JP2006261984A - Communication method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a communication method for evading deterioration in throughput, when there are interferences among base stations. <P>SOLUTION: In the communication method which can be materialized in a communications system, adopting CSMA and evade the collision of packets due to a hidden terminal by RTS/CTS, in the case that an STA 3 which has transmitted an RTS frame cannot receive a CTS frame to the RTS frame, at a prescribed timing from an AP 1 to which the present terminal belongs, the AP 1 being in a transmission inhibition state, in order to avoid the collision of the packets, the STA 3 waits for a polling frame from the AP 1. The AP 1 transmits the polling frame to the STA 3, when The STA 3 is put into a transmittable state, after receiving the RTS frame to the present station from the STA 3 in the period of the transmission inhibition state; and when the STA 3 receives the polling frame from the AP 1, the STA 3 transmits a prescribed data frame to the AP 1. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a communication method in a digital wireless communication system employing CSMA (Carrier Sense Multiple Access), which is one of random access technologies, and particularly to a communication method capable of avoiding performance degradation due to interference. .

  Hereinafter, a conventional communication method will be described. CSMA, which is one of random access technologies, is employed in a communication system that performs wireless packet communication, for example. In the CSMA, a plurality of wireless terminals constituting the system perform carrier sense on a wireless channel prior to wireless packet transmission. When recognizing that the channel is in use (channel busy), the wireless packet transmission is refrained, and then the wireless packet is transmitted when the channel is not used (channel idle).

  However, in the above communication system, when the wireless terminals use directional antennas, when the distance between the wireless terminals is long, or when there are obstacles that block radio waves between the wireless terminals, etc. There are situations where the transmission signals of other wireless terminals cannot be received directly. A wireless terminal that cannot recognize its presence even though it constitutes the same communication system is called a “hidden terminal”. Since the carrier sense does not function effectively between the wireless terminals corresponding to the hidden terminals, there is a case where one side starts transmitting the wireless packet while the other side is transmitting the wireless packet. For example, a radio packet collision occurs at a radio base station existing at an intermediate position between both radio terminals, and normal communication cannot be performed.

  In the following Non-Patent Document 1, the hidden terminal problem is solved by RTS (Request to Send) / CTS (Clear to Send) of DCF (Distributed Coordination Function) using CSMA / CA (Carrier Sense Multiple Access with Collision Avoidance). Improve.

  Here, a communication method between the base station (AP) and the wireless terminal (STA) in the wireless LAN system employing the CSMA will be described. Here, it is assumed that communication is performed in a state where STA1 belongs to AP1 and STA2 and STA3 belong to AP2. Further, it is assumed that the STAs 1 to 3 use directional antennas directed to the APs to which the STAs 1 and 3 belong, and the APs 1 and 2 use omnidirectional antennas. It is assumed that AP1 is not in the communication range of AP2, but is in the interference range, and AP2 is not in the communication range of AP1, but is in the interference range. The RTS, CTS, DATA, and ACK packet formats used in this system include a packet type field for distinguishing packets, a channel use period field according to a transmission / reception sequence, a reception address field, and a transmission address field (CTS, ACK). ) And an error check field for checking bit errors of the packet.

  First, AP2 transmits a DATA frame to STA2. In this system, a wireless terminal (STA) that detects (carrier sense) a wireless signal corresponding to a frame is in a transmission prohibited state due to wireless line busy. Further, a frame used in this system includes virtual carrier sense information called NAV (Net Allocation Vector), and indicates, for example, a channel use time limit in communication with a wireless terminal corresponding to a reception address. Accordingly, wireless terminals other than the reception address are prohibited from transmitting until the time (time period) specified in the NAV. That is, here, the STA 3 is in a transmission prohibited state.

  Next, when the STA 3 transmits an RTS frame to the AP 2 in a state where the channel use period is reached and the transmission prohibition state is released, the AP 2 transmits a CTS frame to the STA 3. Then, the STA3 that has received the CTS frame transmits a DATA frame to the AP2, and after the reception is completed, the AP2 returns an ACK frame to the STA3.

  As described above, in the conventional wireless LAN system, when there are hidden terminals due to interference between a plurality of wireless terminals (STAs) operating at the same frequency, collision of packets can be avoided by carrier sense or by NAV. .

Wireless LAN standard IEEE802.11

  However, in the above-described conventional communication method, for example, AP1 is in a transmission prohibited state by carrier-sensing a packet of AP2 operating at another same frequency, and in this transmission prohibited state, AP1 receives an RTS frame from STA1. However, it is impossible to transmit the CTS frame to STA1. Since STA1 cannot receive the CTS frame at a predetermined timing, it retransmits the RTS frame to STA1 after a random waiting time that becomes longer according to the number of retransmissions. As described above, there is a problem in that the throughput is extremely reduced due to STA1 repeating retransmission of the RTS frame to AP1.

  The present invention has been made in view of the above, and an object of the present invention is to obtain a communication method capable of avoiding a decrease in throughput when there is interference between base stations.

  In order to solve the above-described problems and achieve the object, the communication method according to the present invention can be implemented in a communication system employing CSMA (Carrier Sense Multiple Access), and RTS (Request to Send) / In a communication method capable of avoiding packet collision by a hidden terminal by CTS (Clear to Send), a base station to which a wireless terminal that has transmitted an RTS frame belongs to its own terminal that is in a transmission prohibited state in order to avoid packet collision A polling waiting step for waiting for a polling (QoS CF-Poll: Contention Free Polling) frame from the base station when the CTS frame for the RTS frame cannot be received from the station at a predetermined timing, and the base station prohibits transmission When the RTS frame for the local station is received from the wireless terminal during the state period, the transmission prohibited state is released A polling step for transmitting a polling frame to the wireless terminal when the wireless terminal is ready for transmission; and when the wireless terminal receives the polling frame from the base station, the wireless terminal transmits a predetermined data frame to the base station. And a data transmission step of transmitting.

  According to the present invention, a base station that is in a transmission-prohibited state due to the influence of a hidden terminal transmits a polling frame to the wireless terminal at the time when the base station shifts to a transmittable state, and the wireless terminal receives the polling frame from the base station Then, since the data frame is transmitted to the base station, even when there is interference between the base stations, the number of RTS retransmissions by the wireless terminal is reduced, and a decrease in throughput can be avoided. Play.

  Embodiments of a communication method according to the present invention will be described below in detail with reference to the drawings. Note that the present invention is not limited to the embodiments.

Embodiment 1 FIG.
FIG. 1 is a diagram showing a configuration of a communication system capable of realizing a communication method according to the present invention, and shows a communication state between a base station (AP) and a wireless terminal (STA). In this communication system, it is assumed that packet communication is performed in a state where STA3 belongs to AP1 and STA4 and STA5 belong to AP2. In addition, the AP1 exists in the interference range of AP2, and the AP2 exists in the interference range of AP1. Further, it is assumed that the STAs 3 to 5 use directional antennas directed to the APs to which the STAs 3 belong, and the APs 1 and 2 use omnidirectional antennas.

  FIG. 2 is a diagram illustrating a frame format of the communication system according to the first embodiment. The frame here includes a packet type field for distinguishing packets, a channel usage period field indicating a channel usage period (expiration date) according to a wireless packet transmission / reception sequence, a reception address field (destination address field), and transmission An address field (source address field) (not included in CTS and ACK) and an error check field to which an operation result for checking a bit error of the packet is added.

  That is, the RTS frame includes a packet type field (RTS), a channel usage period field, a reception address field, a transmission address field, and an error check field. The CTS frame includes a packet type field (CTS), a channel use period field, a reception address field, and an error check field.

  The DATA frame includes a packet type field (DATA), a channel use period field, a reception address field, a transmission address field, an address field, DATA (actual data), and an error check field. The ACK frame includes a packet type field (ACK), a channel use period field, a reception address field, and an error check field.

  The QoSCF-Poll (Quality of Service Contention Free Polling) frame includes a packet type field (CF-Poll), a channel use period field, a reception address field, a transmission address field, an address field, and an error check field. Detailed fields of IEEE802.11 and fields commonly added to each wireless packet depending on the modulation / demodulation method are omitted.

  Here, the communication method of the first embodiment will be specifically described with reference to the drawings. FIG. 3 is a diagram illustrating a communication method according to the first embodiment. Here, as an example, a communication method between a base station (AP) and a wireless terminal (STA) in a wireless LAN system based on IEEE802.11 will be described. The vertical axis in the figure is the time axis, and the arrow indicates a frame from the AP to the STA or a frame from the STA to the AP.

  First, AP2 transmits an IEEE802.11 DATA frame to STA4 (FIG. 3, step S1). The frame includes virtual carrier sense information called NAV. For example, a channel use period in communication with a wireless terminal corresponding to the reception address is indicated. Therefore, wireless terminals other than the reception address are prohibited from transmitting until the time specified in the NAV. That is, here, since STA5 exists in the communication range of AP2, STA5 is in a transmission prohibited state (step S2). Next, the STA 4 that has completed the reception of the DATA frame returns an ACK frame to the AP 2 (step S3).

  On the other hand, AP1 that received the interference frame from AP2 in the process of step S1 is in a transmission prohibited state due to carrier sense (step S4). For example, as shown in the figure, it is a case where an RTS frame is received from STA3. However, the CTS frame cannot be returned (step S6). In STA3, since the CTS frame is not returned even if the predetermined time has elapsed, the STA3 waits for the timing to retransmit the RTS frame (step S7).

  Next, in the first embodiment, AP1 that has received the interference frame from AP2 in the process of step S1 is in a state in which the frame can be normally received and the NAV information can be read, and becomes busy due to virtual carrier sense. It is determined whether the carrier sense is possible but the frame cannot be normally received and the carrier sense is busy (interference / NAV determination) (step S8). When the transmission is prohibited due to carrier sense, AP1 performs reception processing (reception at the time of interference) (step S9). Note that when the AP 1 is in a transmission prohibited state due to the NAV, the reception process may be stopped in order to reduce power consumption.

  Next, AP1 that has received the RTS frame in the process of step S5 reads the transmission address corresponding to STA3 and the channel use period (hereinafter referred to as polling target information) from the received RTS frame, and temporarily polls the list (interference The polling target information is stored in the information list regarding the RTS from the STA that could not transmit the CTS frame due to the influence (step S10). FIG. 3 shows a case where AP1 receives an RTS frame from STA3 once. However, when receiving multiple frames from a plurality of STAs, non-overlapping polling target information is stored in a temporary polling list. To do.

  Next, the STA 3 performs processing (polling response preparation) in case polling is returned at the time when the CTS frame is not returned (step S6) even after a predetermined time has elapsed. The STA 3 here prepares for a polling response by using the DATA frame prepared for the case where the CTS is returned (step S11). The STA 3 prepares a polling response by moving data from a CTS response buffer to a polling response preparation buffer, for example. Then, the STA 3 waits for the polling return during the RTS retransmission waiting period (step S12).

  Next, in the AP 1 that is in a transmittable state, the polling target information is stored in the temporary polling list because the CTS frame could not be transmitted due to the influence of interference even though the RTS frame was received. Polling (polling frame transmission) is performed based on the temporary polling list (polling target information) (step S13). FIG. 3 shows a case where AP1 only performs polling to STA3. However, when a plurality of items (polling target information) are stored in the temporary polling list, AP1 corresponds to each item. Perform polling sequentially. The polling from AP1 to STA3 here is performed by information on transmission permission (QoSCF-Poll).

  Next, the STA3 that has received the polling frame in the process of step S13 reads the DATA frame prepared in the process of step S11, and transmits the read DATA frame to the AP1 (step S14). At the same time, the STA 3 cancels the waiting for the timer waiting for the RTS retransmission timing, and performs the RTS retransmission canceling process by discarding the RTS frame scheduled for retransmission (step S15).

  Next, AP1 that has completed the reception of the DATA frame from STA3 discards the polling target information related to STA3 from the temporary polling list (step S16). AP1 returns an ACK frame to STA3 (step S17).

  In FIG. 3, the case where the STA3 receives the polling frame is described, but the retransmission timing from the STA3 to the AP may be earlier than the transmission timing of the polling from the AP1 to the STA3. In this case, the STA 3 cancels the polling waiting state at the time when the RTS frame is retransmitted (step S18).

  Thereafter, in STA3, if the CTS frame is not returned even after the predetermined time has elapsed, the STA3 waits for the timing to retransmit the RTS frame (step S7), and performs the processes of steps S8 to S16.

  As described above, when the STA 3 resends the RTS frame or receives the polling frame, it is possible to avoid the duplicate processing by stopping the resending process of the RTS frame or the polling waiting state. Become.

  As described above, according to the first embodiment, when a predetermined base station is in a transmission prohibited state due to the influence of a hidden terminal even though there is an access from a wireless terminal, the transmission is enabled. At the time of transition, a polling frame for transmitting a DATA frame that could not be received is transmitted, and the wireless terminal is allowed to transmit the DATA frame. In addition, when the CTS cannot be received from the base station, the wireless terminal enters a polling waiting state from the base station and lengthens the RTS retransmission timing. As a result, even when there is interference between base stations, the number of RTS retransmissions by the wireless terminal is reduced, and a reduction in throughput can be avoided.

  Further, since the STA 3 prepares for a polling response when the CTS frame is not returned from the AP 1 even after a predetermined time has elapsed, it is possible to quickly transmit the data frame when the polling frame is received from the AP 1. .

Embodiment 2. FIG.
Hereinafter, the communication method according to the second embodiment will be described. In Embodiment 2, when interference between base stations occurs, the transmission antenna is switched to omnidirectional and the CTS frame is transmitted to the wireless terminal, and then the transmission antenna is switched to directional and the data frame is transmitted to the wireless terminal. Send. Note that the configuration of the communication system is the same as that of FIG. 1 of the first embodiment described above, so the same reference numerals are given and the description thereof is omitted. Each frame format used in the present embodiment is the same as that in FIG. 2 of the first embodiment described above.

  FIG. 4 is a diagram illustrating a communication method according to the second embodiment. First, AP1 transmits an IEEE802.11 DATA frame to STA3 (FIG. 4, step S30). AP2 cannot receive a frame for a predetermined period, but determines the presence or absence of interference by detecting a signal capable of carrier sense (step S31). That is, here, AP1 transmits a DATA frame to STA3, and thereby AP2 determines that there is interference. On the contrary, when AP2 is receiving interference, it is assumed that the own station may cause interference. Next, the STA 3 that has completed the reception of the DATA frame returns an ACK frame to the AP 1 (step S32).

  In Embodiment 1, when AP2 transmits DATA to STA4, the DATA frame is transmitted. However, AP2 in Embodiment 2 switches the transmission antenna to omnidirectional (step S33), and STA4 A CTS frame is transmitted to (step S34). The CTS frame includes NAV (virtual carrier sense information) and indicates, for example, a channel use period in communication with a wireless terminal corresponding to the reception address. Therefore, wireless terminals other than the reception address are prohibited from transmitting until the time specified in the NAV. That is, here, since STA5 exists in the communication range of AP2, STA5 is in a transmission prohibited state (step S35).

  Next, AP2 switches the transmission antenna to directivity (step S36), and transmits a DATA frame to STA4 (step S37). Here, since the directivity of the transmitting antenna is directed to STA4, AP1 cannot perform carrier sense and does not cause interference. Then, the STA 4 that has completed the reception of the DATA frame returns an ACK frame to the AP 2 (step S38).

  On the other hand, AP1 that received the interference frame from AP2 in the process of step S34 is in a transmission prohibited state due to carrier sense (step S39). However, since this interference frame is a relatively short CTS frame that does not include the data body, the period during which AP1 is in the transmission prohibited state is very short. AP1 here determines whether or not transmission prohibition by NAV is prohibition of transmission by carrier sense, as in Embodiment 1, and in the case of transmission prohibition by carrier sense, performs reception processing even at the time of interference And

  For example, as shown in the figure, when AP1 receives an RTS frame from STA3 (step S40), the period during which AP1 is in a transmission-inhibited state is very short. ). Thereby, the STA 3 can receive the CTS frame at a predetermined timing, and can transmit the prepared DATA frame to the AP 1 (step S42). Then, AP1 that has completed receiving the DATA frame returns an ACK frame to STA3 (step S43). Thereby, the transmission of data between AP1 and STA3 is completed without performance degradation.

  Note that the sequence in which the DATA frame is transmitted following the CTS frame is a sequence accepted by IEEE802.11g. However, by transmitting the CTS frame using a compatible modulation method, the peripheral wireless terminals are prohibited from transmitting. After that, the DATA frame is transmitted using an incompatible modulation method, and there is no need to switch the antenna directivity.

  In the second embodiment, AP2 transmits a CTS frame before transmitting a DATA frame, but AP2 transmits a polling frame (QoSCF-Poll) for itself instead of the CTS frame. It is good as well. AP1, which has received the polling frame from AP2 in the process of step S34, is in a transmission prohibited state due to carrier sense (step S39).

  Thus, according to the second embodiment, when a predetermined base station is interfered by DATA transmitted from another base station, the base station receiving the interference is omnidirectional to the wireless terminal. After transmitting the CTS with the antenna, the wireless terminal is made to transmit DATA with the directional antenna. In addition, even when a base station that interferes with another base station receives a CTS from a base station that is experiencing interference, if the transmission is prohibited by carrier sense, reception processing is also performed during interference. I decided to do it. As a result, even when there is interference between base stations, the period during which the base station that interferes with other base stations is busy is shortened, and the performance of data transmission between the base station and the wireless terminal is shortened. Deterioration can be reduced.

  Further, the sequence of transmitting the DATA frame following the CTS frame has an effect of reducing the error rate of the DATA frame and an effect of suppressing leakage of information included in the DATA frame to the wireless terminal other than the destination.

  As described above, the communication method according to the present invention is useful for a communication method of a communication system that employs CSMA, and is particularly suitable for avoiding a decrease in throughput.

It is a figure which shows the structure of the communication system which can implement | achieve the communication method concerning this invention. It is a figure which shows the frame format of a communication system. 3 is a diagram illustrating a communication method according to Embodiment 1. FIG. 6 is a diagram illustrating a communication method according to Embodiment 2. FIG.

Explanation of symbols

1, 2 Base station (AP)
3-5 Wireless terminal (STA)

Claims (9)

In a communication method that can be realized in a communication system adopting CSMA (Carrier Sense Multiple Access), and that can avoid collision of packets by hidden terminals by RTS (Request to Send) / CTS (Clear to Send),
When the wireless terminal that has transmitted the RTS frame cannot receive the CTS frame for the RTS frame at a predetermined timing from the base station to which its own terminal that is in a transmission prohibited state in order to avoid packet collision, Polling waiting step waiting for polling (QoS CF-Poll: Contention Free Polling) frame from the station,
When the base station receives an RTS frame addressed to itself from the wireless terminal during the transmission prohibited state, and when the transmission prohibited state is canceled and the transmission is ready, the polling frame is sent to the wireless terminal. A polling step for sending
When the wireless terminal receives a polling frame from the base station, the wireless terminal transmits a predetermined data frame to the base station; and
A communication method comprising:   2. The polling waiting step according to claim 1, wherein the wireless terminal waits for a polling frame from the base station for a predetermined period until the wireless terminal retransmits the RTS to the base station. Communication method.   When the wireless terminal does not receive a polling frame from the base station in a predetermined period after the polling waiting step until the wireless terminal retransmits the RTS to the base station, the wireless terminal The communication method according to claim 2, further comprising a polling cancellation step in which the wireless terminal cancels the polling waiting from the base station when the RTS frame is retransmitted to the base station.   The communication method according to any one of claims 1 to 3, wherein the data transmission step includes a transmission stop step of stopping retransmission processing of the RTS frame from the wireless terminal to the base station. .   The polling waiting step includes a polling response preparation step in which the wireless terminal moves a data frame stored in a response buffer transmitted in response to a CTS frame from the base station to a polling response buffer to the base station. The communication method according to any one of claims 1 to 4, further comprising: In the polling step, when an RTS frame addressed to the base station is received from the wireless terminal during a period in which the base station is in a transmission prohibited state, the source address of the RTS frame and a NAV (Network Allocation Vector) are received from the received RTS frame. A polling list storage step of extracting information and storing it as polling list information;
6. The communication method according to claim 1, wherein in the polling step, the base station transmits a polling frame to the wireless terminal based on the polling list information. Before the polling waiting step, the base station determines whether or not the transmission prohibited state is a transmission prohibited state due to busy with a carrier sense rather than a transmission prohibited state due to NAV; and
An RTS reception step of receiving, from the wireless terminal, an RTS frame addressed to the own station when the base station determines that the transmission is prohibited due to carrier sense;
The communication method according to claim 1, further comprising: In a communication method employing CSMA (Carrier Sense Multiple Access),
The non-directionality in which the second base station that has determined that there is interference with the first base station switches the transmission antenna to non-directional and transmits a CTS (Clear to Send) frame to the first wireless terminal Sending step;
A directional transmission step of switching the transmission antenna to directivity and transmitting a data frame to the first wireless terminal;
The first base station that receives a CTS frame transmitted by the second base station and is in a transmission-inhibited state is not in a transmission-inhibited state by NAV (Network Allocation Vector) but busy due to carrier sense. A transmission prohibition state determination step for determining whether or not a transmission prohibition state;
An RTS reception step of receiving an RTS frame addressed to the local station from a second wireless terminal when the first base station determines that transmission is prohibited due to carrier sense;
When the first base station receives an RTS frame addressed to the first base station from the second wireless terminal during a transmission prohibited state, and when the transmission prohibited state is canceled and transmission is possible A CTS transmission step of transmitting a CTS frame corresponding to the RTS frame to the wireless terminal of the younger brother 2;
A communication method comprising: In a communication method employing CSMA (Carrier Sense Multiple Access),
The second base station that has determined that there is interference with the first base station switches the transmission antenna to omnidirectional and sends a polling frame (QoS CF-Poll: Contention Free Polling) addressed to itself. An omnidirectional transmission step for transmission;
A directional transmission step of switching the transmission antenna to directivity and transmitting a data frame to the first wireless terminal;
The first base station that receives a polling frame transmitted by the second base station and is in a transmission prohibited state is not in a transmission prohibited state by a NAV (Network Allocation Vector) but is busy due to carrier sense. A transmission prohibition state determination step for determining whether or not a transmission prohibition state;
An RTS reception step of receiving an RTS frame addressed to the local station from a second wireless terminal when the first base station determines that transmission is prohibited due to carrier sense;
When the first base station receives an RTS frame addressed to the first base station from the second wireless terminal during a transmission prohibited state, and when the transmission prohibited state is canceled and transmission is possible A CTS transmission step of transmitting a CTS frame corresponding to the RTS frame to the wireless terminal of the younger brother 2;
A communication method comprising:

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