JP5754815B2 - Wireless communication system and wireless communication method - Google Patents

Description

  The present invention relates to a wireless communication system and a wireless communication method for performing uplink communication from a plurality of terminals to an access point.

  In wireless communication, it is desired to improve the use efficiency of frequency resources in order to accommodate a huge number of terminals in a limited frequency resource and provide a high-quality communication service. For this reason, for example, a CSMA / CA (Carrier Sense Multiple Access with Collision Avoidance) method is used as an autonomous distributed medium access control method for frequency sharing in an unlicensed band. (For example, refer nonpatent literature 1). In Non-Patent Document 1, in the CSMA / CA system defined as DCF (Distributed Coordination Function), each terminal acquires a transmission right in a random manner, and the terminal that has acquired the transmission right occupies radio resources. And transmit. In this case, acquisition of the transmission right is irrelevant to the propagation path status of each terminal, and a single terminal occupies the acquired radio resource. For this reason, in a time selective fading environment and a frequency selective fading environment, sufficiently high frequency utilization efficiency cannot be obtained, and radio resources cannot be utilized sufficiently effectively, and high throughput can be obtained. Can not.

  On the other hand, in the fading environment as described above, a technique for improving throughput by introducing priority control type CSMA / CA that facilitates acquisition of a transmission right by a terminal having a good propagation path condition has been proposed (for example, Non-patent document 2). However, in the case of this method, the transmission right is more easily acquired as the terminal has a better propagation path condition, and the transmission right acquisition frequency varies depending on the terminal. Similarly to Non-Patent Document 1, since a single terminal occupies the acquired radio resource, there is a partially low frequency band in the frequency-selective fading environment, and the radio resource is sufficient. It cannot be used and high throughput cannot be obtained.

IEEE 802.11-2007, IEEE Standard for Information technology-Telecommunications and information exchange between systems-Local and metropolitan area networks-Specific requirements-Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications Chan-soo Hwang, John M. Cioffi, “Opportunistic CSMA / CA for achieving multi-user diversity in wireless LAN IEEE Transactions on Wireless Communications, vol. 8, no. 6, pp. 2972-2982, June 2009.

  As described above, in the wireless communication methods according to Non-Patent Documents 1 and 2, acquisition of a transmission right does not sufficiently consider the propagation path status of each terminal, and occupies the wireless resources acquired by a single terminal. For this reason, there is a problem in that radio resources cannot be used sufficiently effectively and high throughput cannot be obtained under a time selective, frequency selective, and spatial selective fading environment.

  The present invention has been made in view of such circumstances, and an object of the present invention is to provide a radio communication system and a radio communication method capable of effectively using radio resources and increasing throughput.

  In order to solve the above-described problem, the present invention is a wireless communication system that performs uplink multi-user communication in which information is transmitted from a plurality of terminals to an access point, and the plurality of terminals are at least once or more. A carrier sense execution unit that performs carrier sense of the sub-channel, a sub-channel assignment determination unit that determines whether a sub-channel is assigned from the access point according to carrier sense by the carrier sense execution unit, and the sub-channel assignment A random backoff time determination unit that determines a random backoff time of carrier sense for the second and subsequent times by the carrier sense execution unit when the determination unit determines that a subchannel is not allocated, and the access point Is a free sub-channel according to the carrier sense from the terminal. A free subchannel determination unit that determines whether or not there is a channel, and a subchannel that is assigned to a terminal that has acquired a transmission right from the free subchannels when the subchannel determination unit determines that there is a free channel And a sub-channel determination unit that determines a sub-channel.

  According to the present invention, in the above invention, the empty subchannel determination unit divides the communication channel into subchannels according to a time axis, a frequency axis, a spatial axis, or a combination thereof, and is based on a propagation path condition of each subchannel. Thus, an unassigned free subchannel is determined.

  The present invention is characterized in that, in the above invention, the random back-off time determination unit determines the random back-off time of carrier sense so that the better the propagation path condition of the empty sub-channel, the shorter it becomes. .

  In order to solve the above-described problem, the present invention provides a wireless communication method for performing uplink multi-user communication in which information is transmitted from a plurality of terminals to an access point. Performing the carrier sense of the plurality of terminals, determining whether the plurality of terminals are assigned subchannels from the access point according to carrier sense, and assigning the subchannels to the plurality of terminals. Determining a random backoff time of the carrier sense when it is determined that it is not, a step of performing the second and subsequent carrier senses based on the random backoff time when the plurality of terminals, The access point determines whether there is a free subchannel according to the carrier sense from the terminal. And a step of determining a subchannel to be allocated to a terminal that has acquired a transmission right from among the free subchannels when the access point determines that there is a free channel. It is a communication method.

  According to the present invention, it is possible to effectively use radio resources and increase throughput.

It is a block diagram which shows the structure of access point (base station) AP and a terminal by this embodiment. It is a flowchart for demonstrating operation | movement of access point AP and a terminal by this embodiment. In this embodiment, it is a conceptual diagram which shows the concrete communication protocol applied to the IEEE802.11 wireless LAN system. In this embodiment, it is a conceptual diagram which shows the example of the determination method of the presence or absence of an empty subchannel. In this embodiment, it is a conceptual diagram which shows the relationship between the propagation path condition of an empty subchannel, and random backoff time. In this embodiment, it is a conceptual diagram which shows the example of the allocation method of a subchannel. It is a conceptual diagram which shows the MAC (Media Access Control) protocol of a prior art. It is a conceptual diagram which shows the relationship between the number of terminals and maximum throughput in this invention and a prior art.

Hereinafter, an embodiment of the present invention will be described with reference to the drawings.
A. Embodiment FIG. 1 is a block diagram showing main configurations of an access point (base station) AP and a terminal in a wireless communication system of the present embodiment. In FIG. 1, the access point AP includes a free subchannel determination unit 10, a subchannel determination unit 11, and a multiuser transmission success / failure response unit 12. The free subchannel determination unit 10 determines whether there is a free subchannel from the propagation path status of each subchannel, according to carrier sense from each terminal. The subchannel determination unit 11 determines a subchannel to be allocated to the terminal that has acquired the transmission right from among the free subchannels. The multiuser transmission success / failure response unit 12 responds to the success / failure of uplink multiuser transmission from a plurality of terminals (transmission of ACK).

  Terminals # 1 and # 2 (assuming two terminals in this embodiment) include a carrier sense execution unit 20, a subchannel allocation determination unit 21, and a random backoff time determination unit 22. The carrier sense execution unit 20 executes normal carrier sense for the first time, and if the carrier sense cannot acquire a subchannel, repeats carrier sense until a subchannel can be acquired according to a random backoff time described later. Run. The subchannel allocation determination unit 21 determines whether a subchannel is allocated by the access point AP according to the carrier sense by the carrier sense execution unit 20. The random back-off time determination unit 22 determines the random back-off time so that the sub-channel is not allocated, and the shorter the channel condition (reception SNR, etc.) of the empty sub-channel is, the shorter it is.

  In this embodiment, the access point AP estimates the propagation path status from the received SNR, etc. according to the carrier sense by the terminal, determines whether or not the allocation is possible based on the propagation path status, and as long as there is an empty subchannel, The carrier sense is received, and after the second RTS (Request To Send), the terminal estimates the propagation path condition from the reception SNR of CTS (Clear To Send) and determines the back-off time based on the propagation path condition. Then, the access point AP accesses the allocated terminal and performs user communication with the access point AP by a multiple access method such as OFDMA. That is, in order to realize multi-user communication, the access point AP does not centrally map a plurality of terminals to each subchannel, but each terminal performs CSMA / CA multiple times to perform autonomous decentralization. To map. Thereby, since each terminal autonomously selects a subchannel with a good propagation path condition and performs user communication, it is possible to effectively use radio resources.

  FIG. 2 is a flowchart for explaining operations of the access point AP and the terminal according to the present embodiment. First, each terminal performs normal carrier sense (step S1). For example, the transmission right of the terminal is acquired by the CSMA / CA method used in the IEEE 802.11 wireless LAN system.

  Next, the access point AP receives the carrier sense and determines whether there is an empty subchannel (step S2). More specifically, the access point AP estimates the propagation path condition of each subchannel with the terminal that has acquired the transmission right when the transmission right is acquired. Here, each subchannel is a radio resource obtained by dividing a radio resource by a time axis, a frequency axis, or a space axis. The access point AP determines the presence / absence of an empty subchannel from the propagation path status of each subchannel with the terminal that has acquired the transmission right in step S1.

  If there is an empty subchannel (YES in step S2), the terminal shifts to k (k ≧ 2) CSMA / CA implementation (step S3). That is, terminals other than the terminal that has acquired the transmission right in step S1 try to acquire the transmission right according to CSMA / CA. Here, a random backoff value in CSMA / CA is determined so that a terminal that can effectively use an empty subchannel can preferentially acquire a transmission right.

  Next, the access point AP determines a subchannel (step S4). The access point AP determines a subchannel to be allocated to the terminal that has acquired the transmission right in step S3 from among the free subchannels. Then, it returns to step S2 and repeats the process mentioned above.

  On the other hand, when there are no free subchannels (NO in step S2), it is determined that radio resources are already fully utilized, and the process shifts to multiuser transmission (step S5). Uplink multiuser transmission addressed to the access point AP is performed from a plurality of terminals determined through a plurality of carrier senses in steps S1 to S4. The access point AP responds with the success or failure of multiuser transmission (step S6). Specifically, the access point AP checks whether or not the data transmitted from each terminal has been successfully received, and returns an ACK.

  FIG. 3 is a conceptual diagram showing a specific communication protocol applied to the IEEE 802.11 wireless LAN system in the present embodiment. In FIG. 3, in (1), each terminal performs carrier sense of a DIFS (DCF Inter Frame Space) time and a random backoff time according to the CSMA / CA method. Here, the random back-off time is determined in the same manner as in the normal CSMA / CA system.

  In (2), the terminal having the shortest random backoff time transmits the RTS frame to the access point AP and acquires the transmission right. In FIG. 3, the terminal # 1 acquires the transmission right and transmits the RTS frame. The access point AP estimates the propagation path status of each subchannel of the terminal # 1 from the received RTS frame. Based on the estimation result, a subchannel to be assigned to terminal # 1 and a subchannel not to be assigned to terminal # 1 (empty subchannel) are determined. For example, as shown in FIG. 4, when there are four subchannels, a method of setting an empty subchannel when the received signal power to noise power ratio (received SNR) of each subchannel is smaller than a predetermined threshold γ. Can be considered.

  In (3), the access point AP transmits a CTS frame addressed to all terminals in the group. An empty subchannel number is described in the CTS frame. In addition, each terminal estimates a propagation state of an empty subchannel from the received CTS frame.

  In (4), each terminal starts random backoff again according to the CSMA / CA scheme. Here, the random back-off time is determined so as to become shorter as the propagation state (reception SNR, etc.) of the empty subchannel is better. FIG. 5 is a conceptual diagram showing the relationship between the propagation state of the empty subchannel and the random backoff time (CW (Contention Window)) in the present embodiment. It can be seen that the better the received SNR, the smaller the value of CW is.

  In (5), the terminal having the shortest random backoff time for the second time transmits a 2nd-RTS frame to the access point AP and transmits it as a candidate terminal using an empty subchannel. In FIG. 3, it is assumed that terminal # 2 transmits a 2nd-RTS frame. The access point AP estimates the propagation path condition of the terminal # 2 from the 2nd-RTS frame.

  In (6), the access point AP is based on the channel conditions of the terminal # 1 that has acquired the transmission right by the first CSMA / CA and the terminal # 2 that has acquired the transmission right by the second CSMA / CA. A subchannel used by each terminal is determined in uplink multiuser communication. For example, as shown in FIG. 6, the received SNR of each subchannel is compared between the terminal # 1 that has acquired the first transmission right and the terminal # 2 that has acquired the second transmission right. A method of assigning subchannels is conceivable. The result of allocating the subchannel is notified in the ARBI frame.

In (7), a terminal to which a subchannel is assigned transmits data to the access point AP using a multiple access scheme such as OFDMA or SDMA (multiuser communication).
In (8), the access point AP checks the success or failure of data reception from each terminal and returns an ACK. The ACK describes the success or failure of receiving data transmitted from each terminal in multiuser communication.

  In the present embodiment, when there is only one terminal for downlink (or uplink) link communication, that is, when single user communication is actually performed between a single terminal and an access point AP or a base station BS. Includes multi-carrier multiplexing transmission system OFDM (Orthogonal Frequency Division Multiplexing), multi-antenna multiplexing transmission system SDM (Space Division Multiplexing), multi-antenna space-time code system STC (Space Time Code), or interleaver multiplexing transmission system IDM (Interleaved Division Multiplexing) or a combination of these multiple transmission schemes is applied.

  Further, when there are a plurality of terminals in downlink (or uplink) link communication, that is, when multi-user communication is performed between a plurality of full-scale terminals and an access point AP or a base station BS, multicarrier multiple OFDMA (Orthogonal Frequency Division Multiple Access), multi-antenna multiple access method SDMA (Space Division Multiple Access), interleaver multiple access method IDMA (Interleaved Division Multiple Access), or a combination of these multiple access methods, or A combination of these multiple access schemes and the multiplex transmission scheme, for example, OFDMA-SDMA, OFDMA-SDM, OFDM-SDMA, or the like is applied. However, whatever transmission method is applied does not exceed the scope of the present invention. Further, different multiple access methods may be applied to multiple access multiple times. For example, the first time is OFDMA-SDM and the second time is SDMA-OFDM.

B. Numerical Experiment Results of Invention The quantitative effect according to the embodiment of the present invention will be described. As a conventional technique to be compared, a system that acquires a transmission right by the CSMA / CA method defined as DCF in the IEEE 802.11 standard is targeted.

  FIG. 7 is a conceptual diagram showing a conventional MAC (Medium Access Control) protocol. In this case, in the prior art shown in FIG. 7, acquisition of the transmission right by the CSMA / CA method is performed only once, and single-user communication is performed between the terminal that has acquired the transmission right and the access point AP. .

  FIG. 8 is a conceptual diagram showing the relationship between the number of terminals and the maximum throughput in the present invention and the prior art. Note that parameters conforming to IEEE802.11g, which is a wireless LAN standard, are applied to both the prior art and the invention. An orthogonal frequency multiple access OFDMA scheme is used as a multiple access scheme for realizing uplink multiuser communication used in the inventive technology. For simplicity, the inventive technique is set here to perform CSMA / CA up to two times.

  As shown in FIG. 8, it can be seen that the maximum throughput is improved in the present invention in which the CSMA / CA is performed twice at the maximum as compared with the conventional technique in which the CSMA / CA is performed only once. Of course, since the effectiveness of the present invention is not limited to OFDMA, the same effect can be obtained by applying space division multiple access SDMA or interleave division multiple access IDMA.

  According to the above-described embodiment, multi-user communication that uses acquired radio resources by a plurality of users is introduced, and simultaneous communication terminals in multi-user communication are determined in an autonomous and distributed manner. Effective use can be achieved and throughput can be increased.

AP access point 10 Free subchannel determination unit 11 Subchannel determination unit 12 Multiuser transmission success / failure response unit # 1, # 2 Terminal 20 Carrier sense execution unit 21 Subchannel allocation determination unit 22 Random backoff time determination unit

Claims (4)

A wireless communication system for performing uplink multi-user communication for transmitting information from a plurality of terminals to an access point,
The plurality of terminals are:
A carrier sense execution unit that performs at least one carrier sense;
A subchannel allocation determination unit that determines whether a subchannel is allocated from the access point according to carrier sense by the carrier sense execution unit;
A random backoff time determination unit that determines a random backoff time of carrier sense for the second and subsequent times by the carrier sense execution unit when it is determined by the subchannel allocation determination unit that no subchannel is allocated. ,
The access point is
A free subchannel determination unit that determines whether there is a free subchannel according to carrier sense from the terminal;
And a subchannel determining unit that determines a subchannel to be allocated to a terminal that has acquired a transmission right from among the free subchannels when the subchannel determining unit determines that there is a free channel. Communications system. The empty subchannel determination unit
A communication channel is divided into subchannels according to a time axis, a frequency axis, a space axis, or a combination thereof, and an unassigned subchannel is determined based on a propagation path condition of each subchannel. The wireless communication system according to claim 1. The random back-off time determination unit
The wireless communication system according to claim 1, wherein the random back-off time of carrier sense is determined so that the better the propagation path condition of the empty subchannel, the shorter. A wireless communication method for performing uplink multi-user communication for transmitting information from a plurality of terminals to an access point,
The plurality of terminals performing a first carrier sense;
The plurality of terminals determining whether a subchannel has been allocated from the access point according to carrier sense;
Determining a random backoff time of the carrier sense if the plurality of terminals are determined not to be assigned subchannels;
The plurality of terminals performing carrier sense for the second and subsequent times based on the random backoff time;
The access point determining whether there is a free subchannel according to carrier sense from the terminal;
And determining a subchannel to be allocated to a terminal that has acquired a transmission right from among the free subchannels when the access point determines that there is a free channel.

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