KR101787760B1 - Method and apparatus for feedback in a multi-user multi-input multi-output communication system - Google Patents

Abstract

The present invention relates to a method for feedback of channel quality information in a multiple input multiple output communication system supporting multiple users, the method comprising the steps of: receiving a signal from a base station; determining an average signal to interference noise ratio (SINR) to interference plus plus noise ratio), determining a lower limit value 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 BACKGROUND OF THE INVENTION 1. Field of the Invention [0001]

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

Recently, in the wireless communication system, a downlink channel in a multi-user multiple-input multiple-output (MU-MIMO) communication system in which one base station simultaneously supports several users Has progressed. In the downlink channel, inter-user interference becomes the biggest problem. Therefore, a zero-forcing (ZF) beamforming method and a block diagonalization (BD) method for designing a precoder have been widely used as methods for solving this problem. In this precoder design, it is presupposed that the transmitting side acquires information on an accurate downlink channel. On the basis of this, the receiver performs the scheduling algorithm of the user, thereby achieving a higher sum-rate performance.

However, in a practical wireless communication environment, it is difficult to obtain information on an accurate downlink channel at the transmitting end. Therefore, the receiver quantizes channel information with respect to the transmitting side based on a code book recognized in advance, and obtains channel direction information (CDI), which is one of the quantized codebook indexes, To the transmitting side. The CDI indicates the direction of the corresponding channel. The transmitter receiving the CID can design a precoder that cancels channel interference using the CID.

When there are a plurality of users in the MU-MIMO communication system, a performance gain, that is, a multi-user diversity gain can be obtained by allocating users suitable for the transmission environment. To this end, the user transmits channel quality information (CQI: Channel Quality Information) corresponding to the channel characteristics as well as the CDI of the channel to the sender. The transmitter receiving the CDI and the CQI transmitted from the corresponding user can perform more efficient scheduling using the CDI and the CQI.

In a conventional method, when a scheduling is performed in a vector channel having a single antenna at the receiving end, there is a method of analyzing the performance of ZF beamforming. Alternatively, if the receiver has multiple antennas, then there is a way to achieve performance gains by reducing errors due to channel quantization using receiver-side combiners. In another method, there is a method of obtaining the scheduling gain by reflecting the quantization error to the CQI. In the above-mentioned conventional methods, there is a restriction condition that each user has to perform communication through only one data layer, that is, single mode communication.

Meanwhile, a method of using the BD method in a multi-mode communication in which a transmitting side supports a plurality of data layers to a user has been proposed. In this case, the BD technique is used only for a predetermined user using CDI, and the CQI expression scheme for scheduling when a large number of users exist is not 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 the performance degradation caused by the error due to quantization can be caused.

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

The present invention proposes a method and apparatus for a transmitter to feedback channel quality information in an 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 precoding is used to support multi-stream in an MU-MIMO communication system.

A method according to an embodiment of the present invention comprises: A method for feedback of channel quality information in a multiple input multiple output (MIMO) communication system supporting multiple users, the method comprising: receiving a signal from a base station; receiving an average signal to interference plus noise (SINR) And determining a lower limit value 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 includes: A terminal for feedback of channel quality information in a multiple input multiple output communication system supporting multiple users, comprising: a receiver for receiving a signal from a base station; a signal to interference plus noise ratio (SINR) An average SINR calculator for calculating a lower limit value of the average SINR and determining a lower limit value of the average SINR as channel quality information; and a transmitter for feeding back the channel quality information to the base station.

In the MU-MIMO communication system, when precoding is used to support multi-streams in a MU-MIMO communication system, a CQI for maximizing a scheduling gain is used as a reception signal's average signal-to- The base station determines the lower limit value of the SINR to be lower than the lower limit of the SINR and feeds back the lower limit value of the determined average SINR to the base station to provide more accurate channel quality information to the base station, .

1 is a configuration diagram of an MU-MIMO communication system according to an embodiment of the present invention;
2 is a schematic configuration diagram of a terminal according to an embodiment of the present invention;
3 is a flowchart illustrating an operation of a terminal according to an embodiment of the present invention.
4 is a flowchart illustrating an operation of calculating a lower limit value of an average SINR of a UE according to an embodiment of the present invention;
5 is a graph showing a CDF (cumulative distribution function) of the SINR average value 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 showing another example of a sum rate according to the number of users according to an embodiment of the present invention.
FIG. 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; FIG.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. In the following description, only parts necessary for understanding the operation according to the present invention will be described, and the description of other parts will be omitted so as not to obscure the gist of the present invention.

The present invention proposes a CQI calculation method and apparatus for maximizing a scheduling gain when precoding is used in a MU-MIMO communication system to support a multi-stream. As a scheduling scheme, semi-orthogonal user selection (SUS) scheme is considered. The present invention proposes a method and apparatus for calculating a new CQI for minimizing interference to other users in consideration of an error caused by codebook quantization rather than considering only the existing channel size.

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

Lt; / RTI > terminals are present.

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 the terminals

And selects the terminal

When transmitting information to the UEs at the same time,

Th < / RTI > terminal is expressed as 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,

The

Th &lt; RTI ID = 0.0 &gt; terminal &lt; / RTI &gt;

to be.

Represents the number of antennas of the base station 100,

Represents the number of reception antennas of each terminal.

The

Lt; th &gt; terminal and the size is

to be. That is,

Lt; RTI ID =

Data streams are transmitted.

Is a 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

Lt; RTI ID = 0.0 &gt; BD method &lt; / RTI &

). &Lt; / RTI &gt; The BD method receives a data stream corresponding to the number of receive antennas. In the embodiment of the present invention, the BD method is used as an example, but it should be noted that the present invention is not applied to the BD method alone. That is, the present invention is applicable to a case of receiving a data stream that is more or less than the number of reception 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,

The signal to interference plus noise ratio (SINR) at the receiving end of the i < th > terminal is expressed by Equation (2).

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,

Lt; RTI ID = 0.0 &gt; channel &lt; / RTI &gt;

) Other channel of the terminal

), It is impossible to acquire the instantaneous SINR. Therefore, in the embodiment of the present invention,

And the average SINR is calculated.

(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 an embodiment of the present invention

The terminal of the < RTI ID = 0.0 >

) Other than the channel of the other terminal

), And uses the average SINR instead of the instantaneous SINR as the CQI information. At this time, the lower limit value of the average SINR is determined according to Equation (3).

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.

The

Eigen-value decomposition &quot; of &lt; / RTI &gt; At this time,

The

On the orthonormal basis, i.e.,

. Also,

Lt;

The

of

Eigen-value. &Lt; / RTI &gt;

(2) Preliminary are preliminary procedures necessary for calculating the denominator term and denominator of Equation (3).

(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 a code word in a codebook, and the size

to be. The codebook

Codeword &lt; / RTI &gt;

The

And an orthogonal basis of the left-null space.

The

The upper triangular matrix and the diagonal terms are all positive numbers

.

The

A unitary matrix,

The

The upper triangular matrix and diagonal terms are both positive

.

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

의 삼각 법칙을 만족한다.
The weighted cog distance

The triangular law of

First, the process of calculating the numerator term of Equation (3) is as follows.

(3) Molecular term calculation of average SINR (Desired Signal)

The molecular term of the average SINR is expressed as Equation (6) using the definition of the weighted chordal distance and the trigonometric rule.

이고,

는

로 정의한다. In the second inequality of Equation (6) above,

ego,

The

.

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 UEs having a channel matrix with a maximum orthogonal channel among the UEs as a scheduling target. Through this selection operation, the performance degradation due to the rank deficiency phenomenon occurring when the precoding matrix is generated can be prevented. Therefore, the matrix of each terminal is orthogonal to each other

The precoding matrix of the ith user is very similar to the original channel matrix (

). finally

Is close to '0'. Therefore, in the embodiment of the present invention,

To

.

Next, the procedure of calculating the denominator of Equation (3) is as follows.

(4) Computation of the denominator of average SINR (Interfernce Signal)

In Equation (3) The denominator of the average SINR is developed as shown in Equation (7) below.

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

And the decomposition of the decomposition.

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 definition

And is distributed unformed on the left null space of FIG. therefore,

Follows the beta distribution, and by definition of the beta distribution

.

(5) Average SINR Lower limit value  And discussion

The lower limit value of the average SINR is calculated using the values calculated in Equation (4) to Equation (6), as shown in Equation (8).

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 in Equation (8)

Therefore, At this time,

The lower limit value of the average SINR is determined. Then, the lower limit value of the determined average SINR

As a 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

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

silver

Lt; / RTI &gt; Therefore, the average SINR is

, And the channel size (&lt; RTI ID = 0.0 &gt;

) Is used as the 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

. Therefore, the average SINR is

That is, the quantization error. Therefore, in the case of having a high SNR

The conventional method using CQI information does not take into account the magnitude of the quantization error. Therefore, the performance degradation can not be avoided at the high SNR in the user scheduling. However, in the embodiment of the present invention, by using the SINR as the CQI information, proper performance can be guaranteed not only at a low SNR but also at a high SNR.

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

2, the UE 200 includes a receiving unit 205, an average SINR calculating unit 210, and a transmitting unit 215.

The receiving unit 205 receives the data stream transmitted from the base station and transmits the data stream to the average SINR calculating unit 210. The average SINR average calculation unit 210 calculates the lower limit value of the average SINR for the received data stream using Equation (8), and transmits the calculated lower limit value to the transmission unit 215. The transmission unit 215 transmits the calculated lower limit value of the average SINR to the base station as CQI information.

3 is a flowchart illustrating an operation of a UE according to an embodiment of the present invention.

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

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

Referring to FIG. 4, in step 400, the UE calculates the average SINR term using the weighted cadal distance definition and the trigonometric rule as shown in Equation (5), and proceeds to step 405.

In step 405, the UE decomposes the matrix composed of the orthogonal vices of the channel matrix to calculate the denominator of the average SINR, and proceeds to step 410. In step 410,

In step 410, the MS calculates the average SINR using the calculated molecular term and the denominator, and then proceeds to step 415. In step 415, In step 415, the mobile station determines a lower limit value of the average SINR according to the power level of 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 the simulation results using the average SINR according to the embodiment of the present invention, with reference to FIG. 5 to FIG. In this case, the BD method is used for precoding and the SUS algorithm is used for scheduling.

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

Referring to FIG. 5, the CDF of the SINR average value of the scheduled users is shown. In this case, the number of users is 10 and the number of users is 30. As shown in the graph, when the number of users is 10 and the number of users is 30, the performance of the case using the average SINR is compared with the case of using the channel magnitude and the average SINR as CQI, It is considerably higher than when using the size of the channel. In other words, when the size of the channel is used as the CQI, the change of the multiuser diversity gain is insignificant even if the number of users increases. On the other hand, when the average SINR is used as the CQI, the SINR increases as the number of users increases, so that the performance gain increases. Therefore, it is possible to accurately predict the actual SINR through the lower limit value 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. Assume that Nt = 4, Nr = 2, and 10 bits are used for CDI.

Referring to FIG. 6, in a substantially limited feedback situation, as compared with the ideal environment, the higher the transmission power due to the interference due to the quantization error, the more the combining rate is limited. When the SNR is high, the number of users is 30 and the gain of 30% and 40% is 100, respectively. That is, when SNR is high, the sum rate is greatly affected by interference. At this time, when the channel size is used as the CQI, the interference due to the quantization error can not be reflected. Therefore, the sum rate is lower than that in the case of using the average SINR according to the embodiment of the present invention 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. Assume that Nt = 6, Nr = 2, and 15 bits are used for CDI.

Referring to FIG. 7, when the channel size is used as the CQI, the interference due to the quantization error is not reflected. Therefore, when the number of users is 30 and 100, the average SINR Is used as the CQI, a gain of 20% is obtained as compared with the case of using the channel size as the 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, when 5 bits and 10 bits are used for CDI bits, the number of quantization bits is changed to 2, 4, and 6, respectively, and quantization bits are not quantized. 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, in the case of having bits for the same CDI, the higher the number of quantization bits, the greater the sum rate. For example, if the bits for CDI are 10 bits, the number of quantization bits is increased to avoid loss caused by quantization.

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, but is capable of various modifications within the scope of the invention. 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.

Claims (10)

A method for feedback of channel quality information by a terminal in a multiple input multiple output communication system supporting multiple users,
Receiving a signal from a base station,
Calculating a lower limit value of an average signal-to-interference plus noise ratio (SINR) for the signal;
Determining a lower limit value of the average SINR as channel quality information, and feeding back the channel quality information to the base station,
Calculating a lower limit value of the average SINR,
Calculating a molecular term of the average SINR using a definition of a weighted chordal distance and a trigonometric rule.
The method according to claim 1,
Calculating a lower limit value of the average SINR,
Calculating a denominator of the average SINR using an orthogonal basis of a matrix of channels for the set channel and the base station;
Calculating the average SINR using the numerator term and the denominator;
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 according to claim 1,
Wherein the terminal is in a state of not recognizing a channel between the base station and other terminals in addition to the channel.
The method according to claim 1,
The channel quality information may include:
Wherein the base station is used for generating a precoding matrix according to a block diagonalization scheme.
3. The method of claim 2,
Wherein the step of calculating the denominator of the average SINR comprises:
A matrix composed of a codeword constituting a codebook and an orthogonal basis of a left-null space of the codeword, and an upper triangular matrix and a unitary matrix of the codeword And performing decomposition of the matrix.
A terminal for feeding back channel quality information in a multiple input multiple output communication system supporting multiple users,
Comprising: a receiver for receiving a signal from a base station;
Calculating a molecular term of an average signal-to-interference plus noise ratio (SINR) for the signal using the definition of the weighted chordal distance and the trigonometric rule, and calculating a lower limit value of the average SINR An average SINR calculator for determining a lower limit value of the average SINR as channel quality information,
And a transmitter for feeding back the channel quality information to the base station.
The method according to claim 6,
The average SINR calculator calculates the average SINR,
Calculating a denominator of the average SINR using a decomposition of a matrix consisting of an orthogonal basis of a channel matrix for the set channel with the base station and calculating the average SINR using the numerator term and the denominator; And determines a lower limit value of the average SINR according to a power level of a signal transmitted from the base station.
The method according to claim 6,
Wherein the terminal is unable to recognize a channel between the base station and other terminals in addition to the channel.
The method according to claim 6,
The channel quality information may include:
Wherein the base station is used for generating a precoding matrix according to a block diagonalization scheme.
8. The method of claim 7,
The average SINR calculator calculates the average SINR,
A matrix composed of a codeword constituting a codebook and an orthogonal basis of a left-null space of the codeword, and an upper triangular matrix and a unitary matrix of the codeword Wherein the decomposition of the matrix is performed.

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