Abstract:
The present invention can be achieved by a base station in which a terminal is accommodated and the terminal is communicated with another terminal, and which includes: a data rate operation/management unit which operates and stores a time-averaged data rate Rave of the terminal; a threshold value operation/management unit which obtains a current communication data rate R of the terminal to operate and store R/Rave; and a delay management unit which controls to reduce delay on the basis of the R/Rave.

Description:
CLAIM OF PRIORITY 
     The present application claims priority from Japanese patent application serial no. 2007-304561, filed on Nov. 26, 2007, the content of which is hereby incorporated by reference into this application. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to a base station, and particularly to a base station from which wireless data of VoIP are transmitted to a terminal. 
     As a voice calling system using data communications, Voice over IP (VoIP) is given. The VoIP is a technique by which voice as an analog signal is converted into a digital signal to be IP packetized, so as to make a call using an IP network such as the existing Internet. In making a call using the VoIP, it is important to secure the quality of voice and to reduce a delay time. To secure the quality of voice and to reduce a delay time are also important in providing VoIP services using a wireless data communication system. 
     In a wireless data communication system, a wireless base station transmits data while changing a communication data rate in accordance with conditions of wireless environment where a user terminal utilizing data communication services is located. In general, the data is transmitted at a high data rate to a user terminal which is located in a good wireless environment. On the contrary, the data is transmitted at a low data rate to a user terminal which is located in a bad wireless environment. By appropriately changing the communication data rate, the data can be transmitted to the user terminal with high probability. 
     As described above, the communication data rate between the base station and the terminal differs in the wireless data communication system depending on the conditions of wireless environment where the terminal is located. In this case, the number of data slots necessary for transmitting one packet differs in accordance with the communication data rate. In general, in the case where data communications are performed at a low data rate, the number of necessary data slots is increased. 
     With reference to  FIGS. 1 ,  2 A and  2 B, there will be described a relation between the data rate and the number of necessary slots. Here,  FIG. 1  is a diagram for explaining a configuration of a voice packet and the number of slots. Further,  FIGS. 2A and 2B  are diagrams, each explaining slot assignment. 
     In  FIG. 1 , a voice packet  1  is modulated with a modulation signal, and is assigned to slots  1 - 1  to  1 -n. When the data rate of the modulation signal is 153.6 kbits/s, n is 4. Further, when the data rate of the modulation signal is 38.4 kbits/s that is one-fourth of 153.6 kbits/s, n is 16 that is four times of 4. Specifically, when the communication data rate is 153.6 kbits/s, the packet is assigned to 4 slots. When the communication data rate is 38.4 kbits/s, the packet is assigned to 16 slots. 
       FIG. 2A  shows slot assignment in the case where the communication data rate is 153.6 kbits/s. In addition,  FIG. 2B  shows slot assignment in the case where the communication data rate is 38.4 kbits/s. In  FIGS. 2A and 2B , one slot has 1.67 ms of time in a 1×Evolution Data Only (1×EV-DO) system, and the slots are transmitted with an interval of three slots from the time one slot is transmitted to the time the next slot of the same voice packet is transmitted. Thus, it takes 6.68 ms (1.67 ms×four slots). Therefore, in the case of a low data rate such as 38.4 kbits/s in  FIG. 2B , it takes up to 106.88 ms (=1.67 ms×4×16) to transmit voice data of one packet, so that delay occurs. 
     As described above, when the number of data slots to transmit one packet is increased, delay occurs. In the case where delay is increased at the time of using VoIP, there is a problem such as difficulty of hearing voice. Further, an increase in the number of slots used decreases the number of available users. 
     SUMMARY OF THE INVENTION 
     The present invention is to reduce delay caused when wireless environment where a terminal is located deteriorates. 
     The above-described object can be achieved by a base station in which a terminal is accommodated and the terminal is communicated with another terminal, and which includes: a data rate operation/management unit which operates and stores a time-averaged data rate Rave of the terminal; a threshold value operation/management unit which obtains a current communication data rate R of the terminal to operate and store R/Rave; and a delay management unit which controls to reduce delay on the basis of the R/Rave. 
     As a first concrete method, the wireless base station manages a determination index for each terminal, and determines whether or not the wireless environment of the terminal deteriorates, by using a current data rate and the index. In the case where the wireless environment deteriorates, the wireless base station skips and discards packets which are supposed to be transmitted to the terminal, prior to a process of signal conversion in the wireless base station. Accordingly, although the voice is interrupted, delay is reduced. 
     As a second method, the wireless base station assigns packets to slots in accordance with the data rate of the terminal and transmits the same. At this time, the wireless base station manages a determination index for each terminal, and determines whether or not the wireless environment of the terminal deteriorates, by using a current data rate and the index. In the case where the wireless environment of the terminal deteriorates, the wireless base station terminates the transmission of the slots during the transmission process, and assigns the resource left by terminating the transmission of the slots, to transmission of the next slot or other users. Accordingly, although the voice is interrupted, delay is reduced. 
     As a third method, in a demodulation process of the slots transmitted from the wireless base station is performed at the terminal, when a packet with a larger sequence number can be demodulated first, the wireless base station terminates transmission of data with a smaller sequence number. Accordingly, although the voice is interrupted, delay is reduced. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred embodiments of the present invention will now be described in conjunction with the accompanying drawings, in which; 
         FIG. 1  is a diagram for explaining a configuration of a voice packet and the number of slots; 
         FIG. 2A  is a diagram for explaining slot assignment in the case where a communication data rate is 153.6 kbits/s; 
         FIG. 2B  is a diagram for explaining slot assignment in the case where a communication data rate is 38.4 kbits/s; 
         FIG. 3  is a block diagram of a 1×EV-DO system; 
         FIG. 4  is a functional block diagram of a wireless base station; 
         FIG. 5  is a functional block diagram of a terminal; and 
         FIG. 6  is a diagram for explaining slot assignment. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, embodiments of the present invention will be described with reference to the drawings. It should be noted that substantially the same constituent elements are given the same reference numerals, and the explanations thereof will not be repeated. 
     [First Embodiment] 
     A 1×EV-DO system as a wireless data communication system will be described with reference to  FIG. 3 . Here,  FIG. 3  is a block diagram of the 1×EV-DO system. 
     In  FIG. 3 , a 1×EV-DO system  500  includes terminals  100 - 1  and  100 - 2 , wireless base stations  110 - 1  and  110 - 2 , and IP Switches (IP-SWs)  130 - 1  and  130 - 2 . The terminal  100 - 1  is accommodated in the base station  110 - 1 . In addition, the terminal  100 - 2  is accommodated in the base station  110 - 2 . It should be noted that the IP-SWs  130  are connected to the Internet  150 . 
     With reference to  FIG. 4 , a configuration of the wireless base station will be described. Here,  FIG. 4  is a functional block diagram of the wireless base station. In  FIG. 4 , the wireless base station  110  includes a wireless signal transceiver unit  201 , a modulation/demodulation processing unit  211 , a base station control unit  221 , a line interface unit  231 , a data rate operation/management unit  241 , a threshold value operation/management unit  251 , a delay management unit  261 , and an antenna  271 . 
     The wireless signal transceiver unit  201  includes a received signal processing unit  203  which receives a wireless signal from the terminal with the antenna  271  to be converted into a digital signal, and a transmission signal processing unit  202  which converts a digital signal into a wireless signal to be transmitted from the antenna  271 . 
     The modulation/demodulation processing unit  211  includes a demodulation processing unit  213  which demodulates the digital signal converted by the received signal processing unit  203 , and a modulation processing unit  212  which modulates the digital signal to be transmitted to the terminal in accordance with the wireless environment of the terminal. The line interface unit  231  is an interface with the IP-SW  130 . The base station control unit  221  has monitoring and controlling functions of the wireless base station  110 . 
     The data rate operation/management unit  241  operates a time-averaged data rate for each terminal, and stores and manages the same. The threshold value operation/management unit  251  performs a threshold value operation/determination for each terminal by using the average value stored in the data rate operation/management unit and a current data rate. The threshold value operation/management unit  251  performs the following threshold value operation:
 
 Th=Ri/Ri _ave  (Formula 1)
 
     where Ri_ave represents an average data rate of a terminal i, Ri represents a current data rate, and Th represents a threshold value. 
     In the case where the current value Ri continuously falls below the average value Ri_ave (Th&lt;1), the delay management unit  261  assumes that the wireless environment of the terminal i deteriorates. The delay management unit  261  monitors the threshold value operation/management unit  251  to manage the delay of the accommodated terminal. 
     With reference to  FIG. 5 , a configuration of the terminal will be described. Here,  FIG. 5  is a functional block diagram of the terminal. In  FIG. 5 , the terminal  100  includes an antenna  361 , a transceiver unit  301 , a modulation/demodulation processing unit  311 , a control unit  321  which controls the entire terminal  100 , an internal memory  351 , a speaker  341 , and a microphone  331 . Further, the transceiver unit  301  includes a transmission processing unit  302  and a reception processing unit  303 . Further, the modulation/demodulation processing unit  311  includes a modulation processing unit  312  and a demodulation processing unit  313 . 
     Referring to  FIG. 3 , there will be described a case in which the terminals  100 - 1  and  100 - 2  perform VoIP communications using the 1×EV-DO system  500 . The terminals  100 - 1  and  100 - 2  notify the base station  110  of wireless environment information as a data rate control signal, irrespective of presence or absence of data communications when they are located within service areas of the wireless base stations  110 - 1  and  110 - 2 . 
     The terminals  100 - 1  and  100 - 2  start the VoIP communications from the service area of the wireless base station  110 - 1  and the service area of the wireless base station  110 - 2 , respectively. 
     In the terminal  100 - 2 , a voice signal is converted from voice data into voice packets by digital signal processing of the modulation processing unit  312 , and the voice packets are transmitted to the wireless base station  110 - 2  while being superimposed on a wireless signal by the transmission processing unit  302 . The wireless signal is converted into the voice packets by the received signal processing unit  203  of the wireless base station  110 - 2 , and the voice packets are demodulated into the voice data by the demodulation processing unit  213 . Then, the voice data is transmitted to the line interface unit  231  of the wireless base station  110 - 1  through the line interface unit  231 , the IP-SW  130 - 2 , the Internet  150 , and the IP-SW  130 - 1 . 
     The data received by the line interface unit  231  of the wireless base station  110 - 1  is modulated into packet data in the modulation processing unit  212  in accordance with the data rate notified from the terminal  100 - 1 , and is divided into slots. The data which is divided into slots is superimposed on a wireless signal by the transmission signal processing unit  202  to be transmitted to the terminal  100 - 1 . The wireless signal received by the terminal  100 - 1  is converted into voice packets by the reception processing unit  303 , and the voice packets are converted into the voice data by the demodulation processing unit  313 , so that the voice signal reaches through the speaker  341 . 
     Hereinafter, an explanation will be given focusing on the wireless base station  110 - 1  and the terminal  100 - 1 . 
     The terminal  100 - 1  notifies the wireless base station  110 - 1  of wireless environment information as a data rate control signal, irrespective of presence or absence of data communications when the terminal  100 - 1  is located within the service area of the wireless base station  110 - 1 . The wireless base station  110 - 1  operates the average value R 1 _ave (here, i=1) using the received data rate value with the data rate operation/management unit  241 , and stores the same. 
     In accordance with the data rate notification from the terminal  100 - 1 , the modulation processing unit  212  in the wireless base station  110 - 1  modulates voice packets Y 1 , Y 2 , and so on to be transmitted to the terminal  100 - 1  so as to be divided into slots. If a data rate of 76.8 kbits/s is required, the voice packets Y 1 , Y 2 , and so on are transmitted in the order from a slot Y 1 - 1  to a slot Y 1 - 8 , from a slot Y 2 - 1  to a slot Y 2 - 8 , and so on. 
     Here, a threshold value operation is performed using (Formula 1) with the average value R 1 _ave stored in the data rate operation/management unit  241  and the current data rate R 1 . In the case where the current value R 1  during the data communications continuously falls below R 1 _ave (Th&lt;1) and the wireless environment of the terminal  100 - 1  possibly deteriorates, the delay management unit  261  skips and discards the voice packets to be modulated. When the threshold value Th is 1.0 or larger, the delay management unit  261  does not perform the skipping process. When the threshold value Th is 0.5 or larger and smaller than 1.0, the delay management unit  261  performs one skipping process for five packets. When the threshold value Th is 0.5 or smaller, the delay management unit  261  performs one skipping process for three packets. Such a skipping process of the packets reduces delay. Further, although the quality of voice deteriorates, Enhanced Variable Rate Codec (EVRC) is used as voice codec, so that communications are possible without causing continuous interruptions of voice. Further, the slots which are supposed to be transmitted become available by the skipping of packets, so that other users can use the slots and the number of terminals to be accommodated in the base stations is increased. 
     [Second Embodiment] 
     The terminal  100 - 1  notifies the base station  110 - 1  of wireless environment information as a data rate control signal, irrespective of presence or absence of data communications when the terminal  100 - 1  is located within the service area of the wireless base station  110 - 1 . 
     The wireless base station  110 - 1  operates the average value R 1 _ave (i=1) using the received data rate value with the data rate operation/management unit  241 , and stores the same. 
     In accordance with the data rate requirement from the terminal  100 - 1 , the modulation processing unit  212  in the wireless base station  110 - 1  modulates voice packets Y 1 , Y 2 , and so on to be transmitted to the terminal  100 - 1  so as to be divided into slots. If a data rate of 76.8 kbits/s is required, the voice packets Y 1 , Y 2 , and so on are divided into slots Y 1 - 1  to Y 1 - 8 , Y 2 - 1  to Y 2 - 8 , and so on by the modulation processing unit  212 , and are transmitted to the terminal  100 - 1  through the transmission signal processing unit  202 . 
     The terminal  100 - 1  allows the demodulation processing unit  313  therein to demodulate the packet Y 1  using the slot Y 1 - 1 . If the packet Y 1  can be demodulated, ACK is transmitted to the wireless base station  110 - 1 . If not, NAK is transmitted to the wireless base station  110 - 1 . In the case where ACK is returned from the terminal  100 - 1 , the wireless base station  110 - 1  transmits Y 2 - 1  without transmitting Y 1 - 2 . In the case where NAK is returned, the wireless base station  110 - 1  transmits the next slot Y 1 - 2 . The terminal  100 - 1  demodulates the packet Y 1  using the slots Y 1 - 1  and Y 1 - 2 . If the packet Y 1  can be demodulated, ACK is returned. If not, NAK is returned. In the case of ACK, the wireless base station  110 - 1  transmits a slot Y 2 - 1 . In the case of NAK, the wireless base station  110 - 1  transmits the next slot Y 1 - 3 . The terminal  100 - 1  demodulates the packet Y 1  using the slots Y 1 - 1 , Y 1 - 2  and Y 1 - 3 . If the packet Y 1  can be demodulated, ACK is returned. If not, NAK is returned. In the case of ACK, the wireless base station  110 - 1  transmits the slot Y 2 - 1 . In the case of NAK, the wireless base station  110 - 1  transmits the next slot Y 1 - 4 . The above-described procedure is performed up to a slot Y 1 - 16  until the packet Y 1  can be demodulated. 
     Here, a threshold value operation is performed using (Formula 1) with the average value R 1 _ave stored in the data rate operation/management unit  251  in the wireless base station  110 - 1  and the current data rate R 1 . In the case where the current value R 1  during the data communications continuously falls below R 1 _ave and the wireless environment of the terminal  100 - 1  possibly deteriorates, the wireless base station  110 - 1  does not transmits the next slot even when NAK is returned from the terminal  100 - 1 , but transmits the slot Y 2 - 1  by terminating at the slot Y 1 - 5 . 
     By skipping the slots, the quality of voice at the skipped parts deteriorates. However, delay is reduced. In addition, voice is not continuously interrupted and the connection is not cut, so that communications are possible. Further, the slots which are supposed to be transmitted can be assigned to other users by the skipping of slots, so that the number of terminals to be accommodated is increased. 
     [Third Embodiment] 
     With reference to  FIG. 6 , a third embodiment will be described. Here,  FIG. 6  is a diagram for explaining slot assignment. The third embodiment is an embodiment in which slots Y 1 - 1  and Y 2 - 1  generated from different voice packets are continuously transmitted. 
     In  FIG. 6 , the data transmitted from the wireless base station  110 - 1  is demodulated from the slot Y 1 - 1  to the packet Y 1  at the demodulation processing unit  313  in the terminal  100 - 1 . If the packet Y 1  can be demodulated, the terminal  100 - 1  transmits ACK to the wireless base station  110 - 1 . If not, the terminal  100 - 1  transmits NAK to the wireless base station  110 - 1 . Here, it is assumed that the packet Y 1  can not be demodulated, so that the terminal  100 - 1  transmits NAK. 
     Next, the terminal  100 - 1  demodulates the packet Y 2  using the slot Y 2 - 1  transmitted right after the slot Y 1 - 1 . If the packet Y 2  can be demodulated, the terminal  100 - 1  transmits ACK to the wireless base station  110 - 1 . If not, the terminal  100 - 1  transmits NAK to the wireless base station  110 - 1 . Here, it is assumed that the packet Y 2  can be demodulated, so that the terminal  100 - 1  transmits ACK. 
     Next, the wireless base station  110 - 1  transmits Y 1 - 2  because the transmission result of the slot Y 1 - 1  is NAK. The terminal  100 - 1  demodulates the packet Y 1  using the slots Y 1 - 1  and Y 1 - 2 . If the packet Y 1  can be demodulated, the terminal  100 - 1  transmits ACK. If not, the terminal  100 - 1  transmits NAK. Further, the wireless base station  110 - 1  transmits Y 3 - 1  because the transmission result of the slot Y 2 - 1  is ACK. 
     Here, since the packet Y 2  that is a sequence subsequent to the packet Y 1  can be demodulated, the wireless base station  110 - 1  terminates the transmission process of the packet Y 1  irrespective of ACK or NAK of the demodulation result after transmission of the slot Y 1 - 2 , and assigns and transmits a slot Y 4 - 1  that is a slot of another packet. 
     In the case where the packet Y 2  can be demodulated prior to the packet Y 1  that is not demodulated, the terminal  100 - 1  discards the slots Y 1 - 1  and Y 1 - 2 . 
     Accordingly, although the quality of voice deteriorates, delay is reduced. Further, the slots which are supposed to be transmitted can be assigned to other users, so that the number of terminals to be accommodated is increased. 
     According to the present embodiments, delay can be reduced in VoIP communications, and the number of terminals accommodated in the base station can be increased.