KR101512636B1 - Method for Providing Time-Domain Differential Feedback and Apparatus Therefor - Google Patents

Method for Providing Time-Domain Differential Feedback and Apparatus Therefor Download PDF

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KR101512636B1
KR101512636B1 KR1020140012537A KR20140012537A KR101512636B1 KR 101512636 B1 KR101512636 B1 KR 101512636B1 KR 1020140012537 A KR1020140012537 A KR 1020140012537A KR 20140012537 A KR20140012537 A KR 20140012537A KR 101512636 B1 KR101512636 B1 KR 101512636B1
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codebook
tap
channel
base station
feedback
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KR1020140012537A
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Korean (ko)
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김윤성
양기석
임기홍
김현명
조용상
전요셉
강윤성
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에스케이텔레콤 주식회사
포항공과대학교 산학협력단
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B7/00—Radio transmission systems, i.e. using radiation field
    • H04B7/02—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas
    • H04B7/04—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas
    • H04B7/0413—MIMO systems
    • H04B7/0417—Feedback systems
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B7/00—Radio transmission systems, i.e. using radiation field
    • H04B7/02—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas
    • H04B7/04—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas
    • H04B7/0413—MIMO systems
    • H04B7/0456—Selection of precoding matrices or codebooks, e.g. using matrices antenna weighting

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  • Computer Networks & Wireless Communication (AREA)
  • Signal Processing (AREA)
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  • Mobile Radio Communication Systems (AREA)

Abstract

The present invention is related to a method for time domain differential feedback and a device for the same. An embodiment of the present invention relates to a method to reduce the overhead when making the feedback of channel information estimated by the terminal to the base station in a wireless communications system using a multiple-input and multiple-output (MIMO) antenna. In particular, the method and an apparatus for time domain differential feedback provide improved transmission rate with the same feedback amount by using a time domain feedback based on the differential feedback method in a wind-band frequency selective channel with a time correlation, and enable users to set or select codebooks for feedback.

Description

[0001] The present invention relates to a time-domain differential feedback method, and more particularly,

This embodiment relates to a time domain differential feedback method and an apparatus therefor.

The contents described below merely provide background information related to the present embodiment and do not constitute the prior art.

A frequency-domain differential feedback scheme used in a MIMO (Multiple Input Multiple Output) antenna system will be described. A base station using a plurality of transmit antennas may use a signal-to-interference and noise ratio (MIMO) scheme of a received signal of a mobile station using MIMO beamforming or multiuser MIMO (MU-MIMO) SINR) and antenna gain.

In order to perform multi-input / output beamforming applied to a standard of a conventional cellular mobile communication system such as an LTE (Long Term Evolution) system, a base station must recognize channel information between each antenna element and a terminal. The channel information estimation is performed by each terminal using a reference signal transmitted from the base station. The estimated channel information is transmitted from the terminal to the base station using feedback. At this time, the feedback information is quantized and transmitted due to the restriction of frequency and time resources. If the feedback bits are not sufficiently utilized in such a transmission process, a large amount of quantization error occurs, and the occurrence of errors causes inaccuracy and performance loss of beamforming.

Various techniques have been studied to reduce performance loss due to quantization error of channel information feedback and overhead due to feedback. Differential feedback is one of the typical feedback overhead reduction schemes, which improves the accuracy of feedback by using the correlation between adjacent channels.

FIG. 1 is a diagram illustrating a codebook updating method of existing frequency-domain differential feedback in a channel having a correlation over time. The first feedback feeds back the codeword closest to the estimated channel using the basic codebook. The first estimated channel and the second estimated channel have a correlation over time, not a completely independent channel.

For each feedback, the codebook is updated based on the channel correlation in the time domain. At this time, the new codebook quantizes only the region having a high degree of correlation with the previously selected codebook index. Therefore, it is possible to set a codebook having a higher accuracy when all regions are quantized, and accordingly the accuracy of feedback can be improved. However, the frequency-domain feedback must quantize and feed back all the channel information for each subband. Therefore, as the frequency band to be considered becomes wider, the feedback overhead increases. Also, since the existing scheme is limited to a single tap channel model, application to a frequency selective channel, which is a more general channel model, is not considered.

The present embodiment relates to a technique for reducing overhead when channel information estimated by a mobile station is fed back to a base station in a wireless communication system using a multiple input / output (MIMO) antenna. By using time-domain feedback applying a differential feedback scheme in a selective channel of a wide-band frequency, especially with time correlation, it is possible to provide a time-domain differential Feedback method and a device therefor.

According to an aspect of the present invention, a codebook setting unit for setting a base codebook to have the same codebook as a base station communicating with the base station; A feedback unit that estimates a channel with the base station when receiving a pilot signal from the base station, extracts a code word corresponding to the channel, and feeds back an index corresponding to the code word to the base station; A codebook updating unit for generating an update codebook in which the basic codebook is updated based on the code word; And a data transmission / reception unit for receiving data based on a quantized channel corresponding to the codeword from the base station.

According to another aspect of the present invention, there is provided a codebook setting unit for setting a basic codebook so as to have the same codebook as a communication terminal; A pilot transmitting unit for transmitting a pilot signal for each antenna port to the terminal; A codebook updating unit that receives an index corresponding to the pilot signal from the terminal and generates an update codebook that updates the basic codebook according to a codeword corresponding to the index; And a data transmission / reception unit for extracting a quantized channel corresponding to the codeword from the update codebook, and transmitting the data to the terminal based on the quantized channel.

According to another aspect of the present invention, there is provided a method for transmitting a codebook, the method comprising: setting a basic codebook to have the same codebook in a terminal and a base station; Transmitting, by the base station, a pilot signal for each antenna port to the terminal; Estimating a channel with the base station, extracting a codeword corresponding to the channel, and feeding back an index corresponding to the codeword to the base station when the terminal receives a pilot signal from the base station; Generating an update codebook in which the base code and the terminal update the basic codebook according to the codeword; And extracting a quantized channel corresponding to the codeword from the update codebook in the base station and transmitting data based on the quantized channel to the terminal. do.

As described above, according to the present embodiment, in a wireless communication system using a multiple input / output (MIMO) antenna, it is possible to reduce overhead when channel information estimated by a terminal is fed back to a base station .

According to the present embodiment, by using the time-domain feedback using the differential feedback scheme in the selective channel of the wide-band frequency with time correlation, it is possible not only to provide an improved transmission rate with the same amount of feedback, There is an effect that can provide a choice.

According to the present embodiment, the quantization error is reduced while using the same amount of feedback information as the frequency-domain channel information differential feedback technique, so that the base station can acquire channel information closer to the actual channel. Accordingly, it is possible to perform more accurate beamforming, thereby improving the performance of a multi-user input / output system.

1 is a diagram illustrating a codebook updating method of frequency-domain differential feedback in a channel having a correlation over time.
2 is a diagram illustrating codebook-based vector quantization of frequency-domain channel information.
Figure 3 is a diagram illustrating codebook-based vector quantization of time domain channel information.
4A to 4C are views showing a system model according to the present embodiment.
5 is a diagram illustrating a codebook updating method according to the present embodiment.
FIG. 6 is a diagram illustrating a technique for dividing an entire codebook according to the present embodiment into codebooks for size and direction and setting them separately.
7 is a diagram illustrating an operation of an embodiment for operating a differential feedback technique according to the present embodiment.
8 is a diagram for explaining environmental variables of a computer simulation for performance evaluation according to the present embodiment.
FIG. 9 is a diagram illustrating a performance of a rate sum combining function according to size and directional codebook bit distribution when the power delay distribution characteristic of the ETU channel according to the present embodiment is recognized.
FIG. 10 is a diagram illustrating a performance of a rate sum combining function according to size and direction codebook bit distribution when the power delay distribution characteristic of the ETU channel according to the present embodiment is not known.
FIG. 11 is a diagram illustrating a sum rate performance of techniques according to the present embodiment and a general technique.

Hereinafter, the present embodiment will be described in detail with reference to the accompanying drawings.

Recently, the development of high integration antenna array technology has made it possible to utilize a large number of antenna elements, thereby increasing the amount of feedback required for channel information transmission. Therefore, effective feedback reduction techniques are required to improve performance even with limited feedback.

While normal frequency domain feedback requires more feedback amount in proportion to the number of subbands, time domain feedback requires feedback proportional to the number of taps in the channel impulse response (CIR). In the case of a broadband channel, the number of subbands increases proportionally to the bandwidth, while the number of taps in the channel impulse response (CIR) is less than the total number of subbands because the taps exist sparse do. Therefore, for the same amount of feedback, more feedback can be assigned to the channel quantization using time domain feedback, thereby improving the accuracy and performance of the feedback.

2 is a diagram illustrating codebook-based vector quantization of frequency-domain channel information. Since only the direction information of the channel is fed back in the frequency domain, the codebook is distributed only on the unit sphere surface of the same dimension as the code word.

On the other hand, as shown in FIG. 3, the channel information feedback in the time domain needs to transmit both the size and the direction, and therefore the codebook having code words of different sizes and directions is distributed in the entire space instead of the unit sphere. 3 is a diagram illustrating codebook-based vector quantization of time domain channel information.

As shown in FIG. 2 and FIG. 3, in order to quantize the tap of each channel impulse response (CIR) in the time domain, quantization in a manner different from the frequency domain is required. This embodiment is proposed to apply the differential feedback to the time domain, and in particular, a frequency selective channel having time correlation is modeled. At this time, in order to apply differential feedback effectively, a description will be given below of a codebook update, a codebook setting, and a codebook selection.

2, 3, and 6 shown in this embodiment are diagrams showing a kind of conceptual diagram in which arrows indicate values that codewords may have when a channel vector has three components in total. If the channel vector has three components in total, the horizontal line corresponds to the 'x axis', the oblique line to the 'y axis', and the vertical line to the 'z axis'. The channel elements to be fed back are basically expressed in the form of a vector (or matrix), but when the number of components of the channel is four or more, the channel vector can not be expressed in space.

4A to 4C are diagrams showing a system model according to the present embodiment.

In order to describe the present embodiment more specifically, it is assumed that the base station 410 has N T antennas and that a total of K users (terminals) are OFDM (Orthogonal Frequency Division Multiplex) systems each having N R = 1 antenna do. At this time, the channel between the base station 410 and each terminal 420 may be represented by L channel matrices using a tapped-delay line model.

Figure 112014010899815-pat00001
Is the l-th tap of the channel between the τ-th frame (Frame) The base station and the i-th terminal, and its variance is σ 2 l. A channel having a correlation with respect to time can be modeled as a first-order Markov model as shown in Equation (1). A frame refers to a certain unit, size or boundary of information transmitted in data communication.

Figure 112014010899815-pat00002

h i, τ [l] denotes the l-th tap of the channel between the base station and the i-th terminal in the t-th frame. and τ denotes a frame index. i denotes a terminal index. l denotes a channel tap index. ε i is a coefficient of correlation, which means modeling with ε i = J 0 (2πf D T). J 0 (·) means the Zero Order Bessel Function of the First Kind. f D means the Doppler frequency. T means the feedback period. Z i, τ [l] is a complex Gaussian random variable, an average 0 covariance matrix

Figure 112014010899815-pat00003
.
Figure 112014010899815-pat00004
Means a random variable indicating an arbitrary update.

4B illustrates a base station 410 according to an embodiment of the present invention. The base station 410 includes a codebook setting unit 422, a communication unit 430, and a codebook updating unit 440. The components included in the base station 410 are not limited thereto.

The codebook setting unit 422 sets the basic codebook to have the same codebook as the terminal 420 to communicate with. The codebook setting unit 422 sets the codebook (h i, t [l]) of the channel allocated between the base station and the i-th terminal in the t-th frame or the base codebook

Figure 112014010899815-pat00005
). ≪ / RTI > In other words, the codebook setting unit 422 sets the basic codebook using [Equation 3] or [Equation 4].

The codebook setting unit 422 sets the codebook setting unit 422 to use the first tap (h iτ [l]) of the channel between the base station and the i-th terminal in the tau-th frame, variance of the l th channel tap (Power Delay Profile value) (σ 2 l), τ size of the l th channel tap coefficients for the i-th terminal in the second frame (l standard deviation of the second channel tap value (by σ l) normalized value) (α i, τ [l ]) , and τ the base station and the i-th on the τ-th frame based on the direction (g i, r [l] ) in the l th channel tap coefficients for the i-th terminal in the second frame, And calculates the lth tap (h i, τ [l]) of the channel allocated between the terminals.

The codebook setting unit 422 sets a scalar for quantizing the sizes (? I,? [L]) of the l-th tap channel, A code word corresponding to the bit index b in the codebook (

Figure 112014010899815-pat00006
) corresponding to the bit index b of the vector codebook for quantizing the direction g i, t [l] of the l-th tap channel
Figure 112014010899815-pat00007
), a Scalar codebook for quantizing the size ([alpha] i, [l]) of the l-th tap channel
Figure 112014010899815-pat00008
) And a vector codebook for quantizing the direction g i, t [l] of the l-th tap channel
Figure 112014010899815-pat00009
) Of the first tap channel.

The communication unit 430 includes a pilot transmission unit 432 and a data transmission / reception unit 434. The pilot transmission unit 432 transmits a pilot signal for each antenna port to the terminal 420. A pilot is a signal that transmits the characteristics of a system for display, testing, or control. The data transmission / reception unit 434 extracts the quantized channel corresponding to the codeword from the update codebook, forms the multiple input / output beamforming based on the quantized channel, and transmits the data to the terminal 420 through the multiple input beamforming .

The codebook updating unit 440 receives an index corresponding to the pilot signal from the terminal 420 and generates an update codebook that updates the basic codebook according to the codeword corresponding to the index.

The codebook updating unit 440 updates the basic codebook based on the codeword (c l ?, B ) corresponding to the bit index b among the codebooks for quantizing the l-th tap channel in the ? In other words, the codebook updating unit 440 uses the equation (2).

When the codebook updating unit 440 explains the process of using Equation (2), the codebook updating unit 440 updates the codebook updating unit 440 based on the variance value? 2 l of the l-th channel tap, The code word corresponding to the bit index b (

Figure 112014010899815-pat00010
), Design parameters for the frame (τ) (τ δ), and τ-1 l-th tap estimate of the channel allocated between the base station and the i-th terminal in the second frame (
Figure 112014010899815-pat00011
(C l ?, B ) corresponding to the bit index b among the codebooks for quantizing the l-th tap channel in the? Th frame.

4C shows a terminal 420 according to the present embodiment includes a codebook setting unit 450, a communication unit 460, and a codebook updating unit 470. The components included in the terminal 420 are not necessarily limited thereto.

The codebook setting unit 450 sets the basic codebook to have the same codebook as that of the base station 410 with which it communicates. When setting the basic codebook for quantizing each tap of the channel impulse response (CIR) in the time domain, the codebook setting unit 450 separately sets the entire codebook into a scalar codebook for the size and a vector codebook for the direction. Then, the codebook setting unit 450 sets the entire codebook as an equation 4 by combining the scalar codebook and the vector codebook.

The codebook setting unit 450 sets the basic codebook based on the l-th tap (h i, τ [l]) of the channel allocated between the base station and the i-th terminal in the τth frame or the basic codebook of the l-th tap channel. In other words, the codebook setting unit 450 uses [Equation 3] or [Equation 4].

The codebook setting unit 450 sets the codebook setting unit 450 to use the first tap h iτ [l] of the channel between the base station and the i-th terminal in the t- th frame, l th channel tap the variance (σ 2 l), l th channel tap size (a value normalized by σ l) of the coefficient for the i-th terminal in the τ-th frame (α i, τ [l]) and τ th Th tap (h i, τ [l] of the channel allocated between the base station and the i-th terminal in the t- th frame based on the direction (g i, r [l]) of the l-th channel tap coefficient for the i- ]).

The codebook setting unit 450 sets a scalar for quantizing the sizes of the 1-th tap channel , [alpha] i, [lambda] [l], using the codebook setting unit 450. [ A code word corresponding to the bit index b in the codebook (

Figure 112014010899815-pat00012
), a codeword corresponding to the bit index b of the vector codebook for quantizing the direction g i, t [l] of the l-th tap channel (
Figure 112014010899815-pat00013
), a Scalar codebook for quantizing the size ([alpha] i, [l]) of the l-th tap channel
Figure 112014010899815-pat00014
) And a vector codebook for quantizing the direction g i, t [l] of the l-th tap channel
Figure 112014010899815-pat00015
) Of the first tap channel.

The communication unit 460 includes a feedback unit 462 and a data transmission / reception unit 464. When receiving the pilot signal from the base station, the feedback unit 462 estimates a channel with the base station 410, extracts a code word corresponding to the channel, and feeds back an index corresponding to the code word to the base station 410. The feedback unit 462 performs vector-quantization on the channel based on the basic codebook in the time domain and performs differential feedback to the base station 410.

The feedback unit 462 forms each of the taps of the channel impulse response (CIR) in a time domain in the first feedback transmission with a matrix of the number of reception antennas x the number of transmission antennas. Thereafter, the feedback unit 462 quantizes the matrix (the number of reception antennas x the number of transmission antennas) based on the basic codebook, and feeds back the quantized signals to the base station 410. The feedback unit 462 forms a matrix consisting of the number of reception antennas x the number of transmission antennas by changing the value of each tap of the channel impulse response (CIR) in the time domain between the second feedback and the feedback interval. Thereafter, the feedback unit 462 quantizes the matrix (the number of reception antennas x the number of transmission antennas) based on the update codebook, and feeds back the result to the base station 410.

When extracting the codeword using the basic codebook, the feedback unit 462 independently extracts the codeword for each of the scalar codebook and the vector codebook. The feedback unit 462 feeds back each tap position of the channel impulse response (CIR) for the channel to the base station in a bitmap manner. When the length of the channel impulse response is equal to or greater than the length of the CP applied to the system, the feedback unit 462 feeds back each tap position to the base station 410 in the bitmap manner only by the length of the CP.

The feedback unit 462 compares the lth tap estimation value of the channel allocated between the base station and the i < th >

Figure 112014010899815-pat00016
) Or the direction estimate value of the lth channel tap coefficient for the i < th > terminal in the <
Figure 112014010899815-pat00017
) And a size estimation value (normalized by? 1 ) of the l-th channel tap coefficient for the i-th terminal in the?
Figure 112014010899815-pat00018
). ≪ / RTI > In other words, the feedback unit 462 uses [Equation 5] or [Equation 6].

The feedback unit 462 may be configured so that the feedback unit 462 uses the l-th tap ((h i, τ [l]) of the channel allocated between the base station and the i- ]) And a codeword (c l ?, B ) corresponding to a bit index b among the codebooks for quantizing the l-th tap channel in the? Th frame, a channel allocated between the base station and the i- The lth tap estimate (

Figure 112014010899815-pat00019
).

The feedback unit 462 calculates a difference (?) Between the channel information in the previous frame? -1 and the channel information in the current frame? In accordance with Equation (6) H i, τ [l]) and l-th tap in the channel direction (g i, τ [l] ) code words corresponding to the bit index b of the vector codebook to quantize the (

Figure 112014010899815-pat00020
The direction estimate value of the l-th channel tap coefficient for the i < th >
Figure 112014010899815-pat00021
).

The feedback unit 462 calculates the difference between the channel information in the previous frame? -1 and the channel information in the current frame? In accordance with Equation (6) H i, τ [l]), the direction estimate of the l-th channel tap coefficient for the i-th terminal in the τ th frame

Figure 112014010899815-pat00022
), Design parameters for the frame (τ) (δ τ), the l-th channel scalar for quantizing the variance (σ 2 l) and l th size of the tap channel (α i, τ [l]) of the tab (scalar ) Code word corresponding to the bit index b in the codebook (
Figure 112014010899815-pat00023
(The value normalized by? 1 ) of the l-th channel tap coefficient for the i-th terminal in the?
Figure 112014010899815-pat00024
).

The feedback unit 462 newly estimates a channel with the base station 410 when receiving a newly received pilot signal from the base station. The feedback unit 462 extracts a codeword corresponding to the newly estimated channel, and feeds back an index corresponding to the codeword to the base station 410.

The data transmission / reception unit 464 receives data from the base station 410 based on the quantized channel corresponding to the codeword.

The codebook updating unit 470 generates an update codebook that updates the basic codebook based on the code word. The codebook updating unit 470 updates the basic codebook based on the codeword (c l ?, B ) corresponding to the bit index b among the codebooks for quantizing the l-th tap channel in the ? In other words, the codebook updating unit 470 uses the equation (2).

When the codebook updating unit 470 explains the process of using Equation (2), the codebook updating unit 470 updates the variance value (? 2 l ) of the l-th channel tap, The code word corresponding to the bit index b (

Figure 112014010899815-pat00025
), Design parameters for the frame (τ) (τ δ), and τ-1 l-th tap estimate of the channel allocated between the base station and the i-th terminal in the second frame (
Figure 112014010899815-pat00026
), The codeword (c l ?, B ) corresponding to the bit index b among the codebooks for quantizing the l-th tap channel in the? Th frame is calculated.

5 is a diagram illustrating a codebook updating method according to the present embodiment.

Hereinafter, each process performed in FIG. 5 may be performed in the base station 410 and the terminal 420.

N R × N T matrix of l to quantize the second tab, the basic codebook which may be represented by B bits (Bit)

Figure 112014010899815-pat00027
, The first codebook is searched for an optimum codeword for channel estimation using the basic codebook as it is. The codebook C l r of the sub-frame? -Th, which is newly updated thereafter, is updated using Equation (2).

Figure 112014010899815-pat00028

c l ?, b denotes a codeword corresponding to the bit index b among the codebooks for quantizing the l-th tap channel in the? th frame. δ τ is a design parameter for the frame (τ). l denotes a channel tap index. and τ denotes a frame index. and b denotes a bit index. B is the number of bits allocated to the codebook.

Figure 112014010899815-pat00029
Quot; refers to a base codebook. σ 2 l means the dispersion value (power delay profile value) of the l-th channel tap.

Figure 112014010899815-pat00030
Denotes a codeword corresponding to the bit index b of the basic codebook for quantizing the l-th tap channel.

Figure 112014010899815-pat00031
Denotes the l-th tap estimation value of the channel between the base station and the i-th terminal in the (t-1) th frame.

δ τ is a design parameter that is set (designed) by reflecting the quantization error according to the codebook size and the correlation of the channel with time. Since the differential feedback is a method of tracking and updating the actual channel, the feedback value converges to the actual channel over time. Since the quantization error is large at the beginning of the update and decreases with time, δ τ should also be set as a decreasing function with time for fast convergence. Also, after a sufficient time has elapsed, the value of < RTI ID = 0.0 >

Figure 112014010899815-pat00032
As shown in FIG.

The basic codebook used for time-domain differential feedback of the l-th tap

Figure 112014010899815-pat00033
Must quantize the complex space of the dimension. Using the fact that the channel can be separated into two independent terms as shown in Equation (3), a codebook for quantizing the size and direction of the channel is separately set and combined.

Figure 112014010899815-pat00034

h i, τ [l] denotes the l-th tap of the channel between the base station and the i-th terminal in the t-th frame. l denotes a channel tap index. and τ denotes a frame index. i denotes a terminal index. and? 1 denotes a standard deviation value of the l-th channel tap. α i and τ [l] denote the magnitude (normalized by σ l ) of the l-th channel tap coefficient for the i-th terminal in the τth frame. g i, τ [l] denotes the direction of the l-th channel tap coefficient for the i-th terminal in the τ th frame.

Codebook for size

Figure 112014010899815-pat00035
, The codebook for the direction
Figure 112014010899815-pat00036
, And each codebook is quantized into bits of B M and B D. At this time, the proposed codebook is expressed as [Equation (4)] as an element product of two codebooks. 6 is a diagram visualizing [Equation 4].

Figure 112014010899815-pat00037

Figure 112014010899815-pat00038
Denotes a basic codebook of the l-th tap channel.
Figure 112014010899815-pat00039
Denotes a Scalar codebook for quantizing the size ([alpha] i, [l]) of the l-th tap channel.
Figure 112014010899815-pat00040
Denotes a vector codebook for quantizing the direction g i, t [l] of the l-th tap channel. and b denotes a bit index.
Figure 112014010899815-pat00041
Denotes a codeword corresponding to the bit index b among the scalar codebook for quantizing the size ([alpha] i, [l]) of the l-th tap channel.
Figure 112014010899815-pat00042
Denotes a codeword corresponding to the bit index b of the vector codebook for quantizing the direction g i, t [l] of the l-th tap channel.

The description of each codebook setting is as follows.

Figure 112014010899815-pat00043
Is a Scalar codebook for quantizing the size ([alpha] i [l]) of the channel. Since the elements of each channel matrix are complex Gaussian random variables, the square of the channel size α 2 l [l] follows the gamma distribution of (N T , 1). Thus, the channel size follows a generalized gamma distribution defined by (1, 2N T , 2)). Scalar quantization for this
Figure 112014010899815-pat00044
Is set to minimize the mean square error (MSE) using the Lloyd-Max algorithm according to the distribution of channel sizes.
Figure 112014010899815-pat00045
Is a vector codebook for quantizing the channel direction (g [ tau ] [l]). In order to uniformly quantize the complex vector space of dimension N T, and uses the spherical codebook (Spherical Codebook) the N T dimensional codebook converts it into complex vector of the real vector space of dimension 2N T.
Figure 112014010899815-pat00046
and
Figure 112014010899815-pat00047
Is different depending on the power delay profile (PDP) of the channel.

Figure 112014010899815-pat00048
The proposed codebook setting method for each tap requires information on the position and variance of each tap. However, when the terminal 420 of the wireless communication system transmits the channel information to the base station 410, the information on the position of the channel tap and the dispersion value of each tap may not be included. Since the position of the channel tap is a variable that does not change over a very long time compared to the feedback transmission period, once it is transmitted, it does not consume further transmission resources and does not have additional feedback overhead.

The channel tap position can be generated by feeding N ch bits equal to the total channel length in a bit map manner. Assuming that the channel length is smaller than the CP (Cyclic Prefix) length N CP , A method of feeding back CP bits may be used.

[Example] When N ch = 16, N CP = 8 and the tap position is 1, 2, 4, 8, 16

When using N ch bits: 1101000100000001

When using N CP bits: 11010001

A separate codebook setting method applicable when the variance value of each tap is unknown is as follows. Since the dispersion value of each tap differs for each channel model and tab, it is impossible to set an optimal codebook for a general case. Accordingly, a uniform quantization scheme that can operate robustly against mismatching of the distribution of tap dispersion values is proposed. To achieve uniform quantization, it is important to set an appropriate upper limit, which is the channel size value that a single tap channel will have on average

Figure 112014010899815-pat00049
.

There are two methods for finding the optimal codeword for channel estimation using the updated codebook. The first is to use the square error between the actual channel and the codeword as the criterion for best representing the channel. The calculation formula for this is shown in Equation (5).

Figure 112014010899815-pat00050

h i, τ [l] denotes the l-th tap of the channel between the base station and the i-th terminal in the t-th frame. c l ?, b denotes a codeword corresponding to the bit index b among the codebooks for quantizing the l-th tap channel in the? th frame.

Figure 112014010899815-pat00051
Denotes the l-th tap estimation value of the channel between the base station and the i < th >

At this time, the number of calculations required to find an optimal code word is 2B, which is equal to the number of codebooks. The second scheme is to find the optimal codeword for each codebook by using the fact that codebooks for size components and direction components are independently set.

Figure 112014010899815-pat00052
, The scale for each codebook is expressed by Equation (6).

Figure 112014010899815-pat00053

Figure 112014010899815-pat00054
Denotes the direction estimate value of the l-th channel tap coefficient for the i < th > △ H i, τ [l] means a difference between channel information of a previous frame (τ-1) channel information and the current frame (τ) of from.
Figure 112014010899815-pat00055
Denotes a codeword corresponding to the bit index b of the vector codebook for quantizing the direction g i, t [l] of the l-th tap channel.
Figure 112014010899815-pat00056
Denotes a magnitude estimation value (a value normalized by sigma l ) of an i-th channel tap coefficient for an i-th terminal in a tau-th frame. δ τ is a design parameter for the frame (τ). σ 2 l means the dispersion value of the l-th channel tap.
Figure 112014010899815-pat00057
Denotes a codeword corresponding to the bit index b of the scalar codebook for quantizing the size ([alpha] i, [l]) of the l-th tap channel.

The final channel estimate is

Figure 112014010899815-pat00058
to be. In the second method, the number of calculations required to find an optimal codeword is
Figure 112014010899815-pat00059
, Which has fewer computation times and complexity than the first method.

When there are N T transmit antennas and one receive antenna per terminal, the channel in each frame is an average vector of 0,

Figure 112014010899815-pat00060
Of the complex Gaussian with a covariance matrix of Covariance
Figure 112014010899815-pat00061
Quot;). Thus, the channel may be anywhere in the N T dimension space, but the probability that the channel exists in the N T dimension space decreases as the radius increases from the origin. Therefore, the basic codebook for quantizing the channel must also be designed in accordance with the distribution of the complex Gaussian described above, so that the quantization error can be minimized. The spheres shown in FIG. 5 mean that, when a channel exists according to the distribution of the complex Gaussian described above, the probability that the channel exists on the N T -dimensional space is the same on the same position as the radius.

7 is a diagram illustrating an operation of an embodiment for operating a differential feedback technique according to the present embodiment.

The base station 410 sets a basic codebook so as to have the same codebook as the terminal to communicate with (S710). In step S710, the base station can set the basic codebook using Equation (3) or Equation (4). At step S710, the terminal 420 sets a basic codebook to have the same codebook as the base station with which it communicates (step S720). In step S720, the terminal 420 may set the basic codebook using Equation (3) or Equation (4).

Equation (3) in the steps S710 and S720 is a formula indicating that the channel information can be separated into size information and direction information. Since a channel can be separated by components, a codebook for quantizing a channel component can be set separately from a codebook for size information and a codebook for direction information, as shown in Equation (4).

As it is shown in Equation 4, since the base codebook C represented by a combination of size codebook C M, direction codebook C D, the base station 410 and terminal 420, but may have a C, to C M without C It may have a codebook in the form of C D , and it may be advantageous to have a codebook in the form of C M and C D without C. It is more difficult to extract codewords in C M and C D , respectively, by using [Equation (6)] rather than extracting with C [Equation (5) Is lower.

The base station 410 transmits a pilot signal for each antenna port (S730). The pilot signal itself transmitted from the base station 410 in step S730 is a process necessary for determining the channel between the base station 410 and the terminal 420 at the receiving end of the terminal 420. The pilot signal is transmitted separately from the codebook do.

The terminal 420 estimates a channel with the base station 410 when receiving the pilot signal from the base station 410. The terminal 420 extracts an optimal codeword corresponding to the estimated channel, and feeds back an index corresponding to the extracted codeword to the base station 410 (S740). In step S740, the terminal 420 uses Equation (5) or Equation (6).

There are two methods for finding the optimal codeword in the codebook.

The first scheme is that the terminal 420 uses Equation (5)

Figure 112014010899815-pat00062
, And the calculated
Figure 112014010899815-pat00063
And an index corresponding to the codeword corresponding to the codeword is fed back to the base station 410.

The second scheme is that the terminal 420 uses Equation (6)

Figure 112014010899815-pat00064
,
Figure 112014010899815-pat00065
Respectively.
Figure 112014010899815-pat00066
,
Figure 112014010899815-pat00067
And an index corresponding to the codeword corresponding to the codeword is fed back to the base station 410.

The first and second schemes yield the same results theoretically, but the complexity is lower in the second scheme.

Steps S710 to S740 refer to the first frame (frame 0) between the BS 410 and the MS 420. [

The base station 410 updates the basic codebook according to the codeword corresponding to the index fed back from the terminal 420 (S750). In step S750, the base station 410 can update the basic codebook using Equation (2). If the terminal 420 feeds back the index using Equation (5) to the base station 410 in step S740, the base station 410 updates the basic codebook using Equation (2), but in step S740 When the terminal 420 feeds back the index using Equation (6) to the base station 410, the base station 410 does not need to update the base codebook.

The terminal 420 updates the basic codebook according to the codeword corresponding to the index (S760). In step S760, the terminal 420 may update the basic codebook using Equation (2). If the terminal 420 uses Equation 5 in step S740, the terminal 420 updates the basic codebook using Equation 2. In step S740, if the terminal 420 determines that Equation 6 ], The terminal 420 does not need to update the basic codebook.

In other words, when the terminal 420 uses the expression (6) in step S740, the codebook update of the expression (2) is not absolutely necessary. The channel estimation value can be updated from the value in the previous frame as shown in Equation (6) by using the index found for the size and direction codebook, so that the optimal channel estimation value can be obtained without updating the entire codebook.

The base station 410 forms multiple input beamforming based on the codeword (quantized channel information) corresponding to the index, and transmits the data to the terminal 420 using the formed multiple input beamforming (S770). In step S770, the base station 410 uses multi-input beamforming when transmitting data to the terminal 420, wherein the beamforming vector used is determined based on the quantized channel information. The quantized channel information may be determined by retrieving a codeword corresponding to feedback information (codeword index) sent from the terminal 420 to the base station 410 from a codebook of the base station 410.

The base station 410 transmits a new pilot signal for each antenna port (S772). When receiving a new pilot signal from the base station 410, the terminal 420 estimates a channel with the base station 410. The terminal 420 extracts an optimal codeword corresponding to the estimated channel, and feeds back the index corresponding to the extracted codeword to the base station 410 (S780). In step S780, the terminal 420 uses Equation (5) or Equation (6).

There are two methods for finding the optimal codeword in the codebook. The first scheme is that the terminal 420 uses Equation (5)

Figure 112014010899815-pat00068
, And the calculated
Figure 112014010899815-pat00069
And an index corresponding to the codeword corresponding to the codeword is fed back to the base station 410. The second scheme is that the terminal 420 uses Equation (6)
Figure 112014010899815-pat00070
,
Figure 112014010899815-pat00071
Respectively.
Figure 112014010899815-pat00072
,
Figure 112014010899815-pat00073
And an index corresponding to the codeword corresponding to the codeword is fed back to the base station 410.

Steps S750 to S780 refer to subsequent frames (frames 1, 2, ...) between the base station 410 and the terminal 420.

Although it is described in FIG. 7 that steps S710 to S780 are sequentially executed, the present invention is not limited thereto. In other words, Fig. 7 is not limited to the time-series order, since it would be applicable to changing or executing the steps described in Fig. 7 or executing one or more steps in parallel. As described above, the differential feedback technique according to the present embodiment described in FIG. 7 can be implemented in a program and recorded in a computer-readable recording medium.

When wireless communication is performed between the base station 410 and the terminal 420, they must share the same basic codebook. In the first frame (frame 0), the terminal 420 finds an optimal index for the channel estimated based on the basic codebook. At this time, either [Equation 5] or [Equation 6] can be used as a scale. The base station 410 obtains channel information using a code having a corresponding index based on the feedback. Since the base station 410 and the terminal 420 share the same codebook index from the next frame (frames 1, 2, ...), the codebook is updated using Equation (2) 420). Thus, without additional transmission to the codebook, base station 410 and terminal 420 can have the same codebook in every frame.

By applying the time-domain channel information differential feedback technique for a frequency-selective channel with temporal correlation as described above, the quantization error is reduced while using positive feedback information such as the frequency-domain channel information differential feedback technique, It is possible to obtain channel information close to the actual channel. Accordingly, more accurate beamforming is possible, thereby improving the performance of the multi-terminal input / output system.

In the case of applying the feedback technique described in this embodiment, the increase of the sum-rate due to the increase of the signal-to-noise ratio is confirmed by a computer simulation. The calculation formula of the transmission rate sum is shown in Equation (7).

Figure 112014010899815-pat00074

R SUM denotes a sum-rate. P is the transmit power. K is the total number of terminals (users). i denotes a terminal index. v i denotes a precoding vector of the i-th terminal. v j denotes a precoding vector of the j-th terminal. H i denotes a channel of the i-th terminal. And H H i denotes Hermitization for the channel of the i th terminal.

To ensure the fairness of the evaluation results, the amount of overhead used for time domain feedback and frequency domain feedback was adjusted to be the same. When using ETU (Extended Typical Urban) channel and 20 MHz bandwidth, which are widely used in mobile communication systems, there are '9' time domain channel tap and '100' frequency domain resource block. When '11 bits' are assigned to '9' tabs in time domain feedback, a total of 9 × 11 = 99 bits of overhead is used. Since 4 × 1 MIMO of 3GPP LTE uses '4' frequency-domain feedback bits, it is necessary to generate '25' subbands by grouping '100' resource blocks into '4' If four 'feedback bits are allocated, the frequency domain feedback will utilize a total of' 100 bits' overhead similar to that in time domain feedback. Based on this, performance evaluation was performed by computer simulation.

8 is a diagram for explaining environmental variables of a computer simulation for performance evaluation according to the present embodiment.

As the experimental environment, the system bandwidth is 20 MHz and the carrier frequency of 2.5 GHz is used. The MISO (Multi Input Single Output) system having four antennas of the base station 410 and one antenna of the terminal 420 is assumed and the ETU channel is assumed between the transmitting and receiving ends. Each terminal 420 schedules four terminals 420 to one base station 410 in a low-speed fading situation in which the terminal 420 moves at a speed of 3 km / h. The signal to noise ratio was '10 dB'.

FIG. 9 is a diagram illustrating a sum rate performance according to size and directional codebook bit distribution when the power delay distribution characteristic of an ETU channel according to the present embodiment is recognized. FIG. And the bit rate distribution between the size and the direction codebook when the distribution characteristics are not known.

FIGS. 9 and 10 illustrate the performance of sum rate performance according to bit allocation between codebooks when the power delay distribution characteristic of the ETU channel is recognized or not. When recognizing the power delay distribution characteristic, the performance is most favorable when allocating '1 bit' to the size shown in FIG. 9 and '10 bits' to the direction. On the other hand, referring to FIG. 10, when the distribution characteristic is not known, the performance is best when allocating '2 bits' for size and '9 bits' for direction.

FIG. 11 is a diagram illustrating a sum rate performance of techniques according to the present embodiment and a general technique.

Assuming perfect channel feedback, the sum rate is approximately 6 BPS / Hz, which is the theoretical maximum. If the channel is completely delivered only in the first time period and is not notified after that, the channel is changed over time due to the movement of the terminals 420, and the sum rate is decreased.

Tracking channel changes using differential feedback increases the sum of the transmission rates because it can more accurately convey channel information over time. Using the proposed time-domain differential feedback, the performance of the proposed scheme is 5.53 BPS / Hz when the power delay characteristics of the channel are recognized, and 89% when the performance of 92% of the theoretical maximum is recognized as 5.38 BPS / Hz. Can be achieved. Compared with the case where the general frequency domain differential feedback is 2.68 BPS / Hz when applying Grassmannian Line Packing and 2.65 BPS / Hz when applying random vector quantization, which is about 44% of the theoretical maximum value, The performance of the sum rate of the transmission rate can be improved.

Also, the performance change according to the codeword estimation technique can be confirmed in FIG. In the case of using a minimum mean square error (MMSE) scheme using Equation (5) as a codeword estimation scale and dividing a codebook into sizes and directions as proposed in the present embodiment, The performance difference of the series connection (Cascade) which found the word separately is very small within 0.01 BPS / Hz. From this, it can be confirmed that the estimation complexity can be reduced while maintaining the transmission rate sum performance by estimating the codeword in the serial connection formula as proposed in the present embodiment.

The foregoing description is merely illustrative of the technical idea of the present embodiment, and various modifications and changes may be made to those skilled in the art without departing from the essential characteristics of the embodiments. Therefore, the present embodiments are to be construed as illustrative rather than restrictive, and the scope of the technical idea of the present embodiment is not limited by these embodiments. The scope of protection of the present embodiment should be construed according to the following claims, and all technical ideas within the scope of equivalents thereof should be construed as being included in the scope of the present invention.

As described above, the present embodiment is applied to the field of time-domain differential feedback, and in a wireless communication system using a MIMO antenna, it is possible to reduce the overhead of feedback of channel information estimated by a terminal to a base station It is a useful invention that produces an effect.

410: base station 420:
422: codebook setting unit 432: pilot transmission unit
434: Data transmitting / receiving unit 440: Codebook updating unit
450: codebook setting unit 462:
464: Data transmitting / receiving unit 470: Codebook updating unit

Claims (18)

Estimates a channel with a base station using a pilot signal received from a communicating base station, converts the channel into a channel impulse response in a time domain, A codebook setting unit for setting a codebook;
A codebook updating unit for generating a tab-specific update codebook in which the tap-specific basic codebook is updated based on the code word extracted from the previous feedback and each tap dispersion size for each tap of the channel impulse response;
Feedbacks each tap position of the channel impulse response to the base station in a bitmap manner, extracts a codeword corresponding to each tap of the channel impulse response based on the tap update codebook, A feedback unit for feedback to the base station; And
A data transmission / reception unit for receiving data based on a quantized channel corresponding to the codeword from the base station,
And a second terminal. The method according to claim 1,
The codebook setting unit,
A Scalar codebook for quantizing the l < th > tap size in the channel impulse response and a vector codebook for quantizing the l < th > tap direction, wherein the scalar codebook and the vector codebook are combined, Th < th > tap. 3. The method of claim 2,
The codebook setting unit,
Wherein the scalar codebook is set to a scalar quantization for a gamma distribution or an even distribution when setting the basic codebook for each tap, and the vector codebook is set to a spherical codebook. The method according to claim 1,
The codebook updating unit,
And generates a tap-specific update codebook by scaling the tap-specific basic codebook to a dispersion size of each tap when the first feedback transmission is performed. delete The method according to claim 1,
Wherein the feedback unit comprises:
Each tap position of the channel impulse response is fed back to the base station in a bit map manner, and when the length of the channel impulse response is equal to or greater than the length of a CP (Cyclic Prefix) applied to the system, Feedback to the base station. delete delete delete The method according to claim 1,
Wherein the feedback unit comprises:
th tap estimation value of the channel impulse response allocated between the base station and the i < th >
Figure 112014117754767-pat00079
) Or the direction estimate value of the lth tap (
Figure 112014117754767-pat00080
) And a size estimation value (normalized by? L ) of the l-th tap (
Figure 112014117754767-pat00081
And extracting the index based on the index. 11. The method of claim 10,
Wherein the feedback unit comprises:
th tap (h i, τ [l] ) of the channel impulse response allocated between the base station and the i-th terminal in the τth frame and the code word corresponding to the bit index b of the codebook for quantizing the l th tap ( l l , < / RTI >< RTI ID = 0.0 >
Figure 112014117754767-pat00082
). ≪ / RTI > 11. The method of claim 10,
Wherein the feedback unit comprises:
A previous frame (τ-1) and the difference between the current frame (τ) (△ H i, τ [l]) and the direction of the l-th tap of the l-th tap of the channel impulse response (gi, τ [l] ) Corresponding to the bit index b of the vector codebook for quantizing
Figure 112014117754767-pat00104
) Of the l-th tap
Figure 112014117754767-pat00105
). ≪ / RTI > 13. The method of claim 12,
Wherein the feedback unit comprises:
The difference (? H i,? [L]) in the previous frame (? -1) and the current frame (?) For the l-th tap of the channel impulse response ,
Figure 112014117754767-pat00106
), Design parameters for the frame (τ) (δτ), the l-th tap the variance (σ 2 l) and scalar (Scalar for quantizing the size (α i, τ [l]) in the l-th tap of a) A code word corresponding to the bit index b in the codebook (
Figure 112014117754767-pat00107
(The value normalized by? 1 ) of the l-th tap
Figure 112014117754767-pat00108
). ≪ / RTI > The method according to claim 1,
The codebook updating unit,
Th frame feedback transmission after the first feedback transmission, the codeword extracted from the τ-1 th frame for each tap of the channel impulse response, the dispersion size of each tap, the design variable δ τ of the τ th frame, And generates a tap-specific update codebook using the basic codebook. A codebook setting unit for setting a basic codebook for each tap based on the tap size of the time domain channel impulse response received from the communicating terminal;
A codebook update unit for generating a tab-specific update codebook in which the tab-specific basic codebook is updated based on the code word received through the previous feedback for each tap of the channel impulse response and each tap dispersion size; And
A data transmission / reception unit for extracting a quantized channel corresponding to the codeword from the tap-specific update codebook, and transmitting data to the terminal based on the quantized channel,
And a base station. 16. The method of claim 15,
The codebook setting unit,
A scalar codebook for quantizing the l-th tap size in the channel impulse response and a vector codebook for quantizing the l-th tap direction, wherein the scalar codebook and the vector codebook are combined, Th tap of the first codebook. 16. The method of claim 15,
The codebook updating unit,
Th frame feed-back after the first feedback transmission, the codeword received in the τ-1 th frame for each tap of the channel impulse response, the dispersion size of each tap, the design variable δ τ of the τ th frame, And generates a tap-specific update codebook using the basic codebook. Setting a basic codebook for each tap of the time domain channel impulse response to be the same in the terminal and the base station;
Generating a tap-specific update codebook in which the terminal and the base station update the tap-specific basic codebook based on the codeword extracted from the previous feedback and each tap dispersion size for each tap of the channel impulse response;
The terminal feeds back each tap position of the channel impulse response to the base station in a bitmap manner, extracts a codeword corresponding to each tap of the channel impulse response based on the tap update codeword, To the base station; And
Extracting a quantized channel corresponding to the codeword from the update codebook in the base station, and transmitting data based on the quantized channel to the terminal
Time differential feedback method.
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