A demodulation method for Chirp spread spectrum modulation with configurable spreading factor based on FFT

Abstract

The invention discloses a demodulation method of Chirp spread spectrum modulation with configurable spreading factor based on FFT, including receiving a CSS signal sent by a CSS signal modulator, inputting it into an orthogonal demodulation module, obtaining a baseband receiving signal; multiplying the baseband receiving signal with a local dechirp signal, the result is a single-frequency signal, and obtaining the frequency of the single-frequency signal through FFT operation. In the leading symbol stage of the burst frame, frame header detection and symbol synchronization are performed according to the single-frequency frequency value and amplitude size. In the data symbol stage, the frequency value of the single-frequency signal obtained by multiplying the baseband signal with the dechirp signal reflects the symbol modulation information, the decision module finds the frequency value corresponding to the maximum amplitude point from the FFT output, the inverse mapping and parallel-to-serial conversion module inversely maps the frequency into modulated data information, and then restores the transmission sequence after parallel-to-serial conversion to complete the demodulation end data reception. The invention realizes signal detection, symbol synchronization and receiving functions, and realizes low complexity, and verifies the feasibility of the method through simulation.

Description

Technical Field

[0001] The invention relates to a demodulation method of Chirp spread spectrum modulation with configurable spread spectrum factor based on FFT, belonging to the technical field of wireless data communication. Background Art

[0002] In recent years, with the development and increasingly widespread application of Internet of Things (IoT) technology, IoT data transmission increasingly requires long-distance communication technology with narrow bandwidth, low power consumption and long transmission distance. The short-distance, high-speed wireless transmission technology currently used cannot meet the needs of the development of IoT. Different from the wireless access technology used in traditional mobile communication network technology, Low-Power Wide-Area Network (LPWAN) is a wide-area low-power radio communication network technology with the characteristics of long transmission distance, narrow bandwidth, low speed, long standby time and easy deployment. As an emerging IoT technology, it has developed rapidly and has been widely used in industry, agriculture, transportation, health and other fields.

[0003] Sigfox, LoRa, and NB-IoT are some of the more typical technologies in the current LPWAN. Among them, NB-IoT has a faster modulation rate than Sigfox and LoRa. It uses single-carrier frequency division multiple access (FDMA) for uplink communication, orthogonal FDMA (OFDMA) for downlink communication, and uses quadrature phase shift keying (QPSK) modulation. It has the advantages of synchronous communication and high service quality, but its OFDM / FDMA access mode requires the device to consume more energy. Sigfox uses DBPSK uplink and GFSK downlink to send data slowly and in small amounts. This technology uses a very narrow channel bandwidth, and the receiver needs to monitor a small spectrum segment, thereby reducing the impact of noise and the power consumption of the antenna, and can achieve remote transmission. However, SigFox's physical layer communication technology and network protocol specifications are private, and the network deployment is the responsibility of SigFox. LoRa uses Chirp Spread Spectrum (CSS) modulation technology that combines broadband linear frequency modulation and spread spectrum coding. It uses the spread factor to balance the relationship between transmission distance, bandwidth and rate. Although it occupies a wider frequency band, it has stronger anti-interference ability and higher receiver sensitivity. Compared with NB-IoT, it has the advantage of low power consumption. Unlike SigFox, LoRa's specification protocol LoRaWAN is open for access, so LoRa has become an increasingly popular modulation scheme in LPWAN communications.

[0004] However, as LoRa is a patented technology of Semtech, its physical layer details have always been a secret trade secret. Research on CSS modulation and demodulation technology will help develop autonomous and controllable LPWAN technology to avoid the risk of LoRa chip supply interruption. Summary of the invention

[0005] The purpose of the present invention is to overcome the deficiencies in the prior art and provide a demodulation method for Chirp spread spectrum modulation with configurable spreading factor based on FFT to achieve signal detection, symbol synchronization and reception functions.

[0006] To achieve the above object, the present invention is implemented by adopting the following technical solutions:

[0007] In a first aspect, the present invention provides a demodulation method for Chirp spread spectrum modulation with configurable spreading factor based on FFT, the method is applicable to a CSS signal receiver, the CSS signal receiver includes an orthogonal demodulation module, a CSS demodulation module, a frame header detection and synchronization module, a decision module, an inverse mapping and parallel-to-serial conversion module, the CSS demodulation module includes a local signal generator and an FFT module, the method includes:

[0008] Receive the CSS signal sent by the CSS signal modulator, input it into the orthogonal demodulation module, and obtain the baseband receiving signal;

[0009] Sending the baseband receiving signal to the CSS demodulation module, wherein the local signal generator in the CSS demodulation module continuously generates a dechirp signal to multiply the baseband receiving signal;

[0010] The result of multiplying the baseband received signal and the dechirp signal is input into the FFT module for FFT operation to obtain the maximum frequency amplitude value and its frequency position, and then output to the frame header detection and synchronization module and the decision module;

[0011] The frame header detection and synchronization module performs frame header detection and symbol synchronization according to the input maximum frequency amplitude value and its frequency position, obtains a frame header detection signal and a synchronization adjustment signal, and returns the synchronization adjustment signal to the local signal generator so that the local signal generator modifies the phase of the dechirp signal according to the synchronization adjustment signal;

[0012] The judgment module is started by the frame header detection signal. The judgment module obtains the single-frequency signal frequency according to the maximum frequency amplitude value and its frequency position input by the FFT module and outputs it to the inverse mapping and parallel-to-serial conversion module;

[0013] The inverse mapping and parallel-to-serial conversion module maps the single-frequency signal frequency output by the decision module into a symbol modulation value, and then restores the SF bit symbol value into a bit stream through parallel-to-serial conversion to complete the data reception at the demodulation end.

[0014] Further, the process of the CSS signal modulator sending the CSS signal includes:

[0015] According to the spreading factor SF of the CSS signal, each SF bit represents a chirp symbol, the symbol value is the modulation value q, and the front end of the transmitted bit stream is a plurality of leading symbols with a modulation value of 0 for receiver synchronization;

[0016] The transmission bit stream is grouped through serial-to-parallel conversion and input into the CSS signal modulator;

[0017] The CSS signal modulator generates a linear frequency modulation signal s(n) according to the modulation value q, and the expression is as follows:

[0018]

[0019] in:

[0020]

[0021] In the above formula (1) (2): is the phase of the i-th chirp symbol, which is only present when n∈[(i-1)2 SF +1,i2 SF ] contains a value, BW is the signal bandwidth, dir is the frequency sweep direction, and the value is 1 or -1, F s is the sampling frequency, and F is taken to satisfy the Nyquist theorem. s =BW, is the frequency sweep start position of the i-th chirp symbol, representing the modulation information, and takes values ​​[0,2 SF -1], which changes at the beginning of each symbol;

[0022] The linear frequency modulation signal is then orthogonally modulated to output the CSS signal.

[0023] Furthermore, the CSS signal sent by the receiving CSS signal modulator is input into the orthogonal demodulation module to obtain a baseband receiving signal, wherein the baseband receiving signal r(n) is expressed as follows:

[0024]

[0025] In the above formula (3): Δf c is the carrier frequency offset.

[0026] Further, the sending of the baseband received signal to the CSS demodulation module, wherein the local signal generator in the CSS demodulation module continuously generates a dechirp signal to multiply the baseband received signal, comprises:

[0027] The baseband received signal r(n) is input into the CSS demodulation module. The local signal generator in the module continuously generates a dechirp signal to multiply r(n). The dechirp signal is the conjugate of the pilot signal, and its expression is:

[0028]

[0029] in:

[0030]

[0031] In the above formulas (4) and (5): 0 and θ 0 are the frequency and phase offset respectively, is the phase of the i-th dechirp symbol, only in [(i-1)2 SF +1,i2 SF ] has a value. Since the modulation value of each dechirp symbol is 0, the dechirp signal d(n) is also a periodic signal. corr =mod(d corr +binM,2 SF ) is an adjustment symbol with an initial value of 0. It is carried by the synchronization adjustment signal output by the frame header detection and synchronization module and is used to offset the out-of-step information and correct the local signal phase. binM is the frequency of the maximum amplitude value obtained by the subsequent FFT operation.

[0032] Furthermore, the result of multiplying the baseband received signal and the dechirp signal is input into the FFT module for FFT operation to obtain the maximum frequency amplitude value and its frequency position, and output to the frame header detection and synchronization module and the decision module, including:

[0033] The CSS demodulation module performs an M = 2^SF FFT operation on the result of multiplying the baseband received signal and the dechirp signal. Assume that the delay between the transmitted signal and the dechirp signal is an integer multiple of the sampling interval, set to n d , the received baseband signal is multiplied by the dechirp signal, the formula is as follows:

[0034]

[0035] The above formula is the frequency [mod(F i +n d -d corr ,BW)+Δf] complex sinusoidal signal;

[0036] The CSS demodulation module loads the multiplication results (6) into the FFT buffer in order. The size of the buffer is M = 2. SF, each time a data point is loaded, the fft counter increases by one. When the buffer is full and the fft counter is equal to M, the counter is reset to one and the data in the buffer is used as M=2 SF Point FFT operation:

[0037]

[0038] According to the M-point FFT operation, the maximum frequency amplitude value valM and its frequency position binM are obtained and output to the frame header detection and synchronization module and the judgment module.

[0039] Furthermore, in the preamble receiving stage, the frame header detection and synchronization module performs frame header detection and symbol synchronization according to the input maximum frequency amplitude value and its frequency position to obtain a frame header detection signal, including:

[0040] The frame header detection and synchronization module performs frame header detection and symbol synchronization according to the input maximum frequency amplitude value valM and its frequency position binM. If valM is less than the decision threshold, the baseband received signal has not arrived, the buffer is cleared and the fft counter is set to 1, the signal is continued to be received and the data is loaded into the FFT buffer. If it is greater than the threshold, the received signal has arrived, and then it is determined whether binM is equal to 0. If it is 0, it means that the received signal is synchronized with the local dechirp signal. If it is not 0, then at this time:

[0041] mod(F i +n d -d corr ,2 SF )+Δf=n d +Δf=binM (8)

[0042] Substitute equation (8) into the phase correction symbol d corr =mod(d corr +binM,2 SF ) and then put it into formula (6)

[0043] mod(F i +n d -d corr ,2 SF )+Δf=mod(F i +n d -binM,2 SF )+Δf=F i (9)

[0044] That is, the transmitted signal is aligned with the local signal at the beginning of the next data point, and then when receiving the leading symbol, the maximum frequency amplitude of the FFT of each M point will be at F iAt the frequency, when the next data point arrives, modify the fft buffer loading index fftLoadIdx = binM, reload the data after fftLoadIdx, at this time the reloaded data is the result of multiplying the synchronized local signal with the baseband received signal, until the buffer is full, because this FFT counter fails to accumulate to M, so clear the buffer and do not perform FFT operation, set the counter to 1, when the next FFT buffer starts to load, the received signal and the local signal are aligned, the first point loaded in the buffer is the result of multiplying the first point in the current sent chirp and the current dechirp signal, and perform M-point FFT operation after the buffer is full, with the maximum frequency at F i Frequency position, that is, binM=F i At this point, the receiver has completed frame header detection and symbol synchronization through this module, and outputs the frame header detection signal to the decision module.

[0045] Furthermore, the frame header detection signal is used to start the decision module, and the decision module obtains the single-frequency signal frequency according to the input maximum frequency amplitude value and its frequency position and outputs it to the inverse mapping and parallel-to-serial conversion module, including:

[0046] The judgment module is started by the frame header detection signal. At this time, the received signal and the local signal are aligned. The frequency value of the single-frequency signal v(n) obtained by multiplying the received baseband signal with the synchronized dechirp signal reflects the modulation data information carried by the symbol. At this time, the FFT buffer is full and an M-point FFT operation is performed. The FFT module finds the maximum amplitude point and its corresponding frequency value binM from the result of this operation and outputs it to the judgment module to obtain the frequency value F of the single-frequency signal v(n). i =binM and output to the inverse mapping and parallel-to-serial conversion module.

[0047] Compared with the prior art, the present invention has the following beneficial effects:

[0048] The present invention provides a demodulation method for Chirp spread spectrum modulation with configurable spreading factor based on FFT. For Chirp spread spectrum modulated signals, FFT technology is used for receiving and demodulating. The phase of the local signal is adjusted by multiplying the received signal and the local dechirp signal and performing FFT operation to realize signal detection, symbol synchronization and receiving functions. The demodulation method proposed in the present invention has low implementation complexity and the feasibility of the method is verified by simulation. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] Figure 1 It is a model diagram of the sending and receiving system of the present invention;

[0050] Figure 2It is a structural diagram of a demodulation end receiver of the present invention;

[0051] Figure 3 is a demodulator symbol synchronization flow chart of the present invention;

[0052] Figure 4 It is the FFT result simulation diagram of the present invention;

[0053] Figure 5 It is a simulation diagram of the sending and receiving verification of the present invention. DETAILED DESCRIPTION

[0054] The present invention will be further described below in conjunction with the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solution of the present invention, and cannot be used to limit the protection scope of the present invention.

[0055] Example 1

[0056] This embodiment introduces a demodulation method for Chirp spread spectrum modulation with configurable spreading factor based on FFT. The method is applicable to a CSS signal receiver. The CSS signal receiver includes an orthogonal demodulation module, a CSS demodulation module, a frame header detection and synchronization module, a decision module, an inverse mapping and parallel-to-serial conversion module. The CSS demodulation module includes a local signal generator and an FFT module. The method includes:

[0057] Receive the CSS signal sent by the CSS signal modulator, input it into the orthogonal demodulation module, and obtain the baseband receiving signal;

[0058] Sending the baseband receiving signal to the CSS demodulation module, wherein the local signal generator in the CSS demodulation module continuously generates a dechirp signal to multiply the baseband receiving signal;

[0059] The result of multiplying the baseband received signal and the dechirp signal is input into the FFT module for FFT operation to obtain the maximum frequency amplitude value and its frequency position, and then output to the frame header detection and synchronization module and the decision module;

[0060] The frame header detection and synchronization module performs frame header detection and symbol synchronization according to the input maximum frequency amplitude value and its frequency position, obtains a frame header detection signal and a synchronization adjustment signal, and returns the synchronization adjustment signal to the local signal generator so that the local signal generator modifies the phase of the dechirp signal according to the synchronization adjustment signal;

[0061] The judgment module is started by the frame header detection signal. The judgment module obtains the single-frequency signal frequency according to the maximum frequency amplitude value and its frequency position input by the FFT module and outputs it to the inverse mapping and parallel-to-serial conversion module;

[0062] The inverse mapping and parallel-to-serial conversion module maps the single-frequency signal frequency output by the decision module into a symbol modulation value, and then restores the SF bit symbol value into a bit stream through parallel-to-serial conversion to complete the data reception at the demodulation end.

[0063] The demodulation method of Chirp spread spectrum modulation with configurable spreading factor based on FFT provided in this embodiment specifically involves the following steps:

[0064] Step 1: Generate bit stream data. According to the spreading factor SF of the CSS signal, each SF bit represents a chirp symbol, and the symbol value is the modulation value q. The front end of the transmitted bit stream is a plurality of (not less than 4) preamble symbols with a modulation value of 0 for receiver synchronization. The number of preamble symbols can be changed according to the channel conditions. When the signal-to-noise ratio is too low, the number of preamble symbols should be increased.

[0065] Through serial-to-parallel conversion, the transmitted bit stream is grouped and input into the CSS signal modulator. The CSS signal modulator generates a linear frequency modulation signal according to the modulation value q, and the expression is as follows:

[0066]

[0067] in:

[0068]

[0069] In the above formula (1) (2): is the phase of the i-th chirp symbol, which is only present when n∈[(i-1)2 SF +1,i2 SF ] contains a value. BW is the signal bandwidth. dir is the frequency sweep direction, which can be 1 or -1. s is the sampling frequency, and F is taken to satisfy the Nyquist theorem. s =BW. is the frequency sweep start position of the i-th chirp symbol, representing the modulation information, and takes values ​​[0,2 SF -1], which changes at the beginning of each symbol.

[0070] The linear frequency modulation signal (1) is then orthogonally modulated to output the CSS signal (see Appendix Figure 1 ).

[0071] Step 2: Send the CSS signal and it will reach the receiver after being transmitted through the channel. The structure of the CSS signal receiver is as shown in the attached figure. Figure 2 As shown in the figure, it consists of an orthogonal demodulation module, a CSS demodulation module, a frame header detection and synchronization module, a decision module, an inverse mapping module and a parallel-to-serial conversion module. The transmitted signal first passes through the orthogonal demodulation module to obtain a baseband received signal:

[0072]

[0073] In the above formula (3): Δf c is the carrier frequency offset.

[0074] Step 3: The baseband received signal r(n) is input into the CSS demodulation module. The local signal generator in the module continuously generates a dechirp signal to be multiplied by r(n). The dechirp signal is the conjugate of the pilot signal, and its expression is:

[0075]

[0076] in:

[0077]

[0078] In the above formulas (4) and (5): 0 and θ 0 are the frequency and phase offset respectively. is the phase of the i-th dechirp symbol, only in [(i-1)2 SF +1,i2 SF ] has a value. Since the modulation value of each dechirp symbol is 0, the dechirp signal d(n) is also a periodic signal. corr =mod(d corr +binM,2 SF ) is the adjustment symbol, with an initial value of 0, carried by the synchronization adjustment signal output by the frame header detection and synchronization module, used to offset the out-of-step information and correct the local signal phase. binM is the frequency of the maximum frequency amplitude value obtained by the subsequent FFT operation.

[0079] Then the CSS demodulation module performs an M = 2^SF FFT operation on the result of multiplying the baseband received signal and the dechirp signal. Assume that the delay between the transmitted signal and the dechirp signal is an integer multiple of the sampling interval, set to n d , the received baseband signal is multiplied by the dechirp signal:

[0080]

[0081] because and is a periodic signal, and generally takes F s =BW=2 SF , then the above formula can be further transformed into:

[0082]

[0083] The above formula is the frequency [mod(F i +n d -d corr,BW)+Δf] complex sinusoidal signal.

[0084] The CSS demodulation module loads the multiplication results (6) into the FFT buffer in order. The size of the buffer is M = 2. SF The fft counter is incremented by one for each data point loaded. When the buffer is full and the fft counter is equal to M, the counter is reset to 1 and the data in the buffer is used as M=2. SF Point FFT operation:

[0085]

[0086] According to the M-point FFT operation, the maximum frequency amplitude value valM and its frequency position binM are obtained and output to the frame header detection and synchronization module and the judgment module.

[0087] Step 4: During the preamble symbol reception phase, the frame header detection and synchronization module performs frame header detection and symbol synchronization according to the input maximum frequency amplitude value valM and its frequency position binM (see the attached judgment flow chart). Figure 3 ). If valM is less than the decision threshold (the decision threshold can be set according to channel conditions and transmission requirements), the baseband received signal has not arrived, the buffer is cleared and the fft counter is set to 1, the signal is continued to be received and the data is loaded into the FFT buffer. If it is greater than the threshold, the received signal has arrived, and then it is determined whether binM is equal to 0. If it is 0, it means that the received signal is synchronized with the local dechirp signal. If it is not 0, then there is

[0088] mod(F i +n d -d corr ,2 SF )+Δf=n d +Δf=binM (8)

[0089] Substitute equation (8) into the phase correction symbol d corr =mod(d corr +binM,2 SF ) and then put it into formula (6)

[0090] mod(F i +n d -d corr ,2 SF )+Δf=mod(F i +n d -binM,2 SF )+Δf=F i (9)

[0091] That is, the transmitted signal is aligned with the local signal at the beginning of the next data point, and then when receiving the leading symbol, the maximum frequency amplitude of the FFT of each M point will be at F i At the frequency. At the same time, when the next data point arrives, modify the fft buffer's loading index fftLoadIdx = binM, and reload the data after fftLoadIdx. At this time, the reloaded data is already the result of multiplying the synchronized local signal with the baseband received signal. Until the buffer is full, because this FFT counter has not accumulated to M, the buffer is cleared and no FFT operation is performed, and the counter is set to 1. When the next FFT buffer starts to load, the received signal and the local signal are aligned, and the first point loaded in the buffer is the result of multiplying the first point in the current transmitted chirp and the current dechirp signal. After the buffer is full, perform M-point FFT operation, and the maximum amplitude frequency is at F i Frequency position, that is, binM=F i At this point, the receiver has completed frame header detection and symbol synchronization through this module, and outputs the frame header detection signal to the decision module.

[0092] It should be added that in order to reduce the impact of noise and offset, the maximum amplitude can be scaled by 0.75 and compared with the amplitude of the two adjacent frequency points. If it is less than, the local signal is shifted left or right by one data point by modifying binM, and the next leading symbol is received again; if it is greater, the receiving end has completed symbol synchronization.

[0093] Step 5: After the frame header detection and synchronization module completes symbol synchronization, the judgment module is started through the frame header detection signal (see Appendix Figure 3 ), the receiver enters the data receiving stage. At this time, the received signal and the local signal have been aligned. The frequency value of the single-frequency signal v(n) obtained by multiplying the received baseband signal with the synchronized dechirp signal reflects the modulation data information carried by the symbol. At this time, the FFT buffer is full and an M-point FFT operation is performed. The decision module finds the maximum amplitude point and its corresponding frequency value binM from the output of the FFT module to obtain the frequency F of the single-frequency signal v(n). i =binM and output to the inverse mapping and parallel-to-serial conversion module.

[0094] Supplementary explanation: When the delay (phase difference) between the received signal and the local dechirp signal is less than one sampling period, the synchronization accuracy of the frame header detection and symbol synchronization module is subject to the sampling frequency. Too high a sampling frequency will bring great computing pressure to the receiver. A multi-phase filter can be used to improve the synchronization accuracy: for example, 16 multi-phase filters are used, and the delay difference between each multi-phase filter is 1 / 16 of the sampling period. When the decision module is started, the baseband received signal passes through the multi-phase filter group and is multiplied by the dechirp signal that completes the symbol synchronization. FFT operations are performed to output 16 maximum frequency amplitude values. The decision module selects the one with the largest maximum frequency amplitude value as the optimal filter output. The received signal after this filter is most accurate with the local dechirp signal.

[0095] Step 6: Inverse mapping and parallel-to-serial conversion module according to The starting frequency is mapped into a symbol modulation value, and then the SF bit symbol value is restored into a bit stream through parallel-to-serial conversion to complete the data reception at the demodulation end.

[0096] Simulation Case

[0097] Assume that the spreading factor SF = 7, that is, the length of a chirp symbol is M = 2 SF = 128 points, the leading symbol is set to 4, and the calculation of n is performed without considering noise and other offsets. d When the received signal is equal to 0, 32, 64 and 96, the received signal is multiplied by the local signal and then the M-point FFT result is taken. The result is shown in the attached Figure 5 . Based on the Matlab simulation, it is verified that the frequency point with the maximum frequency is binM=n d +Δf=n d .

[0098] Assume that the spreading factor SF=7, the leading symbol is set to 4, and the symbol value of the data payload is set to 128 0s plus an array: 0, 1, 0, 3, 0, 5, 0, 7, ..., 0, 127. Assume that the signal-to-noise ratio SNR of the Gaussian white noise channel is -6db, the delay is 288 sampling points, and the carrier frequency offset is -0.09 / (2^SF*2). After the demodulation model of the present invention is used for the simulation of transmission and reception, the data payload and the demodulated data are compared as shown in the attached figure. Figure 5 As shown, it can be seen that in this simulation case: the accuracy of the demodulated symbol value is very high after synchronization is completed.

[0099] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the technical principles of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A demodulation method for Chirp spread spectrum modulation with configurable spreading factor based on FFT, It is characterized in that The method is applicable to a CSS signal receiver, which includes an orthogonal demodulation module, a CSS demodulation module, a frame header detection and synchronization module, a decision module, an inverse mapping and parallel-to-serial conversion module, and the CSS demodulation module includes a local signal generator and an FFT module. The method includes: Receive the CSS signal sent by the CSS signal modulator, input it into the orthogonal demodulation module, and obtain the baseband receiving signal; Sending the baseband receiving signal to the CSS demodulation module, wherein the local signal generator in the CSS demodulation module continuously generates a dechirp signal to multiply the baseband receiving signal; The result of multiplying the baseband received signal and the dechirp signal is input into the FFT module for FFT operation to obtain the maximum frequency amplitude value and its frequency position, and then output to the frame header detection and synchronization module and the decision module; The frame header detection and synchronization module performs frame header detection and symbol synchronization according to the input maximum frequency amplitude value and its frequency position, obtains a frame header detection signal and a synchronization adjustment signal, and returns the synchronization adjustment signal to the local signal generator so that the local signal generator modifies the phase of the dechirp signal according to the synchronization adjustment signal; The judgment module is started by the frame header detection signal. The judgment module obtains the single-frequency signal frequency according to the maximum frequency amplitude value and its frequency position input by the FFT module and outputs it to the inverse mapping and parallel-to-serial conversion module; The inverse mapping and parallel-serial conversion module maps the frequency of the single-frequency signal output by the decision module into a symbol modulation value, and then restores the SF bit symbol value into a bit stream through parallel-serial conversion to complete the data reception at the demodulation end; The result of multiplying the baseband received signal and the dechirp signal is input into the FFT module for FFT operation to obtain the maximum frequency amplitude value and its frequency position, and output to the frame header detection and synchronization module and the judgment module, including: The CSS demodulation module performs an M = 2^SF FFT operation on the result of multiplying the baseband received signal and the dechirp signal. Assume that the delay between the transmitted signal and the dechirp signal is an integer multiple of the sampling interval, set to n d , the received baseband signal is multiplied by the dechirp signal, the formula is as follows: The above formula is the frequency [mod(F i +n d -d corr ,BW)+Δf] complex sinusoidal signal; where v(n) is the single frequency signal obtained by multiplying the baseband signal and the dechirp signal; F i is the frequency sweep start position of the i-th chirp symbol, F s is the sampling frequency, BW is the signal bandwidth, d corr To adjust the symbol, SF is the spreading factor of the CSS signal; The CSS demodulation module loads the multiplication results (6) into the FFT buffer in order. The size of the buffer is M = 2. SF , each time a data point is loaded, the fft counter increases by one. When the buffer is full and the fft counter is equal to M, the counter is reset to one and the data in the buffer is used as M=2 SF Point FFT operation: According to the M-point FFT operation, the maximum frequency amplitude value valM and its frequency position binM are obtained and output to the frame header detection and synchronization module and the judgment module.

2. The demodulation method of Chirp spread spectrum modulation with configurable spreading factor based on FFT according to claim 1, It is characterized in that The process of the CSS signal modulator sending the CSS signal includes: According to the spreading factor SF of the CSS signal, each SF bit represents a chirp symbol, the symbol value is the modulation value q, and the front end of the transmitted bit stream is a plurality of leading symbols with a modulation value of 0 for receiver synchronization; The transmission bit stream is grouped through serial-to-parallel conversion and input into the CSS signal modulator; The CSS signal modulator generates a linear frequency modulation signal s(n) according to the modulation value q, and the expression is as follows: in: In the above formula (1) (2): is the phase of the i-th chirp symbol, which is only present when n∈[(i-1)2 SF +1,i2 SF ] contains a value, BW is the signal bandwidth, dir is the frequency sweep direction, and the value is 1 or -1, F s is the sampling frequency, and F is taken to satisfy the Nyquist theorem. s =BW, is the frequency sweep start position of the i-th chirp symbol, representing the modulation information, and takes values ​​[0,2 SF -1], which changes at the beginning of each symbol; The linear frequency modulation signal is then orthogonally modulated to output the CSS signal.

3. The demodulation method of Chirp spread spectrum modulation with configurable spreading factor based on FFT according to claim 2, It is characterized in that The CSS signal sent by the receiving CSS signal modulator is input into the orthogonal demodulation module to obtain a baseband receiving signal, wherein the baseband receiving signal r(n) is expressed as follows: In the above formula (3): Δf c is the carrier frequency offset.

4. The demodulation method of Chirp spread spectrum modulation with configurable spreading factor based on FFT according to claim 3, It is characterized in that The sending of the baseband receiving signal to the CSS demodulation module, wherein the local signal generator in the CSS demodulation module continuously generates a dechirp signal to multiply the baseband receiving signal, comprises: The baseband received signal r(n) is input into the CSS demodulation module. The local signal generator in the module continuously generates a dechirp signal to multiply r(n). The dechirp signal is the conjugate of the pilot signal, and its expression is: in: In the above formulas (4) and (5): 0 and θ 0 are the frequency and phase offset respectively, is the phase of the i-th dechirp symbol, only in [(i-1)2 SF +1,i2 SF ] has a value. Since the modulation value of each dechirp symbol is 0, the dechirp signal d(n) is also a periodic signal. corr =mod(d corr +binM,2 SF ) is an adjustment symbol with an initial value of 0. It is carried by the synchronization adjustment signal output by the frame header detection and synchronization module and is used to offset the out-of-step information and correct the local signal phase. binM is the frequency of the maximum amplitude value obtained by the subsequent FFT operation.

5. The demodulation method of Chirp spread spectrum modulation with configurable spreading factor based on FFT according to claim 4, It is characterized in that The frame header detection and synchronization module performs frame header detection and symbol synchronization according to the input maximum frequency amplitude value and its frequency position to obtain a frame header detection signal, including: The frame header detection and synchronization module performs frame header detection and symbol synchronization according to the input maximum frequency amplitude value valM and its frequency position binM. If valM is less than the decision threshold, the baseband received signal has not arrived, the buffer is cleared and the fft counter is set to 1, the signal is continued to be received and the data is loaded into the FFT buffer. If it is greater than the threshold, the received signal has arrived, and then it is determined whether binM is equal to 0. If it is 0, it means that the received signal is synchronized with the local dechirp signal. If it is not 0, then at this time: mod(F i +n d -d corr ,2 SF )+Δf=n d +Δf=binM (8) Substitute equation (8) into the phase correction symbol d corr =mod(d corr +binM,2 SF ) and then put it into formula (6) mod(F i +n d -d corr ,2 SF )+Δf=mod(F i +n d -binM,2 SF )+Δf=F i (9) That is, the transmitted signal is aligned with the local signal at the beginning of the next data point, and then when receiving the leading symbol, the maximum frequency amplitude of the FFT of each M point will be at F i At the frequency, when the next data point arrives, modify the fft buffer loading index fftLoadIdx = binM, reload the data after fftLoadIdx, at this time the reloaded data is the result of multiplying the synchronized local signal with the baseband received signal, until the buffer is full, because this FFT counter fails to accumulate to M, so clear the buffer and do not perform FFT operation, set the counter to 1, when the next FFT buffer starts to load, the received signal and the local signal are aligned, the first point loaded in the buffer is the result of multiplying the first point in the current sent chirp and the current dechirp signal, and perform M-point FFT operation after the buffer is full, with the maximum frequency at F i Frequency position, that is, binM=F i At this point, the receiver has completed frame header detection and symbol synchronization through this module, and outputs the frame header detection signal to the decision module.

6. The demodulation method of Chirp spread spectrum modulation with configurable spreading factor based on FFT according to claim 5, It is characterized in that The frame header detection signal is used to start the decision module, and the decision module obtains the single-frequency signal frequency according to the input maximum frequency amplitude value and its frequency position and outputs it to the inverse mapping and parallel-to-serial conversion module, including: The judgment module is started by the frame header detection signal. At this time, the received signal and the local signal are aligned. The frequency value of the single-frequency signal v(n) obtained by multiplying the received baseband signal with the synchronized dechirp signal reflects the modulation data information carried by the symbol. At this time, the FFT buffer is full and an M-point FFT operation is performed. The FFT module finds the maximum amplitude point and its corresponding frequency value binM from the result of this operation and outputs it to the judgment module to obtain the frequency value F of the single-frequency signal v(n). i =binM and output to the inverse mapping and parallel-to-serial conversion module.

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