CN1885845A - Non-data assistant orthogonal frequency-division multiplexing system frequency offset tracing method - Google Patents

Abstract

The non-data-assist frequency bias track method for OFDM SYSTEM comprises: to OFDM signal in time domain, synchronizing data frame, synchronous capturing, and removing loop prefix to obtain data signal; converting into frequency-domain carrier signal by FFT, multiplying the sub-carrier signal and compensator output to obtain the modified signal; demodulating and estimating phase difference, using low-pass filter to remove noise effect; finally, multiplying the signal with compensation signal. This invention saves hardware resource, improves frequency utility, and convenient to implementation as low complexity.

Description

A kind of non-data-aided orthogonal frequency-division multiplexing system frequency offset tracing method

Technical field

The present invention relates to a kind of frequency deviation tracking method, particularly orthogonal frequency division multiplex OFDM system carries out frame synchronization to the OFDM symbol that time domain receives, synchronization acquistion and go circulation prefix processing, obtain the data-signal of current OFDM symbol correspondence, the current data signal is done fast fourier transform FFT computing become the subcarrier in frequency domain signal, by the phase difference extractor analyzing and processing of this signal is obtained the phase difference estimation signal, utilize the phase difference estimation signal to finish correction to sub-carrier signal, output to next step and carry out demodulation process, return the non-data-aided ofdm system that step a imports next OFDM symbol simultaneously, belong to wireless communication technology field.

Background technology

(Orthogonal Frequency Division Multiplexing OFDM) by mutually orthogonal subcarrier parallel transmission data, is a kind of data transfer mode efficiently to OFDM.Compare with the traditional communication means, adopt the wireless data communication system of OFDM technology to have characteristics such as anti-frequency selective fading, band efficiency height, can satisfy people to transmitting the requirement of data, voice and image simultaneously, so receive people's concern more and more widely.In recent years, OFDM had obtained further investigation and extensive use at wireless communication field, as digital audio broadcasting (DAB), and digital video broadcasting (DVB) and high definition TV (HDTV) etc.The agreement of employing OFDM scheme has the HIPERLAN/2 of IEEE 802.11a and ETSI etc.

OFDM is a kind of multi-carrier transmission technology, it is to very responsive synchronously, owing to Doppler frequency shift, transmit and receive the influence of factors such as the crystal oscillator frequency of machine is inconsistent, the carrier frequency of ofdm system can be offset, cause between subcarrier and disturb, influence the effect of demodulation.So accurate synchronisation measures is extremely important to correct demodulating ofdm signal.

In the ofdm system, the simultaneous techniques of frequency deviation generally is divided into two classes, data ancillary technique and non-data ancillary technique.The data ancillary technique is undertaken having reduced the transmission rate of valid data synchronously by sending known training sequence or pilot signal, thus this structural limitations its range of application.The correlation that is replicated part in non-data ancillary technique general using Cyclic Prefix and the data is carried out synchronously, perhaps utilizes the virtual carrier of OFDM partly to carry out having higher band efficiency synchronously.

The simultaneous techniques of frequency deviation generally is divided into two processes, capturing frequency deviation and frequency offset trackings.The existing a lot of ripe algorithms of capturing frequency deviation process can be used, such as maximal possibility estimation (Maximum Likelihood Estimation, MLE), least mean-square error (Minimize Mean Square Error, MMSE), Schmidl﹠amp; Cox method etc.And aspect frequency offset tracking, existing algorithm major part all needs to send pilot data, and operand is bigger, and band efficiency is low, and software and hardware is realized all more complicated.

Summary of the invention

The present invention seeks to the problems referred to above, the technical scheme of the frequency deviation tracking method of a kind of non-data-aided OFDM (OFDM) digital communication system is provided at the prior art existence.This method does not rely on known training sequence or pilot signal, is not subjected to the restriction of Cyclic Prefix length yet, still can keep good performance when having lower complexity, fully saves the band resource of ofdm system, has extensive applicability.

Technical scheme of the present invention:

At first receiving terminal OFDM symbol that time domain is received carries out frame synchronization, synchronization acquistion and goes circulation prefix processing, obtains the data-signal of current sign correspondence.Secondly data-signal is done the FFT computing, become the sub-carrier signal of frequency domain.With the sub-carrier signal of gained and the signal multiplication of compensating signal generator output, the sub-carrier signal of output through revising.Also input phase difference extractor when entering next step demodulation through the sub-carrier signal of revising carries out the estimation of phase difference.Then the phase signal input low pass filter that extracts is removed The noise, input offset signal generator.At last with compensating signal and FFT computing gained signal multiplication.

Beneficial effect of the present invention: the present invention is a kind of non-data-aided orthogonal frequency-division multiplexing system frequency offset tracing method, it calculates phase difference according to the unique characteristics of the data on each subcarrier, produce corresponding complex signal according to the phase signal of removing noise effect, carry out phase compensation at frequency domain.When programming realized, hardware resource had been saved in getting plural phase bit arithmetic and can reusing existing module of frequency offset tracking part.Because whole, improved band efficiency without the pilot tone auxiliary operation.Compare with the synchronization scenario of existing document record, this method has less operand, lower implementation complexity and preferable performance.Above-mentioned synchronous tracking scheme can be applicable in the various ofdm communication systems.

Description of drawings

Fig. 1 is the frequency offset tracking FB(flow block) in the ofdm system;

The 32QAM planisphere of the untreated signal that is subjected to frequency offset interference when Fig. 2 is signal to noise ratio 30dB;

Pass through the 32QAM planisphere after native system is followed the tracks of processing synchronously during Fig. 3 signal to noise ratio 30dB.

Embodiment

Below in conjunction with accompanying drawing with by embodiment the specific embodiment of the present invention is described further:

This non-data-aided orthogonal frequency-division multiplexing system frequency offset tracing method, its characteristics are: may further comprise the steps:

A. OFDM (OFDM) system OFDM symbol that time domain is received carries out frame synchronization, synchronization acquistion and goes circulation prefix processing, obtains the data-signal of current OFDM symbol correspondence;

B. the current data signal is done fast fourier transform FFT computing and become the subcarrier in frequency domain signal, by the phase difference extractor analyzing and processing of this signal is obtained the phase difference estimation signal, utilize the phase difference estimation signal to finish correction to sub-carrier signal, output to next step and carry out demodulation process, return step a simultaneously and import next OFDM symbol.

Maximal possibility estimation MLE method is adopted in synchronization acquistion among the step a, or other data are auxiliary and non-data-aided synchronization acquiring method.

The sub-carrier signal R that phase difference extractor input among the step b is revised _n', carry out the estimation of phase difference :

Δ＝angle(R _n′)-angle(F(R _n′))

Wherein, angle (x) is for asking the function of independent variable phase place;

F (R _n')=f (real (R _n'))+if (imag (R _n')), f (x) is for taking from the nearest constellation point coordinate function of variable correspondence.

Described method, its operand maximum ask phase function angle (x), when software is realized, need not to write, effective economize on hardware resource is characterized in that: synchronization acquistion is adopted and is met the algorithm that the MLE algorithm described in the claim 2 or other needs utilization are asked phase function angle (x).

Take from the nearest constellation point coordinate function f (x) of variable correspondence, can be expressed as a piecewise function for M-ary orthogonal amplitude modulation(PAM) MQAM:

$f\left(x\right)=\left\{\begin{array}{cc}{k}_{i-2}& x<\frac{3k_{i-1}-k_i}{2},i=2-n\\ k_{i-1}& \frac{3k_{i-1}-k_i}{2}\&le;x<\frac{k_{i-1}+k_i}{2},2-n\&le;i\&le;n\\ k_i& x\&GreaterEqual;\frac{k_{i-1}+k_i}{2},i=n\end{array}\right.$

Wherein, for M-ary orthogonal amplitude modulation(PAM) MQAM, the sub-carrier signal R of correction _n' real part and imaginary part span all be k _i(i=± 1, ± 2, ± 3 ... ± n), the symbol of following target symbology value.

This method comprises that further phase difference  need import low pass filter and remove the noise jamming processing, obtains signal delta  '.

This method further comprises the sub-carrier signal R that the input of phase difference extractor is revised _n' be N point sub-carrier signal R by frequency domain _nSignal C with compensating signal generator output _nGained multiplies each other.

This method further comprises the N point sub-carrier signal R of frequency domain _nBe owing among the b of claim 1, the N point data signal r of current OFDM symbol correspondence _n(n=0,1 ..., N-1) carry out fast fourier transform FFT computing gained.

This method further comprises the signal C of compensating signal generator output _nCan be expressed as:

C _n＝cosΔ′+jsinΔ′。

Embodiment

The present invention is applied in the 32QAM ofdm system, and an OFDM symbol lengths is 16 points.

1, receiving terminal OFDM symbol that time domain is received obtains 16 point data signal r of current sign earlier through frame synchronization, synchronization acquistion with go circulation prefix processing _n(n=0,1 ..., 15).Wherein maximal possibility estimation MLE method is adopted in synchronization acquistion, and its formula is:

Λ(δ′，θ)＝|γ(m)|-ρφ(m)

Wherein $\mathrm{\&gamma;}\left(m\right)=\underset{n=m}{\overset{m+N_G-1}{\mathrm{\&Sigma;}}}{r}_n^{*}\&CenterDot;r_{n+N},$ $\mathrm{\&phi;}\left(m\right)=\frac{1}{2}\underset{n=m}{\overset{m+N_G-1}{\mathrm{\&Sigma;}}}({\left|r_n\right|}^2+{\left|r_{n+N}\right|}^2),$ $\mathrm{\&rho;}=\frac{\mathrm{SNR}}{\mathrm{SNR}+1}$

Timing offset θ, the estimation formulas of decimal frequency bias δ ' is

$\widehat{\mathrm{\&theta;}}=\arg \max \left(\mathrm{\&Lambda;}({\mathrm{\&delta;}}^{\&prime;},\mathrm{\&theta;})\right)$

${\mathrm{\&delta;}}^{\&prime;}=\frac{1}{2\mathrm{\&pi;}}\mathrm{angle}\left(\mathrm{\&gamma;}\left(\widehat{\mathrm{\&theta;}}\right)\right)$

In the following formula, r _n, N, N _GRepresent n sample point, FFT length of window that receiving terminal obtains respectively, CP length.Argmax (Λ (δ ', θ)) expression ask Λ (δ ', the value of parameter m when θ) reaching maximum, the phase place of plural number is asked in angle (x) expression, SNR is a signal to noise ratio, and γ (m) is the correlation that is replicated part in CP and the OFDM symbol, and φ (m) is the energy value of received signal.

2, with r _nCarry out 16 FFT computings, obtain 16 sub-carrier signal R of frequency domain _n

3, with R _nSignal C with compensating signal generator output _nCarry out multiplying, the sub-carrier signal R that output is revised _n'.

4, R _n' input 32QAM demodulator enters the phase difference extractor simultaneously, carries out the estimation of phase difference.

The estimation formulas of phase difference  is as follows:

Δ＝angle(R _n′)-angle(F(R _n′))

Wherein, F (R _n')=f (real (R _n'))+if (imag (R _n'))

For 32QAM, R _n' real part and the span of imaginary part all be k _i(i=± 1, ± 2, ± 3), the symbol of following target symbology value.

F (x) is defined as a piecewise function (n=3):

$f\left(x\right)=\left\{\begin{array}{cc}{k}_{i-2}& x<\frac{3k_{i-1}-k_i}{2},i=2-n\\ k_{i-1}& \frac{3k_{i-1}-k_i}{2}\&le;x<\frac{k_{i-1}+k_i}{2},2-n\&le;i\&le;n\\ k_i& x\&GreaterEqual;\frac{k_{i-1}+k_i}{2},i=n\end{array}\right.$

5, the phase difference estimation value Δ  with output is input to low pass filter elimination The noise.Δ  ' is input to compensating signal generator then, obtains signal C _n:

C _n＝cosΔ′+jsinΔ′

6, with C _nData R with the subcarrier of FFT output _nMultiply each other and export R _n'.

7, return step 1, import 16 point data of next symbol.

Claims (9)

1. non-data-aided orthogonal frequency-division multiplexing system frequency offset tracing method, it is characterized in that: this method may further comprise the steps:

A. OFDM (OFDM) system OFDM symbol that time domain is received carries out frame synchronization, synchronization acquistion and goes circulation prefix processing, obtains the data-signal of current OFDM symbol correspondence;

B. the current data signal is done fast fourier transform FFT computing and become the subcarrier in frequency domain signal, by the phase difference extractor analyzing and processing of this signal is obtained the phase difference estimation signal, utilize the phase difference estimation signal to finish correction to sub-carrier signal, output to next step and carry out demodulation process, return step a simultaneously and import next OFDM symbol.

2. non-data-aided orthogonal frequency-division multiplexing system frequency offset tracing method according to claim 1 is characterized in that: maximal possibility estimation MLE method is adopted in synchronization acquistion among the step a, or other data are auxiliary and non-data-aided synchronization acquiring method.

3. non-data-aided orthogonal frequency-division multiplexing system frequency offset tracing method according to claim 1 is characterized in that: the sub-carrier signal R that the phase difference extractor input among the step b is revised _n', carry out the estimation of phase difference :

Δ＝angle(R _n′)-angle(F(R _n′))

Wherein, angle (x) is for asking the function of independent variable phase place;

F (R _n')=f (real (R _n'))+if (imag (R _n')), f (x) is for taking from the nearest constellation point coordinate function of variable correspondence.

4. method according to claim 3, its operand maximum ask phase function angle (x), when software is realized, need not to write, effective economize on hardware resource is characterized in that: synchronization acquistion is adopted and is met the algorithm that the MLE algorithm described in the claim 2 or other needs utilization are asked phase function angle (x).

5. method according to claim 3 is characterized in that: take from the nearest constellation point coordinate function f (x) of variable correspondence, can be expressed as a piecewise function for M-ary orthogonal amplitude modulation(PAM) MQAM:

$f\left(x\right)=\left\{\begin{array}{cc}{k}_{i-2}& x<\frac{{3k}_{i-1}-k_i}{2},i=2-n\\ k_{i-1}& \frac{{3k}_{i-1}-k_i}{2}\&le;x<\frac{k_{i-1}+k_i}{2},2-n\&le;i\&le;n\\ k_i& x\&GreaterEqual;\frac{k_{i-1}+k_i}{2},i=n\end{array}\right.$

Wherein, for M-ary orthogonal amplitude modulation(PAM) MQAM, the sub-carrier signal R of correction _n' real part and imaginary part span all be k _i(i=± 1, ± 2, ± 3 ... ± n), the symbol of following target symbology value.

6. method according to claim 3 is characterized in that: this method comprises that further phase difference  need import low pass filter and remove the noise jamming processing, obtains signal delta  '.

7. method according to claim 3 is characterized in that: this method further comprises the sub-carrier signal R that the input of phase difference extractor is revised _n' be N point sub-carrier signal R by frequency domain _nSignal C with compensating signal generator output _nGained multiplies each other.

8. method according to claim 7 is characterized in that: this method further comprises the N point sub-carrier signal R of frequency domain _nBe owing among the b of claim 1, the N point data signal r of current OFDM symbol correspondence _n(n=0,1 ..., N-1) carry out fast fourier transform FFT computing gained.

9. method according to claim 7 is characterized in that: this method further comprises the signal C of compensating signal generator output _nCan be expressed as:

C _n=cos Δ  '+jsin Δ  ', wherein Δ  ' comes from claim 6.