KR100916724B1 - Apparatus and method for estimation/compensation of high frequency distortion for orthogonal frequency division multiplexing system - Google Patents

Abstract

The present invention relates to an apparatus for estimating and compensating for signal distortion in an orthogonal frequency division multiplexing wireless system. The present invention relates to an apparatus for estimating signal distortion of a transmission modem of an orthogonal frequency division multiplexing (OFDM) system. A test signal generation unit for generating and transmitting the test signal to the transmission modem, a switch control unit for controlling a switch to set a path of the test signal transmitted to the transmission modem, and receiving a test signal in which signal distortion has occurred through the set path. The calculation unit calculates an average of the signal sizes of the subcarriers and the signal amplitude of each subcarrier using the average of the signal sizes of each subcarrier calculated by the calculator and the signal size of each subcarrier of the test signal generated by the test signal generator. DAC including distortion estimator for obtaining corresponding compensation value There is an effect that it is possible to collectively compensate for signal distortion in the transmission modem, regardless of the kind.

OFDM, subcarrier, test signal, digital to analog converter (DAC)

Description

APPARATUS AND METHOD FOR ESTIMATION / COMPENSATION OF HIGH FREQUENCY DISTORTION FOR ORTHOGONAL FREQUENCY DIVISION MULTIPLEXING SYSTEM}

The present invention relates to an apparatus for estimating and compensating for signal distortion in an orthogonal frequency division multiplexing wireless system. More particularly, the transmission signal of an orthogonal frequency division multiplexing system includes a digital filter and a digital / analog of a transmission modem. The present invention relates to an apparatus and method for estimating and compensating for distortion occurring in a transmission signal by a frequency characteristic of a digital-to-analog converter (hereinafter, referred to as a DAC).

The orthogonal frequency division multiplexing wireless system includes a transmission modem for transmitting data and a reception modem for processing a received signal as shown in FIG. 1, and the transmission modem is a digital signal processing unit for processing digital signals and an analog signal for processing. It is divided into analog signal processor. The digital signal processing unit and the analog signal processing unit use a plurality of filters. In particular, the low pass filter (hereinafter referred to as LPF) used in the digital signal processing unit is used in the analog signal processing unit in the process of filtering signals. The DAC generates distortion in frequency characteristics in the process of converting a digital signal into an analog signal.

2 illustrates the frequency characteristics of the low pass filter (LPF). When 1.0 on the horizontal axis is a sampling frequency of the digital signal, the low pass filter (LPF) passes a frequency component of 0.43 or less based on the sampling frequency. Above 0.43, the size is reduced to -80 dB. Here, a passband of 0.43 or less generates ripple of about ± 1 [dB].

FIG. 3 is a diagram illustrating the concept of sample-and-hold of a digital-to-analog converter (DAC), in which a DAC is generally sample-and-hold to convert a digital signal into an analog signal. The concept of the method is mainly used. The sample-and-hold method is equivalent to the rectangular function that maintains digital values located at the sample period Ts interval for Ts time.In the formula, the Convolution operation is repeated for each digital value. same. This convolution operation is equivalent to the multiplication between the frequency spectra of the two functions in the frequency domain. In addition, the sample-and-hold function has a rectangular shape in the time domain, but is the same as the sinc function in the frequency domain.

Therefore, the DAC frequency characteristic based on the sample-and-hold method is a sinc function having a sampling frequency of 1.0 as shown in FIG. 4 (a), and the highest frequency component of the signal passing through the digital filter. If it is located at 0.43, signal attenuation of about -2.8 [dB] occurs at that position as shown, which corresponds to unwanted signal distortion.

In order to compensate for the signal distortion of the DAC, a digital signal is compensated by including an inverse sinc component in the digital filter coefficient. 5 illustrates the frequency characteristics of the low pass filter (LPF) to which the inverse sinc function is applied, and it can be seen that the magnitude of the frequency characteristics of the pass band increases to about 0.43 on the horizontal axis.

However, in an OFDM wireless system, a signal may be distorted in a transmission modem by a digital filter of a digital signal processor and a DAC of an analog signal processor.

In addition, since the frequency characteristics of the DAC used in the OFDM wireless system vary depending on the type used, it is cumbersome to obtain the number of filters according to the frequency characteristics of each DAC.

The present invention to solve the above problems is to estimate the signal distortion that may occur in the digital filter and the DAC of the transmission modem in the OFDM wireless system to compensate for the signal distortion in the OFM system that can be compensated before the signal transmission It is an object to provide an estimation / compensation apparatus and method.

Another object of the present invention is to provide a method for preliminarily estimating and compensating for signal distortion that may occur in a transmission signal according to frequency characteristics of each DAC in an OFDM system supporting various types of DACs.

In order to achieve the above object, an apparatus for estimating frequency distortion of a transmission modem in an orthogonal frequency division multiplexing (OFDM) system according to the present invention generates a test signal for each subcarrier to estimate signal distortion of the transmission modem. A test signal generation unit configured to transmit the test signal to the transmission modem, a switch control unit controlling a switch to set a path of the test signal transmitted to the transmission modem, and receiving a test signal having a signal distortion through the set path, thereby receiving each subcarrier A calculation unit for calculating an average of signal amplitudes of the subcarriers and a signal amplitude of each subcarrier of the subcarriers of the test signal generated by the test signal generator and a signal magnitude of each subcarrier calculated by the calculator And a distortion estimating unit for obtaining a compensation value.

On the other hand, in the orthogonal frequency division multiplexing (OFDM) system of the present invention, a method for compensating for frequency distortion of a transmission modem stores each compensation value obtained from a signal distortion estimating apparatus, and transmits an OFDM symbol. The signal is compensated by multiplying the compensation value corresponding to a subcarrier.

In addition, the method for estimating the frequency distortion of a transmission modem in an orthogonal frequency division multiplexing (OFDM) system according to the present invention includes (a) establishing a test path of a test signal and transmitting the test signal to the set test path. (B) oversampling a test signal transmitted to the transmission modem, (c) receiving a test signal in which signal distortion has occurred via the test path, and calculating an average of signal sizes of each subcarrier; and d) obtaining a compensation value corresponding to the signal distortion of each subcarrier using the average of the calculated signal magnitudes of each subcarrier and the signal magnitude of each subcarrier of the generated test signal.

Meanwhile, the method for estimating and compensating for the frequency distortion of a transmission modem in an orthogonal frequency division multiplexing (OFDM) system further includes (e) compensating the signal by multiplying a compensation value corresponding to each subcarrier among the compensation values. It is characterized by including.

As described above, the method for estimating and compensating for signal distortion in the OPM system according to the present invention can estimate signal distortion that may be caused by a digital filter or a DAC in a transmission modem, thereby easily compensating for signal distortion before signal transmission. It works.

In addition, the present invention has the effect that it is possible to collectively compensate for signal distortion of the transmission modem regardless of the type of various DACs that can be used for the transmission modem of the OPM system.

In addition, the present invention has the effect that the transmission signal can be compensated in advance using a compensation value of each subcarrier stored in advance, regardless of a complicated circuit configuration.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. However, in describing in detail the operating principle of the preferred embodiment of the present invention, if it is determined that the detailed description of the related known functions or configurations may unnecessarily obscure the subject matter of the present invention, the detailed description thereof will be omitted.

In addition, the same reference numerals are used for parts having similar functions and functions throughout the drawings.

6 is a diagram illustrating characteristics of a plurality of functions connected in series. (a) shows a case where a plurality of functions h ₁ (t), h ₂ (t), ..., h _n (t) are connected in series, and the frequency characteristics of each function are H ₁ (f), When H ₂ (f), ..., H _n (f), the frequency characteristics of all functions including all functions are expressed as the product of the frequency characteristics of each function in the frequency domain as shown in (b).

Therefore, the signal distortion of a digital filter and a DAC on a transmission signal of a transmission modem of a system employing an orthogonal frequency division multiplexing scheme can be known from the product of two frequency characteristic functions.

Accordingly, the present invention measures all signal distortions generated in the transmission modem of the OFDM system using a test signal, estimates a compensation value for compensating the measured signal distortion, and applies the subcarrier of the OFDM symbol to the digital signal of the transmission modem. An apparatus and method for precompensating signal distortion that may occur in a filter or a DAC. EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated concretely.

7 is a diagram illustrating a configuration of an apparatus for estimating and compensating for signal distortion in an OFDM system according to the present invention.

Referring to FIG. 7, an OFDM system for estimating and compensating signal distortion includes a frequency processor 700, a transmitter 720, an RF transceiver 730, a receiver 740, and the like, including an estimation and compensation device 710. The signal distortion estimating and compensating apparatus includes a test signal generator 711, a switch controller 712, a calculator 713, a distortion estimator 714, and a distortion compensator 715.

First, in order to estimate the signal distortion of the digital filter and the DAC using the test signal, a path of the test signal is determined by controlling the switches SW1, SW2, and SW3 using the switch controller 712. The switch controller 712 may control the switch differently according to the case of estimating only the distortion of the digital filter 725 and the case of estimating the total distortion of the digital filter 725 and the DAC 726.

Estimated Target SW1 SW2 SW3 Distortion Estimation of Digital Filters - a a Digital Filter + DAC Distortion Estimation a b a

Referring to FIG. 7, when only the output distortion of the digital filter 725 is to be estimated, as shown in Table 1, SW 1 is set to OFF, SW2 and SW3 are set so that the switches are in contact with a, and the output distortion of the transmission modem is shown. When estimating, as shown in Table 1, SW1 is in contact with a, SW2 is in contact with b, and SW3 is contacted with a.

When the test signal is routed, the test signal generator 711 generates a test signal to estimate the distortion of the output signal of the digital filter and the transmission modem. Since signal distortion needs to be estimated for each subcarrier, a test signal is generated for the usable subcarriers. Here, the values transmitted through each subcarrier are determined to be easy to calculate. For example, 1 + 0j can be used as the transmitted value.

The test signal generated as described above is transmitted to the transmitter 720. However, when the values of all subcarriers input to the IFFT 721 are the same value, the output of the IFFT 721 becomes an impulse value, which exceeds the representable range of the data path, causing saturation. Therefore, before inputting the IFFT 721, the value of the subcarrier is multiplied by the pseudo-random bit sequence (PRBS) so that the phase is not the same while maintaining the size.

This test signal is passed to the IFFT 721 to convert a signal in the frequency domain into a signal in the time domain by inverse Fourier transform. The converted signal is reconverted to a signal in the time domain at the P / S 722, a CP is added at the C / P adder 723 as necessary, and more than doubled while passing through the upsampling unit 724. It becomes over-sampling. Oversampling is typically two or four times more common. For example, when the nominal sample rate is 10 Msamples / sec, when the over-sample rate is 4 times, the sample rate of the DAC is 40 Msamples / sec. In addition, if necessary, the test signal output from the IFFT 721 may be repeatedly transmitted two or more times. Since the loop-back delay between the transmitter 720 and the receiver 740 is not known, the transmission is repeatedly performed at least two times. In addition, the signal distortion may be estimated by obtaining the average power for each subcarrier using the repeatedly transmitted test signal.

When estimating only the distortion of the digital filter 725 under the control of the switch controller 712, the test signal passes through the LPF 725 and is immediately transmitted to the memory unit 751, and when estimating the total distortion of the transmission modem. After passing through the LPF 725 and the DAC 726 is delivered to the ADC 741 of the receiver 740. Here, since the test signal passing through the DAC 726 is an analog signal, it is converted back into a digital signal by the ADC 741 of the receiver.

As a result, the test signal generates distortion due to ripple in the LPF 725, and signal distortion occurs due to signal conversion by a sample / hold method in the DAC 726.

The test signal generated by the signal distortion in the receiving unit is transferred to the memory unit 751, and the memory unit 751 temporarily stores the received test signal. The memory unit 751 is capable of storing at least one OFDM sample length or more.

The test signal stored in the memory unit 751 performs decimation according to the over-sampling rate in the input signal processor 752 to generate an OFDM symbol equal to the over-sampling rate. Is generated and passed to the FFT 746 via the S / P 745. The FFT 746 converts a signal in the time domain into a signal in the frequency domain and transmits the signal to the calculator 713.

In this case, when the FFT size is 1024, one OFDM symbol excluding CP is composed of 1024 samples, and when the transmission modem has twice the over-sample rate, one OFDM symbol is composed of 2048 samples in total. do. The same applies to the ADC 741 of the receiving modem. In the case of double oversampling, when the first sample of every 2 samples is collected, one OFDM symbol capable of performing an FFT is formed, and when the second sample is collected, another OFDM symbol may be configured. Signal distortion can be obtained using the average power for each subcarrier using the plurality of signal powers for each subcarrier of the plurality of OFDM symbols.

The test signal converted from the FFT 746 into a signal in the frequency domain is transferred to the calculation unit 713, and the calculation unit 713 calculates an average size of each subcarrier of the test signal output from the FFT 746 and averages the result. Find the value. That is, the average of a plurality of OFDM symbols that are outputs of the FFT 746 is obtained.

When the received test signal is expressed in the frequency domain, it is expressed by Equation 1 below.

만큼 신호크기의 변화가 발생하고,

[rad] 만큼 위상의 변화가 발생한다. 여기서, k는 부반송파 번호이며, m은 FFT로 출력된 반복횟수이다. Where the distortion

Changes in signal size

The change in phase occurs by [rad]. Where k is the subcarrier number and m is the number of repetitions output to the FFT.

When the signal average is obtained for each subcarrier as shown in Equation 2, only the signal magnitude component remains. Although the distortion component is a fixed value that does not change over time, the average of several OFDM symbols is intended to increase the estimation accuracy by minimizing the effects of unwanted random interference such as noise. Here, c _k is a PRBS value to be multiplied by the k-th subcarrier (subcarrier).

The distortion estimator 714 calculates a distortion compensation value by using an average value of power for each subcarrier obtained by the calculator 713. The distortion compensation value is obtained by comparing the power value of each subcarrier with respect to the test signal generated by the test signal generator 711 and the average value of the test signal power for each subcarrier obtained by the calculator 713.

<Equation 3> represents a compensation value for the distortion magnitude for each subcarrier when the test signal X _k is (1 + j0).

As shown in FIG. 8, the distortion compensator 715 stores distortion compensation values of respective subcarriers obtained by the distortion estimator 714. The subcarrier of the OFDM symbol to be transmitted is multiplied by a corresponding compensation value to compensate for signal distortion that may occur in the transmission modem.

Equation 4 represents signal distortion compensation in the distortion compensator 715 as an expression.

X ' _k = c _k × X _k × L _k

는 신호의 IFFT(721) 입력이다. 또한, 여기서 L_k 는 k번째 부반송파에 대한 왜곡 보상치를 나타낸다. Here, c _k is a PRBS value to be multiplied by the k-th subcarrier (subcarrier) has a value of -1 or 1. However, it is also available in 2 bits, whereby (1 + j0), (-1 + j0), (0 + j1) and (0-j1).

Is the IFFT 721 input of the signal. In addition, L _k represents a distortion compensation value for the k-th subcarrier.

The signal distortion estimator / compensation apparatus according to the present invention estimates the signal distortion of the test signal, and when the distortion estimator 714 obtains the distortion compensation value, repeatedly transmits the compensated test signal again using the obtained distortion compensation value. By estimating the signal distortion, the accuracy of the obtained compensation value can be confirmed again. That is, the performance of the test can be tested by checking whether the compensated compensation error falls below a predetermined threshold by receiving the compensated test signal. Here, the compensation error may be defined as a difference between the reception value and the target value for each subcarrier with respect to the FFT output of the received signal looped back in the same manner as the estimation process after applying the estimated compensation value to the test signal.

9 is a view showing another embodiment of a method for estimating and compensating for distortion of a signal according to the present invention.

In the OFDM transmission and reception system, two modes are used to compensate for the signal distortion in advance by using the compensation value calculated by estimating the signal distortion of the transmission modem.

The first is "estimate". It estimates the signal distortion of the transmitting modem to calculate the compensation for the available subcarriers.

The second is "confirm". This confirms the accuracy of the compensation calculated in the estimation mode.

First, an estimation mode for signal distortion is set (S110).

When the estimation mode is set, the switch controller controls the switch to set the path of the test signal in order to measure the signal distortion at the transmitter. (S120) As shown in Table 1, the switch is appropriately controlled to set the path.

When the switch control for the routing is performed, initial compensation values to be compensated for the IFFT input values are set (S130). Here, the initial compensation values are set to (1 + j0). Thus, L _k = 1 + jO. In addition, L _k is a distortion compensation value for the k-th subcarrier as a compensation value to precompensate distortion occurring in an OFDM symbol in the future.

Next, a test signal is generated. (S140) A test signal transmitted through each subcarrier is determined to be easy to calculate. For example, the test signal can be set to 1 + j0. However, to avoid saturation at the IFFT output, the PRBS sequence is multiplied by the values of all subcarriers before the IFFT input.

In addition, the generated test signal is compensated using the compensation value. However, if the initial compensation value is set to 1 + j0, the compensated test signal maintains the original test signal.

When the test signal is generated, the test signal is transmitted to the transmission modem of the OFDM transmission and reception system (S150). That is, the test signal is transmitted to the IFFT input of the transmission modem.

The test signal generated by the signal distortion in the transmission modem is received by looping back through the path set by the switch (S160).

 Then, iteration i is set to 0 (S170).

The received test signal is decimated at a nominal sampling rate by the input signal processor and input to the FFT 850. In other words, the input signal processor 840 receives an oversample rate of the transmission modem. The decimation is performed according to the over-sample rate, and the FFT is performed by inputting an OFDM symbol equal to the over-sample rate to the FFT.

When the FFT is executed and the power value for each subcarrier is obtained, the powers of the respective subcarriers are summed (S190).

Next, the repetition number is set to i = i + 1 (S200), and the size of the repetition number i and the preset number A is compared (S210).

If i is smaller than A, the process returns to step S180 of performing an FFT on the decimated OFDM symbol.

If i is not less than A, an average FFT output for each subcarrier power is obtained (S220).

When the calculated average FFT output for each subcarrier power, it is determined whether or not there is "estimated mode" is set (S230), if the estimated mode is set, and stores obtain the compensation value L _k for each subcarrier. In operation S240, a check mode is set (S250), and a test signal is generated using the compensation value L _k (S140).

If the estimation mode is not set, a compensation error, which is a difference between the received value and the target value, is calculated for each subcarrier in the FFT output for the test signal to which the compensation value is applied (S260).

When the compensation error is calculated, the magnitude of the compensation error is compared with the set threshold value (S270). Here, the threshold value is a negligible compensation error and may be appropriately set in some cases.

If the compensation error is greater than the threshold value, the process returns to the estimation mode again, and returns to the step (S140) of generating a test signal (S280).

If the compensation error is not greater than the threshold, the process of obtaining the distortion compensation value is terminated (S290).

When the distortion compensation value for each subcarrier is obtained through this process, the distortion compensation value is stored in the distortion compensation unit, and when the OFDM symbol including general data is transmitted according to the operation of the OFDM system, the distortion of the signal for each subcarrier of the OFDM symbol is calculated. Reward in advance.

The present invention described above is not limited to the above-described embodiments and the accompanying drawings, and it is common in the art that various substitutions, modifications, and changes can be made without departing from the technical spirit of the present invention. It will be apparent to those skilled in the art.

1 is a view showing the configuration of a typical OFDM transmission and reception system.

2 is a diagram illustrating frequency characteristics of a low pass filter (LPF).

3 is a diagram illustrating the concept of sample-and-hold of a digital-to-analog converter (DAC).

4 is a diagram illustrating frequency characteristics of a digital-to-analog converter (DAC).

5 is a diagram illustrating a frequency characteristic for reverse sink compensation by the prior art.

6 is a diagram illustrating characteristics of a plurality of functions connected in series.

7 is a diagram illustrating a configuration of an apparatus for estimating and compensating for signal distortion in an OFDM system according to the present invention.

8 is a diagram illustrating a configuration of an apparatus for compensating for distortion of a signal according to the present invention.

9 is a view showing another embodiment of a method for estimating and compensating for distortion of a signal according to the present invention.

Claims (17)

An apparatus for estimating signal distortion of a transmission modem in an orthogonal frequency division multiplexing (OFDM) system, A test signal generator for generating a test signal for each subcarrier and estimating the signal distortion of the transmission modem to the transmission modem; A switch control unit controlling a switch to set a path of a test signal transmitted to the transmission modem; A calculation unit configured to receive a test signal generated with signal distortion through the set path and calculate an average of signal sizes of respective subcarriers; And a distortion estimator for calculating a compensation value corresponding to the signal distortion of each subcarrier by using the average of the signal magnitudes of each subcarrier calculated by the calculator and the signal size of each subcarrier of the test signal generated by the test signal generator. And the test signal is oversampled in the transmission modem. The method of claim 1, wherein the test signal generator Apparatus for estimating signal distortion in an OFDM system, characterized by multiplying each subcarrier value by a pseudo-random bit sequence (PRBS) to generate test signals of different phases while maintaining the magnitude of the test signal of each subcarrier. The method of claim 1, wherein the test signal is At least two or more times after the IFFT output of the transmitting modem. delete The method of claim 1, The method may further include an input signal processor configured to receive the test signal in which the signal distortion occurs and perform decimation according to an oversample rate to generate a plurality of OFDM symbols equal to the oversample rate, and to transmit each OFDM to the calculator. An apparatus for estimating signal distortion in an OFDM system. The method of claim 5, wherein the calculation unit The signal distortion estimation apparatus of claim 1, wherein the signal size for each subcarrier is obtained by dividing the signal size for each subcarrier of the plurality of received OFDM symbols and dividing by the number of received OFDM symbols. The method of claim 1, wherein the switch control unit And controlling the switch according to a mode for measuring signal distortion of the digital filter of the transmission modem and a mode for measuring signal distortion of the transmission modem. The method of claim 1, And a memory unit for temporarily storing a test signal in which the signal distortion has occurred. The method of claim 1, wherein the distortion estimator And calculating the compensation value by dividing the average of the signal sizes of the subcarriers calculated by the calculator by the signal sizes of the subcarriers of the test signal generated by the test signal generator. An OFDM apparatus comprising a distortion compensation unit configured to store the compensation value obtained from the apparatus for estimating signal distortion according to claim 1, and to compensate the signal by multiplying the compensation value corresponding to each subcarrier of the OFDM symbol when transmitting an OFDM symbol. Signal distortion compensation device in the system. A method for estimating signal distortion of a transmission modem in an orthogonal frequency division multiplexing (OFDM) system, (a) setting a test path of a test signal and transmitting the test signal to the set test path; (b) oversampling a test signal sent to the transmission modem; (c) receiving a test signal in which signal distortion has occurred via the test path and calculating an average of signal sizes of each subcarrier; And (d) a method of estimating a signal distortion in an OFDM system comprising calculating a compensation value corresponding to a signal distortion of each subcarrier by using the average of the calculated signal sizes of each subcarrier and the signal magnitude of each subcarrier of the generated test signal . 12. The method according to claim 11, wherein in step (a) The method of claim 1, wherein the test signal is generated by multiplying each subcarrier value by a pseudo-random bit sequence. The method of claim 11, And transmitting the transmitted test signal at least twice after the IFFT output of the transmitting modem. delete The method of claim 11, wherein step (c) In the OFDM system, receiving the test signal in which the signal distortion is generated, performing decimation according to the oversample rate, generating a plurality of OFDM symbols as much as the oversample rate, and calculating an average of signal sizes of respective subcarriers. Signal distortion estimation method. The method of claim 11, And receiving and temporarily storing the test signal in which the signal distortion has occurred. The method of claim 11, wherein after step (d), and (e) compensating for a signal by multiplying a compensation value corresponding to each subcarrier among the compensation values.

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