WO2011136473A2

WO2011136473A2 - Method for avoiding interference in wireless communication system and apparatus for the same

Info

Publication number: WO2011136473A2
Application number: PCT/KR2011/001995
Authority: WO
Inventors: Dae Won Lee; In Kyu Lee; Kyoung Jae Lee; Yong Ho Seok
Original assignee: Lg Electronics Inc.; Korea University Research And Business Foundation
Priority date: 2010-04-26
Filing date: 2011-03-23
Publication date: 2011-11-03
Also published as: WO2011136473A3

Abstract

Disclosed are a method for avoiding interference in a wireless communication system and an apparatus supporting the same. The method for avoiding interference by an access point (AP) in a wireless local area network (WLAN) system includes: acquiring first effective channel information between the AP and a first station (STA) targeted for multi user-multiple input multiple output (MU-MIMO) transmission; acquiring second effective channel information between the AP and a second STA that receives data from a third STA during the MU-MIMO transmission; and performing the MU-MIMO transmission to a target STA including the first STA by beamforming based on an interference-avoiding precoding that eliminates interference due to data transmission from the third STA to the second STA from a receiving signal of the first STA and interference due to the MU-MIMO transmission from a receiving signal of the second STA.

Description

METHOD FOR AVOIDING INTERFERENCE IN WIRELESS COMMUNICATION SYSTEM AND APPARATUS FOR THE SAME

The present invention relates to wireless communications, and more particularly, to a method for avoiding interference in a wireless communication system and an apparatus supporting the same.

With recent development of information and communication technology, various wireless communication technologies have been developed. Among them, a wireless local area network (WLAN) is a technology based on a radio frequency technology, which allows a mobile terminal such as a personal digital assistant (PDA), a laptop computer, a portable multimedia player (PMP), etc. to wirelessly access Internet in a home, a business or a certain service providing zone.

Since the Institute of Electrical and Electronics Engineers (IEEE) 802 was established as organization for standardization of the WLAN technology on February 1980, a lot of standardization efforts have been made. Under the IEEE 802. 11, the early WLAN technology supported a data rate of 1 to 2Mbps using a frequency of 2.4GHz through frequency hopping, a spread spectrum, infrared communication, etc., but the recent WLAN technology can support a peak data rate of 54Mbps using orthogonal frequency division multiplex (OFDM). Besides, the IEEE 802.11 is putting standards of various technologies to practical use or developing it, including quality of service (QoS) enhancement, access point protocol compatibility, security enhancement, radio resource measurement, wireless access vehicular environment, fast roaming, a mesh network, interworking with an external network, a wireless network management, etc. Also, the IEEE 802.11n is a technical standard relatively recently introduced to overcome a limited data rate which has been considered as a drawback in the WLAN. The IEEE 802.11n is devised to increase network speed and reliability and to extend an operational distance of a wireless network. More specifically, the IEEE 802.11n supports a high throughput (HT), i.e., a data processing rate of up to above 540 Mbps, and is based on a multiple input and multiple output (MIMO) technique which uses multiple antennas in both a transmitter and a receiver to minimize a transmission error and to optimize a data rate. In addition, this standard may use a coding scheme which transmits several duplicate copies to increase data reliability and also may use the OFDM to support a higher data rate.

With the widespread use of the WLAN and the diversification of applications using the WLAN, there is a recent demand for a new WLAN system to support a higher throughput than a data processing rate supported by the IEEE 802.11n. However, the IEEE 802.11n medium access control (MAC)/physical layer (PHY) protocol is not effective in providing a throughput of above 1Gbps. The reason why is because the IEEE 802.11 MAC/PHY protocol is for operating a single station (STA), i.e., a STA having one network interface card (NIC) and thus an additional overhead also increases as a frame throughput increases while intactly maintaining the MAC/PHY protocol of the existing IEEE 802.11n. Finally, there is a limit to throughput enhancement of the wireless communication network while intactly maintaining the existing IEEE 802.11n MAC/PHY protocol, i.e., the single STA architecture.

Accordingly, to achieve a data processing rate of above 1 Gbps, there is required a new system different from the existing IEEE 802.11n MAC/PHY protocol, i.e., different from the single STA architecture. As the next version of the IEEE 802.11n WLAN system, very high throughput (VHT) WLAN system is one of IEEE 802.11 WLAN systems which have recently been proposed to support the data processing rate of above 1 Gbps in a MAC service access point (SAP).

The VHT WLAN system allows a plurality of VHT STAs to simultaneously access and use a channel at the same time so as to efficiently use a radio channel. To this end, transmission based on a multi user multiple input multiple output (MU-MIMO) method using multiple antennas is supported. A VHT access point (AP) can perform spatial division multiple access (SDMA) transmission for transmitting spatially multiplexed data to the plurality of VHT STAs. Since data is simultaneously transmitted by distributing the plurality of spatial streams to the plurality of STAs through the plurality of antennas, the overall throughput of the WLAN system can be enhanced.

In a multiple-user environment where one AP supports the plurality of STAs, there have been researched a multiple transceiving antenna transmission technologies or the like considering the multi users in order to increase the whole channel capacity of the MU-MIMO system considering the multi users. A channel environment for the multi users has to guarantee that a channel matrix is in a good status so that all MU-MIMO techniques can fully use a degree of spatial freedom, and is required to make simultaneous communication possible at respective desired transmission rates without being limited by interference between the multi users. Since the AP in a downlink channel simultaneously transmits a wireless signal to many STAs, each STA receives a signal of another user in addition to a desired signal, which may act as interference. To keep the interference down, the AP filters the channel and eliminates the interference. For example, a zero-forcing filter may be used to relieve the interference.

Meanwhile, the IEEE 802.11e supports a direct link setup (DLS) service where data is directly transmitted between the STAs without passing the AP. In the DLS service, a direct link (DL) is set up between a DLS initiating STA (i.e., a DLS initiator) and a DLS responding STA (i.e., a DLS responder), and then a data frame is directly transmitted/received via the DL. For more details of the DLS service, sections 7.4.3 and of ‘IEEE Standard for Information technology- Telecommunications and information exchange between systems-Local and metropolitan area networks- Specific requirements, Part 11: Wireless LAN Medium Access Control(MAC) and Physical Layer(PHY) Specifications,’ which was established on June 2007, will be referred.

While the MU-MIMO transmission is implemented between the AP and the plural STAs to more efficiently use a radio resource in the WLAN system supporting the MU-MIMO, there may be considered a method to simultaneously perform data transmission using the DLS service between the STAs unsuited for the MU-MIMO transmission. At this time, the MU-MIMO transmission of the AP may act as interference to the STA that receives data based on the DLS service.

In the case where the AP knows channel information, the simplest method to deal with the interference between the STAs may include a zero-forcing method of making all interference signal into zero, or a block diagonalization method, and a transmitter may apply precoding a transmission signal by a pseudoinverse matrix of a channel. However, there is a constraint that the number of STA transmission antennas should be more than the number of total data streams of other users in order to have the ever-present pseudoinverse matrix of the channel. Under a bad channel environment, e.g., when the STAs are closely located, power consumed by eliminating the interference may cause the power to be reduced and thus deteriorate system performance. This results in lowering the reliability of the data transmission and hindering an efficient use of the radio resource, thereby decreasing the overall throughput of the WLAN system. Accordingly, there is a need for considering a method to avoid the interference when the MU-MIMO transmission and the data transmission based on the DLS service are implemented at the same time.

The present invention provides a method for avoiding interference in a WLAN system where MU-MIMO transmission and data transmission based on a DLS service are simultaneously performed, and an apparatus supporting the same.

In an aspect, there is provided a method for avoiding interference by an access point (AP) in a wireless local area network (WLAN) system, the method including: acquiring first effective channel information between the AP and a first station (STA) targeted for multi user-multiple input multiple output (MU-MIMO) transmission; acquiring second effective channel information between the AP and a second STA that receives data from a third STA during the MU-MIMO transmission; and performing the MU-MIMO transmission to a target STA including the first STA by beamforming based on an interference-avoiding precoding that eliminates interference due to data transmission from the third STA to the second STA from a receiving signal of the first STA and interference due to the MU-MIMO transmission from a receiving signal of the second STA, the interference-avoiding precoding matrix being obtained on the basis of a pseudo inverse matrix

that transforms the first effective channel information, the second effective channel information and a certain element of a matrix into 0.

The pseudo inverse matrix may be expressed by the following expression:

where, P_t is a power limit value of the whole WLAN system, the number of ‘0’ in a diagonal matrix is n₀, and the number of ‘1’ is N_r, i.e., in which N_r is a total number of antennas the target STA including the first STA has, and n₀is a total number of antennas the second STA has.

The first effective channel information may be acquired by applying a first receiving filter to a first channel information, the first channel information may be channel information between the AP and the first STA, and the first receiving filter may be acquired on the basis of first interference channel information which is channel information between the third STA and the first STA.

The first channel information may be acquired by transmitting a training request (TRQ) message for requesting a sounding frame to the first STA; receiving the sounding frame from the first STA in response to the TRQ message; and estimating a channel between the AP and the first STA on the basis of the sounding frame.

The first interference channel information may be included in the sounding frame received from the first STA.

The second effective channel information may be acquired by applying a second receiving filter to a second channel information, the second channel information may be channel information between the AP and the second STA, and the second receiving filter may be acquired on the basis of second interference channel information between the third STA and the second STA.

The second interference channel information may be acquired by transmitting a TRQ message for requesting a sounding frame to the second STA; receiving the sounding frame from the second STA in response to the TRQ message; and estimating a channel between the AP and the second STA on the basis of the sounding frame.

The second interference channel information may be included in the sounding frame received from the second STA.

The second STA may receive the data from the third STA through a direct link.

In another aspect, there is provided a method for avoiding interference by an access point (AP) in a wireless local area network (WLAN) system, the method including: acquiring first effective channel information between the AP and a first station (STA) targeted for multi user-multiple input multiple output (MU-MIMO) transmission; acquiring second effective channel information between the AP and a second STA that receives data from a third STA during the MU-MIMO transmission; and performing the MU-MIMO transmission to a target STA including the first STA by beamforming based on an interference-avoiding precoding that eliminates interference due to data transmission from the third STA to the second STA from a receiving signal of the first STA and interference due to the MU-MIMO transmission from a receiving signal of the second STA, the interference-avoiding precoding matrix being obtained on the basis of a pseudo inverse matrix

that transforms the first effective channel information, the second effective channel information and a certain element of a matrix into 0, and a transmission weight matrix for eliminating interference with the target STA.

The pseudo inverse matrix may be expressed by the following expression:

The transmission weight matrix may be determined on the basis of a condition for minimizing a mean square error (MSE) under a total power constraint of the WLAN system.

The transmission weight matrix may be determined on the basis of a condition for minimizing interference-plus-noise power under a power constraint per target STA including the first STA.

In still another aspect, there is provided a wireless local area network (WLAN) apparatus including: a processor; and a transceiver which functionally connects with the processor and transmits and receives a frame, the processor acquiring first effective channel information between the AP and a first station (STA) targeted for multi user-multiple input multiple output (MU-MIMO) transmission; acquiring second effective channel information between the AP and a second STA that receives data from a third STA during the MU-MIMO transmission; and performing the MU-MIMO transmission to a target STA including the first STA by beamforming based on an interference-avoiding precoding that eliminates interference due to data transmission from the third STA to the second STA from a receiving signal of the first STA and interference due to the MU-MIMO transmission from a receiving signal of the second STA, and the interference-avoiding precoding matrix being obtained on the basis of a pseudo inverse matrix

The pseudo inverse matrix may be expressed by the following expression:

As apparent from the foregoing description, in an environment where an MU-MIMO downlink transmission and a DLS service coexist and interference therebetween may arise, an interference MU-MIMO precoder to the DLS is kept down so that the throughput of the WLAN system supporting the MU-MIMO can be enhanced without deteriorating the performance of the DLS service.

FIG. 1 is a view showing an example of a wireless local area network (WLAN) system to which an exemplary embodiment of the present invention can be applied.

FIG. 2 shows a WLAN system to which an exemplary embodiment of the present invention can be applied.

FIG. 3 is a flowchart showing a method of avoiding interference in the WLAN system according to an exemplary embodiment of the present invention.

FIG. 4 is a graph showing a simulation result of a signal to noise ratio (SNR)-sum rate relationship at a multi user multiple input multiple output (MU-MIMO) downlink transmission based on the interference avoiding method according to an exemplary embodiment of the present invention.

FIG. 5 is a graph showing a simulation result of a SNR-frame error rate (FER) relationship at the MU-MIMO downlink transmission based on the interference avoiding method according to an exemplary embodiment of the present invention.

FIG. 6 is a block diagram showing a wireless apparatus in which an exemplary embodiment of the present invention is achieved.

Below, an exemplary embodiment of the present invention will be described in detail with reference to accompanying drawings. The exemplary embodiment to be described later may be usefully applied to a very high throughput (VHT) wireless local area network (WLAN) system supporting a multi user multiple input multiple output (MU-MIMO), and thus the VHT WLAN system will be described by way of example but the technical features of the present invention are not limited thereto. For example, an interference avoiding method proposed by the present invention may be equally applied even when data transmission based on a direct link setup (DLS) service (hereinafter, referred to as a “direct link transmission”) and another direct link transmission are simultaneously performed. The direct link transmission is an example of communication between stations (STAs), which may be equally applied to a Wi-Fi direct service and other direct communications between stations besides an IEEE 802.11 DLS service.

An interference avoiding method proposed by the present invention may be equally applied to a WLAN system operating in a frequency band of 512-698MHz (television (TV) white space), a frequency band of 2.5GHz (2.4-2.4835GHz, low band (LB)), a frequency band of 5GHz (4.9-5.825GHz, high band(HB)), and a frequency band of 60GHz (57-66GHz, ultra band (UB)).

While an access point (AP) transmits a data frame to its paired mobile station by a MU-MIMO downlink transmission method, adjacent two mobile STAs set up a direct link and transmits the data frame. The two mobile STAs having the setup direct link may be regarded as a high definition (HD) TV and a set-top box, respectively. Since an HD TV signal has to be transmitted at a high transmission rate, the two STAs may operate in a closed-loop MIMO method.

In light of transmitting a data frame, the AP can perform an MU-MIMO down link communication to its paired STA through a spatial resource of one frequency without interfering with a receiving signal of the HD TV. Hereinafter, a transceiving terminal that maximizes the performance of the MU-MIMO downlink channel without interfering with the direct link communication, and a method of configuring an operation necessary at this time will be described.

Referring to FIG. 1, a WLAN system includes one or more basic service sets (BSS). The BSS is a set of STAs successfully synchronized and communicating with each other, which is not a concept indicating a specific zone.

The BSS may be divided into an infrastructure BSS, an independent BSS (IBSS), and a personal BSS (PBSS), and FIG. 1 illustrates the infrastructure BSS.

The infrastructure BSS BSS1, BSS2 includes one or more STAs STA1, STA2, STAa, STAb; an access point (AP) that is a STA providing a distribution service; and a distribution system (DS) connecting a plurality of APs. On the other hand, the IBSS does not include the AP, and therefore all STAs are mobile STAs. Further, the IBSS forms a self-contained network since access to the DS is not allowed.

The PBSS is a kind of IEEE 802.11 LAN ad hock network, which is similar to the IBSS. The STAs of the PBSS are directly connected to each other and communication between the STAs is possible. However, there is a PBSS control point (PCP) that serves as a coordinator of the BSS in contrast to the IBSS. The PCP is a STA that acts as a coordinator in the PBSS, solely takes charge of transmission for a beacon frame, and allocates for a service section and a competition-based section.

The STA is an arbitrary functional medium that includes a medium access control (MAC) complying with the IEEE 802.11 standards and a physical layer interface for a wireless medium, which broadly includes all such as an AP, a PCP, a non-AP STA, and a non-PCP station. Further, a STA including a transceiver operating at a frequency of 60GHz will be called a millimeter wave (mmWave STA, mSTA).

The STA for a wireless communication includes a processor, a transceiver, a user interface unit, a display unit, etc. The processor is a functional unit devised to generate a frame to be transmitted via a wireless network or process a frame received via the wireless network, which performs various functions for controlling the STA. Further, the transceiver is a unit functionally connected to the processor and devised to transmit and receive a frame for the STA via the wireless network.

Among the STAs, a mobile terminal to be operated by a user is a non-AP/non-PCP STA, which may indicate the non-AP/non PCP STA if it is just a STA. The non-AP/non-PCP STA may be differently called a terminal, a wireless transmitting/receiving unit (WTRU), user equipment (UE), a mobile station (MS), a mobile terminal, a mobile subscriber unit, etc. In the following description, the STA refers to the non-AP/non-PCP STA as long as there is no special comment.

The AP AP1, AP2 is a functional individual that provides a STA associated thereto with access to the DS via an wireless medium. In the infrastructure BSS including the AP, communication between the non-AP STAs is achieved via the AP in principle, but the non-AP STAs may directly communicate with each other if direct links are set up. Besides the access point, the AP may be called a centralized controller, a base station (BS), a node-B, a base transceiver system (BTS), a site controller, etc.

The plurality of infrastructure BSSs may be connected to one another through the distribution system (DS). The plurality of BSSs connected through the DS is called an extended service set (ESS). The STAs included in the ESS can communicate with each other, and the non-AP STA can move from one BSS to another BSS within one ESS while performing communication without disconnection.

The DS is a mechanism for making one AP communicate with another AP, by which the AP can transmit a frame for the STAs associated with its managing BSS, transmit a frame when one STA moves to another BSS, or transmit a frame to a wired network or the like external network. Such a DS is not necessarily a network, and there is no limit to its type as long as it can provide a modified distribution service complying with the IEEE 802.11. For example, the DS may be a wireless network such as a mesh network, or a physical structure connecting the APs one another.

FIG. 1 shows an example where data frame transmission based on downlink MU-MIMO transmission of an AP 50 to the STA1 10 and the STA2 20 and direct link transmission of a STAb 70 to the STAa 60 are performed at the same time. That is, it is illustrated by way of example that the MU-MIMO transmission and the direct link transmission are simultaneously implemented within the infrastructure BSS. This is nothing but an example of the WLAN system to which the present invention can be applied, and the present invention can be equally applied to the STA operating in the PBSS. In the case of being applied to the PBSS, the AP of FIG. 1 may be a PBSS control point (PCP). In the following exemplary embodiments, it is assumed that the AP 50 performs the MU-MIMO transmission with regard to the STA1 10 and the STA2 20 and the STAb 70 performs the direct link transmission with regard to the STAa 60.

In the example of FIG. 1 and the respective following exemplary embodiments, it will be assumed that the DLS service between the STAa 60 and the STAb 70 is initiated. The initiation of the DLS service between the STAa 60 and the STAb 70 is achieved by an exchange between a DLS request frame and a DLS response frame. For more details of procedures of formatting and exchanging the DLS request frame and the DLS response frame, sections 7.4.3 and of ‘IEEE Standard for Information technology- Telecommunications and information exchange between systems-Local and metropolitan area networks- Specific requirements, Part 11: Wireless LAN Medium Access Control(MAC) and Physical Layer(PHY) Specifications’ will be referred.

In the state that the STAb 70 transmits data to the STAa 60 through a direct link while the AP 50 implements the downlink MU-MIMO transmission with regard to the STA1 10 and the STA2 20, the MU-MIMO transmission of the AP 50 to the STA1 10 and the STA2 20 may act as interference to the STAa 60 when the STAa 60 receives the data from the STAb 70. When performing the MU-MIMO transmission, the AP 50 has to form a beam in consideration of the STAs such as the STAa 60 and the STAb 70 that transmit the data frame through the direct link. At this time, the data transmission of the STAb 70 that performs the direct link transmission may be achieved in the form of directional transmission or Omni-directional transmission. That is, the transmission type of the STAb 70 that performs the direct link transmission in the following exemplary embodiment may be anyone of the directional transmission or the Omni-directional transmission.

Referring to FIG. 2, there are an MU-MIMO group 100 including an AP and a STA which transmit and/or receive a frame through the MU-MIMO downlink transmission, and a direct link group 200 which includes a STA in which the direct link is set up. The two groups operating in a WLAN communication method may be included in one WLAN system or respectively included in separate WLAN systems. It will be assumed that the two groups are under a condition that interference therebetween may arise due to their physical locations and occupied frequency environments.

The MU-MIMO group 100 includes an AP 110 and K STAs 120 targeted for the MU-MIMO transmission (i.e., a STA1 120a, a STA2 120b, …, a STAK 120K). The AP 110 includes N_t antennas. The STA1 120a, the STA2 120b, …, the STAK 120K include n₁ antennas, n₂ antennas, …, n_K antennas, respectively. The total number of receiving antennas the plural STAs 120 have is N_r. P is a precoding matrix for the MU-MIMO transmission of the AP 110 to the STA 120 targeted for the MU-MIMO transmission, which can be expressed by P=[P ₁ P ₂ … P _K].

The direct link group 200 includes a STAd1 210 and a STAd2 220 which are set up for the direct link. The STAd1 210 transmits a frame to the STAd2 220 through the direct link. The STAd1 210 can support the MU-MIMO transmission. To this end, the STAd1 210 uses a precoding matrix P ₀.

The AP 110 has to acquire channel information H ₁, H ₂, …, H ₃ about the STA 120 targeted for the MU-MIMO transmission in order to transmit a frame to each of the STA 120 targeted for the MU-MIMO through the MU-MIMO downlink transmission. The channel information can be acquired by receiving direct data such as a channel matrix or a sounding frame from each STA 120 targeted for the MU-MIMO transmission and by estimating a channel. Alternatively, the channel information may be acquired by receiving a channel feedback.

First, the AP 110 needs to take the interference due to the direct link group 200 into account when performing the MU-MIMO downlink transmission to the STA 120 targeted for the MU-MIMO transmission. Referring to FIG. 2, a direct link wireless signal that the STAd1 210 transmits to the STAd2 220 may generate interference with the STA 120 targeted for the MU-MIMO transmission. Thus, the AP 110 may acquire the channel information about the STA1 120a, the STA2 120b, …, the STAK 120K from the STAd1 210, i.e., the information about the transmission that may generate interference based on the direct link in light of forming a beam to transmit the frame. Since the channel information based on the STAd1 210 is regarded for the AP 110 as channel information that affects interference with a receiving terminal, each channel information H ₁ ^I, H ₂ ^I,…, H ₃ ^I indicates the channel information from the STAd1 210 to each STA 120 targeted for the MU-MIMO transmission on the contrary to H ₁, H ₂,…, H ₃, which means the channel information related to the interference acting on the receiving terminal from the point of view of the AP 110 that performs the MU-MIMO downlink transmission.

The H ₁ ^I, H ₂ ^I,…, H ₃ ^I may be performed when the MU-MIMO targeted STA 120 receives and a direct link wireless signal that the STAd1 210 transmits to the STAd2 220 and estimates it to acquire a covariance matrix. Also, the channel information may be acquired by receiving a sounding frame and estimating a channel, or the channel information included in a certain frame may be directly received from the STAd2 220. Thus, the AP 110 can acquire the H ₁ ^I, H ₂ ^I,…, H ₃ ^I by receiving the sounding frame including the H ₁ ^I, H ₂ ^I,…, H ₃ ^Ifrom each MU-MIMO targeted STA 120. Also, it is possible to acquire the above channel information by receiving a separate frame including the above channel information. The channel information H ₁ ^I, H ₂ ^I,…, H ₃ ^I may be used for obtaining a receiving terminal filter of the STA 210 targeted for the MU-MIMO transmission.

Next, the AP 110 has to consider the interference caused in the direct link group 200 by the MU-MIMO downlink transmission. Referring to FIG. 2, while the AP 110 transmits a wireless signal to each STA 120 targeted for the MU-MIMO transmission, the wireless signal may be received by the STAd2 220, i.e., by the receiving terminal of the direct link group 200. Accordingly, from the point of view of the STAd2 220, the wireless signal corresponds to an interference signal. To ascertain the interference with the STAd2 220, the AP 120 has to acquire channel information H ₀ about the direct link transmission from the STAd1 210 to the STAd2 220 and channel information H ₀ ^Iabout the MU-MIMO downlink transmission of the AP 110 causing the interference with the STAd2 220. This may be achieved by receiving the sounding frame from the STAd2 220 and estimating a channel, or directly receiving a channel matrix about the channel information or a channel feedback.

As above, the AP 110 can acquire the channel information between the AP and the STA and between the STA and the STA, and form a beam based on the channel information, thereby transmitting the data frame to the STA 120 targeted for the MU-MIMO transmission. Below, an exemplary embodiment of the present invention will be described in more detail with reference to FIG. 3.

FIG. 3 is a flowchart showing a method of avoiding interference in the WLAN system according to an exemplary embodiment of the present invention. A channel matrix to be described below shows a data value of the corresponding channel information in the form of matrix, and meanings of them may be used as being mingled. Also, a channel matrix causing interference with another STA among the channel matrixes will be called an interference channel matrix.

Referring to FIG. 3, the AP 110 transmits a training request (TRQ) message to each STA 120 targeted for the MU-MIMO transmission and the STAd2 220 (S310). The TRQ message is a message for requesting the STA, which receives the TRQ message, to transmit a sounding frame.

The STA 120 targeted for the MU-MIMO transmission and the STAd2 220, which received the TRQ message, transmits the sounding frame to the AP 110 (S320). The sounding frame, which the STA 120 targeted for the MU-MIMO transmission transmits to the AP 110, may include interference channel information (e.g., an interference channel matrix) about that the direct link transmission performed by the STAd1 210 cause interference with the STA 120 targeted for the MU-MIMO transmission. Also, the sounding frame, which the STAd2 220 transmits to the AP 110, may include channel information (e.g., a channel matrix) about that the STAd1 210 transmits a wireless signal to the STAd2 through the direct link, and as necessary may include information related to the direct link formed by the STAd1 210 and the STAd2 220, for example, a precoding matrix used by the STAd1 210, and MIMO-based information of the STAd1 210 and the STAd2 220.

The AP 110 performs channel estimation with respect to each STA 120 targeted for the MU-MIMO transmission on the basis of the received sounding frame, and acquires the channel information H ₁, H ₂,…, H _K (S330).

The AP 110, which receives the sounding frame, acquires an interference-avoiding precoding matrix for avoiding the interference in light of the MU-MIMO downlink transmission on the basis of the channel information and the interference channel information between the AP and the STA and/or between the STA and the STA (S340). The acquisition of the interference-avoiding precoding matrix will be described in more detail.

The AP 110, which acquires the interference-avoiding precoding matrix, forms a beam based on the interference-avoiding precoding matrix (S350), and transmits a data frame to each MU-MIMO targeted STA 120 by the MU-MIMO downlink transmission method (S360).

In the foregoing exemplary embodiment, the AP 110 transmits the TRQ message, receives the sounding frame, and performs the channel estimation so as to acquire the channel information H ₁, H ₂,…, H _K about the corresponding STA. However, the channel information may be alternatively acquired by receiving a certain frame (including the sounding frame) in which the channel information about the corresponding STA is included. Also, the interference channel information H ₀ ^I acquired after performing the channel estimation based on the sounding frame received from the STAd2 220 may be directly acquired by receiving a certain frame, in which the channel information is included, from the STAd2 220.

Below, a method of acquiring the interference-avoiding precoding matrix will be described with reference to expressions.

Referring back to FIG. 2, in the MU-MIMO group 100, s is a data symbol vector about the MU-MIMO targeted STA 120, which can be expressed by s=[s ₁ ^T s ₂ ^T … s _K ^T]^T. A noise vector at the receiving terminal of the STA 120 targeted for the MU-MIMO transmission can be expressed by z=[z ₁ ^T z ₂ ^T … z _K ^T]^T. The channel matrix of the AP 110 and the STA 120 targeted for the MU-MIMO transmission can be expressed by H=[H ₁ ^T H ₂ ^T … H _K ^T]^T. A whole receiving filter of the WLAN system is defined by M=diag{M ₁ M ₂ … M _K}. The interference channel where the STAd1 210 causes interference with the STA 120 targeted for the MU-MIMO transmission may be expressed by H ^I=[H ₁ ^IT H ₂ ^IT … H _K ^IT].

In the direct link group 200, s ₀is a data symbol vector transmitted through the direct link transmission, P ₀ is a precoding matrix in the direct link, H ₀is a channel matrix, and H ₀ ^Iis an interference channel matrix.

In the MU-MIMO group 100, a total receiving signal vector y of the MU-MIMO downlink can be expressed by the following expression 1.

[Expression 1]

At this time, the receiving signal vector y _jabout the j^th STA, i.e., the STAj can be expressed by the following expression 2 (where, j is an integer smaller than k, and the STAj indicates one among the STA 120).

[Expression 2]

This shows a state where n_j spatial streams are transmitted with regard to the STAj, and it will be assumed that the state satisfies the condition of the following expression 3.

[Expression 3]

Next, in the direct link group 200, a total receiving signal vector about the direct link will be expressed by the following expression 4.

[Expression 4]

Each of the MU-MIMO targeted STAs 120, which receive the frame through the MU-MIMO downlink transmission in the MU-MIMO group 100, estimates interference generated by the direct link group 120, and whitens the interference and noise through Cholesky factorization. At this time, the matrix to be applied will be called a whitening filter. When an interference signal is received in a certain direction, the whitening filter is generally designed to minimize the corresponding directionality, so that the interference signal can be minimized by applying the whitening filter to the received signal. The procedure of the Cholesky factorization applied to the WLAN system given according to the present invention and the whitening filter

obtained by this procedure can be expressed by the following expression 5. The whitening filter may be a result from applying Cholesky decomposition to a covariance matrix, and the expression 5 shows the procedure of applying the Cholesky decomposition to the covariance matrix with regard to a sum of distribution of HPs signal matrix and distribution of noise.

[Expression 5]

The whitening filter

is expressed by a block diagonal matrix of the whitening filter L ₁, L ₂, …, L _K to be applied to each of the STAs 120 targeted for the MU-MIMO. The whitening filter L of the MU-MIMO targeted STA 120 may include a component related to information about an amplitude and receiving direction of a wireless signal to be received, and a component related to information about noise. In other words, if the information about only the direction of the signal to be received is used, this whitening filter is a zero-forcing type whitening filter. However, this filter takes the noise component into account, and thus becomes a whitening filter considering the minimization of the mean square error (MSE).

If the whitening filter

is applied to the receiving signal vector y of the MU-MIMO group 100, it can be expressed by the following expression 6. At this time, the whitening-filtered channels, i.e., an effective channel matrix

can be expressed by the following expression 7.

[Expression 6]

[Expression 7]

Next, if a receiving filter used by the receiving terminal, i.e., the STAd2 220 for the direct link transmission in the direct link group 200 is

, the receiving signal received by the STAd2 220 can be expressed by the following expression 8. The receiving filter

in the direct link may be determined by the direct link.

[Expression 8]

The channel information about the interference caused in the STAd2 220 by the AP 110 can be expressed by the effective channel matrix

via the receiving filter and the receiving channel.

Each MU-MIMO targeted STA 120 compensates for the channel information, to which the receiving filter

is applied, i.e., compensates for each sub matrix of

by feedback to the AP 110 or reciprocity calibration, so that the AP 110 can use the channel information.

Using this channel information, it is possible to obtain an F matrix, which can be expressed by the following expression 9.

[Expression 9]

F _mb = [F _mb,0 F _mb,1 F _mb,2 … F _mb,K]=H _S ^H(H _S H _S ^H+R _n)^-1

The matrix F is a matrix obtained by pseudo-inversing the channel information H _s, which matches with the channel information H _s. The matching matrix decomposes the channel information H _s to mostly leave only diagonal values when the channel information H _s is multiplied by it , in which non-diagonal values are transformed into very small values. That is, if the corresponding matrix F is used as the precoding matrix, signals can be distinguishably transmitted without interference between them.

Here, R _n is a pseudo-inverse matrix of noise covariance, which can be expressed by the following expression 10.

[Expression 10]

(where, the number of ‘0’ in a diagonal matrix is n₀, and the number of ‘1’ is N_r, i.e.,the total number of antennas from the STA1 to the STAK.) Here, n₀ indicates a dimension of a subspace assigned to a target where the AP transmits no signal and desires to minimize interference, and generally indicates a receiving dimension of the STA with which the AP desires to have no interference . For example, according to an exemplary embodiment of the present invention, n₀may indicate the number of receiving antennas of the STA, e.g., the STAd2 220 with which the AP desires to have no interference.

At this time, if QR decomposition is applied to the sub matrix F _mb,j, and the interference-avoiding precoding matrix is calculated, the following expression 11 is obtained.

[Expression 11]

Accordingly, the interference-avoiding precoding matrix for the whole system can be expressed by the following expression 12.

[Expression 12]

Contrary to a general block diagonalization (BD) technique for avoiding the interference, an interference constraint matrix

according to an exemplary embodiment of the present invention leaves interference between the STAs. In this case, a transmission weight matrix can be obtained in accordance with two references.

If an n_j×n_j weight matrix applied to

is T_j,the receiving signal of one STA in the MU-MIMO group 100 can be expressed by the following expression 13.

[Expression 13]

Here, if

undergoes singular value decomposition (SVD) and is multiplied by U_j ^H, V_j, a signal vector can be expressed by the following expression 14.

[Expression 14]

H _j ^e Q _mb,jserves to constrain interference of data signals transmitted to STAs different from each other, and make channel information so that little interference can be generated to a STA which desires not to cause interference. Here, the matrix V_j is the optimum precoding of a virtual subspace where the corresponding STA can have the most reception in an interference constraint subspace. Here, if a given channel H is multiplied by an arbitrary matrix Q, the subspace indicated by the channel H is distorted by the multiply of the matrix Q and shows another subspace. Further, if the distorted channel information HQ is multiplied again by the matrix V, the distorted subspace indicated by HQ is changed by the matrix V. The matrix V has an effect on changing the changed subspace, and such an already distorted subspace will be called a virtual subspace. In general, the matrix V obtained by taking the SVD with regard to a certain matrix A indicates the optimum precoding in the subspace indicated by the corresponding matrix A. At this time, the matrix U may be an optimum receiving filter matrix when it is assumed that the matrix V is used in the precoding.

Assuming that U_j ^H and V_jare unitary, T_jcan be obtained as follows with respect to the two references.

First, for minimizing the mean square error (MSE) under the total power constraint:

Let T_j be

using a scaling factor β based on the power constraint. Then, the scaling factor β and the transmission weight matrix can be expressed by the following

expressions

15 and 16.

[Expression 15]

[Expression 16]

Second, for minimizing the interference-plus-noise power under the power constraint per STA:

Let T_j be

using a scaling factor p_j based on the power constraint, and assume

and

.

Under the foregoing conditions, the transmission weight matrix can be derived on the basis of the following expression 17.

[Expression 17]

[Expression 18]

If C _jis calculated by the expression 17, it is possible to obtain the transmission weight matrix such as the expression 18, and a coefficient

can be determined by

.

Interference constraint channel blocks are divided into parallel sub channels for single symbol detection. This can be derived by the following expressions 19 to 21.

[Expression 19]

[Expression 20]

[Expression 21]

If the block channel undergoes the SVD based on the expression 19, the precoding matrix and the receiving filter for the STAj can be obtained by the expression 20. Thus, the output of the receiving filter can be expressed by the expression 21. In the state that the transmission subspace is configured in the form of minimizing or constraining the interference of a signal transmitted to the STAj with another STA by the matrix Q _mb,j, the optimum precoding matrix within the transmission subspace is obtained by the SVD, so that the final transmission precoding matrix can be expressed by combination of multiplication between Q _mb,j and V _mb,j. At this time, the optimum receiving filter used in the STAj is U _mb,j ^Hcorresponding to an inverse matrix of U _mb,j obtained by the SVD.

Thus, the SINR with regard to each stream and a sumrate in the MU-MIMO downlink can be expressed by the following expressions 22 and 23.

[Expression 22]

[Expression 23]

Referring to FIG. 4, the MIMO transmission of the DLS was a 4x4 MIMO transmission, and SNR0 thereof was fixed to 5dB. In the MU-MIMO transmission system, the number of receiving STAs was 2, the number of antennas the AP performing the MIMO transmission has was 8, and the number of antennas each receiving STA has was 2.

As shown in the graph, it will be appreciated that a regularized block diagonalization (BD) w/whitening method according to an exemplary embodiment of the present invention improves the performance of the MU-MIMO downlink transmission as compared with the existing simple zero forcing (ZF)-block diagonalization (BD) or regularized BD method. Here, it is designed that the performance of the direct link is not affected by the downlink transmission signal.

Referring to FIG. 5, the DLS transmission was a 4x4 MIMO transmission, and SNR0 thereof was fixed to 10dB. In the MU-MIMO transmission system, the number of receiving STAs was 2, the number of antennas the AP performing the MIMO transmission has was 8, and the number of antennas each receiving STA has was 2. Each of the MU-MIMO transmission mode and the direct link transmission mode involves w/whitening filtering.

First, referring to the development of the graph related to the MU-MIMO transmission, it will be appreciated that the transmission based on the regularized BD method has a lower FER than the transmission based on the ZF-BD method with regard to the same SNR. In other words, the SNR of the transmission based on the regularized BD method is more gained by about 2.5dB than that of the transmission based on the ZF-BD method with regard to the same FER.

Next, referring to the development of the graph related to the direct link transmission, it will be appreciated that there is little difference in the transmission between the regularized BD method and the ZF-BD method. According to an exemplary embodiment of the present invention, the interference caused in the direct link transmission by the MU-MIMO downlink transmission is completely eliminated in the transmission terminal, and it will be thus appreciated that there is little deterioration in the performance of the direct link transmission.

FIG. 6 is a block diagram showing a wireless apparatus in which an exemplary embodiment of the present invention is achieved. The wireless apparatus 600 includes an AP, a PCP or a non-AP/non-PCP STA.

The wireless apparatus 600 includes a processor 610, a memory 620, and a transceiver 630. The transceiver 630 transmits and/or receives a wireless signal, which achieves a physical layer of the IEEE 802.11. The processor 610 functionally connects with the transceiver 630, which achieves a MAC layer of the IEEE 802.11.

The processor 610 and/or the transceiver 630 may include an application-specific integrated circuit (ASIC), other chipsets, a logic circuit and/or a data processing device. The memory 620 may include a read only memory (ROM), a random access memory (RAM), a flash memory, a memory card, a storage medium and/or other storage devices. If the embodiments are realized by software, the foregoing methods may be implemented by a module (procedure, function, etc.) for performing the above functions. The memory 620 may be placed inside the processor 610, or separately placed in the outside and functionally connected to the processor 610 by various known-means.

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. The exemplary embodiments should be considered in descriptive sense only and not for purposes of limitation. Therefore, the scope of the invention is defined not by the detailed description of the invention but by the appended claims, and all differences within the scope will be construed as being included in the present invention.

Claims

A method for avoiding interference by an access point (AP) in a wireless local area network (WLAN) system, the method comprising:
acquiring first effective channel information between the AP and a first station (STA) targeted for multi user-multiple input multiple output (MU-MIMO) transmission;
acquiring second effective channel information between the AP and a second STA that receives data from a third STA during the MU-MIMO transmission; and
performing the MU-MIMO transmission to a target STA comprising the first STA by beamforming based on an interference-avoiding precoding that eliminates interference due to data transmission from the third STA to the second STA from a receiving signal of the first STA and interference due to the MU-MIMO transmission from a receiving signal of the second STA,
the interference-avoiding precoding matrix being obtained on the basis of a pseudo inverse matrix
that transforms the first effective channel information, the second effective channel information and a certain element of a matrix into 0.
The method of claim 1, wherein the pseudo inverse matrix is expressed by the following expression:

where, P_t is a power limit value of the whole WLAN system, the number of ‘0’ in a diagonal matrix is n₀, and the number of ‘1’ is N_r, i.e., in which N_r is a total number of antennas the target STA comprising the first STA has, and n₀is a total number of antennas the second STA has.
The method of claim 1, wherein the first effective channel information is acquired by applying a first receiving filter to a first channel information,
the first channel information is channel information between the AP and the first STA, and
the first receiving filter is acquired on the basis of first interference channel information which is channel information between the third STA and the first STA.
The method of claim 3, wherein the first channel information is acquired by
transmitting a training request (TRQ) message for requesting a sounding frame to the first STA;
receiving the sounding frame from the first STA in response to the TRQ message; and
estimating a channel between the AP and the first STA on the basis of the sounding frame.
The method of claim 4, wherein the first interference channel information is included in the sounding frame received from the first STA.
The method of claim 1, wherein the second effective channel information is acquired by applying a second receiving filter to a second channel information,
the second channel information is channel information between the AP and the second STA, and
the second receiving filter is acquired on the basis of second interference channel information which is channel information between the third STA and the second STA.
The method of claim 6, wherein the second interference channel information is acquired by
transmitting a TRQ message for requesting a sounding frame to the second STA;
receiving the sounding frame from the second STA in response to the TRQ message; and
estimating a channel between the AP and the second STA on the basis of the sounding frame.
The method of claim 7, wherein the second interference channel information is included in the sounding frame received from the second STA.
The method of claim 1, wherein the second STA receives the data from the third STA through a direct link.
A method for avoiding interference by an access point (AP) in a wireless local area network (WLAN) system, the method comprising:
acquiring first effective channel information between the AP and a first station (STA) targeted for multi user-multiple input multiple output (MU-MIMO) transmission;
acquiring second effective channel information between the AP and a second STA that receives data from a third STA during the MU-MIMO transmission; and
performing the MU-MIMO transmission to a target STA comprising the first STA by beamforming based on an interference-avoiding precoding that eliminates interference due to data transmission from the third STA to the second STA from a receiving signal of the first STA and interference due to the MU-MIMO transmission from a receiving signal of the second STA,
the interference-avoiding precoding matrix being obtained on the basis of a pseudo inverse matrix
that transforms the first effective channel information, the second effective channel information and a certain element of a matrix into 0, and a transmission weight matrix for eliminating interference with the target STA.
The method of claim 10, wherein the pseudo inverse matrix is expressed by the following expression:

where, P_t is a power limit value of the whole WLAN system, the number of ‘0’ in a diagonal matrix is n₀, and the number of ‘1’ is N_r, i.e., in which N_r is a total number of antennas the target STA comprising the first STA has, and n₀is a total number of antennas the second STA has.
The method of claim 11, wherein the transmission weight matrix is determined on the basis of a condition for minimizing a mean square error (MSE) under a total power constraint of the WLAN system.
The method of claim 11, wherein the transmission weight matrix is determined on the basis of a condition for minimizing interference-plus-noise power under a power constraint per target STA comprising the first STA.
A wireless local area network (WLAN) apparatus comprising:
a processor; and
a transceiver which functionally connects with the processor and transmits and receives a frame,
the processor
acquiring first effective channel information between the AP and a first station (STA) targeted for multi user-multiple input multiple output (MU-MIMO) transmission;
acquiring second effective channel information between the AP and a second STA that receives data from a third STA during the MU-MIMO transmission; and
performing the MU-MIMO transmission to a target STA comprising the first STA by beamforming based on an interference-avoiding precoding that eliminates interference due to data transmission from the third STA to the second STA from a receiving signal of the first STA and interference due to the MU-MIMO transmission from a receiving signal of the second STA, and
the interference-avoiding precoding matrix being obtained on the basis of a pseudo inverse matrix
that transforms the first effective channel information, the second effective channel information and a certain element of a matrix into 0.
The wireless apparatus of claim 14, wherein the pseudo inverse matrix is expressed by the following expression:

where, P_t is a power limit value of the whole WLAN system, the number of ‘0’ in a diagonal matrix is n₀, and the number of ‘1’ is N_r, i.e., in which N_r is a total number of antennas the target STA comprising the first STA has, and n₀is a total number of antennas the second STA has.