CN105072075A - Multi-threshold decision OFDM synchronization method - Google Patents

Abstract

The invention belongs to the field of mobile communications and in particular relates to a multi-threshold decision OFDM synchronization method. According to the multi-threshold decision OFDM synchronization method, a synchronization sequence is designed reasonably to improve the anti-jamming capability of a system, and a plurality of relative thresholds are set to make a united decision on two correlated peaks, and therefore, the system can be accurately synchronized within a relatively large signal-to-noise ratio range; besides, the synchronization stability of the system is improved.

Description

A kind of OFDM synchronous method of multi-threshold judgement

Technical field

The invention belongs to moving communicating field, particularly relate to a kind of multi-threshold judgement OFDM synchronous method.

Background technology

OFDM technology has the advantages such as high, the anti-fading ability of spectrum efficiency utilance is strong, has become the technological core of next-generation mobile communications at present.And in an ofdm system, synchronous error is a principal element of influential system performance.According to synchronous function, can will synchronously be divided into Timing Synchronization, carrier frequency synchronization and sampling clock 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-aided synchronous method, document 1 " RobustfrequencyandtimingsynchronizationforOFDM (bySchmidlTM; CoxDC.IEEETrans.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 symbol is identical with later half, can be used for time synchronized and frequency precise synchronization, utilize former and later two intersymbol relations to carry out coarse frequency synchronization.Document 2 " PilotassistedchannelestimationforOFDMinmobilecellularsys tems (byFTufvesson; TMaseng.IEEEVTC; vol.3; pp.1639-1643, May4-7,1997.) " proposes to utilize PN sequence 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, but the higher amount of calculation of accuracy is larger by maximum value search.

But, in radio communication particularly military radio communication, the high-power interference of enemy may be there is, and existing synchronized algorithm lacks interference protection measure, often cannot normally work in interference environment.Meanwhile, existing synchronized algorithm is all often adjudicated single relevant peaks by fixed threshold, and this makes synchro system both cannot adapt to the significantly change of signal to noise ratio, also deficient in stability.

Summary of the invention

For the deficiencies in the prior art, the present invention proposes the OFDM synchronous method of a kind of multi-threshold judgement, by appropriate design synchronizing sequence, the antijamming capability of raising system, by arranging multiple relative threshold, cascading judgement is carried out to two relevant peaks, make system can in larger SNR ranges accurate synchronization, improve the stability of synchronization of system.

An OFDM synchronous method for multi-threshold judgement, comprises the steps:

S1, generate frequency domain sequence C (k) at transmitting terminal, time domain sequences c (k) and main synchronizing sequence [x (k), x (k)];

S2, according to formula calculate the slip correlation of Received signal strength and local sequence at receiving terminal, wherein, the signal that receiving terminal receives is the slip correlation that r (k), p (k) represent the kth moment, x ^*k () expression gets conjugation to x (k);

S3, carry out Timing Synchronization, specific as follows:

S31, multiple relative threshold value α=[α is set ₁, α ₂..., α _i], wherein α ₁> α ₂> ... > α _i;

S32, calculating absolute door limit value λ _i[k]=α _i× p _mean[k] k > N _zc, 1≤i≤I, wherein p _mean[k] is correlation mean value k > N _zc, L is the maximum multipath time delay of channel;

S33, to the N received _slot+ N _zcindividual correlation, by p (k) and p (k-N _zc) successively with threshold value λ described in S32 _i[k] compares, if | p (k) |>=λ _i[k] and | p (k-N _zc) |>=λ _i[k], then detect relevant peaks, and current location is denoted as M _i, otherwise, note M _ifor sky, wherein N _slotrepresent the data amount check in each frame period;

S34, compare M successively ₁, M ₂..., M _i, using the value of first non-NULL as sync bit; If M ₁, M ₂..., M _ibe sky, then return S32, receive lower N _slot+ N _zcindividual correlation proceeds synchronous search;

S4, set up synchronization acquistion and tracking, specific as follows:

S41, complete Timing Synchronization described in S3, be designated as synchronous success 1 time, if continuous 3 subsynchronous successes, and every before and after the two hyposynchronous time intervals at scope [N _slot-Δ N, N _slot+ Δ N] in, then think and complete synchronization acquistion, wherein Δ N is timing wander tolerance, and last sync bit is designated as M;

S42, as described in S41, after completing synchronization acquistion, at the N of captured sync bit M _slotsynchronously search for after individual data, hunting zone is [M+N next time _slot-N _cp, M+N _slot+ N _cp], N _cprepresent circulating prefix-length.If in this hunting zone inter-sync success, then sync bit M is updated to current sync position, and continues to perform S42, otherwise, if M=M+N _slot, and perform S43;

S43, as described in S42, the synchronized tracking stage occur first time do not complete synchronous situation, synchronously keep, continue to use sync bit last time, and continue at [M+N _slot-N _cp, M+N _slot+ N _cp] synchronously search for, if this subsynchronous success, then perform S42 and continue synchronized tracking; Otherwise namely there is twice synchronization failure, be designated as step-out, perform S31 and re-start synchronization acquistion.

Further, generate frequency domain sequence C (k) described in S1 at transmitting terminal, time domain sequences c (k) is specific as follows with main synchronizing sequence [x (k), x (k)]:

S11, generate frequency domain sequence C (k)=C at transmitting terminal _f(k) J (k), wherein, C _fk () is Zadoff-Chu (ZC) sequence in permanent envelope zero auto-correlation (CAZAC) sequence, a is for being signal amplitude, and r is radical exponent, r=N _zc-1, the k kth sampling point representing sequence, the scope of k is [0, N _zc-1], N _zcfor ZC sequence length, J (k) is frequency spectrum perception vector,

S12, frequency domain sequence C (k) described in S11 is utilized to generate time domain sequences c (k)=IFFT (C (k));

S13, windowing process is carried out to time domain sequences c (k) described in S12, obtain time domain sequences x (k)=c (k) w (k) that transmitting terminal is launched, wherein, the window function of w (k) for using in time-domain windowed process a ₀=0.3635819, a ₁=0.4891775, a ₂=0.1365995, a ₃=0.0106411;

S14, launch x (k) sequence [x (k), x (k)] described in two S13 as main synchronizing sequence at transmitting terminal.

The invention has the beneficial effects as follows:

By appropriate design synchronizing sequence, avoid interference place frequency range at frequency domain, and inhibit the impact of interference on available frequency band by carrying out windowing in time domain, improve the antijamming capability of system.

Be 201510138262.9 these patents relative to application number, invention increases and multiple relative threshold is set cascading judgement is carried out to two relevant peaks, the thresholding that adaptive selection is different under different signal to noise ratios, while raising synchronization acquistion probability, reduce false synchronization probability, improve net synchronization capability stability.

Accompanying drawing explanation

Fig. 1 synchronizing sequence generates schematic diagram.

The Timing Synchronization schematic diagram of Fig. 2 multi-threshold judgement.

Fig. 3 synchronization acquistion and trace flow figure.

Fig. 4 synchronization acquistion probability and error capture probability.

Fig. 5 step-out probability.

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 N _zc=512, OFDM circulating prefix-length is N _cp=70, timing wander tolerance Δ N=8, data amount check N in the frame period _slot=28730, channel sample frequency is 10MHz, channel model is high-performance cruise scene corresponding in aviation channel, vacant lot wireless channel is single footpath model, cruising condition flying speed of getting off the plane is 2 Mach, in channel, institute's plus noise is additive white Gaussian noise, and added interference is the partial-band jamming of 20% frequency range.

S1, produce ZC sequence, in order to the signal that avoids interference, by the frequency range corresponding points zero setting of interference signal place at receiving terminal frequency domain, IFFT is done to frequency domain sequence and is converted to time domain sequences, to time domain sequences windowing, suppress the impact of interference on useful frequency range, institute's windowing is Blackman-Nuttall window.Transmitting terminal is launched two sections of synchronizing sequences identical with the time domain sequences after windowing and is used as synchronous.Specific as follows:

S11, generate frequency domain sequence C (k)=C at transmitting terminal _f(k) J (k), wherein, C _fk () is Zadoff-Chu (ZC) sequence in permanent envelope zero auto-correlation (CAZAC) sequence, a is for being signal amplitude, and r is radical exponent, r=N _zc-1, the k kth sampling point representing sequence, the scope of k is [0, N _zc-1], N _zcfor ZC sequence length, J (k) is frequency spectrum perception vector,

S12, frequency domain sequence C (k) described in S11 is utilized to generate time domain sequences c (k)=IFFT (C (k));

S13, windowing process is carried out to time domain sequences c (k) described in S12, obtain time domain sequences x (k)=c (k) w (k) that transmitting terminal is launched, wherein, the window function of w (k) for using in time-domain windowed process a ₀=0.3635819, a ₁=0.4891775, a ₂=0.1365995, a ₃=0.0106411;

S14, to launch described in two S13 at transmitting terminal that x (k) sequence [x (k), x (k)] is as main synchronizing sequence, synchronizing sequence generative process as shown in Figure 1;

S2, according to formula calculate the slip correlation of Received signal strength and local sequence at receiving terminal, wherein, the signal that receiving terminal receives is the slip correlation that r (k), p (k) represent the kth moment, x ^*k () expression gets conjugation to x (k);

S3, carry out Timing Synchronization, first receive 29242 correlations do moving average and calculate threshold value, wherein relative threshold value α=[24,12,6,3], utilize two correlations of thresholding to 512 length of being separated by carry out joint-detection successively, if current threshold value does not detect relevant peaks, then utilize a rear less thresholding again to detect, until relevant peaks detected, and using current time as sync bit; If minimum threshold value does not also detect relevant peaks, then the correlation receiving next frame 29242 length proceeds synchronous search, and timing synchronization procedure as shown in Figure 2;

S4, set up synchronization acquistion and tracking, specific as follows:

S41, complete Timing Synchronization described in S3, be designated as synchronous success 1 time, if continuous 3 subsynchronous successes, and every before and after the two hyposynchronous time intervals at scope [N _slot-Δ N, N _slot+ Δ N] in, then think and complete synchronization acquistion, wherein Δ N is timing wander tolerance, and last sync bit is designated as M;

S42, as described in S41, after completing synchronization acquistion, at the N of captured sync bit M _slotsynchronously search for after individual data, hunting zone is [M+N next time _slot-N _cp, M+N _slot+ N _cp], N _cprepresent circulating prefix-length.If in this hunting zone inter-sync success, then sync bit M is updated to current sync position, and continues to perform S42, otherwise, if M=M+N _slot, and perform S43;

S43, as described in S42, the synchronized tracking stage occur first time do not complete synchronous situation, synchronously keep, continue to use sync bit last time, and continue at [M+N _slot-N _cp, M+N _slot+ N _cp] synchronously search for, if this subsynchronous success, then perform S42 and continue synchronized tracking; Otherwise namely occur twice synchronization failure, be designated as step-out, perform S31 and re-start synchronization acquistion, synchronization acquistion and tracing process are as shown in Figure 3.

The method of the invention is adopted to carry out emulation testing, synchronization acquistion probability curve and error capture probability curve are as shown in Figure 4, can find from figure, due to this programme to multiple thresholding to relevant peaks cascading judgement, improve the stability of synchronization, error capture probability is smaller all the time, and acquisition probability can reach more than 0.9 in signal to noise ratio (SignalNoiseRatio, SNR) for during-14dB.Meanwhile, owing to avoiding interference band in synchronizing sequence design process, make when interference exists, synchro system still can normally work.Time curve table in accompanying drawing 5 is shown in and carries out synchronized tracking, double synchronization failure and cause the probability of step-out, can find no matter whether there is interference, be more than-10dB at SNR, and step-out probability is all smaller.Visible, this programme can be at the wireless channel that there is interference realize stable synchronous.

Claims (2)

1. an OFDM synchronous method for multi-threshold judgement, is characterized in that, comprise the steps:

S1, generate frequency domain sequence C (k) at transmitting terminal, time domain sequences c (k) and main synchronizing sequence [x (k), x (k)];

S2, according to formula calculate the slip correlation of Received signal strength and local sequence at receiving terminal, wherein, the signal that r (k) receives for receiving terminal, p (k) represents the slip correlation in kth moment, x ^*k () expression gets conjugation to x (k);

S3, carry out Timing Synchronization, specific as follows:

S31, multiple relative threshold value α=[α is set ₁, α ₂..., α _i], wherein α ₁> α ₂> ... > α _i;

S32, calculating absolute door limit value λ _i[k]=α _i× p _mean[k] k > N _zc, 1≤i≤I, wherein p _mean[k] is correlation mean value k > N _zc, L is the maximum multipath time delay of channel;

S33, to the N received _slot+ N _zcindividual correlation, by p (k) and p (k-N _zc) successively with threshold value λ described in S32 _i[k] compares, if | p (k) |>=λ _i[k] and | p (k-N _zc) |>=λ _i[k], then detect relevant peaks, and current location is denoted as M _i, otherwise, note M _ifor sky, wherein N _slotrepresent the data amount check in each frame period;

S34, compare M successively ₁, M ₂..., M _i, using the value of first non-NULL as sync bit; If M ₁, M ₂..., M _ibe sky, then return S32, receive lower N _slot+ N _zcindividual correlation proceeds synchronous search;

S4, set up synchronization acquistion and tracking, specific as follows:

S41, complete Timing Synchronization described in S3, be designated as synchronous success 1 time, if continuous 3 subsynchronous successes, and every before and after the two hyposynchronous time intervals at scope [N _slot-Δ N, N _slot+ Δ N] in, then think and complete synchronization acquistion, wherein Δ N is timing wander tolerance, and last sync bit is designated as M;

S42, as described in S41, after completing synchronization acquistion, at the N of captured sync bit M _slotsynchronously search for after individual data, hunting zone is [M+N next time _slot-N _cp, M+N _slot+ N _cp], N _cprepresent circulating prefix-length, if in this hunting zone inter-sync success, then sync bit M is updated to current sync position, and continues to perform S42, otherwise, if M=M+N _slot, and perform S43;

S43, as described in S42, the synchronized tracking stage occur first time do not complete synchronous situation, synchronously keep, continue to use sync bit last time, and continue at [M+N _slot-N _cp, M+N _slot+ N _cp] synchronously search for, if this subsynchronous success, then perform S42 and continue synchronized tracking; Otherwise namely there is twice synchronization failure, be designated as step-out, perform S31 and re-start synchronization acquistion.

2. the OFDM synchronous method of a kind of multi-threshold judgement according to claim 1, it is characterized in that: described in S1, generate frequency domain sequence C (k) at transmitting terminal, time domain sequences c (k) is specific as follows with main synchronizing sequence [x (k), x (k)]:

S11, generate frequency domain sequence C (k)=C at transmitting terminal _f(k) J (k), wherein, C _fk () is Zadoff-Chu (ZC) sequence in permanent envelope zero auto-correlation (CAZAC) sequence, a is for being signal amplitude, and r is radical exponent, r=N _zc-1, the k kth sampling point representing sequence, the scope of k is [0, N _zc-1], N _zcfor ZC sequence length, J (k) is frequency spectrum perception vector,

S12, frequency domain sequence C (k) described in S11 is utilized to generate time domain sequences c (k)=IFFT (C (k));

S13, windowing process is carried out to time domain sequences c (k) described in S12, obtain time domain sequences x (k)=c (k) w (k) that transmitting terminal is launched, wherein, the window function of w (k) for using in time-domain windowed process $w\left(k\right)=a_0-a_{1}cos\left(\frac{2\mathrm{\π}k}{N-1}\right)+a_{2}cos\left(\frac{4\mathrm{\π}k}{N-1}\right)-a_{3}cos\left(\frac{6\mathrm{\π}k}{N-1}\right),$ a ₀＝0.3635819，a ₁＝0.4891775，a ₂＝0.1365995，a ₃＝0.0106411；

S14, launch x (k) sequence [x (k), x (k)] described in two S13 as main synchronizing sequence at transmitting terminal.