CN104717174A - Orthogonal frequency division multiplexing (OFDM) anti-interference synchronization method under complex multipath channel - Google Patents

Orthogonal frequency division multiplexing (OFDM) anti-interference synchronization method under complex multipath channel Download PDF

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CN104717174A
CN104717174A CN201510138262.9A CN201510138262A CN104717174A CN 104717174 A CN104717174 A CN 104717174A CN 201510138262 A CN201510138262 A CN 201510138262A CN 104717174 A CN104717174 A CN 104717174A
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correlation
ofdm
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CN104717174B (en
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甘华强
王军
周鹏
赵阳
陈亚丁
李少谦
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University of Electronic Science and Technology of China
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L27/00—Modulated-carrier systems
    • H04L27/26—Systems using multi-frequency codes
    • H04L27/2601—Multicarrier modulation systems
    • H04L27/2647—Arrangements specific to the receiver only
    • H04L27/2655—Synchronisation arrangements
    • H04L27/2662—Symbol synchronisation
    • H04L27/2663—Coarse synchronisation, e.g. by correlation
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L27/00—Modulated-carrier systems
    • H04L27/26—Systems using multi-frequency codes
    • H04L27/2601—Multicarrier modulation systems
    • H04L27/2647—Arrangements specific to the receiver only
    • H04L27/2655—Synchronisation arrangements
    • H04L27/2657—Carrier synchronisation

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  • Engineering & Computer Science (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • Signal Processing (AREA)
  • Mobile Radio Communication Systems (AREA)
  • Synchronisation In Digital Transmission Systems (AREA)

Abstract

The invention relates to the field of communications, in particular to an orthogonal frequency division multiplexing (OFDM) anti-interference synchronization method under a complex multipath channel with strong interference signals. According to the method, an existing OFDM synchronization scheme is not suitable for a strong-interference complex multipath environment any more, and thus the OFDM anti-interference synchronization method under the complex multipath channel is designed. The method includes the specific steps that a sequence C(k), c(k) and [x(k) and x(k)] is generated; a sliding correlation value is solved; timing synchronization is conducted; frequency offset estimation is carried out. Compared with an existing synchronization method, by the adoption of the scheme, the OFDM symbol timing and frequency estimation accuracy under the complex multipath environment can be improved, and the anti-interference performance of the system is improved.

Description

OFDM anti-interference synchronous method under a kind of complicated multipath channel
Technical field
The present invention relates to the communications field, especially for the complicated multipath channel that there is high reject signal.
Background technology
It is high that OFDM (Orthogonal Frequency Division Multiplexing, OFDM) technology has the availability of frequency spectrum, and the features such as anti-fading ability is strong, have become the technological core of next-generation mobile communications at present.Synchronous error is the principal element affecting ofdm system performance, particularly in complicated multi-path environment synchronous.Existing document has carried out large quantity research to OFDM simultaneous techniques, classifies respectively from different angles to OFDM.According to synchronous function, be divided into Timing Synchronization, carrier frequency synchronization and sampling clock synchronous; Divide according to synchronous scope and precision, be divided into thick synchronous and thin synchronous; According to whether dividing by auxiliary data, be divided into data assist in synchronization method and blind synchronization method.Wherein, data assist in synchronization method synchronization accuracy is higher and computation complexity is lower, but, need to add training sequence, reduce data transmission efficiency.In data assist in synchronization method, document 1 " Robust frequency and timing synchronization for OFDM (by Schmidl T.M; Cox.D.C.IEEE Trans.Commun; 1997,45 (12): 1613-1621.) " first proposed and utilizes two OFDM symbol to carry out time and Frequency Synchronization as training sequence.The first half of first OFDM symbol is identical with later half, can be used for time synchronized and frequency precise synchronization, utilizes former and later two intersymbol relations to carry out coarse frequency synchronization.Document 2 " Pilot assisted channel estimation for OFDM in mobile cellular systems (by F.Tufvesson; T.Maseng.IEEE VTC; vol.3; pp.1639-1643; May 4-7; 1997.) " proposes to utilize pseudo noise (Pseudo-Noise, PN) sequence is as training sequence, Received signal strength is relevant with local sequence, the correlator results of this method has larger output peak value, find sync bit by maximum value search, but the higher amount of calculation of accuracy is larger.Document 3 " A technique for orthogonal frequency division multiplexingfrequency offset correction (by Paul H.Moose.IEEETrans.Commun, 1994, 42 (10): 2908-2914) " propose to send two OFDM sequences repeated at transmitting terminal, utilize the phase difference of these two repetitive sequences to estimate frequency deviation at receiving terminal, the frequency offset estimation accuracy of this method is higher, but the estimation range of frequency deviation is not more than 0.5 subcarrier spacing, if shorten the time cycle of training sequence, frequency offset estimation range can be increased, but estimated accuracy will reduce.
Simultaneously, in complicated multi-path environment, existing OFDM synchronized algorithm faces two severe problems: existing synchronized algorithm is all often in searching relevant peaks maximum when Timing Synchronization, the original position of data window is determined by maximum value position, but, in multidiameter fading channel, the footpath that energy is the strongest is not often Article 1 footpath, can navigate on other footpaths in non-Article 1 footpath when using existing synchronized algorithm, call sign original position is made to move to valid data part, and using the position of current valid data part as fast Fourier change (the Fast Fourier Transformation in OFDM demodulation, FFT) window original position, thus lose the partial data of OFDM symbol head, and afterbody will comprise the data of a rear OFDM symbol, introduce serious intersymbol interference (InterSymbol Interference, and inter-sub-carrier interference (Inter Carrier Interferece ISI), ICI), existing synchronized algorithm lacks interference protection measure, and when there is stronger interference signal, system just cannot work.
Summary of the invention
The present invention is directed to the defect that traditional OFDM synchronous method cannot normally work in the complicated multi-path environment of strong jamming, propose the OFDM anti-interference synchronous method under a kind of complicated multipath channel, described method can not only improve the synchronization accuracy in multidiameter fading channel, also substantially increases the antijamming capability of system.
An OFDM anti-interference synchronous method under complicated multipath channel, concrete steps are as follows:
S1, formation sequence C (k), c (k) and [x (k), x (k)], specific as follows:
S11, generate frequency domain sequence C (k) and time domain sequences c (k) at receiving terminal, wherein, k represents a kth sampling point of sequence, and the scope of k is [0, N zc-1], N zcrepresent sequence length;
S12, windowing process is carried out to time domain sequences c (k) described in S11, obtain x (k)=c (k) w (k), wherein, the time domain sequences that x (k) launches for transmitting terminal, the window function of w (k) for using in windowing process;
S13, launch x (k) sequence [x (k), x (k)] described in two S12 as main synchronizing sequence at transmitting terminal;
S2, calculate Received signal strength r (k) and the slip correlation of local synchronization sequence at receiving terminal, correlation value calculation formula is wherein, p (k) represents the slip correlation in kth moment, x *k () expression gets conjugation to x (k), described local synchronization sequence is identical with x (k) sequence described in S12;
S3, carry out Timing Synchronization, specific as follows:
S31, sliding average p is asked to correlation described in S2 meank (), the length of moving average window is N zc+ 1;
S32, threshold value K is set, wherein, the different and change in [4,20] according to signal to noise ratio of the value of K;
S33, by correlation p (k) at sliding window two ends in S31 and p (k+N zc) with S31 described in p meank () compares, if p (k)>=K × p mean(k) and p (k+N zc)>=K × p meank (), then detect relevant peaks, current location is set to timing synchronization position, is denoted as M, forwards S35 to, if do not satisfy condition, then forwards S34 to;
S34, make k=k+1, repeat step S33;
S35, threshold value K described in S32 to be regulated, make M described in S33 in position, Article 1 footpath;
S4, frequency deviation are estimated, specific as follows:
S41, set receiving terminal to receive main synchronizing sequence as r=[y 1(k) y 2(k)], wherein, n is that the IFFT in ofdm system counts;
S42, according to S2, correlation value calculation formula is correlation value calculation is carried out to r described in S41, obtains the phase difference of correlation wherein, M < m < M+N zc;
S43, carry out frequency deviation estimation according to the phase difference of the correlation of the Lt after Article 1 footpath, wherein, M < m < M+Lt, Lt is the maximum multipath time delay of signal.
Further, frequency domain sequence C (k) described in S11 and time domain sequences c (k) select Zadoff-Chu (ZC) sequence in permanent envelope zero auto-correlation (CAZAC) sequence, and the expression formula of described ZC sequence is by the interference signal place frequency range corresponding points zero setting in ZC sequence, obtain C (k)=C f(k) J (k), described C (k) is made inverse fast Fourier transform (Inverse Fast Fourier Transform, IFFT) time domain sequences c (k)=IFFT (C (k)) is converted into, wherein, r is radical exponent, r=N zc-1, J (k) is frequency spectrum perception vector,
Further, described in S12 w ( k ) = a 0 - a 1 cos ( 2 &pi;k N - 1 ) + a 2 cos ( 4 &pi;k N - 1 ) - a 3 cos ( 6 &pi;k N - 1 ) , Wherein, a 0=0.3635819, a 1=0.4891775, a 2=0.1365995, a 3=0.0106411.
Further, K=5 described in S32.
Further, described in S31, the length of moving average window is N zc+ 1-2L, wherein, L represents an edge L data of described moving average window.
The invention has the beneficial effects as follows:
By arranging relative threshold, can be synchronized to more exactly near Article 1 footpath, being judged by the correlation of two segment sync sequences simultaneously, improve the accuracy of Timing Synchronization.
Avoid interference place frequency range at frequency domain, and windowing inhibits the impact of interference on frequency range used, so native system has stronger antijamming capability.Utilize relevant peak-to-peak phase difference when carrying out frequency deviation and estimating, improve the tolerance to Timing Synchronization deviation, and the impact of interference-free signal, reasonably select sequence of correlation values length can the impact of noise decrease simultaneously, improve frequency deviation estimated performance.
Accompanying drawing explanation
Fig. 1 is that local synchronization sequence generates schematic diagram.
Fig. 2 is that Timing Synchronization relevant peaks searches schematic diagram.
Fig. 3 is that under glitch-free Rayleigh channel, frequency deviation estimates mean square error.
Fig. 4 is that under noisy Rayleigh channel, frequency deviation estimates mean square error.
Embodiment
Below in conjunction with embodiment and accompanying drawing, describe technical scheme of the present invention in detail.
The present embodiment adopts Matlab emulation platform to carry out running experiment.
In embodiment, synchronization parameter is as follows: ZC sequence length is 512, channel sample frequency is 10MHz, channel model is Rayleigh channel, its wireless channel and COST207 urban channel similar, direct projection footpath is not had in channel, disperse component time-delay power spectrum is exponential damping type, and maximum delay is 7 μ s, and disperse Doppler power spectra is that typical Jakes composes.In channel, institute's plus noise is additive white Gaussian noise, and added interference is the partial-band jamming of 20% frequency range.
Step 1: synchronizing sequence generates.
Produce ZC sequence C (k) at receiving terminal frequency domain, c (k) and [x (k), x (k)], the expression formula of described ZC sequence is in order to the signal that avoids interference, by the frequency range corresponding points zero setting of interference signal place, obtain C (k)=C fk () J (k), is IFFT to frequency domain sequence and is converted to time domain sequences c (k)=IFFT (C (k)), to time domain sequences windowing, obtain x (k)=c (k) w (k).Windowing disturbs the impact on useful frequency range to suppress, and institute's windowing is Blackman-Nuttall window.Transmitting terminal launches two sections of synchronizing sequences identical with the time domain sequences after windowing, and to be used as temporal frequency synchronous.
Step 2: correlation value calculation.
Produce one section of local sequence identical with the time domain sequences after windowing in this locality, to carry out slip relevant to the signal that receiver receives.Correlation value calculation formula is wherein, p (k) represents the slip correlation in kth moment, x *k () expression gets conjugation to x (k)
Step 3: Timing Synchronization.
Be N by correlation by a length zcthe moving average window of+1=513, calculates the sliding average p of correlation in moving average window mean(k).Bringing impact in order to reduce tufted correlation to mean value, not doing average treatment to L data of moving average window edges at two ends, the actual scope of averaging is the N in the middle of moving average window zc+ 1-2L data.
Threshold value K is set, wherein, value change in [4,20] according to signal to noise ratio difference of K.
When the correlation of moving average window edges at two ends is greater than the K times of mean value in window, can think and synchronous head be detected, current time just can be used as synchronization point.Time in the present embodiment by adjustment doors limit value K=5, sync bit is in position, Article 1 footpath.
Step 4: frequency deviation is estimated.
Due to the strongest footpath can be led in complicated multi-path environment to be no longer often Article 1 footpath, so, search according to the maximum of relevant peaks the method determining timing synchronization position in existing synchronized algorithm no longer applicable.
There will be two relevant peaks, the N after these two relevant peaks because synchronizing sequence is relevant to Received signal strength zcindividual correlation has a fixing phase difference, utilizes this phase difference to carry out frequency deviation estimation.Maximum multipath time delay due to signal is 7 μ s, i.e. 70 sample points, and correlation energy mainly concentrates on 70 correlations after the first footpath, and 70 correlations after correlation position, place, time synchronized determined Article 1 footpath can be utilized to ask phase difference.
Export after utilizing the frequency deviation estimated to carry out compensate of frequency deviation to the signal after time synchronized and do input demodulation.
Adopt the method for the invention to carry out emulation testing, the frequency deviation under Rayleigh channel estimates mean square error as shown in Figure 3 and Figure 4.The method that proposes in the present invention and traditional Schmidl method is utilized to contrast, frequency deviation in glitch-free situation is estimated, the Performance comparision of two kinds of methods is close, and when system exists interference, because conventional method lacks interference protection measure, when jamming-to-signal ratio JSR is 0dB, the frequency offset error that Schmidl method estimates is very large, can think sync fail, and for the method proposed in the present invention, even if when JSR is 40dB, differ also very little with not having noisy system synchronization performance, native system has stronger antijamming capability as can be seen here.

Claims (5)

1. the OFDM anti-interference synchronous method under complicated multipath channel, is characterized in that, comprise the steps:
S1, formation sequence C (k), c (k) and [x (k), x (k)], specific as follows:
S11, generate frequency domain sequence C (k) and time domain sequences c (k) at receiving terminal, wherein, k represents a kth sampling point of sequence, and the scope of k is [0, N zc-1], N zcrepresent sequence length;
S12, windowing process is carried out to time domain sequences c (k) described in S11, obtain x (k)=c (k) w (k), wherein, the time domain sequences that x (k) launches for transmitting terminal, the window function of w (k) for using in windowing process;
S13, launch x (k) sequence [x (k), x (k)] described in two S12 as main synchronizing sequence at transmitting terminal;
S2, calculate Received signal strength r (k) and the slip correlation of local synchronization sequence at receiving terminal, correlation value calculation formula is wherein, p (k) represents the slip correlation in kth moment, x *k () expression gets conjugation to x (k), described local synchronization sequence is identical with x (k) sequence described in S12;
S3, carry out Timing Synchronization, specific as follows:
S31, sliding average p is asked to correlation described in S2 meank (), the length of moving average window is N zc+ 1;
S32, threshold value K is set, wherein, the different and change in [4,20] according to signal to noise ratio of the value of K;
S33, by correlation p (k) at sliding window two ends in S31 and p (k+N zc) with S31 described in p meank () compares, if p (k)>=K × p mean(k) and p (k+N zc)>=K × p meank (), then detect relevant peaks, current location is set to timing synchronization position, is denoted as M, forwards S35 to, if do not satisfy condition, then forwards S34 to;
S34, make k=k+1, repeat step S33;
S35, threshold value K described in S32 to be regulated, make M described in S33 in position, Article 1 footpath;
S4, frequency deviation are estimated, specific as follows:
S41, set receiving terminal to receive main synchronizing sequence as r=[y 1(k) y 2(k)], wherein, n is that the IFFT in ofdm system counts;
S42, according to S2, correlation value calculation formula is correlation value calculation is carried out to r described in S41, obtains the phase difference of correlation wherein, M < m < M+N zc;
S43, carry out frequency deviation estimation according to the phase difference of the correlation of the Lt after Article 1 footpath, wherein, M < m < M+Lt, Lt is the maximum multipath time delay of signal.
2. the OFDM anti-interference synchronous method under a kind of complicated multipath channel according to claim 1, it is characterized in that: frequency domain sequence C (k) described in S11 and time domain sequences c (k) select Zadoff-Chu (ZC) sequence in permanent envelope zero auto-correlation (CAZAC) sequence, and the expression formula of described ZC sequence is by the interference signal place frequency range corresponding points zero setting in ZC sequence, obtain C (k)=C f(k) J (k), described C (k) is made inverse fast Fourier transform (Inverse Fast Fourier Transform, IFFT) time domain sequences c (k)=IFFT (C (k)) is converted into, wherein, r is radical exponent, r=N zc-1, J (k) is frequency spectrum perception vector,
3. the OFDM anti-interference synchronous method under a kind of complicated multipath channel according to claim 1, is characterized in that: described in S12 w ( k ) a 0 - a 1 cos ( 2 &pi;k N - 1 ) + a 2 cos ( 4 &pi;k N - 1 ) - a 3 cos ( 6 &pi;k N - 1 ) , Wherein, a 0=0.3635819, a 1=0.4891775, a 2=0.1365995, a 3=0.0106411.
4. the OFDM anti-interference synchronous method under a kind of complicated multipath channel according to claim 1, is characterized in that: K=5 described in S32.
5. the OFDM anti-interference synchronous method under a kind of complicated multipath channel according to claim 1, is characterized in that: described in S31, the length of moving average window is N zc+ 1-2L, wherein, L represents an edge L data of described moving average window.
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CN110071888A (en) * 2019-02-28 2019-07-30 成都坤恒顺维科技股份有限公司 A kind of rapid time synchronous method in high-speed digital transmission
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CN106713196A (en) * 2015-11-16 2017-05-24 晨星半导体股份有限公司 Estimable frequency deviation receiving circuit and related method
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CN109561042A (en) * 2018-12-17 2019-04-02 电子科技大学 A kind of timing frequency synchronous method of ofdm system receiver
CN110071888A (en) * 2019-02-28 2019-07-30 成都坤恒顺维科技股份有限公司 A kind of rapid time synchronous method in high-speed digital transmission
CN110933007A (en) * 2019-08-20 2020-03-27 熊军 Device and method for eliminating interference aiming at OFDM broadband signals
CN111416782A (en) * 2020-03-18 2020-07-14 华南理工大学 OFDM system frequency offset estimation analysis method based on null carrier
CN111416782B (en) * 2020-03-18 2021-10-26 华南理工大学 OFDM system frequency offset estimation analysis method based on null carrier

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