JP2009206573A - Wireless communication device and wireless communication method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve throughput by properly setting an initial value of a contention window during channel congestion or inactivity. <P>SOLUTION: A wireless communication device includes: a first means which not only counts the number of transmission successes but also resets the number of transmission failures to zero when receiving an ACK signal after packet transmission and not only counts the number of transmission failures but also resets the number of transmission successes to zero when not receiving an ACK signal within a prescribed time after packet transmission; and a second means which not only reduces an initial value CWmin of the contention window but also resets the number of transmission successes to zero when the number of transmission successes reaches a first prescribed value, and not only increases the initial value CWmin of the contention window but also resets the number of transmission failures to zero when the number of transmission failures reaches a second prescribed value. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a wireless communication system in which a plurality of wireless stations communicate with each other such as a wireless LAN (Local Area Network) or a PAN (Personal Area Network), and in particular, CSMA / CA (Carrier Sense Multiple Access with Collision Avoidance). The present invention relates to a wireless communication apparatus and a wireless communication method that perform random access control based on the above.

  Examples of wireless network standards include IEEE802.11 (Non-patent Document 1) and IEEE802.15.

  Here, CSMA (Carrier Sense Multiple Access) is a connection method corresponding to multiple access by performing carrier sense when transmission is started. In wired communication, there is CSMA / CD (Carrier Sense Multiple Access with Collision Detection) as a multiple access method. The carrier sense of the channel is performed before the transmission is started, and the transmission is started when the channel is idle. When a collision is detected, data is retransmitted after a random time calculated by the backoff algorithm.

  However, in wireless communication, since a difference between transmission power and reception power is large and it is difficult to detect a collision, a method of avoiding a collision by CSMA / CA is adopted. CSMA / CA performs channel carrier sense before starting transmission, and generates random backoff time from the contention window (CW) range when the channel is idle, and the backoff time. The packet is transmitted after waiting for this period. There is a high probability that collisions will be avoided because the back-off times of multiple radio stations will be random, but if the ACK signal is not received within the specified time, it will be judged as transmission failure and the contention window will be expanded. Expand the range of random generation of backoff time. If the transmission fails after that, the contention window is further expanded. However, when a subsequent packet is transmitted after successful transmission or packet discard, the contention window is reset to the initial value CWmin.

FIG. 7 shows a configuration example of a radio communication system to which CSMA / CA is applied. In the figure, the wireless communication system has a configuration in which a plurality of wireless stations 51, 52, and 53 in a communication area perform wireless communication with each other. Each radio station includes an antenna 54, a transmission / reception unit 55, and a control unit 56. The control unit 56 of each radio station performs access control by CSMA / CA when communication is started.
IEEE802.11, "Wireless LAN Medium Access Control (MAC) and Physical (PHY) Layer Specifications", June 2007

  According to the IEEE 802.11 standard, as shown in FIG. 4, it is determined that the channel is congested as the number of retransmissions increases, and the contention window is expanded to reduce the occurrence range of backoff time for collision avoidance. Widen (binary exponent backoff). CWmin shown in FIG. 4 indicates an initial value of the contention window, and CWmax indicates an upper limit value of the contention window. Here, the contention window reaches the upper limit value CWmax at the third retransmission, the transmission success or the number of retransmissions reaches the specified value at the fourth retransmission, the packet is discarded, and the contention window is displayed when a subsequent packet is transmitted. A state where the initial value CWmin is reset is shown.

  Even in such a channel congestion situation, when a subsequent packet is transmitted after successful transmission or packet discard, the contention window is reset to the initial value CWmin. The process is repeated and throughput decreases (delay time increases).

  An object of the present invention is to provide a wireless communication apparatus and a wireless communication method capable of improving throughput by appropriately setting an initial value of a contention window when a channel is congested or quiet.

  In the first invention, carrier sense is performed on a channel used for communication, and when the channel is idle, a random back-off time is generated from the range of the contention window, and only the period of the back-off time is generated. Packet transmission is performed after waiting, and when the ACK signal that is a reception response is not received within a predetermined time after packet transmission, the contention window is expanded and transmission processing from carrier sense is performed again. In a wireless communication device that resets the contention window to the initial value CWmin when discarded, the number of successful transmissions is counted when an ACK signal is received after packet transmission, and the number of transmission failures is reset to 0. Transmission failure when ACK signal is not received in time A first means for counting the number and resetting the number of successful transmissions to 0, and reducing the initial value CWmin of the contention window when the number of successful transmissions reaches the first specified value and reducing the number of successful transmissions to 0 And a second means for expanding the initial value CWmin of the contention window and resetting the number of transmission failures to 0 when the number of transmission failures reaches a second specified value.

  The second means in the first invention may be configured such that the initial value CWmin of the contention window is reduced by CWmin / N, and the initial value CWmin of the contention window is increased by CWmin * N.

  In the second invention, carrier sense is performed on a channel used for communication, and when the channel is idle, a random back-off time is generated from the range of the contention window, and only the period of the back-off time is generated. Packet transmission is performed after waiting, and when the ACK signal that is a reception response is not received within a predetermined time after packet transmission, the contention window is expanded and transmission processing from carrier sense is performed again. In a wireless communication method in which the contention window is reset to the initial value CWmin when discarded, the number of successful transmissions is counted when an ACK signal is received after packet transmission, and the number of transmission failures is reset to 0. Transmission failure when ACK signal is not received in time The number of successful transmissions is reset to 0, and when the number of successful transmissions reaches the first specified value, the initial value CWmin of the contention window is reduced and the number of successful transmissions is reset to 0, When the number of transmission failures reaches the second specified value, the initial value CWmin of the contention window is expanded and the number of transmission failures is reset to zero.

  In the second invention, the initial value CWmin of the contention window may be reduced by CWmin / N, and the initial value CWmin of the contention window may be increased by CWmin * N.

  The present invention expands the initial value CWmin of the contention window when the number of transmission failures reaches the second specified value continuously, for example, CWmin * N, and after subsequent transmission success or packet discard When the subsequent packet is transmitted, the initial value of the enlarged contention window is reset. As a result, the initial value of the contention window increases when the channel is congested, the backoff time generation range is expanded, and the collision probability is reduced, so that throughput can be improved (delay time is reduced) compared to the conventional case. it can.

  In addition, when the number of successful transmissions reaches the first specified value after successful transmission, the initial value CWmin of the contention window is reduced, for example, to CWmin / N, so that the expanded contention window The initial value can be returned to the original CWmin. In addition, when the transmission succeeds when the channel is quiet, the initial value of the contention window is changed from CWmin to CWmin / N, thereby reducing the range of occurrence of backoff time and improving throughput (delay time). Decrease).

  FIG. 1 shows an embodiment of a wireless communication apparatus of the present invention. Here, an example is shown in which the wireless communication apparatus and the wireless communication method of the present invention are realized by the control unit 56 of each wireless station shown in FIG.

  In FIG. 1, a control unit 56 performs carrier sense, transmits / receives reception packets and transmission packets, transmits / receives ACK signals and controls retransmission processing, and a reception response generation unit 12 that generates ACK signals. A transmission buffer 13 for buffering transmission packets corresponding to retransmission processing, a contention window control unit 14 for controlling a contention window according to the number of retransmissions, and counting the number of consecutive transmission successes and transmission failures. The transmission success / failure counting unit 15 is provided.

  FIG. 2 shows a processing procedure of the wireless communication apparatus and the wireless communication method of the present invention. This processing procedure is executed by the transmission / reception control unit 11, the contention window control unit 14, and the transmission success / failure counting unit 15 shown in FIG.

  When the transmission / reception controller 11 transmits a packet (S1), it is determined whether or not an ACK signal as a reception response is received within a predetermined time (S2). Here, when the ACK signal is received, the contention window control unit 14 is notified, and the contention window in the next transmission process is reset to the initial value CWmin (S3). Also, the transmission success / failure count unit 15 is notified to reset the transmission failure number Nf to 0, and the transmission success number Ns is incremented (S4). Next, it is determined whether or not the number of successful transmissions Ns has reached the prescribed value Nsth of the number of successful transmissions (S5). If it has reached, the initial value CWmin of the contention window is reduced to CWmin / N and transmission fails. The number Nf is reset to 0 (S6), and the process returns to the subsequent packet transmission process.

  On the other hand, when the ACK signal is not received within a predetermined time, the transmission / reception control unit 11 notifies the contention window control unit 14 and sets the upper limit of the contention window based on the CSMA / CA binary exponent backoff algorithm. Enlarge until the value is reached (S7). Further, the transmission success / failure count unit 15 is notified to reset the transmission success number Ns to 0, and the transmission failure number Nf is incremented (S8). Next, it is determined whether or not the number of transmission failures Nf has reached the prescribed value Nfth of the number of transmission failures (S9). If it has reached, the initial value CWmin of the contention window is expanded to CWmin * N and transmission is successful. The number Ns is reset to 0 (S10), and the process returns to the subsequent packet transmission process.

FIG. 3 shows an example of controlling the initial value of the contention window according to the present invention.
In FIG. 3, CWmin represents an initial value (specified value) of the contention window, and CWmax represents an upper limit value of the contention window. As the number of retransmissions increases, it is determined that the channel is congested, and the contention window is expanded to widen the range of occurrence of backoff time for collision avoidance (binary exponent backoff). Here, the contention window reaches the upper limit value CWmax at the third retransmission, the number of transmission failures Nf reaches the specified value Nfth = 3 of the number of transmission failures, and the initial value of the contention window expands to CWmin * N. The transmission failure number Nf is reset to zero. The contention window for the fourth retransmission is retransmitted with the upper limit value CWmax. If the transmission is successful or the number of retransmissions reaches a specified value and the packet is discarded, the contention window is transmitted when the subsequent packet is transmitted. Starts from the initial value CWmin * N. As a result, the occurrence range of the back-off time is expanded, the collision probability is reduced, the transmission is successful, and the throughput is improved.

  When the transmission success is continued three times, the initial value of the contention window is reduced to CWmin / N, and the transmission success number Ns is reset to zero. As a result, the initial value of the contention window (CWmin * N in the previous enlargement process) returns to the original specified value CWmin. Further, when the transmission success is continued three times, the initial value of the contention window is reduced to CWmin / N, and the transmission success number Ns is reset to zero. The initial value of the contention window at this time is 1 / N of the specified value CWmin, and the generation range of the back-off time is narrowed to improve the throughput.

  In the processing algorithm of the present embodiment shown in FIG. 2, it is assumed that the initial value of the contention window exceeds the upper limit value CWmax of the contention window or falls below the initial specified value (lower limit value) CWmin. However, the enlargement / reduction processing of the initial value of the contention window according to the present embodiment may be performed within the range of the specified upper limit value and lower limit value of the contention window.

  FIG. 5 shows a simulation evaluation example in terms of throughput in the present invention. FIG. 6 shows an example of simulation evaluation seen from the delay time in the present invention. In FIG. 5 and FIG. 6, the broken line is the conventional configuration, and the solid line is the configuration of the present invention.

  As shown in FIG. 5 and FIG. 6, in the configuration of the present invention, even when the number of radio stations increases and the channel is congested, throughput reduction and delay can be achieved by performing processing for expanding the initial value of the contention window. It can be seen that the increase in time can be suppressed.

The figure which shows embodiment of the radio | wireless communication apparatus of this invention. The flowchart which shows the process sequence of the radio | wireless communication apparatus and radio | wireless communication method of this invention. The figure which shows the example of control of the initial value of the contention window by this invention. The figure which shows the example of expansion and reset of the contention window of IEEE802.11 standard. The figure which shows the example of simulation evaluation (throughput) in this invention. The figure which shows the example of simulation evaluation (delay time) in this invention. The figure which shows the structural example of the radio | wireless communications system to which CSMA / CA is applied.

Explanation of symbols

DESCRIPTION OF SYMBOLS 11 Transmission / reception control part 12 Reception response generation part 13 Transmission buffer 14 Contention window control part 15 Transmission success / failure count part 51,52,53 Radio station 54 Antenna 55 Transmission / reception part 56 Control part

Claims (4)

Carrier sense is performed on the channel used for communication, and when the channel is idle, a random back-off time is generated from the range of the contention window, and packet transmission is performed after waiting for the back-off time period. If the ACK signal that is a reception response is not received within a predetermined time after packet transmission, the contention window is expanded, the transmission processing from the carrier sense is performed again, and the contention is performed when the packet transmission is successful or the packet is discarded. In a wireless communication device that resets the window to the initial value CWmin,
When the ACK signal is received after the packet transmission, the number of successful transmissions is counted and the number of transmission failures is reset to 0. When the ACK signal is not received within a predetermined time after the packet transmission, the number of transmission failures is counted And a first means for resetting the number of successful transmissions to 0,
When the number of successful transmissions reaches a first specified value, the initial value CWmin of the contention window is reduced and the number of successful transmissions is reset to 0, and the number of failed transmissions reaches a second specified value. And a second means for enlarging the initial value CWmin of the contention window and resetting the number of transmission failures to zero. The wireless communication device according to claim 1,
The second means is configured to reduce the initial value CWmin of the contention window to CWmin / N (N is an integer), and to increase the initial value CWmin of the contention window to CWmin * N. A wireless communication device. Carrier sense is performed on the channel used for communication, and when the channel is idle, a random back-off time is generated from the range of the contention window, and packet transmission is performed after waiting for the back-off time period. If the ACK signal that is a reception response is not received within a predetermined time after packet transmission, the contention window is expanded, the transmission processing from the carrier sense is performed again, and the contention is performed when the packet transmission is successful or the packet is discarded. In the wireless communication method for resetting the window to the initial value CWmin,
Counting the number of successful transmissions when receiving the ACK signal after the packet transmission and resetting the number of transmission failures to 0,
Counting the number of transmission failures when not receiving the ACK signal within a predetermined time after transmitting the packet and resetting the number of successful transmissions to 0,
When the number of successful transmissions reaches a first specified value, the initial value CWmin of the contention window is reduced and the number of successful transmissions is reset to 0,
When the number of transmission failures reaches a second specified value, the initial value CWmin of the contention window is expanded and the number of transmission failures is reset to zero. The wireless communication method according to claim 3,
The wireless communication method, wherein the initial value CWmin of the contention window is reduced by CWmin / N (N is an integer), and the initial value CWmin of the contention window is enlarged by CWmin * N.

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