Method of aligning interference in wireless local area network

There is provided a method of aligning interference in a wireless local area network. A method of receiving a frame includes receiving a first frame from a first access point associated with a terminal, comparing a signal level of the first frame with a preset data rate, performing a pre-procedure for interference alignment when the signal level of the first frame is less than the preset data rate, receiving a second frame from the first access point, and aligning interference of the second frame based on information obtained through the pre-procedure. According to the present invention, it is possible to control interference in an overlapping band.

CLAIM FOR PRIORITY

This application claims priority to Korean Patent Application No. 10-2013-0141977 filed on Nov. 21, 2013, No. 10-2013-0158882 filed on Dec. 19, 2013, and No 10-2014-0161410 filed on Nov. 19, 2014 in the Korean Intellectual Property Office (KIPO), the entire contents of which are hereby incorporated by reference.

BACKGROUND

1. Technical Field

Example embodiments of the present invention relate to wireless local area network technology, and more specifically, to interference alignment technology for controlling interference in an overlapping band.

2. Related Art

As information and communication technologies develop, various wireless communication technologies are developing. Among them, a wireless local area network (WLAN) is technology for wirelessly accessing the Internet at home or companies or specific service providing areas using mobile terminals such as personal digital assistants (PDAs), laptop computers, portable multimedia players (PMPs), smart phones, and tablet PCs based on wireless frequency technology.

Institute of Electrical and Electronics Engineers (IEEE) 802.11 has been under development as standards for wireless local area network technology. Wireless local area network technology according to the IEEE 802.11a standard operates based on an orthogonal frequency division multiplexing (OFDM) scheme and may provide a maximum transfer rate of 54 Mbps in a 5 GHz band. Wireless local area network technology according to the IEEE 802.11b standard operates based on a direct sequence spread spectrum (DSSS) scheme and may provide a maximum transfer rate of 11 Mbps in a 2.4 GHz band. Wireless local area network technology according to the IEEE 802.11g standard operates based on the OFDM scheme or the DSSS scheme and may provide a maximum transfer rate of 54 Mbps in the 2.4 GHz band.

Wireless local area network technology according to the IEEE 802.11n standard operates based on the OFDM scheme in the 2.4 GHz band and the 5 GHz band, and when a multiple input multiple output-OFDM (MIMO-OFDM) scheme is used, may provide a maximum transfer rate of 300 Mbps for four spatial streams. Wireless local area network technology according to the IEEE 802.11n standard may support up to a channel bandwidth of 40 MHz and may provide a maximum transfer rate of 600 Mbps in this case.

As such wireless local area networks are being spread more actively and applications using the same become more diverse, a need for new wireless local area network technology for supporting a throughput higher than a data processing rate supported by IEEE 802.11n is increasing. Very high throughput (VHT) wireless local area network technology is one of the IEEE 802.11 wireless local area network technologies proposed to support a data processing rate of 1 Gbps or more. Among them, IEEE 802.11ac is developing as a standard for providing the VHT in a band of 5 GHz or less, and IEEE 802.11ad is developing as a standard for providing the VHT in a 60 GHz band.

As use of the wireless local area network has recently been increased, neighbor access points are highly likely to use an overlapping band. Accordingly, there is a problem in that communication performance decreases in the overlapping band between neighbor access points.

SUMMARY

In order to address the above problems, example embodiments of the present invention are provided to a method of aligning interference for controlling interference in an overlapping band between neighbor access points in a wireless local area network.

In some example embodiments, a method of receiving a frame that is performed in a terminal, the method includes receiving a first frame from a first access point associated with the terminal, comparing a signal level of the first frame with a preset data rate, performing a pre-procedure for interference alignment when the signal level of the first frame is less than the preset data rate, receiving a second frame from the first access point, and aligning interference of the second frame based on information obtained through the pre-procedure.

The performing of the pre-procedure may include obtaining information on at least one neighbor access point operated in a channel overlapping an operation channel of the first access point, and performing a procedure for estimating a channel between the first access point and the at least one neighbor access point.

In the obtaining of information on at least one neighbor access point, information on the at least one neighbor access point operated within a preset channel range based on an operation channel of the first access point may be obtained.

The obtaining of information on at least one neighbor access point may include, transmitting a probe request frame to the at least one neighbor access point operated in the overlapping channel, and receiving a probe response frame that is a response for the probe request frame from the at least one neighbor access point.

The probe request frame may include information indicating that a frame transmission and reception procedure based on interference alignment is started.

The probe response frame may include at least one of operation channel information, bandwidth information and antenna information of the at least one neighbor access point.

The performing of the procedure for estimating a channel may include receiving a null data packet (NDP) request frame from the first access point, and transmitting an NDP response frame that is a response for the NDP request frame to each of the first access point and the at least one neighbor access point.

The performing of the pre-procedure for interference alignment may include, obtaining information on at least one neighbor access point operated in a channel overlapping an operation channel of the first access point, transmitting the information on at least one neighbor access point to the first access point, performing a procedure for estimating a channel between the first access point and the at least one neighbor access point, and announcing completion of the pre-procedure for interference alignment.

The obtaining of information on at least one neighbor access point may include, transmitting a probe request frame to the at least one neighbor access point operated in the overlapping channel, and receiving a probe response frame that is a response for the probe request frame from the at least one neighbor access point.

The probe request frame may include information indicating that a frame transmission and reception procedure based on interference alignment is started.

The probe response frame may include at least one of operation channel information, bandwidth information and antenna information of the at least one neighbor access point.

The performing of the procedure for estimating a channel may include, receiving an NDP request frame from the first access point, transmitting an NDP announcement frame for announcing transmission of an NDP response frame to each of the first access point and the at least one neighbor access point, and transmitting the NDP response frame to the first access point and the at least one neighbor access point.

The NDP announcement frame may include identification information of each of the first access point and the at least one neighbor access point.

The announcing of completion of the pre-procedure for interference alignment may include, transmitting an interference alignment (IA) request to send (RTS) frame indicating completion of the pre-procedure for interference alignment to each of the first access point and the at least one neighbor access point, and receiving an IA clear to send (CTS) frame that is a response for the IA RTS frame from each of the first access point and the at least one neighbor access point.

In other example embodiments, a terminal includes a processor, and a memory in which at least one program command executed through the processor is stored, wherein the at least one program command causes the terminal to execute, receiving a first frame from a first access point associated with the terminal, comparing a signal level of the first frame with a preset minimum data rate, performing a pre-procedure for interference alignment when the signal level of the first frame is less than the preset minimum data rate, receiving a second frame from the first access point, and aligning interference of the second frame based on information obtained through the pre-procedure.

The performing of the pre-procedure for interference alignment may include, obtaining information on at least one neighbor access point operated in a channel overlapping an operation channel of the first access point, and performing a procedure for estimating a channel between the first access point and the at least one neighbor access point.

The obtaining of information on at least one neighbor access point may include, transmitting a probe request frame to the at least one neighbor access point operated in the overlapping channel, and receiving a probe response frame that is a response for the probe request frame from the at least one neighbor access point.

The performing of the procedure for estimating a channel may include, receiving a null data packet (NDP) request frame from the first access point, and transmitting an NDP response frame that is a response for the NDP request frame to each of the first access point and the at least one neighbor access point.

The performing of the pre-procedure for interference alignment may include, obtaining information on at least one neighbor access point operated in a channel overlapping an operation channel of the first access point, transmitting the information on at least one neighbor access point to the first access point, performing a procedure for estimating a channel between the first access point and the at least one neighbor access point, and announcing completion of the pre-procedure for interference alignment.

The announcing of completion of the pre-procedure for interference alignment may include, transmitting an interference alignment (IA) request to send (RTS) frame indicating completion of the pre-procedure for interference alignment to each of the first access point and the at least one neighbor access point, and receiving an IA clear to send (CTS) frame that is a response for the IA RTS frame from each of the first access point and the at least one neighbor access point.

DESCRIPTION OF EXAMPLE EMBODIMENTS

While the invention can be modified in various ways and take on various alternative forms, specific embodiments thereof are shown in the drawings and will be described in detail.

It should be understood, however, that there is no intent to limit the invention to the particular forms disclosed, but on the contrary, the invention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention.

Hereinafter, exemplary embodiments of the invention will be described in detail with reference to the accompanying drawings. In order to facilitate overall understanding of the invention, like reference numerals in the drawings denote like elements, and thus the description thereof will not be repeated.

Throughout this specification, the term “station (STA)” refers to any functional medium including medium access control (MAC) and a physical layer interface of a wireless medium according to specifications of the Institute of Electrical and Electronics Engineers (IEEE) 802.11 standards. The STA may be classified as an STA serving as an access point (AP) or an STA serving as a non-access point (non-AP). The STA serving as the AP may be simply called an AP, and the STA serving as the non-AP may be simply called a terminal.

The STA may include a processor and a transceiver, and may further include a user interface, a display device, and the like. The processor may refer to a unit configured to generate a frame to be transmitted via a wireless network or process a frame received via the wireless network and may perform several functions for controlling the STA. The transceiver refers to a unit functionally connected to the processor and configured to transmit and receive a frame via the wireless network for the STA.

The AP may refer to a centralized controller, a base station (BS), a radio access station, a node B, an evolved node B, a relay, a mobile multihop relay (MMR)-BS, a base transceiver system (BTS), a site controller or the like, and may include some or all functions thereof.

The term “terminal (that is, a non-AP) may refer to a wireless transmit/receive unit (WTRU), user equipment (UE), a user terminal (UT), an access terminal (AT), a mobile station (MS), a mobile terminal, a subscriber unit, a subscriber station (SS), a wireless device, a mobile subscriber unit or the like, and may include some or all functions thereof.

Here, the term “terminal” may refer to a desktop computer, a laptop computer, a tablet PC, a wireless phone, a mobile phone, a smart phone, a smart watch, smart glass, an e-book reader, a portable multimedia player (PMP), a portable game console, a navigation device, a digital camera, a digital multimedia broadcasting (DMB) player, a digital audio recorder, a digital audio player, a digital picture recorder, a digital picture player, a digital video recorder, a digital video player or the like which are capable of communication.

FIG. 1is a block diagram illustrating an embodiment of a station for performing methods according to the present invention.

As illustrated inFIG. 1, a station100may include at least one processor110, a memory120, and a network interface device130configured to perform communication via a network. Also, the station100may further include an input interface device140, an output interface device150, a storage device160, and the like. Components included in the station100may be connected through a bus170to communicate with each other.

The processor110may execute a program command stored in the memory120and/or the storage device160. The processor110may refer to a central processing unit (CPU), a graphics processing unit (GPU) or a dedicated processor in which methods according to the present invention are performed. The memory120and the storage device160may include a volatile storage medium and/or non-volatile storage medium. For example, the memory120may include a read only memory (ROM) and/or a random access memory (RAM).

Embodiments of the present invention are applied to a wireless local area network system according to IEEE 802.11, and may be applied to other communication systems in addition to the wireless local area network system according to IEEE 802.11.

For example, embodiments of the present invention may be applied to a mobile Internet such as a wireless personal area network (WPAN), a wireless body area network (WBAN), a Wireless Broadband Internet (WiBro) or world interoperability for microwave access (WiMax), a 2G mobile communication network such as global system for mobile Communication (GSM) or code division multiple access (CDMA), a 3G mobile communication network such as wideband code division multiple access (WCDMA) or CDMA 2000, a 3.5G mobile communication network such as high speed downlink packet access (HSDPA) or high speed uplink packet access (HSUPA), a 4G mobile communication network such as Long Term Evolution (LTE) or LTE-Advanced, a 5G mobile communication network, and the like.

FIG. 2is a conceptual diagram illustrating an embodiment of a configuration of a wireless local area network system according to IEEE 802.11.

As illustrated inFIG. 2, the wireless local area network system according to IEEE 802.11 may include at least one basic service set (BSS). The BSS refers to a set of stations STA1, STA2(AP1), STA3, STA4, STA5(AP2), STA6, STA7, and STA8which can be successfully synchronized to communicate with each other and does not refer to a concept of a specific area.

The BSS may be classified as an infrastructure BSS or an independent BSS (IBSS). Here, BSS1and BSS2refer to the infrastructure BSS, and BSS3refers to the IBSS.

BSS1may include a first terminal STA1, a first access point STA2(AP1) configured to provide a distribution service, and a distribution system (DS) configured to connect a plurality of access points STA2(AP1) and STA5(AP2). In BSS1, the first access point STA2(AP1) may manage the first terminal STA1.

BSS2may include a third terminal STA3, a fourth terminal STA4, a second access point STA5(AP2) configured to provide a distribution service, and a distribution system (DS) configured to connect a plurality of access points STA2(AP1) and STA5(AP2). In BSS2, the second access point STA5(AP2) may manage the third terminal STA3and the fourth terminal STA4.

BSS3refers to the IBSS operated in an ad-hoc mode. BSS3has no access point serving as an entity (a centralized management entity) that performs a management function in the center. That is, in BSS3, terminals STA6, STA7and STA8may be managed in a distributed manner. In BSS3, all terminals STA6, STA7and STA8may refer to a mobile terminal and build a self-contained network since accessing the distribution system is prohibited.

The access points STA2(AP1) and STA5(AP2) may provide access to the distribution system (DS) for terminals STA1, STA3and STA4connected thereto through a wireless medium. Communication among terminals STA1, STA3and STA4in BSS1and BSS2is generally performed through the access points STA2(AP1) and STA5(AP2), or when a direct link is set, direct communication among the terminals STA1, STA3and STA4is possible.

A plurality of infrastructures BSSs may be connected through the distribution system. The plurality of BSSs connected through the distribution system are called an extended service set (ESS). Entities STA1, STA2(AP1), STA3, STA4and STA5(AP2) included in the ESS may communicate with each other. Any terminal STA1, STA3or STA4in the same ESS may seamlessly communicate and move from one BSS to another BSS.

The distribution system is a mechanism for one access point to communicate with another access point. According to the distribution system, the access point may transmit a frame for terminals connected to the BSS that it manages or transmit a frame for any terminal that has moved to another BSS. Also, the access point may transmit and receive a frame via an external network such as a wired network. This distribution system does not necessarily have a network structure, but may have any structure as long as a predetermined distribution service specified in the IEEE 802.11 standards can be provided. For example, the distribution system may be a wireless network such as a mesh network or a physical structure connecting access points.

Meanwhile, as the number of stations in the wireless local area network increases, contention for accessing a non-overlapping channel increases. Channel settings in a 2.4 GHz industrial scientific medical (ISM) band are as follows.

FIG. 3is a conceptual diagram illustrating channel settings in a 2.4 GHz ISM band.

As illustrated inFIG. 3, in the 2.4 GHz ISM band, a channel1301, a channel5305, a channel9309and a channel13313may be set not to overlap. Since successive channels such as the channel1301, a channel2302, and a channel3303may interfere with one another, use of the channel1301, the channel5305, the channel9309and the channel13313, which are non-overlapping channels, is required as a policy. However, as the number of stations significantly increases, non-overlapping channels301,305,309and313are used to be shared on a time axis by a plurality of stations in the 2.4 GHz ISM band. Accordingly, there are problems in that frequency efficiency of the non-overlapping channels301,305,309and313decreases and performance of the wireless local area network ultimately decreases.

In order to address such problems, a channel overlapping a neighbor channel may be used. However, in this case, interference between neighbor channels occurs and transmission performance decreases. For example, when the channel1301and the channel2302are simultaneously used, a frequency may overlap by 15 MHz. When the channel1301and the channel3303are simultaneously used, a frequency may overlap by 10 MHz. When the channel1301and a channel4304are simultaneously used, a frequency may overlap by 5 MHz. For example, when a first transmitter operates in the channel1301and a second transmitter operates in the channel2302, a receiver positioned in an overlapping area between the channel1301and the channel2302may not reliably receive a frame due to interference between the neighbor channels301and302.

However, when interference alignment (IA) technology is used, interference of a frame transmitted through an overlapping band may be minimized. In order to apply interference alignment technology, signaling and a protocol related to information exchange between stations participating in interference alignment are necessary. Hereinafter, the signaling and protocol related to information exchange between stations participating in interference alignment will be described. That is, in a centralized topology (that is, an environment having a controller) or a decentralized topology (that is, an environment having no controller), a method of exchanging information between stations constituting an overlapped basic service set (OBSS) and a criterion and method for a specific station to participate in interference alignment will be described.

A Frame Transmission and Reception Method Based on Interference Alignment in a Centralized Topology

FIG. 4is a conceptual diagram illustrating a centralized topology to which interference alignment technology is applied.

As illustrated inFIG. 4, a controller400may be connected to access points401,402and403via wireless or wired communication and may control the access points401,402and403. The first access point401may configure BSS1that is a range covered by its own signal and operate in the channel1301. The second access point402may configure BSS2that is a range covered by its own signal and operate in the channel2302. The third access point403may configure BSS3that is a range covered by its own signal and operate in the channel3303. Here, the BSS1, the BSS2and the BSS3may overlap.

A first terminal411may be positioned in an area in which the BSS1, the BSS2and the BSS3overlap. When the first terminal411communicates with the first access point401through the channel1301, 15 MHz thereof may be interfered with by the second access point402and 10 MHz thereof may be interfered with by third access point403.

Meanwhile, since a second terminal412is not positioned in an area in which the BSS1, the BSS2and the BSS3overlap, when the second terminal412communicates with the second access point402through the channel2302, it may not be interfered with by the other access points401and403. Since a third terminal413is not positioned in an area in which the BSS1, the BSS2and the BSS3overlap, when the third terminal413communicates with the third access point403through the channel3303, it may not be interfered with by the other access points401and402.

Since unique access points may be simply provided in public places and downtowns in which main facilities are densely provided, the above OBSS may be frequently generated. When interference alignment technology is not applied, since the first terminal411may receive a signal from the first access point401while being interfered with by the neighbor access points402and403, a decoding error may be generated. Also, when the first access point401transmits a signal to the first terminal411at a low rate in order to minimize interference from the neighbor access points402and403, throughput may decrease.

FIG. 5is a flowchart illustrating a frame transmission and reception method based on interference alignment according to an embodiment of the present invention.FIGS. 6A and 6Bare conceptual diagrams illustrating a frame transmission and reception method based on interference alignment according to an embodiment of the present invention.

As illustrated inFIGS. 5, 6A and 6B, the first terminal STA1may refer to the first terminal411positioned in the area in which the BSS1, the BSS2and the BSS3overlap inFIG. 4. The first access point AP1may refer to the first access point401(that is, an access point operated in the channel1301) configuring the BSS1inFIG. 4. The second access point AP2may refer to the second access point402(that is, an access point operated in the channel2302) configuring the BSS2inFIG. 4. A third access point AP3may refer to the third access point403(that is, an access point operated in the channel3303) configuring the BSS3inFIG. 4. A controller (C) may refer to the controller400configured to control the access points401,402and403inFIG. 4. Here, the first terminal STA1may refer to a terminal associated with the first access point AP1. The first terminal STA1and the access points AP1, AP2and AP3may operate in a 2.4 GHz band.

A frame transmission and reception procedure based on interference alignment may be started by the first terminal STA1positioned in the area in which the BSSs overlap. The first terminal STA1connected to the first access point AP1may transmit or receive a frame to or from the first access point AP1and compare a signal level of any frame received from the first access point AP1with a preset threshold value (S500).

That is, the first terminal STA1may compare a signal level of any frame with a preset threshold value based on the following Formula 1.
log2(1+SINR)<Rthreshold[Formula 1]

In Formula 1, the left-hand side may refer to a ratio of an interference signal and noise level from the neighbor access points AP2and AP3with respect to a signal level received from the first access point AP1, from a viewpoint of the first terminal STA1. In Formula 1, the right-hand side may refer to a minimum data rate required by the first terminal STA1.

When the left-hand side is less than the right-hand side in Formula 1 (that is, when an outage occurs), it may refer to the fact that the minimum data rate required by the first terminal STA1is not satisfied. That is, it may refer to the fact that frame transmission and reception between the first terminal STA1and the first access point AP1are unreliable. When this situation continues, the first terminal STA1has difficulty in successfully decoding the frame received from the first access point AP1due to interference from the neighbor access points AP2and AP3. Therefore, when the left-hand side is less than the right-hand side in Formula 1, the first terminal STA1may start the frame transmission and reception procedure based on interference alignment. On the other hand, when the left-hand side is greater than the right-hand side in Formula 1, the first terminal STA1may transmit or receive a frame to or from the first access point AP1according to a method in the related art (S501).

When the frame transmission and reception procedure based on interference alignment is started, the first terminal STA1may perform a scanning (for example, active scanning or passive scanning) procedure in order to obtain information on neighbor access points (S502). Active scanning in the related art refers to searching all channels (for example, 13 channels in Korea) in the 2.4 GHz band. Here, the first terminal STA1may scan channels of N−3 to N+3 instead of all channels. Here, N may refer to a channel number in which the first terminal STA1currently operates. For example, when the outage occurs while the first terminal STA1transmits or receives a frame to or from the first access point AP1through the channel5305, the first terminal STA1may scan from the channel2302to a channel8(not illustrated).

Meanwhile, since orthogonality between channels is maintained in a 5 GHz band, the first terminal STA1may identify a bandwidth (for example, 20 MHz, 40 MHz, 80 MHz or 160 MHz) used by access points and then determine a channel range to be scanned. That is, when bands used by access points overlap, the first terminal STA1may move to a channel in which a corresponding access point operates and scan.

Here, since the first terminal STA1operates in the channel1301, it is assumed that scanning on the channel2302and the channel3303is performed. The first terminal STA1may transmit a probe request frame600through the channel3303. In this case, when a channel is in an idle state during a distributed coordination function (DCF) inter frame space (DIFS), the first terminal STA1may transmit the probe request frame600in a broadcast manner after a contention window according to random backoff.

The probe request frame600may refer to a probe request frame specified in IEEE 802.11 or a short version (for example, a short probe request frame) of the probe request frame. Also, the probe request frame600may refer to a frame specified for the frame transmission and reception procedure based on interference alignment. For example, the probe request frame600may further include at least one of information indicating that the frame transmission and reception procedure based on interference alignment is started, identification information (for example, an association identifier (AID), a partial AID (PAID) and a medium access control (MAC) address) of the first terminal STA1, operation channel information (for example, an operation channel number), bandwidth information and antenna information (for example, the number of antennas).

The third access point AP3operated in the channel3303may receive the probe request frame600transmitted from the first terminal STA1. The third access point AP3may recognize that the frame transmission and reception procedure based on interference alignment is started by the first terminal STA1based on information included in the probe request frame600, and recognize identification information, operation channel information, bandwidth information, antenna information, and the like of the first terminal STA1.

The third access point AP3may transmit a probe response frame601that is a response for the probe request frame600. In this case, when the channel is in an idle state during the DIFS, the third access point AP3may transmit the probe response frame601after a contention window according to random backoff. Here, by transmitting the probe response frame601, the third access point AP3may represent participation in the frame transmission and reception procedure based on interference alignment. On the other hand, when the third access point AP3does not participate in the frame transmission and reception procedure based on interference alignment, a response for the probe request frame600may not be transmitted.

The probe response frame601may refer to a probe response frame specified in IEEE 802.11 or a short version (for example, a short probe response frame) of the probe response frame. Also, the probe response frame601may refer to a frame specified for the frame transmission and reception procedure based on interference alignment. For example, the probe response frame601may further include at least one of information indicating participation in the frame transmission and reception procedure based on interference alignment, identification information (for example, a service set identifier (SSID) and a basic service set identifier (BSSID)) of the third access point STA3, operation channel information, bandwidth information, and antenna information.

When the probe response frame601is received, the first terminal STA1may recognize that the third access point AP3serves as an interference source to the first terminal STA1and participation in the frame transmission and reception procedure based on interference alignment based on information included in the probe response frame601. Also, the first terminal STA1may recognize identification information, operation channel information, bandwidth information, antenna information, and the like of the third access point AP3.

When all responses for the probe request frame600are received in the channel3303(or when a waiting time set for receiving a response for the probe request frame600is terminated), the first terminal STA1may move to the channel2302and transmit a probe request frame602. In this case, when the channel2302is in an idle state during the DIFS, the first terminal STA1may transmit the probe request frame602in a broadcast manner after a contention window according to random backoff. The probe request frame602may include the same information as the probe request frame600transmitted through the channel3303. For example, the probe request frame602may include at least one of information indicating that the frame transmission and reception procedure based on interference alignment is started by the first terminal STA1and identification information, operation channel information, bandwidth information and antenna information of the first terminal STA1.

The second access point AP2operated in the channel2302may receive the probe request frame602transmitted from the first terminal STA1. The second access point AP2may recognize that the frame transmission and reception procedure based on interference alignment is started by the first terminal STA1based on information included in the probe request frame602, and recognize identification information, operation channel information, bandwidth information, antenna information, and the like of the first terminal STA1.

The second access point AP2may transmit a probe response frame603that is a response for the probe request frame602. In this case, when the channel is in an idle state during the DIFS, the second access point AP2may transmit the probe response frame603after a contention window according to random backoff. Here, by transmitting the probe response frame603, the second access point AP2may represent participation in the frame transmission and reception procedure based on interference alignment. On the other hand, when the second access point AP2does not participate in the frame transmission and reception procedure based on interference alignment, a response for the probe request frame602may not be transmitted.

The probe response frame603may include the same information as the probe response frame601. For example, the probe response frame603may further include at least one of information indicating participation in the frame transmission and reception procedure based on interference alignment, and identification information, operation channel information, bandwidth information and antenna information of the second access point STA2.

The first terminal STA1may receive the probe response frame603transmitted from the second access point AP2. The first terminal STA1may recognize that the second access point AP2serves as an interference source to the first terminal STA1based on information included in the probe response frame603and participation in the frame transmission and reception procedure based on interference alignment. Also, the first terminal STA1may recognize identification information, operation channel information, bandwidth information, antenna information, and the like of the second access point AP2.

Also, when all responses for the probe request frame602are received in the channel2302(or when a waiting time set for receiving a response for the probe request frame602is terminated), the first terminal STA1may move to the channel4304and transmit a probe request frame (not illustrated) and receive a probe response frame (not illustrated) that is a response for the probe request frame (not illustrated).

When the scanning procedure is completed, the first terminal STA1may identify an overlapping band based on the scanning result (S503). That is, the first terminal STA1may identify that some band between the channel1301in which the first terminal STA1operates and the channel2302in which the second access point AP2operates overlaps and identify that some band between the channel1301in which the first terminal STA1operates and the channel3303in which the third access point AP3operates overlaps. The first terminal STA1may recognize that interference alignment technology is applied to a subcarrier to be transmitted through the overlapping band.

Then, each of the access points AP1, AP2and AP3may transmit neighbor access point information (NAI) frames604,605and606to the controller (C) via wired or wireless communication. Hereinafter, configurations of the NAI frames604,605and606will be described.

FIG. 7is a block diagram illustrating a configuration of an NAI frame.

As illustrated inFIG. 7, the NAI frame may include a frame control field710having a 2-octet size, a duration field720having a 2-octet size, a receiver address (RA) field730having a 6-octet size, a transmitter address (TA) field740having a 6-octet size, an NAI field750having a 2-octet size, a basic service set identifier (BSSID) field760having a 6-octet size, and a frame check sequence (FCS) field770having a 4-octet size.

The NAI field750may include an operation channel field751having a 4-bit size, an antenna field752having a 3-bit size, a bandwidth field753having a 4-bit size, and a reserved field754having a 5-bit size. Also, the NAI field750may further include identification information of a terminal that is interfered with by a corresponding access point. The operation channel field751may represent an operation channel number of the corresponding access point. The antenna field752may represent the number of transmission antennas of the corresponding access point and the like. The bandwidth field753may represent a bandwidth used by the corresponding access point and the like.

Referring again toFIGS. 5, 6A and 6B, the third access point AP3may transmit the NAI frame604including at least one of identification information of the first terminal STA1interfered with by the third access point AP3, and identification information, operation channel information, bandwidth information and antenna information of the third access point AP3to the controller (C). The second access point AP2may transmit the NAI frame605including at least one of identification information of the first terminal STA1interfered with the second access point AP2, and identification information, operation channel information, bandwidth information and antenna information of the second access point AP2to the controller (C). The first access point AP1may transmit the NAT frame606including at least one of identification information of the first terminal STA1connected to the first access point AP1, and identification information, operation channel information, bandwidth information and antenna information of the first access point AP1to the controller (C).

The controller (C) may identify a terminal interfered with by the second access point AP2and the third access point AP3based on information included in the NAI frames604,605and606received from the access points AP1, AP2and AP3. Also, the controller (C) may identify the overlapping band based on information included in the NAI frames604,605and606and information (for example, information on a band in which each of the access points AP1, AP2and AP3connected to the controller (C) operates) that is stored in its own database in advance, and determine a subcarrier to be transmitted through the overlapping band as a subcarrier to which interference alignment technology may be applied. That is, the controller (C) may determine a subcarrier to be transmitted in an overlapping band between the channel1301and the channel2302and an overlapping band between the channel1301and the channel3303as a subcarrier to which interference alignment technology may be applied.

The controller (C) may transmit an NAI frame607including information on a band in which a subcarrier to which interference alignment technology is applied may be transmitted to each of the access points AP1, AP2and AP3via wired or wireless communication. Here, the NAI frame607may be configured as similar to or the same as the NAI frame illustrated inFIG. 7.

For example, the NAI field of the NAI frame607may include only information on the band in which a subcarrier to which interference alignment technology is applied may be transmitted. Also, the NAI frame607may further include the NAI field (that is, operation channel information, bandwidth information, antenna information, and the like of the first access point AP1) of the first access point AP1, the NAI field (that is, operation channel information, bandwidth information, antenna information, and the like of the second access point AP2) of the second access point AP2, and the NAI field (that is, operation channel information, bandwidth information, antenna information, and the like of the third access point AP3) of the third access point AP3. In this case, each of the access points AP1, AP2and AP3that have received the NAI frame607may directly determine the band in which a subcarrier to which interference alignment technology is applied may be transmitted based on information included in the NAI frame607.

That is, when each of the access points AP1, AP2and AP3receives the NAI frame607, it is possible to recognize the band in which a subcarrier to which interference alignment technology is applied may be transmitted. Also, each of the access points AP1, AP2and AP3may recognize that NDP response frames609,610and611may be transmitted from the first terminal STA1in order to estimate channel information after the NAI frame607.

Among the access points AP1, AP2and AP3that have received the NAI frame607, the first access point AP1connected to the first terminal STA1may transmit a null data packet (NDP) request frame608to the first terminal STA1in order to estimate information on a channel used for interference alignment. In this case, when the channel1301is in an idle state during the DIFS, the first access point AP1may transmit the NDP request frame608to the first terminal after a contention window according to random backoff. Hereinafter, a configuration of the NDP request frame608will be described.

FIG. 8is a block diagram illustrating a configuration of an NDP request frame.

As illustrated inFIG. 8, the NDP request frame may include a frame control field810having a 2-octet size, a duration field820having a 2-octet size, a receiver address field830having a 6-octet size, a transmitter address field840having a 6-octet size, an NDP indication field850having a 1-octet size, and an FSC field860having a 4-octet size. Here, the NDP indication field850may represent a request for transmission of the NDP response frame.

Referring again toFIGS. 5, 6A and 6B, the first terminal STA1may receive the NDP request frame608from the first access point AP1(S504). The first terminal STA1may transmit the NDP response frames609,610and611to each of the access points AP1, AP2and AP3in response to the NDP request frame608such that each of the access points AP1, AP2and AP3may estimate channel information (S505). That is, the first terminal STA1may transmit the NDP response frame609to the first access point AP1after a short inter frame space (SIFS) from a reception end time of the NDP request frame608.

Then, the first terminal STA1may transmit the NDP response frame610to the second access point AP2through the channel2302. In this case, when the channel2302is in an idle state during the DIFS, the first terminal STA1may transmit the NDP response frame610to the second access point AP2after a contention window according to a random backoff procedure. Also, the first terminal STA1may transmit the NDP response frame611to the third access point AP3through the channel3303. In this case, when the channel3303is in an idle state during the DIFS, the first terminal STA1may transmit the NDP response frame611to the third access point AP3after a contention window according to a random backoff procedure. Hereinafter, configurations of the NDP response frames609,610and611will be described.

FIG. 9is a block diagram illustrating a configuration of an NDP response frame.

As illustrated inFIG. 9, the NDP response frame may include a legacy-short training field (L-STF)910, a legacy-long training field (L-LTF)920, a legacy-signal (L-SIG) field930, a very high throughput (VHT)-SIG-A1 field940, a VHT-SIG-A2 field950, a VHT-STF960, at least one VHT-LTF970, a VHT-SIG B field980, and the like. That is, the NDP response frame may be the same as the NDP frame specified in IEEE 802.11.

Referring again toFIGS. 5, 6A and 6B, each of the access points AP1, AP2and AP3that have received the NDP response frames609,610and611may identify a desired channel and an interference channel according to channel reciprocity. This is called analog local channel information feedback or implicit local channel information feedback.

That is, the first access point AP1may estimate channel information from the first terminal STA1to the first access point AP1based on the NDP response frame609and estimate channel information from the first access point AP1to the first terminal STA1based on the estimated information. The second access point AP2may estimate channel information from the first terminal STA1to the second access point AP2based on the NDP response frame610and estimate channel information from the second access point AP2to the first terminal STA1based on the estimated information. The third access point AP3may estimate channel information from the first terminal STA1to the third access point AP3based on the NDP response frame611and estimate channel information from the third access point AP3to the first terminal STA1based on the estimated information.

Accordingly, each of the access points AP1, AP2and AP3may estimate channel information to the first terminal STA1and obtain a precoder, a decoder and the like for interference alignment based on the estimated channel information. Each first access point AP1may provide the precoder, the decoder and the like for interference alignment to the first terminal STA1. Then, each of the access points AP1, AP2and AP3may pre-code the frame and transmit the pre-coded frames612,613and614.

Meanwhile, the first terminal STA1may receive the pre-coded frame614from the first access point AP1and align interference of the pre-coded frame614based on information obtained from the first access point AP1(S506).

Frame Transmission and Reception Method Based on Interference Alignment in a Distributed Topology

FIG. 10is a conceptual diagram illustrating a decentralized topology to which interference alignment technology is applied.

As illustrated inFIG. 10, a first access point1001may configure BSS1that is a range covered by its own signal and operate in the channel1301, a second access point1002may configure BSS2that is a range covered by its own signal and operate in the channel2302, and a third access point1003may configure BSS3that is a range covered by its own signal and operate in the channel3303. Here, the BSS1, the BSS2and the BSS3may overlap.

A first terminal1011may be positioned in an area in which the BSS1, the BSS2and the BSS3overlap. When the first terminal1011communicates with the first access point1001through the channel1301, 15 MHz thereof may be interfered with by the second access point1002and 10 MHz thereof may be interfered with by the third access point1003. Meanwhile, since a second terminal1012is not positioned in the area in which the BSS1, the BSS2and the BSS3overlap, when the second terminal1012communicates with the second access point1002through the channel2302, it may not be interfered with by the other access points1001and1003. Since a third terminal1013is not positioned in the area in which the BSS1, the BSS2and the BSS3overlap, when the third terminal1013communicates with the third access point1003through the channel3303, it may not be interfered with by the other access points1001and1002.

Meanwhile, the OBSS may occur when access points are provided in an unplanned manner. The station has difficulty in reliably transmitting and receiving a frame in the OBSS. However, when the first terminal1011positioned in the overlapping area aligns interference from the other access points1002and1003in a desired direction, it is possible to minimize interference of a frame received from the first access point1001. In a decentralized topology in which the access points1001,1002and1003are distributed and disposed, each of the stations1001,1002,1003and1011may reliably transmit and receive a frame through a coordination procedure (that is, a sharing procedure of identification information, operation channel information, bandwidth information, antenna information, and the like of a station).

FIG. 11is a flowchart illustrating a frame transmission and reception method based on interference alignment according to another embodiment of the present invention.FIGS. 12A and 12Bare conceptual diagrams illustrating a frame transmission and reception method based on interference alignment according to another embodiment of the present invention.

As illustrated inFIGS. 11, 12A and 12B, the first terminal STA1may refer to the first terminal1011positioned in the area in which the BSS1, the BSS2and the BSS3overlap inFIG. 10. The first access point AP1may refer to the first access point1001(that is, an access point operated in the channel1301) configuring the BSS1inFIG. 10. The second access point AP2may refer to the second access point1002(that is, an access point operated in the channel2302) configuring the BSS2inFIG. 10. The third access point AP3may refer to the third access point1003(that is, an access point operated in the channel3303) configuring the BSS3inFIG. 10. Here, the first terminal STA1may refer to a terminal connected to the first access point AP1. The first terminal STA1, and the access points AP1, AP2and AP3may operate in the 2.4 GHz band.

The frame transmission and reception procedure based on interference alignment may be started by the first terminal STA1positioned in the area in which the BSSs overlap. The first terminal STA1connected to the first access point AP1may transmit or receive a frame to or from the first access point AP1and compare a signal level of any frame received from the first access point AP1with a preset threshold value (S1100).

That is, the first terminal STA1may compare a signal level of any frame with a preset threshold value based on Formula 1. When the left-hand side is less than the right-hand side in Formula 1 (that is, when an outage occurs), it may refer to the fact that the minimum data rate required by the first terminal STA1is not satisfied. That is, it may refer to the fact that transmission between the first terminal STA1and the first access point AP1is unreliable. When this situation continues, the first terminal STA1may not successfully decode the frame received from the first access point AP1due to interference from the neighbor access points AP2and AP3. Therefore, when the left-hand side is less than the right-hand side in Formula 1, the first terminal STA1may start the frame transmission and reception procedure based on interference alignment. On the other hand, when the left-hand side is greater than the right-hand side in Formula 1, the first terminal STA1may transmit or receive a frame to or from the first access point AP1according to a method in the related art (S1101).

When the frame transmission and reception procedure based on interference alignment is started, the first terminal STA1may perform a scanning (for example, active scanning or passive scanning) procedure in order to obtain information on the neighbor access points AP2and AP3(S1102). Active scanning in the related art refers to searching all channels (for example, 13 channels in Korea) in the 2.4 GHz band. Here, the first terminal STA1may scan channels of N−3 to N+3 instead of all channels in order to perform interference alignment in the overlapping band. Here, N may refer to a channel number in which the first terminal STA1currently operates.

Meanwhile, since orthogonality between channels is maintained in a 5 GHz band, the first terminal STA1may identify a bandwidth (for example, 20 MHz, 40 MHz, 80 MHz or 160 MHz) used by access points and then determine a channel range to be scanned. That is, when bands used by access points overlap, the first terminal STA1may move to a channel in which a corresponding access point operates and scan.

Here, since the first terminal STA1operates in the channel1301, it is assumed that scanning on the channel2302and the channel3303is performed. The first terminal STA1may transmit a probe request frame1200through the channel3303. In this case, when the channel is in an idle state during the DIFS, the first terminal STA1may transmit the probe request frame1200in a broadcast manner after a contention window according to random backoff.

The probe request frame1200may refer to a probe request frame specified in IEEE 802.11 or a short version (for example, a short probe request frame) of the probe request frame. Also, the probe request frame1200may refer to a frame specified for the frame transmission and reception procedure based on interference alignment. For example, the probe request frame1200may further include at least one of information indicating that the frame transmission and reception procedure based on interference alignment is started, and identification information (for example, an AID, a PAID, and a MAC address), operation channel information (for example, an operation channel number), bandwidth information and antenna information (for example, the number of antennas) of the first terminal STA1.

The third access point AP3may transmit a probe response frame1201that is a response for the probe request frame1200. In this case, when the channel is in an idle state during the DIFS, the third access point AP3may transmit the probe response frame1201after a contention window according to random backoff. Here, by transmitting the probe response frame1201, the third access point AP3may represent participation in the frame transmission and reception procedure based on interference alignment. On the other hand, when the third access point AP3does not participate in the frame transmission and reception procedure based on interference alignment, a response for the probe request frame1200may not be transmitted.

The probe response frame1201may refer to a probe response frame specified in IEEE 802.11 or a short version (for example, a short probe response frame) of the probe response frame. Also, the probe response frame1201may refer to a frame specified for the frame transmission and reception procedure based on interference alignment. For example, the probe response frame1201may further include at least one of information indicating participation in the frame transmission and reception procedure based on interference alignment, and identification information (for example, an SSID and a BSSID), operation channel information, bandwidth information and antenna information of the third access point STA3.

The first terminal STA1may receive the probe response frame1201transmitted from the third access point AP3. The first terminal STA1may recognize that the third access point AP3serves as an interference source to the first terminal STA1based on information included in the probe response frame1201. Also, the first terminal STA1may recognize identification information, operation channel information, bandwidth information, transmitting antenna information, and the like of the third access point AP3.

When all responses for the probe request frame1200are received in the channel3303(or when a waiting time set for receiving a response for the probe request frame1200is terminated), the first terminal STA1may move to the channel2302and transmit a probe request frame1202. In this case, when the channel2302is in an idle state during the DIFS, the first terminal STA1may transmit the probe request frame1202in a broadcast manner after a contention window according to random backoff. The probe request frame1202may include the same information as the probe request frame1200. For example, the probe request frame1202may further include at least one of information indicating that the frame transmission and reception procedure based on interference alignment is started, and identification information, operation channel information, bandwidth information and antenna information of the first terminal STA1.

The second access point AP2operated in the channel2302may receive the probe request frame1202transmitted from the first terminal STA1. The second access point AP2may recognize that the frame transmission and reception procedure based on interference alignment is started by the first terminal STA1based on information included in the probe request frame1202and recognize identification information, operation channel information, bandwidth information, antenna information, and the like of the first terminal STA1.

The second access point AP2may transmit a probe response frame1203that is a response for the probe request frame1202. In this case, when the channel2302is in an idle state during the DIFS, the second access point AP2may transmit the probe response frame1203after a contention window according to random backoff. Here, by transmitting the probe response frame1203, the second access point AP2may represent participation in the frame transmission and reception procedure based on interference alignment. On the other hand, when the second access point AP2does not participate in the frame transmission and reception procedure based on interference alignment, a response for the probe request frame1202may not be transmitted.

The probe response frame1203may include the same information as the probe response frame1201. For example, the probe response frame1203may further include at least one of identification information, operation channel information, bandwidth information and antenna information of the second access point STA2.

The first terminal STA1may receive the probe response frame1203transmitted from the second access point AP2. The first terminal STA1may recognize that the second access point AP2serves as an interference source to the first terminal STA1based on information included in the probe response frame1203. Also, the first terminal STA1may recognize identification information, operation channel information, bandwidth information, antenna information, and the like of the second access point AP2.

Also, when all responses for the probe request frame1202are received in the channel2302(or when a waiting time set for receiving a response for the probe request frame1202is terminated), the first terminal STA1may move to the channel4304and transmit a probe request frame (not illustrated), and receive a probe response frame (not illustrated) that is a response for the probe request frame (not illustrated).

When the scanning procedure is completed, the first terminal STA1may identify an overlapping band based on the scanning result. That is, the first terminal STA1may identify that some band between the channel1301in which the first terminal STA1operates and the channel2302in which the second access point AP2operates overlaps and identify that some band between the channel1301in which the first terminal STA1operates and the channel3303in which the third access point AP3operates overlaps. The first terminal STA1may recognize that interference alignment technology is applied to a subcarrier to be transmitted through the overlapping band.

Then, the first terminal STA1may generate an NAI frame1204including the scanning result and transmit the generated NAI frame1204to the first access point AP1(S1103). The NAI frame1204may be the same as the NAI frame described above with reference toFIG. 7.

That is, the NAI frame1204may include a frame control field, a duration field, a receiver address field, a transmitter address field, an NAI field, a BSSID field and an FCS field. The NAI field may include an operation channel field, an antenna field, a bandwidth field and a reserved field. As many NAI fields as the number of neighbor access points may be included in the NAI frame1204. For example, when there are three neighbor access points, three NAI fields may be included in the NAI frame1204.

Here, the NAI frame1204may further include the NAI field (that is, operation channel information, bandwidth information, antenna information and the like of the second access point AP2) of the second access point AP2, and the NAI field (that is, operation channel information, bandwidth information, antenna information and the like of the third access point AP3) of the third access point AP3.

The first access point AP1may receive the NAI frame1204from the first terminal STA1. The first access point AP1may identify that the first terminal STA1is interfered with by the second access point AP2and the third access point AP3based on information included in the NAI frame1204. Also, the first access point AP1may recognize identification information, operation channel information, bandwidth information, antenna information and the like of the second access point AP2and the third access point AP3which interfere with the first terminal STA1.

When reception of the NAI frame1204is completed, the first access point AP1may transmit an NAI frame1205to the second access point AP2and the third access point AP3via wired or wireless communication. The NAI frame1205may include the same information as information included in the NAT frame1204. Also, the NAI frame1205may further include operation channel information, bandwidth information and antenna information and the like of the first access point AP1in addition to information included in the NAI frame1204. In addition, the NAI frame1205may further include information on an overlapping band in which a subcarrier to which interference alignment technology is applied may be transmitted.

When the NAI frame1205is received from the first access point AP1, the second access point AP2may identify an overlapping band between the second access point AP2and the first access point AP1and an overlapping band between the second access point AP2and the third access point AP3based on information included in the NAI frame1205. Also, the third access point AP3may receive the NAI frame1205from the first access point AP1. The third access point AP3may identify an overlapping band between the third access point AP3and the first access point AP1and an overlapping band between the third access point AP3and the second access point AP2based on information included in the NAI frame1205.

Then, the first access point AP1may transmit an NDP request frame1206to the first terminal STA1in order to estimate information on a channel used for interference alignment. In this case, when the channel is in an idle state during the DIFS in the channel1301, the first access point AP1may transmit the NDP request frame1206to the first terminal after a contention window according to random backoff. The NDP request frame1206may be the same as the NDP request frame described above with reference toFIG. 8.

The first terminal STA1may receive the NDP request frame1206from the first access point AP1(S1104). In order to announce transmission of NDP response frames1208,1209and1210to each of the access points AP1, AP2and AP3, the first terminal STA1may transmit an NDP announcement frame1207(S1105). That is, the first terminal STA1may transmit the NDP announcement frame1207to the first access point AP1after the SIFS from a reception end time of the NDP request frame1206. In addition, the first terminal STA1may move to the channel2302and transmit the NDP announcement frame1207to the second access point AP2, or move to the channel3303and transmit the NDP announcement frame1207to the third access point AP3. Hereinafter, a structure of the NDP announcement frame1207will be described.

FIG. 13is a block diagram illustrating a configuration of an NDP announcement frame.

As illustrated inFIG. 13, the NDP announcement frame may include a frame control field1310having a 2-octet size, a duration field1320having a 2-octet size, a receiver address field1330having a 6-octet size, a transmitter address field1340having a 6-octet size, a sounding dialog token field1350having a 1-octet size, at least one node information field1360having a 2-octet size and an FCS field1370having a 4-octet size.

The node information field1360may include identification information of an access point that will receive the NDP response frame transmitted after a corresponding NDP announcement frame is transmitted. When the NDP response frame is transmitted to each of the plurality of access points after the NDP announcement frame is transmitted, as many node information fields1360as the number of plurality of access points may be included in the NDP announcement frame.

When the plurality of node information fields1360are included in the NDP announcement frame, a sequence of the plurality of node information fields1360positioned in the NDP announcement frame may refer to a sequence in which the NDP response frame is transmitted to each of the plurality of access points. For example, the NDP response frame may be transmitted first to an access point indicated by the first node information field1360in the NDP announcement frame, and the NDP response frame may be transmitted second to an access point indicated by the second node information field1360.

Referring again toFIGS. 11, 12A and 12B, the NDP announcement frame1207may include sequentially a node information field1indicating the first access point AP1, a node information field2indicating the second access point AP2, and a node information field3indicating the third access point AP3.

The first access point AP1that has received the NDP announcement frame1207may recognize that the NDP response frame1208may be transmitted to the first access point AP1first among the access points AP1, AP2and AP3based on information included in the NDP announcement frame1207, and set a network allocation vector (NAV) timer based on a duration field included in the NDP announcement frame1207in order to receive the NDP response frame1208.

The second access point AP2that has received the NDP announcement frame1207may recognize that the NDP response frame1209may be transmitted second to the second access point AP2among the access points AP1, AP2and AP3based on information included in the NDP announcement frame1207and set the NAV timer based on a duration field included in the NDP announcement frame1207in order to receive the NDP response frame1209.

The third access point AP3that has received the NDP announcement frame1207may recognize that the NDP response frame1210may be transmitted third to the third access point AP3among the access points AP1, AP2and AP3based on information included in the NDP announcement frame1207, and set the NAV timer based on a duration field included in the NDP announcement frame1207in order to receive the NDP response frame1210.

Meanwhile, after the NDP announcement frame1207is transmitted, the first terminal STA1may transmit the NDP response frames1208,1209and1210to each of the access points AP1, AP2and AP3(S1106). That is, the first terminal STA1may transmit the NDP response frame1208to the first access point AP1through the channel1301. After the NDP response frame1208is transmitted, the first terminal STA1may move to the channel2302and transmit the NDP response frame1209to the second access point AP2. After the NDP response frame1209is transmitted, the first terminal STA1may move to the channel3303and transmit the NDP response frame1210to the third access point AP3. Here, the NDP response frames1208,1209and1210may be the same as the NDP response frame described above with reference toFIG. 9.

Each of the access points AP1, AP2and AP3that have received the NDP response frames1208,1209and1210may identify a desired channel and an interference channel according to channel reciprocity. This is called analog local channel information feedback or implicit local channel information feedback. That is, the first access point AP1may estimate channel information from the first terminal STA1to the first access point AP1based on the NDP response frame1208and estimate channel information from the first access point AP1to the first terminal STA1based on the estimated information. The second access point AP2may estimate channel information from the first terminal STA1to the second access point AP2based on the NDP response frame1209and estimate channel information from the second access point AP2to the first terminal STA1based on the estimated information. The third access point AP3may estimate channel information from the first terminal STA1to the third access point AP3based on the NDP response frame1210and estimate channel information from the third access point AP3to the first terminal STA1based on the estimated information.

Accordingly, each of the access points AP1, AP2and AP3may estimate channel information to the first terminal STA1and obtain a precoder, a decoder and the like for interference alignment based on the estimated channel information. The first access point AP1may provide the precoder, the decoder and the like for interference alignment to the first terminal STA1.

After the NDP response frames1208,1209and1210are transmitted, the first terminal STA1may notify the access points AP1, AP2and AP3of completion of a pre-procedure for frame transmission and reception based on interference alignment (S1107). That is, the first terminal STA1may transmit an IA request to send (RTS) frame1211to the first access point AP1through the channel1301. When the IA RTS frame1211is received, the first access point AP1may recognize that the pre-procedure for frame transmission and reception based on interference alignment is completed, and transmit an IA clear to send (CTS) frame1212that is a response for the IA RTS frame1211to the first terminal STA1.

Also, the first terminal STA1may transmit the IA RTS frame1213to the second access point AP2through the channel2302. When the IA RTS frame1213is received, the second access point AP2may recognize that the pre-procedure for frame transmission and reception based on interference alignment is completed and transmit an IA CTS frame1214that is a response for the IA RTS frame1213to the first terminal STA1.

Also, the first terminal STA1may transmit an IA RTS frame1215to the third access point AP3through the channel3303. When the IA RTS frame1215is received, the third access point AP3may recognize that the pre-procedure for frame transmission and reception based on interference alignment is completed and transmit an IA CTS frame1216that is a response for the IA RTS frame1215to the first terminal STA1.

Hereinafter, configurations of the IA RTS frames1211,1213and1215and configurations of the IA CTS frames1212,1214and1216will be described.

FIG. 14is a block diagram illustrating a configuration of an IA RTS frame.

As illustrated inFIG. 14, the IA RTS frame may include a frame control field1410having a 2-octet size, a duration field1420having a 2-octet size, a receiver address field1430having a 6-octet size, a transmitter address field1440having a 6-octet size, and an FCS field1450having a 4-octet size.

FIG. 15is a block diagram illustrating a configuration of an IA CTS frame.

As illustrated inFIG. 15, the IA CTS frame may include a frame control field1510having a 2-octet size, a duration field1520having a 2-octet size, a receiver address field1530having a 6-octet size, a transmit opportunity (TxOP) field1540having a 1-octet size and an FCS field1550having a 4-octet size. Here, the TxOP field1540may indicate TxOP for an access point that has transmitted the corresponding IA CTS frame.

Referring again toFIGS. 11, 12A and 12B, each of the access points AP1, AP2and AP3may pre-code the frame and transmit pre-coded frames1217,1218and1219. Meanwhile, the first terminal STA1may receive the pre-coded frame1219from the first access point AP1and align interference of the pre-coded frame1219based on information obtained from the first access point AP1(S1108).

FIG. 16is a block diagram illustrating another embodiment of a station for performing methods according to the present invention.

As illustrated inFIG. 16, a station1600may include a frame transmission and reception unit1610in the related art configured to transmit and receive a frame according to the IEEE 802.11 standards in the related art, and an interference alignment-based frame transmission and reception unit1620configured to transmit and receive a frame based on interference alignment technology. The frame transmission and reception unit1610in the related art may include a channel access unit1611, a transmission preparing unit1612in the related art and an individual transmission unit1613. The interference alignment-based frame transmission and reception unit1620may include an interference control start determining unit1621, an overlapping band searching unit1622, an AP information sharing unit1623, a channel information estimating unit1624, an interference control preparing unit1625and a cooperative transmission unit1626.

The channel access unit1611may perform a function of accessing a channel. That is, the channel access unit1611may be involved in a scanning procedure (for example, active scanning or passive scanning), a certification procedure (for example, transmission of a certification request frame and reception of a certification response frame), and a connection procedure (for example, transmission of a connection request frame and reception of a connection response frame).

After accessing the channel, the interference control start determining unit1621may perform a function of determining whether the frame transmission and reception procedure based on interference alignment is started based on a signal level of the received frame. That is, the interference control start determining unit1621may perform operation S500described above with reference toFIG. 5and operation S1100described with reference toFIG. 11. When the signal level of the received frame is greater than a preset threshold value, the frame transmission and reception procedure based on interference alignment may not be started. In this case, the transmission preparing unit1612in the related art and the individual transmission unit1613may transmit and receive a frame according to the IEEE 802.11 standards in the related art.

On the other hand, when the signal level of the received frame is less than the preset threshold value, the frame transmission and reception procedure based on interference alignment may be started. In this case, the frame transmission and reception procedure based on interference alignment may be performed by the overlapping band searching unit1622, the AP information sharing unit1623, the channel information estimating unit1624, the interference control preparing unit1625and the cooperative transmission unit1626.

The overlapping band searching unit1622may perform a function of searching for an overlapping band between access points. That is, the overlapping band searching unit1622may perform operation S502and operation S503described above with reference toFIG. 5, and perform operation S1102described above with reference toFIG. 11. The AP information sharing unit1623may perform a function of sharing information on the neighbor access point found through the scanning procedure. That is, the AP information sharing unit1623may be involved in the procedure of transmitting and receiving the NAI frames604,605,606and607described above with reference toFIGS. 6A and 6Band the procedure of transmitting and receiving the NAI frames1204and1205described above with reference toFIGS. 12A and 12B.

The channel information estimating unit1624may perform a function of estimating a channel between a terminal and an access point. That is, the channel information estimating unit1624may be involved in the procedure of transmitting and receiving the NDP request frame608and the procedure of transmitting and receiving the NDP response frames609,610and611described above with reference toFIGS. 6A and 6B. Also, the channel information estimating unit1624may be involved in the procedure of transmitting and receiving the NDP request frame1206, the procedure of transmitting and receiving the NDP announcement frame1207, and the procedure of transmitting and receiving the NDP response frames1208,1209and1210described above with reference toFIGS. 12A and 12B.

The interference control preparing unit1625may perform a function of indicating that the pre-procedure for the frame transmission and reception procedure based on interference alignment is completed. That is, the interference control preparing unit1625may perform the procedure of transmitting and receiving the IA RTS frames1211,1213and1215and the procedure of transmitting and receiving the IA CTS frames1212,1214and1216described above with reference toFIGS. 12A and 12B.

The cooperative transmission unit1626may perform a function of transmitting and receiving a frame to which interference alignment technology is applied. That is, the cooperative transmission unit1626may be involved in the procedure of transmitting and receiving the pre-coded frames612,613and614described above with reference toFIGS. 6A and 6Band the procedure of transmitting and receiving the pre-coded frames1217,1218and1219described above with reference toFIGS. 12A and 12B.

According to the present invention, it is possible to address an interference problem between neighbor access points through interference alignment. When information necessary for interference alignment between access points is shared in the centralized topology (or in the decentralized topology), interference alignment may be applied to the overlapping band. Also, interference alignment may be applied to only the subcarrier transmitted through the overlapping band. Accordingly, it is possible to improve reliability and frequency efficiency of the wireless local area network system.

According to the present invention, it is possible to control interference in an overlapping band between neighbor access points in a wireless local area network.

Embodiments of the present invention may be implemented in the form of program instructions that can be performed through various computer units and recorded in computer readable media. The computer readable media may include a program instruction, a data file, a data structure, or combinations thereof. The program instruction recorded in the computer readable media may be specially designed and prepared for the embodiments of the invention or may be an available well-known instruction for those skilled in the field of computer software.

The computer readable media may refer to a hardware device that is specially made to store and perform the program instruction such as a ROM, a RAM, or a flash memory. The hardware device may be configured as at least one software module in order to perform operations of embodiments of the present invention and vice versa. The program instruction may refer to a machine code generated by a compiler and a high-level language code that can be executed in a computer based on an interpreter or the like.

While the present invention has been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the present invention as defined by the appended claims.