Protocol for cooperation communication between access points in overlapped basic service set (OBSS) environment

A method for cooperative beamforming communication, is described including negotiating, at a first access point, a cooperative transmit beamforming with a second access point having an overlapped basic service set (OBSS) area with the first access point, and transmitting, from the first access point, a clear-to-send (CTS) signal to the second access point. The method, upon detecting at the first access point a completion of transmission of the CTS signal, performs at the first access point the cooperative transmit beamforming with the second access point during a time duration of the cooperative transmit beamforming and after a predetermined time interval has elapsed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the priority benefit of Korean Patent Application No. 10-2014-0041808, filed on Apr. 8, 2014, Korean Patent Application No. 10-2014-0041809, filed on Apr. 8, 2014, Korean Patent Application No. 10-2015-0048169, filed on Apr. 6, 2015, and Korean Patent Application No. 10-2015-0048170, filed on Apr. 6, 2015, in the Korean Intellectual Property Office, the disclosures of which are incorporated herein by reference.

BACKGROUND

1. Field of the Invention

Embodiments of the present invention relate to a media access control (MAC) protocol of a wireless local area network (WLAN), and more particularly, to a protocol for transmitting data through cooperation between access points (APs) in an overlapped basic service set (OBSS) environment.

2. Description of the Related Art

Currently, multiple input multiple output (MIMO) communication technology for transmitting and receiving data using two or more antennas has been generalized in a wireless communication system. Further, according to standards starting from an Institute of Electrical and Electronics Engineers (IEEE) 802.11n standard, a wireless local area network (WLAN) allows two or more transmit antennas to be available. According to the IEEE 802.11n standard, up to four transmit antennas may be used. According to an IEEE 802.11ac standard, up to eight transmit antennas may be used. In addition, when using a plurality of transmit antennas, data may be transmitted using a transmit beamforming technology to improve the signal reception performance. The IEEE 802.11ac standard includes a downlink multi-user (MU) MIMO technology that enables an access point (AP) to concurrently transmit beamformed data frames to a plurality of stations (STAs).

However, according to a recent increase in the complexity of a WLAN environment, an overlapped basic service set (OBSS) frequently appears. Here, the OBSS indicates an overlapping area between a BSS corresponding to a service area covered by a single AP and a BSS covered by another AP. In an OBSS environment, the transmission efficiency may be degraded and thus, a method for performing an efficient transmission even in the OBSS environment is required.

SUMMARY

According to an aspect, there is provided a cooperative beamforming communication method including negotiating, by a first access point, a cooperative transmit beamforming with a second access point having an overlapped basic service set (OBSS) area with the first access point; transmitting, by the first access point, a clear-to-send (CTS) to the second access point; and performing, by the first access point, the cooperative transmit beamforming with the second access point during a cooperative transmit beamforming duration determined through the negotiation after a predetermined time interval is elapsed, when transmission of the CTS is completed.

The cooperative beamforming communication method may further include performing, by the first access point, cooperative sounding with the second access point. The performing of the cooperative sounding may include acquiring, by the first access point, a precoding matrix to be used for the cooperative transmit beamforming.

A duration field of the CTS may be set as a time when the cooperative transmit beamforming duration is terminated.

The predetermined time interval may be a short inter-frame space (SIPS).

The performing of the cooperative transmit beamforming may include transmitting, by the first access point, a first beamformed data frame to a first station positioned in the OBSS area; and transmitting, by the second access point, a second beamformed data frame to a second station positioned in the OBSS area.

According to another aspect, there is provided a cooperative beamforming communication method including negotiating, by a first access point, a cooperative transmit beamforming with a second access point having an BSS area with the first access point; resetting, by the first access point, a network allocation vector (NAV) of the first access point in response to a CTS associated with the negotiation received from the second access point; and performing, by the first access point, the cooperative transmit beamforming with the second access point during a cooperative transmit beamforming duration determined through the negotiation after a predetermined time interval is elapsed, when reception of the CTS is completed.

The cooperative beamforming communication method may further include performing, by the first access point, cooperative sounding with the second access point. The performing of the cooperative sounding may include acquiring, by the first access point, a precoding matrix to be used for the cooperative transmit beamforming.

A duration field of the CTS may be set as a time when the cooperative transmit beamforming duration is terminated.

The predetermined time interval may be a SIFS.

The performing of the cooperative transmit beamforming may include transmitting, by the first access point, a first beamformed data frame to a first station positioned in the OBSS area; and transmitting, by the second access point, a second beamformed data frame to a second station positioned in the OBSS area.

According to still another aspect, there is provided a cooperative beamforming communication method including negotiating a cooperative transmit beamforming between a plurality of access points having an OBSS area; transmitting, by at least one of the access points, a CTS to one or more other access points among the access points; resetting, by the one or more other access points, a NAV of the one or more other access points in response to the CTS; and performing, by the plurality of access points, the cooperative transmit beamforming during a cooperative transmit beamforming duration determined through the negotiation.

The cooperative beamforming communication method may further include performing cooperative sounding between the plurality of access points. The performing of the cooperative sounding may include acquiring, by each of the plurality of access points, a precoding matrix to be used for the cooperative transmit beamforming.

A duration field of the CTS may be set as a time when the cooperative transmit beamforming duration is terminated.

The performing of the cooperative transmit beamforming may include transmitting, by the one or more access points, a first beamformed data frame after a predetermined time interval is elapsed, when transmission of the CTS is completed; and transmitting, by the one or more other access points, a second beamformed data frame after a predetermined time interval is elapsed, when reception of the CTS is completed. The predetermined time interval may be a SIFS.

According to still another aspect, there is provided an access point to perform cooperative beamforming communication, the access point including a communicator configured to transmit and receive a frame associated with negotiation about a cooperative transmit beamforming with one or more other access points having an OBSS area with the access point, to transmit a CTS to the one or more other access points, and to perform the cooperative transmit beamforming with the one or more other access points; and a controller configured to control the communicator to generate a frame associated with the negotiation and to transmit the generated frame to the one or more other access points, to control the communicator to process the frame associated with the negotiation and transmit a CTS to the one or more other access points in response to the frame associated with the negotiation received from the other access point to the communicator, and to control the communicator to perform the cooperative transmit beamforming with the one or more other access points during a cooperative transmit beamforming duration determined through the negotiation after a predetermined time interval is elapsed, when transmission of the CTS is completed.

The controller may also control the communicator to perform cooperative sounding with the one or more other access points. The performing of the cooperative sounding may include acquiring, by the access point, a precoding matrix to be used for the cooperative transmit beamforming.

A duration field of the CTS may be set as a time when the cooperative transmit beamforming duration is terminated.

The predetermined time interval may be a SIFS.

According to still another aspect, there is provided an access point to perform cooperative beamforming communication, the access point including a communicator configured to transmit and receive a frame associated with negotiation about a cooperative transmit beamforming with one or more other access points having an OBSS area with the access point, to receive a CTS from the one or more other access points, and to perform the cooperative transmit beamforming with the one or more other access points; and a controller configured to control the communicator to generate a frame associated with the negotiation and to transmit the generated frame to the one or more other access points, and to control the communicator to process the frame associated with the negotiation in response to the frame associated with the negotiation received the one or more other access points to the communicator, to reset a NAV in response to the CTS received by the communicator from the other access point, to reset a NAV in response to the CTS received by the communicator from the one or more other access points, and to perform the cooperative transmit beamforming with the one or more other access points during a cooperative transmit beamforming duration determined through the negotiation after a predetermined time interval is elapsed, when reception of the CTS is completed.

The controller may also control the communicator to perform cooperative sounding with the one or more other access points. The performing of the cooperative sounding may include acquiring, by the access point, a precoding matrix to be used for the cooperative transmit beamforming.

A duration field of the CTS may be set as a time when the cooperative transmit beamforming duration is terminated.

The predetermined time interval may be a SIFS.

According to still another aspect, there is provided a cooperative beamforming communication method including negotiating, by a first access point, a cooperative transmit beamforming with a second access point having an OBSS area with the first access point; transmitting, by the first access point, a cooperative transmission request frame to the second access point; receiving, by the first access point, a cooperative transmission response frame that is transmitted from the second access point to the first access point in response to the cooperative transmission request frame; and performing, by the first access point, the cooperative transmit beamforming with the second access point during a cooperative transmit beamforming duration determined through the negotiation after a predetermined time interval is elapsed, when reception of the cooperative transmission response frame is completed.

The cooperative beamforming communication method may further include performing, by the first access point, cooperative sounding with the second access point. The performing of the cooperative sounding may include acquiring, by each of the first access point and the second access point, a precoding matrix to be used for the cooperative transmit beamforming.

A duration field of the cooperative transmission request frame and a duration field of the cooperative transmission response frame may be set as a time when the cooperative transmit beamforming duration is terminated.

The predetermined time interval may be a SIFS.

According to still another aspect, there is provided a cooperative beamforming communication method including negotiating, by a first access point, a cooperative transmit beamforming with one or more access points having an OBSS area with the first access point; broadcasting, by the first access point, a cooperative transmission request frame; receiving, by the first access point, a cooperative transmission response frame that is transmitted from the one or more access points to the first access point in response to the cooperative transmission request frame; and performing, by the first access point, the cooperative transmit beamforming with the one or more access points during a cooperative transmit beamforming duration determined through the negotiation after a predetermined time interval is elapsed, when reception of the cooperative transmission response frame is completed.

The one or more access points may include a second access point and a third access point. The receiving of the cooperative transmission response frame may include receiving a first cooperative transmission response frame transmitted from the second access point to the first access point; and receiving a second cooperative transmission response frame transmitted from the third access point to the first access point after the second access point transmits the first cooperative transmission response frame to the first access point. Here, the cooperative beamforming communication method may further include transmitting, by the first access point, a cooperative transmission start frame. The performing of the cooperative transmit beamforming may include transmitting, by each of the first access point, the second access point, and the third access point, a beamformed data frame when transmission of the cooperative transmission start frame is completed.

The one or more access points may include a second access point and a third access point. The receiving of the cooperative transmission response frame may include receiving a first cooperative transmission response frame transmitted from the second access point to the first access point; transmitting, by the first access point, a cooperative transmission response poll frame to the third access point in response to the first cooperative transmission response frame transmitted from the second access point to the first access point; and receiving a second cooperative transmission response frame transmitted from the third access point to the first access point in response to the cooperative transmission response poll frame. Here, the performing of the cooperative transmit beamforming includes transmitting, by each of the first access point, the second access point, and the third access point, a beamformed data frame when reception of the second cooperative transmission response frame is completed.

The cooperative beamforming cooperation method may further include performing cooperative sounding between the first access point and the one or more access points. The performing of the cooperative sounding may include acquiring, by each of the first access point and the one or more access points, a precoding matrix to be used for the cooperative transmit beamforming.

A duration field of the cooperative transmission request frame and a duration field of the cooperative transmission response frame may be set as a time when the cooperative transmit beamforming duration is terminated.

According to still another aspect, there is provided an access point to perform cooperative beamforming communication, the access point including a communicator configured to transmit and receive a frame associated with negotiation about a cooperative transmit beamforming with one or more other access points having an OBSS area with the access point, to transmit a cooperative transmission request frame to the one or more other access points, and to receive a cooperative transmission response frame that is transmitted from the one or more other access points in response to the cooperative transmission request frame; and a controller configured to control the communicator to generate a frame associated with the negotiation and to transmit the generated frame to the one or more other access points, and to control the communicator to process the frame associated with the negotiation in response to the frame associated with the negotiation received by the communicator from the one or more other access points, to generate the cooperative transmission request frame to be transmitted to the one or more other access points, and to perform the cooperative transmit beamforming with the one or more other access points during a cooperative transmit beamforming duration determined through the negotiation after a predetermined time interval is elapsed, when reception of the cooperative transmission response frame is completed.

The controller may also control the communicator to perform cooperative sounding with the one or more other access points. The performing of the cooperative sounding may include acquiring, by the access point, a precoding matrix to be used for the cooperative transmit beamforming.

A duration field of the cooperative transmission request frame and a duration field of the cooperative transmission response frame may be set as a time when the cooperative transmit beamforming duration is terminated.

The predetermined time interval may be a SIFS.

According to still another aspect, there is provided an access point to perform cooperative beamforming communication, the access point including a communicator configured to transmit and receive a frame associated with negotiation about a cooperative transmit beamforming with one or more other access points having an OBSS area with the access point, to broadcast a cooperative transmission request frame, and to receive a cooperative transmission response frame that is transmitted from the one or more access points to the first access point in response to the cooperative transmission request frame; and a controller configured to control the communicator to generate a frame associated with the negotiation and to transmit the generated frame to the one or more other access points, and to control the communicator to process the frame associated with the negotiation in response to the frame associated with the negotiation received by the communicator from the one or more other access points, to generate the cooperative transmission request frame, and to perform the cooperative transmit beamforming with the one or more access points during a cooperative transmit beamforming duration determined through the negotiation, when reception of the cooperative transmission response frame is completed.

The one or more access points may include a second access point and a third access point. The communicator may also receive a first cooperative transmission response frame transmitted from the second access point, and may receive a second cooperative transmission response frame transmitted from the third access point after the second access point transmits the first cooperative transmission response frame.

The communicator may also transmit a cooperative transmission start frame. Also, the controller may control the communicator to transmit a beamformed data frame when transmission of the cooperative transmission start frame is completed.

The one or more access points may include a second access point and a third access point. The communicator may receive a first cooperative transmission response frame transmitted from the second access point, may transmit a cooperative transmission response poll frame to the third access point in response to the first cooperative transmission response frame, and may receive a second cooperative transmission response frame transmitted from the third access point in response to the cooperative transmission response poll frame. Also, the controller may control the communicator to transmit a beamformed data frame when reception of the second cooperative transmission response frame is completed.

The controller may control the communicator to perform cooperative sounding with the one or more access points. The performing of the cooperative sounding may include acquiring, by the access point, a precoding matrix to be used for the cooperative transmit beamforming.

A duration field of the cooperative transmission request frame and a duration field of the cooperative transmission response frame may be set as a time when the cooperative transmit beamforming duration is terminated.

DETAILED DESCRIPTION

The terms used herein may be most widely used general terms that are selected based on functions in the proposed disclosure. However, the terms may be modified based on the intent or custom of those skilled in the art or the appearance of new art.

Also, in some cases, some terms may be arbitrarily used by the applicant to help the understanding and/or for convenience of description. In this example, the detailed meaning thereof will be described in a corresponding description portion. Accordingly, the terms used in the following description should be understood based on meanings of the terms and the overall description of the specification.

FIG. 1illustrates an environment in which basic service sets (BSSs) covered by access points (APs) of a wireless local area network (WLAN) overlap.

Referring toFIG. 1, a BSS121is covered by an AP111and a BSS122is covered by an AP112. Stations (STAs)131and133may belong to the BSS121, and STAs132and134may belong to the BSS122. An overlapping area may be present between the BSSs121and122of the APs111and112. The STA131belonging to the BSS121and the STA132belonging to the BSS122may be positioned in the overlapping area. Here, when the BSSs121and122use the same frequency channel, the APs111and112and the STAs131and132may hear all of the transmitting frames. Accordingly, the transmission efficiency may be degraded in the overlapping area. A situation in which the degradation in the transmission efficiency occurs will be further described with reference toFIG. 2.

An AP according to an embodiment may include a controller and a communicator. The controller may generate a frame to be transmitted from the communicator, and the controller may also process a frame received by the communicator. The controller may control the communicator to communicate with a communicator with an STA using a multiple input multiple output (MIMO) communication method, and may determine an STA to perform communication through the communicator. The controller may be embodied using an integrated circuit (IC) chip, a microprocessor, and a mini computer. The communicator of the AP may communicate with a communicator with another AP or the communicator of the STA. The communicator may include a variety of communication modules such as an antenna, a modulator/demodulator, a frequency processing apparatus, and a filtering apparatus.

FIG. 2illustrates a situation in which the transmission efficiency degraded in an environment in which BSSs covered by APs of a WLAN overlap according to the related art.

Referring toFIG. 2, in an example in which an overlapped BSS (OBSS) area is present between AP1and AP2, and STA1and STA2are present in the OBSS area, when AP1transmits a frame, channels of AP2and STA2are in a busy state. Similarly, when STA1transmits a frame, the channels of AP2and STA2are in a busy state. Also, when AP2transmits a frame, channels of AP1and STA1are in a busy state. Similarly, when STA2transmits a frame, the channels of AP1and STA1are in a busy state. That is, compared to an environment in which an OBSS is absent, a busy state in which a transmission is impossible increases, which may lead to degrading the transmission efficiency. In particular, when a duration field of a frame transmitted from a neighboring BSS is not zero, a network allocation vector (NAV) is set as a value of the duration field and thus, the busy state in which the transmission is impossible may continue until the NAV is terminated, thereby degrading the transmission efficiency. Accordingly, there is a need for a method of preventing the transmission efficiency from being degraded in an OBSS environment. Proposed herein is a method that enables two or more APs to concurrently transmit data even in the OBSS environment, thereby enhancing the transmission efficiency in the OBSS environment.

FIG. 3is a flowchart illustrating a method of performing cooperative transmit beamforming between APs according to an embodiment.

In operation310, a negotiation about a cooperative transmit beamforming may be performed between neighboring APs having an OBSS area. Through the negotiation, APs to participate in the cooperative transmit beamforming and a cooperative transmit beamforming duration may be determined According to an embodiment, a leading AP to lead the cooperative transmit beamforming may be determined among the APs determined to participate in the cooperative transmit beamforming, through the negotiation. Operation310may be performed through wired or wireless communication between the APs.

In operation320, a cooperative sounding process for the cooperative transmit beamforming between the APs associated with the negotiation, that is, the APs to participate in the cooperative transmit beamforming may be performed. The cooperative sounding process may include a process of acquiring channel information between a terminal, that is, a beamformer to perform transmission and a terminal, that is, a beamformee to perform reception in the cooperative transmit beamforming. According to an embodiment, the cooperative sounding process may include an operation of acquiring a precoding matrix to be used for the cooperative transmit beamforming. The precoding matrix may be configured to eliminate or minimize interference during the cooperative transmit beamforming Operation320may be performed through wired or wireless communication between the APs.

In operation330, NAVs of APs associated with the negotiation may be reset and NAVs of WLAN equipments unassociated with the negotiation may be set. That is, a protection may be performed to enable the transmission during the cooperative transmit beamforming duration by resetting the NAVs of the APs participating in the cooperative transmit beamforming, and to disable the transmission during the cooperative transmit beamforming duration by setting the NAVs of the WLAN equipments that do not participate in the cooperative transmit beamforming.

According to an embodiment, an AP leading a cooperative transmit beamforming among APs participating in the cooperative transmit beamforming may transmit a clear-to-send (CTS) in which an AP participating in cooperative transmit beamforming is set as a receiver address (RA). According to an Institute of Electrical and Electronics Engineers (IEEE) 802.11 standard, a WLAN equipment receiving a CTS in which the WLAN equipment is set as a receiver address may reset a NAV, and a WLAN equipment receiving a CTS in which the WLAN equipment is not set as a receiver address may set a NAV based on a value set in a duration field of the CTS. Accordingly, when the AP leading the cooperative transmit beamforming transmits a CTS in which an AP participating in the cooperative transmit beamforming is set as a receiver address and a duration field is set as a time when the cooperative transmit beamforming duration is terminated, the AP participating in the cooperative transmit beamforming may reset a NAV and a WLAN equipment that does not participate in the cooperative transmit beamforming may set a NAV until the cooperative transmit beamforming duration is terminated.

According to another embodiment, an AP leading a cooperative transmit beamforming among APs participating in the cooperative transmit beamforming may transmit a cooperative transmission (CoTx) request frame in which an AP participating in the cooperative transmit beamforming is set as a receiver address. When the AP leading the cooperative transmit beamforming transmits a CoTx request frame in which an AP participating in the cooperative transmit beamforming is set as a receiver address and a duration field is set as a time when a cooperative transmit beamforming duration is terminated, the AP participating in the cooperative transmit beamforming may reset a NAV and a WLAN equipment that does not participate in the cooperative transmit beamforming may set a NAV until the cooperative transmit beamforming duration is terminated.

Accordingly, the APs associated with the negotiation may concurrently perform the cooperative transmit beamforming, and the WLAN equipments unassociated with the negotiation may not transmit a frame during the cooperative transmit beamforming duration.

In operation340, the APs associated with the negotiation may perform the cooperative transmit beamforming during the cooperative transmit beamforming duration. Each of the APs associated with the negotiation may transmit beamformed data to an STA included in a BSS covered by a corresponding AP. For example, when AP1and AP2are associated with a negotiation, AP1may transmit a beamformed data frame to STA1and an AP2may transmit a beamformed data frame to STA2. The beamformed data frames may be concurrently transmitted from AP1and AP2during the cooperative transmit beamforming duration. According to an embodiment, a transmission of a beamformed data frame may be performed using a downlink MU-MIMO technology.

As described above, according to embodiments, the transmission efficiency in an OBSS environment may be enhanced by providing a cooperative transmit beamforming method that enables two or more APs to concurrently perform a transmit beamforming in the OBSS environment.

FIG. 4illustrates an example of a protocol for performing cooperative transmit beamforming between APs according to an embodiment.

Referring toFIG. 4, AP1and AP2may be determined to participate in cooperative transmit beamforming through negotiation about the cooperative transmit beamforming Here, a cooperative sounding process for the cooperative transmit beamforming between AP1and AP2may be performed. For example, each of AP1and AP2may acquire a precoding matrix to be used for the cooperative transmit beamforming through the cooperative sounding process. AP1leading the cooperative transmit beamforming may transmit a CTS in which AP2is set as a receiver address (RA), so that AP2may reset a NAV. Here, a duration field of the CTS is set as a time when the cooperative transmit beamforming duration is terminated and thus, a WLAN equipment unassociated with the negotiation may set a NAV until the cooperative transmit beamforming duration is terminated. AP1and AP2may concurrently perform the cooperative transmit beamforming during the cooperative transmit beamforming duration. For example, AP1may transmit a beamformed data frame to STA1belonging to a BSS covered by AP1and, at the same time, AP2may transmit a beamformed data frame to STA2belonging to a BSS covered by AP2. According to an embodiment, AP1may transmit a beamformed data frame to STA1after a predetermined time interval is elapsed, when transmission of a CTS is completed. AP2may transmit a beamformed data frame to STA2after a predetermined time interval is elapsed when reception of the CTS is completed. Here, the predetermined time interval may be a short inter-frame space (SIFS).

FIG. 5illustrates another example of a protocol for performing cooperative transmit beamforming between APs according to an embodiment.

Referring toFIG. 5, AP1and AP2may be determined to participate in a cooperative transmit beamforming through negotiation about the cooperative transmit beamforming Here, a cooperative sounding process for the cooperative transmit beamforming between AP1and AP2may be performed. For example, each of AP1and AP2may acquire a precoding matrix to be used for the cooperative transmit beamforming through the cooperative sounding process. AP1leading the cooperative transmit beamforming may transmit a CoTx request frame in which AP2is set as a receiver address (RA), so that AP2may reset a NAV. Here, a duration field of the CoTx request frame may be set as a time when a cooperative transmit beamforming duration is terminated and thus, a WLAN equipment unassociated with the negotiation may set a NAV until the cooperative transmit beamforming duration is terminated. AP2receiving the CoTx request frame may transfer an acknowledgement (ACK) frame about the CoTx request frame to AP1by transmitting a CoTx response frame in which AP1is set as a receiver address. Here, a duration field of the CoTx response frame is set as a time when the cooperative transmit beamforming duration is terminated and a WLAN equipment unassociated with the negotiation may set a NAV until the cooperative transmit beamforming duration is terminated. Here, AP1and AP2may concurrently perform the cooperative transmit beamforming during the cooperative transmit beamforming duration. For example, AP1may transmit a beamformed data frame to STA1belonging to a BSS covered by AP1and, at the same time, AP2may transmit a beamformed data frame to STA2belonging to a BSS covered by AP2. According to an embodiment, AP1may transmit a beamformed data frame to STA1after a predetermined time interval is elapsed, when reception of a CoTx response frame is completed. AP2may transmit a beamformed data frame to STA2after a predetermined time interval is elapsed, when transmission of the CoTx response frame is completed. Here, the predetermined time interval may be a SIFS.

As described above, according to embodiments, the transmission efficiency in an OBSS environment may be enhanced by providing a cooperative transmit beamforming method that enables two or more APs to concurrently perform a transmit beamforming in the OBSS environment.

FIG. 6illustrates a format of a CoTx request frame according to an embodiment.

The CoTx request frame may include a frame control (FC) field, a duration field, a receiver address (RA) field, a transmitter address (TA) field, and a frame check sequence (FCS) field as basic fields that constitute a WLAN MAC frame. According to an embodiment, the CoTx request frame may include a CoTx control field and a plurality of CoTx AP info fields to perform a cooperative transmit beamforming. The CoTx control field may include information about the number of APs participating in the cooperative transmit beamforming and information, for example, information constituting L-LENGTH and VHT-SIGA in the case of VHT PPDU, used to make a temporal length of a beamformed data frame equal to a temporal length of a un-beamformed data frame, for example, L-SIG and VHT-SIGA in the case of VHT PPDU. The number of CoTx AP info fields may be the same as the number of APs participating in the cooperative transmit beamforming. The CoTx AP info fields may include information indicating the APs that participate in the cooperative transmit beamforming, respectively. Information indicating each of the APs that participate in the cooperative transmit beamforming may include a BSS ID or information capable of identifying a corresponding AP.

FIG. 7illustrates an environment in which BSSs covered by APs of a WLAN overlap.

Referring toFIG. 7, a BSS121is covered by an AP111, a BSS122is covered by an AP112, and a BSS123is covered by an AP113. STAs131and134may belong to the BSS121, STAs132and135may belong to the BSS122, and STAs133and136belong to the BSS123. An overlapping area may be present among the BSSs121,122, and123of the APs111,112, and113. The STA131belonging to the BSS121, the STA132belonging to the BSS122, and the STA133belonging to the BSS123may be positioned in the overlapping area. Here, when the BSSs121,122, and123use the same frequency channel, the APs111,112, and123and the STAs131,132, and133may hear all of the transmitting frames. Accordingly, the transmission efficiency may be degraded in the overlapping area. A method of preventing the degradation in the transmission efficiency in an OBSS environment is required.

According to embodiments, the transmission efficiency in an OBSS environment may be enhanced by providing a method that enables three or more APs to concurrently transmit data even in the OBSS environment.

FIG. 8illustrates another example of a protocol for performing cooperative transmit beamforming between APs according to an embodiment. InFIG. 8, APs participating in the cooperative transmit beamforming sequentially respond to a single CoTx request frame.

Referring toFIG. 8, AP1, AP2, and AP3may be determined to participate in the cooperative transmit beamforming through negotiation about the cooperative transmit beamforming. A cooperative sounding process for the cooperative transmit beamforming among AP1, AP2, and AP3may be performed. For example, each of AP1, AP2, and AP3may acquire a precoding matrix to be used for the cooperative transmit beamforming through the cooperative sounding process. AP1leading the cooperative transmit beamforming may transmit a CoTx request frame in which a broadcast address (BC) is set as a receiver address. Here, a duration field of the CoTx request frame is set as a time when a cooperative transmit beamforming duration is terminated and thus, a WLAN equipment unassociated with the negotiation may set a NAV until the cooperative transmit beamforming duration is terminated. Also, a first CoTx AP info field of the CoTx request frame may include information indicating AP2and a second CoTx AP info field of the CoTx request frame may include information indicating AP3. Accordingly, AP2and AP3participating in the cooperative transmit beamforming may rest a NAV. AP2associated with the first CoTx AP info field may transfer an ACK frame about the CoTx request frame to AP1by transmitting a CoTx response frame in which AP1is set as a receiver address. Here, a duration field of the CoTx response frame is set as a time when the cooperative transmit beamforming duration is terminated and thus, the WLAN equipment unassociated with the negotiation may set a NAV until the cooperative transmit beamforming duration is terminated. AP3associated with a subsequent CoTx AP info field may transfer an ACK frame about the CoTx request frame to AP1by transmitting a CoTx response frame in which AP1is set as a receiver address. Here, a duration field of the CoTx response frame is set as a time when the cooperative transmit beamforming duration is terminated and thus, the WLAN equipment unassociated with the negotiation may set a NAV until the cooperative transmit beamforming duration is terminated. AP1, AP2, and AP3may perform the cooperative transmit beamforming during the cooperative transmit beamforming duration. For example, AP1may transmit a beamformed data frame to STA1belonging to a BSS covered by AP1, AP2, at the same time, may transmit a beamformed data frame to STA2belonging to a BSS covered by AP2, and AP3, at the same time, may transmit a beamformed data frame to STA3belonging to a BSS covered by AP3. According to an embodiment, AP1and AP2may transmit beamformed data frames to STA1and STA2, respectively after a predetermined time interval is elapsed, when reception of a last CoTx response frame is completed. AP3associated with a last CoTx AP info field may transmit a beamformed data frame to STA3after a predetermined time interval is elapsed, when transmission of a CoTx response frame is completed. Here, the predetermined time interval may be a SIFS.

FIG. 9illustrates still another example of a protocol for performing cooperative transmit beamforming between APs according to an embodiment. InFIG. 9, APs participating in the cooperative transmit beamforming sequentially respond to a single CoTx request frame.

In the embodiment ofFIG. 9, when one of the APs participating in the cooperative transmit beamforming does not respond or one of CoTx response frames is lost, it may be difficult to determine a transmission start point in time of a beamformed data frame. To clarify the transmission start point in time of the beamformed data frame, AP1leading the cooperative transmit beamforming may transmit a CoTx start frame. The CoTx start frame may be transmitted after a predetermined time interval is elapsed, when reception of a last CoTx response frame is completed. According to an embodiment, AP1may transmit a beamformed data frame to STA1after a predetermined time interval is elapsed, when transmission of the CoTx start frame is completed. AP2and AP3may transmit beamformed data frames to STA2and STA3, respectively, after a predetermined time interval is elapsed, when reception of the CoTx start frame is completed. Here, the predetermined time interval may be a SIFS.

FIG. 10illustrates still another example of a protocol for performing cooperative transmit beamforming between APs according to an embodiment. InFIG. 10, an AP associated with a first CoTx AP info field of a CoTx request frame may respond immediately after receiving the CoTx request frame, and each of APs associated with remaining CoTx AP info fields of the CoTx request frame may respond after receiving a CoTx response poll frame.

Referring toFIG. 10, AP1, AP2, and AP3may be determined to participate in the cooperative transmit beamforming through negotiation about the cooperative transmit beamforming. A cooperative sounding process for the cooperative transmit beamforming among AP1, AP2, and AP3may be performed. For example, each of AP1, AP2, and AP3may acquire a precoding matrix to be used for the cooperative transmit beamforming through the cooperative sounding process. AP1leading the cooperative transmit beamforming may transmit a CoTx request frame in which a BC is set as a receiver address. Here, a duration field of the CoTx request frame is set as a time when a cooperative transmit beamforming duration is terminated and thus, a WLAN equipment unassociated with the negotiation may set a NAV until the cooperative transmit beamforming duration is terminated. Also, a first CoTx AP info field of the CoTx request frame may include information indicating AP2and a second CoTx AP info field of the CoTx request frame may include information indicating AP3. Accordingly, AP2and AP3participating in the cooperative transmit beamforming may rest a NAV. AP2associated with the first CoTx AP info field may transfer an ACK frame about the CoTx request frame to AP1by transmitting a CoTx response frame in which AP1is set as a receiver address. Here, a duration field of the CoTx response frame is set as a time when the cooperative transmit beamforming duration is terminated and thus, the WLAN equipment unassociated with the negotiation may set a NAV until the cooperative transmit beamforming duration is terminated. AP1leading the cooperative transmit beamforming may transmit a CoTx response poll frame in which AP3associated with a subsequent CoTx AP info field is set as a receiver address. AP3receiving the CoTx response poll frame may transfer an ACK frame about the CoTx response poll frame to AP1by transmitting a CoTx response frame in which AP1is set as a receiver address. Here, a duration field of the CoTx response frame is set as a time when the cooperative transmit beamforming duration is terminated and thus, the WLAN equipment unassociated with the negotiation may set a NAV until the cooperative transmit beamforming duration is terminated. AP1, AP2, and AP3may perform the cooperative transmit beamforming during the cooperative transmit beamforming duration. For example, AP1may transmit a beamformed data frame to STA1belonging to a BSS covered by AP1, AP2, at the same time, may transmit a beamformed data frame to STA2belonging to a BSS covered by AP2, and AP3, at the same time, may transmit a beamformed data frame to STA3belonging to a BSS covered by AP3. According to an embodiment, AP1and AP2may transmit beamformed data frames to STA1and STA2, respectively, after a predetermined time interval is elapsed, when reception of a last CoTx response frame is completed. AP3associated with a last CoTx AP info field may transmit a beamformed data frame to STA3after a predetermined time interval is elapsed, when transmission of a CoTx response frame is completed. Here, the predetermined time interval may be a SIFS.

As described above, according to embodiments, the transmission efficiency in an OBSS environment may be enhanced by providing a cooperative transmit beamforming method that enables three or more APs to concurrently perform a transmit beamforming in the OBSS environment.

FIG. 11illustrates an example in which BSSs covered by APs of a WLAN overlap. Referring toFIG. 11, a BSS121is covered by an AP111and a BSS122is covered by an AP112. STAs131and133may belong to the BSS121, and STAs132and134may belong to the BSS122. An overlapping area may be present between the BSSs121and122of the APs111and112. The STA131belonging to the BSS121and the STA132belonging to the BSS122may be positioned in the overlapping area. Dissimilar to the embodiment ofFIG. 1orFIG. 7, the APs111and112are positioned outside an OBSS area and the STAs131and132are positioned in the OBSS area. In this environment, since a transmission start point in time of a beamformed data frame using direct communication between APs cannot be known, an STA positioned in an OBSS area may be employed as a relay equipment.

FIG. 12illustrates still another example of a protocol for performing cooperative transmit beamforming between APs according to an embodiment.FIG. 12illustrates a method of using an STA positioned in an OBSS area between APs participating in the cooperative transmit beamforming as a relay equipment.

Referring toFIG. 12, AP1and AP2may be determined to participate in the cooperative transmit beamforming through negotiation about the cooperative transmit beamforming. According to an embodiment, AP1may recognize that AP1and AP2are positioned outside an OBSS area and STA1belonging to a BSS covered by AP1is positioned in the OBSS area, through the negotiation. A cooperative sounding process for the cooperative transmit beamforming between AP1and AP2may be performed. For example, each of AP1and AP2may acquire a precoding matrix to be used for the cooperative transmit beamforming through the cooperative sounding process. AP1leading the cooperative transmit beamforming may transmit a CoTx request frame in which STA1that is a relay equipment is set as a receiver address. Here, a duration field of the CoTx request frame is set as a time when the cooperative transmit beamforming duration is terminated and thus, a WLAN equipment unassociated with the negotiation may set a NAV until the cooperative transmit beamforming duration is terminated. STA1receiving the CoTx request frame may transmit a CoTx request frame in which a BC is set as a receiver address, so that AP2may reset a NAV. AP2receiving the CoTx request frame may transfer an ACK frame about the CoTx request frame to STA1by transmitting the CoTx response frame in which STA1is set as a receiver address. Here, a duration field of the CoTx response frame is set as a time when the cooperative transmit beamforming duration is terminated and thus, the WLAN equipment unassociated with the negotiation may set a NAV until the cooperative transmit beamforming duration is terminated. STA1may receive a CoTx response frame from APs participating in the cooperative transmit beamforming and then transmit a CoTx start frame. The CoTx start frame may be transmitted after a predetermined time interval is elapsed, when reception of a last CoTx response frame is completed. AP1and AP2may concurrently perform the cooperative transmit beamforming during the cooperative transmit beamforming duration. For example, AP1may transmit a beamformed data frame to STA1belonging to a BSS covered by AP1and, at the same time, AP2may transmit a beamformed data frame to STA2belonging to a BSS covered by AP2. According to an embodiment, AP1and AP2may transmit beamformed data frames to STA1and STA2, respectively, after a predetermined time interval is elapsed, when reception of a CoTx start frame is completed. Here, the predetermined time interval may be a SIFS.

As described above, according to embodiments, the transmission efficiency in an OBSS environment may be enhanced by providing a cooperative transmit beamforming protocol that enables two or more APs to concurrently perform transmit beamforming in the OBSS environment by using an STA positioned in an OBSS area as a relay equipment when APs having the OBSS area are positioned outside the OBSS area.