Patent Publication Number: US-2007110064-A1

Title: Multicast transmission method, multicast transmission system, mobile station and base station

Description:
TECHNICAL FIELD  
      The present invention relates to an automatic repeat request (ARQ) technique for improving frame use efficiency in multicast signal transmission to increase throughput in a multicast transmission system.  
      In addition, the present invention relates to a technique for improving throughput of the whole system by improving receiving quality of a multicast signal in a mobile station or by shortening time required for communication of a multicast signal in a multicast transmission system.  
     BACKGROUND ART  
       FIG. 1  is a block diagram of a conventional multicast transmission system. As shown in the figure, the system includes a base station  1  and a mobile station  11 . The base station  1  includes a multicast signal input terminal  2 , an ARQ processor  3 , a transmitter  4  and a receiver  5 . The mobile station  11  includes a multicast signal output terminal  12 , an error detection/ARQ processor  13 , a transmitter  14  and a receiver  15 .  
      In the base station  1 , a multicast signal input from the multicast signal input terminal  2  is received by the ARQ processor  3 . The ARQ processor  3  sends the multicast signal to the transmitter  4  after adding error detection code such as CRC to the multicast signal such that an error can be detected slot by slot after dividing the multicast signal into slots. The transmitter  4  sends the signal from the ARQ processor  3  to each mobile station after modulating the signal into a carrier wave.  
      In the mobile station  11 , the receiver  15  receives and demodulates the multicast signal  7 , and outputs the multicast signal  7  to the error detection/ARQ processor  13 . The error detection/ARQ processor  13  performs error detection for the received multicast signal  7  slot by slot. When an error is detected in the multicast signal  7 , the error detection/ARQ processor  13  outputs a retransmission request (NACK (negative acknowledgement)) signal to the transmitter  14  in a random timing which is predetermined in each mobile station. Then, the transmitter  14  sends NACK to the base station  1  via an uplink channel  6 . In addition, when there is no error in the multicast signal  7 , the mobile station  11  receives multicast signals  7  hereafter without outputting any signal.  
      The receiver  5  in the base station  1  receives the signal via the uplink channel  6  and demodulates the received signal, and outputs the received signal to the ARQ processor  3 . When the received signal is NACK, the ARQ processor  3  stops sending the multicast signal input from the multicast signal input terminal  2  temporarily, and retransmits the multicast signal required by NACK. When the received signal is not NACK, the ARQ processor  3  sends a next multicast signal.  
      Next, operation of ARQ will be described with reference to  FIG. 2 .  FIG. 2  shows an example in which one base station and three mobile stations exist.  
      As for a multicast signal  1  sent by a slot  1 , since mobile stations  1 - 3  do not detect any error in the received multicast signal  1 , the mobile stations  1 - 3  enter a waiting state for a next slot  2 .  
      As for a multicast signal  2  sent by the next slot  2 , since the mobile station  3  do not detect any error in the received multicast signal  2 , the mobile station  3  enters a waiting state for a next slot  3 . However, the mobile station  1  and the mobile station  2  detect an error in the received multicast signal  2 , the mobile station  1  and the mobile station  2  send NACK to the base station. Since the mobile station  1  adopts Δt as the random timing, the mobile station  1  sends NACK after elapsing Δt from receiving the multicast signal  2 . Since the mobile station  2  adopts  2 Δt as the random timing, the mobile station  2  sends NACK after elapsing  2 Δt from receiving the multicast signal  2 . Then, since the base station receives NACK from the two mobile stations, the base station retransmits the multicast signal  2  in a next slot  3 .  
      As for the multicast signal  2  retransmitted by the slot  3 , as in the case of receiving the previous slot, the mobile station  3  does not detect any error and the mobile stations  1  and  2  detect an error. Since the mobile station  1  adopts Δt for setting the random timing, the mobile station  1  sends NACK after elapsing Δt from receiving the multicast signal  2 . Likewise, since the mobile station  2  adopts Δt for setting the random timing, the mobile station  2  sends NACK after elapsing Δt from receiving the multicast signal  2 . In this case, since the base station can not detect NACK due to collision of NACK signals from the two mobile stations, a multicast signal  3  is sent in a next slot  4 .  
      Then, since the multicast signal  3  sent by the slot  4  is different from a multicast signal which is required by the mobile stations  1  and  2 , the multicast signal  3  becomes an error in the mobile stations  1  and  2  even when the mobile stations do not detect any error in the received signal. As a result, retransmission is requested again.  
      As mentioned above, in the conventional multicast transmission ARQ technique, since a reserved interval is necessary for the retransmission request, capacity is wasted. In addition, there is a large possibility that a malfunction for sending multicast signals after collision of NACK signals occur.  
      Further, in the conventional multicast transmission, the base station receives NACK as a retransmission request and sends a multicast signal which is required by the retransmission request. However, when there exists an mobile station in which receiving quality is low, retransmission is repeated due to the mobile station so that delay time is increased and throughput of the whole system is lowered.  
     DISCLOSURE OF THE INVENTION  
      The present invention is contrived in view of the above-mentioned problems. A first object of the present invention is to provide a multicast transmission method, a system, a mobile station and a base station in which the reserved interval for the retransmission request is reduced so that frame use efficiency is improved and throughput is increased in multicast signal transmission.  
      A second object of the present invention is to provide a multicast transmission method, a system, a mobile station and a base station in which receiving quality of a mobile station which receives a multicast signal is improved or time required for communication of the multicast signal is reduced so that throughput of the whole system is improved.  
      The first object can be achieved by the following configuration.  
      The present invention is a multicast transmission method in a multicast transmission system in which the same information is transmitted from a base station to a plurality of mobile stations, the multicast transmission method including the steps of:  
      a mobile station in the mobile stations sending a retransmission request signal to the base station when the mobile station detects an error in a received multicast signal; and  
      the base station judging whether a received signal indicates a retransmission request according to receiving quality of the received signal, and retransmitting a multicast signal corresponding to the retransmission request when the received signal indicates the retransmission request.  
      According to the invention, since the base station judges whether a received signal indicates a retransmission request according to receiving quality of the received signal, it becomes unnecessary to keep a reserved interval for avoiding collision of NACK signals as a conventional method so that the mobile station can send the retransmission request signal at any timing. Therefore, frame use efficiency improves and throughput increases.  
      In the above-mentioned configuration, the base station may use receiving power as the receiving quality, and judge that the received signal is the retransmission request from the mobile station when receiving power of the received signal is greater than a threshold.  
      By using the receiving power as the receiving quality, retransmission request can be recognized by using a noise level as a threshold for example.  
      In the above-mentioned configuration, the mobile station may send spreading code as the retransmission request signal, and the base station obtains receiving quality of the spreading code, and the base station judges that the received signal is the retransmission request when the receiving quality is greater than a threshold.  
      By using the spreading code as the receiving quality, retransmission request can be recognized by using a correlation value as a threshold for example.  
      In the above-mentioned configuration, the base station may perform pass diversity for receiving a signal from the mobile station. Accordingly, powers of retransmission request signals which are dispersed on a time axis can be combined so that the receiving quality of the received signal from the mobile station can be improved.  
      The second object can be achieved by the following configuration.  
      The present invention is a multicast transmission method in a multicast transmission system in which the same information is transmitted from a base station to a plurality of mobile stations, the multicast transmission method including the steps of:  
      a mobile station sending a retransmission request signal to the base station when the mobile station detects an error in a multicast signal; and  
      the base station monitoring a receiving state of the multicast signal in the mobile stations, and changing a multicast transmission method to conform to the receiving state according to a result of monitoring, and sending a multicast signal.  
      The receiving state can be monitored by receiving the retransmission request signal for example. By using the result of the monitoring, an antenna can be controlled such that directivity to the corresponding mobile station can be increased, so that a multicast signal can be sent in which the intensity of the multicast signal conforms to the receiving state of the mobile station. According to this configuration, the number of mobile stations which need retransmission can be decreased successively. As a result, antenna gain to the mobile stations which need retransmission can be further increased so that retransmission can be decreased rapidly. Therefore, the conventional problem in that throughput of the whole system decreases due to retransmission request and retransmission can be solved.  
      In addition, the present invention is a multicast transmission method in a multicast transmission system in which the same information is transmitted from a base station to a plurality of mobile stations, the multicast transmission method including the steps of:  
      a mobile station sending a retransmission request signal to the base station when the mobile station detects an error in a multicast signal; and  
      the base station determining directivity of an antenna on the basis of an incoming wave from the mobile station, and retransmitting a multicast signal by using the directivity of the antenna.  
      In addition, the present invention is a multicast transmission method in a multicast transmission system in which the same information is transmitted from a base station to a plurality of mobile stations, the multicast transmission method including the steps of:  
      a mobile station sending a retransmission request signal to the base station when the mobile station detects an error in a multicast signal; and  
      the base station retransmitting a multicast signal corresponding to the retransmission request signal after changing a transmission method when the base station receives the retransmission request signal from the mobile station.  
      In the above configuration, the transmission method to be changed is an antenna directivity, a modulation method, a transmission speed, a spreading modulation method, error correction code, or a coded ratio.  
      Also, according to the above-mentioned invention, it becomes possible that the multicast signal can be retransmitted or transmitted such that the transmission method conforms to the receiving state of the mobile station. Thus, the number of the retransmission request from the mobile stations can be decreased so that the conventional problem in that throughput of the whole system decreases can be solved.  
      In addition, the present invention is a multicast transmission method in a multicast transmission system in which the same information is transmitted from a base station to a plurality of mobile stations, the multicast transmission method including the steps of:  
      a mobile station measuring receiving quality of a multicast signal, and judging whether the mobile station sends a retransmission request signal according to a result of the measuring; and  
      the base station retransmitting a multicast signal corresponding to the retransmission request signal when the base station receives the retransmission request signal from the mobile station.  
      According to the invention, since the mobile station judges whether it sends a retransmission request from the result of measuring the receiving quality of the multicast signal, the mobile station can send the retransmission request only when the receiving quality is good as mentioned later, for example. Accordingly, the possibility of detecting an error in a retransmitted multicast signal is lowered. That is, the number retransmissions can be decreased as a whole.  
      In the above configuration, in the multicast transmission method, when the mobile station detects an error in the multicast signal, the mobile station sends the retransmission request signal when the receiving quality is better than a predetermined value, and the mobile station may store the retransmission request signal when the receiving quality is not better than a predetermined value; and the mobile station may send the retransmission request signal which is stored when receiving quality becomes better than a predetermined value.  
      In the above configuration, the receiving quality is receiving power of a received multicast signal, a ratio (CIR) between received multicast signal and interference power, an error rate of bit, packet or slot of a received multicast signal, or, the number of bit errors which were corrected or likelihood obtained when decoding error correction code. Accordingly, various parameters can be used as the receiving quality.  
      In the above configuration, when the base station sends a retransmitting multicast signal or when the base station sends a new multicast signal after sending a retransmitting multicast signal, the base station sends the retransmitting multicast signal or the new multicast signal by using a specific channel which is occupied for communication between a mobile station which receives the retransmitting multicast signal or the new multicast signal and the base station.  
      According to the present invention, time required for transmitting the multicast signal can be shortened.  
      In the above configuration, if the mobile station receives a retransmitted multicast signal without an error after sending a retransmission request signal to the base station when detecting an error in a received multicast signal, the mobile station does not perform error detection for a multicast signal which includes the same information as the retransmitted multicast signal and which is further retransmitted after receiving the retransmitted multicast signal; and  
      when the mobile station does not detect any error in a received multicast signal, the mobile station does not send any signal.  
      Accordingly, the number of retransmission requests can be decreased.  
      Other objects, features and advantages of the present invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a block diagram of a conventional multicast transmission system;  
       FIG. 2  is a figure showing an example of an automatic repeat request (ARQ) operation according to a conventional technology;  
       FIG. 3  is a block diagram of a multicast transmission system of an example 1-1 of the present invention;  
       FIG. 4  is a figure showing an example of an automatic repeat request (ARQ) operation in the configuration of the example 1-1 of the present invention shown in  FIG. 3 ;  
       FIG. 5  is a block diagram of a multicast transmission system of an example 1-2 of the present invention;  
       FIG. 6  is a block diagram of a multicast transmission system of an example 1-3 of the present invention;  
       FIG. 7  is a block diagram of a multicast transmission system of an example 2-1 of the present invention;  
       FIG. 8  is a figure showing an example for controlling base station antenna directivity in the example 2-1 of the present invention;  
       FIG. 9  is a block diagram of a multicast transmission system of an example 2-2 of the present invention;  
       FIG. 10  is a flowchart showing an operation of the multicast transmission system of an example 2-2 of the present invention;  
       FIG. 1l  is a block diagram of a multicast transmission system of an example 2-3 of the present invention;  
       FIG. 12  is a figure showing an example of an operation for changing transmission speed and modulation method in the multicast transmission system of the example 2-3 of the present invention. 
    
    
     PREFERRED EMBODIMENTS FOR CARRYING OUT THE INVENTION  
     First Embodiment  
      First, a first embodiment corresponding to the first object of the present invention will be described by using examples.  
     EXAMPLE 1-1  
      An example 1-1 of the present invention will be described with reference to  FIG. 3  and  FIG. 4 . In this example, when the base station detects receiving power which is greater than a threshold, the base station recognizes it as a retransmission request for a multicast signal. In this example, the threshold is determined as a noise level.  
       FIG. 3  is a block diagram of a multicast transmission system of the first example. A base station  121  includes a receiving power measuring device  122 , and an error detection/ARQ processor  133  in a mobile station  131  performs an operation different from the error detection/ARQ processor  13  in the conventional mobile station  11 . These are different points from the conventional multicast transmission system.  
      In the base station  121 , a multicast signal input from the multicast signal input terminal  102  is received by an ARQ processor  103 . The ARQ processor  103  sends the multicast signal to the transmitter  104  after adding error detection code such as CRC to the multicast signal such that an error can be detected slot by slot after dividing the multicast signal into slots. The transmitter  104  sends the signal from the ARQ processor  103  to each mobile station after modulating the signal into a carrier wave.  
      In the mobile station  131 , the receiver  115  receives and demodulates the multicast signal  107 , and outputs the multicast signal  107  to the error detection/ARQ processor  133 . The error detection/ARQ processor  133  performs error detection for the received multicast signal  107  slot by slot. When an error is detected in the multicast signal  107 , the error detection/ARQ processor  133  outputs a signal for requesting retransmission to the transmitter  114 . Then, the transmitter  114  sends the signal to the base station  121  via an uplink channel  106 . The signal for requesting retransmission may be a fixed bit pattern. In addition, when there is no error in the multicast signal  107 , the mobile station  131  receives multicast signals hereafter without outputting any signal. In the above-mentioned operation, it is not necessary to predetermine timing for the mobile station to send the retransmission request signal when the base station performs stored demodulation.  
      After receiving the uplink channel  106 , the receiver  105  in the base station  121  outputs the received signal to a receiving power measurement device  122 . The receiving power measurement device  122  measures the receiving power of the retransmission request signal from the mobile station, then, when the measured value is greater than the noise level, the receiving power measurement device  122  sends NACK to the ARQ processor  103 . The ARQ processor  103  stops sending the multicast signal received from the multicast signal input terminal  102  temporarily, and retransmits the multicast signal requested by NACK. When the receiving power is equal to or lower than the noise level, the receiving power measurement device  122  does not send any signal to the ARQ processor  103 . Thus, the ARQ processor  103  sends a next multicast signal. Accordingly, the receiving power can be used as the receiving quality.  
      In the following, the operation of ARQ will be described concretely with reference to  FIG. 4 .  FIG. 4  shows an operation in which one base station and three mobile stations exist.  
      As for a multicast signal  1  sent by the slot  1 , since the mobile stations  1 - 3  do not detect an error in the multicast signal  1 , the mobile stations  1 - 3  enter a waiting state for a next slot  2 .  
      As for the multicast signal  2  sent by the next slot  2 , since the mobile station  3  does not detect an error in the received multicast signal  2 , the mobile station  3  enters a waiting state for receiving the next slot  3 . However, since the mobile stations  1  and  2  detect an error in the received multicast signal  2 , the mobile stations  1  and  2  send a signal for requesting retransmission. Then, although collision of signals for requesting retransmission from two mobile stations occurs, the base station recognizes that retransmission of the multicast signal  2  is requested because the receiving power measured by the receiving power measuring device is greater than the noise level. Thus, the base station retransmits the multicast signal  2  in the next slot  3 .  
      As for the multicast signal  2  retransmitted by the slot  3 , as in the case of above-mentioned slot  2 , since the mobile station  3  does not detect any error and the mobile station  1  and  2  detect an error, the mobile station  1  and  2  send a signal for requesting retransmission to the base station again. Then, the base station recognizes that retransmission of the multicast signal  2  is requested by measuring the receiving power, then, the base station retransmits the multicast signal  2  in the next slot  4 .  
      In the slot  4 , the multicast signal  2  requested by the mobile stations  1  and  2  is retransmitted. Since the mobile stations do not detect an error in the multicast signal  2 , the mobile stations enter a waiting state for receiving a next slot. While receiving and transmitting a series of multicast signals, the above-mentioned operation is repeated.  
      In this way, since the base station determines whether a received signal is a retransmission request by using receiving power level as a receiving quality instead of using the NACK signal itself, the base station can detect a retransmission request even when collision of the retransmission request signals from a plurality of mobile stations occurs. Therefore, since the reserved interval for sending the retransmission request signal can be shortened, the frame use efficiency improves, in which the reserved interval was determined having an adequate margin such that collision does not occur conventionally. That is throughput can be increased.  
     EXAMPLE 1-2  
      An example 1-2 of the present invention will be described with reference to  FIG. 5 . In this example, orthogonal GOLD code which is one of spreading codes is used as the retransmission request signal.  
       FIG. 5  is a block diagram of a multicast transmission system of the example 1-2. A base station  141  includes a receiving quality detector  142 , and an error detection/ARQ processor  153  in a mobile station  151  performs an operation different from the error detection/ARQ processor  13  in the conventional mobile station  11 . These are different points from the conventional multicast transmission system.  
      In the base station  141 , a multicast signal input from the multicast signal input terminal  102  is received by an ARQ processor  103 . The ARQ processor  103  sends the multicast signal to the transmitter  104  after adding error detection code such as CRC to the multicast signal such that an error can be detected slot by slot after dividing the multicast signal into slots. The transmitter  104  sends the signal from the ARQ processor  103  to each mobile station after modulating the signal into a carrier wave.  
      In the mobile station  151 , the receiver  115  receives and demodulates the multicast signal  107 , and outputs the multicast signal  107  to the error detection/ARQ processor  153 . The error detection/ARQ processor  153  performs error detection for the received multicast signal  107  slot by slot. When an error is detected in the multicast signal  107 , the error detection/ARQ processor  153  outputs the orthogonal GOLD code which is a retransmission request signal. When there is no error in the multicast signal  107 , the mobile station  151  receives multicast signals hereafter without outputting any signal.  
      The orthogonal Gold code is one example of spreading codes which has a sharp peak in the autocorrelation value. When using such code, a correlator is used as the receiving quality detector  142  in the base station  141 , and a correlation value obtained by the correlator is the receiving quality.  
      In the base station  141 , the correlation value is obtained by inputting the received signal which includes equal to or more than one orthogonal Gold codes to the receiving quality detector  142 . When the correlation value obtained by the receiving quality detector  142  is greater than a threshold, the received signal is recognized as the retransmission request. In addition, by associating an orthogonal Gold code with a numbered multicast signal uniquely, the base station can determine which multicast signal needs to be sent. That is, the mobile station  151  sends the orthogonal Gold code which is uniquely associated with the number of the multicast signal which the mobile station  151  needs to be retransmitted. The base station obtains correlation values between the received orthogonal Gold code and every possible orthogonal Gold code. Then, when a correlation value is greater than a threshold, the base station retransmits a multicast signal having the number which uniquely corresponds to the orthogonal Gold code used for obtaining the correlation value greater than the threshold. In addition, since the more the number of the mobile stations, the greater the correlation value is. Thus, when the base station sends a multicast signal in which the corresponding correlation value is greater than a threshold value and is the largest, it becomes possible that the number of re-retransmission request signals which may occur hereinafter can be decreased.  
      In this case, since distances between each mobile station which sends the retransmission request signal and the base station are different, and multipath occurs, there may be some timings in which the correlation value takes a peak. Thus, by adopting pass diversity, in the same way as Rake receive in the CDMA system, it is possible to improve ability for detecting receiving quality by synthesizing receiving powers of the retransmission request signals dispersed on the time axis.  
      In addition, when the timing for sending the retransmission request signal differs by each mobile station, there is a possibility in that receiving quality can not be detected accurately due to that correlation detection timing in the receiving quality detector  142  of the base station  141  is unknown. However, this problem can be solved when the receiving quality detector  142  performs stored demodulation.  
      Accordingly, since the mobile stations can send signals without considering collision of retransmission request signals at the base station, the reserved interval for sending retransmission request signal can be decreased, in which the reserved interval was determined having an adequate margin such that collision does not occur conventionally. Thus, the frame use efficiency improves. In addition, the retransmission request signal can be detected even when the collision of the retransmission request signal occurs.  
     EXAMPLE 1-3  
      An example 1-3 of the present invention will be described with reference to  FIG. 6 . In this example, error correction code which is one of spreading codes is used as the retransmission request signal.  
       FIG. 6  is a block diagram of a multicast transmission system of the example 1-3. A base station  161  includes a receiving quality detector  162 , and an error detection/ARQ processor  173  in a mobile station  171  performs an operation different from the error detection/ARQ processor  13  in the conventional mobile station  11 . These are different points from the conventional multicast transmission system.  
      In the base station  161 , a multicast signal input from the multicast signal input terminal  102  is received by an ARQ processor  103 . The ARQ processor  103  sends the multicast signal to the transmitter  104  after adding error detection code such as CRC to the multicast signal such that an error can be detected slot by slot after dividing the multicast signal into slots. The transmitter  104  sends the signal from the ARQ processor  103  to each mobile station after modulating the signal into a carrier wave.  
      In the mobile station  171 , the receiver  115  receives and demodulates the multicast signal  107 , and outputs the multicast signal  107  to the error detection/ARQ processor  173 . The error detection/ARQ processor  173  performs error detection for the received multicast signal  107 . When an error is detected in the multicast signal  107 , the mobile station  171  sends the error correction code which is a retransmission request signal. When there is no error in the multicast signal  107 , the mobile station  171  receives multicast signals hereafter without sending any signal.  
      The correlation vale of the spreading code is inversely proportional to the number of usable codes. That is, as for a spreading code such as the orthogonal Gold code in which cross-correlation value with other orthogonal Gold code becomes zero, the number of the spreading codes becomes the number of bits which form one code. However, as the cross-correlation value becomes closer to 1, the number of usable spreading codes can be increased. The error correction code is one of the spreading codes in which the correlation value is allowed to some extent so that the number of usable codes can be increased. When using such code, an error correction decoder is used as the receiving quality detector  162  of the base station  161  in which the number of correctable bits obtained when the decoder performs decoding, distance between codes, or likelihood used in decoding is used as the receiving quality. The kind of the code which is used as the spreading code is determined according to the number of codes which are desired to be used, the size of the receiving quality detector, propagation environment between the base station and the mobile station which receives the multicast signal and the like.  
      The base station  161  obtains the receiving quality by inputting the received signal in which equal to or more than one error correction codes are included to the receiving quality detector  162 . In the process of the receiving quality detector  162  which is a decoder, distances between the received error correction code and every possible error correction code are obtained first. Here, the number of correctable bits or the likelihood can also be used. Then, since the shorter the distance between the codes is, the more definite the signal is, a reciprocal of the distance between the codes is obtained and when the reciprocal is greater than a threshold, the signal is determined to be the retransmission request. Then, the base station retransmits a multicast signal of a number uniquely corresponding to the error correction code used for obtaining the reciprocal of the distance of the codes which is greater than the threshold.  
      Accordingly, since the mobile stations can send signals without considering collision of retransmission request signals at the base station, the reserved interval for sending retransmission request signal can be decreased, in which the reserved interval was determined for having an adequate margin such that collision does not occur conventionally. Thus, the frame use efficiency improves. In addition, the retransmission request signal can be detected even when the collision of the retransmission request signals occurs.  
      Although, in the examples 1-2 and 1-3, examples have been described in which the orthogonal Gold code and the error correction code which are spreading codes are used as the retransmission request signal, other spreading codes can also be used such as BCH code, Reed-Solomon code, convolutional code, Preparata code, orthogonal code, biorthogonal code, Gold code, Gold-like code, orthogonal convolutional code, comma-free code, Turbo code and the like.  
      As mentioned above, according to the conventional technique, the collision of the retransmission request signals is avoided by providing the reserved interval. On the other hand, according to the first embodiment of the present invention, following effect can be obtained. Since the retransmission request can be recognized even when the collision of the retransmission request signals occurs, the reserved interval can be shortened so that frame use efficiency can be improved and throughput is increased in the multicast signal transmission.  
     Second Embodiment  
      In the following, a second embodiment corresponding to the second object of the present invention will be described using examples.  
     EXAMPLE 2-1  
      The example 2-1 of the present invention will be described with reference to  FIGS. 7 and 8 .  FIG. 7  is a block diagram of the multicast transmission system in the example 2-1.  
      The base station  221  includes a multicast signal input terminal  202 , an ARQ processor  203 , a transmitter  204 , a receiver  205 , a duplexer  222 , a weight controller  223  and array antenna elements  224 . The mobile station  231  includes a multicast signal output terminal  212 , an error detection/ARQ processor  213 , a transmitter  214  and a receiver  215 .  
      In the base station  221 , a multicast signal input from the multicast signal input terminal  202  is received by the ARQ processor  203 . The ARQ processor  203  sends the multicast signal to the transmitter  204  after adding error detection code such as CRC to the multicast signal such that an error can be detected slot by slot after dividing the multicast signal into slots. The transmitter  204  sends the signal from the ARQ processor  203  to the duplexer  222  after modulating the signal into a carrier wave.  
      In this embodiment, in the first multicast signal transmission (multicast signal transmission which is not retransmission), since the weights for each array antenna element  224  of the weight controller  223  are the same, the directivities of the antennas of the base station become the same for every direction. The carrier wave from the duplexer  222  is transmitted, from the base station antennas having the directivity, to each mobile station  231  which receives the multicast signal.  
      In the mobile station  231 , the receiver  215  receives and demodulates the multicast signal  207 , and outputs the multicast signal  207  to the error detection/ARQ processor  213 . The error detection/ARQ processor  213  performs error detection for the received multicast signal  207  slot by slot. When an error is detected in the multicast signal  207 , the error detection/ARQ processor  213  outputs a signal for requesting retransmission to the transmitter  214 . Then, the transmitter  214  sends the signal to the base station  221  via an uplink channel  206 . In this example, NACK which is used when the ARQ normally requests retransmission is used as a signal for retransmission request in this example. In addition, when there is no error in the multicast signal  207 , the mobile station  231  receives multicast signals hereafter without outputting any signal.  
      The base station  221  inputs an incoming wave, received by each array antenna, sent from a plurality of mobile stations which request retransmission to the weight controller  223 . The weight controller  223  inputs the incoming waves from each mobile station to the duplexer  222  by multiplying weights so as to separate the incoming waves from each mobile station. According to the weight obtained at this time, the directivity of the base station antenna is controlled such that gain is increased to each mobile station which requested retransmission.  
      Next, after the receiver  205  receives the received signal from the duplexer  222  and demodulates the received signal, the demodulated retransmission request signal is input into the ARQ processor  203 . The ARQ processor  203  temporarily stops sending the multicast signal which is input from the multicast signal input terminal  202 , and sends the multicast signal which is requested to be retransmitted. At this time, the multicast signal is retransmitted to each mobile station which requested retransmission by using antenna directivity such that gain becomes large for each mobile station which requested retransmission by using the weight values obtained when receiving the incoming waves.  
      The above-mentioned operation is repeated until the retransmission request from each mobile station stops.  
      Since the number of mobile stations which request retransmission decreases by repeating retransmission, it becomes possible to increase antenna gain for mobile stations which request retransmission gradually so that receiving quality of a multicast signal to be retransmitted.  
      Next, the above-mentioned operation will be described concretely by using  FIG. 8 . FIGS.  8  shows an operation when three mobile stations exist.  
      When sending an multicast signal for the first time, since weights for each array antenna element provided from the weight controller are the same, the base station sends the multicast signal with the same antenna gains for each direction.  
      As shown in  FIG. 8 ( a ), since an MS (mobile station)  1  does not detect an error in the received multicast signal, the MS 1 does not send any signal to the base station, and enters a waiting state for a next new multicast signal. However, since an MS 2 and an MS 3 detect an error in the received multicast signal, they requests retransmission.  
      Then, as shown in  FIG. 8 ( b ), the BS (base station) increases antenna gain for the directions to the positions where the MS 2 and the MS 3 reside by separating incoming waves from the MS 2 and the MS 3 obtained by each array antenna element by multiplying weight value provided from the weight controller  23 .  
      When sending the signal for a second time (first retransmission), the multicast signal is sent by the base station antennas in which gain for directions of the MS 2 and MS 3 is increased by receiving retransmission request signals for the multicast signal sent first. In this case, since the MS 2 does not detect any error in the retransmitted multicast signal, the MS 2 does not send any signal to the base station and enters a state of waiting a next new multicast signal. However, since the MS 3 detects an error in the retransmitted multicast signal, the MS 3 requests retransmission again. Here, since the MS 1 did not detect any error when receiving the multicast signal for the first time, the MS 1 does not perform error detection for the multicast signal sent for the second time.  
      As shown in  FIG. 8 ( c ), the base station is controlled such that receiving power from the MS 3 becomes the largest by multiplying incoming wave by the weight value from the weight controller  23 . That is, the base station antenna gain becomes the largest in the direction of the MS 3.  
      When sending the multicast signal for a third time (second retransmission), the multicast signal is re-retransmitted by using an base station antenna in which gain of the direction of the MS 3 is increased by the request of re-retransmission for the retransmitted multicast signal. Then, since the MS 3 does not detect any error in the received re-retransmitted multicast signal, the MS 3 does not send any signal to the mobile station, and enters state of waiting a next new multicast signal. Since the MS 1 and the MS 2 did not detect an error in the multicast signal, they do not perform error detection for the re-retransmitted multicast signal.  
      According to the above-mentioned operation, receiving quality is improved by increasing base station antenna gain for each mobile station which requests retransmission. Thus, the possibility of repeating retransmission decreases so that throughput of the whole system improves.  
     EXAMPLE 2-2  
      Next, an example 2-2 of the present invention will be described.  FIG. 9  is a block diagram of a multicast transmission system of the example 2-2. A base station  241  includes a multicast signal input terminal  202 , an ARQ processor  203 , a transmitter  204 , and a receiver  205 . The mobile station  251  includes a multicast signal output terminal  212 , an error detection/ARQ processor  253 , a transmitter  214 , a receiver  215 , and a receiving power measuring part  252 .  
      The operation of the multicast transmission system of the example 2-2 will be described by using  FIG. 9  and a flowchart of  FIG. 10 .  
      The base station  241  provides an multicast signal input from the multicast signal input terminal  202  to the ARQ processor  203 . The ARQ processor  203  divides the input multicast signal into slots. After that, the ARQ processor adds error detection code such as CRC to the multicast signal such that an error can be detected slot by slot, and sends the multicast signal to the transmitter  204 . The transmitter  204  modulates the signal from the ARQ processor  203  into a carrier wave and sends it to each mobile station (step  1 ).  
      In the mobile station  251 , the receiver  215  receives and demodulates the multicast signal  207 . After that, the receiver  215  outputs the signal to the error detection/ARQ processor  253 . In addition, a receiving power value of the multicast signal measured by the receiving power measuring part  252  is input to the error detection/ARQ processor  253  (step  2 ).  
      The error detection/ARQ processor  253  performs error detection for the received multicast signal  207  slot by slot, and outputs a retransmission request signal to the transmitter  214  when the receiving power value is equal to or greater than a predetermined value and an error is detected (steps  3 - 5 ). When the receiving power value is smaller than a predetermined value and the error is detected, the retransmission request signal is not output to the transmitter  214 , and is stored in a memory in the error detection/ARQ processor  253  (steps  3 ,  4 ,  6 ). When an error is not detected, the mobile station enters a state of waiting for a next multicast signal regardless of receiving power value (steps  3 ,  7 ).  
      The error detection/ARQ processor  253  performs receiving processing of multicast signals received hereafter irrespective of whether the retransmission signal is stored or not, and stores a retransmission request signal corresponding to a received multicast signal in which an error is detected as long as the receiving power value is smaller than the predetermined value (steps  3 ,  4 ,  6  and steps  6 ,  8 ). Then, when the receiving power value becomes equal to or greater than a predetermined value, the stored retransmission request signal is output successively (steps  8 ,  5 ). The transmitter  214  sends the input retransmission request signal to the base station  241  via an uplink channel  206  (step  5 ).  
      It is possible to use a specific channel used specifically for communication between the mobile station and the base station for transmitting the retransmission request signal from the mobile station and for retransmitting the multicast signal corresponding to the retransmission request signal. By using the specific channel, since the time required for transmitting the multicast signal is not dominated by a mobile station in which receiving state is bad, the time can be decreased.  
      The receiver  205  in the base station  241  outputs the received signal to the ARQ processor  203  after receiving/demodulating an uplink channel  206 . When the ARQ processor  203  receives a retransmission request signal, the ARQ processor temporarily stops sending a multicast signal input from the multicast signal input terminal  202 , and sends the multicast signal requested by the retransmission request signal again (step  9 ). When the ARQ processor  203  does not receive the retransmission request signal, the ARQ processor  203  sends a next multicast signal.  
      As mentioned above, since the retransmission request is sent when the receiving power value is equal to or greater than a predetermined value, the multicast signal is retransmitted when receive state of a mobile station which receives the multicast signal is good. Therefore, the possibility of retransmitting the multicast signal equal to or more than two times decreases so that throughput of the whole system improves.  
     EXAMPLE 2-3  
      Next, an example 2-3 of the present invention will be described with reference to  FIG. 11  and  FIG. 12 . In this example, 64QAM, 16QAM, and QPSK are used as modulation methods.  
       FIG. 11  is a block diagram of a multicast transmission system in the example 2-3. The base station  261  includes a multicast signal input terminal  202 , an ARQ processor  203 , a transmitter  264  and a receiver  205 . The transmitter  264  includes a modulation parameter control part  265  and modulation/sending part  266 . The mobile station  271  includes a multicast signal output terminal  212 , an error detection/ARQ processor  213 , a transmitter  214  and a receiver  275 . The receiver  275  includes a modulation parameter estimation part  276  and a receiving/modulation part  277 .  
      In the base station  261 , a multicast signal input from the multicast signal input terminal  202  is provided to the ARQ processor  203 . The ARQ processor  203  divides the input multicast signal into slots. After that, the ARQ processor adds error detection code such as CRC to the multicast signal such that an error can be detected slot by slot, and sends the multicast signal to the transmitter  264 . The modulation parameter control part  265  in the transmitter  264  controls the modulation/sending part  266  such that 64QAM modulation is performed in which bit rate is the highest since the multicast signal is sent for the first time. The modulation/sending part  266  modulates the multicast signal into 64QAM, and sends it to each mobile station.  
      In the mobile station  271 , the multicast signal is received by the receiving/modulation part  277  in the receiver  275 , and the received signal is input to the modulation parameter estimation part  276 . The modulation parameter estimation part  276  detects that the multicast signal is 64QAM, and controls the receiving/demodulation part  277  for demodulating the modulated signal. Then, the receiving/demodulation part  277  outputs the multicast signal to the error detection/ARQ processor  213  after demodulating the multicast signal of 64QAM. The error detection/ARQ processor  213  performs error detection of the received multicast signal  207 . When the multicast signal  207  includes an error, the error detection/ARQ processor  213  outputs a retransmission request signal to the transmitter  214 . The transmitter  214  sends the retransmission request signal to the base station  261  via an uplink channel  206 . When an error is not detected in the multicast signal, any signal is sent to the base station, and the mobile station enters a state of waiting for a new multicast signal.  
      The base station  261  provides the retransmission request signal received by the receiver  205  to the ARQ processor  203 . The ARQ processor  203  temporarily stops sending a multicast signal input from the multicast signal input terminal  202 , and sends the multicast signal which is requested to be retransmitted to the modulation/sending part  266  in the transmitter  264  again, and informs the modulation parameter control part  265  that the multicast signal is for retransmission. Then, the modulation parameter control part  265  controls the modulation/sending part  266  such that 16QAM modulation is performed in which bit rate is lower than that of transmission of the first time. Then, the modulation/sending part  266  retransmits the multicast signal to each mobile station which requested retransmission after modulating the multicast signal by 16QAM.  
      At this time, it is possible that retransmission is performed while changing the modulation method and the like according to the number of mobile stations which receive arbitrary specified multicast signal or a ratio of mobile stations which receive a multicast signal to all base stations in an area. For example, it is possible that retransmission is informed to the modulation parameter control part  265  and retransmission is performed when the ratio of the mobile stations which receive the multicast signal is, for example, equal to or more than 1%.  
      In addition, it is also possible that the modulation method for sending new multicast signals sent after retransmission is performed is changed from that for the retransmission when the ratio of the mobile stations which receive the multicast signal is, for example, equal to or greater than 30%. That is, antenna directivity control, modulation method, transmission speed, spread modulation, error correction code or coding ratio can be changed for each of a multicast signal to be retransmitted and a multicast signal which is sent after the retransmission according to receiving state of each mobile station which receives a multicast signal.  
      In the mobile station  271 , in the same way as when it receives a multicast signal previously, the modulation parameter estimation part  276  detects that the multicast signal is 16QAM. After that, the receiving/demodulation part  277  demodulates the multicast signal and the multicast signal is input into the error detection/ARQ processor  213 . Then, error detection is performed. When an error is detected, retransmission is requested. When an error is not detected, any signal is sent to the base station, and the mobile station enters a state of waiting for a new multicast signal.  
      Then, the above-mentioned operation is repeated until retransmission request from each mobile station stops.  
      By using a modulation method such that required Eb/No (Eb/No required for obtaining the same error rate) becomes low, the bit error rate improves. Therefore, the possibility of re-retransmitting (second retransmission) a multicast signal decreases so that throughput of the whole system improves, wherein the Eb/No is energy per bit to noise power density ratio.  
      In the above-mentioned example, although receiving power is used as the receiving quality, signal to interference ratio (CIR), error rate of bit, packet or slot, or, the number of bit errors which were corrected or likelihood obtained when the error correction code is decoded can be also used as the receiving quality.  
      Next, the above-mentioned operation will be described concretely by using  FIG. 12 .  FIG. 12  shows a case where one base station and five mobile stations exist. After the base station divides a multicast signal into slots, adds error detection code and sends the slots to each mobile station. In this embodiment, when equal to or more than 50% of mobile stations which receive the multicast signal request retransmission, or, when there is no retransmission request for three consecutive slots, the transmission speed of a multicast signal which is newly sent after that is changed. In addition, the modulation method is changed when retransmitting.  
      As shown in  FIG. 12 , when sending the multicast signal in the first slot, the base station sends the multicast signal at a transmission speed of 1 Mbps and by using the modulation method 64QAM. Since all of the mobile stations MS 1-MS 5 received the multicast signal without error, all of the mobile stations do not send any signal and enter a state of waiting for a next new multicast signal.  
      When sending the multicast signal in the second slot, in the same way as in the first slot, the base station sends the multicast signal at a transmission speed 1 Mbps and by using the modulation method 64QAM. In this slot, since the MS 1 detects an error in a received multicast signal, the MS 1 requests retransmission to the base station. Since the MS 2-MS 5 received the multicast signal without an error, the MS 2-MS 5 do not send any signal and enter a state of waiting for receiving a next new multicast signal.  
      When sending the multicast signal in the third slot, which is retransmission for the second slot, the base station retransmits the multicast signal which is sent in the second slot at the transmission speed 1 Mbps and by using 16QAM in which required Eb/No decreases by one stage. Since the MS 1 received the multicast signal without an error, the MS 1 does not send any signal, and enters a state of waiting for receiving a next new multicast signal. The MS 2-MS 5 does not perform error detection for the retransmitted multicast signal and remains in the state of waiting for receiving a new multicast signal.  
      In the fourth slot transmission, the base station sends the multicast signal by turning back the modulation method to 64QAM by one stage and at the transmission speed 1 Mbps since retransmission ends at the third slot. At this time, since the MS 1-MS 3 detect an error in the multicast signal, the MS 1-MS 3 request retransmission. Since the MS 4 and the MS 5 could receive the multicast signal without an error, the MS 4 and the MS 5 does not send any signal and enters a state of waiting for a next new multicast signal.  
      In the case of transmission in the fifth slot which is retransmission for the fourth slot, since equal to or more than 60% of mobile stations detected an error and requested retransmission in the previous signal receiving, the transmission speed becomes 100 kbps which is lower by one stage. In addition, since retransmission is performed at this slot, the modulation method becomes 16QAM in which is required Eb/No is lowered by one stage. Then, the multicast signal is retransmitted by using the transmission speed and the modulation method. Then, the MS 1 detects an error again and requests re-retransmission. Since the MS 2 and the MS 3 receive the signal without an error, the MS 2 and the MS 3 do not send any signal and enter a state of waiting for receiving a next new multicast signal. In addition, the MS 4 and the MS 5 do not perform error detection of the retransmitted multicast signal and remain in the state of waiting for receiving the new multicast signal.  
      In the transmission in the sixth slot which is re-retransmission for the fourth slot, the modulation method becomes QPSK in which the required Nb/No is lowered further by one stage, and the multicast signal is re-retransmitted at a transmission speed 100 kbps and by QPSK. At this time, the MS 1 could receive the re-retransmitted without an error, the MS 1 does not send any signal and enters a state of waiting for receiving a next new multicast signal. The MS 2-MS 5 does not perform error detection of the re-retransmitted multicast signal and remain in a state of waiting for receiving a new multicast signal.  
      In the transmission in the seventh slot, since retransmission has ended, the base station sends a new multicast signal at the transmission speed of 100 kbps and by turning back the modulation method to 16QAM. Since all of the mobile stations MS 1-MS 5 receive the multicast signal without an error, the mobile stations do not send any signal and enter a state of waiting for a next new multicast signal.  
      In the eighth slot and ninth slot, like in the seventh slot, the multicast signal is transmitted at the transmission speed 100 kbps and by using the modulation method 16QAM. Since all mobile stations receive the multicast signal without an error, the mobile stations do not send any signal and enters a state of waiting for receiving a next new multicast signal.  
      In the tenth slot transmission, since the multicast signal has been received for three consecutive slots without an error so that the base station did not receive a request of retransmission, the transmission speed becomes 1 Mbps which is one stage higher. Then, a new multicast signal is transmitted at the transmission speed 1 Mbps by modulation method 64QAM.  
      The above-mentioned operation is repeated until transmission of a series of multicast signals ends.  
      As mentioned above, according to the present invention on the second embodiment, a new multicast signal is sent on the basis of a receiving state of mobile stations which received a previous multicast signal. In addition, the number of mobile stations subjected to retransmission is decreased on the basis of the receiving state and the multicast signal is transmitted so as to conform to receiving state of remaining mobile stations. Accordingly, receiving quality improves, and duration of communicating multicast signals decreases. Thus, throughput of the whole system improves.  
      As mentioned above, the first embodiment and the second embodiment of the present invention have been described. However, it is also possible that to combine the first embodiment and the second embodiment and use the combined system.  
      That is, the base station judges whether there is a retransmission request according to receiving quality of a received signal, and, when the base station judges that there is a retransmission request, the base station transmits a multicast signal corresponding to the retransmission request. While performing this operation, the base station monitors receiving state of mobile stations and sends a multicast signal by changing transmission method such that the transmission method conforms to receiving state of mobile stations in the basis of the result of monitoring.  
      Accordingly, frame use efficiency improves and the number of mobile stations subjected to retransmission can be decreased successively. Therefore, throughput of the whole system can be further improved.  
      The present invention is not limited to the specifically disclosed embodiments, and variations and modifications may be made without departing from the scope of the invention.