]    [0056]FIG. 2 shows a data processing flow in the database  113 . The database  113  is made up of a CPU  201 , a storage medium such as a ROM which stores a control program, and a RAM serving as a work memory. The database  113  performs predetermined control by a stored program method. The database  113  stores data representing arrival directions θ 1  to θ M  which are estimated by the arrival direction estimation units  111   1  to  111   M  and correspond to the first to Mth users, and user identification numbers U 1  to U M . The database  113  also stores data representing data rates R 1  to R M  on the downlink channels of the first to Mth users assigned by the channel control unit  114 , and the user identification numbers U 1  to U M . A user distribution table is crated from these data (step S 101 ).  
         [0057]    [0057]FIG. 3 shows an example of a data table processed by the database. In FIG. 3, the distribution of downlink data transmission rates required by users for respective arrival directions is stored as a user distribution table  201  in the table form. “USER No.” in the user distribution table  201  represents the user identification numbers U 1  to U M  assigned by the arrival direction estimation units  111   1  to  111   M . The second field “DOWNLINK DATA RATE” represents the data transmission rate of a downlink transmission signal assigned by the channel control unit  114 . In the first embodiment, R Hi  represents a high data transmission rate; R Mi , an intermediate data transmission rate; and R Lo , a low data transmission rate. The third field “ARRIVAL DIRECTION” represents reception signal arrival directions estimated by the arrival direction estimation units  111   1  to  111   M .  
         [0058]    Referring back to FIG. 2, the average of the data transmission rates of downlink channels is calculated from the data rates R 1  to R M  in the user distribution table  201  (step S 102 ). The calculated average is multiplied by a coefficient, and the product is set as a threshold (step S 103 ). This coefficient is an arbitrary coefficient set in advance as an index optimal for use environment conditions such as the number of users and the user distribution. In the first embodiment, the threshold is experimentally determined for the average user distribution of the base station by using as a reference the average of data transmission rates for all the users. At first, the average is set as a default value, and perturbation is conducted to determine a threshold so as to minimize the total transmission power of an average base station. The total transmission power of the base station is observed every day.  
         [0059]    By using this threshold, the user distribution table  201  is divided into two, A and B tables (step S 104 ). Note that the mode may be calculated instead of the average in step S 102 , and the calculated mode may be multiplied by a coefficient to obtain a threshold in step S 103 .  
         [0060]    [0060]FIGS. 4A and 4B show an example when the user distribution table is divided into A and B tables. The A and B tables are attained using as the threshold a value between the data transmission rates R Hi  and R Mi . In an A table  202  shown in FIG. 4A, “No.” represents a number assigned for convenience. “DOWNLINK DATA RATE” represents the data transmission rates R Mi  and R Lo  lower than the threshold. Similar to the table in FIG. 3, “ARRIVAL DIRECTION” represents an estimated reception signal arrival direction. A B table  203  shown in FIG. 4B is identical to the A table  202  except that “DOWNLINK DATA RATE” represents the data transmission rate R Hi  higher than the threshold.  
         [0061]    Referring back to FIG. 2, a number M A  of users belonging to the A table  202  and a number M B  of users belonging to the B table  203  are obtained (step S 105 ), and the number M A  of users and the number M B  of users are compared (step S 106 ). If the number M A  of users is smaller than the number M B  of users, arrival directions contained in the A table  202  representing the number M A  of users are set as null generation direction candidates (step S 107 ). If the number M A  of users is larger than the number M B  of users, arrival directions contained in the B table  203  representing the number M B  of users are set as null generation direction candidates (step S 108 ).  
         [0062]    In this manner, the directions of users who demand downlink transmission signals at data transmission rates belonging to a low-density distribution out of the downlink data transmission rate distribution of the first to Mth users are defined as null generation directions. The coefficient of the threshold prevents the number M A  of users and the number M B  of users from being equal to each other. After processing, null generation direction candidates  118  (FIG. 1) from the database  113  are output to the transmission directivity pattern control units  112   1  to  112   M .  
         [0063]    [0063]FIG. 17A shows the functional block of the CPU  201 . In FIG. 17A, the CPU  201  comprises functional blocks: a distribution table creation unit  201   a  for performing processing in step S 101 , a table division unit  201   b  for performing processing in steps S 102  to S 104 , a comparison unit  201   c  for performing processing in steps S 105  and S 106 , and a null generation direction determination unit  201   d  for performing processing in steps S 107  and S 108 . The CPU  201  for executing database control is installed in the database  113 , but may be arranged outside the database  113 .  
         [0064]    Each of the transmission directivity pattern control units  112   1  to  112   M  is formed from a DSP  202  having a CPU, a storage medium such as a ROM which stores a control program, and a RAM serving as a work memory. The operations of the transmission directivity pattern control units  112   1  to  112   M  are the same, and only the operation of the transmission directivity pattern control unit  112   1  will be explained.  
         [0065]    The transmission directivity pattern control unit  112   1  calculates each antenna weight for generating the transmission directivity pattern of the first user on the basis of the data  116   1  representing the arrival direction of the first user, and the null generation direction candidate  118  output from the database  113 .  
         [0066]    [0066]FIG. 5 shows a processing flow of calculating an antenna weight in the transmission directivity pattern control unit  112   1 . The transmission directivity pattern control unit  112   1  calculates a protection area H on the basis of input of the arrival direction θ 1  estimated by the arrival direction estimation unit  111   1  (step S 111 ):  
           H =θ 1 ±Δθ/2  (1)  
         [0067]    Δθ in equation (1) is given by  
         Δθ= k·θ   BW   (2)  
         [0068]    The numerical value θ BW  in equation (2) is the half power angle of an adaptive antenna in the radio base station. The coefficient k is generally a value of “2” to “3”, but can take various values depending on the characteristics of the apparatus.  
         [0069]    After the protection area H is set, a clock count C of a counter (not shown) for counting clocks is reset to a numerical value “0” (step S 112 ). One arrival direction is selected at random from null generation direction candidates selected in steps S 107  and S 108  (step S 113 ). Whether the selected arrival direction is an angle positioned outside or inside the protection area H is checked (step S 114 ). If the arrival direction falls outside the protection area H, the selected arrival direction is saved (step S 115 ). If the arrival direction falls within the protection area H, processing in steps S 113  and S 114  is repeated.  
         [0070]    After the arrival direction is saved in step S 115 , the clock count “C” of the counter is incremented (step S 116 ), and whether the clock count “C” reaches a preset value L is checked (step S 117 ). If YES in step S 117 , antenna weights which have nulls in L selected arrival directions and generate a main beam in the arrival direction of the first user are calculated (step S 118 ). If NO in step S 117 , processing in steps S 113  to S 117  is repeated.  
         [0071]    [0071]FIG. 17B shows the functional block of the DSP (CPU)  202  of the transmission directivity pattern control unit  112   1 . In FIG. 17B, the DSP  202  has functional blocks: a protection area setting unit  202   a  for performing processing in step S 111 , a terminal selection unit  202   b  for performing processing in steps S 113  to S 117 , and a weight calculation unit  202   c  for performing processing in step S 118 .  
         [0072]    The preset value L will be explained. In the adaptive antenna, a predetermined restriction is posed on a freely adjustable null direction. This is called the degree of freedom of the antenna. Letting N (N is a positive integer) be the number of antenna elements, the degree of freedom of the antenna is given by  
         Degree of Freedom of Antenna= N− 1  (3)  
         [0073]    When the main beam generation direction is adjusted, the degree of freedom of the antenna is decremented by “1” and is given by  
         Degree of Freedom of Antenna= N− 2  (4)  
         [0074]    As represented by equations (3) and (4), the direction in which the null can be freely controlled suffers the predetermined restriction. The maximum value of the preset value L is the degree of freedom of the antenna given by equation (4).  
         [0075]    Arrival directions falling within the protection area H are not selected as null generation directions on the basis of the protection area H given by equation (1). In other words, no null is generated to users present in the protection area H of the first user. In this case, no null is generated within a predetermined angle including the direction of the first user. A decrease in the antenna gain of the main beam of the transmission directivity pattern of the first user can be reduced, and an increase in transmission power can be prevented. Accordingly, interference with another user due to an increase in transmission power can be reduced.  
         [0076]    [0076]FIG. 6 shows the transmission directivity pattern of the first user generated by the transmission directivity pattern control unit. In FIG. 6, θ 1  represents a direction in which a first user  301  exists out of a plurality of users within the cells of the radio base station; θ 2  to θ 5 , directions which are selected as null generation direction candidates and in which second to fifth users  302  to  305  exist; and θ 6  to θ 9 , directions which are not selected as null generation directions and in which sixth to ninth users  306  to  309  exist.  
         [0077]    In FIG. 6, all the null generation direction candidates falling within the protection area H are not selected as null generation direction candidates. Since user directions falling within the protection area H are not set as null generation direction candidates, a decrease in the antenna gain of the main beam can be reduced. Antenna weights are so obtained as to generate the transmission directivity pattern of the first user in which the main beam is directed to the arrival direction θ 1  and nulls are directed to angles θ 2 , θ 3 , . . . , θ 5 .  
         [0078]    Referring back to FIG. 1, to calculate each antenna weight by the transmission directivity pattern control unit  112   1 , a desired wave/interference wave distribution is simulated by signal processing which is as a conventional antenna weight calculation method, and the MMSE algorithm is applied to a simulation signal. In the transmission directivity pattern control unit  112   1  a transmission signal  119   1  from a transmission signal generation unit (not shown) is weighted by each calculated antenna weight to generate the transmission directivity pattern of the first user. Transmission signals weighted by the transmission directivity pattern control unit  112   1  undergo spread of multiplying them by the spreading code of a channel assigned by the channel control unit  114 . The resultant signals are output as transmission signals  120   1  to  120   N  to the D/A converters  108   1  to  108   N  .  
         [0079]    The same processing is done by the M transmission directivity pattern control units  112   1  to  112   M  when M users exist within the cells of the radio base station of the transmission directivity control apparatus  101 .  
         [0080]    [0080]FIG. 7 shows transmission directivity patterns generated by the transmission directivity pattern control units corresponding to a plurality of users. FIG. 7 shows the transmission directivity patterns of two users  310 A and  310 B out of a plurality of users present within the cells of the radio base station. θA and θB represent the directions of users  311 A and  311 B selected as null generation direction candidates, and θN represents the direction of a user  311 N not selected as a null generation direction. The transmission directivity pattern of the user  310 A is a directivity pattern  312 A generated by directing the main beam toward the user  310 A. At this time, in step S 113  of FIG. 5, a null generation direction is selected at random for the user  310 A. A null is generated to the user  311 A present in the direction θA selected at random from selected null generation direction candidates.  
         [0081]    Similarly, the transmission directivity pattern of the user  310 B is a directivity pattern  312 B generated by directing the main beam toward the user  310 B. A null is generated to the user  311 B present in the direction θB selected at random from selected null generation direction candidates.  
         [0082]    The direction of a null θA 3  among the arrival directions of users will be described to explain the situation of a direction in which the null is directed. The direction θA 3  to which the null is generated is the direction of a user  311 A 3  selected from null generation direction candidates in generating a transmission directivity pattern for the user  310 A. The null θA 3  prevents generation of any null to the user  310 B because the user  310 B is not selected as a null generation direction. Even if many users exist, this processing is performed for all the users. The possibility of directing a null toward the direction θA 3  of the user  311 A 3  by the transmission directivity patterns of all the users is decreased. However, some users selected as null generation direction candidates direct nulls to the direction θA 3 .  
         [0083]    As described above, a predetermined restriction is posed on a freely adjustable null direction. In the first embodiment, a null generation direction is selected at random from null generation direction candidates under the restriction on the degree of freedom. Even if the number of interference waves arriving at the radio base station exceeds the degree of freedom of the adaptive antenna, nulls equivalently having a depth to a certain degree are generated by generating transmission directivity patterns for a plurality of radio terminals. For example, as shown in FIG. 7, the direction to the user  311 A 3  is not selected as a null generation direction in generation of a transmission directivity pattern for the user  310 B, but is selected as a null generation direction candidate for each user. As far as the direction is selected as a null generation direction candidate, this direction may be selected at high possibility as a null generation direction with respect to the transmission directivity pattern of another user. As a result, interference with many users can be effectively suppressed.  
         [0084]    Referring back to FIG. 1, the transmission signals  120   1  to  120   N  output from the transmission directivity pattern control units  112   1  to  112   M  are input to the transmission unit  105 . The transmission signals are converted into analog signals by the D/A converters  108   1  to  108   N  , and modulated and amplified by the transmitters  109   1  to  109   N . The amplified transmission signals are transmitted from the antenna elements  102   1  to  102   N  via the transmission/reception demultiplexing circuits  103   1  to  103   N .  
         [0085]    &lt;Modification of First Embodiment&gt; 
         [0086]    [0086]FIG. 8 shows a modification of the database. In a user distribution table  204  according to the modification, “DIRECTION” represents an angular range when the direction in which the directivity can be adjusted by the transmission directivity control apparatus  101  of the first embodiment is equally divided into Q.  
         [0087]    The angular range is defined by  
         Angular Range=θ 1 ±Δθ/2  (5)  
         [0088]    Δθ in equation (5) is given by equation (2).  
         [0089]    In this modification, angular ranges after equal division in the field “DIRECTION” of the user distribution table  204  are values calculated by sequentially adding Δθ to equation (5). “DOWNLINK DATA RATE X NUMBER OF USERS” is a value calculated by adding the data transmission rates of downlink transmission signals assigned by the channel control unit  114  for users present within each range.  
         [0090]    [0090]FIGS. 9A and 9B show an example when the user distribution table  204  is divided into A and B tables. In A and B tables  205  and  206 , “DIRECTION” and “DOWNLINK DATA RATE X NUMBER OF USERS” represent the same items as those in FIG. 8. In this example, the threshold is experimentally determined for the average user distribution of the base station by using as a reference the average of data transmission rates for all the angular ranges. Similar to the first embodiment, the average is first set as a default value, and perturbation is conducted to determine a threshold so as to minimize the total transmission power of an average base station. The total transmission power of the base station is observed every day. The A table  205  shows a case in which a total of the downlink data transmission rates of users present within each of Q divided angular ranges is smaller than the threshold. To the contrary, the B table  206  shows a case in which a total of the downlink data transmission rates of users present within each of Q divided angular ranges is larger than the threshold.  
         [0091]    Processing in the database  113  is the same as that in the first embodiment, and a transmission directivity pattern is generated by the same operation as that in the first embodiment. In addition to the effects of the first embodiment, the modification effectively suppresses interference with many radio terminals because nulls are directed to radio terminals concentrated in a predetermined direction.  
         [0092]    &lt;Second Embodiment&gt; 
         [0093]    [0093]FIG. 10 shows a transmission directivity control apparatus in a radio base station using CDMA according to the second embodiment of the present invention. In a transmission directivity control apparatus  401  of the second embodiment, the same reference numerals as in the transmission directivity control apparatus  101  of the first embodiment denote the same parts, their operations are substantially the same, and a detailed description thereof will be omitted. In the second embodiment, transmission directivity generation units  402   1  to  402   M  replace the transmission directivity generation units  110   1  to  110   M  in the first embodiment.  
         [0094]    The transmission directivity generation units  402   1  to  402   M  are respectively constituted by arrival direction estimation/transmission power control units  403   1  to  403   M  for receiving reception signals from A/D converters  107   1  to  107   N  estimating the arrival directions of the first to Mth users, and controlling downlink transmission power, and transmission directivity pattern control units  404   1  to  404   M  for controlling the transmission directivity patterns of the first to Mth users. The outputs of the arrival direction estimation/transmission power control units  403   1  to  403   M  are connected to a database  405  such as a magnetic disk which stores output data, and the transmission directivity pattern control units  404   1  to  404   M . The database  405  is connected to the transmission directivity pattern control units  404   1  to  404   M .  
         [0095]    The arrival direction estimation/transmission power control units  403   1  to  403   M  are formed from DSPs each having a CPU, a storage medium such as a ROM which stores a control program, and a RAM serving as a work memory.  
         [0096]    The arrival direction estimation/transmission power control units  403   1  to  403   M  execute despreading for input reception signals  115   1  to  115   N , similar to the first embodiment. The arrival direction estimation/transmission power control units  403   1  to  403   M  assign identification numbers to the reception signals  115   1  to  115   N , and estimate their arrival directions. The format of a reception signal discriminated from other reception signals designates the transmission power of a downlink transmission signal. Based on these reception signals, the arrival direction estimation/transmission power control units  403   1  to  403   M  extract downlink transmission powers designated by users.  
         [0097]    Data  406   1  to  406   M  representing user identification numbers U 1  to U M  output from the arrival direction estimation/transmission power control units  403   1  to  403   M  arrival directions θ 1  to θ M , and downlink transmission powers P 1  to P M  are stored in the database  405 .  
         [0098]    [0098]FIG. 11 shows a data processing flow in the database  405 . The database  405  is made up of a CPU, a storage medium such as a ROM which stores a control program, and a RAM serving as a work memory. The database  405  performs predetermined control by a stored program method. The database  405  creates a user distribution table in which the stored user identification numbers U 1  to U M , arrival directions θ 1  to θ M  and downlink transmission powers P 1  to P M  correspond to each other (step S 121 ).  
         [0099]    [0099]FIG. 12 shows an example of a table structure processed by the database. In the second embodiment, the distribution of the downlink transmission powers of users is stored as a user distribution table  207  in the table form. In the user distribution table  207 , “USER No.” represents the user identification numbers U 1  to U M  assigned by the arrival direction estimation/transmission power control units  403   1  to  403   M . “DOWNLINK DATA RATE” represents the downlink transmission powers P 1  to P M  extracted by the arrival direction estimation/transmission power control units  403   1  to  403   M . “ARRIVAL DIRECTION” represents reception signal arrival directions from users who demand downlink transmission powers listed up in “DOWNLINK DATA RATE”.  
         [0100]    In FIG. 11, the average of the transmission powers of downlink channels is calculated from the arrival directions θ 1  to θ M , user identification numbers U 1  to U M , and downlink transmission powers P 1  to P M  stored in the user distribution table  207  (step S 122 ). The calculated average is multiplied by a coefficient to set a threshold (step S 123 ). This coefficient is an arbitrary coefficient which flexibly copes with the use environment, similar to the first embodiment. By using this threshold, the user distribution table  207  is divided into two, A and B tables (step S 124 ). Note that the mode may be used in place of the average.  
         [0101]    [0101]FIGS. 13A and 13B show an example when the user distribution table is divided into A and B tables. In an A table  208  shown in FIG. 13A, “No.” represents a number assigned for convenience. “DOWNLINK DATA RATE” represents a data transmission power lower than the threshold. “ARRIVAL DIRECTION” is the same as that of FIG. 12, and a description thereof will be omitted. A B table  209  shown in FIG. 13B is identical to the A table  208  except that “DOWNLINK DATA RATE” represents a downlink transmission power higher than the threshold.  
         [0102]    A number M A  of users belonging to the A table  208  and a number M B  of users belonging to the B table  209  are obtained (step S 125 ), and the number M A  of users and the number M B  of users are compared (step S 126 ). If the number M A  of users is smaller than the number M B  of users, arrival directions contained in the A table  208  representing the number M A  of users are set as null generation direction candidates (step S 127 ). If the number M A  of users is larger than the number M B  of users, arrival directions contained in the B table  209  representing the number M B  of users are set as null generation direction candidates (step S 128 ). In this way, null generation direction candidates are selected from the data  406   1  to  406   M  stored in the database  405 . The coefficient of the threshold prevents the number M A  of users and the number M B  of users from being equal to each other. Data  407  representing the selected null generation direction candidates are respectively input to the transmission directivity pattern control units  404   1  to  404   M . Processing in the transmission directivity pattern control units  404   1  to  404   M  is the same as that in the first embodiment, and the subsequent operation is also the same.  
         [0103]    &lt;Modification of Second Embodiment&gt; 
         [0104]    [0104]FIG. 14 shows a modification of the database in the second embodiment. In a user distribution table  210  according to the modification, “DIRECTION” represents an angular range when the direction in which the directivity can be adjusted by the transmission directivity control apparatus  401  of the second embodiment is equally divided into Q. The angular range is the same as a divided angular range described in the modification of the first embodiment. “DOWNLINK DATA RATE X NUMBER OF USERS” is a value calculated by adding the transmission powers of downlink transmission signals extracted by the arrival direction estimation/transmission power control units  403   1  to  403   M  for users present within each range.  
         [0105]    [0105]FIGS. 15A and 15B show an example when the user distribution table  210  shown in FIG. 14 is divided into A and B tables. In an A table  211  shown in FIG. 15A and a B table  212  shown in FIG. 15B, “DIRECTION” and “DOWNLINK DATA RATE X NUMBER OF USERS” represent the same items as those in FIG. 14. The A table  211  shows a case in which a total of the downlink transmission powers of users present within each of Q divided angles is smaller than the threshold. The B table  212  shows a case in which a total of downlink transmission powers is larger than the threshold.  
         [0106]    As described in the second embodiment and its modification, a null generation direction is selected in accordance with power actually transmitted to each user. This selection considers the power difference depending on the distance between users with the same data rate. Hence, interference with users who perform communication at low data rate with small transmission power is more efficiently suppressed.  
         [0107]    &lt;Third Embodiment&gt; 
         [0108]    The third embodiment adopts TDMA (Time Division Multiple Access) or FDMA (Frequency Division Multiple Access). TDMA and FDMA communications realize SDMA (Space Division Multiple Access) in which a plurality of users are spatially multiplexed onto the same time channel or same frequency channel by a directivity pattern.  
         [0109]    [0109]FIG. 16 shows a transmission directivity control apparatus in a radio base station using TDMA/FDMA according to the third embodiment of the present invention. In a transmission directivity control apparatus  501  of the third embodiment, the same reference numerals as in the transmission directivity control apparatus  101  of the first embodiment denote the same parts, their operations are substantially the same, and a detailed description thereof will be omitted.  
         [0110]    In the third embodiment, transmission directivity generation units  502   1  to  502   M  replace the transmission directivity generation units  110   1  to  110   M  in the first embodiment. An arrival direction estimation unit  503  common to users replaces the arrival direction estimation units  111   1  to  111   M  of the first embodiment. The arrival direction estimation unit  503  does not have an arrangement of performing despreading for respective users and separating signals received by the antenna elements of a CDMA radio base station. For this reason, a spatially multiplexed uplink transmission signal is common to all the users.  
         [0111]    The transmission directivity generation units  502   1  to  502   M  are respectively constituted by transmission directivity pattern control units  504   1  to  504   M  for controlling the transmission directivity patterns of the first to Mth users. The output of the arrival direction estimation unit  503  is connected to a database  505  such as a magnetic disk which stores output data, and the transmission directivity pattern control units  504   1  to  504   M . The database  505  is connected to a channel control unit  114  for assigning a channel to the user, and the transmission directivity pattern control units  504   1  to  504   M .  
         [0112]    The arrival direction estimation unit  503  is formed from a DSP having a CPU, a storage medium such as a ROM which stores a control program, and a RAM serving as a work memory. The arrival direction estimation unit  503  receives reception signals  115   1  to  115   N , and detects the directions of spatially multiplexed users by using the MUSIC algorithm.  
         [0113]    The database  505  is made up of a CPU, a storage medium such as a ROM which stores a control program, and a RAM serving as a work memory. The database  505  performs predetermined control by a stored program method. The database  505  creates a data table from arrival directions θ 1  to θ M , user identification numbers U 1  to U M , and downlink channel data rates R 1  to R M  of respective users. A null generation direction candidate  507  for each transmission directivity pattern is determined from the database. Data processing in the third embodiment is substantially the same as that in the first embodiment shown in FIG. 1, and a description thereof will be omitted.  
         [0114]    Also when SDMA is realized, a database made up of the data rates (multiple values) of downlink time channels or frequency channels, and the directions of users multiplexed onto the same time slot or same frequency channel is created. A downlink transmission directivity pattern is determined using this database by the same processing as that in the first embodiment, and interference is efficiently suppressed on the downstream channel.  
         [0115]    A signal prepared by multiplexing a downlink transmission power control signal onto the uplink transmission signal of each user can also be employed. The downlink transmission power of each user can also be extracted from this signal. With this arrangement, a database comprised of the transmission powers and arrival directions of users is created. A downlink transmission directivity pattern is created by the same processing as that in the second embodiment. Accordingly, interference is efficiently suppressed on the downstream channel.  
         [0116]    &lt;Other Modifications&gt; 
         [0117]    In the first embodiment, a null generation direction is selected at random from null generation direction candidates. When the transmission powers of radio terminals as null generation direction candidates are high, radio terminals with higher transmission powers can be sequentially selected to adjust the directivity pattern. In this case, if only a few radio terminals exhibit high transmission powers, the directivity pattern concentratedly directs nulls to the high-transmission-power radio terminals. Radio interference with the high-transmission-power radio terminals can be effectively suppressed. When the transmission powers of radio terminals as null generation direction candidates are low, radio terminals with lower transmission powers can be sequentially selected to adjust the directivity pattern. In this case, if only a few radio terminals exhibit low transmission powers, the directivity pattern concentratedly directs nulls to the low-transmission-power radio terminals. Radio interference with the low-transmission-power radio terminals can be effectively suppressed.  
         [0118]    In the first embodiment, the protection area is set, and radio terminals outside the protection area are selected as null directions. A null generation direction candidate most different from the main beam direction can also be selected as a null direction. In this case, a decrease in the antenna gain of the main beam can be similarly reduced, and an increase in transmission power can be prevented.  
         [0119]    The above-described embodiments have exemplified a transmission directivity control apparatus for controlling the transmission directivity. The present invention can also be applied to a reception directivity control apparatus for controlling the reception directivity. Even the reception directivity control apparatus exhibits a given relationship between the main beam and null of the reception directivity pattern with respect to a plurality of time-overlapping radio terminals. To generate the reception directivity pattern of a certain radio terminal, a main beam in a direction from which radio waves are most strongly received is directed to the radio terminal, and nulls in directions from which radio waves are weakly received are directed to the remaining radio terminals. The same directivity control as those in the first to third embodiments can be achieved by adopting a database using the reception powers or transmission rates of radio terminals received by the radio base station, and a reception directivity pattern control unit for generating a reception directivity pattern.  
         [0120]    As has been described above, according to the present invention, when a group of high-transmission-power radio terminals is selected as null generation candidates, transmission directivity patterns having nulls toward the high-transmission-power radio terminals are generated. This suppresses interference with the high-transmission-power radio terminals, increases the ratio of signal power to interference power (SIR) in the high-transmission-power radio terminals, and reduces necessary transmission power. Controlling transmission power yields the effect of reducing transmission power to the high-transmission-power radio terminals, and suppressing interference with low-transmission-power radio terminals. When a group of low-transmission-power radio terminals is selected as null generation candidates, nulls are generated toward the low-transmission-power radio terminals. Interference with the low-transmission-power radio terminals can be suppressed. Since the directivity pattern is generated based on transmission power in transmission, the power difference depending on the distance between radio terminals with the same transmission rate can be considered.  
         [0121]    The threshold is calculated by multiplying the average or mode of transmission power by a predetermined coefficient, and can flexibly cope with use environment conditions such as the number of users and the user distribution. For example, the average is first set as a default value, and perturbation is conducted to determine a threshold so as to minimize the average of total transmission power. The total transmission power is observed every day. As a result, interference can be optimally suppressed.  
         [0122]    A protection area is set in the direction of a desired radio terminal, and no null is generated in the protection area. A decrease in the antenna gain of the main beam by null generation can be reduced, an increase in transmission power can be prevented, and interference with other radio terminals can be suppressed.  
         [0123]    Radio terminals are selected at random to adjust the directivity pattern. This directivity pattern distributively weakens radio waves to many selected radio terminals. Radio interference with many radio terminals can be effectively suppressed. Even if the number of interference waves arriving at the radio base station exceeds the degree of freedom of the adaptive antenna, nulls equivalently having a depth to a certain degree are generated by generating transmission directivity patterns for a plurality of radio terminals. Radio interference with many radio terminals can be suppressed.  
         [0124]    The use of a divided angular range can prevent concentrated selection of radio terminals in close directions with respect to directions in which a plurality of radio terminals exist. Close radio terminals are concentratedly selected to generate nulls in close directions. Nulls are distributively generated in many directions, which provides a more efficient effect over the entire distribution.  
         [0125]    Since the transmission rate in transmission to a radio terminal is used, the transmission rate in assigning a channel can be easily exploited.  
         [0126]    Since the reception directivity pattern is adjusted similar to the transmission directivity pattern, an effect corresponding to the reception directivity pattern can be attained.