JP5639292B2 - Wireless communication method and wireless communication system - Google Patents

Description

  The present invention relates to a radio communication method and radio communication system for suppressing interference between cells.

  In recent years, as a high-speed wireless access system using the 2.4 GHz band or the 5 GHz band, access points (base station apparatuses) based on the IEEE802.11g standard, the IEEE802.11a standard, and the like have been widely used. In systems based on these standards, an orthogonal frequency division multiplexing (OFDM) modulation scheme, which is a technology for stabilizing the characteristics in a multipath fading environment, is used, and a transmission rate of 54 Mbps is realized at the maximum. doing. The transmission rate here is a transmission rate on the physical layer. The effective data throughput for the user has an upper limit of about 30 Mbps because the transmission efficiency in the MAC (Medium Access Control) layer is about 50 to 70%. However, IEEE 802.11n is scheduled to be standardized in 2010 and aims to realize high-speed communication exceeding 100 Mbps by using MIMO (Multiple input multiple output) technology.

  Further, the communication speed of the wired LAN has been increasing due to the spread of FTTH (Fiber to the home), and it is expected that further increase in the transmission speed will be required in the wireless LAN in the future. As a technique for further increasing the transmission speed in the wireless LAN, the multi-user MIMO transmission technique is attracting the most attention. In the multi-user MIMO transmission technique, independent signals that are different from each other at the same frequency and the same timing are transmitted from a plurality of transmission antennas to a plurality of communication partners having a plurality of antennas. Further, this is a technique that realizes improvement in downlink throughput by regarding the entire receiving antennas of a plurality of communication partners as a huge receiving array (see, for example, Non-Patent Document 1).

Q. H. Spencer, "Zero-forcing methods for downlink spatial multiplexing in multiuser MIMO channel," IEEE Trans. On Signal Processing, Vol 52, Issue 2, pp.461-471, 2004

  However, as the wireless LAN communication speed and the number of users increase, the problem of interference from adjacent communication cells is increasing. Since wireless LANs normally use CSMA / CA (Carrier Sense Multiple Access / Collision Avoidance) to prevent packet collisions, there is a problem that communication opportunities decrease as the number of wireless communication terminals in the receivable area increases.

  FIG. 16 is a diagram for explaining a decrease in transmission rate in an overlapping communication cell in a conventional communication method. The state of the transmission / reception with respect to the time of the radio | wireless communication terminals WT1-WT4 is represented. A block in Tx represents that the wireless communication terminal is transmitting, and a block in Rx represents that the wireless communication terminal is receiving a transmission signal having a corresponding number. Here, WT1 and WT2 and WT3 and WT4 are wireless communication terminals belonging to the same communication cells C1 and C2, respectively. Since communication is performed only between wireless communication terminals belonging to the same communication cell, WT1 and WT2 and WT3 and WT4 communicate with each other, but WT1 and W3 and WT2 and WT4 do not communicate with each other. Here, it is assumed that the communication between WT1 and WT2 is received by WT3 with a magnitude greater than the receivable power, and the transmission signal of WT3 is also received by WT1 and WT2.

  FIG. 16 shows that the transmission signal 3 is transmitted from the WT 4 and received by the WT 3. The WT 1 starts transmitting the transmission signal 1 at the timing when this communication ends. Here, in WT1, the transmission signal 3 transmitted by WT4 is not received or is not recognized because the reception level is low. In other words, the WT 4 is a “hidden terminal” as viewed from the communication cell C1. On the other hand, for transmission signal 1 by WT1, since it is possible to detect that WT1 communication is originally started in WT3, according to CSMA / CA, WT3 transmission is not performed in principle. Since signal transmission is performed in a short time after the end of reception without exceptionally detecting the signal, signal 4 is transmitted even though the signal from WT1 is received. In such a case, since the signal 4 is received by the WT 2 as 4 ″, this signal becomes an interference signal. In particular, the signal 1 ′ in the WT 2 is decoded when it overlaps with the timing detection or channel estimation block at the head of the signal block. As a result, WT1 does not receive the ACK signal, and therefore retransmits using signal 2, which greatly reduces the throughput. Also, there is a problem that the same signal is transmitted many times, so that the received signals 6 ′ and 2 ″ are received at the same time in the WT 3 and the signal quality is reduced.

  Another problem in the conventional method will be described with reference to FIG. In FIG. 17, WT2 and WT3 can detect each other's communication, and wireless communication terminals WT1 and WT2 and WT3 and WT4 belong to communication cells C1 and C2, respectively. Communication by Request to Send (RTS) and Clear to Send (CTS) is performed by a communication cell C1 composed of WT1 and WT2, with 5 corresponding to the RTS and 6 corresponding to the CTS signal. The CTS signal has a role of reserving a channel by setting a Network Allocation Vector (NAV) for the detected wireless communication terminal, and prevents terminals other than the terminal that reserved the channel from transmitting for a certain period of time. In FIG. 17, since WT3 receives CTS by the received signal 6 ″, NAV is set as indicated by 8. At this time, WT4 transmits signal 4, and WT3 decodes this signal. However, since the timing for transmitting the ACK still corresponds to the NAV section, the ACK cannot be transmitted, it is determined that the transmission is failed in WT4, and retransmission is performed. Due to the influence of the “terminal”, there is a problem that retransmission of data increases, resulting in a decrease in throughput.

  Further, although the problem has been described with respect to ACK and CTS, the same problem arises with signals other than ACK and CTS, such as ACK and CTS, which are shorter than the bit length of the data signal portion. For example, even a channel information notification signal in multi-user MIMO communication may cause interference in other communications depending on the conditions of the “hidden terminal” and “exposed terminal”.

  As described above, the ACK signal, the CTS signal, or the signal similar to the CTS signal causes interference with other communication cells, or the signal that is similar to the ACK signal, the CTS signal, or the CTS signal cannot be transmitted. There is a problem that decreases.

  The present invention has been made in view of such circumstances, and suppresses occurrence of inter-cell interference by transmitting an ACK signal when interference is received in an overlapping communication cell. An object of the present invention is to provide a wireless communication method and a wireless communication system.

  The present invention is a wireless communication method for suppressing interference between cells in a communication system using a plurality of frequency channels, in which some of the plurality of communication cells overlap, and is used in other communication cells. Another communication cell frequency channel estimation step for estimating a frequency channel, a self-communication cell signal processing step for receiving a control signal or a data signal from a radio communication terminal of the self-communication cell and decoding a signal from the radio communication terminal; An ACK signal for notifying successful reception after completion of data communication from a frequency channel that is not used or used less frequently by other communication cells, and a CTS signal for notifying that reception is ready after receiving a transmission request signal At least one CCTS signal having a function of notifying channel information after receiving the channel information request signal Control signal frequency channel selection step to determine the frequency channel to transmit, and using the determined frequency channel, ACK signal notifying the successful reception after the end of data communication, ready to receive after receiving the transmission request signal And a control signal transmission step of transmitting at least one of a CTS signal and a CCTS signal having a function of notifying channel information after receiving the channel information request signal.

  The present invention takes into account the deterioration of the accuracy of channel information when a CTS signal or CCTS signal is sent from a communication device that is a communication partner to notify that reception is ready after receiving a transmission request signal. The method further includes a control signal-considering data transmission step of changing a modulation mode composed of a modulation method and a coding rate of a data signal to be transmitted to one having a low transmission bit.

  The present invention is a wireless communication system in which a part of a plurality of communication cells overlap and a plurality of frequency channels are used to suppress interference between cells, and the frequency channels used in other communication cells Other communication cell frequency channel estimation means for estimating the own communication cell signal processing means for receiving a control signal or a data signal from the wireless communication terminal of the own communication cell and decoding the signal from the wireless communication terminal; An ACK signal that notifies successful reception after the end of data communication from a frequency channel that is not used or used less frequently in a communication cell, and a CTS signal that notifies that reception is ready after receiving a transmission request signal After receiving the information request signal, at least one of the CCTS signals having a function of notifying channel information is transmitted. Control signal frequency channel selection means for determining a channel, an ACK signal for notifying successful reception after completion of data communication, and a CTS signal for notifying that reception is ready after receiving a transmission request signal And a control signal transmitting means for transmitting at least one of the CCTS signals having a function of notifying the channel information after receiving the channel information request signal.

  The present invention takes into account the deterioration of the accuracy of channel information when a CTS signal or CCTS signal is sent from a communication device that is a communication partner to notify that reception is ready after receiving a transmission request signal. And a control signal-considering data transmitting means for changing a modulation mode composed of a modulation method and a coding rate of a data signal to be transmitted to one having a low transmission bit.

  According to the present invention, when interference is received in an overlapping communication cell, it is possible to suppress the occurrence of inter-cell interference by transmitting an ACK signal, thereby increasing throughput. The effect of being able to be obtained.

It is a figure showing the communication method in 1st Embodiment of this invention. It is a flowchart which shows operation | movement of the 1st transmission method in 1st Embodiment. It is a flowchart which shows operation | movement of the 2nd transmission method in 1st Embodiment. It is a flowchart which shows operation | movement of the 3rd communication method in 1st Embodiment. It is a figure showing the communication method in 2nd Embodiment of this invention. It is a flowchart which shows operation | movement of the 1st transmission method in 2nd Embodiment. It is a flowchart which shows operation | movement of the 2nd transmission method in 2nd Embodiment. It is a figure showing the communication method in 3rd Embodiment of this invention. It is a figure showing the modification of the communication method in 3rd Embodiment of this invention. It is a figure showing the communication method in 4th Embodiment of this invention. It is a flowchart which shows operation | movement of the 1st transmission method in 4th Embodiment. It is a flowchart which shows operation | movement of the 2nd transmission method in 4th Embodiment. It is a flowchart which shows the modification of operation | movement of the 2nd transmission method in 4th Embodiment. It is a figure showing the communication method of the 2nd form in 4th Embodiment of this invention. It is a flowchart which shows the transmission operation corresponding to FIG. It is explanatory drawing which shows the fall of the transmission rate in the overlap communication cell in the conventional communication system. It is explanatory drawing which shows the other problem in the conventional system.

<First Embodiment>
Hereinafter, a wireless communication method and a wireless communication system according to a first embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a diagram illustrating a communication method according to the first embodiment of the present invention. As in the case of FIG. 16, immediately after receiving the transmission signal 3 from the wireless communication terminal WT4, the wireless communication terminal WT1 starts communication, and sends an ACK signal by the reception interference signal 1 ″ corresponding to this communication signal 1 The wireless communication terminal WT3 is receiving a signal at the timing. However, by eliminating or making the received signal 4 ″ at the wireless communication terminal WT2 small enough to be ignored, the wireless communication terminal WT2 is able to receive the signal 1 of the wireless communication terminal WT1. Decoding can be performed, and communication can be completed by the ACK signal 5.

  Next, a first transmission method for transmitting the transmission signal 4 will be described. First, transmission is performed by reducing the transmission signal 4 by DdB from the normal transmission power. By reducing the transmission power in this way, the interference power to the radio communication terminals WT1 and WT2 is reduced, and transmission is performed with power that cannot be detected by the radio communication terminals WT1 and WT2, thereby using CSMA / CA. An ACK signal is transmitted without affecting the existing wireless communication terminals WT1 and WT2. In order to perform this communication, it is necessary to estimate reception power for transmission signals from the wireless communication terminals WT1 and WT2 in advance.

Here, the thermal noise power estimated by the wireless communication terminal WT3 is P _N [dBm], the reception power of the transmission signal of the wireless communication terminal WT1 is P ₁₃ [dBm], and the reception power of the transmission signal of the wireless communication terminal WT2 is P _23. [DBm], the minimum detection signal-to-noise ratio of the communication signal is S _S [dB], the maximum inter-cell interference power-to-noise ratio allowed in the wireless communication terminals of other communication cells is S _max [dB], and the wireless communication terminal P ₃₄ [dBm] is the received power at the wireless communication terminal WT 4 when transmission is performed with normal transmission power from the WT 3, and the transmission power individual difference margin prepared for the difference in transmission power that may occur in different communication apparatuses. When transmitting M and ACK, let D [dB] be a power reduction coefficient that decreases from normal transmission power. Then, the signal-to-noise power ratios S ₁ [dB] and S ₂ [dB] at which the ACK transmitted by the wireless communication terminal WT3 is received by the wireless communication terminals WT1 and WT2 are respectively
S ₁ = D + P ₃₁ −P _N (1)
S ₂ = D + P ₃₂ −P _N (2) P ₃₁ and P ₃₂ can be estimated as P ₃₁ = P ₁₃ and P ₃₂ = P ₂₃ from the signal powers P ₁₃ and P ₂₃ transmitted from the wireless communication terminal WT3 and received by WT1 and WT2. However, since it is not necessarily the same depending on the transmission method and the individual device difference, it is also possible to take into account the difference in power between P ₃₂ and P ₂₃ of P ₃₁ and P ₁₃ .

Therefore, using the margin of transmission power difference,
S ₁ = D + M + P ₃₁ −P _N (3)
It can be estimated that S ₂ = D + M + P ₃₂ −P _N (4). Here, it can be interpreted that the wireless communication terminal WT3 can transmit transmission power that is M [dB] higher than the wireless communication terminals WT1 and WT2. Since the transmission power of each communication device needs to be transmitted such that these reception levels are lower than the maximum inter-cell interference power-to-noise ratio S _max ,
S _max ≧ max (S ₁ , S ₂ ) (5)
It can be expressed.

Therefore,
S _max ≧ D + M + max (P ₃₁ , P ₃₂ ) −P _N (6)
D ≦ S _max −M−max (P ₃₁ , P ₃₂ ) + P _N (7)
It can be expressed as.

This ACK needs to be receivable by the wireless communication terminal WT4. Therefore,
P ₃₄ + D−P _N ≧ S _S (8)
It is necessary to satisfy D ≧ −P ₃₄ + P _N + S _S (9). From Equation (6) and Equation (7), the transmission power at the time of ACK transmission is
In the case of a relationship satisfying −P ₃₄ + P _N + S _S ≦ D ≦ S _max −M−max (P ₃₁ , P ₃₂ ) + P _N (10), the first transmission method can be used. (10) from the left side of the formula and the expression on the right side of the equation can be obtained condition of the power required for the P _34.
−P ₃₄ + P _N + S _S ≦ S _max −M−max (P ₃₁ , P ₃₂ ) + P _N (11)
−P ₃₄ ≦ S _max −M−max (P ₃₁ , P ₃₂ ) −S _S (12)
P ₃₄ ≧ max (P ₃₁ , P ₃₂ ) + (S _S −S _max + M) (13)

Therefore, it can be derived that P ₃₄ needs to be larger than max (P ₃₁ , P ₃₂ ) by (S _S −S _max + M) dB or more. For example, the thermal noise _PN is −90 dBm, P ₃₁ is −75 dBm, P ₃₂ is −80 dBm, the margin M is 1 dB, the minimum detection signal-to-noise ratio S _S is 5 dB, and the maximum inter-cell interference power vs. noise date S _max is − Assuming 5 dB, the equation (10) is
−P ₃₄ + (− 90) + (5) ≦ D ≦ (−5) -1-(− 75) + (− 90) (14)
−P ₃₄ −85 ≦ D ≦ −21 (15) Two things can be understood from the equation (10). If P ₃₄ is -64dB smaller than, (11) ~ (13) D is not satisfying equation. When D exists, power reduction that satisfies −P ₃₄ −85 ≦ D ≦ −36 may be performed to transmit ACK.

Two examples of transmission guidelines are shown. When transmitting to wireless communication terminal WT4 with the minimum transmission power at which signal power is detected, transmission is performed with a transmission power of −P ₃₄ −85 + X ₁ [dB]. X ₁ is an offset value given so as not to fall below the minimum detection sensitivity is set to a value of 0 or more. Or a case of performing transmission at a maximum power which does not give interference power to the adjacent communication cell, in this case, can transmit at -36-X 2 _[dB]. X ₂ is an offset value given so as not to exceed the maximum inter-cell interference power to noise ratio, is set greater than or equal to zero. Even if there are a large number of wireless communication terminals in adjacent communication cells when viewed from the wireless communication terminal WT3, the number of wireless communication corresponding to the coefficient in max in the expressions (10) and (11) to (13) The transmission power is similarly determined by increasing the number of terminals and setting max (P ₃₁ , P ₃₂ ,..., P _3N ). Here, N is the number of wireless communication terminals in the adjacent communication cell.

In addition, P ₃₁ , P ₃₂ ,..., P _3N can be all wireless communication terminals existing in the preliminarily estimated adjacent cells, and the estimated power is stored for each wireless communication terminal. Only the wireless communication terminal designated by the wireless communication terminal that performs transmission may be set.
Note that setting the transmission power to a low value increases the possibility that the ACK signal will fail to transmit. Therefore, in the transmission of the ACK signal, a low modulation scheme or the like corresponding to the transmission power may be used. Thereby, the effect of improving the reception success probability of the ACK signal can be obtained.

  Next, the operation of the first transmission method in the first embodiment will be described with reference to FIG. FIG. 2 is a flowchart showing the operation of the first transmission method in the first embodiment. A wireless communication terminal using the communication method of the present invention first detects a wireless communication terminal that does not belong to its own communication cell but can detect a signal and estimates received power (step S101). When a transmission signal is transmitted from a wireless communication terminal belonging to the own communication cell, this signal is received and decoded. Here, when decoding is performed without any problem (step S102), ACK is transmitted, but it is determined whether there is interference from wireless communication terminals other than the desired communication partner during transmission of ACK (step S103).

  This detection of interference may be performed during or after data decoding, or may be determined by detecting RTS or the like before data decoding. If there is no received signal from another communication cell, ACK is transmitted (step S104), and the process ends. When there are received signals from other communication cells, it is confirmed whether those signal conditions satisfy the expressions (11) to (13) (step S105). If they are satisfied, transmission power control is performed using the coefficient D. ACK is transmitted (step S104), and the process ends. If not satisfied, ACK is transmitted as usual (step S104). Alternatively, in a system that assigns the highest priority to not giving interference to other communications, ACK may not be completed and the process may return to Start (dotted line from Step S105 to Start shown in FIG. 2). Alternatively, the value of the transmission power used for ACK may be determined in advance, and after step S102, transmission power control (step S106) may always be performed for transmission (from step S102 to step S106 in FIG. 2). dotted line).

  Next, the second transmission method will be described. In the second transmission method, the transmission signal 4 is transmitted using a band different from the frequency channel used by the wireless communication terminals WT1 and WT2, which are adjacent communication cells, without affecting the communication of the adjacent communication cell. An ACK signal is transmitted. In order to perform this communication, it is necessary to estimate the frequency channel used for the transmission signals from the wireless communication terminals WT1 and WT2. Here, the frequency channels that the wireless communication terminals WT1 and WT2 are using for communication are F5 and F6, the frequency channels that can be communicated with the wireless communication terminals WT3 and WT4 are F5, F6, F7, and F8, and both communication cells are primary. Assume that the frequency channel is F5. When the ACK signal is transmitted using the same frequency band as that of the data signal, or transmitted through the primary frequency channel, there arises a problem that the ACK signal shown in FIG. 16 becomes an interference signal. In the present invention, the radio communication terminal WT3 detects the frequency channel used in the communication cell C1, and transmits the ACK signal using the frequency channels F7 and F8 not used in the communication cell C1. By controlling in this way, the interference power to adjacent communication cells can be significantly reduced. In addition, when the adjacent communication cell performs communication using F5, F6, F7, and F8, ACK cannot be transmitted by this method. The determination of whether or not ACK transmission is possible is a determination of whether there is a frequency channel that can be transmitted and received by the own communication cell other than the frequency channel used by the adjacent communication cell.

Next, the operation of the second transmission method in the first embodiment will be described with reference to FIG. FIG. 3 is a flowchart showing the operation of the second transmission method in the first embodiment. When a transmission signal is transmitted from a wireless communication terminal belonging to the own communication cell, this signal is received and decoded.
Here, when decoding is performed without any problem (step S202), ACK is transmitted, but it is determined whether there is interference from wireless communication terminals other than the desired communication partner at the time of transmission of ACK (step S203). This detection of interference may be performed during or after data decoding, or may be determined by detecting RTS or the like before data decoding. If there is no received signal from another communication cell, ACK is transmitted (step S204), and the process ends.

  On the other hand, when there is a received signal from another communication cell, a radio communication terminal that estimates a frequency channel used by the other communication cell and sends a transmission signal using a frequency channel other than the frequency channel used by the other communication cell (Step S205), if communication is possible, select a frequency channel that is not used by the adjacent communication cell (step S206), transmit ACK (step S206), and the process ends. If there is no frequency channel that does not cause interference, ACK is transmitted as usual (step S204). Alternatively, in a system that assigns the highest priority to not giving interference to other communications, ACK may not be completed and the process may return to Start (dotted line from Step S105 to Start in FIG. 3).

  Next, the third transmission method will be described. The third transmission method is similar to the second transmission method, but differs in that a frequency channel for transmitting / receiving ACK is determined in advance. In order to perform this communication, the radio communication terminal WT3 or the radio communication terminal WT4 needs to estimate in advance the frequency channel used for the transmission signals from the radio communication terminals WT1 and WT2. The frequency channels that the wireless communication terminals WT1 and WT2 are using for communication are F5 and F6, the frequency channels that can be communicated with the wireless communication terminals WT3 and WT4 are F5, F6, F7, and F8. Assume F5. In the third transmission method, when the radio communication terminal WT3 or the radio communication terminal WT4 detects the frequency channel used in the communication cell C1, and the radio communication terminal WT3 transmits an ACK in the communication of the communication cell C2, the frequency is It is determined in advance to transmit an ACK signal using channels F7 and F8. By controlling in this way, the interference power to adjacent communication cells can be significantly reduced. Further, in this transmission method, it is also possible to notify in advance about the frequency band used for the ACK signal in the wireless communication terminals WT3 and WT4.

Next, the operation of the third communication method in the first embodiment will be described with reference to FIG. FIG. 4 is a flowchart showing the operation of the third communication method in the first embodiment. A wireless communication terminal using the communication method of the present invention first detects a wireless communication terminal that does not belong to its own communication cell but can detect a signal and estimates a frequency channel used by another communication cell (step S300).
A frequency channel that is not used by another communication cell or that is used infrequently, and that can be used by the wireless communication terminal that is the communication partner of the own wireless communication terminal and a frequency channel that is used for ACK between the wireless communication terminals. (Step S301). When a transmission signal is transmitted from a wireless communication terminal belonging to the own communication cell, this signal is received and decoded. Here, when decoding is performed without any problem (step S302), ACK is transmitted using a predetermined frequency channel (step S304).

  Here, after step S302, it may be determined whether or not there is a reception signal (interference signal) from another communication cell, and it is determined whether to change the frequency channel of the ACK signal (step S303). This corresponds to a case where a frequency channel that is not assumed by another communication cell is used, or the other communication channel is not communicating at all. The determination of the signal from the other communication cell may be performed during the decoding of data from the wireless communication terminal as the communication partner, after the decoding, or by detecting the RTS before decoding the data. It may be left. In step S303, the frequency channel of the ACK signal is changed according to the usage status of the frequency channel of the other communication cell, or it is confirmed that the other communication cell is not transmitting, and the frequency channel of the ACK signal is the same as that of the data signal. A frequency channel may be used, a primary frequency channel may be used, or both a primary frequency channel and a predetermined frequency channel may be used. After the frequency channel is changed (step S305), ACK is transmitted (step S304), and the process ends.

Second Embodiment
Next, a second embodiment of the present invention will be described with reference to FIG. The form of communication is the same as that shown in FIG. Here, wireless communication terminals WT2 and WT3 can detect each other's communication, and wireless communication terminals WT1 and WT2 and wireless communication terminals WT3 and WT4 belong to communication cells C1 and C2, respectively. Communication by Request to Send (RTS) and Clear to Send (CTS) is performed by the communication cell C1 including the wireless communication terminals WT1 and WT2, and the signal 5 corresponds to the RTS signal and the signal 6 corresponds to the CTS signal. For the CTS signal, NAV is set to the detected wireless communication terminal so that only the designated terminal can transmit for a certain period of time. In FIG. 17, since the wireless communication terminal WT3 receives the CTS by the received signal 6 ″, the NAV is set as indicated by the signal 8. The communication cell C1 and the communication cell C2 use the same frequency channel. In this case, even when the wireless communication terminal WT3 decodes the transmission signal 3 from the wireless communication terminal WT4, the wireless communication terminal WT3 cannot transmit the ACK signal.

Next, the first transmission method in the second embodiment will be described. In the first transmission method, transmission is performed by reducing the transmission signal 7 by RdB from the normal transmission power. By reducing the transmission power in this way, the interference power to the radio communication terminals WT1 and WT2 is reduced, and transmission is performed with power that cannot be detected in the radio communication terminals WT1 and WT2, thereby using RTS / CTS. An ACK signal can be transmitted without affecting the existing wireless communication terminals WT1 and WT2. In order to perform this communication, it is necessary to estimate reception power for transmission signals from the wireless communication terminals WT1 and WT2 in advance. Here, the thermal noise power estimated by the wireless communication terminal WT3 is P _N [dBm], the reception power of the transmission signal of the wireless communication terminal WT1 is P ₁₃ [dBm], and the reception power of the transmission signal of the wireless communication terminal WT2 is P _23. [DBm], the minimum detection signal-to-noise ratio of the communication signal is S _S [dB], the maximum inter-cell interference power-to-noise ratio allowed in the wireless communication terminals of other communication cells is S _max [dB], and the wireless communication terminal P ₃₄ [dBm] is the received power at the wireless communication terminal WT 4 when transmission is performed with normal transmission power from the WT 3, and the transmission power individual difference margin prepared for the difference in transmission power that may occur in different communication apparatuses. When transmitting M and ACK, let D [dB] be a power reduction coefficient that decreases from normal transmission power. Thereby, the signal reception power-to-noise ratio in the communication cells C2 and C1 can be defined in the same manner as in the relationships of the expressions (1) to (15).

The signal 7 is transmitted by determining D so as to satisfy the expression (10). When transmitting to the wireless communication terminal WT4 with the minimum transmission power at which signal power is detected, transmission is performed with a transmission power of −P ₃₄ + P _N + S _S + X ₁ [dB]. X ₁ is an offset value given so as not to fall below the minimum detection sensitivity is set to a value of 0 or more. Alternatively, transmission is performed with maximum power that does not give interference power to adjacent communication cells. In this case, transmission is performed with S _max −M−max (P ₃₁ , P ₃₂ ) + P _N −X ₂ [dB]. It can be carried out. X ₁ is an offset value given so as not to exceed the maximum inter-cell interference power to noise ratio, is set to a value of 0 or more. Even if there are a large number of wireless communication terminals in adjacent communication cells as seen from the wireless communication terminal WT3, the coefficient in max in the equations (10) to (13) is increased to the corresponding number of wireless communication terminals, By setting max (P ₃₁ , P ₃₂ ,..., P _3N ), the transmission power can be similarly determined. Here, N is the number of wireless communication terminals in the adjacent communication cell.

P ₃₁ , P ₃₂ ,..., P _3N may be all wireless communication terminals existing in the adjacent cell estimated in advance, or the estimated power is stored for each wireless communication terminal, Only the wireless communication terminal designated by the wireless communication terminal that performs transmission may be set.

  Next, the operation of the first transmission method in the second embodiment will be described with reference to FIG. FIG. 6 is a flowchart showing the operation of the first transmission method in the second embodiment. A wireless communication terminal using the communication method of the present invention first detects a wireless communication terminal that does not belong to its own communication cell but can detect a signal and estimates received power (step S401). When a transmission signal is transmitted from a wireless communication terminal belonging to the own communication cell, this signal is received and decoded. Here, when the CTS signal is received, the requested NAV section is set (step S402). If a reception signal is detected during this period, the reception completion time is estimated, and it is determined whether the ACK signal is in the NAV section (step S403). Since the ACK signal is transmitted as usual outside the NAV section, it is not shown in the figure. If it is within the NAV section, it is confirmed whether there is a transmission power control attenuation coefficient D that satisfies the equations (11) to (13) (step S404). The power is reduced and ACK is transmitted (step S405), and the process ends. If it is not satisfied in step S404, the communication is not completed and the process returns to Start.

  Next, the second transmission method in the second embodiment will be described. The second transmission method affects the communication of other communication cells by transmitting the ACK signal 7 using a band different from the frequency channel of the CTS signal indicated by the received signal 6 ″ in the wireless communication terminal WT3. The frequency channels used by the wireless communication terminals WT1 and WT2 for communication are F5 and F6, and the frequency channels that can be communicated by the wireless communication terminals WT3 and WT4 are F5, F6, F7, Assuming F8, the CTS signal is considered to have NAV set in F5 and F6 In such a case, in the second transmission method, the frequency channel of F7 or F8 or F7 and F8 is used, and the ACK signal is changed. Send.

  Next, the operation of the second transmission method in the second embodiment will be described with reference to FIG. FIG. 7 is a flowchart showing the operation of the second transmission method in the second embodiment. The wireless communication terminal using the communication method of the present invention detects the CTS signal, acquires the frequency channel corresponding to the CTS signal, and sets the NAV (step S501). When a signal that needs to transmit ACK in the NAV section is received (step S502), it is determined whether the wireless communication terminals WT3 and WT4 can communicate with each other in a frequency channel corresponding to CTS (step S503). If not, an ACK signal is transmitted on a channel other than the frequency channel corresponding to CTS (step S504), and the process ends. If no frequency channel is available, ACK is not transmitted and the process returns to Start.

<Third Embodiment>
Next, a third embodiment of the present invention will be described with reference to FIG. Here, wireless communication terminals WT2 and WT3 can detect each other's communication, and wireless communication terminals WT1 and WT2 and wireless communication terminals WT3 and WT4 belong to communication cells C1 and C2, respectively. Communication by request to send (RTS) and Clear to send (CTS) is performed by the communication cell C1 including the wireless communication terminals WT1 and WT2, and the signals 3 and 5 are RTS signals, and the signals 4 and 6 are CTS signals. It corresponds. For the CTS signal, NAV is set to the detected wireless communication terminal so that only the designated terminal can transmit for a certain period of time. In FIG. 8, since the wireless communication terminal WT3 receives the CTS by the received signal 6 ″, the NAV is set as indicated by the signal 8. At this time, the wireless communication terminal WT4 cannot receive the CTS signal 6. Therefore, the RTS signal is transmitted to the wireless communication terminal WT3, so that the wireless communication terminal WT3 receives the RTS signal in the NAV section, and since it is originally a NAV section, the CTS signal cannot be returned. The present invention makes it possible to transmit a CTS signal.

The first transmission method and the second transmission method of the third embodiment can be realized by replacing the data signal with the RTS signal and the ACK with the CTS signal in the transmission method shown in FIGS. In the first transmission method, in the transmission operation shown in FIG. 6, when an RTS signal is received in the NAV section (step S403), whether a CTS signal can be transmitted is determined from equation (10) (step S404), and transmission is possible. In step S405, a CTS signal is transmitted by transmission power control.
In the second transmission method, in the transmission operation shown in FIG. 7, an RTS signal is received in the NAV section (step S502), and there is a frequency channel that can be used other than the frequency channel corresponding to the CTS received in step S501. If it can be transmitted, a CTS signal is transmitted using a frequency channel not used in CTS (step S504). Here, in the second transmission method in the third embodiment, several transmission methods are possible as the CTS signal.

  The CTS transmission method of the wireless communication terminal WT3 in the third embodiment will be described with reference to first and second CTS transmission methods. Here, the radio communication terminals WT1 and WT2 of the communication cell C1 communicate using the frequency channels F5 and F6, and the radio communication terminals WT3 and WT4 of the communication cell C2 communicate using the frequency channels F5 to F8. It shall be. In the first CTS transmission method, the wireless communication terminal WT3 detects the CTS from the wireless communication terminal WT2 without detecting the RTS from the wireless communication terminal WT1, and the RTS from the wireless communication terminal WT4 is detected. F7 or F8 or F7 and F8 is used to transmit a CTS signal, and NAV is set to the frequency channel used for the terminal capable of detecting this signal, while F5 to F8 are set for the wireless communication terminal WT4. To notify that transmission is allowed.

  Here, since the wireless communication terminal WT4 can use F5 and F6 (frequency used by the wireless communication terminal WT1), since the wireless communication terminal WT4 transmits RTS after CTS transmission of the wireless communication terminal WT2, This is because the radio communication terminal WT4 cannot decode the signal of the radio communication terminal WT2, in other words, it can be expected that the transmission signal of the radio communication terminal WT4 does not interfere with the radio communication terminal WT2. In this CTS transmission method, since the wireless communication terminal WT3 cannot newly set NAV for F5 and F6, the transmission signal of the wireless communication terminal WT4 that is a hidden terminal for the communication cell C1 in the frequency channel of F5 and F6 is wireless. Although no interference occurs between the communication terminals WT1 and WT2, the transmission signals of the radio communication terminals WT1 and WT2 can interfere with the radio communication terminal WT3. In order to reduce the influence of the interference from the hidden terminal, in the transmission signal of the wireless communication terminal WT4, the modulation mode determined from the modulation method and the coding rate of F5 and F6 is set to one corresponding to the one having few transmission bits. Or by setting all of the modulation modes F5 to F8 to ones with a small number of transmission bits, it is possible to reduce the probability that a packet error will occur compared to an interference signal by a hidden terminal.

  In addition, as a second CTS transmission method, a CTS signal is transmitted using F7 or F8, or F7 and F8, and the frequency channel used in the CTS signal of the wireless communication terminal WT3 is used for the wireless communication terminal WT4. Notify you to send. In these CTS transmission methods for requesting a reduction in the number of transmission mode bits of the transmission signal of the wireless communication terminal WT4, an increase in packet length accompanying a reduction in throughput and an increase in transmission power per frequency band accompanying a reduction in frequency channels Can occur. Therefore, the wireless communication terminals WT3 and WT4 need to communicate so as to solve the problems caused by these.

  Regarding the increase in the packet length, the number of frequency channels to be designated in the wireless communication terminal WT3 and the frequency channel and the section of the NAV used by the RTS signal of the wireless communication terminal WT4 are referred to. As shown in FIG. 8, the transmission signal 4 can be transmitted as a CTS signal notifying this section. For example, when the RTS signal 3 designating F5 to F8 is received, and this is a NAV section having a length T, this is used when notifying that the F7 and F8 are newly used by the CTS signal. Since the frequency channel to be transmitted is halved, a CTS signal designating a 2V-long NAV section is transmitted as the transmission signal 4. Further, when the signal 7 is transmitted from the wireless communication terminal WT4, the wireless communication terminal WT4 may also designate a new length of NAV corresponding to 2T.

  Also, as the transmission power of the wireless communication terminal WT4, when F7 and F8 are designated, the transmission power is set to double the transmission power per frequency channel as compared with the case of transmitting from F5 to F8. It may be defined and transmitted, or transmission may be performed while maintaining transmission power per frequency channel when originally attempting to transmit at F5 to F8.

  Alternatively, the wireless communication terminals WT3 and WT4 communicate with each other so as to solve the problem caused by the increase in packet length accompanying a decrease in throughput and the increase in transmission power per frequency band accompanying a decrease in frequency channel. You may make it communicate. First, the wireless communication terminal WT3 uses the signal 4 to request the wireless communication terminal WT4 to specify a transmission mode with a low transmission bit or limit the number of frequency channels to be used. Based on the signal 4, the wireless communication terminal WT4 newly determines the new transmission mode assignment / frequency channel assignment and the NAV length in the wireless communication terminals WT3 and WT4, and makes a transmission request again with the signal 9. If there is no problem, the wireless communication terminal WT notifies the completion of reception preparation by the signal 10, sets the NAV, and the communication from the wireless communication terminal WT4 to the wireless communication terminal WT3 can be realized by the signal 7.

<Fourth embodiment>
Next, a fourth embodiment of the present invention will be described with reference to FIG. FIG. 10 shows that wireless communication terminals WT1 and WT2 belong to communication cell C1 and communicate with each other, and wireless communication terminals WT3 to WT5 belong to communication cell C2 and communicate with each other. ing. Here, the wireless communication terminal WT3 has a multi-user MIMO function and can transmit to a plurality of communication partners. In FIG. 4, the wireless communication terminal WT3 transmits to the wireless communication terminals WT4 and WT5 using the same time and the same frequency channel. However, the wireless communication terminals WT3 and WT4 have sufficiently small interference power from the adjacent communication cell, but the wireless communication terminal WT5 receives the interference power from the adjacent communication cell, and the transmission signal of the wireless communication terminal WT5 is also received. This is an interference with the wireless communication terminals WT1 and WT2. The fourth embodiment is a transmission method for coping with adjacent communication cell interference during such multiuser MIMO communication.

  First, wireless communication terminal WT3 transmits transmission signal 3 to wireless communication terminal WT4 and wireless communication terminal WT5. This is a multi-user RTS signal for notifying that transmission by multi-user MIMO is to be performed. Radio communication terminals WT4 and WT5 transmit a channel CTS (CCTS) signal indicating that they are ready to receive the received multi-user RTS signal by means of transmission signal 4 and transmission signal 5, respectively. Here, the CCTS signal is a signal that enables channel information to be transmitted to a communication partner by feedback of channel information or transmission of a known signal. There is no function of a conventional CTS signal, and a signal for transmitting channel information is also defined as a CCTS signal. At this time, it is assumed that the wireless communication terminals WT1 and WT2 perform communication. Since the wireless communication terminals WT1 and WT2 are installed at positions where the transmission signal of the wireless communication terminal WT3 cannot be detected, transmission is performed freely. However, since the communication of the communication cell C1 is received, the wireless communication terminal WT5 can know that the interference power affects the multiuser MIMO communication from the wireless communication terminal WT3. In the present embodiment, a CTSS signal transmitted by the wireless communication terminal WT5 is shown.

  Next, the first CTSS transmission method in the fourth embodiment will be described. Here, in the first CTSS transmission method, the wireless communication terminal WT5 estimates the reception level of the transmission signals of the wireless communication terminals WT1 and WT2 in the communication cell C1 and the reception power between the wireless communication terminal WT3 which is the communication partner. Then, the signal 5 is transmitted by reducing the transmission power by the coefficient D so as to satisfy the expression (10). At this time, multi-user MIMO transmission of the wireless communication terminal WT3 is performed for transmission from the wireless communication terminal WT3 to the wireless communication terminal WT5 using channel information obtained by a transmission signal from the wireless communication terminal WT5 to the wireless communication terminal WT3. When performing open-loop multi-user MIMO transmission for acquiring channel information, the estimation accuracy of channel information is greatly reduced. Therefore, the radio communication terminal WT3 uses a modulation scheme and coding rate used for transmission to the radio communication terminal WT5. The modulation mode consisting of is changed to one with a low transmission bit and transmitted.

  In addition, when multi-user MIMO transmission is performed using feedback information of channel information during communication from the wireless communication terminal WT3 to the wireless communication terminal WT5 included in the transmission signal 5, the transmission signal 5 is transmitted at a low information rate. The modulation mode corresponding to the low information bit is selected in consideration of the transmission quality degradation due to the increase of the delay time from the reception of the signal 3 used for channel estimation. Alternatively, when the signal 5 has a function of setting the NAV, it is assumed that an interference signal is received because an area in which this signal can be received becomes small. In order to increase resistance to such interference signals, the modulation mode may be set small.

  Next, the operation of the first transmission method in the fourth embodiment will be described with reference to FIG. FIG. 11 is a flowchart showing the operation of the first transmission method in the fourth embodiment. A wireless communication terminal using the communication method of the present invention first detects a wireless communication terminal that does not belong to its own communication cell but can detect a signal and estimates received power (step S601). When a multi-user RTS signal is detected (step S602) and a transmission signal of another communication cell is received before transmitting a CTSS signal (step S603), it is determined whether CTSS can be transmitted by power control (step S604). If there is D that satisfies the relationship of equation (10), the transmission power is suppressed and CTSS transmission is performed (step S605). If not, communication is not completed and the process returns to Start.

  Next, a second CTSS transmission method in the fourth embodiment will be described. Here, in FIG. 10, radio communication terminals WT1 and WT2 of communication cell C1 use frequency channels F5 and F6, and radio communication terminals WT3 to WT6 of communication cell C2 use frequency channels F5 to F8. It shall be. Multi-user RTS signals are received by radio communication terminals WT4 and WT5 in frequency channels F5 to F8. At this time, since the transmission signal of the wireless communication terminal WT3 cannot be detected by the wireless communication terminals WT1 and WT2, the wireless communication terminal WT1 may start transmission. At this time, the wireless communication terminal WT3 receives interference power from other communication cells at F5 and F6. When transmitting CTSS to the wireless communication terminal WT3, the wireless communication terminal WT5 transmits the signal 5 to the wireless communication terminal WT3 using F7, F8, or F7 and F8.

  Here, two cases will be described. In the case of an open-loop method in which multi-user MIMO communication of the wireless communication terminal WT3 uses channel information in transmission from the wireless communication terminal WT5 to the wireless communication terminal WT3, channel information cannot be estimated in F5 and F6 in this method. In addition, multi-user MIMO transmission is not possible in F5 and F6. For example, communication to the wireless communication terminal WT4 can be performed in the frequency channels F5 and F6, and communication to the wireless communication terminal WT5 or communication to the wireless communication terminals WT4 and WT5 can be performed in F7 and F8. Alternatively, channel information from the wireless communication terminal WT3 to the wireless communication terminal WT5 in F5 and F6 is included as feedback information, and in F5 and F6, multi-user MIMO transmission (closed loop method) based on the feedback information is performed, and in F7 and F8 Can also perform multi-user MIMO transmission using an open loop scheme.

  Next, the operation of the second transmission method in the fourth embodiment will be described with reference to FIG. FIG. 12 is a flowchart showing the operation of the second transmission method in the fourth embodiment. First, a multi-user RTS signal is received (step S701), and when interference from another communication cell is received in the NAV section (step S702), the frequency channel used by the other communication cell or the frequency channel specified by the NAV It is determined whether there is a frequency channel that can be transmitted (step S703). If there is no frequency channel that can be transmitted, the process returns to Start and communication is terminated. On the other hand, if there is a frequency channel that can be transmitted, ACK, CTS, or CTSS is transmitted using one or more frequency channels that can be transmitted (step S704).

  In addition, as shown in FIG. 13, by adding the step (step S700) which detects the frequency channel used with another communication cell, without performing determination by step S703 shown in FIG. 12, in step S704, it is ACK, CTS, CCTS. May be transmitted.

  Further, as a second mode of the fourth embodiment, a new NAV or transmission mode is newly set as in the signal 12 based on a new transmission mode designated by the signal 5 or the frequency channel used for transmission in the wireless communication terminal WT3. The set multi-user RTS may be transmitted again. At this time, the CCTS signal may be retransmitted from both the wireless communication terminal WT4 and the wireless communication terminal WT5, or only a specific wireless communication terminal (wireless communication terminal WT5 in FIG. 14) transmits the CCTS signal as shown in FIG. You may make it resend. Thus, by newly setting the NAV, packet collision can be avoided.

The transmission operation corresponding to FIG. 14 is shown in FIG. FIG. 15 is a flowchart showing a transmission operation corresponding to FIG. First, a multi-user RTS signal is detected (step S602), and in the NAV section, when interference that causes a problem in communication conditions specified by RTS is detected (step S603), transmission can be performed without causing interference in other communication cells. It is determined whether there is a correct frequency channel (step S604). If there is a frequency channel that can be transmitted, the AP is notified of interference conditions and channel information (step S605). The wireless communication terminal that has transmitted the multi-user RTS signal based on the notified signal determines the NAV section and the transmission mode again, and transmits the multi-user RTS signal (step S606).
When the multi-user RTS signal is received again and the interference received in the NAV section has no problem with the content specified in the RTS, the CTSS signal is transmitted (step S607), and communication is performed with the wireless communication terminal that has transmitted the RTS signal. To start.

  Thus, in an overlap communication cell environment in which at least one wireless communication terminal detects a signal of a wireless communication terminal belonging to another communication cell, ACK signals, CTS signals, and signals similar to CTS signals have different transmission powers, Alternatively, by transmitting on different frequency channels, it is possible to prevent interference with other communication cells and prevent a decrease in system throughput due to signal retransmission.

  In the prior art, when a certain wireless station is communicating, when another wireless station starts communication, there is a problem that reception fails due to interference with each other. On the other hand, if the interference waves are so far away that the influence of the interference wave can be ignored, reception is successful even if simultaneous communication is performed. However, even in such a case, when a signal is received in the wireless LAN, an ACK is always returned, and further, an ACK is reflexively returned without carrier sense. When one of the wireless stations that started the transmission ends earlier, there is a problem that interference is caused by an ACK returned by the communication partner.

  In the present invention, transmission power control is performed on the ACK, thereby reducing interference caused by the ACK. That is, the transmission power is controlled so that the communication partner can receive and the interference partner does not disturb the communication. As another method, ACK is returned on another channel not used by the interference partner. With such a configuration, since interference due to ACK is reduced, throughput can be increased.

  A computer-readable program for realizing the functions of the wireless communication terminal according to the present invention (processing operations shown in FIGS. 2, 3, 4, 6, 7, 12, 13, and 15). The transmission power control process may be performed by recording in a recording medium, reading the program recorded in the recording medium into a computer system, and executing the program. Here, the “computer system” includes an OS and hardware such as peripheral devices. The “computer-readable recording medium” refers to a storage device such as a flexible medium, a magneto-optical disk, a portable medium such as a ROM and a CD-ROM, and a hard disk incorporated in a computer system. Further, the “computer-readable recording medium” refers to a volatile memory (RAM) in a computer system that becomes a server or a client when a program is transmitted via a network such as the Internet or a communication line such as a telephone line. In addition, those holding programs for a certain period of time are also included.

  The program may be transmitted from a computer system storing the program in a storage device or the like to another computer system via a transmission medium or by a transmission wave in the transmission medium. Here, the “transmission medium” for transmitting the program refers to a medium having a function of transmitting information, such as a network (communication network) such as the Internet or a communication line (communication line) such as a telephone line. The program may be for realizing a part of the functions described above. Furthermore, what can implement | achieve the function mentioned above in combination with the program already recorded on the computer system, what is called a difference file (difference program) may be sufficient.

  In order to suppress interference between cells, it can be applied to an application in which it is essential to perform transmission power control for ACK.

  C1, C2 ... communication cells, WT1, WT2, WT3, WT4, WT5 ... wireless communication terminals

Claims (4)

A wireless communication method for suppressing interference between cells in a communication system using a plurality of frequency channels, where a part of a plurality of communication cells overlaps,
Another communication cell frequency channel estimation step for estimating a frequency channel used in another communication cell;
A self-communication cell signal processing step for receiving a control signal or a data signal from the radio communication terminal of the self-communication cell and decoding a signal from the radio communication terminal;
After have such have frequency channel used in the other communication cell, ACK signal indicating successful reception after the data communication ends, CTS signal indicating that ready to receive a transmission request signal after receiving the channel information request signal A control signal frequency channel selection step for determining a frequency channel for transmitting at least one signal among CCTS signals having a function of notifying channel information after reception;
Using the determined frequency channel, an ACK signal for notifying successful reception after the end of data communication, a CTS signal for notifying that reception is ready after receiving a transmission request signal, and channel information after receiving a channel information request signal And a control signal transmission step of transmitting at least one signal among CCTS signals having a function of notifying. When a CTS signal or CCTS signal notifying that preparation for reception has been completed after receiving a transmission request signal is transmitted from a communication apparatus as a communication partner, transmission is performed in consideration of deterioration in accuracy of channel information. 2. The wireless communication method according to claim 1, further comprising a control signal-considered data transmission step of changing a modulation mode composed of a data signal modulation scheme and a coding rate to one having a low transmission bit. A part of a plurality of communication cells overlap, and a wireless communication system that suppresses interference between cells using a plurality of frequency channels,
Other communication cell frequency channel estimation means for estimating a frequency channel used in another communication cell;
A self-communication cell signal processing means for receiving a control signal or a data signal from a radio communication terminal of the self-communication cell and decoding a signal from the radio communication terminal;
After have such have frequency channel used in the other communication cell, ACK signal indicating successful reception after the data communication ends, CTS signal indicating that ready to receive a transmission request signal after receiving the channel information request signal Control signal frequency channel selection means for determining a frequency channel for transmitting at least one signal among CCTS signals having a function of notifying channel information after reception;
Using the determined frequency channel, an ACK signal for notifying successful reception after the end of data communication, a CTS signal for notifying that reception is ready after receiving a transmission request signal, and channel information after receiving a channel information request signal And a control signal transmission means for transmitting at least one signal among CCTS signals having a function of notifying. When a CTS signal or CCTS signal notifying that preparation for reception has been completed after receiving a transmission request signal is transmitted from a communication apparatus as a communication partner, transmission is performed in consideration of deterioration in accuracy of channel information. 4. The wireless communication system according to claim 3, further comprising control signal consideration data transmission means for changing a modulation mode composed of a data signal modulation scheme and a coding rate to a transmission mode having a low transmission bit.

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