WO2011025145A2

WO2011025145A2 - Method and apparatus of transmitting a sounding frame and communicating data in a wireless local area network system

Info

Publication number: WO2011025145A2
Application number: PCT/KR2010/004501
Authority: WO
Inventors: Yong Ho Seok
Original assignee: Lg Electronics Inc.
Priority date: 2009-08-26
Filing date: 2010-07-12
Publication date: 2011-03-03
Also published as: WO2011025145A3

Abstract

A method of transmitting a sounding frame in a wireless local area network (WLAN) system, performed by a station in a first basic service set (BSS), and wireless apparatus supporting the method are provided. The method includes receiving, from an access point (AP), a request frame for requesting a transmission of a sounding frame, transmitting an indication frame indicating that a third party station in a second BSS sets a network allocation vector (NAV) in order for the third party station to prevent channel access for the NAV and transmitting the sounding frame in response to the request frame after transmitting the indication frame.

Description

METHOD AND APPARATUS OF TRANSMITTING A SOUNDING FRAME AND COMMUNICATING DATA IN A WIRELESS LOCAL AREA NETWORK SYSTEM

The present invention relates to a wireless local area network (WLAN), and more particularly, to a method and apparatus of transmitting a sounding frame and communicating data in a wireless local area network system.

With the advancement of information communication technologies, various wireless communication technologies have recently been developed. Among the wireless communication technologies, a wireless local area network (WLAN) is a technology whereby Internet access is possible in a wireless fashion in homes or businesses or in a region providing a specific service by using a portable terminal such as a personal digital assistant (PDA), a laptop computer, a portable multimedia player (PMP), etc.

Ever since the institute of electrical and electronics engineers (IEEE) 802, i.e., a standardization organization for WLAN technologies, was established in February 1980, many standardization works have been conducted. In the initial WLAN technology, a frequency of 2.4GHz was used according to the IEEE 802.11 to support a data rate of 1 to 2Mbps by using frequency hopping, spread spectrum, infrared communication, etc. Recently, the WLAN technology can support a data rate of up to 54Mbps by using orthogonal frequency division multiplex (OFDM).

To maximize performance and communication capability of a wireless communication system, a multiple input multiple output (MIMO) system has drawn attention in recent years. Being evolved from the conventional technique in which a single transmit (Tx) antenna and a single receive (Rx) antenna are used, a MIMO technique uses multiple Tx antennas and multiple Rx antennas to improve transfer efficiency of data to be transmitted or received. The MIMO system is also referred to as a multiple antenna system. In the MIMO technique, instead of receiving one whole message through a single antenna path, data segments are received through a plurality of antennas and are then collected as one piece of data. As a result, a data transfer rate can be improved in a specific range, or a system range can be increased with respect to a specific data transfer rate.

The MIMO technique includes transmit diversity, spatial multiplexing, and beamforming. The transmit

diversity is a technique in which the multiple Tx antennas transmit the same data so that transmission reliability increases. The spatial multiplexing is a technique in which the multiple Tx antennas simultaneously transmit different data so that data can be transmitted at a high speed without increasing a system bandwidth. The beamforming is used to add a weight to multiple antennas according to a channel condition so as to increase a signal to interference plus noise ratio (SINR) of a signal.

Spatial multiplexing includes single-user spatial multiplexing and multi-user spatial multiplexing. The single-user spatial multiplexing is also referred to as a single user-MIMO (SU-MIMO). The multi-user spatial multiplexing is also referred to as spatial division multiple access (SDMA) or multi user-MIMO (MU-MIMO).

IEEE 802.11n is a technical standard relatively recently introduced to increase network speed and reliability and to extend an operational distance of a wireless network. More specifically, the IEEE 802.11n supports a high throughput (HT), i.e., a data processing rate of up to 540 Mbps or higher, and is based on the MIMO technique to minimize a transmission error and to optimize a data rate. In addition, this standard may use a coding scheme which transmits several duplicate copies to increase data reliability and also may use the OFDM to support a higher data rate.

A basic access mechanism of IEEE 802.11 medium access control (MAC) is a carrier sense multiple access with collision avoidance (CSMA/CA) mechanism combined with binary exponential backoff. The CSMA/CA mechanism is also referred to as a distributed coordinate function (DCF) of the IEEE 802.11 MAC, and basically employs a “listen before talk” access mechanism. In this type of access mechanism, a station (STA) listens a wireless channel or medium before starting transmission. As a result of listening, if it is sensed that the medium is not in use, a listening STA starts its transmission. Otherwise, if it is sensed that the medium is in use, the STA does not start its transmission but enters a delay period determined by the binary exponential backoff algorithm.

The CSMA/CA mechanism also includes virtual carrier sensing in addition to physical carrier sensing in which the STA directly listens the medium. The virtual carrier sensing is designed to compensate for a limitation in the physical carrier sensing such as a hidden node problem. For the virtual carrier sending, the IEEE 802.11 MAC uses a network allocation vector (NAV). The NAV is a value transmitted by an STA, currently using the medium or having a right to use the medium, to anther STA to indicate a remaining time before the medium returns to an available state. Therefore, a value set to the NAV corresponds to a period reserved for the use of the medium by an STA transmitting a corresponding frame.

One of procedures for setting the NAV is a exchange procedure of a request to send (RTS) frame and a clear to send (CTS) frame. The RTS frame and the CTS frame include information capable of delaying frame transmission from receiving STAs by reporting upcoming frame transmission to the receiving STAs. The information may be included in a duration filed of the RTS frame and the CTS frame. After performing the exchange of the RTS frame and the CTS frame, a source STA transmits a to-be-transmitted frame to a destination STA.

As IEEE 802.11 standards have been gradually established and developed, stations conforming to different standards may coexist. A WLAN system needs to consider compatibility with legacy stations operating in the existing WLAN system. The legacy station may be unable to operate in a new WLAN system, or occasionally, the legacy system may interrupt a normal operation of the WLAN system.

The growing number of users and service providers of the WLAN has resulted in the increase in the number of hot spots where the WLAN is available, thereby increasing an overlapping basic service set (OBSS) environment. The OBSS is a BSS operating on the same channel as the station’s BSS and within (either partly or wholly) its basic service area (BSA). In the OBSS environment, the aforementioned compatibility between the legacy station and the existing WLAN system can be a more important issue.

Accordingly, there is a need for a data communication method considering a protection mechanism so that an operation of the legacy system does not act as interference in an OBSS environment where a BSA of an access point (AP) and station supporting MU-MIMO overlaps with that of the existing WLAN system not supporting the MU-MIMO.

The present invention provides a method of avoiding interference caused by a neighbor basic service set (BSS) when data communication is performed using a multi user-multi input multi output (MU-MIMO) scheme in a wireless local area network (WLAN) system operating in an overlapping basic service set (OBSS) environment.

In an aspect of the present invention, a method of transmitting a sounding frame in a wireless local area network (WLAN) system, performed by a station in a first basic service set (BSS), includes receiving, from an access point (AP), a request frame for requesting a transmission of a sounding frame, transmitting an indication frame indicating that a third party station in a second BSS sets a network allocation vector (NAV) in order for the third party station to prevent channel access for the NAV, and transmitting the sounding frame in response to the request frame, after transmitting the indication frame.

The request message may include a list of stations to transmit the sounding frame and to receive data frames.

The request message may further include a scheduled time for each station in the list of stations, the scheduled time indicating when the indication frame is transmitted by each station in the list of stations.

The request frame may further include information for the station to configure the NAV in the indication frame.

The method may further include receiving a steered SDMA data frame from the AP, wherein the AP may simultaneously transmit a plurality of steered SDMA data frames to stations in the list of stations.

The indication frame may include an announcement field indicating that a following frame of the indication frame is the sounding frame.

The indication frame may include a CTS-to-self frame that is a Clear to Send (CTS) frame whose a receiver address (RA) field indicates an address of the station which transmits the CTS frame.

The CTS-to-self frame may be included in a control wrapper frame, and the control wrapper frame further comprises an announcement field indicating that a following frame of the control wrapper frame is the sounding frame.

The sounding frame may be a physical layer protocol data unit (PPDU) that carries no data field.

In another aspect of the present invention, a method of communicating data in a WLAN system, performed by an AP, incldues transmitting a request frame for requesting transmission of sounding frames to the plurality of stations, receiving the sounding frames from the plurality of stations in response to the request message, estimating channels by using the sounding frame to acquire steered SDMA data frames for the plurality of stations, and transmitting the steered SDMA data frames to the plurality of stations.

In still another aspect of the present invention, a wireless apparatus in a wireless local area network (WLAN) system, includes a transceiver for transmitting and receiving a radio signals, and a processor operatively coupled with the transceiver and configured to receive, from an access point (AP), a request frame for requesting a transmission of a sounding frame, to transmit an indication frame indicating that a third party station in a second BSS sets a network allocation vector (NAV) in order for the third party station to prevent channel access for the NAV, and to transmit the sounding frame in response to the request frame, after transmitting the indication frame.

According to the present invention, data transmission/reception can be achieved by using a multi user-multi input multi output (MU-MIMO) scheme without interference caused by a neighbor basic service set (BSS) in an overlapping basic service set (OBSS) environment. Therefore, a data transfer rate can be improved and radio resources can be utilized more efficiently.

FIG. 1 shows an exemplary structure of a wireless local area network (WLAN) system in an overlapping basic service set (OBSS) environment according to an embodiment of the present invention.

FIG. 2 shows an example of downlink (DL) transmission using multi use- multiple input multiple output (MU-MIMO).

FIG. 3 shows an example of a sounding frame transmission method according to an embodiment of the present invention.

FIG. 4 shows a spatial division multiple access (SDMA) data frame transmission method using downlink MU-MIMO according to an embodiment of the present invention.

FIG. 5 is a schematic block diagram of wireless apparatuses implementing an exemplary embodiment of the present invention.

Hereinafter, exemplary embodiments of the present invention will be described in detail according to the accompanying drawings.

The WLAN system includes one or more basic service sets (BSSs). The BSS is a set of stations (STAs) which are successfully synchronized to communicate with one another, and is not a concept indicating a specific region. The BSS can be classified into an infrastructure BSS and an independent BSS (IBSS). The infrastructure BSS includes one or more stations, an access point (AP) as a station for providing a distributions service, and a distribution system (DS) for connecting a plurality of APs. The IBSS consists of only mobile stations, not including the AP, and constructs a self-contained network not capable of accessing the DS.

The infrastructure BSS including the AP is shown in FIG. 1. A basic service area (BSA) shown in FIG. 1 is a local area within a service coverage of the AP. The BSA is an area containing members of the BSS. It may contain members of other BSSs. In FIG. 1, a BSA_1 115 of a BSS_1 110 and a BSA_2 165 of a BSS_2 160 partially overlap with each other, and may use the same channel. That is, an STA_A 130, an STA_X 180, and an STA_Y 190 of FIG. 1 operate in an OBSS.

In downlink MU-MIMO, an AP transmits a training request frame (TRQ) 210 to destination stations, i.e., an STA_1 and an STA_2, which intend to transmit data after channel access. In response to the TRQ 210, the destination stations transmit a transmit sounding physical layer convergence procedure (PLCP) protocol data unit (PPDU) 220 and a sounding PPDU 230. 3rd party stations, not the destination stations, set a network allocation vector (NAV) 270 upon hearing the TRQ 210 and suspend the channel access. Upon receiving the sounding PPDU 220 and the sounding PPDU 230, the AP acquires channel estimation information on the STA_1 and the STA_2. On the basis of the acquired channel estimation information, the AP may transmit a steered spatial division multiple access (SDMA) data frame 240 to the STA_1 and the STA_2. When transmission is successful, the destination stations to the SDMA data is transmitted respond to the AP by transmitting a block acknowledgment (ACK) frame 250 and a block ACK frame 260.

A problem may occur when the 3rd party stations fail to receive the TRQ 210. Referring to FIG. 1, it is assumed that the STA_A 130 of the BSS_1 110 fails to receive the TRQ when an AP_2 170 of the BSS_2 160 transmits the TRQ. If the STA_A 130 fails to receive the TRQ transmitted by the AP_2 170 of the BSS_2 160, the STA_A 130 cannot set an NAV. In such a situation, interference may occur when the STA_1 130 attempts channel access while the AP_2 170 transmits an SDMA data frame to the STA_X 180 and the STA_Y 190. Such a problem may occur frequently in an OBSS environment. For example, if legacy stations are an AP and stations of a neighbor BSS of which a BSA partly or wholly overlaps, the AP and stations of the neighbor BSS may fail to receive the TRQ.

After performing channel access on the basis of the IEEE 802.11 standard, an AP transmits to stations a request frame 310 for requesting a destination station to transmit a sounding frame. The request frame 310 includes information required to set an NAV by a 3rd party station when a station hearing the request frame 310 sets the NAV or when the destination station transmits an indication frame. The request frame 310 may include a transmission duration and a list of destination stations to which an SDMA data frame 340 is to be transmitted using downlink MU-MIMO. The transmission duration indicates a time required for transmitting the SDMA data frame 340. The 3rd party station hearing the request frame 310 may suspend the channel access during the transmission duration by setting an NAV 370.

Destinations stations STA_1 and STA_2 receiving the request frame transmit an indication frame 320 and an indication frame 330 before transmission of a sounding frame 325 and a sounding frame 335. In this case, the transmitted indication frames 320 and 330 are frames having a format that can be heard even by a legacy AP and legacy station not supporting MU-MIMO transmission. Therefore, even if the 3rd party station is either the legacy AP or the legacy station which cannot hear the request frame 310 or when the 3rd party station cannot hear the request frame for other reasons, the 3rd party station can set an NAV 380 by hearing the indication frame 320 and the indication frame 330 transmitted by the destination stations STA_1 and STA_2.

For example, the indication frame 320 may be transmitted as a clear to send (CTS)-to-self frame. The CTS-to-self frame is a CTS frame of which a receiver address (RA) field indicates a MAC address of a station which transmits the CTS-to-self frame. Since it is transmitted in the CTS frame format, legacy stations conforming to a previous version of the IEEE 802.11n standard can set an NAV by hearing this frame.

The indication frame 320 and the indication frame 330 may include an announcement field indicating that the sounding frame 325 and the sounding frame 335 are next frames to be transmitted subsequent to the indication frame 320 and the indication frame 330. The announcement field may be included as one field in the indication frame. Alternatively, the announcement field may use a control wrapper frame of the IEEE 802.11n standard by considering backward compatibility. The control wrapper frame is used to carry any other control frame (i.e., excluding the control wrapper frame) together with a high throughput (HT) control field. When the CTS-to-self frame is used as the aforementioned indication frame, the CTS-to-self frame may be transmitted by being wrapped in the control wrapper frame. In this case, the HT control field includes a null data packet (NDP) announcement subfield to indicate whether an NDP is transmitted subsequently. The NDP is an example of the sounding frame of the present invention, and a detailed explanation thereof will be described below.

The destination station STA_1 transmits the sounding frame 325 after transmitting the indication frame 320. Likewise, the destination station STA_2 also transmits the sounding frame 335 after transmitting the indication frame 330. The sounding frame’s uses include calculation of transmit steering, calculation of recommended modulation and coding scheme (MCS), and calculation of calibration parameters. The sounding frame may be transmitted by using a null data packet (NDP). The NDP is a physical layer convergence procedure (PLCP) protocol data unit (PPDU) that carries no data field. That is, the NDP is a frame in which only a PHY header exists and of which a PHY PLCP service data unit (PSDU) is null. Since there is no information on the MAC header, the NDP is transmitted subsequently when a short inter-frame space (SIFS) elapses after a non-NDP PPUD is transmitted. In the embodiment of the present invention, the destination station may transmit an indication frame and thereafter transmit the NDP after the SIFS elapses.

In a case where a plurality of destination stations transmit an indication frame and a sounding frame, transmission may be performed in sequence to increase transmission reliability. Such transmission can be achieved in various manners, for example, by allowing a request frame to include information indicating a transmission order of an indication frame and a sounding frame of each destination station or information indicating a transmission time, etc. Related explanations will be described below with reference to FIG. 4.

The AP acquires channel estimation information with respect to the STA_1 and the STA_2 by using the sounding frame 325 and the sounding frame 335. Based on the acquired information, the AP transmits the steered SDMA data frame 340 to the destination stations STA_1 and STA_2 by using downlink MU-MIMO. Upon receiving the SDMA data frame by using the downlink MU-MIMO, the destination stations STA_1 and STA_2 transmit a block ACK frame 350 and a block ACK frame 360 to the AP.

FIG. 4 shows an SDMA data frame transmission method using downlink MU-MIMO according to an embodiment of the present invention.

Channel access based on enhanced distributed channel access (EDCA) described herein is introduced as an example of a channel access method specified in the IEEE 802.11 standard, and technical features of the present invention are not affected even if the channel access is performed using another channel access method. Likewise, regarding a TRQ frame, a request frame is transmitted as the TRQ frame for exemplary purposes only. Further, regarding transmission of a CTS-to-self frame and an NDP frame, an indication frame and a sounding frame are transmitted respectively as the CTS-to-self frame and the NDP frame for exemplary purposes only, and thus technical features of the present invention are not limited thereto.

When the channel access is achieved by performing the EDCA conforming to the IEEE 802.11 standard, an AP transmits a TRQ frame 410 to destination stations which intend to transmit an SDMA data frame. It is assumed herein that the destinations stations are an STA_1 and an STA_2. The TRQ frame further includes information indicating that the destination stations are the STA_1 and the STA_2 (i.e., a list of destination stations) and information regarding a transmission order of the indication frame and the sounding frame of the STA_1 and the STA_2 in addition to a transmission duration in which data transmission is performed. A transmission time of each of the stations in the destination station list can be defined or an order of transmissions can be defined. In the example of FIG. 4, the STA_1 performs transmission first and then the STA_2 performs transmission subsequently. According to an order assigned to the TRQ frame 410, the STA_1 first transmits a CTS-to-self frame 420 as an indication frame, and after an SIFS elapses, transmits an NDP frame 425 as a sounding frame. Subsequently, the STA_2 transmits a CTS-to-self frame 430 as an indication frame, and after an SIFS elapses, transmits an NDP frame 435 as a sounding frame. Thereafter, by considering a channel estimation result, the AP transmits a steered SDMA data frame 440 to the STA_1 and the STA_2 by using MU-MIMO. Upon receiving the SDMA data frame 440, the STA_1 transmits a block ACK frame 450 and the STA_2 transmits a block ACK frame 460.

FIG. 5 is a schematic block diagram of wireless apparatuses implementing an exemplary embodiment of the present invention. An AP 500 includes a processor 510, a memory 520, and a transceiver 530, and an STA 550 includes a processor 560, a memory 570, and a transceiver 580. The

transceivers

530 and 580 transmit/receive a radio signal and implement an IEEE 802 physical layer. The processors 510 and 560 are connected with the

transceivers

530 and 580 to implement an IEEE 802 MAC layer. The processors 510 and 560 may implement the above-described transmitting a sounding frame and communicating data.

The processors 510 and 560 and/or the

transceivers

530 and 580 may include an application-specific integrated circuit (ASIC), a different chip set, a logical circuit, and/or a data processing unit. The

memories

520 and 570 may include a read-only memory (ROM), a random access memory (RAM), a flash memory, a memory card, a storage medium, and/or any other storage units. When an exemplary embodiment is implemented by software, the above-described scheme may be implemented as a module (process, function, etc.) performing the above-described functions. The module may be stored in the

memories

520 and 570 and executed by the processors 510 and 560. The

memories

520 and 570 may be disposed within or outside the processors 510 and 560 and connected with the processors 510 and 560 via well-known means.

The aforementioned embodiments include various exemplary aspects. Although all possible combinations for representing the various aspects cannot be described, it will be understood by those skilled in the art that other combinations are also possible. Therefore, all replacements, modifications and changes should fall within the spirit and scope of the claims of the present invention.

Claims

A method of transmitting a sounding frame in a wireless local area network (WLAN) system, performed by a station in a first basic service set (BSS), the method comprising:receiving, from an access point (AP), a request frame for requesting a transmission of a sounding frame;transmitting an indication frame indicating that a third party station in a second BSS sets a network allocation vector (NAV) in order for the third party station to prevent channel access for the NAV; andafter transmitting the indication frame, transmitting the sounding frame in response to the request frame.
The method of claim 1, wherein the request message includes a list of stations to transmit the sounding frame and to receive data frames.
The method of claim 2, wherein the request message further includes a scheduled time for each station in the list of stations, the scheduled time indicating when the indication frame is transmitted by each station in the list of stations.
The method of claim 2, wherein the request frame further includes information for the station to configure the NAV in the indication frame.
The method of claim 2, further comprisingreceiving a steered SDMA data frame from the AP, wherein the AP simultaneously transmits a plurality of steered SDMA data frames to stations in the list of stations.
The method of claim 1, wherein the indication frame comprises an announcement field indicating that a following frame of the indication frame is the sounding frame.
The method of claim 1, wherein the indication frame comprises a CTS-to-self frame that is a Clear to Send (CTS) frame whose a receiver address (RA) field indicates an address of the station which transmits the CTS frame.
The method of claim 7, wherein the CTS-to-self frame is included in a control wrapper frame, and the control wrapper frame further comprises an announcement field indicating that a following frame of the control wrapper frame is the sounding frame.
The method of claim 1, wherein the sounding frame is a physical layer protocol data unit (PPDU) that carries no data field.
A method of communicating data in a WLAN system, performed by an AP, the method comprising:transmitting a request frame for requesting transmission of sounding frames to the plurality of stations; receiving the sounding frames from the plurality of stations in response to the request message;estimating channels by using the sounding frame to acquire steered SDMA data frames for the plurality of stations; andtransmitting the steered SDMA data frames to the plurality of stations.
A wireless apparatus in a wireless local area network (WLAN) system, comprising: a transceiver for transmitting and receiving a radio signals; and a processor operatively coupled with the transceiver and configured to: receive, from an access point (AP), a request frame for requesting a transmission of a sounding frame; transmit an indication frame indicating that a third party station in a second BSS sets a network allocation vector (NAV) in order for the third party station to prevent channel access for the NAV; and after transmitting the indication frame, transmit the sounding frame in response to the request frame.
The station of claim 11, wherein the request frame includes a list indicating one or more destination stations and a scheduled time of transmitting the indication frame and the sounding frame for each of the destination stations.
The station of claim 11, wherein the request frame includes a list indicating one or more destination stations and information of a transmission duration during which an AP transmits data to the one or more destination stations.
A wireless apparatus in a wireless local area network (WLAN) system, comprising:a transceiver for transmitting and receiving a radio signals; and a processor operatively coupled with the transceiver and configured to: transmit a request frame for requesting transmission of sounding frames to the plurality of stations; and receive the sounding frames from the plurality of stations in response to the request message; estimate channels by using the sounding frame to acquire steered SDMA data frames for the plurality of stations; and transmit the steered SDMA data frames to the plurality of stations.