Time-frequency synchronization method for high-speed mobile platform

Abstract

The invention relates to the technical field of wireless communication, in particular to a time-frequency synchronization method for a high-speed mobile platform. Comprising the following steps of: obtaining a leader sequence at a transmitting end based on a constant envelope zero autocorrelation sequence through mathematical deformation such as weight factor, conjugate processing, symmetric transformation and the like; the method comprises the following steps: (1) carrying out mathematical operation on continuously received signals and a local weight factor sequence to obtain a timing measurement sequence, selecting a maximum value, and recording a corresponding position to complete symbol timing synchronization; and (2) extracting a receiving pilot frequency sequence, performing fractional-multiple carrier frequency deviation estimation and compensation, and then performing integral-multiple carrier frequency deviation estimation and compensation. And (3) extracting a receiving pilot frequency sequence, and performing common phase deviation estimation and compensation by using a local preamble sequence. According to the invention, high-accuracy timing acquisition and high-precision frequency offset correction capabilities are realized under the conditions of low signal-to-noise ratio, high dynamic Doppler frequency shift and unmatched crystal oscillators of the receiving and transmitting ends.

Description

Technical Field

[0001] The present invention relates to the field of wireless communication technology, and in particular to a time-frequency synchronization method for a high-speed mobile platform. Background Art

[0002] Unmanned aerial vehicles that can act as flying base stations are seen as a promising means to expand wireless network coverage and improve rate performance. There are several advantages to using aerial base stations. First, due to the high altitude, aerial base stations have more opportunities to establish line-of-sight connections with ground users, thereby alleviating signal blocking and shadow fading. Second, due to the height adjustability and maneuverability of aerial platforms, they can be easily moved to the vicinity of potential ground users and establish reliable connections with low transmit power to achieve flexible deployment to provide fast on-demand communications. The air-to-air channel is mainly dominated by the line-of-sight wireless transmission component. Although there may be limited multipath fading due to ground reflections, its impact is minimal compared to air-to-ground or ground-to-ground channels. In addition, due to the large relative speed between air-to-air platforms, the air-to-air channel may have a high Doppler shift. Air-to-air communications dominated by line-of-sight links can achieve high-capacity link transmission through millimeter waves, but their potential high relative speeds plus the high frequencies of the millimeter wave band may result in excessively high Doppler shifts. Therefore, one of the challenges is to study the technology for quickly and accurately implementing time-frequency synchronization in view of the unique channel characteristics in air-to-air scenarios. Summary of the invention

[0003] The present invention proposes a preamble sequence structure and an algorithm flow for using the preamble sequence at the receiving end to complete symbol timing synchronization and frequency offset estimation and correction. The present invention realizes high-accuracy timing capture and high-precision frequency offset correction capabilities under low signal-to-noise ratio conditions, high dynamic Doppler frequency shift, and mismatched crystal oscillators at the transmitting and receiving ends.

[0004] The technical solution adopted by the present invention is:

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

[0006] The processing process at the transmitter is:

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

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

[0009] (103) Multiplying sequence A by weight factor sequence W obtains sequence C;

[0010] (104) Sequence D is obtained by conjugate symmetry transformation of sequence C;

[0011] (105) Sequence A, sequence B, sequence C, and sequence D are serially spliced ​​in sequence to obtain sequence S;

[0012] (106) The length of the end of sequence S is truncated to N CP The sequence CP is used as the cyclic prefix sequence;

[0013] (107) Serially concatenate the sequence CP and the sequence S to obtain a final preamble sequence SYN; concatenate the preamble sequence SYN and other subsequent signals to obtain a baseband transmission signal, and finally send the signal to a wireless channel after oversampling and up-conversion;

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

[0015] (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 subjected to mathematical operation 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;

[0016] (202) Based on 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

[0017] (203) Sequence S and the received leading sequence Mathematical operations are performed, and the results of the operations are used to estimate and compensate for the common phase deviation of the baseband time domain sequence y.

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

[0019]

[0020] Where q is an arbitrary integer and n can be 0, 1, ..., N / 2-1.

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

[0022]

[0023] In the formula, (·) * Indicates conjugate operation, where n can be 0, 1, ..., N / 2-1.

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

[0025]

[0026] In the formula, the mathematical expression of the nth element of the weight factor sequence W is: n can take values ​​of 0, 1, ..., N / 2-1.

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

[0028]

[0029] In the formula, (·) * Indicates conjugate operation, where n can be 0, 1, ..., N / 2-1.

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

[0031] S(n)=[ABCD]

[0032] Where n is 0, 1, ..., 2N-1.

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

[0034] Furthermore, the specific process of step (201) is as follows:

[0035] 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 and the weight factor sequence W are mathematically operated to obtain a timing metric sequence M(d); the calculation storage window of the receiving end is set to 2N, and the mathematical expression of the timing metric function M(d) is:

[0036]

[0037] Among them, the mathematical expression of R(d) is:

[0038]

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

[0040]

[0041] Wherein, 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 the conjugate operation;

[0042] Then compare multiple consecutive timing measurement sequences M(d) and select the maximum value M max And record the corresponding position That is the timing synchronization point we need, 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) Based on the estimated position Extracting a received pilot sequence p from a continuously received baseband time domain sequence y;

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

[0047]

[0048] In the formula, the coefficient sequence G is obtained by concatenating 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 nth element of G, p(n) is the nth element of p, n is 0, 1, ..., N-1;

[0049] (2023) Use After compensating the baseband time domain sequence y, extract the CP part of the sequence to obtain the sequence p1, and then use the sequence p1 to accurately estimate the fractional carrier frequency deviation. The expression is as follows:

[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 part to obtain the sequence p2; then use the special property of the leading sequence, that is, the normalized integer frequency offset to make sequences A and C cyclically shift right by integer points, and sequences B and D cyclically shift left by integer points, respectively. Perform shift cross-correlation operation on sequence p2 and the local leading sequence in blocks, and select the maximum value from the obtained results to obtain the normalized integer frequency offset estimation, which is 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] In the formula, 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] In the application scenario of dynamic communication of high-speed moving air-to-air platforms, the present invention proposes a low-complexity time-frequency synchronization algorithm based on CAZAC sequence. Under the condition of low signal-to-noise ratio, the leading sequence of the present invention can have the advantages of good timing synchronization performance and high accuracy under the premise of low computational complexity. At the same time, in the case of large Doppler frequency shift and imperfect matching of crystal oscillators at the transmitting and receiving ends, the large carrier frequency deviation generated also has high-precision estimation and compensation performance. BRIEF DESCRIPTION OF THE DRAWINGS

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

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

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

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

[0067] Figure 5 It is a comparison diagram of the mean square error of carrier frequency deviation estimation between the method of the present invention and the Shao algorithm. DETAILED DESCRIPTION

[0068] The present invention is described in detail below with reference to the accompanying drawings.

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

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

[0071] On the transmitter side, there are seven steps:

[0072] (101) Generate a CAZAC (Constant Amplitude Zero Auto Correlation) sequence A of length N / 2, where N is an even number. The mathematical expression of the nth element A(n) in the sequence A is:

[0073]

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

[0075] (102) Sequence B is obtained by performing conjugate symmetry transformation on sequence A. The mathematical expression of the nth element B(n) in sequence B is:

[0076]

[0077] in,(·) * Indicates conjugate operation, where n can be 0, 1, ..., N / 2-1.

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

[0079]

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

[0081]

[0082] Among them, n takes values ​​of 0, 1, ..., N / 2-1.

[0083] (104) Sequence D is obtained by performing conjugate symmetry transformation on sequence C. The mathematical expression of the nth element of sequence D is:

[0084]

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

[0086] (105) Sequence A, sequence B, sequence C and sequence D are serially concatenated 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, and the structure is as follows Figure 1 shown.

[0090] (107) Sequence CP and sequence S are serially concatenated to obtain a final preamble sequence SYN; the mathematical expression of sequence SYN is: SYN = [CP S]; then the preamble sequence SYN and other subsequent partial signals are concatenated to obtain a baseband transmission signal, and finally the signal is sent to a 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, and the continuously received baseband time domain sequence y and the local weight factor sequence W are mathematically operated 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. At this time, the timing metric function M(d) can be defined as:

[0093]

[0094] Among them, the mathematical expression 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 transformation 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) Based on 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) Based on the estimated position The received pilot sequence p is extracted from the continuously received baseband time domain sequence y.

[0102] (2022) Using the weighted repeatability before and after the synchronization sequence, a rough estimate of the fractional carrier frequency deviation of the received pilot sequence p is performed, and the expression is 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)], n is 0, 1, ..., N / 2-1;

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

[0107] (2023) Use After compensating the baseband time domain sequence y, extract the CP part of the sequence to obtain the sequence p1, and then use the sequence p1 to accurately estimate the fractional carrier frequency deviation. The expression is as follows:

[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 part to obtain the sequence p2; then use the special property of the leading sequence, that is, the normalized integer frequency offset to make sequences A and C cyclically shift right by integer points, and sequences B and D cyclically shift left by integer points, respectively. Perform shift cross-correlation operation on sequence p2 and the local leading sequence in blocks, and select the maximum value from the obtained results to obtain the normalized integer frequency offset estimation, which is expressed as follows:

[0111]

[0112] Among them, g takes values ​​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] In the formula, 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 experiment conditions are based on the MATLAB platform, and only the leading sequence of the present invention is included for software simulation testing.

[0119] Figure 3 The following 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. In order to ensure that other variables except 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 leading sequence starts at 2305. Figure 3As shown, due to the repeatability of the sequence, the S&C algorithm will only add one correct correlation point each time the correlation summation is performed, the curve is not very sharp, and there will be a flat-top effect with a length equal to the CP length at the peak of the curve; the Minn algorithm will add two correct correlation values ​​each time the summation is performed, so the curve is sharper than the S&C algorithm, and because there is a difference in positive and negative signs between the front and back halves, the platform effect of the S&C algorithm is overcome, but the curve has a large side lobe; the Park algorithm makes the peak sharper due to the introduction of the conjugate symmetry idea, but there are still small side lobes. Compared with the Shao algorithm, the peak of the method of the present invention is sharper and there are almost no side lobes.

[0120] Figure 4 This is a comparison chart of the timing detection probability of the method of the present invention and several typical synchronization algorithms. Figure 4 In the example, the sequence length is 2N (N=1024). Under the AWGN channel condition with a normalized frequency deviation of 35.555, the performance of the method of the present invention is better than that of the Minn algorithm and the Park algorithm. When the SNR is -6dB, the timing detection probability can reach 100%. When the SNR is less than -6dB, the timing synchronization performance is slightly worse than that of the Shao algorithm. However, the Shao algorithm introduces a complex weight factor in the timing synchronization step, and the computational complexity is much higher than that of the algorithm proposed in this paper. Therefore, the method of the present invention has the advantages of low complexity and good timing synchronization performance at low signal-to-noise ratio, and has stronger comprehensive performance.

[0121] Figure 5 It is a comparison diagram of the mean square error of carrier frequency deviation estimation between the method of the present invention and the Shao algorithm. Figure 5 Considering the situation of large Doppler frequency shift and imperfectly matched crystal oscillators at the transmitting and receiving ends, under different signal-to-noise ratio conditions, the sequence length is 2N (N = 1024), and when the normalized frequency offset ε = 69.575 is added, the frequency offset correction capabilities of the Shao algorithm and the algorithm proposed in this paper are compared. Figure 5 It can be seen that the mean square error performance of the frequency offset estimation of the algorithm proposed in this paper 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 process includes: The processing process at the transmitter is: (101) Generate a sequence A with a length of N / 2 and a constant envelope and zero autocorrelation, where N is an even number; (102) performing conjugate symmetry transformation on sequence A to obtain sequence B; (103) Multiplying sequence A by weight factor sequence W obtains 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) Serially concatenate the sequence CP and the sequence S to obtain a final preamble sequence SYN; concatenate the preamble sequence SYN and other subsequent signals to obtain a baseband transmission signal, and finally send the signal to a 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 subjected to mathematical operation 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) Based on 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 and the received leading sequence Mathematical operations are performed, and the results of the operations are used 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 can be 0, 1, ..., N / 2-1.

3. A 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: In the formula, (·) * Indicates conjugate operation, where n can be 0, 1, ..., N / 2-1.

4. A 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: In the formula, the mathematical expression of the nth element of the weight factor sequence W is: n can take values ​​of 0, 1, ..., N / 2-1.

5. A 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: In the formula, (·) * Indicates conjugate operation, where n can be 0, 1, ..., N / 2-1.

6. A 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 as follows: 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 and the weight factor sequence W are mathematically operated to obtain a timing metric sequence M(d); the calculation storage window of the receiving end is set to 2N, and the mathematical expression of the timing metric function M(d) is: Among them, the mathematical expression of R(d) is: The mathematical expression of P(d) is: Wherein, 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 the conjugate operation; Then compare multiple consecutive timing measurement sequences M(d) and select the maximum value M max And record the corresponding position That is the timing synchronization point we need, and the starting point of the entire frame is obtained. The expression is:

9. A 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) Based on the estimated position Extracting a received pilot sequence p from a continuously received baseband time domain sequence y; (2022) Using the weighted repeatability before and after the synchronization sequence, a rough estimate of the fractional carrier frequency deviation of the received pilot sequence p is performed, and the expression is as follows: In the formula, the coefficient sequence G is obtained by concatenating 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, extract the CP part of the sequence to obtain the sequence p1, and then use the sequence p1 to accurately estimate the fractional carrier frequency deviation. The expression is as follows: 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 part to obtain the sequence p2; then use the special property of the leading sequence, that is, the normalized integer frequency offset to make sequences A and C cyclically shift right by integer points, and sequences B and D cyclically shift left by integer points, respectively. Perform shift cross-correlation operation on sequence p2 and the local leading sequence in blocks, and select the maximum value from the obtained results to obtain the normalized integer frequency offset estimation, which is 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. A time-frequency synchronization method for a high-speed mobile platform according to claim 9, characterized in that: The specific process of step (203) is as follows: Combine the sequence S with the processed received preamble sequence Perform the common phase deviation estimation operation, the expression is: In the formula, 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.

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