KR20110006121A - Transmit antenna and user selection system for multiuser spatial multiplexing system with maximum likelihood receiver - Google Patents

Abstract

PURPOSE: A transmit antenna and user selection system for multiuser spatial multiplexing system is provided to make an ML(Maximum Likelihood) detecting unit detect optimum detecting performance. CONSTITUTION: An MS(Mobile Station)(230) comprises an ML detector(233). The ML detector performs MIMO(Multiple Input Multiple Output) signal detection with a base station(210). The base station selects the MS combination and the transmission antenna combination the minimum distance toward the MIMO channel of the ML detector to maximum. The base station transmits data stream.

Description

TRANSMITMIT ANTENNA AND USER SELECTION SYSTEM FOR MULTIUSER SPATIAL MULTIPLEXING SYSTEM WITH MAXIMUM LIKELIHOOD RECEIVER}

Embodiments of the present invention relate to a system for selecting a transmit antenna and a user when performing ML (maximum likelihood) detection in a multi-user spatial multiplexing system for communication between multi-user multi-antennas.

A multiple input multiple output (MIMO) system refers to a wireless communication system capable of transmitting signals using a plurality of transmitting antennas or receiving signals using a plurality of receiving antennas.

Meanwhile, a wireless communication system is developing into a system for providing a service capable of high-speed, high-capacity data transmission and reception to mobile stations (hereinafter, referred to as "users").

However, unlike a wired channel environment, a wireless channel environment has a variety of factors such as multipath interference, shadowing, propagation attenuation, time-varying noise, interference, and fading. Unavoidable errors occur, resulting in loss of information.

As shown in FIG. 1, when the base station 110 and the mobile station 130 each have a plurality of receive antennas, the base station 110 transmits multiple data to the plurality of users 130, respectively. A system for transmitting streams is defined as a multiuser spatial multiplexing system.

1 illustrates an example of a multi-user spatial multiplexing system in which a base station 110 having four transmit antennas 111 transmits two data streams to two users 130 having two receive antennas 131, respectively. It is shown.

In this case, a block diagonalization method is known as a transmission precoding technique capable of eliminating interference between multiple users generated while the base station 110 transmits multiple streams to the multiple users 130.

In a multi-user spatial multiplexing system, the following two problems can be considered.

First, among the plurality of users requiring data transmission, which user is most efficient to select and transmit?

)에 비해 RF 망(chain)의 수(

)가 큰 상황에서 기지국은

개의 송신 안테나 중에서

개의 안테나를 선택해서 RF 망에 연결해야 하는데, 이때 어떤 기준으로 송신 안테나를 선택할 것인가.Second, the number of transmit antennas the base station has

Compared to the number of RF chains (

), The base station

Of transmit antennas

Two antennas should be selected and connected to the RF network.

 In particular, in the second case, since the RF network of the base station is relatively expensive, having as many transmission antennas as a factor increases the base station cost significantly. Therefore, an antenna selection technique for increasing the number of transmitting antennas themselves and fixing the number of RF networks and selecting and transmitting as many as the number of RF networks among all transmitting antennas is known to greatly improve performance.

As a criterion for selecting a user and an antenna for the above two problems, a criterion of "maximizing the minimum stream signal to noise ratio (SNR)" is used. That is, the base station calculates the SNR for the data stream to be transmitted for each user combination and transmit antenna combination and assigns a minimum value (minimum stream SNR) among them to the user combination and transmit antenna combination. The base station selects the user combination and the transmit antenna combination having the largest minimum stream SNR and transmits the data stream.

"Maximum minimum stream SNR" is optimized for linear detectors such as zero-forcing (ZF) detectors or minimum mean square error (MMSE) detectors. There are two types of methods for MIMO signal detection, one of which is the above-described linear detector and the other is the maximum likelihood detector.

The linear detector uses a receive filter for orthogonalizing the MIMO channel, so the diversity gain is reduced, so that the detection error is relatively large, but it is easy to implement. On the other hand, the ML detector has a very large calculation amount compared to the linear detector, but has an advantage of minimizing detection errors.

Due to the advantages of the ML detector, research on low complexity ML detectors for ML detection of MIMO signals has been actively conducted, and "maximizing the minimum stream SNR" is a selection criterion for selecting a user combination and an antenna combination optimized for a linear detector. In this regard, the optimal choice for an ML detector cannot be achieved with "maximizing the minimum stream SNR."

One embodiment of the present invention provides a new criterion for selecting a user combination and a transmission antenna combination to maximize the system efficiency by serving more users simultaneously in a multi-user spatial multiplexing system.

One embodiment of the present invention provides an optimal selection criteria of a user combination and a transmit antenna combination for an ML detector in selecting a user combination and a transmit antenna combination.

In a multi-user spatial multiplexing system according to an embodiment of the present invention, a multi-user spatial multiplexing system in which a base station transmits a plurality of data streams to a plurality of users corresponding to a mobile station, respectively, wherein the user has a MIMO with the base station. and a maximum likelihood (ML) detector for performing multiple input multiple output (DA) signal detection, wherein the base station selects a user combination and a transmit antenna combination that maximizes the minimum distance for the MIMO channel of the ML detector to select the data stream. send.

In an embodiment of the present invention, the base station may select a user combination and a transmission antenna combination that maximize the minimum distance for the MIMO channel by applying a precoding matrix determined by a block diagonalization scheme.

In an embodiment of the present invention, the base station may select a user combination that maximizes the minimum distance for the MIMO channel by applying a precoding matrix determined by a block diagonalization scheme and a unitary precoding scheme together. .

According to an embodiment of the present invention, in selecting a user combination and a transmission antenna combination, the transmission efficiency of the multi-user spatial multiplexing system may be polarized by applying a selection criterion for the ML detector to have an optimal detection performance. In other words, the ML detector may have optimal detection performance by selecting a user combination and a transmission antenna combination that maximize the minimum distance for the MIMO channel to which the base station intends to transmit the data stream.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. However, the present invention is not limited or limited by the embodiments. Like reference numerals in the drawings denote like elements.

One embodiment of the present invention provides a criterion for a base station to select an optimal user and a transmit antenna in a multi-user multi-antenna downlink communication system.

2 illustrates an example of a multi-user multi-antenna communication system according to an embodiment of the present invention.

In one embodiment of the present invention, the base station 210 and the mobile station 230 each have multiple antennas 211 and 231, and the base station 210 multiples each of the multiple users 230. It can be applied to a multi-user spatial multiplexing system that transmits a data stream of (hereinafter, the antenna 211 of the base station 210 is referred to as a 'transmit antenna', and the antenna 231 of the user 230 is a 'receive antenna' It will be called).

The base station 210 selects the user 230 combination and the transmission antenna 211 combination to transmit multiple data streams. In one embodiment of the present invention, user 230 uses a maximum likelihood (ML) detector 233 to detect MIMO signals between base stations 210.

An embodiment of the present invention provides a criterion for the base station 210 to select an optimal user combination and a transmission antenna combination for the case where the user 230 has the ML detector 233 in the multi-user spatial multiplexing system having the above-described configuration. To provide.

The process of the base station 210 selecting the user combination and the transmit antenna combination will be described in detail.

개의 송신 안테나를 가지고 있으며, k번째 사용자는

개의 수신 안테나를 가지고 있다고 가정한다. 이때, 기지국과

번째 사용자에 대한 채널은

행렬(

)로 나타낼 수 있다. 또한, 기지국이 k명의 사용자에 대해 각각

개의 데이터 스트림을 전송한다고 가정하면, k번째 사용자에 대한

프리코딩 행렬(

)은 블록 대각화(block diagonalization) 방식을 통해 다중 사용자 간 간섭이 없도록 결정할 수 있다.The base station

K transmit antennas, the k-th user

Assume that we have two receiving antennas. At this time, the base station and

For the first user

procession(

) In addition, the base station can be used for k users, respectively.

Suppose you send two data streams, for the kth user

Precoding matrix (

) May be determined such that there is no interference between multiple users through a block diagonalization method.

)을 모 아 수학식 1과 같이 정의한다.That is, the channel of the remaining users except the channel for the kth user (

) Is defined as Equation 1.

이 되는 프리코딩 행렬(

)은 SVD(singular value decomposition)를 통해 영점 특이 값(zero singular value)에 대한 우 특이 벡터(right singular vectors)(

)로 결정된다.At this time,

Precoding matrix (

) Uses right singular vectors (SVs) for zero singular values through SVD (singular value decomposition).

Is determined.

)가 기지국에서 전송하고자 하는 데이터 스트림 수(

)보다 크다면, k번째 사용자의 채널(

)에 대한 SVD를 통해

개의 가장 큰 특이 값(largest singular values)에 해당하는 우 특이 벡터(

)를 얻은 후 최종적으로

를 프리코딩 행렬(

)로 결정한다. 이와 같은 프리코딩 행렬(

)의 결정 과정은 고유모드 선택(eigenmode selection)이라고 한다.If the number of receive antennas of the k-th user (

) Is the number of data streams (

Greater than), the kth user's channel (

Via SVD

Singular vectors corresponding to the largest singular values

) And finally

Is the precoding matrix (

Decide on) Such a precoding matrix (

The decision process of c) is called eigenmode selection.

)와 기지국에서 전송하고자 하는 데이터 스트림의 개수(

)가 같은 경우를 고려한다. 현재 시스템에

명의 사용자가 있다고 가정한다면, 기지국은 이 중에서 K명을 선택해야 한다. 이때, 고려될 수 있는 사용자의 조합은 모두

개이며, 이 중에서 q번째 사용자 조합에 대해 최소 스트림 SNR(signal to noise ratio)(q^MMSS)을 계산하면 수학식 2와 같다.First, the number of receive antennas of the k-th user (

) And the number of data streams that the base station wants to transmit (

Consider the same case. On the current system

Assuming there are users, the base station should select K among them. At this time, any combination of users that can be considered

The minimum stream signal to noise ratio (q ^MMSS ) for the q-th user combination is calculated by Equation 2 below.

는 q번째 사용자 그룹에 대한 k번째 사용자의 채널을 의미하며,

는 MIMO 채널(H)에 대한 최소 스트림 SNR을 의미한다. 상기한 최소 스트림 SNR을 최대화하는 방식은 ZF(zero forcing) 검출기나 MMSE(minimum mean square error) 검출기에 최적화 된 것이다.here,

Is the kth user's channel for the qth user group,

Is the minimum stream SNR for the MIMO channel (H). The method of maximizing the minimum stream SNR is optimized for a zero forcing (ZF) detector or a minimum mean square error (MMSE) detector.

One embodiment of the present invention may select an optimal user combination and antenna combination based on the criteria of "maximizing the minimum distance" rather than "maximizing the minimum stream SNR" for the ML detector.

The q th user combination q ^{MMD maximizing} the minimum distance to the MIMO channel H may be defined as in Equation 3.

는 MIMO 채널 H에 대한 최소 거리(minimum distance)를 의미하며, 기본적으로 수학식 4와 같이 정의된다.here,

Denotes a minimum distance for the MIMO channel H, and is basically defined as in Equation 4.

데이터 심볼 벡터 중 하나이며, 위의 수식을 계산하기 위한 계산 복잡도는 매우 큰 것으로 알려져 있다.Where s ₁ and s ₂ are all sendable

One of the data symbol vectors, and the computational complexity for calculating the above equation is known to be very large.

로 정의하고 여기서 h_i를

벡터로 가정하면 수학식 5의 과정을 통해 최소 거리(

)를 계산할 수 있다.Therefore, one embodiment of the present invention uses a low complexity minimum distance calculation scheme. That is, the MIMO channel

, Where h _i is

Assuming a vector, the minimum distance (

) Can be calculated.

Consider user selection when the eigenmode selection is possible. If precoding is performed with eigenmode selection and then "maximization of minimum stream SNR" is applied, the effective channel is orthogonal. In this case, the result of the ZF detector and the ML detector are the same. The eigenmode selection scheme and "maximizing the minimum stream SNR" yield no diversity gain with the ML detector, only the performance of the ZF detector.

로 프리코딩 행렬을 결정했다면, 본 발명의 일실시예에 따른 방식은 단일 프리코더들의 집합(

)을 정의하고 최소 거리를 최대화하는

의 한 원소를 선택한다. 즉, 수학식 6을 통해 MIMO 채널 H에 대한 최소 거리(

)를 최대화하는

를 선택하고 프리코딩 행렬은

이 된다.Thus, an embodiment of the present invention applies a unitary precoding scheme and "maximum minimum distance" instead of eigenmode selection. That is, in case of eigenmode selection method

Once the precoding matrix is determined, the scheme according to an embodiment of the present invention is a set of single precoders (

) To maximize the minimum distance

Select one element of. That is, the minimum distance for MIMO channel H (Equation 6)

To maximize)

And the precoding matrix

Becomes

After performing the above process for all user combinations, a user combination q ^{UP , MMD} that maximizes the minimum distance for each user combination may be selected as shown in Equation 7 below.

는 q번째 사용자 그룹에 대한 k번째 사용자에 대해 최소 거리를 최대로 하는 단일 프리코더를 의미한다.here,

Denotes a single precoder that maximizes the minimum distance for the kth user for the qth user group.

)에 비해 RF 망의 수 (

)가 큰 상황에서 기지국은

개의 송신 안테나 중에서

개의 안테나를 선택해서 RF 망에 연결해야 한다. 이때, 고려될 수 있는 조합은 모두

이다. 각 안테나 조합에 대해 블록 대각화를 적용할 수 있으며, 이때 p번째 안테나 조합에 대한 k번째 사용자의 프리코딩 행렬을

라고 한다면, 최소 거리를 최대로 하는 안테나의 조합(p^MMD)은 수학식 8을 통해 선택될 수 있다.Next, consider the problem of transmission antenna selection. Number of transmitting antennas (

Compared to the number of RF networks ()

), The base station

Of transmit antennas

Antennas should be selected and connected to the RF network. At this time, any combination that can be considered

to be. Block diagonalization can be applied to each antenna combination, where the k-th user's precoding matrix for the p-th antenna combination is

In this case, the combination p ^MMD of the antenna ^{maximizing the} minimum distance may be selected through Equation 8.

The user combination q ^{UP and MMD} and antenna combination p ^MMD selected above may be selected as shown in Equation (9).

FIG. 3 illustrates a symbol error rate (SER) for a situation in which two users are selected to transmit a data stream in a situation where there are seven transmit antennas and six RF networks in a multi-user spatial multiplexing system.

Referring to FIG. 3, the performance of the ML detector (MMSS + UP + ML detector) according to the maximization of the minimum stream SNR and the single precoding scheme is increased according to the ZF detector (MMSS + Eigenmode +) according to the maximization of the minimum stream SNR and the eigenmode selection method. It can be seen that it is superior to the ZF detector (MMSS + UP + ZF detector) according to the ZF detector or the maximum stream SNR and the single precoding scheme. Moreover, the performance of ML detectors (MMD + UP + ZF detectors) using a single precoding scheme along with the maximization of minimum distances can be applied to maximize the minimum stream SNR as well as ZF detectors under the same conditions. It can be seen that it is very superior to the above-described situations applied.

Thus, in one embodiment of the present invention, the ML detector may have an optimal detection performance by selecting a user combination and a transmission antenna combination based on the criteria of "maximizing the minimum distance".

As described above, the present invention has been described by specific embodiments such as specific components and the like. For those skilled in the art to which the present invention pertains, various modifications and variations are possible.

Therefore, the spirit of the present invention should not be limited to the described embodiments, and all of the equivalents or equivalents of the claims as well as the claims to be described later will belong to the scope of the present invention. .

1 is a view for explaining a conventional multi-user spatial multiplexing system.

2 illustrates an example of a multi-user multi-antenna communication system according to an embodiment of the present invention, and illustrates a multi-user spatial multiplexing system for selecting a user combination and a transmission antenna combination for a case where a user has an ML detector. It is a figure for following.

Figure 3 shows a symbol error rate graph for explaining the system performance of the present invention in selecting a user combination and a transmit antenna combination.

<Explanation of symbols for the main parts of the drawings>

210: base station

230: user

233: ML detector

Claims (5)

A multi-user spatial multiplexing system in which a base station transmits multiple data streams to a plurality of users corresponding to mobile stations, respectively. The user, A maximum likelihood (ML) detector for performing multiple input multiple output (MIMO) signal detection with the base station, The base station comprises: And selecting a user combination and transmit antenna combination that maximizes the minimum distance to the MIMO channel of the ML detector to transmit the data stream. The method of claim 1, The base station comprises: Applying a precoding matrix determined by a block diagonalization method to obtain a user combination that maximizes the minimum distance for the MIMO channel. Equation:

(Where q ^MMD is the qth user combination,

Is the minimum distance for the MIMO channel (H),

Denotes the k-th user precoding matrix for the q-th user combination.) Selected by, multi-user space multiplexing system. The method of claim 1, The base station comprises: A user combination that maximizes the minimum distance to the MIMO channel by applying a precoding matrix determined by a block diagonalization method and a unitary precoding method. Equation:

(Where q ^{UP and MMD} are the qth user combination,

Is the minimum distance for the MIMO channel (H),

Is the k-th user's precoding matrix for the q-th user combination,

Refers to a single precoder that maximizes the minimum distance for the k-th user in the q-th user combination.) Selected by, multi-user space multiplexing system. The method of claim 1, The base station comprises: By applying a precoding matrix determined by the block diagonalization scheme, a combination of transmit antennas that maximizes the minimum distance to the MIMO channel is obtained. Equation:

Where p ^MMD is the pth transmit antenna combination,

Is the minimum distance for the MIMO channel (H),

Denotes the k-th user's precoding matrix for the p-th transmit antenna combination.) Selected by, multi-user space multiplexing system. The method according to any one of claims 2 to 4, The minimum distance for the MIMO channel (H), Equation:

(Here, the MIMO channel (H)

(Where N _s is the number of data streams) and h _i is

Vector (where M _{R and k} are the number of receive antennas of the k-th user). Calculated by the multi-user space multiplexing system.

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