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

Abstract

The invention belongs to the field of mobile communication, in particular to a multi-threshold judgment OFDM synchronization method. An OFDM synchronization method with multi-threshold judgment, through rationally designing the synchronization sequence, the anti-interference ability of the system is improved, and by setting multiple relative thresholds, two correlation peaks are jointly judged, so that the system can operate within a large SNR range Accurate synchronization, improve the synchronization stability of the system.

Description

An OFDM Synchronization Method Based on Multi-Threshold Decision

technical field

The invention belongs to the field of mobile communication, in particular to a multi-threshold judgment OFDM synchronization method.

Background technique

OFDM technology has the advantages of high spectral efficiency utilization rate and strong anti-fading ability, and has become the technical core of the new generation of mobile communication. In OFDM systems, however, synchronization error is a major factor affecting system performance. According to the synchronization function, synchronization can be divided into timing synchronization, carrier frequency synchronization and sampling clock synchronization; according to whether auxiliary data is used, it can be divided into data-assisted synchronization method and blind synchronization method. Among them, the data-assisted synchronization method has higher synchronization accuracy and lower computational complexity, but needs to add training sequences, which reduces the data transmission efficiency.

In the data-assisted synchronization method, the literature 1 "Robust frequency and timing synchronization for OFDM (by SchmidlTM, CoxDC. IEEETrans. Commun, 1997, 45 (12): 1613-1621.)" first proposed the use of two OFDM symbols as training sequences for time and frequency synchronization , the first half of the first symbol is the same as the second half, which can be used for time synchronization and frequency fine synchronization, and the relationship between the two symbols before and after is used for coarse frequency synchronization. Document 2 "Pilot assisted channel estimation for OFDM mobile cellular systems (by FTufvesson, TMaseng. IEEEVTC, vol. 3, pp. 1639-1643, May 4-7, 1997.)" proposes to use the PN sequence as a training sequence to correlate the received signal with the local sequence. The correlation of this method The result of the detector has a large output peak value, and the synchronization position is found through the maximum value search, which has high accuracy but a large amount of calculation.

However, in wireless communication, especially in military wireless communication, there may be high-power interference from the enemy, and the existing synchronization algorithms lack anti-interference measures, and often cannot work normally in the interference environment. At the same time, the existing synchronization algorithms often judge a single correlation peak through a fixed threshold, which makes the synchronization system unable to adapt to large changes in SNR and lacks stability.

Contents of the invention

Aiming at the deficiencies of the prior art, the present invention proposes a multi-threshold judgment OFDM synchronization method, which improves the anti-interference capability of the system by rationally designing the synchronization sequence, and performs joint judgment on two correlation peaks by setting multiple relative thresholds, so that The system can be accurately synchronized within a large range of signal-to-noise ratio, improving the synchronization stability of the system.

A kind of OFDM synchronous method of multi-threshold decision, comprises the steps:

S1. Generate a frequency domain sequence C(k), a time domain sequence c(k) and a primary synchronization sequence [x(k), x(k)] at the transmitter;

S2, according to the formula Calculate the sliding correlation value of the received signal and the local sequence at the receiving end, where the signal received at the receiving end is r(k), p(k) represents the sliding correlation value at the kth moment, and x ^* (k) represents the pair x( k) take the conjugate;

S3, perform timing synchronization, specifically as follows:

S31. Set a plurality of relative threshold values α=[α ₁ ,α ₂ ,…,α _I ], where α ₁ >α ₂ >…>α _I ;

S32. Calculate the absolute threshold value λ _i [k]=α _i ×p _mean [k]k>N _zc , 1≤i≤I, where p _mean [k] is the average value of the correlation value k>N _zc , L is the maximum multipath delay of the channel;

S33. For the received N _slot +N _zc correlation values, compare p(k) and p(kN _zc ) with the threshold value λ _i [k] described in S32 in turn, if |p(k)|≥ λ _i [k] and |p(kN _zc )|≥λ _i [k], then the correlation peak is detected, and the current position is recorded as M _i , otherwise, M _i is recorded as empty, where N _slot represents each frame period The number of data in;

S34. Compare M ₁ , M ₂ ,...,M _I sequentially, and use the first non-empty value as the synchronization position; if M ₁ , M ₂ ,...,M _I are all empty, return to S32 and receive the next N _slot +N _zc related values continue to search synchronously;

S4. Establish synchronous capture and tracking, specifically as follows:

S41. Complete the timing synchronization described in S3, and record it as a successful synchronization once. If the synchronization is successful for 3 consecutive times, and the time interval between each two synchronizations is within the range [N _slot -ΔN, N _slot +ΔN], then it is considered Synchronization acquisition is completed, where ΔN is the tolerance range of timing drift, and the last synchronization position is recorded as M;

S42. As described in S41, after the synchronous capture is completed, the next synchronous search is performed after the N _slot data of the captured synchronous position M, and the search range is [M+N _slot -N _cp , M+N _slot +N _cp ], N _cp indicates the length of the cyclic prefix. If the synchronization is successful within the search range, update the synchronization position M to the current synchronization position, and continue to execute S42, otherwise, set M=M+N _slot , and execute S43;

S43. As described in S42, when the synchronization is not completed for the first time in the synchronization tracking phase, the synchronization is maintained, and the last synchronization position is used, and continues to be in [M+N _slot -N _cp , M+N _slot +N _cp ] for synchronization search, if the synchronization is successful, execute S42 to continue synchronization tracking; otherwise, there will be two synchronization failures, which will be recorded as out-of-synchronization, and execute S31 to perform synchronization capture again.

Further, the generation of frequency domain sequence C(k), time domain sequence c(k) and primary synchronization sequence [x(k), x(k)] at the transmitting end as described in S1 is specifically as follows:

S11. Generate a frequency domain sequence C(k)= _Cf (k)J(k) at the transmitting end, wherein _Cf (k) is Zadoff-Chu(ZC) in the constant envelope zero autocorrelation (CAZAC) sequence sequence, A is the signal amplitude, r is the root index, r=N _zc -1, k represents the kth sample point of the sequence, the range of k is [0,N _zc -1], N _zc is the length of the ZC sequence, J( k) is the spectrum sensing vector,

S12. Using the frequency domain sequence C(k) described in S11 to generate a time domain sequence c(k)=IFFT(C(k));

S13. Perform windowing processing on the time domain sequence c(k) described in S12 to obtain the time domain sequence x(k)=c(k)w(k) transmitted by the transmitter, where w(k) is the time domain sequence c(k) The window function used in window processing, a ₀ =0.3635819, a ₁ =0.4891775, a ₂ =0.1365995, a ₃ =0.0106411;

S14. Transmit the two x(k) sequences [x(k), x(k)] described in S13 at the transmitting end as the primary synchronization sequence.

The beneficial effects of the present invention are:

By rationally designing the synchronization sequence, the frequency band where the interference is located is avoided in the frequency domain, and the influence of interference on the available frequency band is suppressed by windowing in the time domain, which improves the anti-interference ability of the system.

Compared with the patent with application number 201510138262.9, the present invention adds multiple relative thresholds for joint judgment of two correlation peaks, and adaptively selects different thresholds under different signal-to-noise ratios. The probability of false synchronization is reduced, and the stability of synchronization performance is improved.

Description of drawings

Figure 1 Schematic diagram of synchronous sequence generation.

Fig. 2 Schematic diagram of timing synchronization of multi-threshold decision.

Figure 3 is a flow chart of synchronous capture and tracking.

Figure 4 Simultaneous acquisition probability and misacquisition probability.

Figure 5 Out-of-sync probability.

Detailed ways

The technical solution of the present invention will be described in detail below in combination with the embodiments and the accompanying drawings.

In this embodiment, a Matlab simulation platform is used for running experiments.

The synchronization parameters in the embodiment are as follows: the ZC sequence length is N _zc =512, the OFDM cyclic prefix length is N _cp =70, the timing drift tolerance range ΔN=8, the number of data in the frame period N _slot =28730, and the channel sampling frequency is 10MHz , the channel model is the corresponding high-speed cruise scene in the aviation channel, the air-ground wireless channel is a single-path model, the flight speed of the aircraft in the cruise state is Mach 2, the noise added to the channel is additive white Gaussian noise, and the added interference is 20% of the frequency band part of the frequency band interference.

S1. The ZC sequence is generated in the frequency domain at the receiving end. In order to avoid the interference signal, the corresponding point of the frequency band where the interference signal is located is set to zero, and the frequency domain sequence is converted into a time domain sequence by IFFT, and the time domain sequence is windowed to suppress interference. The influence of the frequency band, the added window is Blackman-Nuttall window. The transmitter transmits two synchronization sequences identical to the windowed time domain sequence for synchronization. details as follows:

S11. Generate a frequency domain sequence C(k)= _Cf (k)J(k) at the transmitting end, wherein _Cf (k) is Zadoff-Chu(ZC) in the constant envelope zero autocorrelation (CAZAC) sequence sequence, A is the signal amplitude, r is the root index, r=N _zc -1, k represents the kth sample point of the sequence, the range of k is [0,N _zc -1], N _zc is the length of the ZC sequence, J( k) is the spectrum sensing vector,

S12. Using the frequency domain sequence C(k) described in S11 to generate a time domain sequence c(k)=IFFT(C(k));

S13. Perform windowing processing on the time domain sequence c(k) described in S12 to obtain the time domain sequence x(k)=c(k)w(k) transmitted by the transmitter, where w(k) is the time domain sequence c(k) The window function used in window processing, a ₀ =0.3635819, a ₁ =0.4891775, a ₂ =0.1365995, a ₃ =0.0106411;

S14. Transmit two x(k) sequences [x(k), x(k)] described in S13 at the transmitting end as the main synchronization sequence, and the synchronization sequence generation process is as shown in Figure 1;

S2, according to the formula Calculate the sliding correlation value of the received signal and the local sequence at the receiving end, where the signal received at the receiving end is r(k), p(k) represents the sliding correlation value at the kth moment, and x ^* (k) represents the pair x( k) take the conjugate;

S3, perform timing synchronization, first receive 29242 correlation values for sliding average and calculate the threshold value, wherein the relative threshold value α=[24,12,6,3], use the threshold to pair two correlation values separated by 512 lengths in turn Carry out joint detection, if no correlation peak is detected by the current threshold value, use the latter smaller threshold to re-detect until a correlation peak is detected, and use the current moment as the synchronization position; if the minimum threshold value has not been detected Correlation peak, then receive the correlation value of next frame 29242 lengths and continue to search synchronously, the timing synchronization process is as shown in Figure 2;

S4. Establish synchronous capture and tracking, specifically as follows:

S41. Complete the timing synchronization described in S3, and record it as a successful synchronization once. If the synchronization is successful for 3 consecutive times, and the time interval between each two synchronizations is within the range [N _slot -ΔN, N _slot +ΔN], then it is considered Synchronization acquisition is completed, where ΔN is the tolerance range of timing drift, and the last synchronization position is recorded as M;

S42. As described in S41, after the synchronous capture is completed, the next synchronous search is performed after the N _slot data of the captured synchronous position M, and the search range is [M+N _slot -N _cp , M+N _slot +N _cp ], N _cp indicates the length of the cyclic prefix. If the synchronization is successful within the search range, update the synchronization position M to the current synchronization position, and continue to execute S42, otherwise, set M=M+N _slot , and execute S43;

S43. As described in S42, when the synchronization is not completed for the first time in the synchronization tracking phase, the synchronization is maintained, and the last synchronization position is used, and continues to be in [M+N _slot -N _cp , M+N _slot +N _cp ] to search for synchronization, if this synchronization is successful, then execute S42 to continue synchronous tracking; otherwise, there will be two synchronization failures, recorded as out-of-synchronization, and execute S31 to perform synchronous capture again, and the process of synchronous capture and tracking is shown in Figure 3.

Adopt the method described in the present invention to carry out emulation test, synchronous capture probability curve and miscapture probability curve are as shown in accompanying drawing 4, can find from the figure, because this scheme is to multiple threshold pair correlation peak joint judgment, has improved synchronous stability , the probability of miscapture is always relatively small, and the probability of capture can reach above 0.9 when the signal-to-noise ratio (SignalNoiseRatio, SNR) is -14dB. At the same time, because the interference frequency band is avoided during the synchronization sequence design process, the synchronization system can still work normally when interference exists. The curve in Figure 5 indicates the probability of out-of-synchronization caused by two consecutive synchronization failures during synchronization tracking. It can be found that no matter whether there is interference or not, the probability of out-of-synchronization is relatively small when the SNR is above -10dB. It can be seen that this solution can realize stable synchronization in a wireless channel with interference.

Claims (2)

1. a kind of OFDM synchronization method of multi-threshold decision, is characterized in that, comprises the steps: S1. Generate a frequency domain sequence C(k), a time domain sequence c(k) and a primary synchronization sequence [x(k), x(k)] at the transmitter; S2, according to the formula Calculate the sliding correlation value of the received signal and the local sequence at the receiving end, where r(k) is the signal received at the receiving end, p(k) represents the sliding correlation value at the kth moment, and x ^* (k) represents the pair x( k) take the conjugate; S3, perform timing synchronization, specifically as follows: S31. Set a plurality of relative threshold values α=[α ₁ ,α ₂ ,…,α _I ], where α ₁ >α ₂ >…>α _I ; S32. Calculate the absolute threshold value λ _i [k]=α _i ×p _mean [k]k>N _zc , 1≤i≤I, where p _mean [k] is the average value of the correlation value k>N _zc , L is the maximum multipath delay of the channel; S33. For the received N _slot +N _zc correlation values, compare p(k) and p(kN _zc ) with the threshold value λ _i [k] described in S32 in turn, if |p(k)|≥ λ _i [k] and |p(kN _zc )|≥λ _i [k], then the correlation peak is detected, and the current position is recorded as M _i , otherwise, M _i is recorded as empty, where N _slot represents each frame period The number of data in; S34. Compare M ₁ , M ₂ ,...,M _I sequentially, and use the first non-empty value as the synchronization position; if M ₁ , M ₂ ,...,M _I are all empty, return to S32 and receive the next N _slot +N _zc related values continue to search synchronously; S4. Establish synchronous capture and tracking, specifically as follows: S41. Complete the timing synchronization described in S3, and record it as a successful synchronization once. If the synchronization is successful for 3 consecutive times, and the time interval between each two synchronizations is within the range [N _slot -ΔN, N _slot +ΔN], then it is considered Synchronization acquisition is completed, where ΔN is the tolerance range of timing drift, and the last synchronization position is recorded as M; S42. As described in S41, after the synchronous capture is completed, the next synchronous search is performed after the N _slot data of the captured synchronous position M, and the search range is [M+N _slot -N _cp , M+N _slot +N _cp ], N _cp represents the length of the cyclic prefix, if the synchronization is successful within the search range, then update the synchronization position M to the current synchronization position, and continue to execute S42, otherwise, set M=M+N _slot , and execute S43; S43. As described in S42, when the synchronization is not completed for the first time in the synchronization tracking phase, the synchronization is maintained, and the last synchronization position is used, and continues to be in [M+N _slot -N _cp , M+N _slot +N _cp ] for synchronization search, if the synchronization is successful, execute S42 to continue synchronization tracking; otherwise, there will be two synchronization failures, which will be recorded as out-of-synchronization, and execute S31 to perform synchronization capture again. 2. the OFDM synchronous method of a kind of multi-threshold decision according to claim 1, is characterized in that: described in S1 generates frequency domain sequence C (k) at transmitting end, time domain sequence c (k) and main synchronous sequence [ x(k), x(k)] is as follows: S11. Generate a frequency domain sequence C(k)= _Cf (k)J(k) at the transmitting end, wherein _Cf (k) is Zadoff-Chu(ZC) in the constant envelope zero autocorrelation (CAZAC) sequence sequence, A is the signal amplitude, r is the root index, r=N _zc -1, k represents the kth sample point of the sequence, the range of k is [0,N _zc -1], N _zc is the length of the ZC sequence, J( k) is the spectrum sensing vector, S12. Using the frequency domain sequence C(k) described in S11 to generate a time domain sequence c(k)=IFFT(C(k)); S13. Perform windowing processing on the time domain sequence c(k) described in S12 to obtain the time domain sequence x(k)=c(k)w(k) transmitted by the transmitter, where w(k) is the time domain sequence c(k) The window function used in window processing, $w\left(k\right)=a_0-a_{1}co\mathrm{the\; s}\left(\frac{2\mathrm{\π}k}{N-1}\right)+a_{2}co\mathrm{the\; s}\left(\frac{4\mathrm{\π}k}{N-1}\right)-a_{3}co\mathrm{the\; s}\left(\frac{6\mathrm{\π}k}{N-1}\right),$ a ₀ =0.3635819, a ₁ =0.4891775, a ₂ =0.1365995, a ₃ =0.0106411; S14. Transmit the two x(k) sequences [x(k), x(k)] described in S13 at the transmitting end as the primary synchronization sequence.