Differential coding modulation communication method suitable for large Doppler frequency shift

Abstract

The invention belongs to the field of wireless communication, and particularly relates to wireless communication of aerospace crafts, in particular to a differential coding modulation communication method suitable for large Doppler frequency shift. The method comprises the following steps: 1, setting frame structure parameters; 2, framing is carried out according to power control, a synchronization header, a header and a data segment, and a frame structure is generated; step 3, quantizing and de-spreading the received signal to generate a to-be-synchronized data stream; 4, determining a frame synchronization position and a bit synchronization position, calculating frequency offset by using FFT (Fast Fourier Transform), and calculating channel frequency offset to carry out frequency offset compensation; 5, calculating a correlation value after compensating the data stream, and determining a coarse synchronization position of a receiving end frame; and 6, carrying out five-path parallel correlation peak calculation to obtain a data block bit synchronization position, a position ahead of two sampling points, a position lagging of two sampling points, a position lagging of one sampling point and a position ahead of one sampling point.

Description

Technical Field

[0001] The invention belongs to the field of wireless communication, and in particular relates to wireless communication of aerospace vehicles, and is a differential coding modulation communication method adapting to large Doppler frequency shift. Background Art

[0002] Direct sequence spread spectrum (DSSS) is a type of spread spectrum technology that uses a high-speed pseudo-random spread spectrum sequence to multiply the information waveform and modulate the carrier to expand the signal transmission bandwidth. Direct sequence spread spectrum technology has the advantages of anti-interference, anti-noise, anti-multipath fading, strong anti-decoding and multi-address multiplexing. It has been successfully applied to deep space exploration, remote control, telemetry and navigation communication.

[0003] With the rapid development of spacecraft, the demand for wireless communication is getting higher and higher. It faces the problems of high Doppler frequency shift and low signal-to-noise ratio of received signals caused by the high relative speed of the transmitter and receiver. It is necessary to focus on reducing the impact of high dynamics and low signal-to-noise ratio on receiving performance. Generally speaking, high dynamics usually refers to the relative speed of the aircraft exceeding Mach 3. Under this condition, signal capture and tracking of conventional direct sequence spread spectrum technology becomes difficult, requiring both a relatively wide capture range and a relatively short capture time, while ensuring capture accuracy. However, the capture range and capture time need to be designed in a balanced way. The wider the capture range, the longer the capture time, and the shorter the capture time, the narrower the capture range.

[0004] The core of direct sequence spread spectrum technology is to enhance the strength of received useful signals and improve the received signal-to-noise ratio through correlation operations. Under Gaussian channel conditions, the longer the correlation length, the greater the signal-to-noise ratio gain; but under Doppler frequency shift conditions, the correlation length cannot be increased indefinitely; at the same time, under high dynamic conditions, there are Doppler frequency shift changes. The capture and tracking of direct sequence spread spectrum signals under high dynamic and low signal-to-noise ratio conditions can be solved by improving the communication frame structure design, and the signal capture and tracking has a lower algorithm complexity. Summary of the invention

[0005] The purpose of the present invention is to propose a differential coding modulation communication method that adapts to large Doppler frequency shift. A spread spectrum modulation method combining differential coding based on a PN sequence and base code BPSK modulation is used to design a synchronization head. The receiving end despreads and dedifferentiates the received signal and then correlates it with the PN sequence. Due to the use of differential operation, the phase difference between the two PN sequences before and after is less than 180 degrees, which will not destroy the correlation of the synchronization head and can achieve coherent accumulation. At the same time, differential demodulation is used to reduce the influence of Doppler frequency shift and Doppler frequency shift change rate on signal demodulation.

[0006] The technical solution of the present invention:

[0007] A differential coding modulation communication method adapted to a large Doppler frequency shift, the steps are as follows:

[0008] Step 1: Set frame structure parameters;

[0009] Step 2: Frame according to power control, synchronization header, header and data segment to generate a frame structure;

[0010] Step 3: quantize and despread the received signal;

[0011] Step 4: Determine the frame synchronization position and the bit synchronization position, and use FFT to calculate the frequency deviation;

[0012] Step 5: Calculate the correlation value;

[0013] Step 6: Perform 5-way parallel despreading: bit synchronization position, leading 2 sampling point positions, lagging 2 sampling point positions, lagging 1 sampling point position and leading 1 sampling point position.

[0014] Furthermore, the total length of the frame structure is L, and the power control length is L agc , the synchronization head length is L sync , the header length is L mess , the data segment length is L data , the data segment consists of W data blocks.

[0015] Furthermore, the synchronization header includes two parts: a synchronization header base code and a PN sequence; the length of the PN sequence PNC is N, where N is a natural number;

[0016] The output sequence of the PN sequence after differential encoding is PND, with a length of N, and the reference bit D0 is set to 0. The bit-by-bit differential encoding is performed, and the calculation formula is: PND k =PNC k ⊕D k-1 ; where ⊕ represents XOR, k = 1, 2······N;

[0017] The length of the synchronization header base code SyncB is set to M sync ;

[0018] The output sequence of the synchronization header base code after BPSK modulation is SyncB mod , its length is still M sync ;

[0019] Carry out SyncB-based mod The spread spectrum modulation generates the synchronization header SYNC, and the synchronization header length L sync =M sync* N.

[0020] Furthermore, the header includes a header base code and a header source sequence:

[0021] The length of the header source sequence MessC is R, and the R value is user-defined;

[0022] MessC k After differential encoding, the output sequence is MessD, with a length of R, and the reference bit D0 is set to 0. The bit-by-bit differential encoding is performed, and the calculation formula is: MessD k =MessC k ⊕D k-1 ; Wherein ⊕ represents XOR, k = 1, 2······R, R is a natural number;

[0023] The length of the header source sequence MessC is set to M mess ;

[0024] The header base code MessB is modulated by BPSK and the output sequence is MessB mod , its length is still M mess ;

[0025] Carry out MessB-based mod The spread spectrum modulation generates the header MESS, and the header length is L mess =M mess *R.

[0026] Furthermore, the data segment includes a data segment base code and a data segment source sequence:

[0027] The data segment base code is the same as the header base code, and it remains the same after BPSK modulation. The data segment base code is MessB mod , whose length is M mess ;

[0028] The data segment consists of W data blocks, and the data segment sequence is decomposed into W data block source sequences;

[0029] The length of the data block source sequence DataB is Q, and the Q value is user-defined;

[0030] After differential encoding, the output sequence of DataB is DataBD, with a length of Q, and the reference bit D0 is set to 0. The bit-by-bit differential encoding is performed, and the calculation formula is: DataBD k =DataB k ⊕D k-1 ; Wherein ⊕ represents exclusive OR; k = 1, 2······Q, Q is a natural number;

[0031] Carry out MessB-based mod The spread spectrum modulation generates a data block BLOCK, the data block length L block =M mess* Q.

[0032] Furthermore, the header base code length Mmess and synchronization header base code length M sync The relationship between them is: mess =i*M sync , i is an integer greater than 1;

[0033] M sync Satisfy η(f d )≥0.9;

[0034] η(f d ) is the Doppler frequency shift f d The peak impact on the synchronization head coherence accumulation satisfies: where f d is the Doppler shift, C is the speed of light, v is the relative speed of the sender and receiver, and f is the carrier frequency; M sync is the synchronization header base code length;

[0035] T sync is the code width of the synchronization header, which is the inverse of the synchronization header code symbol rate; T is the coherent accumulation time, T = L sync *T sync .

[0036] The synchronization header base code and the header base code are spread spectrum codes and can be PN sequences.

[0037] Further, the quantization and despreading process is as follows:

[0038] Its feature is to simplify the number of quantization bits of the received signal while retaining complete effective information. The receiving end first performs 16-phase quantization on the received spread spectrum signal, that is, the signals in different angle ranges are mapped to 16 phases respectively. The received signal is correlated with the local ZC to obtain the correlation value, and then despreading is performed to obtain the despreading information.

[0039] The received signal (16-phase quantization) is conjugate-multiplied and then accumulated with the local SP54 (16-phase quantization). The relevant algorithm is as follows:

[0040]

[0041] Further, the process of determining the frame synchronization position and the bit synchronization position is as follows:

[0042] The syn_code of the synchronization header is used for rough estimation, and the frequency deviation estimation method in the frequency domain is used. That is, after the timing is completed, the N peaks in the first correlation result are taken from the timing position and multiplied by the syn_code after local difference. The correlation result is obtained by M-point FFT, and then the maximum value is obtained by modulus. The frequency deviation is calculated based on the maximum value and its corresponding subscript position, and the first-level frequency deviation fre_set1 is estimated without rough correction.

[0043] The frequency deviation calculated by FFT is:

[0044]

[0045] Where b is the position where the FFT output sequence has the largest modulus value, F symb is the synchronization header symbol rate, M is the number of FFT points, and the process of calculating the correlation value is as follows:

[0046] The signal after despreading is represented by b(0), b(1), ..., b(M-1), then the differential decoding process can be expressed as:

[0047]

[0048] Perform correlation processing to obtain a preliminary frame synchronization position.

[0049] Further, the 5-way parallel despreading process is as follows:

[0050] Separately 1 Perform offset-related peak search.

[0051] In order to ensure the correlation performance of long sequences, multiple frequency slots need to be searched. Code capture is a two-dimensional search process that is performed simultaneously on the time axis and the frequency axis. The frequency deviation is reduced to less than 1 / 5 of the original by using 5-way parallelism. Since the traditional sliding correlation method requires each search unit to be captured separately, it takes a lot of time. Therefore, when using the time domain correlation method for capture, multiple search units can often be captured, and the higher the degree of parallelism, the faster the capture speed, but the hardware resources consumed also increase accordingly.

[0052] Beneficial effects of the present invention:

[0053] The present invention discloses a differential coding modulation communication method adapting to large Doppler frequency shift. A spread spectrum modulation method combining differential coding based on a PN sequence and base code BPSK modulation is adopted to design a synchronization head. A receiving end despreads and dedifferentiates a received signal and then correlates it with the PN sequence. Due to the adoption of differential operation, the phase difference between the front and rear PN sequences is less than 180 degrees, which will not destroy the correlation of the synchronization head and can realize coherent accumulation. Meanwhile, differential demodulation is adopted to reduce the influence of Doppler frequency shift and Doppler frequency shift change rate on signal demodulation. BRIEF DESCRIPTION OF THE DRAWINGS

[0054] Figure 1 is a signal transmission generation flow chart in an implementation example of the present invention;

[0055] Figure 2 is a schematic diagram of the frame structure length in an implementation example of the present invention;

[0056] Figure 3 is a schematic diagram of synchronization header generation in an implementation example of the present invention;

[0057] Figure 4 It is a schematic diagram of header generation in an implementation example of the present invention;

[0058] Figure 5 is a schematic diagram of data block generation in an implementation example of the present invention;

[0059] Figure 6 Schematic diagram of the effect of Doppler frequency offset on synchronization head coherent accumulation peak value in an implementation example of the present invention;

[0060] Figure 7 Block diagram of the receiver with high-Doppler frequency offset resistance;

[0061] Figure 8 Block diagram of multi-channel synchronous search for correlation peak synchronous reception. DETAILED DESCRIPTION

[0062] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0063] The present invention discloses a differential coding modulation communication method adapting to large Doppler frequency shift, comprising the following steps:

[0064] The signal transmission includes the agreed basic frame structure, synchronization header generation, header and data segment generation, framing, etc. Figure 1 shown.

[0065] S1 agreed basic frame structure

[0066] The frame structure consists of power control, synchronization header, header and data segment. The frame length is L, the power control length is L agc , the synchronization head length is L sync , the header length is L mess , the data segment length is L data , the data segment consists of W data blocks, such as Figure 2 shown.

[0067] S2 sync header, header and data segment generation

[0068] S2.1 is a synchronous head design according to S1, characterized in that, Figure 3 As shown:

[0069] a) The synchronization header design includes two parts: the synchronization header base code and the PN sequence;

[0070] b) the length of the PN sequence PNC is N, where N is a natural number;

[0071] c) After differential encoding, the output sequence of the PN sequence is PND, with a length of N, and the reference bit D0 is set to 0. The bit-by-bit differential encoding is performed, and the calculation formula is: PND k =PNC k ⊕D k-1 ; where ⊕ represents XOR,

[0072] k=1,2······N;

[0073] d) The length of the synchronization header base code SyncB is set to M sync ;

[0074] e) The output sequence of the synchronization header base code after BPSK modulation is SyncB mod , its length is still M sync ;

[0075] f) Implement SyncB-based PND mod Spread spectrum modulation generates synchronization header SYNC, the synchronization header length

[0076] L sync =M sync* N.

[0077] S2.2 is a header design according to S1, characterized in that, Figure 4 As shown:

[0078] a) Header design includes header base code and header source sequence;

[0079] b) The length of the header source sequence MessC is R, and the value of R is user-defined;

[0080] c)MessC k After differential encoding, the output sequence is MessD, with a length of R, and the reference bit D0 is set to 0. The bit-by-bit differential encoding is performed, and the calculation formula is: MessD k =MessC k ⊕D k-1 ; Wherein ⊕ represents XOR, k = 1, 2······R, R is a natural number;

[0081] d) The length of the header source sequence MessC is set to M mess ;

[0082] e) The header base code MessB is modulated by BPSK and the output sequence is MessB mod , its length is still

[0083] M mess ;

[0084] f) Carry out MessB-based mod Spread spectrum modulation generates the header MESS, the header length

[0085] L mess =M mess* R.

[0086] S2.3 is designed according to the data segment described in S1, characterized in that, Figure 5 As shown:

[0087] a) Data segment design includes data segment base code and data segment source sequence;

[0088] b) The data segment base code is the same as the header base code, and it remains the same after BPSK modulation, that is, the data segment base code is MessB mod , whose length is M mess ;

[0089] c) A data segment is composed of W data blocks, and a data segment sequence is decomposed into W data block source sequences;

[0090] d) The length of the data block source sequence DataB is Q, and the Q value is user-defined;

[0091] e) After differential encoding, the output sequence of DataB is DataBD, with a length of Q, and the reference bit D0 is set to 0. The bit-by-bit differential encoding is performed, and the calculation formula is: DataBD k =DataB k ⊕D k-1 ; Wherein ⊕ represents exclusive OR; k = 1, 2······Q, Q is a natural number;

[0092] f) Carry out MessB-based mod The spread spectrum modulation generates a data block BLOCK, the data block length L block =M mess* Q.

[0093] S2.4 is designed based on the synchronization header base code and the header base code described in S2.1, S2.2 and S2.3, and is characterized by:

[0094] a) Header base code length M mess and synchronization header base code length M sync The relationship between them is: mess =

[0095] i*M sync , i is an integer greater than 1;

[0096] b)M sync The selection must satisfy η(f d )≥0.9;

[0097] c)η(f d ) is the Doppler frequency shift f d The peak impact on the synchronization head coherence accumulation satisfies:

[0098] where f d is the Doppler shift, C is the speed of light, v is the relative speed of the sender and receiver, and f is the carrier frequency; M sync is the synchronization header base code length;

[0099] T sync is the code width of the synchronization header, which is the inverse of the synchronization header code symbol rate; T is the coherent accumulation time, T = L sync *T sync .

[0100] d) The synchronization header base code and the header base code are spread spectrum codes and can be PN sequences.

[0101] S3 Framing

[0102] The frame structure is generated by assembling the frames in the order of synchronization header, header and data segment.

[0103] S4 Reception

[0104] S4.1 Design the receiving logic based on the synchronization header design described in S1

[0105] a) The received signal enters the synchronization module after quantization and despreading

[0106] b) Use the first correlation value to make a second difference and then accumulate to determine the frame synchronization position and bit synchronization position, and use FFT to calculate the frequency deviation

[0107] c) The receiving end first performs 16-phase quantization on the received spread spectrum signal, that is, the signals in different angle ranges are mapped to 16 phases respectively, and the received signal is correlated with the local ZC to obtain the correlation value

[0108] d) The synchronization module adopts a 5-way synchronization search mode and performs 5-way parallel despreading at the same time: bit synchronization position, leading 2 sampling point positions, lagging 2 sampling point positions, lagging 1 sampling point position and leading 1 sampling point position, which can quickly complete the synchronization search function.

[0109] Figure 6 A schematic diagram describing the effect of Doppler frequency shift on the synchronization head coherent accumulation peak value in an implementation example of the present invention. The Doppler frequency shift value is ±26kHz, the synchronization head base code symbol rate is 6.4MHz, and the coherent accumulation length is generally selected to be controlled within 70, that is, the synchronization head length L sync Controlled between 0-70.

[0110] The above is only a specific embodiment of the present invention, and the present invention is described in detail. The unexplained part is a conventional technology. However, the protection scope of the present invention is not limited to this. Any changes or substitutions that can be easily thought of by any technician familiar with the technical field within the technical scope disclosed by the present invention should be included in the protection scope of the present invention. The protection scope of the present invention shall be based on the protection scope of the claims.

Claims

1. A differential coding modulation communication method adapted to large Doppler frequency shift, characterized in that: The steps are as follows: Step 1: Set frame structure parameters; Step 2: Frame according to power control, synchronization header, header and data segment to generate a frame structure; Step 3: quantize and despread the received signal; Step 4: Determine the frame synchronization position and the bit synchronization position, and use FFT to calculate the frequency deviation; Step 5: Calculate the correlation value; Step 6: Perform five-way parallel despreading, the five ways include: bit synchronization position, 2 sampling point positions ahead, 2 sampling point positions behind, 1 sampling point position behind and 1 sampling point position ahead.

2. The method according to claim 1, characterized in that: The total length of the frame structure is L, and the power control length is L agc , the synchronization head length is L sync , the header length is L mess , the data segment length is L data , the data segment consists of W data blocks.

3. The method according to claim 2, characterized in that: The synchronization header includes two parts: the synchronization header base code and the PN sequence; the length of the PN sequence PNC is N, where N is a natural number; The output sequence of the PN sequence after differential encoding is PND, with a length of N, and the reference bit D0 is set to 0. The bit-by-bit differential encoding is performed, and the calculation formula is: PND k =PNC k ⊕D k-1 ; where ⊕ represents XOR, k = 1, 2······N; The length of the synchronization header base code SyncB is set to M sync ; The output sequence of the synchronization header base code after BPSK modulation is SyncB mod , its length is still M sync ; Carry out SyncB-based mod The spread spectrum modulation generates the synchronization header SYNC, and the synchronization header length L sync =M sync* N.

4. The method according to claim 3, characterized in that: The header includes the header base code and the header source sequence: The length of the header source sequence MessC is R, and the R value is user-defined; MessC k After differential encoding, the output sequence is MessD, with a length of R, and the reference bit D0 is set to 0. The bit-by-bit differential encoding is performed, and the calculation formula is: in represents XOR, k = 1, 2, ..., R, R is a natural number; The length of the header source sequence MessC is set to M mess ; The header base code MessB is modulated by BPSK and the output sequence is MessB mod , its length is still M mess ; Carry out MessB-based mod The spread spectrum modulation generates the header MESS, and the header length is L mess =M mess* R.

5. The method according to claim 4, characterized in that: The data segment includes the data segment base code and the data segment source sequence: The data segment base code is the same as the header base code, and it remains the same after BPSK modulation. The data segment base code is MessB mod , whose length is M mess ; The data segment consists of W data blocks, and the data segment sequence is decomposed into W data block source sequences; The length of the data block source sequence DataB is Q, and the Q value is user-defined; After differential encoding, the output sequence of DataB is DataBD, with a length of Q, and the reference bit D0 is set to 0. The bit-by-bit differential encoding is performed, and the calculation formula is: in represents exclusive OR; k = 1, 2······Q, where Q is a natural number; Carry out MessB-based mod The spread spectrum modulation generates a data block BLOCK, the data block length L block =M mess* Q.

6. The method according to claim 5, characterized in that: Header base code length M mess and synchronization header base code length M sync The relationship between them is: mess =i*M sync , i is an integer greater than 1; M sync Satisfy η(f d )≥0.9; η(f d ) is the Doppler frequency shift f d The peak impact on the synchronization head coherence accumulation satisfies: where f d is the Doppler shift, C is the speed of light, v is the relative speed of the sender and receiver, and f is the carrier frequency; M sync is the synchronization header base code length; T sync is the code width of the synchronization header, which is the inverse of the synchronization header code symbol rate; T is the coherent accumulation time, T = L sync *T sync ; The synchronization header base code and the header base code are spread spectrum codes, which are PN sequences.

7. The method according to claim 6, characterized in that: The quantization and despreading process is as follows: The receiving end first performs 16-phase quantization on the received spread spectrum signal, maps the signals in different angle ranges to 16 phases respectively, correlates the received signal with the local ZC, obtains the correlation value, and performs despreading processing.

8. The method according to claim 7, characterized in that: The process of determining the frame synchronization position and the bit synchronization position is as follows: The syn_code of the synchronization header is used for rough estimation. The frequency deviation estimation method in the frequency domain is used. After the timing is completed, the N peaks in the first correlation result are taken from the timing position and multiplied with the syn_code after local difference. The correlation result is obtained by M-point FFT, and then the modulus is taken to find the maximum value. The frequency deviation is calculated based on the maximum value and its corresponding subscript position, and the first-level frequency deviation fre_set1 is estimated without rough correction. The frequency deviation calculated by FFT is: Where b is the position where the FFT output sequence has the largest modulus value, F symb is the synchronization header symbol rate, and M is the number of FFT points.

9. The method according to claim 8, characterized in that: The process of calculating the correlation value is as follows: The despread signal is represented as b(0), b(1), …, b(M-1), then the differential decoding process can be expressed as: Perform correlation processing to obtain a preliminary frame synchronization position.

10. The method according to claim 9, characterized in that: The process of performing 5-way parallel despreading is as follows: Separately f2π-ts 、e f2π-2ts 、e f2π2ts 、e f2π2ts , 1 to search for offset-related peaks.

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