KR101580155B1 - Apparatus and method for beamforming in multi-cell multi-antena system - Google Patents

Abstract

The present invention relates to a method and apparatus for beamforming considering interference in a multi-cell multi-antenna system, comprising: acquiring channel and interference information of a terminal belonging to a base station; receiving a channel and interference information of a terminal belonging to the cooperative base station, Based on the obtained channel and interference information, a signal of the own terminal in consideration of a reception beamforming vector of each terminal based on the matched filter, The beamforming gain of the UE belonging to itself can be increased while minimizing the interference to the cooperative base station UE and calculating the optimal beamforming scheme using the iterative algorithm. Which can overcome the computational delay and performance degradation of the system.

Multi-cell, interference cancellation, beamforming, transmit vector, MIMO, base station cooperation

Description

TECHNICAL FIELD [0001] The present invention relates to an apparatus and a method for beamforming in a multi-cell multi-antenna system,

The present invention relates to a beamforming method and apparatus considering interference in a multi-cell multi-antenna system, and more particularly, to a method and apparatus for determining a transmission / reception beamforming vector through cooperation among base stations in a multi- .

Due to the rapid growth of the wireless mobile communication market, various multimedia services are required in a wireless environment. Accordingly, in recent years, studies have been made on a multiple antenna system (e.g., MIMO (Multiple Input Multiple Output)) capable of efficiently using a limited frequency while increasing the capacity of transmission data and increasing the speed of data transmission in order to provide the multimedia service .

The multi-antenna system transmits data using independent channels for each antenna, thereby improving transmission reliability and transmission rate compared to a single antenna system without additional frequency or transmission power allocation. In addition, the multi-antenna system can be extended to a multi-user multi-antenna system supporting multi-user. That is, the multi-antenna system of the multi-user environment can increase the frequency efficiency by sharing the space resources secured through the multiple antennas simultaneously by several users.

In the multi-antenna system of the multi-user environment, a beam forming technique is used to eliminate interference between users. For example, the multi-antenna system can remove interference between users using co-ordinated beamforming (Coordinated Beamforming) techniques.

In the case of using the cooperative beamforming scheme, the base station and the plurality of terminals can precondition the transmission signals and perform combining on the reception signals to distinguish signals of the respective terminals. Here, pre-processing means multiplying a transmission signal by a preprocessing vector, that is, a transmission beamforming vector, and combining means multiplying a reception signal by a coupling vector, that is, a reception beamforming vector. To this end, the BS must obtain a transmission beamforming vector and a reception beamforming vector for each MS.

In the prior art, when determining a transmission beamforming vector and a reception beamforming vector of a terminal belonging to a base station, the base station acquires channel information of the base station and information on an interference channel to minimize interference to other base station terminals, a nulling BF technique has been proposed for determining a transmission beamforming vector in a null space. Also, conventionally, a technique has been proposed in which a base station performs beamforming such that a beamforming gain of a terminal belonging to the base station is maximized, and each terminal uses only a plurality of reception antennas thereof to reduce interference.

However, the nulling BF technique of the conventional technique has a disadvantage in that the beamforming gain due to cooperation between the base stations is limited because the base station only performs beamforming on its own channel, and there is a disadvantage that the gain of a plurality of receiving antennas . Also, if a plurality of reception antennas of the UE are utilized, the complexity of the transmission unit is greatly increased because a repetitive algorithm must be used.

Also, since the interference cancellation performance is limited by the relatively small number of antennas of the UE, the technique of maximizing the beamforming gain of the base station and eliminating the interference of the UE has a problem that the performance is reduced. There is a problem that performance deterioration due to interference becomes serious.

It is an object of the present invention to provide a beamforming method and apparatus considering interference of neighboring cells in a multi-antenna system of a multi-cell multi-user environment.

It is another object of the present invention to provide a method and apparatus for determining a transmit / receive beamforming vector through cooperation among base stations in a multi-antenna system in a multi-user environment.

It is still another object of the present invention to provide a method and apparatus for determining a transmit beamforming vector that maximizes a signal for a mobile station in consideration of a receive beamforming vector in a multi-antenna system in a multi-user environment.

It is another object of the present invention to provide a method and apparatus for determining a receive beamforming vector using matching information of an effective channel in a multi-antenna system in a multi-user environment.

It is another object of the present invention to provide a method and apparatus for determining a transmission beamforming vector that minimizes interference to a terminal of a cooperative base station by exchanging channel information with a cooperative base station in a multi-antenna system of a multi-user environment.

According to a first aspect of the present invention, there is provided a method of performing beamforming by cooperation between base stations in a multi-antenna system, the method comprising: receiving, by the base station, channel information and interference information of a terminal belonging to the base station; Acquiring channel information and interference information of a terminal belonging to the cooperative base station from the cooperative base station, and calculating a maximum signal of the terminal according to the received beamforming vector of each terminal based on the obtained channel information And calculating a transmit / receive beamforming vector that minimizes interference to a terminal of the cooperative base station.

According to a second aspect of the present invention, there is provided an apparatus for performing beamforming through cooperation between base stations in a multi-antenna system, the base station including: A reception unit for acquiring channel information and interference information of a terminal belonging to the cooperative base station from the base station; and a receiving unit operable to maximize a signal of the own terminal in consideration of a reception beamforming vector of each terminal based on the obtained channel information, And a beamforming control unit for calculating a transmission beamforming vector that minimizes interference with the transmission beamforming vector.

The present invention relates to a multi-antenna system in a multi-user environment, in which a coherent reception beamforming vector of an effective channel is considered on the basis of channel information and interference information of a cooperative base station, It is possible to increase the beamforming gain of the UE belonging to the cooperative base station terminal while minimizing the interference to the cooperative base station terminal and determine the system delay and performance deterioration due to the use of the iterative algorithm There is an effect that can be overcome.

Hereinafter, preferred embodiments of the present invention will be described with reference to the accompanying drawings. In the following description, a detailed description of known functions and configurations incorporated herein will be omitted when it may make the subject matter of the present invention rather unclear.

In the present invention, in the multi-user environment of the multi-user environment, based on the channel information and the interference information of the cooperative base station and the channel information and the interference information of the cooperative base station, A description will be given of a technique for determining a transmission / reception beamforming vector to be transmitted / received.

FIG. 1 illustrates a cooperation environment between base stations in a multi-antenna system according to the present invention.

As shown in FIG. 1, in the present invention, it is assumed that two base stations cooperate with each other in order to improve the performance of a terminal existing at a cell edge. Also, in the present invention, it is assumed that each base station knows only the signal of the terminal belonging to itself in order to reduce the load on the backhaul or the upper network. It is assumed that each UE measures interference information between a neighboring base station performing cooperating with channel information between itself and a base station belonging to itself and handover it to the base station. That is, each base station can know downlink channel information and interference information for all users belonging to the base station through an uplink sounding channel or a feedback channel, It is assumed that the downlink channel information and the interference information of the UE belonging to the cooperative base station are acquired through the backhaul.

In addition, the present invention assumes a flat fading environment in which channels are slowly changed. This is because it is reasonable to assume that each subcarrier is assumed to be flat fading when the Orthogonal Frequency Division Multiplexing (MIMO) scheme is used in the future, and that the channel slowly changes with respect to a terminal that moves slowly.

In the present invention, the number of user terminals that can transmit data at one time is two, and one stream is used for each user. In consideration of the downlink environment, the number of transmit antennas (Nt) and the number of receive antennas Nr) are plural.

Hereinafter, a process of determining a beamforming vector by two base stations in cooperation with each other according to the present invention will be described using an equation.

When a base station performs beamforming on a transmission signal using a transmission beamforming vector and the terminal considers downlink communication using a reception beamforming vector, a signal received by a terminal belonging to each base station is expressed by Equation 1 > and Equation (2).

Equation (1) represents a signal received by a terminal 1 belonging to a first base station, and Equation (2) represents a signal received by a terminal 2 belonging to a second base station.

2, x _k denotes a transmission signal of a terminal belonging to the k-th base station, f _k denotes a transmission beamforming vector of the k-th base station, w _k denotes a transmission beamforming vector of the k-th base station, denotes the reception beamforming vector of the UE belonging to the k-th base station. Also, H _k denotes a channel between the k th BS and the k th BS, and G _k denotes an interference channel between the k th BS and the cooperative BS. Also, n _k denotes a Gaussian noise vector of a terminal belonging to the k-th base station.

Here, each base station uses a pilot beamforming scheme to transmit a reception beamforming vector w _k to a terminal belonging to the base station. The pilot beamforming scheme allocates a dedicated pilot so that the Node B does not use the UE repeatedly, then performs beamforming of the pilot using a transmission beamforming vector corresponding to each UE, channel, and then a matched filter is formed and used as a reception beamforming vector. That is, the reception beamforming vector w _k is determined by a transmission beamforming vector as shown in Equation (3).

The <Equation 3> to look at, k receiving beamforming vector (w _k) of the terminal k belonging to the second base station is the transmit vector for the k channel between the second BS and the MS (H _k) and the k-th base station (f _k) .

At this time, the optimal transmission beamforming vector can be expressed as Equation (4) below on the assumption that the reception beamforming vector is a matched filter.

,

,

That is, each base station must determine a transmission beamforming vector with which interference is maximized while the signal size of the terminal belonging to itself is maximized as in Equation (4).

이 성립하며, 이는 하기 <수학식 5>와 같이 나타낼 수 있다.Referring to Equation (4)

, Which can be expressed as Equation (5) below.

In Equation (5),? Is represented by constants a ₁ and b ₁ that are not zero. That is,? = A ₁ / b ₁ . Assuming that the transmission beamforming vector f ₁ is not a zero vector, Equation (5) always satisfies Equation (6).

의 일반화된 아이겐벡터(generalized eigenvector)들을 구함으로써 얻을 수 있음을 알 수 있다. 즉, 상기 f2는

의 아이겐 벡터가 된다. 여기서, 상기 (·)^# 은 행렬의 pseudo-inverse를 나타낸다.Referring to Equation (6), f2

Of the generalized eigenvectors of the eigenvectors. That is,

Eigenvectors of the &lt; / RTI &gt; Here, the (-) ^# denotes the pseudo-inverse of the matrix.

If f ₂ is obtained as described above, f ₁ can be obtained using the following Equation (7).

에 직교(orthogonal)하는 벡터를 구하면, 그 벡터가 바로 f₁이 된다. 만일 기지국이 2개의 안테나를 가지고 있다면,

의 아이겐벡터는 2 개가 구해지는데 f2는 이 두 개의 아이겐 벡터 중 하나로 정해지고 f1은 나머지 하나가 된다. 이 경우 하기 <수학식 10>을 이용하여 전송률 합을 최대로 하는 아이겐벡터들의 쌍으로 송신 빔포밍 벡터를 구성한다.That is, as shown in Equation (7), by using the previously obtained f ₂

, The vector is immediately f ₁ . If the base station has two antennas,

The eigenvectors of the two eigenvectors are obtained, f2 is defined as one of these two eigenvectors, and f1 is the other. In this case, a transmission beamforming vector is constructed with pairs of eigenvectors maximizing the sum of the transmission rates using Equation (10) below.

와 f1은 하기 <수학식 8>과 같이 정의할 수 있으며, 상기 기지국이 4개의 안테나를 가지고 있다면, 상기

와 f1은 하기 <수학식 9>과 같이 정의할 수 있다.Here, if the base station has two antennas,

And f1 can be defined as Equation (8) below. If the base station has four antennas,

And f1 can be defined as Equation (9) below.

Here, a plurality of eigenvectors may be obtained by performing the generalized eigen-analysis. In this case, a pair of eigenvectors maximizing the sum of the transmission rates may be calculated using Equation (10) &Lt; / RTI >

는 첫 번째 기지국에서 자신에게 속한 단말 1로 송신한 신호의 전력을 나타내며, 상기

는 두 번째 기지국이 단말 1에 미치는 간섭 신호의 전력을 나타낸다. 또한,

는 상기 두 번째 기지국에서 자신에게 속한 단말 2로 송신한 신호의 전력을 나타내며, 상기

는 상기 첫 번째 기지국이 상기 단말 2에 미치는 간섭 신호의 전력을 나타낸다. 본 기술은 기지국에서의 전송 전력 최적화 (transmit power optimization) 는 고려하고 있지 않으며 동일 전력 전송 (equal power transmission)을 기반으로 한다.In Equation (10)

Represents the power of a signal transmitted from the first base station to the terminal 1 belonging to the first base station,

Represents the power of the interference signal of the second base station on the terminal 1. Also,

Represents the power of a signal transmitted from the second base station to the terminal 2 belonging to the second base station,

Represents the power of an interference signal of the first base station to the terminal 2. [ The technique does not consider transmit power optimization at the base station and is based on equal power transmission.

As described above, according to the present invention, the transmission beamforming vector can be obtained from the viewpoint of making the interference zero (0) through the generalized eigen-analysis, and in view of using the matching of effective channels The reception beamforming vector can be obtained.

Hereinafter, the structure and operation of a base station for determining a beamforming vector as described above will be described.

3 illustrates a block diagram of a base station in a multi-antenna system according to an embodiment of the present invention.

3, the base station includes a plurality of channel codecs and interleavers 300-1 to 300-N, a plurality of modulators 310-1 to 310-N, a beamformer 320, A plurality of summers 330-1 to 330-N, a plurality of antennas 340-1 to 340-N, a Modulation and Coding Scheme (MCS) controller 350, a beamforming controller 360, And a control information receiving unit 370.

Each of the plurality of channel codecs and interleavers 300-1 to 300-N channel-codes and interleaves a bit stream to be transmitted through a corresponding transmission stream. The plurality of modulators 310-1 to 310-N modulate the coded bit stream provided from the channel codec and the interleaver 300 corresponding to the modulators 310-1 to 310-N to generate a complex symbol ).

The beamformer 320 performs transmission beamforming on the symbols to be transmitted through each transmission stream. Here, one stream is allocated to one UE, and the beamforming unit 320 performs beamforming using a transmission beamforming vector for the UE to which the corresponding transmission stream is allocated, (330-1 to 330-N). Here, the transmission beamforming vector for each terminal is determined by the beamforming controller 360. [ The beamforming unit 320 preferentially performs transmission beamforming on a dedicated pilot symbol of each terminal, and after the beamformed dedicated pilot symbol is transmitted, the plurality of modulators 310-1 to 320-N And then performs transmission beamforming on the data symbols provided from the transmitter.

Each of the plurality of summers 330-1 to 330-N adds symbols provided from the beamforming unit 320 and transmits the summed symbols through the plurality of antennas 340-1 to 340-N.

The MCS controller 350 determines a coding scheme to be used in the plurality of channel codecs and interleavers 300-1 to 300-N according to the channel information and downlink channel quality information provided from the control information receiver 370, The modulation methods to be used in the modulation units 310-1 to 310-N of the modulation method.

The beamforming controller 360 determines transmission beamforming vectors for the UEs using the channel information and the channel information provided from the control information receiver 370. [ That is, as shown in Equations (1) to (9), the beamforming controller 360 calculates the maximum beamwidth of the terminal belonging to the cooperative base station while maximizing the signal of the terminal belonging to itself, And calculates a transmission beamforming vector of the cooperative base station, that is, a transmission beamforming vector pair, which minimizes interference. Here, if a plurality of transmission beamforming vector pairs are obtained, the beamforming controller 360 selects a transmission beamforming vector pair that maximizes a transmission rate sum as shown in Equation (10). At this time, the beamforming controller 360 controls and processes a function for transmitting information on the selected beamforming vector pair to the corresponding cooperative base station.

And analyzes the channel information and the control information received through the normal link sounding channel or the feedback channel of the terminals belonging to the control information receiver 370 itself. For example, the channel information and control information receiver 370 may receive the downlink channel quality information and the interference information through the uplink sounding channel or the feedback channel and provide the downlink channel quality information and the interference information to the MCS controller 350, And provides the downlink channel information and the interference information to the beamforming controller 360. [ Herein, the downlink channel information of each UE includes channel information of the Node B itself and each of the UEs, and includes information on interference channels of the UEs.

In addition, the channel information and control information receiver 370 acquires downlink channel information of a terminal belonging to the cooperative base station by performing information exchange with a cooperative base station through a backhaul, and transmits the downlink channel information to the beamforming controller 360 to provide.

FIG. 4 illustrates an operation procedure of a base station for determining a transmission beamforming vector in a multi-antenna system according to an embodiment of the present invention. Herein, the base station is referred to as a base station i, the cooperative base station of the base station i is referred to as a base station j, the terminal belonging to the base station i is referred to as a terminal i, and the terminal belonging to the base station j is referred to as a terminal j.

Referring to FIG. 4, in step 401, the base station i obtains channel information (i.e., channel information between the base station i and the terminal i) and interference channel information of the base station j on the terminal i Gi. For example, the base station i acquires the channel information (Hi, Gi) through an uplink sounding channel or a feedback channel of the terminal i.

In step 403, the BS exchanges channel information of the MS with the cooperative BS through the backhaul. That is, the base station i transmits the channel information (Hi, Gi) acquired in step 401 to the cooperative base station j, and acquires channel information (Hj) between the base station j and the terminal j from the base station j, j of the interference channel.

Then, in step 405, the BS i calculates, based on the obtained channel information, the transmission beamforming vector fi of the BS i, which maximizes the signal of each MS, considering the reception beamforming vector of each MS, The transmission beamforming vector fj of the base station j, that is, the transmission beamforming vector pair fi, fj is calculated. That is, as shown in Equation (4), the base station i establishes a relational expression for minimizing the interference to the terminal j belonging to the cooperative base station while maximizing the signal of the terminal i in consideration of the reception beamforming vector of each terminal, And calculates a transmission beamforming vector of the base station i itself and the cooperative base station j according to a generalized eigenvector analysis technique. Here, the process of calculating the transmission beamforming vector according to the generalized eigenvector analysis technique is as shown in Equation (4) to Equation (9).

Then, the base station i checks whether a plurality of transmission beamforming vector pairs (fi, fj) are obtained in step 407. If only one transmission beamforming vector pair is acquired, the base station i goes directly to step 411, When the beamforming vector pair is obtained, in step 409, one transmission beamforming vector pair having the maximum transmission rate sum is selected as shown in Equation (10).

In step 413, the base station i calculates a reception beamforming vector pair (wi, wj) to be used in each terminal based on the transmission beamforming vector pair in step 411. In step 413, And the reception beamforming vector wj of the terminal j belonging to the cooperative base station j. In step 413, the base station i transmits the reception beamforming vector (wi) information of the terminal i to the terminal i belonging to the base station i.

In step 417, the BS i transmits a beamformed signal to the terminal i using its own transmission beamforming vector fi for the terminal i, and ends the algorithm according to the present invention.

While the present invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not limited to the disclosed embodiments. Therefore, the scope of the present invention should not be limited by the illustrated embodiments, but should be determined by the scope of the appended claims and equivalents thereof.

1 illustrates a cooperative environment between base stations in a multi-antenna system according to the present invention;

2 is a diagram illustrating a multi-antenna system model for performing cooperation between base stations according to an embodiment of the present invention;

3 is a block diagram of a base station in a multi-antenna system according to an embodiment of the present invention, and Fig.

4 is a diagram illustrating an operation procedure of a base station for determining a transmission beamforming vector in a multi-antenna system according to an embodiment of the present invention.

Claims (8)

A method of operating a base station for performing beamforming through cooperation between base stations in a multi-antenna system, Receiving channel information and interference information of a terminal belonging to the base station; Receiving channel information and interference information of a terminal belonging to the cooperative base station from a cooperative base station; Determining reception beamforming vectors based on the channel information and the interference information; Transmitting the reception beamforming vectors through a dedicated pilot so as not to be used redundantly by the terminals belonging to the base station; The first transmission beamforming vector of the base station minimizing the interference to the terminal of the cooperative base station while maximizing the signal of the terminal of the base station in consideration of the channel information, the interference information and the reception beamforming vectors of the terminals And determining a second transmit beamforming vector of the cooperative base station. A base station apparatus for performing beamforming through cooperation between base stations in a multi-antenna system, A receiver for receiving channel information and interference information of a terminal belonging to the base station and channel information and interference information of a terminal belonging to the cooperative base station from a cooperative base station; A transmission unit for transmitting the reception beamforming vectors through a dedicated pilot so as not to be used redundantly by the terminals belonging to the base station; Calculates the reception beamforming vectors based on the channel information and the interference information, and maximizes the signal of the terminal of the base station in consideration of the channel information, the interference information, and the reception beamforming vectors of the terminals And a beamforming controller for determining a first transmission beamforming vector of the base station and a second transmission beamforming vector of the cooperative base station that minimizes interference to a terminal of the cooperative base station. The method according to claim 1, Wherein the determining of the first transmission beamforming vector of the base station and the second transmission beamforming vector of the cooperative base station comprises: Determining a pair of vectors having a maximum of a sum of a transmission rate of the first transmission beamforming vector and a transmission rate of the second transmission beamforming vector. The method of claim 3, Wherein the pair of vectors and the receive beamforming vector are formed of matched filters. The method according to claim 1, Receiving channel information and interference information of a terminal belonging to the base station, Receiving downlink channel information between the base station and the terminal and interference information from the cooperative base station to the terminal through an uplink sounding channel or a feedback channel. The method of claim 2, Wherein the beamforming control unit determines a pair of vectors in which the sum of the transmission rates of the first transmission beamforming vector and the second transmission beamforming vector is the maximum. The method of claim 6, Wherein the pair of vectors and the receive beamforming vector are formed of matched filters. The method of claim 2, Wherein the receiving unit receives the downlink channel information between the base station and the terminal and the interference information from the cooperative base station to the terminal through an uplink sounding channel or a feedback channel.

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