JP6212007B2 - Wireless communication system and wireless communication method - Google Patents

Description

  The present invention relates to a radio communication system and a radio communication method that are configured by a base station and a plurality of terminal stations under its control and perform access control of the terminal station.

  In access control according to the IEEE 802.11 wireless communication standard, each terminal station performs random back-off control for avoiding carrier sense and collision. That is, the channel is idle for the DIFS time before transmitting the packet, and the packet is transmitted after the channel is idle until the random backoff value generated from the range of the random number generation range (CW) becomes 0. CSMA / CA (Carrier Sense Multiple Access with Collision Avoidance) method is used. If the channel becomes busy before the random backoff value becomes 0, no packet is transmitted, and carrier sensing is continued for the remaining random backoff value period when the channel becomes idle next time.

  On the other hand, the base station that has received the packet normally transmits an acknowledgment ACK in order to notify the terminal station that is the transmission source. Therefore, if the terminal station that is the transmission source of the packet does not receive the ACK within a predetermined time, the terminal station determines that the transmission has failed, expands CW up to CWmax as the upper limit, and based on the generated random backoff value To retransmit. That is, whenever the number of retransmissions increases, it is determined that the channel is congested, and the CW is expanded to vary the random back-off value to avoid collision. If the packet transmission is successful, CW is reset to CWmin (for example, 15).

  Here, in wireless communication, if there is a distance between wireless stations or there are obstacles that do not allow radio waves to pass between the wireless stations, the state in which the wireless signals do not reach each other (the radio wave environment where carrier sense does not function) It can happen. In this case, since carrier sense does not function effectively, even if random back-off control is performed, the frequency of frame collision increases, which causes a deterioration in throughput characteristics.

  Therefore, in the IEEE 802.11 wireless communication standard, an RTS (Request to Send) / CTS (Clear to Send) control procedure is used to cope with an environment where carrier sense does not function effectively, that is, an “environment where a hidden terminal exists”. Is provided.

FIG. 5 shows an example of a conventional RTS / CTS control procedure.
In FIG. 5, it is assumed that the terminal stations 1 and 2 and the terminal station 3 are in a hidden terminal relationship. The terminal stations 1, 2, and 3 perform carrier sense before data frame transmission, and the terminal station 1 whose random back-off value is first 0 after DIFS transmits the RTS to the base station 0 that is the destination of the data frame Send. Since the terminal station 1 and the terminal station 2 can sense each other, the terminal station 2 receives the RTS transmitted by the terminal station 1 and stops the back-off counter. On the other hand, since the terminal station 1 and the terminal station 3 cannot sense each other, the terminal station 3 cannot receive the RTS transmitted by the terminal station 1 and continues the back-off counter. The base station 0 that is the destination of the data frame returns a CTS to the terminal station 1 via SIFS after receiving the RTS. When receiving the CTS transmitted from the base station 0, the terminal station 3 stops the back-off counter.

  Here, the RTS and CTS frames have a duration field in which a scheduled period for using the channel is described. The terminal station 2 reserves a reservation signal (NAV) only during the period described in the RTS frame transmitted by the terminal station 1. By setting, transmission is prohibited and collision is prevented. The terminal station 3 sets a reservation signal (NAV) only during the period described in the CTS frame transmitted by the base station 0, thereby prohibiting transmission and preventing collision. The terminal station 1 that has received the CTS for RTS transmits a data frame while the terminal stations 2 and 3 prohibit transmission, and the base station 0 that has received the data frame also transmits an ACK signal and terminates communication. To do. As described above, if RTS / CTS is normally transmitted / received between the terminal station 1 and the base station 0, the data frame and the ACK signal are exchanged without being interrupted thereafter.

IEEE Standard for Information technology Telecommunications and information exchange between systems Local and metropolitan area networks Specific requirements: Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications

  By the way, in an environment where many terminal stations exist in the area of the base station 0, there are many cases where a plurality of terminal stations are in a relationship of hidden terminals. In an environment where such a hidden terminal exists, it is possible to avoid collision of data frames by the RTS / CTS control procedure as shown in FIG.

  However, in the terminal stations 1 and 2 and the terminal station 3 that are in the positional relationship of the hidden terminals, since carrier sense does not function with each other, collision between RTSs may occur. For example, as shown in the latter half of FIG. 5, the terminal station 3 continues the backoff counter because the terminal stations 1 and 2 cannot detect the RTS even if the random backoff value is 0 and RTS is transmitted. When the random back-off value of 2 becomes 0 and RTS is transmitted, the RTSs transmitted by the terminal stations 2 and 3 collide with each other. However, even if an RTS collision occurs, since the back-off control is started after each CTS timeout, the time loss is smaller than the data frame collision, and the merit of improving the throughput characteristics by the RTS / CTS control procedure is great.

  FIG. 6 shows an example of the effect of improving the throughput characteristics by the conventional RTS / CTS control. In FIG. 6, A shows the throughput characteristics in the case of standard CSMA / CA control (without RTS / CTS control) in an environment where there is no hidden terminal. Even without RTS / CTS control, predetermined throughput characteristics can be realized by CSMA / CA control (random backoff control).

  B shows the throughput characteristic in the case of standard CSMA / CA control (without RTS / CTS control) in an environment where all terminal stations are in the positional relationship of hidden terminals. Since carrier sense does not function even when CSMA / CA control is performed, collision of data frames occurs frequently, and the throughput characteristic is remarkably deteriorated.

  C shows the throughput characteristics when RTS / CTS control is performed in an environment where there is no hidden terminal. With CSMA / CA control and RTS / CTS control, collision of data frames can be reliably avoided and good throughput characteristics can be realized. However, when the number of terminal stations is small, the upper limit of the throughput can be suppressed by the overhead associated with transmission / reception of the RTS / CTS frame.

  D shows the throughput characteristics when RTS / CTS control is performed in an environment where all terminal stations are in the positional relationship of hidden terminals. Since data frame collision is avoided by the RTS / CTS control, the throughput can be significantly improved compared to the throughput characteristic B in which the RTS / CTS control is not performed. However, since there is a possibility that RTSs transmitted by terminal stations that are in a positional relationship as hidden terminals will collide with each other, the throughput is lower than the throughput characteristics A and C in an environment where there are no hidden terminals.

  The present invention provides a wireless communication system and a wireless communication method capable of reducing the RTS collision in RTS / CTS control even in an environment where a hidden terminal exists, and realizing a throughput comparable to that in an environment where no hidden terminal exists. The purpose is to provide.

A first aspect of the present invention is a wireless communication system in which a plurality of terminal stations under a base station acquire a data frame transmission right and perform communication by random backoff control and RTS / CTS control by the CSMA / CA method. When the random back-off value by the random back-off control becomes 0, the terminal station determines whether the RTS can be transmitted according to the RTS transmission probability having a characteristic that decreases from the initial value according to the number of retransmissions of the RTS or data frame. If it is determined that transmission is possible, RTS is transmitted based on RTS / CTS control, and if transmission is not possible, a transition is made to a standby mode for waiting for transmission of RTS , and another station transmitted from the base station during the standby mode. Control to return from the standby mode when a CTS addressed to the destination is received, or when a predetermined timeout period elapses after entering the standby mode. Access control means for performing is provided.

In the wireless communication system of the first invention, the access control means transmits an RTS and does not return a CTS within the first timeout period, or transmits a data frame and confirms reception within the second timeout period. If the ACK is not returned, a setting to lower the R TS transmission probability depending on the number of retransmissions of the RTS or data frame, when the CTS is returned, or the number of retransmissions of the RTS when acknowledgment ACK is returned reset the filters to re-set the R TS transmission probability to the initial value. The access control means, the setting is as R TS transmission probability providing a predetermined lower limit value.

A second invention is a radio communication method in which a plurality of terminal stations under the control of a base station acquire a data frame transmission right and perform communication by random backoff control and RTS / CTS control by the CSMA / CA method. When the random backoff value by the random backoff control becomes 0, the access control means of the terminal station determines the RTS or RTS according to the RTS transmission probability having a characteristic that decreases from the initial value according to the number of retransmissions of the data frame. If the transmission is possible, the RTS is transmitted based on the RTS / CTS control. If the transmission is not possible, the mobile station shifts to the standby mode for waiting for the RTS transmission , and transmits from the base station during the standby mode. From the standby mode when a CTS addressed to another station is received, or when a predetermined timeout period elapses after entering the standby mode. Control to return.

In the wireless communication method of the second invention, the access control means transmits an RTS and does not return a CTS within the first timeout period, or transmits a data frame and confirms reception within the second timeout period. If the ACK is not returned, a setting to lower the R TS transmission probability depending on the number of retransmissions of the RTS or data frame, when the CTS is returned, or the number of retransmissions of the RTS when acknowledgment ACK is returned reset the filters to re-set the R TS transmission probability to the initial value. The access control means, the setting is as R TS transmission probability providing a predetermined lower limit value.

  According to the present invention, in a wireless communication system that performs communication by acquiring a data frame transmission right by random backoff control and RTS / CTS control by the CSMA / CA method, the random backoff value by random backoff control becomes zero. RTS is not transmitted immediately, and whether or not RTS can be transmitted is determined according to a predetermined RTS transmission probability. If transmission is not possible, RTS transmission is stopped and a transition is made to the standby mode. As a result, even if the random back-off value coincides by chance or the RTS transmitted by the other station cannot be detected due to the positional relationship of the hidden terminal, the RTS transmission is controlled according to the RTS transmission probability. Therefore, RTS collision can be avoided and throughput characteristics can be improved.

  When the terminal station that has entered the standby mode receives a CTS addressed to another station, it can return from the standby mode and shift to the normal mode. In this case, the reservation signal (NAV) is set only during the period described in the CTS frame.

  Further, even when all the terminal stations enter the standby mode and the base station does not transmit CTS, each terminal station can return from the standby mode after the timeout time elapses and shift to the normal mode. In this case, random backoff control by the CSMA / CA method is started.

It is a flowchart which shows the example of an access control procedure of the terminal station in this invention. It is a time chart which shows the operation example of Example 1 of this invention. It is a time chart which shows the operation example of Example 2 of this invention. It is a figure which shows an example of the improvement effect of the throughput characteristic by this invention. It is a time chart which shows the example of the conventional RTS / CTS control procedure. It is a figure which shows an example of the improvement effect of the throughput characteristic by the conventional RTS / CTS control.

  In the wireless communication system of the present invention, in order to avoid RTS collision between hidden terminals, each terminal station determines whether or not RTS can be transmitted according to a predetermined RTS transmission probability after the random backoff value becomes 0 through DIFS. The terminal station that has made the determination and has stopped transmitting the RTS enters the standby mode, and returns to the normal mode from the standby mode when receiving the CTS addressed to the other station transmitted from the base station. As a result, the other terminal station can transmit the RTS while one of the terminal stations in the hidden terminal relationship is in the standby mode. The terminal station that has entered the standby mode returns to the normal mode from the standby mode upon reception of the CTS, and sets a reservation signal (NAV) only for the period described in the CTS frame.

  In addition, since it is assumed that all the terminal stations are in the standby mode, the terminal station that has entered the standby mode performs control to return from the standby mode to the normal mode when a predetermined timeout time has elapsed. As a result, even if all the terminal stations enter the standby mode and the base station does not transmit CTS, each terminal station returns to the normal mode after the time-out period elapses, and random backoff control by the CSMA / CA method is performed. Start.

FIG. 1 shows an example of an access control procedure of a terminal station in the present invention.
In FIG. 1, the terminal station performs random back-off control via DIFS (S1), and determines whether RTS transmission is possible or not according to a predetermined RTS transmission probability when the random back-off value becomes 0 (S2). ). Here, if the RTS can be transmitted, the RTS is transmitted according to the normal RTS / CTS control, and data transmission is performed after the CTS addressed to the own station is returned (S3). Here, the RTS transmission probability is adjusted according to the success or failure of the RTS or data transmission (S4), which will be described in detail later.

  On the other hand, if the RTS cannot be transmitted, the RTS transmission is stopped and the standby mode is entered (S5), and when the CTS addressed to another station is received, the normal mode is restored (S6). For example, NAV setting corresponding to the CTS frame is performed, and the process returns to step S1. Alternatively, when a predetermined time-out period elapses after entering the standby mode, the normal mode is restored and the process returns to step S1 (S7).

FIG. 2 shows an operation example of the first embodiment of the present invention.
In FIG. 2, it is assumed that the terminal stations 1 and 2 and the terminal station 3 are in a hidden terminal relationship. The terminal stations 1, 2, and 3 perform carrier sense before data frame transmission, and the terminal station 1 whose random back-off value first becomes 0 after DIFS determines whether or not RTS can be transmitted according to a predetermined RTS transmission probability. In this case, the RTS is transmitted to the base station 0 which is the destination of the data frame. Since the terminal station 1 and the terminal station 2 can sense each other, the terminal station 2 receives the RTS transmitted by the terminal station 1, stops the back-off counter, and performs a reservation signal (NAV) only for the period described in the RTS frame. Set.

  On the other hand, since the terminal station 1 and the terminal station 3 cannot sense each other, the terminal station 3 does not receive the RTS transmitted by the terminal station 1 and continues the back-off counter. Then, when the random back-off value becomes 0, the terminal station 3 determines whether or not RTS can be transmitted according to a predetermined RTS transmission probability. Here, the terminal station 3 stops RTS transmission and enters a standby mode. Thereby, the RTS transmitted from the terminal station 1 is received by the base station 0 without colliding. In some cases, the terminal station 3 transmits an RTS to cause an RTS collision. At this time, control is performed to reduce the probability of the next RTS transmission. Details will be described later.

  After receiving the RTS, the base station 0 returns a CTS to the terminal station 1 via SIFS, and the terminal station 1 receives the CTS. The terminal station 3 also receives the CTS transmitted from the base station 0, returns from the standby mode to the normal mode, and sets a reservation signal (NAV) only for the period described in the CTS frame. The terminal station 1 that has received the CTS for RTS transmits a data frame while the terminal stations 2 and 3 prohibit transmission, and the base station 0 that has received the data frame also transmits an ACK signal and terminates communication. To do.

  At the next timing, the terminal station 2 whose random back-off value first becomes 0 after DIFS stops transmission of RTS according to a predetermined RTS transmission probability and enters a standby mode. Next, the terminal station 3 whose random back-off value has become 0 transmits the RTS to the base station 0 according to a predetermined RTS transmission probability. At this time, since the terminal station 1 and the terminal station 3 cannot sense each other, the terminal station 1 does not receive the RTS transmitted by the terminal station 3, continues the back-off counter, and the random back-off value becomes 0. Whether or not RTS transmission is possible is determined according to the RTS transmission probability, and here, the RTS transmission is stopped and the standby mode is entered. Thereby, the RTS transmitted by the terminal station 3 is received by the base station 0 without colliding. In some cases, the terminal station 1 transmits an RTS to cause an RTS collision. At this time, control is performed to reduce the probability of the next RTS transmission. Details will be described later.

  After receiving the RTS, the base station 0 returns a CTS to the terminal station 3 via SIFS, and the terminal station 3 receives the CTS. The terminal station 1 also receives the CTS transmitted from the base station 0, returns from the standby mode to the normal mode, and sets a reservation signal (NAV) only for the period described in the CTS frame. The terminal station 2 has returned to the normal mode after the timeout time in the standby mode has elapsed, and has entered random backoff control, but sets a reservation signal (NAV) only for the period described in the received CTS frame. To do. The terminal station 3 that has received the CTS for RTS transmits a data frame while the terminal stations 1 and 2 prohibit transmission, and the base station 0 that has received the data frame also transmits an ACK signal and terminates communication. To do.

FIG. 3 shows an operation example of the second embodiment of the present invention.
In FIG. 3, it is assumed that the terminal stations 1 and 2 and the terminal station 3 are in a hidden terminal relationship. The terminal stations 1, 2 and 3 perform carrier sense before data frame transmission, and the terminal station 2 whose random back-off value first becomes 0 after DIFS determines whether or not RTS can be transmitted according to a predetermined RTS transmission probability. In this case, the transmission of the RTS is stopped and the standby mode is entered. Next, the terminal stations 1 and 3 whose random back-off values have become 0 each stop transmission of RTS according to a predetermined RTS transmission probability and enter a standby mode. That is, the terminal stations 1, 2, and 3 enter the standby mode all at once. In this case, since the CTS is not transmitted from the base station 0, each terminal station performs control to return from the standby mode to the normal mode when a predetermined timeout time elapses.

  The terminal station that has entered the normal mode performs random backoff control via DIFS, and determines whether or not RTS transmission is possible according to a predetermined RTS transmission probability when the random backoff value becomes zero. Here, the terminal station 1 transmits the RTS to the base station 0. The terminal station 2 receives the RTS transmitted from the terminal station 1, stops the back-off counter, and sets the reservation signal (NAV) only for the period described in the RTS frame. When the terminal station 3 does not receive the RTS transmitted by the terminal station 1 and continues the back-off counter and the random back-off value becomes 0, the terminal station 3 stops transmitting the RTS according to a predetermined RTS transmission probability. To enter standby mode. Thereby, the RTS transmitted from the terminal station 1 is received by the base station 0 without colliding.

  After receiving the RTS, the base station 0 returns a CTS to the terminal station 1 via SIFS, and the terminal station 1 receives the CTS. The terminal station 3 also receives the CTS transmitted from the base station 0, returns from the standby mode to the normal mode, and sets a reservation signal (NAV) only for the period described in the CTS frame. The terminal station 1 that has received the CTS for RTS transmits a data frame while the terminal stations 2 and 3 prohibit transmission, and the base station 0 that has received the data frame also transmits an ACK signal and terminates communication. To do.

Here, setting and updating of the RTS transmission probability will be described.
In the conventional random back-off control, if the random back-off values are different between the terminal stations, the RTS transmitted by one terminal station is received by the other terminal station, and the back-off counter is stopped. Does not occur. However, even if the random back-off values are different between terminal stations in a hidden terminal relationship, carrier sense cannot be performed, so the back-off counter is continued, and RTS may eventually collide. Alternatively, if the random backoff values between the terminal stations are the same, the RTS may collide regardless of the presence or absence of the hidden terminal. Therefore, as in the present invention, when the random back-off value becomes 0, the RTS transmission is determined according to the RTS transmission probability, thereby reducing the RTS collision caused by the hidden terminal and the RTS / CTS control. Throughput can be expected based on However, when it is determined whether or not RTS transmission is possible according to the RTS transmission probability, if transmission is possible at a plurality of terminal stations, an RTS collision cannot be avoided.

The present invention determines that an RTS collision or a data collision has occurred when an RTS is transmitted and no CTS is returned during the timeout period or when a data frame is transmitted and no ACK is returned during the timeout period. Then, control is performed to reduce the next RTS transmission probability. For example, the RTS transmission probability is set as follows.
RTS transmission probability = C · N ^-R

  Here, C and N are constants, 1 ≧ C> 0, N> 1, and R is the number of times of RTS or data retransmission (initial transmission is 0). Therefore, the initial value of the RTS transmission probability is C, and the RTS transmission probability decreases as the number of retransmissions R increases. When the RTS transmission probability is 0.50, for example, it is determined that transmission is possible when a random number is subtracted from a predetermined range and, for example, it is odd or even.

  The number of retransmissions R is updated in the following cases. When the CTS for the RTS does not return within the timeout period, when the data is transmitted and the ACK does not return during the timeout period, it is estimated that the transmission has failed due to a collision, and the number of retransmissions R is incremented by one. To increase. In addition, when the CTS for the RTS is returned, the data is transmitted and the ACK is returned, the retransmission count R is reset to the initial value 0. It is assumed that the RTS transmission probability is calculated immediately after the number of retransmissions R is updated.

  The RTS transmission probability set based on the above setting formula decreases as the number of retransmissions R increases. For example, when there are many hidden terminals, RTS collisions are likely to occur and the number of retransmissions R is likely to increase. In such a situation, since it is necessary to refrain from RTS transmission, the RTS transmission probability is lowered based on the above setting formula. However, as the value of the constant N increases, the rate of decrease in the RTS transmission probability with the increase in the number of retransmissions R increases. Therefore, terminal stations that fail to transmit RTS have fewer opportunities to transmit RTS, while terminal stations that have successfully transmitted RTS always maintain a high RTS transmission probability and continue to transmit RTS. Unfairness occurs.

  In order to deal with such unfairness, the value of the constant N may be set to about 2, 3 and 4, for example, and a lower limit of the RTS transmission probability may be set. For example, when C = 1 and N = 2, the RTS transmission probability is halved to 1,0.5, 0.25, 0.125, 0.0625, 0.03125,... In the case of = 4, it becomes 1/4 such as 1,0.25, 0.0625, 0.015625,. Here, assuming that the lower limit of the RTS transmission probability is “0.03”, for example, when N = 2, the RTS transmission probability 0.03125 of the number of retransmissions R = 5 becomes the lower limit, and when N = 4, the RTS transmission of the number of retransmissions R = 2. Probability 0.0625 is the lower limit. Note that the RTS transmission probability is not limited to the setting formula for the RTS transmission probability as long as the RTS transmission probability is a characteristic that decreases according to the RTS or the number of data retransmissions R.

  By the way, each time the number of retransmissions R increases, it is also effective to double the random number range CW for generating the random backoff value within the range of CWmax and combine it with the conventional method for reducing the probability that the random backoff value matches. is there. In this case, when the number of retransmissions R is reset or when the number of retransmissions R exceeds a predetermined value, CW is reset to CWmin, but it is also effective to optimally control CWmin according to the number of terminal stations.

FIG. 4 shows an example of the effect of improving the throughput characteristics according to the present invention.
4, C and D are the conventional throughput characteristics shown in FIG. 6, and C shows the throughput characteristics when RTS / CTS control is performed in an environment where there are no hidden terminals. D shows the throughput characteristics when RTS / CTS control is used in an environment where all terminal stations are in a hidden terminal position relationship, but the throughput is lower than the throughput characteristics C due to RTS collision.

  E, according to the present invention, performs RTS / CTS control in an environment where all terminal stations are in a hidden terminal position relationship, and determines whether or not RTS can be transmitted based on the RTS transmission probability. The throughput characteristic improved from the characteristic D is shown. The RTS transmission control based on the RTS transmission probability according to the present invention reduces the RTS collision probability even in the situation where there is a hidden terminal, and can avoid the RTS collision caused by the matching of random backoff values even in the situation where there is no hidden terminal. The effect of improving the throughput is great.

  Further, in addition to the RTS transmission control based on the RTS transmission probability, CWmin, which is a CSMA / CA parameter, is optimally controlled according to the number of terminal stations, thereby further improving the throughput characteristics.

0 Base station 1, 2, 3 Terminal station

Claims (6)

In a wireless communication system in which a plurality of terminal stations under the control of a base station perform communication by acquiring a transmission right of a data frame by random backoff control and RTS / CTS control by the CSMA / CA method.
When the random back-off value by the random back-off control becomes 0, the terminal station performs RTS or RTS transmission according to the RTS transmission probability having a characteristic that decreases from the initial value according to the number of retransmissions of the data frame . determining whether transmission, and transmits an RTS based on the RTS / CTS control if send, if unsendable shifts to the standby mode to wait for transmission of RTS, from the base station during該待motor mode It comprises access control means for performing control to return from the standby mode when a CTS addressed to another station is received, or when a predetermined timeout period elapses after shifting to the standby mode. Wireless communication system. The wireless communication system according to claim 1, wherein
Said access control means, when said the CTS within a first timeout period to send the RTS is not returned, or the case where transmit data frames in the second timeout period not returned acknowledgment ACK in the a RTS or set to reduce the pre Symbol R TS transmission probability depending on the number of retransmissions of the data frame, when the CTS is returned, or retransmits the RTS when the acknowledgment ACK is returned wireless communication system, characterized by resetting the previous SL R TS transmission probability to the initial value by resetting the count. The wireless communication system according to claim 2,
Wireless communication system, wherein the access control means are pre Symbol R TS transmission probability providing a predetermined lower limit value as set. In a wireless communication method in which a plurality of terminal stations under a base station acquire a data frame transmission right by RSMA / CTS random backoff control and RTS / CTS control and perform communication.
When the random backoff value by the random backoff control becomes 0, the access control means of the terminal station has an RTS transmission probability having a characteristic that decreases from the initial value according to the number of retransmissions of RTS or the data frame. depending determines whether transmission of the RTS and sends a RTS based on the RTS / CTS control if send, if unsendable shifts to the standby mode to wait for transmission of RTS, in該待motor mode When the CTS addressed to another station transmitted from the base station is received, or when a predetermined timeout period elapses after shifting to the standby mode, control to return from the standby mode is performed. Communication method. The wireless communication method according to claim 4, wherein
Said access control means, when said the CTS within a first timeout period to send the RTS is not returned, or the case where transmit data frames in the second timeout period not returned acknowledgment ACK in the a RTS or set to reduce the pre Symbol R TS transmission probability depending on the number of retransmissions of the data frame, when the CTS is returned, or retransmits the RTS when the acknowledgment ACK is returned radio communication method characterized by resetting the previous SL R TS transmission probability to the initial value by resetting the count. The wireless communication method according to claim 5, wherein
It said access control means, a radio communication method, wherein a pre Symbol R TS transmission probability is set to provide a predetermined lower limit value.

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