KR20020053229A - Apparatus and method for compensating ofdm spectrum distortion in ofdm communication system - Google Patents

Abstract

PURPOSE: An apparatus for compensating for a distortion of an OFDM spectrum and a method thereof are provided to remove a distortion component of the OFDM spectrum in an orthogonal frequency division multiple communication system. CONSTITUTION: A pattern generator(201) generates a specific pattern to obtain a distortion coefficient by an analog element in a system. A shaper(202) multiplies the specific pattern by a stored distortion compensation coefficient to compensate for a distortion generated in a transmission signal. An OFDM modulator(203) reverse-high-Fourier transforms the distortion compensated signal. A DAC and LPF(204) converts and filters an output of the OFDM modulator(203) into an analog signal and outputs a base band signal. An IF/RF converter(205) converts an output of the DAC and LPF(204) into an intermediate frequency. A switch(206) switches an output of the IF/RF converter(205) to an RF/IF converter(207). The RF/IF converter(207) reduces a frequency of a radio frequency signal. An ADC and LPF(208) filters a signal from the RF/IF converter(207) and outputs an OFDM signal. An OFMD demodulator(209) demodulates the OFDM signal from the ADC and LPF(208).

Description

Apparatus and method for compensating distortion of the UFDM spectrum in an orthogonal frequency division multiplexing system {APPARATUS AND METHOD FOR COMPENSATING OFDM SPECTRUM DISTORTION IN OFDM COMMUNICATION SYSTEM}

The present invention relates to an orthogonal frequency division multiple (OFDM) communication system, and more particularly, to an apparatus and a method for removing distortion components of an OFDM spectrum.

In general, OFDM technology is applied to digital transmission technologies such as digital audio broadcasting (DAB), digital television, wireless local area network (WLAN), and wireless asynchronous transfer mode (WATM). It is widely applied. The OFDM scheme is a multi-carrier technology in which data to be transmitted is divided into a plurality of modulation data and then transmitted in parallel. However, due to the complexity of the structure, it has not been widely used, but it has been widely used due to the development of various digital signal processing technologies including the Fast Fourier Transform (FFT) and the Inverse FFT (IFFT). have. The OFDM scheme is similar to the conventional FDM, but above all, it is possible to obtain optimal transmission efficiency in high-speed data transmission by maintaining orthogonality between subcarriers. Recently, various implementation technologies, such as OFDM / TDMA and OFDM / CDMA, have been proposed using the OFDM scheme for high-speed data transmission such as WATM.

In the conventional OFDM system, the ratio of the number of used subcarriers to the total number of subcarriers is about 80%. The reason for this margin is to use it for special purposes such as future synchronization, and to minimize the influence of the filter characteristics of the system as much as possible. If the ratio of subcarriers used is 95% or more, performance degradation of the system occurs.

1 shows a transmission apparatus for compensating for such degradation in performance according to the prior art.

Referring to FIG. 1, the encoder 101 (FEC Encoder) encodes input transmission data and outputs a code symbol. Here, the encoder 101 outputs a code symbol having a capability of forward error correction (FEC). For example, a convolutional coder and a turbo coder can be used. The interleaver 102 interleaves and outputs the code symbols from the encoder 101 so as to be strong against a burst error or the like according to a given rule. The modulator 103 modulates the code symbol from the interleaver 102 into a complex signal by a prescribed mapping scheme, converts it to correspond to each subcarrier, and outputs it.

The shaper 104 compensates for the distortion generated in the transmission signal and outputs the modulated signal from the modulator 103. Here, the frequency shaper 104 compensates the distortion generated in the transmission signal before the IFFT due to the frequency characteristics of the analog device including the low pass filter, the band pass filter, and the like of the system. For example, first, there is a method of correcting only the magnitude of a complex signal inputted with an amplitude look-up table at a predetermined ratio. However, this method has a problem that is applicable only to differential modulation. Secondly, it is also used as a complex filter, which corrects both the phase and amplitude of the input complex signal.

An inverse fast freer converter (IFFT) 104 inverts the fast compensated distortion signal from the frequency shaper 104 and loads it on different subcarriers having orthogonality in the frequency domain, and maps the subcarrier carrying information back to the time domain. To print.

As described above, conventional transmitters undergo frequency shaping prior to input of the IFFT 104. The transmission bits, which have undergone channel encoding and interleaving, are converted to complex values through a prescribed mapping scheme and then correspond to each subcarrier. In this case, the frequency shaper 104 compensates the distortion occurring in the transmission signal before the IFFT 104 due to the frequency characteristic of the analog device including the filters of the system. That is, the conventional transmitter has to remove the component before transmission and transmit it if the distortion component generated in the signal spectrum due to the channel characteristics affects the performance of the system. In addition, there is a problem that a different frequency shaper must be used according to the modulation scheme of the modulator 103.

Accordingly, an object of the present invention is to provide an apparatus and method for removing distortion components of an OFDM spectrum in an orthogonal frequency division multiplexing communication system.

According to a first aspect of the present invention for achieving the above object, a transmission path in a transmitter of an orthogonal frequency division multiplexing system having a transmission path for frequency shaping a signal converted to a complex value, performing OFDM modulation after frequency modulation; A distortion compensation device for compensating for the spectral distortion of the signal output from the transmission path, the transmission path for outputting the RF signal by the frequency up-adjustment after the OFDM modulation and the specific pattern during the initialization; A feedback circuit connected to the output of the transmission path and frequency-adjusted and digitized the distorted RF signal to generate an OFDM demodulated signal, and then outputting the distorted specific pattern by OFDM demodulation; And a distortion estimator connected to the feedback circuit and generating a distortion compensation coefficient used for the frequency shaping by analyzing the distorted specific pattern.

According to a second aspect of the present invention, a spectrum of a signal output from the transmission path in a transmitter of an orthogonal frequency division multiplexing system having a transmission path for frequency shaping a signal converted into a complex value, performing OFDM modulation after frequency modulation, and then adjusting the frequency up A method for compensating for distortion may include: performing OFDM modulation on a specific pattern during initialization; Converting the OFDM modulated signal into an RF signal; Converting the RF signal into a baseband signal; OFDM demodulating the baseband signal to generate the distorted specific pattern; And analyzing the distorted characteristic pattern to generate a distortion compensation coefficient.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a diagram showing a transmission apparatus of an orthogonal frequency division multiplexing (OFDM) communication system according to the prior art.

2 is a diagram illustrating a transmission apparatus of an orthogonal frequency division multiple communication system according to an exemplary embodiment of the present invention.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. In describing the present invention, if it is determined that the detailed description of the related known function or configuration may unnecessarily obscure the subject matter of the present invention, the detailed description thereof will be omitted.

In the present invention described below, the distortion compensation coefficient of the transmission signal is obtained after analyzing the filter effect of the system itself in each initialization process before performing the communication in an orthogonal frequency division multiplexing (OFDM) communication system, and using the distortion compensation coefficient for frequency shaping. It is about the method of compensating for predistortion.

2 illustrates a transmission apparatus of an orthogonal frequency division multiple communication system according to an exemplary embodiment of the present invention.

Referring to FIG. 2, the pattern generator 201 generates a specific pattern for obtaining a distortion coefficient by an analog device or the like in the system. For example, all 0's or all 1's of a predetermined size are generated. The shaper 202 compensates for the distortion generated in the transmission signal by multiplying the specific pattern from the pattern generator 201 and the previously stored distortion compensation coefficient during communication. Here, the frequency shaper 202 compensates for distortion occurring in the transmission signal before the IFFT due to the frequency characteristics of the analog device including the low pass filter, the band pass filter, and the like of the system. That is, the frequency shaper 202 pre-distorts and outputs a signal (see FIG. 1) converted into a complex value with an estimated distortion compensation coefficient.

Meanwhile, according to the present invention, the frequency shaper 202 passes the specific pattern to the OFDM modulator 209 without performing distortion compensation while performing initialization before performing communication. Then, when a distortion compensation coefficient is provided from the distortion estimator 210, the actual transmission signal is multiplied by the distortion compensation coefficient to remove the distortion component and provided to the OFDM modulator 203.

The OFDM modulator 203 inverses the fast compensator of the distortion-compensated signal from the frequency corrector 104, loads them on different subcarriers having orthogonality in the frequency domain, and maps the subcarriers carrying information back to the time domain. A digital analog converter (DAC) and a low pass filter (LPF) 204 convert the output from the OFDM modulator 203 into an analog signal, filter it, and output a baseband signal. The frequency up converter 205 converts the output from the DAC / LPF 204 into an intermediate frequency and then up-converts the radio frequency signal to output the radio frequency signal.

The switch 206 switches the output of the frequency raising converter 205 to the frequency lowering converter 207 for a predetermined time during the initialization operation before full communication under the control of an upper controller (not shown) according to the present invention. In other words, it switches to a feedback circuit according to the invention. Here, the feedback circuit uses a normal receiving path. On the other hand, in the normal mode, the output of the frequency rising converter 205 is switched to the antenna.

The frequency down converter 207 converts and outputs a baseband signal by frequency downping the radio frequency signal received through the switch 206. An analog digital converter (ADC) and a low pass filter (LPF) 208 filter the signal from the frequency down converter 207, convert it into a baseband complex signal, and output it. The OFDM demodulator 209 demodulates the OFDM signal from the ADC / LPF 208 and converts each subcarrier in the frequency domain into original chip data for output. Here, the data demodulated by the OFDM demodulator 209 becomes a specific pattern generated by the pattern generator 201.

The distortion estimator 210 analyzes the data from the OFDM demodulator 209 to estimate the distortion level by an analog device in the system, and generates a distortion compensation coefficient according to the estimated distortion level. The distortion compensation coefficient thus obtained is provided to the frequency shaper 202, and the frequency shaper 202 compensates the distortion generated in the transmission signal with the distortion compensation coefficient. In this case, since the distortion estimator 210 can use the equalizer of the handset as it is, no additional hardware is required.

The operation based on the configuration of FIG. 2 is as follows.

The present invention analyzes the filter effect of the system itself at every system initialization process before performing communication and compensates for the distortion component according to the frequency shaping. That is, OFDM modulation is performed after compensating for attenuation that appears toward both ends of the bandwidth through a frequency shaping process.

First, in a full-fledged initialization process, OFDM patterns are modulated by the OFDM modulator 203 and converted into analog signals through the DAC 204. The analog OFDM signal is converted into an RF signal through an IF conversion and an RF conversion 205 in the frequency rising converter 204. The RF signal is input to the receiver via a switch 206. The switch 206 used at this time operates only during the initialization process of the system for obtaining the distortion compensation coefficient.

The signal input to the receiver (or feedback circuit) is converted into an OFDM signal which is a baseband complex signal through the frequency down converter 207 and the ADC / LPF 208. The OFDM signal is demodulated in a specific pattern transmitted by the transmitter in the OFDM demodulator 209. Here, the demodulated specific pattern has a distortion component. Accordingly, the distortion estimator 210 analyzes the specific pattern to estimate the distortion degree due to system filter characteristics, and obtains a distortion compensation coefficient based on the estimated distortion degree. The distortion compensation coefficient is then provided to the frequency shaper 202. Then, in the subsequent communication process, the frequency shaper 202 predistorts the transmission signal using the distortion compensation coefficient and then provides it to the OFDM modulator (IFFT).

Meanwhile, in the detailed description of the present invention, specific embodiments have been described, but various modifications are possible without departing from the scope of the present invention. Therefore, the scope of the present invention should not be limited to the described embodiments, but should be determined not only by the scope of the following claims, but also by those equivalent to the scope of the claims.

As described above, according to the present invention, a specific pattern is passed through a transmission path of a system for each initialization process before communication is performed, and a specific pattern that has passed the transmission path is demodulated and analyzed by a reception path, thereby causing an analog element included in the system. The signal distortion component is obtained, and the transmission signal is frequency-shaped with the distortion component and transmitted.

That is, the distortion component of the signal spectrum generated by the analog element of the transmission path can be removed, thereby improving system performance. In addition, the present invention has an advantage that can be applied to all systems regardless of the modulation scheme. On the other hand, all the configurations according to the present invention have the advantage that the present invention can be easily applied in the existing system without additional configuration by using the existing configuration.

Claims (7)

A transmitter for an orthogonal frequency division multiplexing system having a transmission path for frequency shaping a signal converted into a complex value, performing OFDM modulation, and then adjusting the frequency up, the distortion compensation device for compensating for spectral distortion of the signal output from the transmission path. In The transmission path for outputting an RF signal by frequency up-adjusting the OFDM-modulated and analogized specific pattern during initialization; A feedback circuit connected to the output of the transmission path and frequency-adjusted and digitized the distorted RF signal to generate an OFDM demodulated signal, and then outputting the distorted specific pattern by OFDM demodulation; And a distortion estimator coupled to the feedback circuit, the distortion estimator generating a distortion compensation coefficient used for the frequency shaping by analyzing the distorted specific pattern. The method of claim 1, wherein the transmission path, A channel encoder for channel coding the transmission data, An interleaver for interleaving the output from the channel encoder; A modulator for converting the output from the interleaver into a complex value by a prescribed mapping scheme; A frequency shaper for distortion-compensating the output from the modulator; An IFFT for OFDM modulating the output from the frequency corrector, And a radio unit for frequency-adjusting the output from the IFFT. The method of claim 2, And the frequency shaper pre-distorts the output of the modulator using the distortion compensation coefficients to remove distortion components of the OFDM spectrum. The method of claim 1, wherein the feedback circuit, A radio unit for converting the RF signal into a baseband signal by adjusting the frequency down; An ADC for converting the output from the radio into a digital signal; And a duplex period for OFDM demodulating the output from the ADC to output the specific pattern. An apparatus for generating a distortion compensation coefficient for compensating distortion of an OFDM spectrum in an orthogonal frequency division multiple communication system, A modulator for OFDM modulating a specific pattern during initialization; A frequency up converter for outputting an RF signal by frequency up-adjusting the output from the modulator; A frequency down converter for converting the RF signal from the radio into a baseband signal; An OFDM demodulator for OFDM demodulating the baseband signal; And a distortion estimator for analyzing the OFDM demodulated signal to generate a distortion compensation coefficient. A method for compensating for spectral distortion of a signal output from a transmission path in a transmitter of an orthogonal frequency division multiple communication system having a transmission path for frequency shaping a signal converted into a complex value, performing OFDM modulation, and then adjusting the frequency up; Performing OFDM modulation on a specific pattern during initialization; Converting the OFDM modulated signal into an RF signal; Converting the RF signal into a baseband signal; OFDM demodulating the baseband signal to generate the distorted specific pattern; Analyzing the distorted characteristic pattern to generate a distortion compensation coefficient. The method of claim 6, And said distortion compensation coefficient is used for frequency shaping the signal converted into said complex value to remove distortion components of OFDM spectrum.