KR20000010178A - Equalizing method and equalizer for a ofdm receiver - Google Patents

Abstract

PURPOSE: A method of compensating a channel distortion by reducing an interpolation interval in a frequency domain and a channel characteristic varying with time and an equalizer for performing the method are provided. CONSTITUTION: A method of compensating channel distortions of a plurality of carriers transmitted through a plurality of sub-channels using distributed pilots, each of which has a predetermined symbol period, the method which includes the steps of: a) estimating a value of the channel characteristic based on positions of the distributed pilots extracted from received the plurality of carriers; b) updating the characteristics of the plurality of sub-channels through which ones among the distributed pilots are transmitted every symbols and interpolating the characteristics of the sub-channels through which the other distributed pilots are transmitted by predicting the characteristics of the sub-channels based on the characteristics of sub-channels through which neighboring distributed pilots are transmitted; and c) compensating the channel distortions based on all characteristic values of the sub-channels. The method is very effective when equalizing a distortion by a selected frequency.

Description

Equalization Methods and Equalizers for ODF Receivers

The present invention relates to a receiver for receiving a signal transmitted by Orthogonal Frequency Division Multiplexing (OFDM), and more particularly, to an equalization method and an equalizer for compensating for channel distortion.

OFDM is a type of multi-carrier modulation in digital signal transmission, in which data is carried on a plurality of subcarriers in parallel in a long symbol period while using a frequency band used in a single carrier data transmission. In this OFDM scheme, since symbols are determined in the frequency domain, an equalizer in the frequency domain is required to compensate for channel distortion with respect to the received symbol. To this end, the OFDM scheme transmits a distributed pilot. However, this distributed pilot is not transmitted on a predetermined subchannel as shown in FIG. 1, but is transmitted on a different subchannel for each symbol with a predetermined pattern. In FIG. 1, the x axis is a time axis and the y axis corresponds to a frequency axis.

That is, as shown in FIG. 1, the distributed pilot is repeated with a period of 4 symbols, while the distributed pilot is transmitted on the first and last subchannels (●), while the second start point is changed every 12 symbols with 12 subchannel periods. In order to accurately estimate the channel characteristics, it is necessary to determine the location where the distributed pilot is transmitted, and then calculate an estimated value according to the characteristics of the subchannel (●). After the estimated values of the pilot are interpolated, the overall subchannel characteristics are estimated. However, when interpolating for each symbol, since the interval between distributed pilots is 11 subchannels, a compensation error increases when frequency distortion occurs. In addition, if the channel characteristics are tracked for each symbol, since 11 subchannels between distributed pilots are required, an interpolation filter also requires a sophisticated filter.

However, "European Telecommunication Standard" Draft ETS 300 744, March 1997 proposed by the European Telecommunication Standards Institute (ETSI) does not describe the detailed configuration of the equalizer for compensating for channel distortion.

The technical problem to be achieved by the present invention is to provide an equalization method that accurately compensates for channel distortion by compensating for the characteristics of the channel changing over time and reducing the interpolation interval on the frequency axis.

Another technical problem to be achieved by the present invention is to provide an equalization method that accurately compensates for channel distortion by compensating for characteristics of a channel changing over time and reducing interpolation intervals on a frequency axis.

1 illustrates an example of a distribution of a conventional distributed pilot.

2 shows an example of an equalization coefficient updating method proposed by the present invention.

3 is a detailed block diagram of an equalizer for an OFDM receiver according to the present invention.

In order to solve the above technical problem, the present invention provides a method for compensating for channel distortion of a plurality of carriers transmitted on a plurality of subchannels by using a distributed pilot having a predetermined number of symbol periods: (a) Extracting a distributed pilot from a carrier of the channel and estimating a channel characteristic value at the extracted distributed pilot position; (b) updating, by each symbol, the subchannel characteristics transmitted by the distributed pilot to a new value, and predicting and interpolating the subchannel characteristics transmitted by the remaining distributed pilots from the characteristics of the subchannels transmitted by the neighboring distributed pilots; (c) compensating for the channel distortion based on the total subchannel characteristic values.

In order to solve the above other technical problem, the present invention provides a method for estimating subchannel characteristics of a plurality of carriers transmitted on a plurality of subchannels by using a distributed pilot having a predetermined number of symbol periods. Update the characteristics of the transmitted subchannels, predict the characteristics of the subchannels from which the previous distributed pilots were transmitted from the time-varying values of the adjacent distributed pilots, and frequency the characteristics of the subchannels from which the remaining distributed pilots were not transmitted. The equalization method obtained by interpolation of an axis.

In order to solve the above other technical problem, the present invention provides an orthogonal frequency division multiplexing receiver for receiving a plurality of carriers through a plurality of subchannels including a distributed pilot having a predetermined number of symbol periods: An extractor for extracting a distributed pilot; A channel estimator estimating a subchannel characteristic value from the distributed pilot extracted by the extractor using coordinates of a previously known distributed pilot; The characteristics of the subchannels in which the distributed pilots are transmitted every new symbol from the channel estimator are updated to new values, and the characteristics of the subchannels in which the adjacent distributed pilots are transmitted are temporally updated. A channel characteristic updater for predicting by giving a weight to the changed value; An interpolator for estimating characteristics of the entire channel by interpolating characteristics of the subchannels from which the remaining distributed pilots are not transmitted, from the characteristics of the subchannels provided from the channel characteristic updater, on the frequency axis; And a digital filter outputting data compensated for channel distortion by multiplying a plurality of received carriers by an inverse number of coefficients interpolated in the interpolator.

Hereinafter, exemplary embodiments of an equalization method and an equalizer for an OFDM receiver according to the present invention will be described with reference to the accompanying drawings.

First, the equalization coefficient updating method proposed by the present invention will be described with reference to FIG. 2.

STEP 1: Find out which of the 4 symbols is the received symbol, extract the distributed pilot from the received symbol, and estimate the channel characteristic value at the extracted distributed pilot position.

STEP 2: The characteristic (●) of the current subchannel in which the distributed pilot is transmitted every symbol is updated with a new value (equalization coefficient) every 12 subchannels, and the characteristic of the subchannel in which the previous distributed pilot was transmitted (□). ) Predicts the value by weighting the degree to which the characteristics of adjacent newly updated subchannels change with the time base. The characteristic of the subchannel, where no remaining scattered pilot is transmitted at all, estimates the characteristics of the entire channel using interpolation on the frequency axis.

In more detail, as shown in FIG. 2, by calculating a subchannel characteristic in which a current pilot is transmitted, a deviation (δ1) in which the characteristic of the subchannel is changed in one symbol period can be obtained, and neighboring pilots are transmitted. The deviation (δ2) in which the characteristic of the subchannel has changed can be obtained. Interpolation is performed to estimate the characteristics of the subchannel (○) through which data is transmitted.In this case, in order to adapt well to the frequency-selective distortion, the subchannel characteristics of the position to be interpolated are first predicted and then interpolated on the frequency axis. The interpolation interval can be reduced to two subchannels. That is, as shown in FIG. 2, the characteristic? Of the subchannel as the reference for interpolation is obtained by adding or subtracting the deviation obtained by multiplying the characteristic of the previous subchannel by a weight according to the distance between the deviation δ1 and the deviation δ2. Available For example, the characteristic? Of the subchannel as a reference for the seventh interpolation is located at an intermediate distance, and thus is calculated as 1 / 2δ1 + 1 / 2δ2. Since the change over time of the channel acts almost the same for all subchannels, the characteristics of the reference subchannels adapted to the change in time can be obtained by this calculation. Next, we estimate the characteristics of all subchannels using FIR filter interpolation using the window as the frequency axis.

Here, in the interpolation method, if the characteristics of the subchannels do not change significantly, it is assumed that the characteristics of the subchannels to which the pilot is transmitted are the same as those of the subchannels to which the next two data are transmitted. Another method is to calculate the characteristics of two subchannels through which data is transmitted by linear interpolation, and another method is to use a windowed finite impulse response (FIR) filter with higher interpolation performance. You can also use interpolation.

However, using constant interpolation or linear interpolation simplifies the structure, but the accuracy is lower than the interpolation by the window FIR filter, and using the window FIR filter, the accuracy is higher, but the position of the scattered pilot Errors can occur at the beginning of a symbol because they are continuous at the beginning. Accordingly, in the present invention, constant interpolation or linear interpolation is used at the beginning of the symbol, and the equalization performance is improved as a whole by using interpolation by a windowed FIR filter.

STEP 3: If you estimate the characteristics of the subchannels to which data is transmitted in STEP 2, you know the characteristics of the entire subchannels. Therefore, the amplitude and phase distortion, i.e., multiply the data of each subchannel by the inverse of the characteristics of the subchannels. , To compensate for channel distortions.

Here, since the equalizer compensates for the frequency distortion in units of symbols in the frequency domain, assuming that the channel characteristics are H (f), the transmitted data is X (f), and the received data is R (f). Can be represented.

R _k (f) = H _k (f) X _k (f)

Where k represents the index of the subchannel.

The subchannel to which the pilot is transmitted can be represented as in Equation 2, and the amplitude and phase characteristics of the subchannel can be obtained from Equation 3.

R _k (f) = H _k (f) P _k (f)

P (f) is a distributed pilot.

For example, in the case of the 8K mode of DVB-T, the number of subchannels obtained from the distributed pilot of the modified distribution shown in FIG. 2 is 6816/3, and the remaining 6816 * 2/3 is obtained using the characteristics of the subchannels. The interpolation characteristics of the data subchannel are obtained by using interpolation. Constant interpolation used at the beginning of every symbol can be expressed as Equation 4.

H _{p (k)} (f) = H _{d (1)} (f) = H _{d (2)} (f)

Next, the linear interpolation used at the beginning of every symbol is briefly described as follows. If the characteristic of the channel through which the distributed pilot is transmitted is H _{P (k)} (f), the characteristic of the channel where the next pilot is transmitted may be referred to as H _{P (k + 1)} (f). The characteristics of the two scattered pilot sub-channel a sub-channel data is transmitted between H _{d (1) (f)} and H _{d (2) (f)} When d _{H d (1) (f)} , H d (2) (f) can be obtained from equations (5) and (6).

Constant or linear interpolation is performed up to the beginning of every symbol (here, the 12th carrier) and then converted to interpolation by a windowed FIR filter, which is a lowpass filter whose sampling frequency is three times the carrier frequency. Design

Therefore, when the characteristics of the distributed pilot are filtered, the characteristics of the two subchannels therebetween are calculated and output. That is, if the characteristic of the filter is F (f), the characteristics of the entire channel can be obtained by simply convolving the characteristics of the distributed pilot and the characteristic F (f) of the filter. That is, the characteristic H (f) of all the channels can be obtained as follows.

H (f) = H _p (f) ⋅F (f)

After obtaining the characteristics of all the subchannels in the above manner, multiply the received data by the inverse of the characteristics of the subchannels to obtain an equalized output as shown in Equation (8).

If the equalization method according to the present invention described above does not change the position of the distributed pilot since the position of the distributed pilot does not change, it is not necessary to track the position of the distributed pilot and the interval between the distributed pilots becomes two subchannels. Although the channel characteristics can be estimated by calculating the values, interpolation by the windowed FIR filter is used together to improve the interpolation performance. After estimating the characteristics of the entire subchannels, equalization in the frequency domain can be done simply by multiplying the received signal by the inverse of the channel characteristics, which is very simple since it has a simple one-tap equalizer structure.

Next, the equalizer for the OFDM receiver according to the present invention will be described with reference to FIG.

In FIG. 3, the guard interval remover 102 removes a cyclic prefix (CP) corresponding to the guard interval from the received data. Since the guard band eliminator 104 outputs the data from the guard interval eliminator 102 in the time domain, fast guard transform (FFT) is performed to convert the data into the frequency domain in symbol units. Remove guard bands included in FFT data. Here, each symbol is composed of 6817 carriers in the 8K mode, which is transmitted in a symbol period (Ts) duration, which is a valid symbol part interval and guard interval for transmitting 6817 carriers. It consists of 1704 carriers in the 2K mode, which is transmitted in a symbol period (Ts) duration, this period (Ts) is a valid symbol part interval and guard interval for transmitting 1704 carriers Include.

The frame synchronizer 105 generates a frame synchronizer from the received data. The distributed pilot extractor 106 generates a distributed pilot from 6817 carriers per symbol when the guard band output from the guard band remover 104 is removed according to the frame synchronization generated from the frame synchronizer 105, i.e., in 8K mode. Extract.

Since the distributed pilot is repeated in four symbol periods, knowing the position of the distributed pilot of one symbol can automatically determine the distributed pilot position of the four symbols. Thus, if the distributed pilot extractor 106 knows exactly the distributed pilot position of one symbol, it outputs the level value received at the already known distributed pilot position for the next three symbols.

Here, although the boosted power level (4/3) ² of the received distributed pilot, the distributed pilot according to each distributed pilot position of 4 symbol periods in transmission is (4 / 3,0) or (-4 / 3,0) Has the coordinate value of. Since this information is already known to the distributed pilot determiner 108, the coordinate values for the distributed pilot extracted from the distributed pilot extractor 106 are output to the channel estimator 110.

The channel estimator 110 estimates the channel using the coordinate values corresponding to the distributed pilot positions output from the distributed pilot determiner 108 and outputs channel characteristic values at the distributed pilot positions. That is, the channel estimator 110 compares the boosted level of the distributed pilot extracted from the distributed pilot extractor 106 with the original coordinate value, and compensates for the amplitude distortion (r) and the value for compensating the phase distortion ( θ )

The channel characteristic updater 112 updates the characteristics of the subchannels to which the distributed pilots are transmitted for each symbol to new values for each of the 12 subchannel characteristics, and the characteristics of the subchannels to which the previous distributed pilot was transmitted are adjacently updated. The distribution pilot predicts the value by weighting the value of the subchannels on which the distributed pilot is temporally changed.

The coefficient interpolator 114 does not transmit a variance pilot for every symbol provided from the channel characteristic updater 112 to estimate unknown subchannel characteristics (○) between the estimated subchannels in the channel characteristic updater 112. The interpolated equalization coefficients are output by interpolating the subchannel characteristics of the subchannels. The coefficient interpolation by the coefficient interpolator 114 maintains the channel interpolation interval on the frequency axis as two subchannels. Coefficient interpolation may use linear interpolation, constant interpolation, or windowed FIR filter interpolation.

The frequency domain equalizer 116 may be configured as a filter having one tap, that is, a multiplier, and inverse of the data from which the guard band is output from the guard band remover 104 and coefficients interpolated by the coefficient interpolator 114. Multiply by to output the data compensated for the channel distortion.

Meanwhile, the above-described embodiments of the present invention can be written as a program that can be executed on a computer. And it can be implemented in a general-purpose digital computer for operating the program from a medium used in the computer. The media includes storage media such as magnetic storage media (e.g., ROM, floppy disks, hard disks, etc.), optical reading media (e.g., CD-ROM, DVD, etc.) and carrier waves (e.g., transmitted over the Internet). . A method for compensating for channel distortion of a plurality of carriers transmitted on a plurality of subchannels by using a distributed pilot having the predetermined number of symbol periods, the method comprising: extracting a distributed pilot from a plurality of received carriers and extracting Estimating a channel characteristic value of the sub-channel characteristic, wherein the sub-channel characteristic transmitted by the distributed pilot for each symbol is updated to a new value for each sub-channel of the second predetermined number, and the neighboring distributed pilot is updated with the sub-channel characteristic transmitted by the remaining distributed pilot. Predicting and interpolating characteristics of the transmitted subchannel and compensating for channel distortion based on the total subchannel characteristic values are stored as program code executable by a computer.

And functional program code and code segments for implementing the present invention can be easily inferred by programmers in the art to which the present invention belongs.

The equalization method of the present invention can exhibit improved equalization performance with respect to frequency selective distortion, and has an effect of reducing the amount of computation while reducing the interval between distributed pilots while being well adapted to a channel that changes in time. In addition, the equalizer of the present invention does not equalize in units of four symbols, but equalizes every symbol, thereby reducing the memory required.

Claims (14)

A method for compensating for channel distortion of a plurality of carriers transmitted on a plurality of subchannels using a distributed pilot having a predetermined number of symbol periods: (a) extracting distributed pilots from a plurality of received carriers and estimating channel characteristic values at the extracted distributed pilot positions; (b) updating, by each symbol, the subchannel characteristics transmitted by the distributed pilot to a new value, and predicting and interpolating the subchannel characteristics transmitted by the remaining distributed pilots from the characteristics of the subchannels transmitted by the neighboring distributed pilots; (c) compensating for channel distortion based on the total subchannel characteristic values. 4. The equalization method of claim 1, further comprising: interpolating the subchannels to which the distributed pilot is not transmitted in step (b). The equalization method of claim 1, wherein the characteristic value of the subchannel as the reference for interpolation in step (b) is predicted by adding a weight to the characteristic value of the subchannels to which neighboring distributed pilots are transmitted. 4. The equalization method according to claim 3, wherein the characteristic value of the subchannel as a reference for interpolation is estimated for each distributed pilot position having a predetermined number of subchannels. 5. The equalization method according to claim 4, wherein the predetermined number is three. The subchannel characteristic as a reference for interpolation is determined by a variation of a subchannel characteristic of a current pilot transmitted for one symbol period and a variation of a characteristic of a subchannel transmitting a neighboring pilot, respectively. Equalization method characterized in that it is obtained by adding or subtracting the deviation obtained by multiplying the weight according to the distance. 2. The method of claim 1, wherein in order to estimate a characteristic value of a subchannel through which data is transmitted in step (b), coefficients interpolated by predetermined interpolation of channel characteristic values of a distributed pilot having a predetermined number of neighboring subchannel periods. Equalizing method further comprising the step of outputting. 8. The method of claim 7, wherein multiplying the received carrier by an inverse of the interpolated coefficients compensates for channel distortion. 8. The method of claim 7, wherein the predetermined interpolation is one of constant interpolation, linear interpolation, and interpolation by a windowed FIR filter. 8. The equalization method according to claim 7, wherein one of constant interpolation and linear interpolation is used for a predetermined first carrier of each symbol, and interpolation by a windowed FIR filter is used for the remaining carriers. A method for estimating subchannel characteristics of a plurality of carriers transmitted over a plurality of subchannels using a distributed pilot having a predetermined number of symbol periods: For each symbol, the characteristics of the subchannels in which the distributed pilots are transmitted are updated, and the characteristics of the subchannels in which the previous distribution pilots are transmitted are estimated from the values of the time-varying variation of the adjacent distributed pilots. Equalization method characterized in that to obtain the characteristics of the sub-channel not transmitted the remaining distributed pilot by interpolation of the frequency axis. An equalizer for compensating for channel distortion of a plurality of carriers transmitted on a plurality of subchannels using a distributed pilot having a predetermined number of symbol periods: For each symbol, the characteristics of the subchannels in which the distributed pilots were transmitted are updated, and the characteristics of the subchannels in which the previous distributed pilots were transmitted are weighted to values of temporal changes in the characteristics of the subchannels in which the adjacent updated pilots are transmitted. Channel characteristic updating means for predicting and interpolating, on the frequency side, characteristics of subchannels for which the remaining distributed pilots are not transmitted; And And compensating means for compensating for channel distortion using the received plurality of carriers using the total subchannel characteristic values. 13. The method of claim 12, wherein the channel characteristic updating means interpolates a channel characteristic value at a distributed pilot position having a predetermined number of subchannel periods adjacent to estimate a characteristic of a subchannel through which data between the distributed pilots is transmitted. And outputting coefficients interpolated by the equalizer. In an orthogonal frequency division multiplexing receiver for receiving a plurality of carriers over a plurality of subchannels including a distributed pilot having a predetermined number of symbol periods: An extractor for extracting distributed pilots from a plurality of received carriers; A channel estimator estimating a subchannel characteristic value from the distributed pilot extracted by the extractor using coordinates of a previously known distributed pilot; The characteristics of the subchannels in which the distributed pilots are transmitted every new symbol from the channel estimator are updated to new values, and the characteristics of the subchannels in which the adjacent distributed pilots are transmitted are temporally updated. A channel characteristic updater predicting the weighted value of the changed value; An interpolator for estimating the characteristics of the entire channel by interpolating the characteristics of the subchannels from which the remaining distributed pilots are not transmitted from the characteristics of the subchannels provided from the channel characteristic updater on the frequency axis; And And a digital filter outputting data compensated for channel distortion by multiplying a plurality of received carriers by an inverse of the coefficient interpolated in the interpolator.