JP2012023670A - Receiver for ofdm transmission system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To solve the conventional problems of involving great increase of calculation amount, because of required analysis in a frequency region where a notch occurs, for removing a frequency selective fading and of no easy system replacement in an improvement method requiring modification in the transmitter side.SOLUTION: A receiver for orthogonal frequency division multiplexing (OFDM) transmission system is characterized in that: there is a time-domain adaptive equalization unit performing adaptive equalization processing in a time-domain prior to a synchronous processing unit converting to a base band signal; and the time-domain adaptive equalization unit has an FIR filter with a weighting coefficient calculated by Godard algorithm.

Description

The present invention relates to wireless communication and is a technology applicable to a receiving apparatus in OFDM (Orthogonal Frequency Division Multiplexing) communication.

In wireless communication, a signal transmitted from an antenna often has a multipath environment in which a plurality of propagation paths exist due to, for example, reflection of a building or a natural object. In this case, since the environment differs depending on the propagation path, the intensity, phase, and transmission time of the signal reaching the receiver change depending on the propagation path, resulting in signal distortion.

In wideband wireless communication, frequency selective fading occurs due to the multipath, and the reception level decreases at the frequency where the fading occurs and its surrounding frequencies. The quality of communication using will deteriorate.

In general single-carrier broadband wireless communication, when frequency selective fading occurs, all data in the band is affected and communication quality deteriorates. In the worst case, all data can be lost.

On the other hand, in multi-carrier broadband wireless communication, when the frequency selective fading occurs, the data portion transmitted using subcarriers included in the frequency region (notch region) in which the reception level locally decreases due to the fading, Degradation of communication quality mainly occurs.

The most common means for avoiding such deterioration of communication characteristics due to frequency selective fading in OFDM communication is equalization processing. That is, pilot subcarriers are embedded at specific intervals among a plurality of subcarriers in one OFDM symbol, and the propagation path of the pilot subcarriers is estimated in the frequency domain, and data subcarriers between the pilot subcarriers are estimated. For, equalization processing in the time direction and frequency direction is performed by interpolating the obtained propagation path estimation results of pilot subcarriers.

However, with this method, if pilot subcarriers come into the notch region, an error occurs in the equalization process (see Non-Patent Document 1).

Further, for example, in Patent Document 1, two types of OFDM signals are generated and synthesized while controlling the phase, thereby avoiding deterioration of communication characteristics due to frequency selective fading.

JP 2010-74417 A

(Nobuyuki Miki, Digital Terrestrial Television Broadcasting, Trikes)

According to the above prior art, the invention must be applied to each of the transmission side and the reception side, and the replacement of the device is not easy. Further, as a guideline for removing frequency selective fading, analysis in a frequency region where a notch occurs is required, which causes a problem that a calculation amount is significantly increased.

Further, as described above, there is a problem that an error occurs in the equalization processing when the pilot subcarrier comes to the notch region.

In order to solve the above-mentioned problems, the present invention takes the following means to smooth the notch. In other words,
In a receiving apparatus in an orthogonal frequency division multiplexing (OFDM) transmission system,
A receiving apparatus in an OFDM transmission system having a time domain adaptive equalization unit that performs adaptive equalization processing in a time domain before a synchronization processing unit that converts to a baseband signal.

That is, first, in the receiver, OFDM demodulation is measured after the multipath component is removed or reduced by the adaptive equalizer in the time domain.

The present invention also provides
The time domain adaptive equalization unit is a receiving apparatus in the OFDM transmission system, which has an FIR filter in which a weighting factor is calculated by a Godard algorithm.

That is, secondly, as a means for updating the FIR filter in the adaptive equalizer, a Godard algorithm that can significantly reduce the amount of calculation is applied, although it is not an OFDM signal constant line signal.

According to the present invention, since timing synchronization is measured based on a signal obtained by flattening notches due to multipath, correlation peaks can be detected with high accuracy and the CN ratio can be improved.

Further, according to the present invention, adaptive processing can be realized with high accuracy and a low amount of computation.

It is a block diagram of the conventional OFDM receiver. It is a block diagram of the synchronizing part of an OFDM receiver. It is a block diagram including the present invention. It is a figure of the FIR filter used by this invention. It is an experimental result by this invention.

A preferred embodiment of the present invention will be described with reference to the drawings.

A conventional OFDM receiver to which the present invention is applied is shown in FIG. An OFDM reception signal received by the antenna 101 is converted into a baseband signal by the synchronization unit 103. A detailed block diagram of the synchronization unit 103 is shown in FIG.

FIG. 2 shows a configuration in which an OFDM reception signal is quadrature modulated (down-converted) using a cosine wave and a sine wave of a carrier frequency to form a baseband signal. That is, with respect to the oscillator 203 having the center frequency fc, the sine wave and the received signal are mixed by 201a, and the cosine wave and the received signal are mixed by 201b. At this time, a signal having the center frequency 2fc is generated. The signal is removed. The baseband signal is converted into a sampling / digital signal using the analog / digital conversion circuits 209a and 209b, and each signal is transmitted to the next stage.

Next, the Fourier transform unit 105 converts the data into frequency domain data, and the primary demodulation unit 107 performs symbol detection to convert the data into binary data. Further, the frequency domain equalization unit 109 performs channel estimation. In general, the guard interval (redundant signal: CP) is composed of a copy of the end portion of the OFDM symbol, and since the OFDM symbol is transmitted after the CP and the channel characteristics can be considered flat when viewed in units of subcarriers. The frequency domain equalization unit 109 may be simple, and the frequency domain equalization unit does not perform complicated equalization processing according to the situation.

The gist of the present invention is to insert a time-domain equalization unit 301 into the conventional OFDM receiver as described above, as shown in FIG.

That is, in the OFDM receiver, the time-domain OFDM signal is input to the time-domain adaptive equalization unit 301, thereby flattening notches due to multipaths that appear later in frequency conversion, and the synchronization unit 103 performs baseband processing. The signal is converted into a signal, and the baseband signal is Fourier-transformed at 105 to obtain an OFDM signal in the frequency domain. Then, primary demodulation is performed at 107, and the result is input to the frequency domain equalization unit 109. Is obtained. As described above, by using the time domain adaptive equalization unit, the flattened signal can be demodulated and an improvement in the CN ratio (Carrier to Noise ratio) can be expected.

Next, the configuration and operation of the time domain adaptive equalizer will be described with reference to FIG. Reference numerals 411 to 41N denote delay units, 421 to 42N denote weighting coefficients, 43 denotes an adder, and 44 denotes an adaptive processing unit, which includes a filter unit composed of an FIR filter and an adaptive processing unit that updates the coefficient of the FIR filter. Has been.

The FIR filter is a general-purpose filter, and delay devices 411 to 41N multiply weighting coefficients 421 to 42N respectively for the latest signal and a total of N signals from one time before to N-1 times before. The summation is calculated by the adder 43, and the weighting coefficient is updated by the adaptive processing unit 44 so as to minimize the error between the addition result and the signal input at the next time.

Here, as an adaptation means of the adaptation processing unit 44, a Godard algorithm (CMA: Constant Modulus Algorithm) is used by utilizing the fact that the time domain OFDM signal is a constant square line signal.

The Godard algorithm is based on the assumption that a transmission signal is a constant envelope signal such as a frequency modulation (FM) wave or a phase modulation (PM) wave, and the maximum of the DU ratio (Desired to Unsigned signal ratio). The most important feature is that it is a blind algorithm that does not require a replica of a desired signal. Therefore, this Godard algorithm can be applied to a radio signal demodulator with a simple configuration.

The tap coefficient update formula by the Godard algorithm is given by the following (Equation 1) as is well known. E (n) in (Equation 1) is given by (Equation 2).
(Equation 1)
w (n + 1) = w (n) + μu * (n) e (n)
(Equation 2)
e (n) = y (n) (R− | y (n) | ^ 2)
w (n + 1), w (n): Tap coefficient (vector)
u (n): input signal (vector)
e (n): error signal y (n): equalized output signal {y (n) = w * (n) u (n) = yi + jyq}
R: Constant μ: Step size “*” represents a complex conjugate.

Here, R is a constant, and R can be obtained from the average power of the time-domain OFDM wave, and the adaptation process can be realized by a simple expression.

Note that the step size μ changes the speed approaching the optimum value and the residual error amount depending on this value. Generally, the larger the value, the faster the convergence, but the larger the residual error, the smaller the value, the slower the convergence. However, the residual error is small.

As described above, it is possible to reduce the influence of multipath such as frequency selective fading by regarding the OFDM wave as a constant-angle line signal and applying the time domain adaptive equalization unit including the FIR filter to the OFDM receiver. It can be realized with a calculation amount.

FIG. 5 shows an experimental result by computer simulation when the time domain adaptive equalization unit of the present invention is used. (A) is an experimental result when the time domain adaptive equalization unit is not inserted, and (b) is an experimental result when the time domain adaptive equalization unit is inserted.

As shown in the figure, not only is the notch improved, but the power value is 1. (B) has a power value of 1. for locations where E + 00 is exceeded. It can be seen that it is flattened to E + 00. That is, since the dynamic range of received power is reduced, there is an advantage that the accuracy of various calculations can be increased.

Note that the present invention is to insert a time domain adaptive equalization unit in the OFDM receiver, and the configurations of the Fourier transform unit and the primary demodulation unit, which are the subsequent calculations, are not limited at all.

Further, the order of the FIR filter used in the present invention is arbitrary, and it is not limited at all.

101 ... antenna, 103 ... synchronizing unit,
105 ... Fourier transform unit 107 ... Primary demodulation unit,
109 ... frequency domain equalization unit,
201a, 201b ... mixer, 203 ... oscillator,
205 ... Phase shifter, 207a, 207b ... Low pass filter,
209a, 209b ... AD converter,
301 ... time domain adaptive equalization unit,
411-41N ... delay device, 421-42N ... weighting coefficient,
43 ... adder, 44 ... adaptive processing section.

Claims (2)

In a receiving apparatus in an orthogonal frequency division multiplexing (OFDM) transmission system,
A receiving apparatus in an OFDM transmission system, comprising a time domain adaptive equalization unit that performs adaptive equalization processing in the time domain before a synchronization processing unit that converts to a baseband signal. The receiving apparatus in the OFDM transmission system, wherein the time domain adaptive equalization unit includes an FIR filter in which a weighting factor is calculated by a Godard algorithm.