KR100695005B1 - Channel Estimation Apparatus and Method of OFDM based Receiver - Google Patents

Abstract

The present invention relates to a channel estimation apparatus and method for an orthogonal frequency division multiplexing receiver for effectively eliminating noise during channel estimation in a noisy environment such as a wireless channel environment to improve reception quality by estimating a more accurate radio channel. A channel estimation apparatus of an orthogonal frequency division multiplexing receiver, the apparatus comprising: signal processing means for averaging an impulse signal, which is a time domain preamble signal transmitted from the outside, for each section; Noise removal means for removing noise by inserting zeros after the maximum delay in the impulse signal sequence transmitted from the signal processing means; Signal selecting means for discriminating whether the data signal is transmitted from the outside or the time domain preamble signal from which the noise transmitted through the noise removing means is removed; Control means for controlling the signal selection means; Channel response estimating means for estimating a channel response by converting a time domain impulse response from the signal selecting means into a frequency domain; Channel coefficient storage means for storing a coefficient for the channel response estimated by the channel response estimation means; And multiplication means for multiplying the channel coefficient compensated in the frequency domain of the channel response estimating means with the signal received through the channel coefficient storage means to output the received data compensated for by the channel.

OFDM, WLAN, Channel, Estimation, Frequency Domain, Time Domain, Fourier, Zero Insertion, Average

Description

Channel Estimation Apparatus and Method of Orthogonal Frequency Division Multiplexing Receiver

1 is a schematic configuration diagram of a general orthogonal frequency division multiple communication system;

2A is a schematic block diagram of a channel compensator in the orthogonal frequency division multiplexing receiver of FIG.

2b is a frame structure diagram of an IEEE 802.11a physical layer using a long training signal;

3 is a configuration diagram of an apparatus for estimating a channel of an orthogonal frequency division multiplexing receiver according to the present invention;

4 is a diagram illustrating an intermediate signal between components.

* Explanation of symbols for the main parts of the drawings

31: RF / AD converter 32: synchronizer

33: channel estimation device 331: signal averaging unit

332: zero (0) insertion unit 333: control unit

334: Signal selector 335: Fourier transform (FFT)

336: channel coefficient RAM 337: multiplier

34: demodulator

The present invention relates to a channel estimating apparatus and method for an Orthogonal Frequency Division Multiplexing (OFDM) -based receiver, and more particularly, to data loss due to a radio channel when transmitting data in a radio environment. The present invention relates to a channel estimation apparatus and method of an orthogonal frequency division multiplexing receiver for effectively removing noise in a noisy environment.

Here, Orthogonal Frequency Division Multiplexing (OFDM) is a frequency multiplexing method in which data is distributed and transmitted over a plurality of orthogonal carriers. It refers to a frequency multiplex communication method.

1 is a schematic configuration diagram of a general quadrature frequency division multiple communication system.

As shown in FIG. 1, user data input to the transmitter 11 is serially input and subjected to modulation by the modulator 111, and the modulated serial signal is converted into a parallel signal by the serial / parallel converter 112. The converted parallel signal is orthogonally frequency-transformed through an inverse Fourier transform unit (IFFT) 113 and transmitted. The transmitted data is affected by the radio channel, causing distortion of frequency and phase.

Accordingly, the receiver 12 of FIG. 1 converts the received orthogonal frequency division multiplexed signal into a frequency domain through the Fourier transform unit (FFT) 121 and serializes the converted signal in the parallel / serial converter 122. Convert to a signal. The converted serial signal is transmitted to the channel compensator 123 that compensates for the distorted portion of the wireless channel, and is demodulated to the original user signal through the demodulator 124.

FIG. 2A is a schematic block diagram of a channel compensator in the orthogonal frequency division multiplexing receiver of FIG.

As shown in FIG. 2A, a typical quadrature frequency division multiplexing receiver receives data passing through a radio channel through the RF / AD converter 21. At this time, the received data is mostly distorted in phase and magnitude due to radio channel effects.

Therefore, frequency synchronization is required under the influence of the voltage-controlled oscillators (VCOs) of the receiver, and the synchronization unit 22 is required to compensate for this. The synchronizer 22 also finds the starting point of the data symbol.

The Fourier transformer 23 converts the received data into the frequency domain. Since the signal converted into the frequency domain remains severely distorted by the radio channel, a channel compensator 24 for compensating for the radio channel is required.

Accordingly, the channel compensator 24 generates a reference signal in which the training symbol ROM (241) in which the transmitted training symbols are stored does not have distortion due to noise and phase. Since the received signal is a signal affected by the radio signal, and the training symbol ROM 241 is a reference signal, the radio channel estimation in the frequency domain is a complex division of the received signal into the training symbol by the complex divider 242. You can get it by

In general, in the WLAN modem (IEEE802.11a), since 64 long training signals are outputted in two in succession, the delay unit 243 is provided and averaged in two sections. At this time, the elements used are the adder 244 and the right shifter 245.

On the other hand, the estimated radio channel is stored in the channel coefficient RAM (RAM) 246 and performed by channel compensation through the multiplier 247 with the received signal. The received data whose channel is compensated is demodulated by the demodulator 25.

As described above, most of the related arts use a long training sequence used in a wireless LAN modem (IEEE802.11a), and a method of estimating a channel in the frequency domain has been mainly used. Is as follows.

△ Channel estimation method using long preamble presented by IEEE802.11a

In the case of a modem transmitting burst data such as IEEE802.11a, a long training signal is used to estimate a channel at an early stage of data transmission. In this scheme, a long training symbol corresponding to the entire length of one OFDM symbol is transmitted, and the receiver uses the same to estimate a channel in all subchannels to obtain a channel compensation coefficient. Here, the frame structure of the IEEE802.11a physical layer using the long training signal is as shown in Fig. 2b.

The long training signal is the signal x _n known to the receiver. When passing through a known signal through the channel, the received signal y _n can be expressed as Equation 1 below.

Here, x _n is a transmitted signal, h _n is an impulse response of the channel, w _n is AWGN (Additive White Gaussian Noise), and * is a convolution. If Equation 1 is expressed in the frequency domain, it is given by Equation 2 below.

That is, the channel response estimated in Equation 1 may be estimated as shown in Equation 2 above.

는 채널상에서 존재하는 잡음이 없을 때 추정된 채널 보상 값들이 된다. 이와 같은 방법으로 채널 추정을 하면 잡음에 의해 서 주파수 전 대역에 AWGN이 추가되어 AWGN이 증가할수록 채널 추정값의 오차가 커지게 되는 문제점이 발생한다.Estimated in Equation 2 above

Are estimated channel compensation values when there is no noise present on the channel. If the channel estimation is performed in this way, AWGN is added to the entire frequency band due to noise, and as the AWGN increases, the error of the channel estimation value increases.

△ Channel estimation method using Discrete Fourier Transform (DFT)

In the OFDM system, channel estimation is very important because the performance of the receiver depends on the accuracy of the channel estimation. Channel estimation methods have been studied in the time domain and frequency domain estimation methods. Recently, the channel estimation method using zero insertion based on the Discrete Fourier Transform (DFT) can effectively reduce the influence of AWGN. This proposed method is based on the maximum delay time of the impulse response. One of two DFT-based channel estimation methods is MMSE (Minimum Mean-Square) method, and the other is LS (Least square) method. The MMSE method is very complex to implement and cannot be used without knowing the noise and channel information in advance. On the other hand, the LS method can exhibit good performance with low complexity in terms of implementation.

However, the above methods have a disadvantage in that the influence on noise cannot be reduced. Therefore, a method for estimating a channel more effectively in a noisy environment such as a wireless channel is required.

The present invention has been proposed to meet the above demands, and effectively removes noise during channel estimation in a noisy environment such as a wireless channel environment, thereby estimating a more accurate radio channel and improving reception quality. An object of the present invention is to provide an apparatus and method for estimating a channel of a split multi-based receiver.

Other objects and advantages of the present invention can be understood by the following description, and will be more clearly understood by the embodiments of the present invention. Also, it will be readily appreciated that the objects and advantages of the present invention may be realized by the means and combinations thereof indicated in the claims.

According to an aspect of the present invention, there is provided a channel estimating apparatus for an orthogonal frequency division multiplexing receiver, comprising: signal processing means for averaging an impulse signal, which is a time domain preamble signal, transmitted from an external unit; Noise removal means for removing noise by inserting zeros after the maximum delay in the impulse signal sequence transmitted from the signal processing means; Signal selecting means for discriminating whether the data signal is transmitted from the outside or the time domain preamble signal from which the noise transmitted through the noise removing means is removed; Control means for controlling the signal selection means; Channel response estimating means for estimating a channel response by converting a time domain impulse response from the signal selecting means into a frequency domain; Channel coefficient storage means for storing a coefficient for the channel response estimated by the channel response estimation means; And multiplication means for multiplying the channel coefficient compensated in the frequency domain of the channel response estimating means with the signal received through the channel coefficient storage means to output the received data compensated for by the channel.

In addition, the present invention provides a channel estimation method based on orthogonal frequency division multiplexing, comprising: calculating an average for each section by adding an impulse signal for each section to an impulse signal that is a time domain preamble signal; A noise removal step of removing noise after the guard interval data by inserting zeros after a maximum delay in the average impulse signal sequence; A channel response estimating step of estimating a channel response by converting the noise-reduced time-domain impulse response to a frequency domain through a fast Fourier transform; And multiplying the channel coefficients compensated in the frequency domain by the channel response estimating step with the previous channel coefficients and outputting the received data with the channel compensated.

The above objects, features and advantages will become more apparent from the following detailed description taken in conjunction with the accompanying drawings, whereby those skilled in the art may easily implement the technical idea of the present invention. There will be. In addition, in describing the present invention, when it is determined that the detailed description of the known technology related to the present invention may unnecessarily obscure the gist of the present invention, the detailed description thereof will be omitted. Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings.

Before describing the present invention, a configuration of an orthogonal frequency division multiplexing receiver will be described. An RF / AD converter 31, a synchronizer 32 for searching for frequency synchronization and time synchronization, a channel estimation apparatus 33, and a double It is composed of a grandfather 34, the channel estimation device 33 will be described in more detail as shown in FIG.

3 is a diagram illustrating an embodiment of a channel estimating apparatus of a quadrature frequency division multiplexing receiver according to the present invention.

As shown in FIG. 3, the channel estimation apparatus 33 of the orthogonal frequency division multiplexing receiver according to the present invention is a signal for averaging the impulse signal, which is a time domain preamble signal transmitted from the synchronizer 32, for each section. A zero padding 332 for removing noise by zero insertion into the impulse signal sequence transmitted through the averaging section 331, the signal averaging section 331, and a synchronization section 32. A signal selector 334 for distinguishing between the transmitted data signal and the time domain preamble signal from which the noise transmitted through the zero insertion unit 332 is removed, a controller 333 for controlling the signal selector 334, and Fourier transform unit 335 for estimating the channel response by converting the impulse response of the time domain from which the noise from the signal selector 334 is removed to the frequency domain, and the channel response estimated by the Fourier transform unit 335 Low coefficient for And multiplying the channel coefficient RAM (336) and the channel coefficient compensated in the frequency domain of the Fourier transform unit (335) with the signal received through the channel coefficient RAM (336). A multiplier 337 for delivery to the demodulator 34.

As described above, in the present invention, in order to change the modulated signal to the time domain as shown in FIG. 4, the preamble signal of the frequency domain is changed like the impulse signal sequence 42 of the time domain through the inverse Fourier transformer 41. . Accordingly, the impulse signal string 42 should be designed in consideration of the maximum delay time of the radio channel. This is because the impulse signal sequence 42 prevents the delay component of the first impulse signal sequence from affecting the adjacent impulse signal sequence because the delay is generated by the multipath channel of the radio channel.

If it is composed of 64 sample data in one data symbol, and it is assumed that the impulse signal sequence 42 of one section is set to 16 samples in consideration of the maximum delay of the radio channel, it can be divided into four impulse signal sequence sections.

This signal sequence becomes a modified impulse signal sequence 44 through delay, phase distortion, and signal attenuation through the radio channel environment 43. The modified signal sequence 44 is changed in size by noise independently in four sections. In general, since the radio channel does not change in one OFDM symbol, the influence of the channel is the same for each section.

If the average of the impulse repeated in the four sections through the average and zero insertion unit 45 is reduced the effect of the noise. In this case, since the noise generally has a Gaussian distribution with a mean of 0, the averaged signal converges to 0. Therefore, the more intervals you can reduce the effect of noise. Once averaged, you will have an impulse signal in only one interval.

Then, zero ('0') is inserted into the data value to minimize the effect of noise. Through this process, the newly changed impulse string 46 is changed to the frequency domain through the Fourier transform unit 47 to estimate the channel response of the wireless channel environment.

Referring to the structure of the channel estimation apparatus of the receiver according to the present invention having the structure as described above in detail as follows.

In FIG. 4, the preamble signal in the frequency domain before the inverse Fourier transform unit 41 may be expressed as shown in Equation 3 below, so that the impulse signal sequence 42 in the time domain may come out.

When the inverse Fourier transform is performed using this signal, it can be expressed as Equation 4 below.

는 n=0 일 때 '1'을 갖는 단일 임펄스 함수이다. TS(n)신호는 도 4의 임펄스 신호열(42)과 같이 표현된다. 여기서, 일반적으로 한 개의 OFDM 심볼동안에 채널은 시간적으로 변하지 않는다. 수신신호

은 [수학식 5]와 같이 표현될 수 있다.here,

Is a single impulse function with '1' when n = 0. The TS (n) signal is expressed as the impulse signal sequence 42 of FIG. Here, generally, the channel does not change in time during one OFDM symbol. Receiving signal

May be expressed as shown in [Equation 5].

을 AWGN 잡음을 나타내고

은 채널의 임펄스 응답을 나타낸다. *부호는 콘볼류션을 나타낸다. [수학식 5]는 [수학식 6]과 같이 나타낼 수 있다.here

Represents AWGN noise

Represents the impulse response of the channel. * Indicates a convolution. Equation 5 may be expressed as Equation 6.

이거나 또는

일 때

이 된다. 따라서 [수학식 6]은 채널 임펄스 응답이 L개의 샘플 간격으로 지연되어 연속으로 4개가 존재하는 것을 나타낸다. [수학식 7]과 같이 4개의 임펄스 응답을 평균을 내면 잡음의 분산을 줄일 수 있다.Considering the Kausal system

Or

when

Becomes Therefore, Equation 6 shows that the channel impulse response is delayed by L sample intervals, so that there are four consecutively. As shown in Equation 7, averaging four impulse responses can reduce the variance of noise.

을 나타낸다. 은 서로 독립이며 동일 확률분포를 가진다. 따라서 [수학식 7]에서 잡음 베리언스(noise variance)가 1/4로 줄어듦을 알 수 있다.here

Indicates. Are independent of each other and have the same probability distribution. Therefore, it can be seen that the noise variance is reduced to 1/4 in [Equation 7].

은 하기의 [수학식 8]과 같이 나타낼 수 있다.In an OFDM receiver, four received impulse signals can be considered an ideal impulse channel response. The impulses are averaged into one received impulse signal and are represented by Equation 7 below. Where h (n) is the channel response by the impulse signal string and n (n) is AWGN. As such, the channel estimation error generated by the AWGN can be reduced by averaging the received impulse response signal. When using an N-point FFT, since the (NL) portion of the maximum impulse response is a signal generated by the AWGN, the effect of the AWGN can be reduced by inserting zero into this (NL) portion. Zero Impulse Response Signal

May be represented as in Equation 8 below.

동안에 푸리에 변환을 수행함으로써 채널을 추정할 수 있다. 하기의 [수학식 9]는 주파수 영역에서 추정된 채널을 나타낸 것이다.Here, impulse response with zero insertion

The channel can be estimated by performing Fourier transform. Equation 9 below shows the estimated channel in the frequency domain.

In summary, the present invention divides the data into four sections at intervals of 16 samples in the time domain by using a new preamble in the frequency domain and estimates the channel response. At this time, the impulse signals are transmitted during one OFDM symbol.

On the other hand, if one OFDM symbol is 64 samples, assuming that the maximum delay is 16, the average section may be changed to average, such as 4 sections. For example, when one OFDM symbol is 64 and the maximum delay is 16, the section for averaging may be divided into [4, 3, 2].

That is, the channel estimating apparatus according to the present invention adds an impulse signal for each section and averages the result, and since the data after the guard interval in the averaged impulse response is noise, zero padding is used to remove the noise and Fourier transform. There is an advantage in that the channel estimation response can be found by performing (FFT).

The present invention described above is capable of various substitutions, modifications, and changes without departing from the technical spirit of the present invention for those skilled in the art to which the present invention pertains. It is not limited by the drawings.

As described above, the present invention effectively removes noise during channel estimation in a noisy environment such as a wireless channel environment, and thus has an effective effect of estimating a more accurate wireless channel and improving reception quality.

Claims (8)

In the channel estimation apparatus of orthogonal frequency division multiplexing receiver, Signal processing means for averaging the impulse signal, which is a time domain preamble signal transmitted from the outside, for each section; Noise removal means for removing noise by inserting zeros after the maximum delay in the impulse signal sequence transmitted from the signal processing means; Signal selecting means for discriminating whether the data signal is transmitted from the outside or the time domain preamble signal from which the noise transmitted through the noise removing means is removed; Control means for controlling the signal selection means; Channel response estimating means for estimating a channel response by converting a time domain impulse response from the signal selecting means into a frequency domain; Channel coefficient storage means for storing a coefficient for the channel response estimated by the channel response estimation means; And Multiplication means for multiplying a channel coefficient compensated in the frequency domain of the channel response estimating means with a signal received through the channel coefficient storing means and outputting received data compensated for by the channel; Channel estimation apparatus of the orthogonal frequency division multiplexing receiver comprising a. The method of claim 1, The signal processing means, In order to generate four impulse responses in the time domain, the channel estimation apparatus of the orthogonal frequency division multiplexing receiver, characterized in that for generating a training signal in the frequency domain as shown in [Equation 1]. [Equation 1]

(Where h (n) is the channel response by the impulse signal train and w (n) is AWGN) delete The method of claim 2, The channel response estimating means, Impulse response with zero insertion as shown in [Equation 2] below

A channel estimation apparatus of an orthogonal frequency division multiplexing receiver, characterized in that it estimates a channel by performing a Fourier transform and represents a channel estimated in a frequency domain as shown in Equation 3 below. [Equation 2]

[Equation 3]

The method according to claim 1 or 2, The channel response estimating means, To estimate the channel response, a new preamble in the frequency domain is used to divide the channel into four sections at intervals of 16 samples in the time domain, and an impulse response is generated in each section. Device. In the channel estimation method based on orthogonal frequency division multiplexing, Adding an impulse signal for each section to an impulse signal that is a time domain preamble signal and obtaining an average for each section; A noise removal step of removing noise after the guard interval data by inserting zeros after a maximum delay in the average impulse signal sequence; A channel response estimating step of estimating a channel response by converting the noise-reduced time-domain impulse response to a frequency domain through a fast Fourier transform; And Outputting the channel-compensated received data by multiplying the channel coefficient compensated in the frequency domain by the channel response estimation step with a previous channel coefficient. Orthogonal frequency division multiple based channel estimation method comprising a. The method of claim 6, The channel response estimating step, Impulse response with zero insertion as shown in Equation 4 below

Orthogonal frequency division multiple channel estimation method characterized in that the fast Fourier transform during the estimation of the channel to represent the estimated channel in the frequency domain as shown in Equation 5 below. [Equation 4]

[Equation 5]

The method according to claim 6 or 7, The channel response estimating step, Orthogonal frequency division multiple channel estimation method characterized in that the frequency response is divided into four sections at intervals of 16 samples in the time domain using a new preamble in the frequency domain, and an impulse response is generated in each section. .

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