CN104717174B - A kind of OFDM anti-interference synchronous methods under complexity multipath channel - Google Patents

A kind of OFDM anti-interference synchronous methods under complexity multipath channel Download PDF

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CN104717174B
CN104717174B CN201510138262.9A CN201510138262A CN104717174B CN 104717174 B CN104717174 B CN 104717174B CN 201510138262 A CN201510138262 A CN 201510138262A CN 104717174 B CN104717174 B CN 104717174B
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correlation
ofdm
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CN104717174A (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)
  • Synchronisation In Digital Transmission Systems (AREA)
  • Mobile Radio Communication Systems (AREA)

Abstract

The present invention relates to the communications field, especially for there are the complicated multipath channel of high reject signal.The present invention is directed to existing Orthogonal Frequency Division Multiplexing (Orthogonal Frequency Division Multiplexing, OFDM the problem of) synchronization scenario is no longer applicable in strong jamming complexity multi-path environment, devise the OFDM anti-interference synchronous methods under a kind of complicated multipath channel, specific steps include formation sequence C (k), c (k) and [x (k), x (k)]；Ask for sliding correlation；It is timed synchronization；Carry out offset estimation.Compared to existing synchronous method, the program can improve OFDM symbol timing and the accuracy of Frequency Estimation in complicated multi-path environment, and improve the interference free performance of system.

Description

A kind of OFDM anti-interference synchronous methods under complexity multipath channel
Technical field
The present invention relates to the communications field, especially for there are the complicated multipath channel of high reject signal.
Background technology
Orthogonal Frequency Division Multiplexing (Orthogonal Frequency Division Multiplexing, OFDM) technology has The features such as availability of frequency spectrum is high, and anti-fading ability is strong, becomes the technological core of next-generation mobile communications at present.Synchronous error is Influence a principal element of ofdm system performance, the particularly synchronization in complicated multi-path environment.Existing literature is to OFDM Simultaneous techniques has carried out numerous studies, classifies from different angles to OFDM respectively.According to synchronous function, it is divided into timing Synchronous, carrier frequency synchronization and sampling clock are synchronous；Divide according to synchronous scope and precision, be divided into thick synchronous and thin synchronous； Divide according to whether by auxiliary data, be divided into data assist in synchronization method and blind synchronization method.Wherein, data assist in synchronization side Method synchronization accuracy is higher and computation complexity is relatively low, however, it is desirable to add training sequence, reduces data transmission efficiency.In number According in assist in synchronization method, " the Robust frequency and timing synchronization for OFDM of document 1 (by Schmidl T.M,Cox.D.C.IEEE Trans.Commun,1997,45(12):1613-1621.) " it first proposed Using two OFDM symbols time and Frequency Synchronization are carried out as training sequence.The first half and later half of first OFDM symbol It is identical, available for time synchronization and frequency precise synchronization, coarse frequency synchronization is carried out using former and later two intersymbol relations.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.) " propose to make an uproar using puppet Sound (Pseudo-Noise, PN) sequence will receive signal and local sequence is related, the correlator of this method as training sequence As a result there is larger output peak value, sync bit is found by maximum value search, accuracy is higher but calculation amount is larger.Document 3“A technique for orthogonal frequency division multiplexing frequency offset correction(by Paul H.Moose.IEEE Trans.Commun,1994,42(10):2908-2914) " propose sending out The OFDM sequences that end sends two repetitions are penetrated, estimate frequency deviation using the phase difference of the two repetitive sequences in receiving terminal, it is this The frequency offset estimation accuracy of method is higher, but the estimation range of frequency deviation is not more than 0.5 subcarrier spacing, if shortening training sequence Time cycle, frequency offset estimation range can be increased, but estimated accuracy will reduce.
Meanwhile in complicated multi-path environment, the problem of existing OFDM synchronized algorithms face two sternnesses：Existing synchronization Algorithm is all often to find related peak maximum in Timing Synchronization, and the start bit of data window is determined by maximum value position Put, still, in multidiameter fading channel, the most strong footpath of energy is frequently not first footpath, meeting when using existing synchronized algorithm Navigating on other footpaths in non-first footpath so that symbol initial position moves to valid data part, and with the currently active data Risen as fast Fourier change (Fast Fourier Transformation, FFT) window in OFDM demodulation partial position Beginning position, so as to lose the partial data on OFDM symbol head, and afterbody will include the data of the latter OFDM symbol, introduce tight The intersymbol interference (Inter Symbol Interference, ISI) of weight and inter-sub-carrier interference (Inter Carrier Interferece, ICI)；Existing synchronized algorithm lacks interference protection measure, when there are during stronger interference signal, system is with regard to nothing Method works.
The content of the invention
The defects of present invention can not be worked normally for traditional OFDM synchronous method in strong jamming complexity multi-path environment, The OFDM anti-interference synchronous methods under a kind of complicated multipath channel are proposed, the method can not only improve multidiameter fading channel In synchronization accuracy, also substantially increase the antijamming capability of system.
A kind of OFDM anti-interference synchronous methods under complexity multipath channel, comprise the following steps that：
S1, formation sequence C (k), c (k) and [x (k), x (k)] are specific as follows：
S11, in receiving terminal generation frequency domain sequence C (k) and time domain sequences c (k), wherein, k represents k-th of sampling point of sequence, The scope of k is [0, Nzc- 1], NzcRepresent sequence length；
S12, carry out windowing process to time domain sequences c (k) described in S11, obtains x (k)=c (k) w (k), wherein, x (k) is The time domain sequences of transmitting terminal transmitting, w (k) are the window function used in windowing process；
S13, transmitting terminal launch two S12 described in x (k) sequences [x (k), x (k)] be used as main synchronizing sequence；
S2, calculate the slip correlation for receiving signal r (k) and local synchronization sequence, correlation value calculation formula in receiving terminal ForWherein, p (k) represents the slip correlation at kth moment, x*(k) represent to take x (k) conjugation, The local synchronization sequence is identical with x (k) sequences described in S12；
S3, be timed synchronization, specific as follows：
S31, seek sliding average p to correlation described in S2mean(k), the length of sliding average window is Nzc+1；
S32, set threshold value K, wherein, the value of K is different according to signal-to-noise ratio and changes in [4,20]；
S33, correlation p (k) and the p (k+N by sliding window both ends in S31zc) with S31 described in pmean(k) it is compared, If p (k) >=K × pmean(k) and p (k+Nzc)≥K×pmean(k), then relevant peaks are detected, current location is set to Timing Synchronization Position, is denoted as M, goes to S35, if being unsatisfactory for condition, goes to S34；
S34, make k=k+1, repeat step S33；
S35, be adjusted threshold value K described in S32, makes M described in S33 in first footpath position；
S4, offset estimation, it is specific as follows：
S41, set the received main synchronizing sequence of receiving terminal as r=[y1(k)y2(k)], wherein,N For the IFFT point number in ofdm system；
S42, correlation value calculation formula is according to S2Correlation is carried out to r described in S41 Calculate, obtain the phase difference of correlationWherein, M ＜ m ＜ M+Nzc；
S43, according to the phase difference of Lt correlation after first footpath carry out offset estimation,Wherein, M ＜ m ＜ M+Lt, Lt are the maximum multipath time delay of signal.
Further, frequency domain sequence C (k) described in S11 and time domain sequences c (k) selects permanent envelope zero auto-correlation (CAZAC) Zadoff-Chu (ZC) sequence in sequence, the expression formula of the ZC sequences areBy in ZC sequences Frequency range corresponding points zero setting where interference signal, obtains C (k)=Cf(k) J (k), makees inverse fast Fourier transform by the C (k) (Inverse Fast Fourier Transform, IFFT) is converted into time domain sequences c (k)=IFFT (C (k)), wherein, r is Radical exponent, r=Nzc- 1, J (k) is frequency spectrum perception vector,
Further, described in S12Wherein, a0= 0.3635819, a1=0.4891775, a2=0.1365995, a3=0.0106411.
Further, K=5 described in S32.
Further, the length of sliding average window described in S31 is Nzc+ 1-2L, wherein, L represents the sliding average window L, edge data.
The beneficial effects of the invention are as follows：
It by setting relative threshold, can relatively accurately be synchronized near first footpath, pass through two segment sync sequences Correlation judges at the same time, improves the accuracy of Timing Synchronization.
The frequency range where frequency domain avoids interference, and adding window inhibits influence of the interference to frequency range used, so the system There is stronger antijamming capability.When carrying out offset estimation using the phase difference between relevant peaks, improve inclined to Timing Synchronization The tolerance of difference, and the influence of interference-free signal, while reasonably selection sequence of correlation values length can reduce the influence of noise, Improve offset estimation performance.
Brief description of the drawings
Fig. 1 generates schematic diagram for local synchronization sequence.
Fig. 2 searches schematic diagram for Timing Synchronization relevant peaks.
Fig. 3 is offset estimation mean square error under glitch-free Rayleigh channel.
Fig. 4 is offset estimation mean square error under noisy Rayleigh channel.
Embodiment
With reference to embodiment and attached drawing, the technical solution that the present invention will be described in detail.
The present embodiment carries out running experiment using Matlab emulation platforms.
Synchronization parameter is as follows in embodiment：ZC sequence lengths are 512, and channel sample frequency is 10MHz, and channel model is auspicious Sharp channel, its wireless channel is similar with COST207 urban channels, does not have direct projection footpath in channel, and disperse component time-delay power spectrum is Exponential damping type, maximum delay are 7 μ s, and disperse Doppler power spectra is composed for typical case Jakes.Institute's plus noise is high for additivity in channel This white noise, added interference are the partial-band jamming of 20% frequency range.
Step 1：Synchronizing sequence generates.
ZC sequence Cs (k) are produced in receiving terminal frequency domain, c (k) and [x (k), x (k)], the expression formula of the ZC sequences areFor the signal that avoids interference, by frequency range corresponding points zero setting where interference signal, C (k)=C is obtainedf (k) J (k), does frequency domain sequence IFFT and is converted to time domain sequences c (k)=IFFT (C (k)), to time domain sequences adding window, obtain x (k)=c (k) w (k).Adding window is that institute's adding window is Blackman-Nuttall windows in order to suppress to disturb the influence to useful frequency range. Transmitting terminal launches two sections of synchronizing sequences identical with the time domain sequences after adding window and is used as temporal frequency synchronization.
Step 2：Correlation value calculation.
In the locally generated one section local sequence identical with the time domain sequences after adding window, with the signal that receiver receives into Row slides related.Correlation value calculation formula isWherein, p (k) represents that the slip at kth moment is related Value, x*(k) represent to take x (k) conjugation
Step 3：Timing Synchronization.
By a length it is N by correlationzcThe sliding average window of+1=513, calculates correlation in sliding average window Sliding average pmean(k).Influence is brought on average value in order to reduce tufted correlation, to the L of sliding average window edges at two ends A data do not do average treatment, and scope of actually averaging is the N among sliding average windowzc+ 1-2L data.
Threshold value K is set, wherein, the value of K is different according to signal-to-noise ratio and changes in [4,20].
When the correlation of sliding average window edges at two ends is more than K times of average value in window, it is believed that detect synchronous head, Current time can be used as synchronization point.When in the present embodiment by adjusting threshold value K=5, sync bit is in first footpath institute In position.
Step 4：Offset estimation.
No longer it is often first footpath since most strong footpath can be led in complicated multi-path environment, so, existing synchronous calculation The method searched in method according to the maximum of relevant peaks to determine timing synchronization position is no longer applicable.
Occur two relevant peaks since synchronizing sequence is related to receiving signal, the N after the two relevant peakszcA correlation Value has a fixed phase difference, and offset estimation is carried out using this phase difference.Since the maximum multipath time delay of signal is 7 μ s, That is 70 sample points, correlation energy are concentrated mainly on 70 correlations after the first footpath, using time synchronization institute really 70 correlations after correlation position where first fixed footpath seek phase difference.
Signal detection demodulation is done in output after carrying out frequency deviation compensation to the signal after time synchronization using the frequency deviation estimated.
Emulation testing is carried out using the method for the invention, offset estimation mean square error such as Fig. 3 under Rayleigh channel and Shown in Fig. 4.Contrasted using the method proposed in the present invention and traditional Schmidl methods, in the case of glitch-free Offset estimation, the performance of two methods relatively, and when system exist disturb when, since conventional method lacks anti-interference arrange Apply, when jamming-to-signal ratio JSR is 0dB, the frequency offset error that Schmidl methods estimate is very big, it is believed that sync fail, And the method for being proposed in the present invention, when JSR is 40dB, differed also very with the system synchronization performance not interfered with It is small, it can be seen that the system has stronger antijamming capability.

Claims (5)

1. the OFDM anti-interference synchronous methods under a kind of complexity multipath channel, it is characterised in that include the following steps：
S1, formation sequence C (k), c (k) and [x (k), x (k)] are specific as follows：
S11, in receiving terminal generation frequency domain sequence C (k) and time domain sequences c (k), wherein, k represents k-th of sampling point of sequence, k's Scope is [0, Nzc- 1], NzcRepresent sequence length；
S12, carry out windowing process to time domain sequences c (k) described in S11, obtains x (k)=c (k) w (k), wherein, x (k) is transmitting The time domain sequences of transmitting are held, w (k) is the window function used in windowing process；
S13, transmitting terminal launch two S12 described in x (k) sequences [x (k), x (k)] be used as main synchronizing sequence；
S2, calculate the slip correlation for receiving signal r (k) and local synchronization sequence in receiving terminal, and correlation value calculation formula isWherein, p (k) represents the slip correlation at kth moment, x*(k) represent to take x (k) conjugation, institute It is identical with x (k) sequences described in S12 to state local synchronization sequence；
S3, be timed synchronization, specific as follows：
S31, seek sliding average p to correlation described in S2mean(k), the length of sliding average window is Nzc+1；
S32, set threshold value K, wherein, the value of K is different according to signal-to-noise ratio and changes in [4,20]；
S33, correlation p (k) and the p (k+N by sliding window both ends in S31zc) with S31 described in pmean(k) it is compared, if p (k)≥K×pmean(k) and p (k+Nzc)≥K×pmean(k), then relevant peaks are detected, current location is set to Timing Synchronization position Put, be denoted as M, go to S35, if being unsatisfactory for condition, go to S34；
S34, make k=k+1, repeat step S33；
S35, be adjusted threshold value K described in S32, makes M described in S33 in first footpath position；
S4, offset estimation, it is specific as follows：
S41, set the received main synchronizing sequence of receiving terminal as r=[y1(k) y2(k)], wherein,N is IFFT point number in ofdm system；
S42, correlation value calculation formula is according to S2Correlation value calculation is carried out to r described in S41, Obtain the phase difference of correlationWherein, M ＜ m ＜ M+Nzc；
S43, according to the phase difference of Lt correlation after first footpath carry out offset estimation,Wherein, M ＜ m ＜ M+Lt, Lt are the maximum multipath time delay of signal.
2. the OFDM anti-interference synchronous methods under a kind of complicated multipath channel according to claim 1, it is characterised in that： Frequency domain sequence C (k) described in S11 and time domain sequences c (k) selects the Zadoff- in permanent envelope zero auto-correlation (CAZAC) sequence Chu (ZC) sequence, the expression formula of the ZC sequences areBy frequency where the interference signal in ZC sequences Section corresponding points zero setting, obtains C (k)=Cf(k) J (k), makees inverse fast Fourier transform (Inverse Fast by the C (k) Fourier Transform, IFFT) time domain sequences c (k)=IFFT (C (k)) is converted into, wherein, r is radical exponent, r=Nzc- 1, J (k) is frequency spectrum perception vector,
3. the OFDM anti-interference synchronous methods under a kind of complicated multipath channel according to claim 1, it is characterised in that： Described in S12 <mrow> <mi>w</mi> <mrow> <mo>(</mo> <mi>k</mi> <mo>)</mo> </mrow> <msub> <mi>a</mi> <mn>0</mn> </msub> <mo>-</mo> <msub> <mi>a</mi> <mn>1</mn> </msub> <mi>cos</mi> <mrow> <mo>(</mo> <mfrac> <mrow> <mn>2</mn> <mi>&amp;pi;k</mi> </mrow> <mrow> <mi>N</mi> <mo>-</mo> <mn>1</mn> </mrow> </mfrac> <mo>)</mo> </mrow> <mo>+</mo> <msub> <mi>a</mi> <mn>2</mn> </msub> <mi>cos</mi> <mrow> <mo>(</mo> <mfrac> <mrow> <mn>4</mn> <mi>&amp;pi;k</mi> </mrow> <mrow> <mi>N</mi> <mo>-</mo> <mn>1</mn> </mrow> </mfrac> <mo>)</mo> </mrow> <mo>-</mo> <msub> <mi>a</mi> <mn>3</mn> </msub> <mi>cos</mi> <mrow> <mo>(</mo> <mfrac> <mrow> <mn>6</mn> <mi>&amp;pi;k</mi> </mrow> <mrow> <mi>N</mi> <mo>-</mo> <mn>1</mn> </mrow> </mfrac> <mo>)</mo> </mrow> <mo>,</mo> </mrow> Wherein, a0=0.3635819, a1= 0.4891775, a2=0.1365995, a3=0.0106411.
4. the OFDM anti-interference synchronous methods under a kind of complicated multipath channel according to claim 1, it is characterised in that： K=5 described in S32.
5. the OFDM anti-interference synchronous methods under a kind of complicated multipath channel according to claim 1, it is characterised in that： The length of sliding average window described in S31 is Nzc+ 1-2L, wherein, L represents L, the edge data of the sliding average window.
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