A time-frequency synchronization method for high-speed moving platform

Abstract

The application relates to a time-frequency synchronization method for a high-speed moving platform, which comprises the following steps: at a transmitting end, a preamble sequence is obtained by performing mathematical transformation on a constant envelope zero autocorrelation sequence through a weight factor, conjugate processing and symmetric transformation; at a receiving end, the received signal is processed step by step, (1) the continuously received signal is subjected to mathematical operation with a local weight factor sequence to obtain a timing metric sequence, the maximum value is selected and the corresponding position is recorded to complete symbol timing synchronization; (2) a received pilot sequence is extracted, fractional carrier frequency offset estimation and compensation are carried out, and integer carrier frequency offset estimation and compensation are carried out; (3) the received pilot sequence is extracted, and common phase offset estimation and compensation are carried out by using a local preamble sequence. The application realizes high-accuracy timing acquisition and high-precision frequency offset correction capability under the condition of low signal-to-noise ratio, high dynamic Doppler frequency shift and non-matching of transmitting and receiving end crystal oscillators.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of wireless communication, in particular to a time-frequency synchronization method for high-speed mobile platform. BACKGROUND

[0002] Unmanned aerial vehicles that can act as flying base stations are considered as a promising means to extend the coverage and improve the rate performance of wireless networks. There are several advantages of using aerial base stations. First, due to the higher altitude, aerial base stations have more opportunities to establish line-of-sight connections with ground users, thus mitigating signal blockage and shadowing fading. Second, due to the adjustable height and maneuverability of aerial platforms, it is easy to move to the potential ground users near the upper air and establish reliable connections with low transmission power, so as to achieve flexible deployment to provide fast on-demand communication. The air-to-air channel is mainly controlled by the line-of-sight wireless transmission component. Although there may be limited multipath fading due to ground reflection, its influence is minimal compared with air-to-ground or ground-to-ground channels. In addition, due to the relatively high speed between air-to-air platforms, the air-to-air channel may have a high Doppler shift. The air-to-air communication dominated by line-of-sight links can achieve high-capacity link transmission through millimeter waves, but the potential high relative speed plus the high frequency of the millimeter wave band may result in an excessively high Doppler shift. Therefore, it is a challenge to research fast and accurate time-frequency synchronization technology for the unique channel characteristics in the air-to-air scene. SUMMARY

[0003] The present application proposes a preamble sequence structure, and an algorithm process for completing symbol timing synchronization and frequency offset estimation and correction using the sequence at the receiving end. The present application realizes high-accuracy timing capture and high-precision frequency offset correction capability in the environment of low signal-to-noise ratio, high dynamic Doppler shift and transmitter-receiver crystal oscillator mismatch.

[0004] The technical scheme adopted by the present application is:

[0005] A time-frequency synchronization method for high-speed mobile platform, comprising the following processes:

[0006] The processing process of the transmitting end is:

[0007] (101) generating a sequence A with constant envelope and zero autocorrelation with a length of N / 2, wherein N is an even number;

[0008] (102) performing conjugate symmetric transformation on the sequence A to obtain a sequence B;

[0009] (103) multiplying the sequence A with a weight factor sequence W to obtain a sequence C;

[0010] (104) performing conjugate symmetric transformation on the sequence C to obtain a sequence D;

[0011] (105) sequentially concatenating the sequence A, the sequence B, the sequence C and the sequence D in series to obtain a sequence S;

[0012] (106) taking a sequence CP with a length of N CP at the end of the sequence S as a cyclic prefix sequence;

[0013] (107) concatenating the sequence CP and the sequence S in series to obtain a final preamble sequence SYN; concatenating the preamble sequence SYN and a subsequent other part signal to obtain a baseband transmitting signal, and finally performing oversampling and up-conversion to send the signal to a wireless channel;

[0014] The processing process at the receiving end is as follows:

[0015] (201) obtaining a baseband time domain sequence y by performing down-conversion and down-sampling on the received signal, performing mathematical operation on the continuously received baseband time domain sequence y and a weight factor sequence W to obtain a timing metric sequence M(d); comparing a plurality of continuous timing metric sequences M(d) to select a maximum value M max and record the corresponding position as a required timing synchronization point; wherein the value range of d is set according to actual requirements;

[0016] (202) extracting a received pilot sequence p from the continuously received baseband time domain sequence y according to the estimated position , performing fractional carrier frequency offset estimation and compensation, and then performing integer carrier frequency offset estimation and compensation to obtain a processed received pilot sequence

[0017] (203) performing mathematical operation on the sequence S and the received preamble sequence , and using the operation result to perform common phase offset estimation and compensation on the baseband time domain sequence y.

[0018] Further, the mathematical expression of the nth element A(n) in the sequence A in step (101) is as follows:

[0019]

[0020] wherein q is an arbitrary integer, and n takes values 0, 1,..., N / 2-1.

[0021] Further, the mathematical expression of the nth element B(n) in the sequence B in step (101) is as follows:

[0022]

[0023] wherein (·) * represents performing conjugate operation, and n takes values 0, 1,..., N / 2-1.

[0024] Further, the mathematical expression of the nth element C(n) in sequence C in step (103) is:

[0025]

[0026] In the formula, the mathematical expression of the nth element in weight factor sequence W is: n takes the value of 0, 1,..., N / 2-1.

[0027] Further, the mathematical expression of the nth element D(n) in sequence D in step (104) is:

[0028]

[0029] In the formula, (·) * represents a conjugate operation, and n takes the value of 0, 1,..., N / 2-1.

[0030] Further, the mathematical expression of sequence S in step (105) is:

[0031] S(n) = [A B C D]

[0032] In the formula, n takes the value of 0, 1,..., 2N-1.

[0033] Further, the value of N CP in step (106) is N / 4.

[0034] Further, the specific process of step (201) is:

[0035] The baseband time domain sequence y is obtained by down-conversion and down-sampling processing of the received signal, and the timing metric sequence M(d) is obtained by mathematical operation of the continuously received baseband time domain sequence y and the weight factor sequence W. The calculation and storage window of the receiving end is set to 2N, and the mathematical expression of the timing metric function M(d) is:

[0036]

[0037] In the formula, the mathematical expression of R(d) is:

[0038]

[0039] The mathematical expression of P(d) is:

[0040]

[0041] In the formula, the value of d is set according to actual requirements, y(n) is the nth element of the baseband time domain sequence y, and the mathematical expression of the nth element in the weight factor sequence W is: n takes values ​​of 0, 1, ..., N / 2-1, (·) * Indicates conjugate operation;

[0042] Then compare multiple consecutive timing measurement sequences M(d) and select the maximum value M max And record the corresponding position This is the timing synchronization point we are looking for, and the starting point of the entire frame is obtained. The expression is:

[0043]

[0044] Furthermore, the specific process of step (202) is as follows:

[0045] (2021) According to the estimated position Extracting a received pilot sequence p from the continuously received baseband time domain sequence y;

[0046] (2022) Using the weighted repetitiveness of the synchronization sequence before and after, a rough estimate of the fractional carrier frequency deviation of the received pilot sequence p is performed, which is expressed as follows:

[0047]

[0048] In the formula, the coefficient sequence G is obtained by splicing the weight factor sequence W, and the expression is: G = [W * (n)W * (N / 2-1-n)], n is 0, 1, ..., N / 2-1; G(n) is the n-th element of G, p(n) is the n-th element of p, n is 0, 1, ..., N-1;

[0049] (2023) Use After compensating the baseband time domain sequence y, the CP portion of the sequence is extracted to obtain the sequence p1. Sequence p1 is then used to accurately estimate the fractional carrier frequency deviation, as shown in the following expression:

[0050]

[0051] Where N CP is the length of the sequence CP, p1(n) is the nth element of p1;

[0052] (2024) Use After compensating the baseband time-domain sequence y, extract the sequence S portion to obtain sequence p2. Using the special properties of the preamble sequence, namely the normalized integer frequency offset, sequences A and C are cyclically shifted rightward by integer points, and sequences B and D are cyclically shifted leftward by integer points. Sequence p2 is subjected to block-wise shifted cross-correlation with the local preamble sequence. The maximum value of the obtained results is selected to obtain the normalized integer frequency offset estimate, as expressed as follows:

[0053]

[0054] Where g takes values ​​of 0, 1, ..., N-1, and p2(n) is the nth element of p2;

[0055] (2025) Use Compensate the received signal to obtain the processed received pilot sequence Complete the estimation and compensation of all carrier frequency deviations.

[0056] Furthermore, the specific process of step (203) is as follows:

[0057] Combine the sequence S with the processed received preamble sequence Perform the common phase deviation estimation operation, the expression is:

[0058]

[0059] Where, for The nth element of , S(n) is the nth element of S;

[0060] Then use The common phase deviation of the baseband time domain sequence y is estimated and compensated.

[0061] Compared with the prior art, the present invention has the following beneficial effects:

[0062] For dynamic communication applications involving high-speed air-to-air platforms, this paper proposes a low-complexity time-frequency synchronization algorithm based on the CAZAC sequence. This preamble sequence ensures good timing synchronization performance and high accuracy under low signal-to-noise ratio conditions, while maintaining low computational complexity. Furthermore, it accurately estimates and compensates for large carrier frequency deviations caused by large Doppler shifts and imperfectly matched crystal oscillators between the transmitter and receiver. BRIEF DESCRIPTION OF THE DRAWINGS

[0063] Figure 1 It is a schematic diagram of the synchronization sequence frame structure at the transmitting end of the time-frequency synchronization method of the present invention.

[0064] Figure 2 This is a flow chart of the time-frequency synchronization method of the present invention at the receiving end.

[0065] Figure 3 This is a comparison chart of the timing metric function curves of the method of the present invention and several typical synchronization algorithms under ideal conditions;

[0066] Figure 4 This is a comparison chart of the timing detection probability of the method of the present invention and several typical synchronization algorithms;

[0067] Figure 5 Figure 3 is a comparison chart of the mean square error of carrier frequency offset estimation of the method of the present application and the Shao algorithm. DETAILED DESCRIPTION

[0068] The present application will be described in detail below with reference to the accompanying drawings.

[0069] A time-frequency synchronization method for a high-speed mobile platform, comprising the following processes:

[0070] A time-frequency synchronization method for a high-speed mobile platform, comprising the following processes:

[0071] At the transmitting end, the following seven steps are included:

[0072] (101) A constant amplitude zero auto correlation (CAZAC) sequence A with a length of N / 2 is generated, wherein N is an even number; the mathematical expression of the nth element A(n) in the sequence A is:

[0073]

[0074] wherein q is an arbitrary integer, and because of the excellent zero cyclic autocorrelation of CAZAC, different values can be selected to achieve the isolation of different sequences, and n takes values of 0, 1,..., N / 2-1.

[0075] (102) The conjugate symmetry transformation is performed on the sequence A to obtain a sequence B; the mathematical expression of the nth element B(n) in the sequence B is:

[0076]

[0077] wherein (·) * represents the conjugate operation, and n takes values of 0, 1,..., N / 2-1.

[0078] (103) The sequence A is multiplied by a weight factor sequence W to obtain a sequence C; the mathematical expression of the nth element of the weight factor sequence W is:

[0079]

[0080] The mathematical expression of the nth element of the sequence C is:

[0081]

[0082] wherein n takes values of 0, 1,..., N / 2-1.

[0083] (104) Sequence C is subjected to conjugate symmetric transformation to obtain sequence D; the mathematical expression of the nth element of sequence D is:

[0084]

[0085] Here, n takes values ​​of 0, 1, ..., N / 2-1.

[0086] (105) Sequence A, sequence B, sequence C, and sequence D are serially spliced ​​to obtain sequence S. The mathematical expression of sequence S is:

[0087] S(n)=[ABCD]

[0088] Here, n takes values ​​of 0, 1, ..., 2N-1.

[0089] (106) The end of the sequence S is truncated to a length of N CP The sequence CP is used as the cyclic prefix sequence, specifically N CP The value of is selected according to actual needs, generally N / 4, the structure is as follows Figure 1 shown.

[0090] (107) Sequences CP and S are serially concatenated to obtain the final preamble sequence SYN; the mathematical expression of sequence SYN is: SYN = [CP S]; then the preamble sequence SYN and the subsequent other partial signals are concatenated to obtain a baseband transmission signal, and finally the signal is sent to the wireless channel after oversampling and up-conversion;

[0091] At the receiving end, the processing flow is as follows Figure 2 As shown, it includes the following three steps:

[0092] (201) The received signal is down-converted and down-sampled to obtain a baseband time domain sequence y. The continuously received baseband time domain sequence y is subjected to mathematical operation with the local weight factor sequence W to obtain a timing metric sequence M(d). According to the setting of the preamble sequence length, the calculation storage window of the receiving end can be set to 2N. In this case, the timing metric function M(d) can be defined as:

[0093]

[0094] The mathematical expression for defining R(d) is:

[0095]

[0096] The mathematical expression of P(d) is defined as:

[0097]

[0098] The weight factor sequence W is arranged in the order of [1-1] or [jj]. In the specific engineering implementation, the complex multiplication of the weight factor is simplified to the sign conversion and the exchange of the real and imaginary parts, so the algorithm complexity is greatly reduced. Compare multiple consecutive timing measurement sequences M(d) and select the maximum value M max And record the corresponding position is the required timing synchronization point. According to the calculation result of the timing metric function, the starting point of the entire frame can be obtained, which can be expressed as

[0099]

[0100] (202) According to the estimated position Extract the received pilot sequence p from the continuously received baseband time domain sequence y, perform fractional carrier frequency deviation estimation and compensation, and then perform integer carrier frequency deviation estimation and compensation to obtain the processed received pilot sequence The specific process includes:

[0101] (2021) According to the estimated position The received pilot sequence p is extracted from the continuously received baseband time domain sequence y.

[0102] (2022) Using the weighted repetitiveness of the synchronization sequence before and after, a rough estimate of the fractional carrier frequency deviation of the received pilot sequence p is performed, which is expressed as follows:

[0103]

[0104] Among them, the coefficient sequence G is obtained from the weight factor sequence W, and the expression is:

[0105] G=[W * (n)W * (N / 2-1-n)], where n is 0, 1, ..., N / 2-1;

[0106] G(n) is the n-th element of G, p(n) is the n-th element of p, and n is 0, 1, ..., N-1.

[0107] (2023) Use After compensating the baseband time domain sequence y, the CP portion of the sequence is extracted to obtain the sequence p1. Sequence p1 is then used to accurately estimate the fractional carrier frequency deviation, as shown in the following expression:

[0108]

[0109] Among them, N CP is the length of the sequence CP, and p1(n) is the nth element of p1.

[0110] (2024) Use After compensating the baseband time-domain sequence y, extract the sequence S portion to obtain sequence p2. Using the special properties of the preamble sequence, namely the normalized integer frequency offset, sequences A and C are cyclically shifted rightward by integer points, and sequences B and D are cyclically shifted leftward by integer points. Sequence p2 is subjected to block-wise shifted cross-correlation with the local preamble sequence. The maximum value of the obtained results is selected to obtain the normalized integer frequency offset estimate, as expressed as follows:

[0111]

[0112] Here, g takes values ​​of 0, 1, ..., N-1, and p2(n) is the nth element of p2.

[0113] (2025) Use Compensate the received signal to obtain the processed received pilot sequence Complete the estimation and compensation of all carrier frequency deviations.

[0114] (203) Sequence S is combined with the processed received leading sequence Perform the common phase deviation estimation operation, the expression is:

[0115]

[0116] Where, for The nth element of , S(n) is the nth element of S;

[0117] Then use The common phase deviation of the baseband time domain sequence y is estimated and compensated.

[0118] Figure 3 、 Figure 4 and Figure 5 The simulation results are based on the MATLAB platform and only the leader sequence of the present invention is included for software simulation testing.

[0119] Figure 3 This is a comparison chart of the timing metric function curves of the method of the present invention and several typical synchronization algorithms under ideal conditions. To ensure that other variables other than the algorithm are as equal as possible when comparing performance, the training sequence length used by the synchronization algorithm is 2N (N = 1024), and the correct preamble sequence starts at position 2305. Figure 3As shown, the S&C algorithm increases only one correct correlation point each time, the curve is not very sharp, and there is a flat top effect with a length equal to the CP length at the peak of the curve; the Minn algorithm increases two correct correlation values each time, so the curve is sharper than the S&C algorithm, and because of the difference between the positive and negative signs in the front and back halves, the platform effect of the S&C algorithm is overcome, but there is a large sidelobe in the curve; the Park algorithm makes the peak value more sharp due to the introduction of the conjugate symmetry idea, but there is still a small sidelobe. The peak of the method of the application is sharper than that of the Shao algorithm, and there is almost no sidelobe.

[0120] Figure 4 is a timing detection probability comparison chart of the method of the application and several typical synchronization algorithms. In Figure 4 , the sequence length is 2N (N=1024), and under the condition of an AWGN channel with a normalized frequency offset of 35.555, the performance of the method of the application is better than that of the Minn algorithm and the Park algorithm, and the timing detection probability can reach 100% when the SNR is-6dB, and when the SNR is less than-6dB, the timing synchronization performance is slightly worse than that of the Shao algorithm, but the Shao algorithm introduces a complex weight factor in the timing synchronization step, and the calculation complexity is much higher than that of the algorithm. Therefore, the method of the application has the advantages of low complexity and good timing synchronization performance at low SNR, and has better comprehensive performance.

[0121] Figure 5 is a carrier frequency offset estimation mean square error comparison chart of the method of the application and the Shao algorithm. Figure 5 is a comparison of the frequency offset correction ability of the Shao algorithm and the algorithm of the application under different SNR conditions, considering the case of large Doppler shift and imperfect matching crystal oscillator at the transmitting and receiving ends, with a sequence length of 2N (N=1024) and a normalized frequency offset of ε=69.575. From Figure 5 it can be seen that the frequency offset estimation mean square error performance of the algorithm of the application is one order of magnitude better than that of the Shao algorithm.

Claims

1. A time-frequency synchronization method for a high-speed mobile platform, characterized in that: The following processes are included: The processing process at the transmitter is: (101) Generate a sequence A of length N / 2 with constant envelope and zero autocorrelation, where N is an even number; (102) Sequence A is subjected to conjugate symmetry transformation to obtain sequence B; (103) Multiplying sequence A by weight factor sequence W yields sequence C; (104) Sequence C is subjected to conjugate symmetry transformation to obtain sequence D; (105) Sequence A, sequence B, sequence C, and sequence D are serially spliced ​​in sequence to obtain sequence S; (106) The length of the end of sequence S is truncated to N CP The sequence CP is used as the cyclic prefix sequence; (107) Sequence CP and sequence S are serially concatenated to obtain the final preamble sequence SYN; the preamble sequence SYN and subsequent other partial signals are concatenated to obtain a baseband transmission signal, and finally the signal is sent to the wireless channel after oversampling and up-conversion; The processing process at the receiving end is: (201) The received signal is down-converted and down-sampled to obtain a baseband time domain sequence y, and the continuously received baseband time domain sequence y is mathematically operated with the weight factor sequence W to obtain a timing measurement sequence M(d); multiple consecutive timing measurement sequences M(d) are compared and the maximum value M is selected. max And record the corresponding position is the required timing synchronization point; the value range of d is set according to actual needs; (202) According to the estimated position Extract the received pilot sequence p from the continuously received baseband time domain sequence y, perform fractional carrier frequency deviation estimation and compensation, and then perform integer carrier frequency deviation estimation and compensation to obtain the processed received pilot sequence (203) Sequence S is combined with the received leading sequence Perform mathematical operations and use the results of the operations to estimate and compensate for the common phase deviation of the baseband time domain sequence y.

2. A time-frequency synchronization method for a high-speed mobile platform according to claim 1, characterized in that: The mathematical expression of the nth element A(n) in the sequence A in step (101) is: Where q is an arbitrary integer and n is 0, 1, ..., N / 2-1.

3. The time-frequency synchronization method for a high-speed mobile platform according to claim 2, characterized in that: The mathematical expression of the nth element B(n) in the sequence B in step (101) is: Where, (·) * Indicates conjugate operation, where n is 0, 1, ..., N / 2-1.

4. The time-frequency synchronization method for a high-speed mobile platform according to claim 2, characterized in that: The mathematical expression of the nth element C(n) in the sequence C in step (103) is: Wherein, the mathematical expression of the nth element of the weight factor sequence W is: n takes values ​​of 0, 1, ..., N / 2-1.

5. The time-frequency synchronization method for a high-speed mobile platform according to claim 4, characterized in that: The mathematical expression of the nth element D(n) in the sequence D in step (104) is: Where, (·) * Indicates conjugate operation, where n is 0, 1, ..., N / 2-1.

6. The time-frequency synchronization method for a high-speed mobile platform according to claim 5, characterized in that: The mathematical expression of the sequence S in step (105) is: S(n)=[ABCD] Where n is 0, 1, ..., 2N-1.

7. The time-frequency synchronization method for a high-speed mobile platform according to claim 1, characterized in that: In step (106), N CP The value of is N / 4.

8. The time-frequency synchronization method for a high-speed mobile platform according to claim 1, characterized in that: The specific process of step (201) is: The received signal is down-converted and down-sampled to obtain a baseband time domain sequence y. The continuously received baseband time domain sequence y is mathematically operated with the weight factor sequence W to obtain a timing metric sequence M(d). The calculation storage window at the receiving end is set to 2N. The mathematical expression of the timing metric function M(d) is: The mathematical expression of R(d) is: The mathematical expression of P(d) is: Where, the value of d is set according to actual needs, y(n) is the nth element of the baseband time domain sequence y, and the mathematical expression of the nth element of the weight factor sequence W is: n takes values ​​of 0, 1, ..., N / 2-1, (·) * Indicates conjugate operation; Then compare multiple consecutive timing measurement sequences M(d) and select the maximum value M max And record the corresponding position This is the timing synchronization point we are looking for, and the starting point of the entire frame is obtained. The expression is:

9. The time-frequency synchronization method for a high-speed mobile platform according to claim 8, characterized in that: The specific process of step (202) is as follows: (2021) According to the estimated position Extracting a received pilot sequence p from the continuously received baseband time domain sequence y; (2022) Using the weighted repetitiveness of the synchronization sequence before and after, a rough estimate of the fractional carrier frequency deviation of the received pilot sequence p is performed, which is expressed as follows: In the formula, the coefficient sequence G is obtained by splicing the weight factor sequence W, and the expression is: G = [W * (n)W * (N2-1-n)], n is 0, 1, ..., N / 2-1; G(n) is the nth element of G, p(n) is the nth element of p, n is 0, 1, ..., N-1; (2023) Use After compensating the baseband time domain sequence y, the CP portion of the sequence is extracted to obtain the sequence p1. Sequence p1 is then used to accurately estimate the fractional carrier frequency deviation, as shown in the following expression: Where N CP is the length of the sequence CP, p1(n) is the nth element of p1; (2024) Use After compensating the baseband time-domain sequence y, extract the sequence S portion to obtain sequence p2. Using the special properties of the preamble sequence, namely the normalized integer frequency offset, sequences A and C are cyclically shifted rightward by integer points, and sequences B and D are cyclically shifted leftward by integer points. Sequence p2 is subjected to block-wise shifted cross-correlation with the local preamble sequence. The maximum value of the obtained results is selected to obtain the normalized integer frequency offset estimate, as expressed as follows: Where g takes values ​​of 0, 1, ..., N-1, and p2(n) is the nth element of p2; (2025) Use Compensate the received signal to obtain the processed received pilot sequence Complete the estimation and compensation of all carrier frequency deviations.

10. The time-frequency synchronization method for a high-speed mobile platform according to claim 9, characterized in that: The specific process of step (203) is: Combine the sequence S with the processed received preamble sequence Perform the common phase deviation estimation operation, the expression is: Where, for The nth element of , S(n) is the nth element of S; then use The common phase deviation of the baseband time domain sequence y is estimated and compensated.

Images