KR20120060729A - Method and apparatus for feedback in a multi-user multi-input multi-output communication system - Google Patents

Abstract

The present invention provides a method for a user equipment to feed back channel quality information in a multi-input multi-output communication system supporting multiple users, the method comprising receiving a signal from a base station and an average signal-to-interference noise ratio (SINR) for the signal. calculating a lower limit of Interference plus Noise Ratio, determining a lower limit of the average SINR as channel quality information, and feeding back the channel quality information to the base station.

Description

METHOD AND APPARATUS FOR FEEDBACK IN A MULTI-USER MULTI-INPUT MULTI-OUTPUT COMMUNICATION SYSTEM}

The present invention relates to a method and apparatus for feeding back channel quality information in a multi-input multi-output (MIMO) communication system supporting multiple users.

In the recent wireless communication system, a downlink channel in a multi-user multiple-input multiple-output (MU-MIMO) communication system in which one base station supports multiple users simultaneously Has been going on. Interference between users is the biggest problem in the downlink channel. Therefore, as a method for solving this problem, a zero-forcing (ZF) beamforming method and a block diagonalization (BD) method for designing a precoder are widely used. In such a precoder design, it is assumed that a transmitter acquires information on an accurate downlink channel. And, based on this, the receiver performs a scheduling algorithm of the user, thereby obtaining a higher sum-rate performance.

However, in a practical wireless communication environment, it is difficult for a transmitter to obtain accurate downlink channel information. Therefore, the receiver quantizes information about the channel with the transmitter based on a codebook that is recognized in advance, and receives channel direction information (CDI), which is one of the quantized codebook indexes. Send to the sender. The CDI indicates the direction of the channel. The transmitting side receiving the CID can design a precoder that eliminates interference of the channel by using the CID.

When there are a plurality of users in the MU-MIMO communication system, performance gain, that is, multi-user diversity gain, may be obtained by allocating a user suitable for a transmission environment. To this end, the user transmits not only CDI for his channel but also channel quality information (CQI) corresponding to characteristics of the channel to the transmitter. The transmitting side receiving the CDI and the CQI transmitted from the user can perform more efficient scheduling using the CDI and the CQI.

In the conventional method, there is a method of analyzing performance of ZF beamforming when the receiving side performs scheduling in a vector channel having one antenna. Alternatively, if the receiving side has a plurality of antennas, there is a method of obtaining the performance gain by reducing the error due to channel quantization using the receiving side coupling. Another method is to obtain a scheduling gain by reflecting the quantization error to the CQI. The above-described conventional schemes have a constraint that each user must communicate through only one data layer, that is, single mode communication.

Meanwhile, a method of using a BD method has also been proposed in a multiple mode communication in which a transmitting side supports a plurality of data layers to a user. In this case, the BD technique is used using only CDI for a given user, and a representation method of CQI for scheduling when there are a plurality of users has not been considered. In a simple way, the user can measure his channel size and use the measured channel size as the CQI. In this case, in a limited feedback situation, there is a problem that may cause performance degradation caused by failing to reflect an error due to quantization.

Therefore, when the channel information is transmitted to the transmitting end using a codebook in the MU-MIMO communication system, a CQI expression that is distinct from a system supporting only one user is required.

The present invention proposes a method and apparatus for a transmitter to feed back channel quality information in a MU-MIMO communication system.

The present invention proposes a method and apparatus for calculating an average SINR as a CQI in order to maximize a scheduling gain when using precoding to support multi-stream in a MU-MIMO communication system.

Method according to an embodiment of the present invention; In a multi-input multi-output communication system supporting multiple users, a method for feeding back channel quality information by a terminal, the method comprising: receiving a signal from a base station, and an average signal-to-interference noise ratio (SINR) for the signal; Calculating a lower limit of a ratio), determining a lower limit of the average SINR as channel quality information, and feeding back the channel quality information to the base station.

An apparatus according to an embodiment of the present invention; A terminal for feeding back channel quality information in a multi-input multi-output communication system supporting multiple users, comprising: a receiver for receiving a signal from a base station, and an average signal to interference plus noise ratio (SINR) for the signal; An average SINR calculation unit that calculates a lower limit value of the LSI and determines the lower limit value of the average SINR as channel quality information, and a transmitter that feeds back the channel quality information to the base station.

According to the present invention, when a transmitter uses precoding to support multi-stream in a MU-MIMO communication system, the average signal-to-interference noise ratio of the receiver as a CQI for maximizing a scheduling gain ( By determining the lower limit for Signal to Interference plus Noise Ratio (SINR) and feeding back the lower limit of the determined average SINR to the base station, it is possible to provide more accurate channel quality information to the base station and enable the base station to estimate a more accurate channel. There is.

1 is a block diagram of a MU-MIMO communication system according to an embodiment of the present invention.
2 is a schematic structural diagram of a terminal according to an embodiment of the present invention;
3 is a flowchart illustrating operations of a terminal according to an exemplary embodiment of the present invention.
4 is a flowchart illustrating an operation of calculating a lower limit value of an average SINR of a terminal according to an exemplary embodiment of the present invention.
5 is a graph showing a cumulative distribution function (CDF) of SINR average values according to an embodiment of the present invention.
6 is a graph illustrating an example of a sum rate according to the number of users according to an embodiment of the present invention.
7 is a graph illustrating another example of the sum rate according to the number of users according to an embodiment of the present invention.
8 is a graph illustrating an example of a sum rate according to the number of bits for CID and the number of quantization bits according to an embodiment of the present invention.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. It should be noted that in the following description, only parts necessary for understanding the operation according to the present invention will be described, and descriptions of other parts will be omitted so as not to distract from the gist of the present invention.

The present invention proposes a CQI calculation method and apparatus for maximizing a scheduling gain when a transmitter uses precoding to support multi-stream in a MU-MIMO communication system. As a scheduling technique, a semi-orthogonal user selection (SUS) technique is considered. The present invention proposes a method and apparatus for calculating a new CQI for minimizing interference to other users by considering not only an existing channel norm but also an error generated by codebook quantization.

개의 단말이 존재한다고 가정한다.1 is a block diagram of a MU-MIMO communication system according to an embodiment of the present invention. In an embodiment of the present invention, a single-cell MIMO broadcast channel, one base station 100 and

It is assumed that there are two terminals.

개의 단말들 중

개의 단말을 선택하고, 선택된

개의 단말들에게 동시에 정보를 송신할 경우,

번째 단말의 수신 신호는 하기 <수학식 1>과 같이 나타내어진다.
The base station 100

Of terminals

Terminals and selected

In case of transmitting information to two terminals at the same time,

The received signal of the second terminal is represented by Equation 1 below.

는

번째 단말의 채널 매트릭스(channel matrix)이며 크기는

이다.

는 상기 기지국(100)의 안테나의 수를 나타내고,

는 각 단말의 수신 안테나의 수를 나타낸다.

는

번째 단말에게 송신되는 정보 벡터이며 크기는

이다. 즉, 상기

번째 단말에게

개의 데이터 스트림이 송신됨을 의미한다.

는 프리코딩 매트릭스이며 크기는

이다. here,

Is

Is the channel matrix of the first terminal and the size is

to be.

Represents the number of antennas of the base station 100,

Denotes the number of receive antennas of each terminal.

Is

Information vector transmitted to the first terminal, the size of which is

to be. That is

To the first terminal

Data streams are transmitted.

Is the precoding matrix and the size is

to be.

는 다른 단말이 송신한 신호가

번째 단말에게 수신되지 않도록 BD 방법(

)을 사용하여 결정된다. 상기 BD 방법은 수신 안테나의 수에 대응하는 데이터 스트림을 수신한다. 본 발명의 실시 예에서는 일 예로서 BD 방법을 사용하는 경우를 설명하였으나, 본 발명이 BD 방식에만 적용되는 것이 아님을 유의해야 한다. 즉, 본 발명은 수신 안테나의 수보다 많거나 적은 수의 데이터 스트림을 수신하는 경우에도 적용 가능함은 물론이다.At this time, the precoding matrix

Is a signal transmitted from another terminal

BD method (not to be received by the first terminal)

Is determined using. The BD method receives a data stream corresponding to the number of receive antennas. In the embodiment of the present invention, a case of using the BD method is described as an example, but it should be noted that the present invention is not applied only to the BD method. That is, the present invention is also applicable to the case of receiving more or fewer data streams than the number of receiving antennas.

번째 단말의 수신단에서 신호 대 간섭 잡음비(SINR: Signal to Interference plus Noise Ratio)는 하기 <수학식 2>와 같이 나타내어진다.
in this case,

Signal to interference plus noise ratio (SINR) at the receiving end of the second terminal is represented by Equation 2 below.

번째 단말의 수신단에서는 자신의 채널(

) 이외에 다른 단말의 채널(

)을 인지하지 못하는 상태이므로, 순시 SINR을 획득할 수 없다. 그러므로, 본 발명의 실시 예에서는 상기 <수학식 2>의

를 평균화하여 평균 SINR을 계산한다.
But,

The receiving end of the first terminal own channel (

In addition to the

Instantaneous SINR cannot be obtained. Therefore, in the embodiment of the present invention,

Calculate the average SINR by averaging

(1) average SINR  Calculation

번째 단말의 수신단이 자신의 채널(

) 이외 에 다른 단말의 채널(

)을 모른다고 가정하고, 순시 SINR 대신 평균 SINR을 CQI 정보로 사용한다. 이때, 상기 평균 SINR의 하한값은 하기 <수학식 3>과 같이 결정된다.
In the embodiment of the present invention

The receiving end of the first terminal has its own channel (

Channel other than

) And use the average SINR as the CQI information instead of the instantaneous SINR. At this time, the lower limit of the average SINR is determined as shown in Equation 3 below.

는

의 아이겐-벨류 분해(Eigen-value decomposition)를 나타낸다. 이때,

는

의 직교적인 베이시스(orthonormal basis) 즉,

로 구성되어 있다. 또한,

이며,

는

의

번째 아이겐 벨류(Eigen-value)이다.

Is

Represents Eigen-value decomposition of. At this time,

Is

Orthonormal basis of

Consists of Also,

,

Is

of

The first Eigen-value.

(2) Preliminary are preliminary procedures necessary to calculate the denominator term and denominator term of Equation 3 above.

(2) Preliminary

는 하기 <수학식 4>와 같이 분해된다.

Is decomposed as shown in Equation 4 below.

는 코드북에서 하나의 코드워드를 나타내며 크기는

이다. 상기 코드북은

개의 코드워드(codeword)를 포함한다.

는

의 레프트 널 스페이스(left-null space)의 직교적인 베이시스로 구성되어 있다.

는

상위의 삼각 매트릭스(upper triangular matrix)이며 대각 텀(term)들은 모두 양수이며

을 만족한다.

는

유니터리 매트릭스(unitary matrix)이고,

는

상위의 삼각 매트릭스이며 대각 텀들은 모두 양수이고

을 만족한다.here,

Represents one codeword in the codebook, and the size

to be. The codebook is

Contains two codewords.

Is

It consists of orthogonal basis of left-null space of.

Is

The upper triangular matrix and the diagonal terms are all positive

To satisfy.

Is

Unitary matrix,

Is

The upper triangular matrix and the diagonal terms are all positive

To satisfy.

In addition, the weighted chordal distance to be used in the mathematical development to be described later is defined as in Equation 5 below.

의 삼각 법칙을 만족한다.
The weighted codal distance is

Satisfies the trigonometric law of.

First, a process of calculating the molecular term of Equation 3 is as follows.

(3) Calculation of the molecular term of the average SINR (Desired Signal)

The molecular term of the mean SINR is expressed by Equation 6 using the definition of the weighted chordal distance and the trigonometric law.

이고,

는

로 정의한다. In the second inequality of Equation 6,

ego,

Is

.

번째 유저의 프리코딩 매트릭스는 원래 채널 매트릭스와 매우 유사(

)하게 된다. 결국

는 ‘0’에 가까워지므로, 본 발명의 실시 예에서는 상기 <수학식 5>의

를

으로 가정하기로 한다.
The scheduler of the base station 100 basically selects terminals having the most orthogonal channel matrix among the terminals as a scheduling target. Through this selection operation, it is possible to prevent performance degradation due to rank deficiency that occurs when generating the precoding matrix. Therefore, the matrix of each terminal is orthogonal to each other.

The user's precoding matrix is very similar to the original channel matrix (

) finally

Becomes close to '0', and according to the embodiment of the present invention,

To

Let's assume.

Next, a process of calculating the denominator term of Equation 3 is as follows.

(4) Calculation of the denominator term of the average SINR (Interfernce Signal)

Of Equation 3 The denominator portion of the average SINR develops as shown in Equation 7 below.

의 분해(decomposition)를 이용하여 계산한다. The first equal sign of Equation 7 is expressed as Equation 4 above.

Calculated using the decomposition of.

와

는 정의에 의해

의 레프트 널 스페이스 위에서 언폼(unform)하게 분포되어 있다. 따라서,

는 베타 분포를 따르며, 베타 분포의 정의에 의해

이 된다.
Meanwhile,

Wow

By the definition

It is unformed above the left null space of. therefore,

Follows the beta distribution and is defined by

Becomes

(5) average SINR Lower limit  And reflection

When the lower limit value of the average SINR is calculated using the values calculated in Equations 4 to 6, Equation 8 is expressed as follows.

이므로 성립한다. 이때, 스트림별 전력 레벨을 나타내는

에 따라 평균 SINR의 하한값을 결정한다. 그리고, 결정된 평균 SINR의 하한 값

를 CQI 값으로 결정한다.The second equal sign of Equation 8 is

Is established. At this time, the power level for each stream

Determine the lower limit of the average SINR according to. And, the lower limit value of the determined average SINR

Is determined as the CQI value.

즉, 신호대 잡음 비(SNR: Signal to Noise Ratio)가 매우 낮을 경우, 상기 평균 SINR은

로 근사화된다. 양자화 비트수가 3이상인 경우,

은

보다 상당히 작다. 따라서 평균 SINR은

에 의해 결정되며, 결국 채널 크기(

)를 CQI 정보로 이용하는 기존의 피드백 시스템과 동일하다.
Specifically,

That is, when the signal to noise ratio (SNR) is very low, the average SINR is

Is approximated. If the number of quantization bits is 3 or more,

silver

Considerably smaller than Therefore, the average SINR is

Determined by the channel size, which in turn

) Is the same as the existing feedback system using CQI information.

즉, SNR이 매우 높을 경우, 상기 평균 SINR은

으로 근사화된다. 따라서 평균 SINR은 분모항의

, 즉 양자화 에러에 의해 대부분 결정된다. 따라서, 높은 SNR을 갖는 경우에서는

를 CQI 정보로 이용하는 기존 방식은 양자화 에러의 크기를 고려하지 않으므로, 유저 스케쥴링 시 높은 SNR에서 성능저하를 피할 수 없었다. 그러나, 본 발명의 실시 예에서는 SINR을 CQI 정보로 이용함에 의해서 낮은 SNR 뿐만 아니라 높은 SNR에서도 적절한 성능을 보장할 수 있게 된다.
On the other hand,

That is, when the SNR is very high, the average SINR is

Is approximated. Therefore, the average SINR of the denominator

That is, it is mostly determined by the quantization error. Therefore, in the case of high SNR

Since the conventional method using CQI as the CQI information does not consider the size of the quantization error, performance degradation cannot be avoided at high SNR in user scheduling. However, in the embodiment of the present invention, by using the SINR as the CQI information, it is possible to ensure proper performance at high SNR as well as low SNR.

2 is a schematic structural diagram of a terminal according to an exemplary embodiment of the present invention.

2, the terminal 200 includes a receiver 205, an average SINR calculator 210, and a transmitter 215.

The receiver 205 receives the data stream transmitted from the base station and transmits the received data stream to the average SINR calculator 210. The average SINR average calculation unit 210 calculates a lower limit value of the average SINR for the received data stream and transmits the lower limit value of the average SINR to the transmitter 215. The transmitter 215 transmits the lower limit value of the calculated average SINR to the base station as CQI information.

3 is a flowchart illustrating operations of a terminal according to an exemplary embodiment of the present invention.

Referring to FIG. 3, in step 300, the terminal receives a data stream transmitted from a base station, and proceeds to step 305. In step 305, the terminal calculates the lower limit value of the average SINR for the received data stream using Equation (8), and proceeds to step 310. In step 310, the terminal transmits the calculated lower limit value of the average SINR to the base station as CQI information.

4 is a flowchart illustrating an operation of calculating a lower limit value of an average SINR of a terminal according to an exemplary embodiment of the present invention.

Referring to FIG. 4, in step 400, the terminal calculates the molecular term of the average SINR using the definition of the weighted codal distance and trigonometric law represented by Equation 5, and proceeds to step 405.

In step 405, the UE decomposes a matrix composed of orthogonal bases of the channel matrix, calculates the numerator of the average SINR, and proceeds to step 410.

In step 410, the terminal calculates an average SINR using the calculated molecular term and the numerator, and then proceeds to step 415. In step 415, the terminal determines the lower limit of the average SINR according to the power level for each data stream received from the base station.

Hereinafter, the performance gain of the technique proposed by the present invention will be described with reference to FIGS. 5 to 7 through simulation results using the average SINR according to an embodiment of the present invention. Here, BD method is used for precoding and SUS algorithm is used for scheduling.

5 is a graph showing a cumulative distribution function (CDF) of an average SINR value according to an embodiment of the present invention. Here, suppose that Nt is 4, Nr is 2, and SNR is 20 dB when 10 bits are used for CDI.

Referring to FIG. 5, the CDF for the SINR average value of the scheduled users is shown. In this case, the case where the number of users is 10 and the case of 30 is shown in comparison. As shown in the graph, when the number of users is 10 and the number of users is 30, the magnitude and the average SINR value of each channel are used as the CQI, respectively. As a result, the performance gain is obtained when the average SINR is used. This is considerably higher than when using the size of the channel. In other words, when the channel size is used as the CQI, even if the number of users increases, the change in the multiuser diversity gain is minimal. On the other hand, when the average SINR is used as the CQI, the performance gain increases because the SINR increases as the number of users increases. Therefore, it is possible to accurately predict the actual SINR through the lower limit of the average SINR calculated according to the embodiment of the present invention.

6 is a graph illustrating an example of a sum rate according to the number of users according to an embodiment of the present invention. Here, assume that Nt = 4, Nr = 2, and 10 bits are used for CDI.

Referring to FIG. 6, in a substantially limited feedback situation compared to an ideal environment, the sum rate is limited as the transmission power increases due to interference due to quantization error. If the SNR is high, there are 30 and 40% gains for 30 users and 100 users, respectively. That is, when the SNR is high, the sum rate is greatly affected by the interference. In this case, when the channel size is used as the CQI, since the interference due to the quantization error is not reflected, the sum rate is lower than that when the average SINR according to the embodiment of the present invention is used as the CQI.

7 is a graph illustrating another example of the sum rate according to the number of users according to an embodiment of the present invention. Here, assume that Nt = 6, Nr = 2, and 15 bits are used for CDI.

Referring to FIG. 7, as in FIG. 6, when the channel size is used as a CQI, since the interference due to quantization error is not reflected, the average SINR according to the embodiment of the present invention is 30 and 100, respectively. Using CQI provides 20% gain over using channel size as CQI.

8 is a graph illustrating an example of a sum rate according to the number of bits for CID and the number of quantization bits according to an embodiment of the present invention. Here, Nt = 4 and Nr = 2.

Referring to FIG. 8, the number of quantization bits is changed to 2, 4, 6, and no quantization for each case of using 5 bits and 10 bits as bits for CDI. As a result, when the bit for CDI is 5 bits, it has a lower sum rate than when the bit for CDI is 10 bits. And, with bits for the same CDI, the higher the number of quantization bits, the higher the sum rate. For example, if the bit for CDI is 10 bits, the number of quantization bits is increased to avoid loss caused by quantization.

Meanwhile, in the detailed description of the present invention, specific embodiments have been described, but various modifications are possible without departing from the scope of the present invention. Therefore, the scope of the present invention should not be limited to the described embodiments, but should be determined not only by the scope of the following claims, but also by the equivalents of the claims.

Claims (10)

In a multi-input multi-output communication system supporting multiple users, a method for feeding back channel quality information by a terminal,
Receiving a signal from a base station,
Calculating a lower limit of an average signal to interference plus noise ratio (SINR) for the signal;
Determining the lower limit of the average SINR as channel quality information, and feeding back the channel quality information to the base station.
The method of claim 1,
The process of calculating the lower limit value of the average SINR,
Calculating a molecular term of said average SINR using the definition of a weighted chordal distance and the trigonometric law,
Calculating a denominator numerator of the average SINR using a decomposition of a matrix composed of an orthogonal basis of a channel matrix for the base station and a set channel;
Calculating the average SINR using the molecular term and the numerator;
And determining a lower limit value of an average SINR according to a power level of a signal transmitted from the base station.
The method of claim 1,
And the terminal does not recognize a channel between the base station and other terminals in addition to the channel.
The method of claim 1,
The channel quality information is
And the base station is used when generating a pre-coding matrix according to the block diagonalization method.
The method of claim 1,
The process of calculating the denominator Term of the average SINR,
A matrix consisting of a codeword constituting a codebook, an orthogonal basis of left-null space of the codeword, and an upper triangular matrix and a unitary matrix of the codeword. Channel quality information feedback method comprising the step of performing the decomposition of the matrix.
A terminal for feeding back channel quality information in a multiple input multiple output communication system supporting multiple users,
A receiving unit for receiving a signal from a base station,
An average SINR calculator configured to calculate a lower limit of an average signal to interference plus noise ratio (SINR) for the signal, and determine a lower limit of the average SINR as channel quality information;
And a transmitter for feeding back the channel quality information to the base station.
The method of claim 6,
The average SINR calculation unit,
Using the definition of a weighted chordal distance and trigonometric law, the molecular term of the mean SINR is calculated, and the decomposition of the matrix composed of the orthogonal basis of the channel matrix for the set channel and the base station is solved. Calculate the denominator of the average SINR using the quantum term, calculate the average SINR using the numerator and the numerator, and determine a lower limit of the average SINR according to the power level of the signal transmitted from the base station. Terminal.
The method of claim 6,
The terminal is characterized in that the terminal does not recognize the channel between the base station and the other terminals in addition to the channel.
The method of claim 6,
The channel quality information is
The base station is used when generating a pre-coding matrix according to the block diagonalization method.
The method of claim 6,
The average SINR calculation unit,
A matrix consisting of a codeword constituting a codebook, an orthogonal basis of left-null space of the codeword, and an upper triangular matrix and a unitary matrix of the codeword. Terminal for the decomposition of the matrix.

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