MSK fast frame capture and symbol synchronization method based on super symbols

Abstract

The invention discloses an MSK fast frame capture and symbol synchronization method based on a super symbol, and belongs to the technical field of wireless communication. The method comprises the following steps: firstly, by taking a super symbol as a basic processing unit, completing multi-template complex matching of the super symbol by using Fourier transform on the basis of high-power oversampling processing to form a super symbol matching matrix; and then, on the basis of the super-symbol matching matrix, extracting a corresponding super-symbol sequence from the frame header sequence according to the content of the frame header sequence, completing quasi-coherent combination of the super-symbol sequence by using Fourier transform, extracting the most possible path from multiple paths of quasi-coherent combination results to perform peak judgment, and completing frame capture and symbol synchronization after successful judgment. According to the method, the problem that the symbol boundary is difficult to directly judge through phase jump in the complete MSK modulation waveform is solved, and the requirements of MSK burst communication on frame capture and symbol synchronization are met.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of wireless communication, and in particular to a MSK fast frame capture and symbol synchronization method based on super symbol. BACKGROUND

[0002] MSK (Minimum Shift Keying) is a common digital modulation technology with constant envelope and low out-of-band radiation. MSK demodulation is divided into coherent demodulation and non-coherent demodulation, which have great differences in performance, implementation complexity and dependence on channel estimation. MSK coherent demodulation technology has better anti-noise performance and is compatible with advanced signal processing technology, but has high implementation complexity. MSK non-coherent demodulation technology has strong anti-frequency offset capability and low demodulation complexity, but has a large gap in performance compared with coherent demodulation.

[0003] In scenarios with high requirements for transmission reliability, coherent demodulation is the best choice. MSK coherent demodulation requires accurate carrier synchronization and symbol synchronization. The purpose of carrier synchronization is to obtain the frequency and initial phase of the carrier, and symbol synchronization needs to strictly match the symbol period. If the symbol period estimation is not accurate in the demodulation process, the sampling point will be offset, which will cause symbol decision error. MSK is a continuous phase modulation, and the phase change between symbols is smooth, and there is no obvious jump feature at the symbol boundary, so it is difficult to directly determine the symbol time by phase jump. In burst communication, it is particularly difficult to quickly and accurately estimate symbol synchronization.

[0004] The symbol synchronization of general communication systems adopts the method of oversampling + matched filtering to obtain the best sampling point. In order to ensure the reliability of the algorithm, the communication system needs to estimate the best sampling point by combining multiple symbols, and realizes frame capture and symbol synchronization through sequence matching. When not at the best sampling point, the single symbol matched filtering result is not a peak value, and the sequence matching result is also not a peak value. When at the best sampling point, the single symbol matched filtering result is a peak value, and if the symbol sequence is completely consistent with the frame header sequence, coherent accumulation of symbol information can obtain a correlation peak with large energy, otherwise the information cancellation during coherent accumulation will cause the output energy to be close to zero. In burst communication, the receiver cannot predict the accurate frequency and phase of the received signal before frame header capture. When the demodulated data has frequency offset, even at the best sampling point and the symbol sequence is completely consistent with the frame header sequence, the coherent accumulation of the frame header sequence will still produce certain information cancellation, causing capture failure. SUMMARY

[0005] In view of the problem that the complete MSK modulation waveform is difficult to directly judge the symbol boundary through phase jump, the application provides a MSK fast frame capture and symbol synchronization method based on super symbol.

[0006] The technical scheme adopted by the application is:

[0007] A MSK fast frame capture and symbol synchronization method based on super symbol is applied to a signal receiving end, and is used for frame capture and symbol synchronization of signals sent by a sending end, wherein the sending end bundles four MSK symbols into a group in a frame header part, and forms a super symbol, and forms a super symbol sequence by super symbols carrying different information, and forms a frame header.

[0008] Step 1, processing the received signal to obtain a complex baseband signal;

[0009] Step 2, slidingly extracting data from the complex baseband signal according to the super symbol length of high times oversampling, and performing super symbol matching through Fourier transform to output a super symbol matching matrix;

[0010] Step 3, in the super symbol matching matrix, extracting the corresponding super symbol sequence according to the arrangement form of the frame header sequence, and completing quasi-coherent combination of the super symbol sequence through Fourier transform to obtain a plurality of quasi-coherent combination results;

[0011] Step 4, extracting the path with the maximum energy from the plurality of quasi-coherent combination results to perform peak value judgment, if the peak value is found, step 5 is executed, otherwise step 3 is continuously executed;

[0012] Step 5, determining the position of the symbol best sampling point and the position of the frame header end according to the peak value position, and realizing frame capture and symbol synchronization; in addition, the frequency offset information of the current received frame is obtained according to the serial number of the maximum value in the Fourier transform result of the frame header sequence.

[0013] Further, the four MSK symbols in a super symbol carry the same information.

[0014] The application has the advantages that:

[0015] 1. The method is simple, stable and reliable, and solves the problem of high hardware resource consumption in the traditional method at the cost of a small amount of demodulation performance, and is a method that can complete non-coherent frequency shift keying demodulation under resource limited conditions.

[0016] 2、The application has strong anti-frequency offset capability and does not need additional overhead, and is suitable for a communication scene with large frequency offset.

[0017] 3、The application solves the problem that a complete MSK modulation waveform is difficult to directly pass through phase jump to judge the symbol boundary, and meets the requirements of frame capture and symbol synchronization of MSK burst communication. BRIEF DESCRIPTION OF DRAWINGS

[0018] In order to more clearly illustrate the technical solutions in the embodiments of the application, the drawings needed to be used in the embodiments or prior art description will be briefly introduced below.

[0019] Figure 1 is an example diagram of a super symbol 0 waveform in the embodiments of the application.

[0020] Figure 2 is an example diagram of a super symbol 1 waveform in the embodiments of the application.

[0021] Figure 3 is a schematic diagram of a frame header waveform fragment in the embodiments of the application.

[0022] Figure 4 is a super symbol matching flowchart in the embodiments of the application.

[0023] Figure 5 is a sequence quasi-coherent matching flowchart in the embodiments of the application. DETAILED DESCRIPTION

[0024] The technical solutions of the application will be further described in detail below with reference to the drawings.

[0025] A MSK fast frame capture and symbol synchronization method based on a super symbol is applied to a signal receiving end, and is used for frame capture and symbol synchronization of a signal sent by a sending end, wherein the sending end bundles four MSK symbols into a group in a frame header part, and forms a super symbol, and forms a super symbol sequence by using super symbols carrying different information, and forms a frame header; the method comprises the following steps:

[0026] Step 1, processing a received signal to obtain a complex baseband signal;

[0027] Step 2, slidingly extracting data from the complex baseband signal according to a super symbol length of high times oversampling, and performing super symbol matching through Fourier transform to output a super symbol matching matrix;

[0028] Step 3, in the super symbol matching matrix, extracting a corresponding super symbol sequence according to an arrangement form of a frame header sequence, and completing quasi-coherent combination of the super symbol sequence through Fourier transform to obtain a plurality of quasi-coherent combination results;

[0029] Step 4, extract the path with maximum energy from the multi-path quasi-coherent combining results to perform peak decision, if the peak is found, then execute step 5, otherwise continue to execute step 3;

[0030] Step 5, determine the position of the symbol optimal sampling point and the position of the end of the frame header according to the peak position, and realize frame capture and symbol synchronization; in addition, obtain the frequency offset information of the current received frame according to the sequence number of the maximum value in the Fourier transform result of the frame header sequence.

[0031] The implementation basis of the method is the special frame header design based on super symbol, and the core processing method is quasi-coherent sequence matching based on super symbol matching matrix. The method needs the transmitter to bundle four symbols in the frame header part into a group to form a super symbol, and the information carried by the four symbols in the super symbol is completely the same. The transmitter arranges super symbols carrying different information into a specific super symbol sequence. The receiver filters and extracts the received signal, extracts data from the complex baseband signal according to the super symbol length on the basis of high times oversampling processing, and performs super symbol matching through Fourier transform to output a super symbol matching matrix. The synchronization module of the receiver takes the super symbol as a processing unit, extracts the corresponding super symbol sequence in the super symbol matching matrix according to the arrangement form of the frame header sequence, and completes quasi-coherent combining of the super symbol sequence using Fourier transform to obtain multi-path quasi-coherent combining results. The synchronization decision unit of the synchronization module extracts the most likely path from the multi-path quasi-coherent combining results to perform peak decision, and obtains the position of the symbol optimal sampling point and the position of the end of the frame header after successful decision, and outputs the frame timing information, and then obtains the frequency offset information of the current received frame according to the sequence number of the maximum value in the Fourier transform result of the frame header sequence.

[0032] Among them, the super symbol is composed of four MSK symbols. The baseband waveform of one MSK symbol is the first quarter period of the complex single frequency signal with the same initial phase, so four MSK symbols carrying the same information can be bundled into a group to obtain a complete period of complex single frequency signal. According to the carried information, the super symbol is divided into two kinds, and the two kinds of super symbols are orthogonal.

[0033] The frame header part can select a specific binary sequence according to needs, and the length is N. Each element in the sequence is repeated 4 times to obtain a new sequence with 4 times the length. The transmitter can directly modulate according to the expanded new sequence. The four symbols are regarded as a group, and the output of the modulator is N groups of complete period single frequency signals, and the frequency information is one-to-one corresponding to the selected binary sequence.

[0034] The start end combines the frame head part binary sequence, the pilot part binary sequence and the data part binary sequence to form a complete frame. The phase calculation part in the modulator calculates the phase information of each sampling point in the frame according to the specific content of the frame information. The waveform generation part in the modulator generates the corresponding complex baseband waveform according to the phase of each sampling point. The quadrature upconverter in the modulator realizes the spectrum shift of the complex baseband signal to generate the required intermediate frequency signal. Finally, the DA converter converts the intermediate frequency signal into an analog signal, which is handed over to the subsequent analog signal processing.

[0035] The receiving end obtains a high-fold oversampled complex baseband signal after performing digital down-conversion, filtering and decimation on the received signal. The super-symbol matching module extracts data from the complex baseband signal according to the super-symbol length of the high-fold oversampling, and performs super-symbol level matching. The sequence quasi-coherent matching module extracts corresponding data according to the arrangement form of the frame header sequence in the super-symbol matching result, and performs quasi-coherent combination. The demodulator finds the sequence correlation peak in the sequence quasi-coherent matching result, and then frame capture is completed, that is, symbol synchronization and synchronization are completed.

[0036] When the oversampling multiple is M, each MSK symbol contains M sampling points, and each super-symbol contains 4M sampling points. If the current sampling point is the best sampling point and is in the frame header part, the super-symbol composed of the current sampling point and the previous 4M-1 sampling points is a complete single-frequency signal.

[0037] The super-symbol matching is completed using Fourier transform, and the matching result will appear at two specific frequency points. Each time a sampling data is obtained, the super-symbol matching module performs Fourier transform on the super-symbol composed of the current sampling point and the previous 4M-1 sampling points, and stores the result.

[0038] The sequence quasi-coherent matching first zero-pads the possible frame header sequence, and then completes the matching through Fourier transform. Fourier transform is coherent matching, but super-symbol matching is not coherent matching, so the sequence matching in the present application is a kind of quasi-coherent matching. Zero-padding and then Fourier transform is equivalent to frequency domain interpolation, which can effectively counteract the adverse effects of frequency offset, and can also complete the estimation of system frequency offset.

[0039] For the frame header sequence, there must be one best sampling point in every 4M sampling points, but the demodulator does not know which one is the best sampling point. Therefore, the sequence quasi-coherent matching module needs to perform sequence quasi-coherent matching on each sampling point. The sequence quasi-coherent matching process is as follows: the sequence quasi-coherent matching module takes the current sampling point as the starting point, extracts information on the frequency point corresponding to the frame header frequency every 4M sampling points, and performs sequence quasi-coherent matching through Fourier transform after zero padding. If the current sampling point is the best sampling point and is in the last symbol of the frame header super-symbol sequence, the frame header sequence matching module will output a very large peak value, and the frame capture and symbol synchronization are completed at the same time. Otherwise, the frame header sequence matching module outputs values all less than the above-mentioned peak value, and the frame capture and symbol synchronization are in an incomplete state.

[0040] The following is a more specific example:

[0041] In this example, the initial phase of the complex baseband waveform is 0, the symbol rate is Rs, and the oversampling multiple is M.

[0042] Figure 1 is an example waveform when the super-symbol is 0. When the modulation information is four consecutive 0s, four consecutive MSK symbols can form a segment of complex single-frequency signal, the frequency of which is Rs / 4, and the starting phase and ending phase of the waveform are both 0 phase.

[0043] Figure 2 is an example waveform when the super-symbol is 1. When the modulation information is four consecutive 1s, four consecutive MSK symbols can form a segment of complex single-frequency signal, the frequency of which is -Rs / 4, and the starting phase and ending phase of the waveform are both 0 phase.

[0044] Figure 3 is a segment of the frame header complex baseband waveform, which contains four complete super-symbols. The modulation information sequence of the frame header segment is: 0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 0, 0, 0, 0, and the corresponding super-symbol sequence segment is: 0, 0, 1, 0. The frame header segment in the figure is composed of four complete period complex single-frequency signals with frequencies of Rs / 4, Rs / 4, -Rs / 4, and Rs / 4.

[0045] The frame header part selects a binary sequence with length N according to the requirement, which is the original frame header sequence and also the super-symbol information sequence. The original frame header sequence will affect the performance of the synchronization decision, and a binary sequence with strong correlation such as an m-sequence or a Gold sequence can be selected. Each element in the original frame header sequence is repeated 4 times to obtain the modulation information sequence of the frame header segment. The initial phase of the complex baseband waveform is set to 0, and the modulator performs MSK modulation according to the modulation information sequence. Each four symbols can form a complete period of complex single-frequency signal. The frequency of each segment of the complex single-frequency signal is determined by the original frame header sequence. When the original frame header information is 0, the frequency of the corresponding complex single-frequency signal is Rs / 4, and when the original frame header information is 1, the frequency of the corresponding complex single-frequency signal is -Rs / 4. N segments of the complex single-frequency signal (N super-symbols) constitute a complete frame header waveform.

[0046] Figure 4 is a super-symbol matching flow chart. After the super-symbol matching module obtains the sampling data, it first stores the oversampled complex baseband signal according to the write controller instruction, and informs the sampling data read controller of the storage location; secondly, the read controller sends the latest 4M group of data into the super-symbol waveform complex matcher; finally, the super-symbol waveform complex matcher uses fast Fourier transform to complete the matching of the super-symbol waveform and two orthogonal complex single-frequency signals, and outputs two complex matching results.

[0047] Figure 5 is a sequence quasi-coherent matching flow chart. After the frame header sequence matching module obtains the super-symbol matching result, it stores the two super-symbol matching results according to the write controller instruction, and informs the sampling data read controller of the storage location X. The read controller extracts the data with storage addresses X, X-4M*1, X-4M*2,..., X-4M*(N-1) from the matching result storage matrix. The frame header sequence selector forms a super-symbol sequence by combining the super-symbol matching results at the corresponding frequency points according to the frame header sequence information, and sends the expanded super-symbol sequence into the frame header sequence matcher after zero padding. The frame header sequence matcher uses fast Fourier transform to complete the quasi-coherent combination of the super-symbol sequence, and outputs the quasi-coherent combination results of multiple frame header super-symbol sequences. The matching result processor selects the one with the maximum energy from the multiple quasi-coherent combination results and outputs the corresponding frequency offset information and position information. When the frame peak decision maker identifies the maximum correlation peak in the quasi-coherent combination result, it can be considered that the frame capture and symbol synchronization are completed, and the frame timing information and frequency offset information are output.

[0048] The data storage part of the super-symbol matching and the sequence quasi-coherent matching adopts the same design, so the data storage addresses of the super-symbol matching and the sequence quasi-coherent matching are the same for the same sampling point. When the frame capturing is completed, the data storage address corresponding to the correlation peak is also the position of the best sampling point. The subsequent circuit reads the corresponding waveform from the sampling data storage according to the best sampling point X obtained by the frame header sequence matching module, and after compensation of the frequency offset and the phase offset, can perform MSK coherent demodulation.

[0049] In summary, the present application simultaneously realizes frame capturing and symbol synchronization through two processes: the first step uses a super-symbol as a basic processing unit, uses Fourier transform to complete multi-template complex matching of the super-symbol on the basis of high-multiple oversampling processing, and forms a super-symbol matching matrix; the second step extracts the corresponding super-symbol sequence from the super-symbol matching matrix according to the frame header sequence content, uses Fourier transform to complete quasi-coherent combination of the super-symbol sequence, extracts the most likely one from the multiple quasi-coherent combination results to perform peak value decision, and outputs frame timing information and frequency offset information after successful decision. The present application solves the problem that a completely MSK modulated waveform is difficult to directly determine a symbol boundary through phase jump, and meets the requirements of MSK burst communication on frame capturing and symbol synchronization.

Claims

1. A fast frame capture and symbol synchronization method based on supersymbols using MSK, characterized in that, This method is applied at the signal receiving end for frame capture and symbol synchronization of signals sent from the transmitting end. The transmitting end bundles four MSK symbols into a group in the frame header to form a supersymbol. These supersymbols, carrying different information, are then combined into a supersymbol sequence to form the frame header. The method includes the following steps: Step 1: Process the received signal to obtain a complex baseband signal; Step 2: Extract data from the complex baseband signal by sliding according to the supersymbol length of the high-multiple oversampling, and perform supersymbol matching through Fourier transform to output the supersymbol matching matrix; Step 3: In the supersymbol matching matrix, extract the corresponding supersymbol sequence according to the arrangement of the frame header sequence, and use Fourier transform to complete the quasi-coherent merging of the supersymbol sequence to obtain the multi-path quasi-coherent merging result. Step 4: Extract the path with the highest energy from the multi-path quasi-coherent merging results and perform peak determination. If a peak is found, proceed to step 5; otherwise, continue to step 3. Step 5: Determine the position of the optimal sampling point of the symbol and the position of the end of the frame header based on the peak position to achieve frame acquisition and symbol synchronization; in addition, obtain the frequency offset information of the current received frame based on the index of the maximum value in the Fourier transform result of the frame header sequence.

2. The MSK fast frame capture and symbol synchronization method based on supersymbols according to claim 1, characterized in that, The four MSK symbols in a supersymbol carry the same information.

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