KR20100077973A - Channel estimation method using fast fourier transform - Google Patents

Abstract

PURPOSE: A channel estimation method for using FFT is provided to reduce the influence of the noise by applying the channel deduction technique of the FFT based. CONSTITUTION: A first N-pint FFT(110) changes a radio frequency signal received through antenna into the frequency area by performing FFT. A Pilot/Data subcarrier discriminator(120) discriminates the pilot subcarrier and data subcarrier in the transformed signal to the frequency area. A LS channel estimator(130) estimates the channel value of the corresponding pilot subcarrier as a LS mode. A N-point IFFT(140) changes the outputted result into the time area by being proceed IFFT. A N-point FFT(150) changes the outputted result into the frequency area by being proceed FFT. A channel compensator(160) compensates the data subcarrier by selecting the value with in the location of the Used subcarrier of outputs of the N-point FFT the virtual subcarrier.

Description

CHANNEL ESTIMATION METHOD USING Fast Fourier Transform}

The present invention relates to channel estimation, and more particularly, to channel estimation of aperiodic pilot subcarriers in an OFDMA based system.

The OFDM transmission method is a type of a multi-carrier transmission method using a plurality of carriers and transmits the input data in parallel on a plurality of subcarriers having mutual orthogonality.

In this OFDM transmission scheme, the transmission period is increased in each channel by the number of carriers. In this case, the frequency-selective channel characteristics, which are represented by using a wide band when transmitting high-speed data, are approximated to a non-frequency channel by a narrow banded channel. do. Therefore, the distortion compensation due to the channel is possible only by the equalizer of a single sample, which is simpler than the single carrier system, and is widely used in high-speed data transmission systems in various fields such as multimedia data transmission.

In general, when demodulating a non-coherent signal in an OFDM transmission scheme, the receiver structure can be simplified because channel information and channel estimation are not necessary, and also a training symbol for channel estimation. However, since the pilot tone is not used, the transmission efficiency is superior to the coherent method.

However, this transmission method raises a problem that the detection performance is degraded by about 3 to 4 [dB] compared to the coherent method due to the influence of noise. Therefore, in order to improve the performance of an OFDM system, a coherent signal detection technique cannot be used. For this purpose, accurate channel estimation and equalization are required.

The channel estimation method for OFDM is based on the linear minimum mean square error (LMMSE) and the least square (LS) criteria according to which criterion is used in deriving the channel estimator. It can be classified by a channel estimation method.

Among them, the channel estimation method based on LS is very simple to calculate, but has a disadvantage of being greatly affected by noise. The channel estimation method based on the MMSE estimates the channel in consideration of the influence of noise, and thus shows better estimation performance than the LS reference method.

FIG. 1 is a block diagram illustrating a configuration of an FFT-based channel estimator in a typical OFDMA scheme, and FIG. 2 is an exemplary diagram illustrating a pilot position in a typical OFDMA scheme.

First, referring to FIG. 2, it is assumed that N represents the total number of subcarriers, and a pilot subcarrier is an M subcarrier period.

Next, referring to FIG. 1, the FFT-based channel estimator includes an LS channel estimator 11, an N-point Inverse Fast Fourier Transform (IFFT) 12, an MMSE vaporizer 13, N-point FFT 14 is included.

Referring to FIG. 1, the LS channel estimator 11 first obtains a subchannel response { H _m ^f } from each pilot tone of an OFDM symbol { Y _m } demodulated by an FFT of size N. FIG. This is done by removing the data symbols assigned to the pilot tones from the demodulated received data symbols { Y _m } at each pilot tone position. The LS channel estimator 11 estimates the pilot subchannel and inserts 0 into the remaining data subcarrier positions and outputs the zero.

Then, the N-point IFFT 12 converts the output result into the time domain. The signal obtained in this process becomes the instantaneous impulse response (CIR) of the multipath channel if the pilot tone spacing satisfies the sampling theory. This impulse response has a structure that is repeated M times, and the repetition interval becomes N / M.

The MMSE weighting unit 13 adjusts to have a value similar to the original channel impulse response by multiplying the signal of the first N / M interval by an appropriate coefficient.

Finally, the N-point FFT 14 converts the impulse response into the frequency domain and outputs the frequency domain response of the channel.

However, in the frequency domain, a channel frequency response must be obtained not only at the pilot tone but also at subcarrier positions between the pilot tones. In other words, an interpolation process in the frequency domain is required. To this end, N-N / M zeros are inserted after the impulse response having the obtained N / M samples (zero padding), and then a DFT or FFT of size N is performed on the result.

As described above, the channel estimation method using the MMSE criterion while using the pilot subcarrier has an advantage of tracking the channel change with excellent estimation performance, but it is difficult to implement when the number of subcarriers is large because of a large amount of calculation. have.

However, this problem can be solved using the Discrete-Time Fourier Transform (DFT) based channel estimation method which processes both the frequency domain and the time domain.

However, another problem is that in order to apply the conventional DFT-based channel estimation scheme, pilot tones must exist at equal intervals.

However, as can be seen with reference to FIG. 3, in the Partial Usage SubChannel (PUSC) structure of the IEEE 802.16e Wireless OFDMA system, the pilot subcarriers do not exist at equal intervals, and thus there is a problem that cannot be applied.

Therefore, an object of the present invention is to solve the above-mentioned problem.

Specifically, an object of the present invention is to facilitate channel estimation even when pilot subcarriers do not exist at equal intervals.

In order to achieve the above object, the present invention comprises the steps of estimating the channel value of the pilot subcarrier in the received signal by the least square (LS) method; Placing the estimated channel value at the pilot subcarrier position and inserting zero into the remaining subcarrier position of the signal to perform inverse fast Fourier transform; A half of a subcarrier corresponding to a delay spread of a front end of the signal of the inverse fast Fourier transform time domain and a subcarrier corresponding to a delay spread of the rear end of the signal is selected, and the remaining one of the signals Inserting zero at the subcarrier position and performing fast Fourier transform; And selecting a value at a position other than the virtual subcarrier and the DC subcarrier among the signals of the fast Fourier-changed frequency domain.

In order to achieve the above object, the present invention provides a channel estimator for estimating a channel value of a pilot subcarrier in a received signal using a least square (LS) method; An IFFT performer for performing an inverse fast Fourier transform by placing the estimated channel value at the pilot subcarrier position and inserting zero into the remaining subcarrier positions of the signal; A half of a subcarrier corresponding to a delay spread of a front end of the signal of the inverse fast Fourier transform time domain and a subcarrier corresponding to a delay spread of the rear end of the signal is selected, and the remaining one of the signals An FFT performer for inserting zero at a subcarrier position and performing fast Fourier transform; And a channel compensator for selecting a value at a position other than the virtual subcarrier and the DC subcarrier among the signals of the fast Fourier-changed frequency domain to compensate for the data subcarrier in the signal.

According to the present invention, even if the pilot subcarriers are located without having equal intervals, the effect of noise can be reduced by applying an FFT-based channel estimation technique.

It is to be noted that the technical terms used herein are merely used to describe particular embodiments, and are not intended to limit the present invention. It is also to be understood that the technical terms used herein are to be interpreted in a sense generally understood by a person skilled in the art to which the present invention belongs, Should not be construed to mean, or be interpreted in an excessively reduced sense. In addition, when the technical terms used herein are incorrect technical terms that do not accurately express the spirit of the present invention, they should be replaced with technical terms that can be understood correctly by those skilled in the art. In addition, the general terms used in the present invention should be interpreted as defined in advance or in accordance with the context before and after, and should not be interpreted in an excessively reduced sense.

Also, the singular forms "as used herein include plural referents unless the context clearly dictates otherwise. In this application, terms such as “consisting of” or “having” should not be construed as necessarily including all of the various components, or various steps described in the specification, some of which include some or some of the steps. It should be construed that it may not be, or may further include additional components or steps.

In addition, terms including ordinal numbers, such as first and second, as used herein may be used to describe various components, but the components should not be limited by the terms. The terms are used only for the purpose of distinguishing one component from another. For example, without departing from the scope of the present invention, the first component may be referred to as the second component, and similarly, the second component may also be referred to as the first component.

When an element is referred to as being "connected" or "connected" to another element, it may be directly connected or connected to the other element, but other elements may be present in between. On the other hand, when a component is said to be "directly connected" or "directly connected" to another component, it should be understood that there is no other component in between.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings, and the same or similar components will be given the same reference numerals regardless of the reference numerals, and redundant description thereof will be omitted. In addition, in describing the present invention, when it is determined that the detailed description of the related known technology may obscure the gist of the present invention, the detailed description thereof will be omitted. In addition, it should be noted that the accompanying drawings are only for easily understanding the spirit of the present invention and should not be construed as limiting the spirit of the present invention by the accompanying drawings. The spirit of the present invention should be construed to extend to all changes, equivalents, and substitutes in addition to the accompanying drawings.

Prior to describing the present invention, the downlink PUSC basic unit in the IEEE 802.16e Wireless MAN-OFDMA system is a cluster. In the SISO environment, each cluster is composed of 14 subcarriers on the frequency axis and two symbols on the time axis as shown in FIG. Among them, four pilot subcarriers and 24 data subcarriers. In case of using dedicated pilot, pilot subcarriers are located aperiodically in OFDMA symbol as shown in the figure.

4 is an exemplary diagram illustrating a structure of a channel estimator according to the present invention, and FIG. 5 is an exemplary diagram showing a process according to the operation of the channel estimator according to the present invention.

As can be seen with reference to FIG. 4, the channel estimator according to the present invention includes a first N-point fast Fourier transform (FFT) 110, a pilot / data subcarrier separator 120, and an LS. The channel estimator 130, the N-point Inverse Fast Fourier Transform (IFFT) 140, the second N-point FFT 150, the channel compensator 160, and the determiner 170. It includes.

Referring to Figure 4, the operation is as follows.

The first N-pint FFT 110 performs an FFT on the RF signal received through the antenna and converts the RF signal into a frequency domain. The pilot / data subcarrier separator 120 distinguishes a pilot subcarrier and a data subcarrier from a signal converted into the frequency domain, and outputs the pilot subcarrier to the LS channel estimator 130, wherein the data subcarrier is the channel. Output to the compensator 160.

The LS channel estimator 130 estimates a channel value of the pilot subcarrier in the 2n + 1th symbol, and assumes that the data subcarrier at the same position of the 2n symbol is experienced, and estimates the channel value of the pilot subcarrier in the LS method. do. That is, the channel estimation algorithm is applied on the assumption that the 2n th and 2n + 1 th symbols have experienced the same channel. In this case, in the OFDMA scheme, the frequency axis is affected by the delay profile, and the time axis is influenced by the Doppler frequency. In addition, since the coherence time is long in a Mobile WiMAX system, it can be assumed that the first symbol has experienced a channel of a pilot subcarrier in another symbol at the same subcarrier location in a cluster.

Subsequently, the estimated channel value is placed in the pilot subcarrier position, and 0 is inserted into the remaining subcarrier position and output.

The N-point IFFT 140 performs an IFFT to convert the output result into a time domain. In this case, as shown in FIG. 5, the subcarrier in the front part corresponding to the delay spread section and the last subcarrier corresponding to half of the delay spread are selected, and 0 is inserted into the remaining subcarrier positions.

)을 상기 채널 보상기(160)로 출력한다.The N-point FFT 150 performs an FFT on the output result, converts the result into a frequency domain, and estimates a channel estimate (

) Is output to the channel compensator 160.

The channel compensator 160 compensates the data subcarriers by selecting values at positions of used subcarriers, excluding virtual subcarriers and DC subcarriers, among the output values of the N-point FFT 150.

For the sake of understanding, the above operations will be explained once again as follows.

First, it is assumed that a data value of a pilot subcarrier in a 2n + 1th symbol is experienced by a data subcarrier at the same position of a 2nth symbol.

Subsequently, the channel value of the pilot subcarrier is estimated by the LS method.

Subsequently, the estimated channel value is placed in the pilot subcarrier position, 0 is inserted in the remaining subcarrier positions, and then N point IFFT is performed.

The subcarrier corresponding to delay spread and the subcarrier corresponding to half of delay spread are selected from the front part of the signal in the time domain output by the result of the IFFT, and 0 is inserted into the remaining subcarrier positions. , N point FFT.

Among the values resulting from the N point FFT, a value located at the position of the used subcarrier except for the virtual subcarrier and the DC subcarrier is selected.

6 is an exemplary diagram illustrating a comparison of channel estimates, and FIG. 7 is another exemplary diagram illustrating a comparison of channel estimates.

FIG. 6 shows the case where Eb / No is 5dB, and FIG. 7 shows the case where Eb / No is 30dB. In this case, the thick solid line represents the actual channel value, the solid red line represents the value by FFT estimation, and the thin solid line represents the estimated value by linear interpolation.

As can be seen with reference to FIGS. 6 and 7, according to the present invention, a channel can be estimated close to an actual channel value.

While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments, but, on the contrary, May be modified, modified, or improved.

1 is a block diagram illustrating a configuration of an FFT-based channel estimator in a general OFDMA scheme.

2 is an exemplary view showing the position of a pilot in a typical OFDMA scheme.

3 is an exemplary diagram illustrating a structure of a conventional partial usage subchannel (PUSC) channel.

4 is an exemplary view showing the structure of a channel estimator in accordance with the present invention.

5 is an exemplary diagram illustrating a process according to the operation of the channel estimator according to the present invention.

6 is an exemplary diagram illustrating a comparison of channel estimation values.

7 is another exemplary diagram illustrating a comparison of channel estimation values.

Claims (7)

Estimating a channel value of the pilot subcarrier in the received signal by least square (LS) method; Placing the estimated channel value at the pilot subcarrier position and inserting zero into the remaining subcarrier position of the signal to perform inverse fast Fourier transform; A half of a subcarrier corresponding to a delay spread of a front end of the signal of the inverse fast Fourier transform time domain and a subcarrier corresponding to a delay spread of the rear end of the signal is selected, and the remaining one of the signals Inserting zero at the subcarrier position and performing fast Fourier transform; And selecting a value at a position other than the virtual subcarrier and the DC subcarrier among the signals of the fast Fourier-transformed frequency domain. The method of claim 1, And separating a pilot subcarrier and a data subcarrier from the received signal. The method of claim 1, wherein the estimating step And the 2n th and 2n + 1 th symbols in the signal are set to undergo the same channel environment. The method of claim 1, And based on the selected value, compensating for data subcarriers in the signal. A channel estimator estimating a channel value of the pilot subcarrier in the received signal by least square (LS) method; An IFFT performer for performing an inverse fast Fourier transform by placing the estimated channel value at the pilot subcarrier position and inserting zero into the remaining subcarrier positions of the signal; A half of a subcarrier corresponding to a delay spread of a front end of the signal of the inverse fast Fourier transform time domain and a subcarrier corresponding to a delay spread of the rear end of the signal is selected, and the remaining one of the signals An FFT performer for inserting zero at a subcarrier position and performing fast Fourier transform; And a channel compensator for selecting a value at a position other than the virtual subcarrier and the DC subcarrier among the signals of the fast Fourier-changed frequency domain to compensate for the data subcarrier in the signal. The method of claim 5, And a separator for separating and outputting a pilot subcarrier and a data subcarrier from the received signal. 6. The apparatus of claim 5, wherein the channel estimator And the 2nth and 2n + 1th symbols in the signal are determined to have experienced the same channel environment.

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