KR101481548B1 - Channel Estimation Using Time Domain Window with Various Sizes - Google Patents

Abstract

A channel estimation method using time-domain windows having various sizes will be described.

In performing channel estimation using a time domain window, a frequency domain channel signal generated using a received signal is converted into a time domain channel signal, and a signal outside a time domain window having a predetermined size And outputs a channel signal that is replaced with "0 ", and then performs channel estimation by converting the output channel signal into a frequency domain channel signal, wherein the predetermined size is set to be differently set according to a variance value of the received signal Channel estimation performance can be improved.

Also, in the above process, channel signals are compensated in parallel or serially / repetitively using a plurality of predetermined time domain windows having different sizes, and a channel signal with the smallest variance value on the constellation is selected to perform channel estimation performance .

Time Domain Window Size, Variation

Description

TECHNICAL FIELD [0001] The present invention relates to a channel estimation method using time-domain windows having various sizes,

The following description relates to a method of performing channel estimation using time-domain windows having different sizes according to the state of a received signal.

In a wireless mobile communication system, a fading channel exists between a transmission end and a reception end. Also, as the terminal moves, the quality of the channel changes. In order to demodulate a transmission signal in a fading channel environment, a procedure for estimating and compensating channel information through a pilot signal promised mutually between transmitting and receiving ends is needed. In the changing channel environment, the accuracy of the channel information estimated by the receiver greatly affects the system performance. The following description relates to a technique for estimating reliable channel information in an OFDM system environment.

An OFDM scheme, a DFT-S-OFDM (DFT Spreading OFDM) scheme, and an OFDMA scheme will be briefly described as an example of a communication method of a multi-carrier system for this purpose.

First, orthogonal frequency division multiplexing (OFDM) will be described. The basic principle of OFDM is to divide a data stream having a high-rate into a plurality of data streams having a low transmission rate and to transmit them simultaneously using a plurality of carriers. Each of the plurality of carriers is referred to as a subcarrier. Since there is orthogonality between the plurality of carriers of the OFDM, even if the frequency components of the carriers overlap each other, detection at the receiving end is possible. The data stream having the high-speed data rate is converted into a data stream having a plurality of low data rates through a serial-to-parallel (S / P) converter, and each of the sub- And then the respective data strings are summed and transmitted to the receiving end.

The parallel data streams generated by the serial-to-parallel conversion unit may be transmitted on a plurality of subcarriers by an inverse discrete Fourier transform (IDFT), and the IDFT may be an Inverse Fast Fourier Transform Fast Fourier Transform).

The symbol duration of a sub-carrier having a low data rate is increased, so that the relative signal dispersion in time caused by multipath delay spreading is reduced. Inter-symbol interference can be reduced by inserting a guard interval longer than the delay spread of a channel between OFDM symbols. Also, if a part of the OFDM signal is copied in the guard interval and placed at the beginning of the symbol, the OFDM symbol can be cyclically extended to protect the symbol.

Next, the DFT-S-OFDM scheme will be described. The DFT-S-OFDM scheme is also referred to as SC-FDMA (Single Carrier-FDMA). In the conventional SC-FDMA scheme, spreading is first applied to a DFT matrix in the frequency domain before OFDM signals are generated, and then the result is modulated by a conventional OFDM scheme and transmitted .

1 is a diagram showing a structure of a transmitter of the DFT-S-OFDM scheme.

Several variables are defined to illustrate the operation of the illustrated DFT-S-OFDM transmitter. N denotes the number of subcarriers for transmitting an OFDM signal, and Nb denotes the number of subcarriers for an arbitrary user. In addition, F denotes a discrete Fourier transform matrix, i.e., a DFT matrix, s denotes a data symbol vector, x denotes a vector in which data is dispersed in the frequency domain, and y denotes an OFDM symbol vector to be transmitted in the time domain do.

In SC-FDMA, the data symbols (s) are distributed using a DFT matrix before transmission. This is expressed by the following equation.

는, 데이터 심볼(s)을 분산시키기 위해서 사용된 Nb 크기의 DFT 행렬이다. 이렇게 분산된 벡터(x)에 대하여 일정한 부 반송파 할당 기법에 의해 부 반송파 매핑(subcarrier mapping)이 수행되고, IDFT 모듈에 의해 시간영역으로 변환되어 수신 측으로 전송하고자 하는 신호가 얻어진다. 상기 수신 측으로 전송되는 전송신호는 아래 식과 같다. In Equation (1)

Is a DFT matrix of Nb size used to distribute the data symbols s. Subcarrier mapping is performed by a constant subcarrier allocation technique on the vector x thus dispersed, and the signal is transformed into a time domain by the IDFT module to obtain a signal to be transmitted to the receiver. The transmission signal transmitted to the receiving side is expressed by the following equation.

는 주파수 영역의 신호를 시간 영역의 신호로 변환하기 위해 사용되는 크기 N의 IDFT 행렬이다. 상술한 방법에 의해 생성된 신호 y는, 순환 전치(cyclic prefix)가 삽입되어 전송된다. In Equation (2)

Is an IDFT matrix of size N that is used to transform a frequency domain signal into a time domain signal. The signal y generated by the above method is transmitted by inserting a cyclic prefix.

A method of generating a transmission signal by the above-described method and transmitting the transmission signal to the receiving side is referred to as an SC-FDMA method. The size of the DFT matrix can be variously controlled for a specific purpose.

Next, orthogonal frequency division multiple access (OFDMA) will be described. OFDMA refers to a multiple access method that realizes multiple access by providing each user with a part of a subcarrier available in a modulation system using a plurality of orthogonal subcarriers. OFDMA provides a frequency resource called a sub-carrier to each user, and each frequency resource is provided to a plurality of users independently of one another and does not overlap with each other.

In a wireless mobile communication system, a receiving end performs an operation for estimating fading channel information between transmitting and receiving ends. In a system employing OFDM, DFT-S-OFDM, and OFDMA techniques, channel estimation is generally performed in the frequency domain. That is, the transmitting terminal transmits a signal (pilot or reference signal) promised during transmission / reception, and this signal passes through a channel existing between the transmitting terminal and the receiving terminal and is input to the receiving terminal.

Various channel estimation methods such as a least square (LS) channel estimation method and a DFT based channel estimation method have been provided. Among them, the DFT-based channel estimation method has excellent performance when the channel environment is poor.

The DFT-based channel estimation technique has the advantage that it can reduce the influence of noise by removing signals exceeding the range of the time window of the channel information using the received signal using the time domain window, but in a specific case, The channel estimation performance may deteriorate.

Accordingly, an improved method of performing channel estimation adaptively according to the state of a received signal is provided below.

According to an aspect of the present invention, there is provided a method of performing channel estimation using a time domain window, the method comprising: converting a frequency domain channel signal generated using a received signal into a time domain channel signal; ; Outputting a channel signal obtained by replacing a signal out of the time-domain window range having a predetermined size from the time-domain channel signal by "0 "; And transforming the output channel signal into a frequency domain channel signal, wherein the predetermined size is different from a predetermined value according to a variance value of the received signal.

In this case, the predetermined size may be set to be smaller as the variance value of the received signal becomes larger, and may be measured using a time domain distribution of the received signal within a time domain range of a specific size as a variance value of the received signal Variance values can be used.

According to another aspect of the present invention, there is provided a method of performing channel estimation using a time domain window, the method comprising: converting a frequency domain channel signal generated using a received signal to a time domain channel signal; Outputting channel signals obtained by replacing signals outside the respective time-domain windows of the time-domain channel signals with "0" by using a plurality of preset time-domain windows having different sizes; And measuring a variance value on the constellation diagram of the output channel signals to select a channel signal having the smallest variance value.

In this case, it is possible to set the plurality of preset time-domain windows having different sizes in parallel to output the channel signals. Alternatively, the plurality of preset time-domain windows having different sizes may be sequentially And outputting the channel signals, sequentially measuring a variance value on the constellation diagram of the output channel signals, and selecting an output channel signal having the smallest variance value among the measured variance values.

Here, it is assumed that the channel estimation method is a DFT-based channel estimation method. However, the present invention is not limited to the DFT-based channel estimation method and may be applied to various channel estimation methods using a method of processing a signal out of a corresponding range to "0 & Can be applied.

According to the embodiments of the present invention described above, the channel estimation performance can be improved by performing channel estimation adaptively according to the state of a received signal.

Hereinafter, preferred embodiments according to the present invention will be described in detail with reference to the accompanying drawings. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The following detailed description, together with the accompanying drawings, is intended to illustrate exemplary embodiments of the invention and is not intended to represent the only embodiments in which the invention may be practiced. In the following description, the base station may be replaced with other terms such as "Node B "," eNode B ", and the terminal may be replaced with terms such as "UE "," have.

The following detailed description includes specific details in order to provide a thorough understanding of the present invention. However, those skilled in the art will appreciate that the present invention may be practiced without these specific details. In some instances, well-known structures and devices are omitted or shown in block diagram form around the core functions of each structure and device in order to avoid obscuring the concepts of the present invention. In the following description, the same components are denoted by the same reference numerals throughout the specification.

As described above, the present invention provides an improved channel estimation technique for performing channel estimation adaptively according to the state of a received signal. The LS channel estimation method and the DFT-based channel estimation method will be described in more detail, and how to improve the above-described technique according to the state of the received signal will be described below.

As described above, channel estimation in a system to which OFDM, DFT-S-OFDM, and OFDMA techniques are applied is generally performed in a frequency domain, and a channel is estimated using a pilot promised during transmission and reception. The pilot signal received at the receiving end has the form of a product of the fading channel signal and the original transmitted pilot signal in the frequency domain and can be expressed as follows.

In Equation (3), R [k] denotes a received pilot signal, H [k] denotes a fading channel signal to be estimated at the receiving end, P [k] denotes a promised pilot signal during transmission and reception, and W [k] denotes a white noise signal it means.

It is assumed that all of the four variables are signals in the frequency domain, and k denotes a subcarrier index in the frequency domain.

First, a method of estimating a least square (LS-Least Square) channel will be described.

Figure 2 shows a simplified block diagram of the LS channel estimation technique.

The LS channel estimation scheme is a channel estimation scheme that simply divides the received pilot signal R [k] and the known pilot signal P [k]. That is, the frequency domain received signals R [0], R [1], ..., R [N-1] and the pilot signals P [0], P [ , And H [N-1] are output by dividing the received signal into pilot signals and outputting the channel signals of H '[0], H' [1], ..., H '[N-1] do.

That is, the general expression H [k] of the channel information estimated through the LS channel estimation technique can be expressed as follows.

Next, a DFT (Discrete Fourier Transform) -based channel estimation method will be described. The DFT-based channel estimation method is characterized in that a procedure for reducing the white noise applied at the receiver is inserted.

3 shows a block diagram of a DFT-based channel estimation method.

In FIG. 3, H_LS (k) denotes channel information of the k-th subcarrier estimated through the LS channel estimation scheme. h (m) is the channel information in the m-th sample obtained by converting the H_LS into the time domain through the IDFT operation. h_zero (m) represents an m-th sample of a signal obtained by substituting 0 for a part of the time-domain channel information h (k) which takes an arbitrary time window. Here, it is assumed that 0 <= m <= N-1 and 0 <= k <= N-1.

The operation of the DFT-based channel estimation scheme will be described in more detail as follows.

First, the channel information signal of the frequency domain estimated through the LS channel estimation method is transformed into the signal of the time domain through the IDFT block. In the absence of noise, the channel information in the time domain has a peak value at the time delay point of the multipath.

FIGS. 4 and 5 are conceptual diagrams of channel information in the time domain when there is no noise and noise, respectively.

That is, in the case where there is no noise, the channel information in the time domain can be represented as a distribution having only signal components at t1, t2, t3, t4 and t5 as shown in Fig. However, as shown in FIG. 5, the signal in the time domain of the real channel has a shape like a dotted line to which white noise is applied after interpolating each peak value.

Specifically, in FIG. 5, in the case of a low signal-to-noise ratio (SNR), only a signal due to white noise exists after the time delay t5. Thus, if the time window has a range up to t5, the signal after this time window can be reduced to zero to reduce the effect of white noise. In the DFT-based channel estimation scheme, if the DFT is passed after replacing 0, the channel information with reduced white noise can be obtained by comparing with the channel information of the existing frequency domain.

The advantages and disadvantages of the above two methods will be discussed.

The LS channel estimation method can estimate the reliable channel information because the influence of the white noise is small in the channel exhibiting the high SNR. However, in the case of the channel exhibiting the low SNR, the white noise is reflected in the estimated channel information as it is, .

On the other hand, in the case of the DFT-based channel estimation method, a channel having a low SNR can achieve superior performance to the LS estimation method due to the white noise reduction effect.

As described above, the DFT-based channel estimation method generally assumes information within a fixed time window length in channel information in the time domain to be real channel information and time domain over the fixed window is noise. Therefore, the signal within the fixed window length is maintained as it is, and the signal beyond that is forcibly replaced with 0, thereby reducing the influence of noise. However, in the process of converting the channel information of the frequency domain to the time domain using the IDFT operation, interpolation effects are shown between the peak values shown in FIG. 4, and the channel information is included even after the time t5 which is the maximum multi-path delay value. Therefore, replacing channel information having a length equal to or greater than a fixed window length with 0 has a positive effect of noise reduction effect and a negative effect that channel information is distorted.

Therefore, according to an embodiment of the present invention to be described below, a method of performing channel estimation more efficiently using windows having different sizes according to the channel state based on the DFT-based channel estimation method is proposed.

As mentioned above, the DFT-based channel estimation technique has a problem of channel information distortion by replacing the signal outside the time window range with zero in the time domain channel information. That is, in a channel having a low SNR, performance decreases due to white noise rather than performance degradation due to actual channel information distortion. However, in the case of a channel exhibiting a high SNR, performance due to white noise The performance degradation due to the distortion of the channel information is more critical than the decrease.

Therefore, the present embodiment proposes a method of increasing the reliability of channel estimation by changing the size of the time window, which is a standard for replacing the signal in the time domain with 0, depending on the situation. In particular, unlike the conventional method of setting a time domain window of a fixed size irrespective of the quality of a received signal, a technique of improving the channel estimation performance by estimating the variance of the received signal and varying the size of the time window is proposed do.

6 shows channel information of a time domain estimated in an actual system.

As can be seen from FIG. 6, when the channel environment is good (that is, in the case of a channel exhibiting a high SNR) Can be confirmed. That is, it can be seen that as the time domain variance of the received signal increases, the channel exhibits a poor SNR.

Therefore, the method proposed in the present embodiment is as follows.

First, a frequency domain channel signal generated using a received signal is converted into a time domain channel signal. This can be implemented through an IDFT module as shown in FIG. Then, the variance of the specific interval N is measured with respect to the time-domain channel information obtained through the channel estimation algorithm.

The measured variance is used to determine the size of the time domain window to be used for DFT-based channel estimation using a mapping table between a dispersion value previously set during the transmission / reception interval or through a higher layer signaling and a time domain window size .

An example of a mapping table between the variance value (Var) and the time domain window size (Size) is shown in Table 1 below.

Variance value (Var) Time Window Size (Size) 0 <Var <X1 Size 1 X1 <Var <X2 Size 2 X2 <Var <X3 Size 3 Xn-1 < Var < Xn You n

In Table 1, it is assumed that the size of the time window decreases as the dispersion value becomes larger as the channel becomes worse. That is, assume that size 1 ≤ size 2 ≤ size 3 ≤ ... ≤ size n.

In addition, the threshold values X1, X2, X3, ..., Xn for determining the level of the dispersion value in Table 1 may be set at equal intervals or may be set to have a specific type of interval according to system requirements .

In the present embodiment, the time window size selected according to the mapping table shown in Table 1 is applied to maintain the signal value in the time window according to the same DFT-based channel estimation method as described above with reference to FIG. 3, 0 &quot;.

In the above-described embodiment, the variance may be measured for each received symbol, or an average variance may be used by accumulating several symbols.

Meanwhile, another embodiment of the present invention proposes a method of performing DFT-based channel estimation using a plurality of time domain windows having different sizes.

FIGS. 7 and 8 are diagrams illustrating examples of a method for performing DFT-based channel estimation using a plurality of time domain windows having different sizes according to an embodiment of the present invention.

Specifically, FIG. 7 illustrates a method of performing channel compensation in the same manner as applied to the DFT-based channel estimation technique using a plurality of time domain windows having different sizes in parallel, And a method of selecting a channel signal having the smallest variance value by measuring a variance value.

In the case of FIG. 8, a method of measuring variance of a constellation signal by estimating and compensating a channel by sequentially changing time windows of different sizes from Size 1 to Size n, As shown in FIG.

The improved DFT-based channel estimation method according to the embodiments of the present invention as described above may be referred to in various terms in addition to the "DFT-based channel estimation method"

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The foregoing description of preferred embodiments of the invention disclosed herein has been presented to enable any person skilled in the art to make and use the invention. While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the following claims It can be understood that Accordingly, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

The embodiments of the present invention as described above can be used by a receiving end to perform channel estimation in various wireless communication systems, and the application of the embodiments to a specific communication system need not be limited.

1 is a diagram showing a structure of a transmitter of the DFT-S-OFDM scheme.

Figure 2 shows a simplified block diagram of the LS channel estimation technique.

3 shows a block diagram of a DFT-based channel estimation method.

FIGS. 4 and 5 are conceptual diagrams of channel information in the time domain when there is no noise and noise, respectively.

6 shows channel information of a time domain estimated in an actual system.

FIGS. 7 and 8 are diagrams illustrating examples of a method for performing DFT-based channel estimation using a plurality of time domain windows having different sizes according to an embodiment of the present invention.

Claims (7)

A method for performing channel estimation using a time domain window, Converting a frequency-domain channel signal generated using the received signal into a time-domain channel signal; Measuring a variance value of the received signal; Outputting a channel signal obtained by replacing a signal out of the time-domain window range having a specific size from the time-domain channel signal by "0 "; And Converting the output channel signal into a frequency domain channel signal, Wherein the size of the time domain window has a mapping relationship with a variance value of the predetermined range of the received signal, The mapping relationship is signaled through an upper layer, Wherein the time domain window having the specific size is determined according to the measured variance value of the received signal using the mapping relationship. Channel estimation method. The method according to claim 1, Wherein the size of the time-domain window according to the mapping relation is set to be smaller as the variance value of the predetermined range of the received signal becomes larger, Channel estimation method. The method according to claim 1, Wherein a variance value measured using a time domain distribution of the received signal within a time domain range of a specific size is used as a variance value of the received signal. A method for performing channel estimation using a time domain window, Converting a frequency-domain channel signal generated using the received signal into a time-domain channel signal; Outputting channel signals obtained by replacing signals outside the respective time-domain windows of the time-domain channel signals with "0" by using a plurality of preset time-domain windows having different sizes; And Measuring a variance value on the constellation diagram of the output channel signals and selecting a channel signal having the smallest variance value, And simultaneously outputting the channel signals using a plurality of predetermined time domain windows having different sizes in parallel. delete delete 5. The method according to any one of claims 1 to 4, Wherein the channel estimation method is a DFT-based channel estimation method.

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