Patent Publication Number: US-2022217504-A1

Title: Wireless communication system and wireless communication method

Description:
TECHNICAL FIELD 
     The present disclosure relates to a wireless communication system and a wireless communication method for performing a broadcast/multicast service through broadcast/multicast communication. 
     BACKGROUND ART 
     Examples of the most widely used wireless communication systems that perform autonomous distributed access control include a wireless LAN system based on the IEEE 802.11 standard (Non Patent Literature 1). 
     Typical examples of a service of simultaneously transmitting data to a plurality of destinations include broadcast and multicast. In an IEEE802.11 wireless LAN system, a method of handling group-addressed data, that is, data addressed to a plurality of terminals, is determined. In the IEEE802.11 standard, “10.3.6 Group addressed MPDU transfer procedure” includes regulations related to a basic transfer procedure for broadcast/multicast data, and “11.2 Power management” includes regulations related to a data transfer procedure in a case where a terminal device is operated in a power saving mode. 
       FIG. 5  illustrates a frame transfer procedure when a typical unicast frame in the IEEE802.11 standard is transmitted. When an AP transmits a unicast frame in accordance with an access control procedure and an STA receives the unicast frame normally, the STA returns an ACK. 
       FIG. 6  illustrates a procedure of transmitting a broadcast/multicast frame in the IEEE802.11 standard. An AP transmits a broadcast/multicast frame in accordance with the same access control procedure as that when the AP transmits a unicast frame. However, an STA does not return an ACK even when the STA has received the unicast frame normally. This is because a broadcast/multicast frame is transmitted to a plurality of STAs, and returning an ACK from each STA may cause a collision. Thus, in broadcast/multicast communication, there is no procedure of returning a response after transmission of a frame, which reduces the quality of communication and reliability as compared to unicast communication. 
     CITATION LIST 
     Non Patent Literature 
     
         
         Non Patent Literature 1: IEEE Std 802.11-2016 
       
    
     SUMMARY OF THE INVENTION 
     Technical Problem 
     In a case where communication is performed only unidirectionally in broadcast/multicast communication, there is no response frame from a communication partner serving as a destination as compared to a case where communication is performed bidirectionally. Thus, it is not possible to obtain feedback information such as whether or not communication has been successful or the current state of communication quality. For this reason, in the current circumstances, there is a problem in that, even when a wireless frame is transmitted, the content cannot be received normally due to the frequent occurrence of frame errors caused by a poor communication environment, such as a propagation path, or conversely, an excessively low-speed rate for reducing a frame error rate results in reduction in capacity. 
     An object of the present disclosure is to provide a wireless communication system and a wireless communication method that are capable of improving communication quality and reliability even in the case of unidirectional communication in broadcast/multicast communication. 
     Means for Solving the Problem 
     A first invention is a wireless communication system including a plurality of transmission devices configured to receive broadcast/multicast data transmitted from a broadcast/multicast data transmission device, convert the broadcast/multicast data into wireless frames, and broadcast/multicast the wireless frames to a plurality of reception devices, and the plurality of reception devices configured to receive the wireless frames broadcasted/multicasted from the plurality of transmission devices and acquire the broadcast/multicast data, in which each of the plurality of transmission devices includes a broadcast/multicast transmission unit configured to generate the same wireless frame from the broadcast/multicast data and broadcast/multicast the generated wireless frame at the same transmission timing using the same MCS. 
     The wireless communication system according to the first invention further includes a control device connected to the broadcast/multicast data transmission device and the plurality of transmission devices, the control device includes a unit configured to calculate an interval of a transmission timing in each transmission device in accordance with a data rate of the broadcast/multicast data transmitted from the broadcast/multicast data transmission device and an amount of data to be transmitted at one time in accordance with an MCS of each transmission device, and notify the plurality of transmission devices of the calculated interval. 
     In the wireless communication system according to the first invention, the broadcast/multicast transmission unit of each of the plurality of transmission devices is configured to set a random back-off value in random access control to a common fixed value, and to start to transmit the wireless frame at the transmission timing when a signal is not detected through carrier sensing. 
     In the wireless communication system according to the first invention, the control device includes a unit configured to notify the plurality of transmission devices of a common parameter value to be set in each field of a MAC header of the wireless frame. 
     In the wireless communication system according to the first invention, the plurality of transmission devices each include a unit configured to notify the control device of received power values of mutually received beacon signals, and the control device includes a unit configured to calculate the MCS in accordance with the received power values of the respective transmission devices and notify the plurality of transmission devices of the calculated MCS. 
     A second invention is a wireless communication method including by a plurality of transmission devices, receiving broadcast/multicast data transmitted from a broadcast/multicast data transmission device, converting the broadcast/multicast data into wireless frames, and broadcasting/multicasting the wireless frames to a plurality of reception devices, and by the plurality of reception devices, receiving the wireless frames broadcasted/multicasted from the plurality of transmission devices and acquiring the broadcast/multicast data, in which each of the plurality of transmission devices generates the same wireless frame from the broadcast/multicast data and broadcasts/multicasts the generated wireless frame at the same transmission timing using the same MCS. 
     Effects of the Invention 
     According to the present disclosure, broadcast/multicast data transmitted from a broadcast/multicast data transmission device is broadcasted/multicasted as the same wireless frame from each of a plurality of transmission devices at the same transmission timing using the same MCS, and each reception device receives the same wireless frames in a superimposed manner. This increases an apparent received power value and improves an SINR, whereby it is possible to increase communication capacity. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram illustrating a configuration of an example of a wireless communication system of the present disclosure. 
         FIG. 2  is a timing chart illustrating an example of broadcast/multicast transmission of the wireless communication system of the present disclosure. 
         FIG. 3  is a diagram illustrating a configuration example of a transmission device  100 . 
         FIG. 4  is a diagram illustrating a configuration example of a control device  20 . 
         FIG. 5  is a diagram illustrating a frame transfer procedure when a unicast frame is transmitted. 
         FIG. 6  is a diagram illustrating a procedure of transmitting a broadcast/multicast frame. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
       FIG. 1  illustrates a configuration of an example of a wireless communication system of the present disclosure. 
     In  FIG. 1 , a broadcast/multicast data transmission device  10  such as a content server outputs broadcast/multicast data to be broadcasted or multicasted, and transfers the broadcast/multicast data to a plurality of transmission devices  100 - 1  to  100 -M through a control device  20 . The transmission devices  100 - 1  to  100 -M each convert input broadcast/multicast data into a wireless frame and broadcast/multicast the wireless frame toward reception devices  200 - 1  to  200 -N. Reception devices designated by a group address receive the wireless frames. 
     The present disclosure is characterized in that the plurality of transmission devices  100 - 1  to  100 -M each generate the same wireless frame from the broadcast/multicast data and broadcast/multicast the generated wireless frame at the same transmission timing using the same modulation and coding system (MCS). For example, as illustrated in  FIG. 2 , when transmission devices  1  and  2  transmit the same wireless frames at the same transmission timing using the same MCS, reception devices  1  and  2  receive the same wireless frames from the transmission devices  1  and  2  in a superimposed manner. Thereby, in the reception devices  1  and  2 , an apparent received power value is increased, and an SINR can be improved. 
     Here, the same transmission timing, the same wireless frame, and the same MCS will be described. 
     With Regard to Same Transmission Timing 
     In order for each reception device to receive a plurality of wireless frames from a plurality of transmission devices in a superimposed manner and to perform reception processing on the wireless frames as one wireless frame, the transmission timings of the wireless frames from the plurality of transmission devices need to be the same. Note that, strictly speaking, the reception devices need to simultaneously receive the wireless frames transmitted from the plurality of transmission devices. However, if the plurality of transmission devices simultaneously transmit the wireless frames, a difference in reception time between the reception devices can be absorbed during the reception processing of the wireless frames. 
     In the configuration of  FIG. 1 , the control device  20  determines transmission timing of the respective transmission devices  100 - 1  to  100 -M and notifies the transmission devices of the transmission timing. The transmission timing may be a relative time or may be an absolute time. The control device  20  calculates the interval of the transmission timing in each transmission device based on the data rate of broadcast/multicast data of the broadcast/multicast data transmission device  10  and the amount of data to be transmitted at one time by the transmission device. For example, when the data rate of the broadcast/multicast data is set to 2 Mbps, and the amount of data to be transmitted at one time in accordance with the MCS is set to 1500 bytes×10 packets, the interval of the transmission timing is 1500×8×10/(2×106)=0.06 (sec). 
     Note that this transmission timing indicates the time when a wireless frame starts to be transmitted from an antenna of the transmission device. In a wireless communication system such as a wireless LAN for performing CSMA/CA control in an unlicensed band, carrier sensing is performed before a transmission timing, and random back-off control is performed when a signal transmitted by another device is not detected on a channel. A wireless frame is transmitted when no signal is detected. Thus, it is necessary to set a random back-off value to the same fixed value in the plurality of transmission devices and also use the same carrier sensing time (CS) as illustrated in  FIG. 2 . 
     With Regard to Same Wireless Frame and Same MCS In order that a plurality of wireless frames received in a superimposed manner by each reception device are demodulated as the same frame, for example, wireless frames of OFDM communication should be received at the same symbol position in the same phase. For this, it is necessary to align bit strings of the frames before demodulation in the plurality of transmission devices. 
     In the configuration illustrated in  FIG. 1 , the control device  20  notifies the transmission devices  100 - 1  to  100 -M of a common parameter value to be set in each field of a MAC header of the wireless frame generated from broadcast/multicast data. For example, a MAC address and a BSSID of an individual transmission device are typically set in a transmission address field. However, in a case where the same wireless frame in the present disclosure is generated, a common value is set in the transmission address field. In addition, the transmission devices each do not manage a sequence control value, and the control device  20  sets a sequence number uniquely designated from, for example, an ID of the broadcast/multicast data transmission device  10  and an ID of a traffic flow as a sequence control value. Further, in a case where fragmentation is performed, the control device  20  designates the size thereof in each of the transmission devices. When such designation is performed, the size may be a specified fixed value or may be a given set value. 
     Further, in order for the reception devices to receive wireless frames at the same symbol position in the same phase, it is also necessary to use the same MCS in the respective transmission devices. First, the transmission devices  100 - 1  to  100 -M observe received power values of mutually received beacon signals and calculate approximate distances. The intermediate point of the calculated distances is a point where a total received power value is minimized. For this reason, the transmission devices each obtain an SINR taking device noise into account in consideration of a power value at this point, and select an MCS with which the error rate is considered to be sufficiently small. Each transmission device notifies the control device  20  of the obtained MCS. Then, the control device  20  selects the MCS with the lowest error rate and notifies the transmission devices of the selected MCS as a common parameter. Alternatively, the transmission devices may notify the control device  20  of the observed received power values, and the control device  20  performs the above-described processing, calculates a common MCS, and notifies the transmission devices of the calculated MCS. Then, the transmission devices broadcast/multicast wireless frames using the notified MCS. Note that, in a case where the transmission devices cannot mutually observe the received power values, the transmission devices may be notified of an MCS set by the control device  20 . 
       FIG. 3  illustrates a configuration example of the transmission device  100 . 
     In  FIG. 3 , an input and output unit  101  of the transmission device  100  outputs data transferred from a high-level device to a data determination unit  102 . The data determination unit  102  determines input data. When the input data is broadcast/multicast data transmitted from the broadcast/multicast data transmission device  10 , the data determination unit  102  outputs the input data to a broadcast/multicast data transmission queue  103 . When the input data is other data, the data determination unit  102  outputs the input data to a general transmission queue  104 . Note that, in a case where the transmission device  100  receives only broadcast/multicast data from the broadcast/multicast data transmission device  10  at all times (for example, in a case where the transmission device  100  is a transmission device dedicated to broadcasting), the data determination unit  102  and the general transmission queue  104  are not necessary. A transmission source ID indicating the broadcast/multicast data transmission device  10  is set in the broadcast/multicast data, and the transmission source ID is stored as, for example, a transmission source address of a wireless frame in which the broadcast/multicast data is stored. 
     A wireless frame modulation unit  105  modulates the broadcast/multicast data stored in the broadcast/multicast data transmission queue  103  or the data stored in the general transmission queue  104  in accordance with a wireless frame format. In a case where a wireless access control unit  106  acquires a transmission right through an access procedure corresponding to each queue by CSMA/CA or the like, the wireless access control unit  106  extracts data stored in the corresponding queue and transmits the data as a wireless frame through a wireless modulation and demodulation unit  107  and a wireless input and output unit  108 . 
     In a case where the control device  20  notifies a control information management unit  111  of a transmission timing of the wireless frame, information on a MAC header, information on an MCS, and the like, the control information management unit  111  notifies each of a transmission timing management unit  112 , the wireless frame modulation unit  105 , and the wireless modulation and demodulation unit  107  of the notified information. The wireless frame modulation unit  105  generates the same wireless frame to be broadcasted/multicasted by each transmission device based on the notified information of the MAC header. The transmission timing management unit  112  extracts broadcast/multicast data from the broadcast/multicast data transmission queue  103  in accordance with the transmission timing, and controls the wireless access control unit  106  to start wireless access control in accordance with the transmission timing. At the time of transmitting the wireless frame, the wireless modulation and demodulation unit  107  performs modulation and coding processing by using the MCS common in the transmission devices. 
       FIG. 4  illustrates a configuration example of the control device  20 . 
     In  FIG. 4 , the control device  20  is configured such that a data input unit  21  receives broadcast/multicast data transmitted by the broadcast/multicast data transmission device  10 , and a data management unit  22  including a buffer function manages the broadcast/multicast data and transfers the broadcast/multicast data to the transmission devices through a transmission device input and output unit  23 . An intra-area transmission device information management unit  24 , a parameter management unit  25 , and a transmission timing control unit  26  collect information necessary for each transmission device to generate the same wireless frame from broadcast/multicast data and transmit the generated wireless frame at the same transmission timing using the same MCS. Then, the intra-area transmission device information management unit  24  notifies, through the transmission device input and output unit  23 , each transmission device of the transmission timing of the wireless frame, the information of the MAC header, and the information of the MCS which are generated as described above. 
     REFERENCE SIGNS LIST 
     
         
           10  Broadcast/multicast data transmission device 
           20  Control device 
           21  Data input unit 
           22  Data management unit 
           23  Transmission device input and output unit 
           24  Intra-area transmission device information management unit 
           25  Parameter management unit 
           26  Transmission timing control unit 
           100  Transmission device 
           101  Input and output unit 
           102  Data determination unit 
           103  Broadcast/multicast data transmission queue 
           104  General transmission queue 
           105  Wireless frame modulation unit 
           106  Wireless access control unit 
           107  Wireless modulation and demodulation unit 
           108  Wireless input and output unit 
           111  Control information management unit 
           112  Transmission timing management unit