Abstract:
The present invention is to accommodate a large number of users required in a digital radio communication system and to prevent communication quality from deteriorating. The present invention is to prevent the occurrence of a delay by adding a process for restricting packet assignment for a call having low communication quality. When the average DRC value is equal to or smaller than a threshold, assigned slots are thinned out to improve communication quality for radio terminals close to cell boundaries and to increase the number of radio terminals accommodated. As a method for thinning out assigned slots, packets received from an upper level unit of a radio base station are, for example, discarded before transmission as long as the lowest limit sound quality is maintained. With this operation, the lowest limit sound quality is ensured.

Description:
CLAIM OF PRIORITY 
       [0001]    The present application claims priority from Japanese patent application JP 2008-020626 filed on Jan. 31, 2008 the content of which is hereby incorporated by reference into this application. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to radio base stations and scheduling methods, and more particularly, to a radio base station and a scheduling method for controlling packet assignment according to the quality of radio communication in a digital radio communication system. 
         [0004]    2. Description of the Related Art 
         [0005]    An adaptive rate method may be used in a digital radio communication system as a method for changing the communication speed according to the quality of a radio channel in order to increase the efficiency of frequency use. An upstream communication rate, a downstream communication rate, or both are determined according to the quality of the radio channel. 
         [0006]    In a system that uses an adaptive rate method where an upstream rate and a downstream rate are determined according to the quality of a channel, such as a 1xEV-DO system, a radio terminal determines the downstream rate according to the quality of a downstream channel measured by the radio terminal. 
         [0007]      FIG. 11  shows the relationship between downstream channel quality and determined downstream channel rates in a conventional adaptive rate method. 
         [0008]    In general, the lower the channel quality is, the larger the number of slots used per radio packet; and the higher the channel quality is, the smaller the number of slots used. When a hybrid ARQ method is used as a retransmission method for downstream signal data, if packet recovery is found to be possible at an intermediate slot among assigned slots, slot assignment is cancelled at the slot and the remaining slots can be assigned to another user. With the use of this method, flexible channel assignment is possible according to the quality of the downstream channel. 
         [0009]    Non-patent Document 1: 9.3.1.3.1 Modulation Parameters (pages 9-67 to 9-70), cdma2000 High Rate Packet Data Air Interface Specification, C.S0024-0, V 4.0, The Third Generation Partnership Project 2 (3GPP2) Specifications, URL: http://www.3gpp2.org/Public_html/specs/tsgc.cfm, searched for on Sep. 11, 2007 
       SUMMARY OF THE INVENTION  
       [0010]    In a digital radio communication system, such as a 1xEV-DO system, a packet delay does not make a large influence on quality in conventional best-effort data communication. In recent years, however, it is demanded that an application that requires real-time processing be used in the radio communication system. In that case, the influence on service quality caused by a delay cannot be ignored. In a real-time application system, for example, in a VoIP system, small-size packets need to be sent with a short delay, and, in addition, a large number of accommodated users need to be handled. 
         [0011]    When a user uses a low quality channel, a plurality of slots is assigned to the user at a low communication rate. As a result, the number of users who can be accommodated lowers and a delay occurs for the other users. 
         [0012]    Accordingly, an object of the present invention is to accommodate a large number of users required in a digital radio communication system and to prevent communication quality from deteriorating. 
         [0013]    Another object of the present invention is to prevent the occurrence of a delay by adding a process for restricting packet assignment for a call having low communication quality. 
         [0014]    To solve the above-described drawbacks, for example, when a radio base station in a 1xEV-DO system determines that a downstream channel is congested at a certain level or more by determining, for example, that the slot use rate is equal to or more than a threshold, if the radio base station determines, among users who use an identical application, for example, among VoIP users, a user who uses relatively many slots at the same throughput because of a relatively low quality of the downstream channel by determining that the average data rate control (DRC) value becomes equal to or smaller than a threshold, an assigned slot is thinned out in the present invention. If thinning out occurs in burst, the sound cannot be recovered by interpolation. Therefore, it is necessary to take some measure so as not to thin out assigned slots in burst. In addition, it is possible to use either a constant thinning-out rate or a thinning-out rate changed according to the degree of congestion in the downstream channel. With this, the lowest limit sound quality is guaranteed. It is expected that the thinning-out rate is switched between the two rates described above, for example, for radio terminal users located close to cell boundaries, in many cases. 
         [0015]    The present invention provides a radio communication system having radio terminals and a radio base station that employs adaptive rate control that changes the communication rates according to the quality of radio channels. The radio base station includes a communication-rate selection section, a section for determining the number of use slots assigned according to the selected communication rate and for thinning out an assigned slot, and a section for assigning the thinned-out slot to another radio terminal. The radio communication system ensures a predetermined number of radio terminals accommodated and solves congestion for other radio terminals. 
         [0016]    In the present invention, as the section for thinning out an assigned slot, a section for controlling slot assignment such that thinning out does not prevent data recovery with interpolation. 
         [0017]    In the present invention, as a section for selecting a radio terminal to which slot assignment control is applied, the following three sections may be used: a section for measuring the quality of a radio channel between the radio base station and a radio terminal, a section for selecting a communication rate according to the result of measurement, and a section for performing channel assignment based on the quality of a radio channel by determining the number of use slots assigned according to the selected communication rate and for selecting a radio terminal to which a large number of use slots are assigned because of the low quality radio channel. 
         [0018]    As the section for thinning out an assigned slot, the section for performing slot assignment control, or a section for controlling slot assignment to a radio terminal to which a large number of use slots are assigned, a section for setting a thinning out rate in slot assignment to a constant value irrespective of the degree of congestion of the radio channel may be used. 
         [0019]    As the section for thinning out an assigned slot, the section for performing slot assignment control, or the section for controlling slot assignment to a radio terminal to which a large number of use slots are assigned, a section for changing the thinning out rate in slot assignment according to the degree of congestion of the radio channel may be used. 
         [0020]    According to the first solving means of the present invention, there is provided a radio base station for a radio communication system that employs adaptive rate control where a communication rate is changed according to the quality of a radio channel, the radio base station comprising: 
         [0021]    a radio receiver for receiving an upstream signal that includes request rate information, from a radio terminal; 
         [0022]    a radio base station controller
       for comparing the request rate information included in the upstream signal received by the radio receiver, with a threshold determined in advance,   when the request rate information is equal to or larger than, or larger than the threshold, for determining that the quality of a radio channel with the radio terminal is relatively high, and for performing scheduling such that packet data is to be sent with a short slot, which means a slot length used is short or the number of slots used is small, and   when the request rate information is smaller than, or equal to or smaller than, the threshold, for determining that the quality of the radio channel with the radio terminal is relatively low, and for determining whether, if a part of packet data to be sent is discarded prior to the transmission, the part can be recovered by complement, and for performing scheduling, when the part can be recovered, such that the part is discarded and a corresponding slot is assigned to another packet data to be sent to another radio terminal; and       
 
         [0026]    a radio transmitter for transmitting the packet data to the radio terminal or the other packet data to the other radio terminal according to the scheduling performed by the radio base station controller. 
         [0027]    According to the second solving means of the present invention, there is provided a scheduling method in a radio base station for a radio communication system that employs adaptive rate control where a communication rate is changed according to the quality of a radio channel, the radio base station comprising the steps of: 
         [0028]    receiving an upstream signal that includes request rate information, from a radio terminal; 
         [0029]    comparing the request rate information included in the upstream signal with a threshold determined in advance; 
         [0030]    when the request rate information is equal to or larger than, or larger than the threshold, determining that the quality of a radio channel with the radio terminal is relatively high, and for performing scheduling such that packet data is to be sent with a short slot, which means a slot length used is short or the number of slots used is small, and transmitting the packet data to the radio terminal; and 
         [0031]    when the request rate information is smaller than, or equal to or smaller than, the threshold, determining that the quality of the radio channel with the radio terminal is relatively low, and for determining whether, if a part of packet data to be sent is discarded prior to the transmission, the part can be recovered by complement, and for performing scheduling, when the part can be recovered, such that the part is discarded and a corresponding slot is assigned to another packet data to be sent to another radio terminal, and transmitting the other packet data to the other radio terminal. 
         [0032]    According to the present invention, when an application that requires real-time processing, such as a VoIP application, is applied to a radio communication system that uses adaptive rate control, even if a user uses a low quality channel, the number of users that can be accommodated is prevented from lowering, a packet delay is avoided for other users, and the user is prevented from being subjected to the deterioration of communication quality. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0033]      FIG. 1  shows an example structure of a network using 1xEV-DO. 
           [0034]      FIG. 2  shows an example structure of a radio base station. 
           [0035]      FIG. 3  shows an example structure of a radio terminal. 
           [0036]      FIG. 4  is an operation flowchart of a radio base station controller. 
           [0037]      FIG. 5  shows a packet control example (1). 
           [0038]      FIG. 6  is a call-connection sequence diagram in the packet control example (1). 
           [0039]      FIG. 7  shows a packet control example (2). 
           [0040]      FIG. 8  is a call-connection sequence diagram in the packet control example (2). 
           [0041]      FIG. 9  shows a packet control example (3). 
           [0042]      FIG. 10  shows the packet control example (3). 
           [0043]      FIG. 11  shows the relationship between downstream channel quality and determined downstream channel rates in a 1xEV-DO system. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0044]      FIG. 1  shows the structure of a VoIP network that uses 1xEV-DO, as an example structure of a mobile communication network. 
         [0045]    This network mainly includes radio terminals  101 , radio base stations  102 , a packet control function (PCF)  103 , a packet data serving node (PDSN)  104 , a wired network  105  such as the Internet, and a Session Initiation Protocol server (SIP server)  106 . 
         [0046]      FIG. 2  shows an example structure of the radio base stations  102 . 
         [0047]    Each radio base station  102  mainly includes an antenna  201 , a multiplexer  202 , a radio receiver  203 , a radio transmitter  204 , a demodulator  205 , a modulator  206 , and a radio base station controller  207 . 
         [0048]    The antenna  201  sends and receives signals to and from radio terminals  101 . The multiplexer  202  is used to share one antenna between a signal to the radio receiver  203  and a signal from the radio transmitter  204 . The radio receiver  203  receives an upstream channel signal from a radio terminal  101 . The radio transmitter  204  sends a downstream channel radio signal to a radio terminal  101 . The demodulator  205  demodulates the received upstream channel signal. The modulator  206  modulates the downstream channel signal to be transmitted. The radio base station controller  207  controls the radio receiver  203 , the radio transmitter  204 , the demodulator  205 , and the modulator  206 , controls packet assignment for downstream channel signals, and serves as an interface with the wired network  105 . 
         [0049]      FIG. 3  shows an example structure of the radio terminals  101 . Each radio terminal  101  mainly includes an antenna  301 , a multiplexer  302 , a radio receiver  303 , a radio transmitter  304 , a demodulator  305 , a modulator  306 , a reception quality measurement section  307 , a DRC estimation section  308 , and a radio-terminal controller  309 . 
         [0050]    The antenna  301  sends and receives a signal to and from a radio base station  102 . The multiplexer  302  is used to share one antenna between a signal to the radio receiver  303  and a signal from the radio transmitter  304 . The radio receiver  303  receives a downstream channel signal from the radio base station  102 . The radio transmitter  304  sends an upstream channel radio signal to the radio base station  102 . The demodulator  305  demodulates the received downstream channel signal. The modulator  306  modulates the upstream channel signal to be transmitted. The reception quality measurement section  307  calculates the average and variance of I/Q, which indicate the orthogonal coordinates of the symbol obtained by despreading the demodulated received signal, to obtain the signal to interference ratio (SIR), which indicates signal quality. The DRC estimation section  308  calculates a data rate control (DRC) value, which indicates the rate to be required of the radio base station, based on the SIR. The smaller the DRC value is, the lower the communication rate. The larger the DRC value is, the higher the communication rate. In other words, the radio terminal  101  sets the DRC value larger when the calculated SIR is larger, and sets the DRC value smaller when the calculated SIR is smaller. The radio-terminal controller  309  controls the radio receiver  303 , the radio transmitter  304 , the demodulator  305 , the modulator  306 , the reception quality measurement section  307 , and the DRC estimation section  308 , and serves as an interface with a personal computer when the radio terminal  101  is a data communication card or as a man-machine interface, such as a key or a microphone, when the radio terminal  101  is a mobile phone. 
         [0051]      FIG. 4  is a flowchart of the operation of the radio base station controller  207 . 
         [0052]    The operation of the radio base station controller  207  will be described below with reference to the block diagrams shown in  FIG. 1  and  FIG. 2  and the flowchart shown in  FIG. 4 . 
         [0053]    When processing starts after a call connection is established between the radio base station  102  and each radio terminal  101 , the radio base station controller  207  of the radio base station  102  sends a downstream signal that includes a downstream pilot signal to the radio terminal  101  through the modulator  206 , the radio transmitter  204 , the multiplexer  202 , and the antenna  201  (S 401 ). 
         [0054]    The radio terminal  101  receives the downstream signal and inputs it into the radio-terminal controller  309  and the reception quality measurement section  307  through the antenna  301 , the multiplexer  302 , the radio receiver  303 , and the demodulator  305 . The reception quality measurement section  307  measures the signal quality of the downstream pilot signal included in the downstream signal, and the DRC estimation section  308  selects a DRC value to be required of the radio base station  102  based on the measurement result. Instead of the DRC value, information indicating an appropriate rate required by the radio terminal  101  may be used. The radio-terminal controller  309  sends an upstream signal that includes the DRC value to the radio base station  102  through the modulator  306 , the radio transmitter  304 , the multiplexer  302 , and the antenna  301 . 
         [0055]    The radio base station  102  receives the upstream signal through the antenna  201 , the multiplexer  202 , the radio receiver  203 , and the demodulator  205 , and inputs it into the radio base station controller  207  to receive the DRC value from the radio terminal (S 402 ). The radio base station controller  207  determines a downstream communication rate according to the DCR value reported in the upstream signal from the radio terminal  101 . For example, the radio base station controller  207  can determine the downstream communication rate by calculation with an appropriate equation or by a table or threshold determined in advance such that the smaller the DRC value is, the smaller the communication rate; and the larger the DRC value is, the higher the communication rate. In addition, the radio base station controller  207  determines the downstream communication rate for each of all radio terminals  101  connected to the radio base station  102 , calculates the average communication rate R of each radio terminal  101 , and also calculates the value (DRC/R) obtained by dividing the DRC of each radio terminal  101  by R (S 403 ). An example expression 1 for calculating R will be shown below. 
         [0000]    
       
         
           
             
               
                 
                   
                     R 
                      
                     
                       ( 
                       
                         t 
                         + 
                         1 
                       
                       ) 
                     
                   
                   = 
                   
                     
                       
                         ( 
                         
                           1 
                           - 
                           
                             1 
                             
                               t 
                               c 
                             
                           
                         
                         ) 
                       
                        
                       
                         R 
                          
                         
                           ( 
                           t 
                           ) 
                         
                       
                     
                     + 
                     
                       1 
                       
                         t 
                         c 
                       
                     
                     + 
                     
                       DRC 
                        
                       
                         ( 
                         t 
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   
                      
                     1 
                   
                   ) 
                 
               
             
           
         
       
     
         [0000]    where, DRC(t) indicates the data rate required by the user i at time t, R(t) indicates the average of the data rates required by the users i at time t, and t c  indicates 1000 slots, which equals 1.6 seconds. 
         [0056]    The radio base station controller  207  performs scheduling for the radio terminals  101  in descending order of DRC/R (S 403 ) This scheduling method is called a proportional fairness method. An appropriate other scheduling method may be used. 
         [0057]    As shown in  FIG. 11 , the lower the downstream channel quality is, the larger the number of slots used or the longer the slot length used; and the higher the downstream channel quality is, the smaller the number of slots used or the shorter the slot length used. The radio base station controller  207  next determines whether the DRC value is equal to or larger than a threshold determined in advance (S 404 ). When the DRC value is equal to or larger than the threshold, which means that the downstream channel quality is high, the result of the determination in step S 404  is Yes. In this case, when packet data to be sent to this radio terminal  101  reaches the radio base station  102  through the wired network  105  (S 405 ), the packet data is maintained (S 406 ), scheduling is performed with a short slot (S 407 ), which means that the slot length used is short or the number of slots used is small, and the packet data is sent to the radio terminal  101  (S 414 ). 
         [0058]    In contrast, if the downstream channel quality is low, the number of slots used for the radio terminal  101  is larger than that for a radio terminal  101  having high channel quality. When a large number of users are accommodated, congestion may occur for the other users. As a result, it is expected that the number of users accommodated is reduced and delays occur for the other users. In a VoIP system, for example, a delay directly lowers the audio quality. Therefore, when the DRC value is smaller than the threshold, which means low downstream channel quality, the determination of the radio base station controller  207  in step S 404  is No. In that case, when packet data to be sent to this radio terminal  101  reaches the radio base station  102  through the wired network  105  (S 408 ), the radio base station controller  207  determines whether, if a part of the packet data is discarded before transmission, that part can be recovered by complement (S 409 ). 
         [0059]    For example, a predetermined number of packets that can be discarded is specified within a range where the deterioration of audio quality is permissible. The radio base station controller  207  counts the number of packets discarded and compares the number with the predetermined number in step S 409 . When the number of packets discarded is equal to or smaller than the predetermined number, the radio base station controller  207  determines that the sound can be recovered. If the number of packets discarded exceeds the predetermined number, the radio base station controller  207  determines that the sound cannot be recovered. 
         [0060]    When the sound can be recovered, the radio base station controller  207  discards the part of the packet data (S 410 ), performs scheduling such that the corresponding slot is assigned to another radio terminal (S 411 ), and sends the packet data to the radio terminal  101  (S 414 ). In contrast, if it is determined in step S 409  that the sound cannot be recovered, the radio base station controller  2007  maintains the packet data (S 412 ), performs scheduling with a long slot (S 413 ), which means that the slot length used is long or the number of slots used is large, and sends the packet data to the radio terminal  101  (S 414 ). With this operation, a predetermined number of accommodated radio terminals used by VoIP users is ensured and packet delays of the other users are avoided. 
         [0061]    An example concrete operation in the present embodiment will be described below with a figure. 
         [0062]      FIG. 5  shows a packet control example ( 1 ). In the figure, a first row shows that a VoIP packet group  1 , a VoIP packet group  2 , . . . , and a VoIP packet group n each having a predetermined number of packets are sent at predetermined reaching intervals. Second and subsequent rows show a plurality of slots included in first packets of each VoIP packet in an enlarged viewing manner. 
         [0063]    When a radio terminal  2  has high channel quality, one slot is assigned to each VoIP packet because a VoIP packet is sent by one slot. However, because a radio terminal  1  has low channel quality, it uses many slots, causing congestion for other users when a large number of users are accommodated. In VoIP, packets are discarded in a range where the audio quality appears to be not deteriorated (in a permissible range). For example, packets are discarded up to a predetermined number of (for example, two) packets among n packets, and the number of (for example, three) packets exceeding the predetermined number are not discarded. When packets are discarded, the emptied slots are assigned to other users to solve congestion for them. 
         [0064]      FIG. 6  is a sequence diagram of the above-described series operation. Call connections are completed between a radio base station  603  and the radio terminal  1  ( 601 ), having low reception quality, and the radio terminal  2  ( 602 ), having high reception quality, and VoIP packets reach the radio base station  603  through a wired network  604 . This case is taken as an example and will be described below. 
         [0065]    The radio base station  603  sends a downstream pilot signal  1  ( 606 ) to the radio station  1  ( 601 ) and a downstream pilot signal  2  ( 605 ) to the radio station  2  ( 602 ). The radio terminal  1  ( 601 ), having low reception quality, selects a low DRC value  608  and sends it to the radio base station  603  because a poor result is obtained when the reception quality of the downstream pilot signal  1  ( 606 ) is measured. In contrast, the radio terminal  2  ( 602 ), having high reception quality, selects a high DRC value ( 607 ) and sends it to the radio base station  603  because a good result is obtained when the reception quality of the downstream pilot signal  2  ( 605 ) is measured. This control is always executed during the call connections. In the figure, each radio station sends the DRC value to the radio base station  603  only once, but actually the radio base station always receives a DCR value from each radio terminal at predetermined timing. 
         [0066]    Since data can be sent at a high rate with less slots to a terminal having high reception quality, the radio base station  603  controls scheduling such that a VoIP packet  1 - 2  ( 609 ) is sent with a short slot and sends the packet to the radio terminal  2  ( 602 ), which requests the high DRC value ( 607 ). Since a terminal having low reception quality uses many slots at a low rate to cause congestion for other users, the radio base station  603  discards a VoIP packet  1 - 1  ( 610 ) and transmits no packet. When the next VoIP packet  2 - 1  ( 612 ) and the next VoIP packet  2 - 2  ( 611 ) reach the radio base station  603 , if the reception quality of the radio terminal  1  ( 601 ) and the radio terminal  2  ( 602 ) has not changed, the same processes as for the preceding packets are performed. The radio base station  603  controls scheduling such that the VoIP packet  2 - 2  ( 611 ) is sent with a short slot and sends the packet to the radio terminal  2  ( 602 ). The radio base station  603  discards the VoIP packet  2 - 1  ( 612 ) and transmits no packet. Then, when the next VoIP packet  3 - 1  ( 614 ) and the next VoIP packet  3 - 2  ( 613 ) reach the radio base station  603 , if the reception quality of the radio terminal  1  ( 601 ) and the radio terminal  2  ( 602 ) has not changed, a different process from that for the preceding packet  2 - 1  is performed. The radio base station  603  controls scheduling such that the VoIP packet  3 - 2  ( 613 ) is sent with a short slot and sends the packet to the radio terminal  2  ( 602 ). Because the number of (for example, three) VoIP packets exceeding a threshold are not allowed to be discarded for the radio terminal  1  ( 601 ), the radio base station  603  controls scheduling such that the VoIP packet  3 - 1  ( 614 ) is sent with a long slot and sends the packet to the radio terminal  1  ( 601 ). The same processes as for the VoIP packet  3 - 1  ( 614 ) and the VoIP packet  3 - 2  ( 613 ) are performed until a VoIP packet n- 1  ( 616 ) and a VoIP packet n- 2  ( 615 ) reach the radio base station  603 . 
         [0067]      FIG. 7  shows a packet control example (2). When a radio terminal  2  has high channel quality, one slot is assigned to each VoIP packet because a VoIP packet is sent by one slot. However, because a radio terminal  1  has low channel quality, it uses many slots, causing congestion for other users when a large number of users are accommodated. In VoIP, packets are discarded in a range where the audio quality appears to be not deteriorated. Packets are discarded up to a predetermined number of (for example, one) packets among n packets, and the number of (for example, two) packets exceeding the predetermined number are not discarded. When packets are discarded, the emptied slots are assigned to other users to solve congestion for them. 
         [0068]      FIG. 8  is a sequence diagram of the above-described series operation. Call connections are completed between a radio base station  803  and the radio terminal  1  ( 801 ), having low reception quality, and the radio terminal  2  ( 802 ), having high reception quality, and VoIP packets reach the radio base station  803  through a wired network  804 . This case is taken as an example and will be described below. 
         [0069]    The radio base station  803  sends a downstream pilot signal  1  ( 806 ) to the radio station  1  ( 801 ) and a downstream pilot signal  2  ( 805 ) to the radio station  2  ( 802 ). The radio terminal  1  ( 801 ), having low reception quality, selects a low DRC value  808  and sends it to the radio base station  803  because a poor result is obtained when the reception quality of the downstream pilot signal  1  ( 806 ) is measured. In contrast, the radio terminal  2  ( 802 ), having high reception quality, selects a high DRC value ( 807 ) and sends it to the radio base station  803  because a good result is obtained when the reception quality of the downstream pilot signal  2  ( 805 ) is measured. This control is always executed during the call connections. 
         [0070]    Since data can be sent at a high rate with less slots to a terminal having high reception quality, the radio base station  803  controls scheduling such that a VoIP packet  1 - 2  ( 809 ) is sent with a short slot and sends the packet to the radio terminal  2  ( 802 ), which requests the high DRC value ( 807 ). Since a terminal having low reception quality uses many slots at a low rate to cause congestion for other users, the radio base station  803  discards a VoIP packet  1 - 1  ( 810 ) and transmits no packet. When the next VoIP packet  2 - 1  ( 812 ) and the next VoIP packet  2 - 2  ( 811 ) reach the radio base station  803 , if the reception quality of the radio terminal  1  ( 801 ) and the radio terminal  2  ( 802 ) has not changed, different processes as for the preceding packets are performed. The radio base station  803  controls scheduling such that the VoIP packet  2 - 2  ( 811 ) is sent with a short slot and sends the packet to the radio terminal  2  ( 802 ). Because the number of (for example, two) VoIP packets exceeding a threshold are not allowed to be discarded for the radio terminal  1  ( 801 ), the radio base station  803  controls scheduling such that the VoIP packet  2 - 1  ( 812 ) is sent with a long slot and sends the packet to the radio terminal  1  ( 801 ). The same processes as for the VoIP packet  2 - 1  ( 812 ) and the VoIP packet  2 - 2  ( 811 ) are performed until a VoIP packet n- 1  ( 816 ) and a VoIP packet n- 2  ( 815 ) reach the radio base station  803 . 
         [0071]      FIG. 9  and  FIG. 10  show a packet control example ( 3 ). For example, the state of congestion in a downstream channel is determined by the slot use rate, and the number of packets discarded is increased depending to the state of congestion. The radio base station controller  207  can determine the state of congestion by comparing appropriate data, such as the number of packets used, the slot use rate, the number of slots used, or traffic, with a threshold determined in advance. 
         [0072]    As shown in  FIG. 9 , for example, when it is determined from a low slot use rate that the downstream channel has low congestion, a radio terminal  2  which has high channel quality assigns one slot to each VoIP packet because a VoIP packet is sent by one slot. However, because a radio terminal  1  has low channel quality, it uses many slots, causing congestion for other users when a large number of users are accommodated. In VoIP, packets are discarded in a range where the audio quality appears to be not deteriorated. Packets are discarded up to the first predetermined number of (for example, one) packets among n packets, and the number of (for example, two) packets exceeding the first predetermined number are not discarded. When one packet is discarded, the emptied slots are assigned to other users to solve congestion for them. 
         [0073]    As shown in  FIG. 10 , for example, when it is determined from a high slot use rate that the downstream channel has high congestion, a radio terminal  2  which has high channel quality assigns one slot to each VoIP packet because a VoIP packet is sent by one slot. However, because a radio terminal  1  has low channel quality, it uses many slots, causing congestion for other users when a large number of users are accommodated. In VoIP, packets are discarded in a range where the audio quality appears to be not deteriorated. Packets are discarded up to the second predetermined number of (for example, two) packets among n packets, and the number of (for example, three) packets exceeding the second predetermined number are not discarded. A predetermined number of (for example, two) packets are discarded and the corresponding slots are assigned to radio terminals  3 ,  4 , and  5 . Since a larger number of emptied slots are assigned to other users, congestion is solved for the other users. 
         [0074]    The present invention can be applied, for example, to a radio communication system that provides a service application, such as VoIP or moving-image distribution, where streaming data is used and a short delay is required.