Linear spread spectrum signal modulation and demodulation method based on index modulation

Abstract

This invention discloses a linear frequency modulated (LFM) spread spectrum signal modulation and demodulation method based on index modulation. The method includes the following steps: creating an index set to map the bit information to be transmitted into index combinations; generating a transmitted LFM spread spectrum signal using the index combinations; creating a preamble to generate a transmitted frame signal using the preamble; performing incoherent demodulation on the received frame signal; calculating estimated parameters and setting a threshold based on the estimated parameters; extracting an initial index combination based on the threshold; and updating the initial index combination using an index matching demodulation algorithm to obtain the demodulated bit information. This invention, applied to LFM spread spectrum systems, can improve the transmission rate of the LFM spread spectrum system and simultaneously enable rapid recovery of bit information using a low-complexity demodulation algorithm.

Description

Technical Field

[0001] This invention belongs to the field of communication technology, and more specifically relates to a linear spread spectrum signal modulation and demodulation method based on index modulation in the field of Internet of Things (IoT) technology. This invention can be used to map the information bits to be transmitted onto a designed index, generate a transmitted signal by superimposing multiple linear frequency modulated spread spectrum signals using the index, and demodulate the signal at the receiving end using a low-complexity index matching algorithm. Background Technology

[0002] The Internet of Things (IoT), as a crucial component of the next generation of information technology, aims to connect everything to the internet through technologies such as wireless communication, enabling information exchange and communication, thereby achieving intelligent identification, location, tracking, monitoring, and management of objects. Low-power wide-area networks (LPWANs) are a powerful emerging IoT technology that allows for ubiquitous connectivity between millions of sensors to collect data and collaborate, thus enabling the development of numerous smart IoT applications and fostering a smart lifestyle.

[0003] LPWAN is designed for low-bandwidth, low-power, long-range, and massively connected IoT applications. LoRa, a derivative of chirp spread spectrum (CSS) technology, has become one of the most popular LPWAN technologies due to its advantages in low power consumption and long-range transmission. LoRa can be described as frequency-shift linear frequency modulation (FSK), which includes a frequency-shift keying (FSK) component for a non-linear frequency modulation symbol and an extension part for extending the bandwidth of the non-linear frequency modulation symbol. LoRa achieves its low-power, long-range advantages at the cost of a low transmission bit rate, which significantly limits the network throughput of LPWAN and also restricts future IoT applications.

[0004] Shangshengte Co., Ltd. discloses a transmitter and receiver method for linear frequency modulation (LFM) signals in its patent application "Transmitter, Receiver, and Method for Linear Frequency Modulated Radio Signals" (Patent Application No.: 202110564500.8, Authorization Announcement No. CN 113726347 B). The method involves generating a series of LFM signals using a LFM generator, where each LFM signal carries an element encoded as cyclic shift information and has a phase encoded with error correction codes for the LFM-dependent cyclic shift; then, a modulator and a radio transmitter are used to modulate the series of LFM signals onto a radio signal. Finally, a receiver demodulates the LFM signal. The drawback of this method is that modulating information bits onto a single LFM spread spectrum signal limits the number of information bits that each transmitted symbol can carry, resulting in a low system transmission rate and significantly restricting its practical application.

[0005] Muhammad Hanif et al. proposed a linear frequency modulation (LFM) spread spectrum method based on index modulation in their paper "Frequency-Shift Chirp Spread Spectrum Communications With Index Modulation" (IEEE Internet of Things Journal, IEEE, 2021). The method involves mapping the information to be transmitted onto an index that does not contain overlapping peaks. Then, a signal generator produces a superimposed transmit signal of multiple LFM spread spectrum signals. The receiver multiplies the received signal by the conjugate of a standard chirp signal, uses an FFT to find the two largest peaks in the frequency domain as indices, and finally uses a search-matching method to obtain the decimal value of the information. However, this method still has shortcomings. While it uses a search-matching method to map the index to the decimal value of the information, the index set can contain up to millions of indices, resulting in high algorithm complexity and making it impractical. Furthermore, the method limits the number of indices in the index set because it excludes overlapping peaks, thus limiting the number of information bits that a single transmitted symbol can carry. Summary of the Invention

[0006] The purpose of this invention is to address the shortcomings of the prior art by proposing a linear spread spectrum signal modulation and demodulation method based on index modulation. This method solves the problem of low information rate caused by information being carried on a single linear frequency modulation spread spectrum signal in traditional linear frequency modulation spread spectrum systems, as well as the problem of high algorithm complexity in existing index modulation methods.

[0007] The idea behind this invention is as follows: At the transmitting end, this invention maps the transmitted information to indices by designing an index set. The indices are used to generate a transmitted signal that is the superposition of two linear frequency modulated (LFM) spread spectrum (RFMS) signals, carrying more bits of transmitted information on the two LFMMS signals. This solves the problem of low transmission rate caused by modulating information onto a single LFMMS signal. The index set designed in this invention includes indices with overlapping peaks, increasing the number of indices in the index set and enabling more information bits to be mapped to the indices. This solves the problem of a limited number of mappable information bits caused by an index set lacking overlapping peaks. At the receiving end, the received signal is multiplied by the conjugate of the standard LFMMS signal, and an FFT is performed. The two frequency units with the largest amplitude in the frequency domain are sorted to obtain the indices. This invention uses an index matching formula to map the indices to the decimal values ​​of the information bits, eliminating the need to match the indices with all indices in the index set, thus solving the problem of high complexity in existing technologies that use search matching algorithms during mapping. Finally, the decimal values ​​of the information bits are converted to binary to obtain the original binary bit information.

[0008] The specific steps to achieve the objective of this invention are as follows:

[0009] Step 1: Create an index set based on the spreading factor SF;

[0010] Step 2: Divide the bit information to be sent into equal intervals of SF_data bits, convert the divided bit information into decimal values, and find the index combination {f1,f2} that corresponds to each decimal value from the index set.

[0011] Step 3: Use the mapped index combination to generate the transmitted linear frequency modulated spread spectrum signal, place the preamble signal in front of the linear frequency modulated spread spectrum signal, generate and transmit the frame signal;

[0012] Step 4: Perform noncoherent demodulation on the received frame signal;

[0013] Step 5: Calculate the estimated parameters of the first 8 signals of the demodulated frame signal, and take the value corresponding to the parameter value closest to the estimated parameter as the threshold T;

[0014] Step 6: Sort the signals after the 8th frame signal after demodulation according to their amplitude from smallest to largest to obtain the amplitude corresponding to the sorted frequency unit, and use the amplitude to obtain the initial index combination of demodulated bit information.

[0015] Step 7: Update the initial index combination using the index matching demodulation algorithm to obtain the demodulated decimal bit information;

[0016] Step 8: Convert the demodulated decimal bit information into binary to obtain the demodulated bit information.

[0017] Compared with the prior art, the present invention has the following advantages:

[0018] First, this invention maps transmitted information to indices using an index set, and generates a transmitted signal by superimposing two linear frequency modulated spread spectrum (LFM) signals. This overcomes the shortcomings of existing technologies that modulate information onto a single LFM spread spectrum signal, resulting in low transmission rates. This allows more bits of information to be carried in a single transmitted symbol, thereby increasing the transmission rate of this invention.

[0019] Secondly, the index set designed in this invention includes indexes with overlapping peaks, overcoming the shortcomings of existing technologies such as a limited number of selectable indexes and a small number of mappable information bits. This allows the invention to map more bit information onto the indexes based on the use of bimodal indexes, thereby further improving the transmission rate on top of the increased transmission rate achieved by using the bimodal index scheme.

[0020] Third, this invention uses an index matching formula to map the index to the decimal value of the information bit, overcoming the high complexity of existing technologies that use search matching algorithms during demodulation. This gives the demodulation algorithm of this invention the advantage of low complexity. Attached Figure Description

[0021] Figure 1 This is a flowchart of the present invention;

[0022] Figure 2 This is an example diagram of the index set of the present invention;

[0023] Figure 3 This is an example diagram of the frequency domain after the receiving end of the present invention performs incoherent demodulation on the data;

[0024] Figure 4 This is a flowchart of the index matching demodulation algorithm in this invention;

[0025] Figure 5 This is a comparison chart of throughput in the simulation experiment of this invention. Detailed Implementation

[0026] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0027] Reference Figure 1 The implementation steps of the embodiments of the present invention will be further described below.

[0028] Step 1: Create an index set based on the spreading factor SF.

[0029] All bit information in the set is indexed pairwise as {0,0}, {0,1}, {1,1}, ..., {2} SF -2,2 SF -1},{2 SF ,2 SF Arrangement, such as Figure 2 As shown, SF represents the spreading factor of the linear frequency modulated spread spectrum signal, which is a value preset by the transmitter in the range [7,12].

[0030] Step 2: Divide the bit information to be sent into equal intervals of SF_data bits, convert the divided bit information into decimal values, and find the index combination {f1,f2} that corresponds to each decimal value from the index set.

[0031] Step 2.1, calculate SF_data according to the following formula:

[0032]

[0033] Where SF_data represents the interval value, The expression represents the floor operation, and log2(·) represents the logarithmic operation with base 2. Indicates from [0,2 SF The number of permutations and combinations of any two distinct numbers in 2. SF Indicates from [0,2 SF The number of permutations and combinations of any two identical numbers in the given set.

[0034] Step 2.2: Determine if the number of bits in the random bit information to be sent is divisible by SF_data. If not, pad the end of the bit information with zeros until the padded random bit information can be evenly divided among SF_data bits. For example, if the random bit information to be sent is 010101 and SF_data = 4, the padded bit information will be 01010100. Convert the evenly divided bit information to decimal values.

[0035] Step 2.3: Find the index combination {f1, f2} that maps to each decimal value from the index set. For example, if the decimal bit information is 0, then find the first index {0, 0} in the index set as the index combination.

[0036] Step 3: Use the mapped index combination to generate the transmitted linear frequency modulated spread spectrum signal, place the preamble signal in front of the linear frequency modulated spread spectrum signal, generate and transmit the frame signal.

[0037] Step 3.1: Generate the transmitted linear frequency modulated spread spectrum signal using the mapped index combination. The linear frequency modulated spread spectrum signal is generated by the following formula:

[0038]

[0039] Where c(k) represents the linear frequency modulated spread spectrum signal generated at the k-th sampling point, and the value of k is 0, 1, ..., 2. SF -1, e(·) represents the exponential operation with the natural constant e as the base, j represents the imaginary unit symbol, and π represents pi.

[0040] Step 3.2: Place the preamble signal before the linear frequency modulated spread spectrum signal to generate and transmit the frame signal. The preamble signal consists of 8 upchirps cascaded together, and each upchirp signal is obtained by the following formula:

[0041]

[0042] Where c0(k) represents the upchirp signal generated at the kth sampling point.

[0043] Step 4: Perform noncoherent demodulation on the received frame signal.

[0044] Incoherent demodulation is obtained from the following equation:

[0045] y(m)=abs(FFT(y m ×conj(c0)))

[0046] Where y(m) represents the m-th demodulated signal, m = 1, 2, ..., 8, abs(·) represents the absolute value operation, and FFT(·) represents the fast Fourier transform operation. m This represents the m-th linear frequency modulated spread spectrum signal of the received frame signal, and conj(·) represents the conjugate operation.

[0047] Step 5: Calculate the estimated parameters of the first 8 signals of the demodulated frame signal, and take the value corresponding to the parameter value closest to the estimated parameter as the threshold T.

[0048] The estimated parameters are obtained from the following formula:

[0049]

[0050] Where r represents the estimated parameter related to the signal-to-noise ratio, y(i,j) represents the j-th sample value of the i-th frame signal after demodulation, and y(i,1) represents the 1-th sample value of the i-th demodulated frame signal.

[0051] Step 6: Sort the signals after the 8th frame signal after demodulation according to their amplitude from smallest to largest to obtain the amplitude corresponding to the sorted frequency unit, and use the amplitude to obtain the initial index combination of demodulated bit information.

[0052] Step 6.1: Sort the signals after the 8th frame of the demodulated signal according to their amplitude in ascending order. The incoherent demodulated signals before sorting are as follows: Figure 3 As shown, the horizontal axis represents the frequency unit, with values ​​of 1, 2, ..., 2. SF The vertical axis represents the amplitude corresponding to the frequency unit.

[0053] Step 6.1: Calculate the difference d1 between the first and second amplitudes in the sorting, and the difference d2 between the second and third amplitudes.

[0054] Step 6.2: When d1 > (d2*T), extract the frequency unit value bin(1) corresponding to the first amplitude to obtain the initial index combination {f1',f1'}={bin(1),bin(1)} of the demodulated bit information.

[0055] Step 6.3: When d1≤(d2*T), extract the frequency unit values ​​bin(1) and bin(2) corresponding to the first and second amplitude values ​​respectively to obtain the initial index combination of demodulated bit information {f1',f2'}={sort[bin(1),bin(2)]}, where sort[·] represents sorting operation from smallest to largest.

[0056] Step 7: Update the initial index combination using the index matching demodulation algorithm to obtain the demodulated decimal bit information. (Refer to...) Figure 5 The implementation steps of the index matching demodulation algorithm will be further described.

[0057] Step 7.1: Using the following index matching formula, calculate the decimal demodulation bit information corresponding to the initial index combination:

[0058]

[0059] Among them, f i The decimal demodulation bit information corresponding to the initial index combination, f1' and f i 'Represents two index values ​​in the initial index combination.

[0060] Step 7.2, when f i >2 SF_data When the initial index combination is {f1',f1'}, the frequency unit values ​​corresponding to the amplitudes after the first amplitude in the sorting are extracted sequentially, and the index combination {f1',f1'} is then used. i '} = {sort[bin(1),bin(i)]} is used as the new initial index combination, where bin(i) represents the frequency unit value corresponding to the i-th amplitude in the extracted sort. The decimal demodulation bit information corresponding to the initial index combination is calculated until f is not satisfied. i >2 SF_data Until then, the demodulated decimal bit information is obtained.

[0061] Step 7.3, when f i >2 SF_data When the initial index combination is {f1', f2'}, the frequency unit values ​​corresponding to the amplitudes after the second amplitude in the sorting are extracted sequentially, and the index combination {f1', f2'} is then used. i '} = {sort[bin(1),bin(i)]} is used as the new initial index combination, where bin(i) represents the frequency unit value corresponding to the i-th amplitude in the extracted sort. The decimal demodulation bit information corresponding to the initial index combination is calculated until f is not satisfied. i >2 SF_data Until then, the demodulated decimal bit information is obtained.

[0062] Step 8: Convert the demodulated decimal bit information into binary to obtain the demodulated bit information.

[0063] The effectiveness of this invention can be further demonstrated through the following simulation experiments.

[0064] 1. Simulation experimental conditions.

[0065] The software platform for the simulation experiment of this invention is: Windows 10 operating system and Matlab R2021b.

[0066] The simulation parameters for this invention are: a spreading factor of 9, and 180 random binary bits. The transmission channel environment is an additive white Gaussian noise channel with a signal-to-noise ratio ranging from -20dB to -6dB.

[0067] 2. Simulation content and result analysis.

[0068] The simulation experiment of this invention employs the index modulation and demodulation scheme of this invention, an existing technology, and a traditional linear frequency modulation spread spectrum transmission scheme. Under 15 signal-to-noise ratio (SNR) conditions, the received linear frequency modulation spread spectrum signal is demodulated, and the number of identical binary bits received and transmitted is counted as the throughput under the 15 SNR conditions. The relationship between the obtained throughput and SNR is plotted as follows: Figure 5 The three curves shown.

[0069] In the simulation experiment, the existing technology used refers to: a modulation and demodulation scheme for linear spread spectrum signals based on index modulation proposed by Muhammed Hanif in his paper "Frequency-Shift Chirp Spread Spectrum Communications With Index Modulation" (IEEE Internet of Things Journal, 2021); the traditional linear frequency modulation spread spectrum transmission system refers to: a linear frequency modulation spread spectrum transmission scheme proposed by L. Vangelista in his paper "Frequency Shift Chirp Modulation: The LoRa Modulation" (IEEE Signal Processing Letters).

[0070] The following is combined Figure 5 The simulation diagrams further illustrate the effects of the present invention.

[0071] Figure 5 The horizontal axis represents the signal-to-noise ratio (SNR) in dB, and the vertical axis represents the throughput in bits. Figure 5 The curve marked with a triangle represents the relationship between throughput and signal-to-noise ratio obtained by simulation using the method of this invention. Figure 5 The curve marked with a diamond represents the relationship between throughput and signal-to-noise ratio obtained from simulations using existing technology. Figure 5The curve marked with a circle represents the relationship between throughput and signal-to-noise ratio obtained by simulation using traditional linear frequency modulation spread spectrum technology.

[0072] Combination Figure 5 As can be seen, the throughput obtained by the method of this invention increases continuously with the increase of the signal-to-noise ratio (SNR). Within the SNR range of [-20, -6] dB, the throughput obtained by the method of this invention is consistently greater than that of the traditional linear frequency modulation scheme and existing technologies. Furthermore, the index matching algorithm used in the demodulation scheme of this invention has lower complexity than the search matching algorithm in existing technologies, and the demodulation method of this invention requires less time than existing technologies.

Claims

1. A method for modulating and demodulating a chirp spread spectrum signal based on index modulation, characterized in that, According to the spreading factor SF, an index set is created, two linear frequency spread spectrum signals are superimposed to generate a sending signal, and an index matching demodulation algorithm is used to update the initial index combination; the steps of the method include the following: Step 1, according to the spreading factor SF, an index set is created; Step 2, dividing the bit information to be transmitted by 1 bit interval, converting the divided bit information into a decimal value, finding a mapped index combination corresponding to each decimal value from an index set ; and ; Step 3, using the mapped index combination to generate the transmitted linear frequency spread spectrum signal, placing the preamble signal in front of the linear frequency spread spectrum signal to generate and send a frame signal; Step 4, non-coherent demodulation is performed on the received frame signal; Step 5, calculate the estimated parameters of the first 8 signals of the demodulated frame signal, and take the value corresponding to the parameter value closest to the estimated parameter as the threshold ; Step 6, the signal after the demodulated frame signal is sorted according to its amplitude from small to large, and the amplitude corresponding to the sorted frequency unit is obtained, and the initial index combination of the demodulated bit information is obtained using the amplitude; Step 7, using the index matching demodulation algorithm to update the initial index combination, the demodulated decimal bit information is obtained; Step 8, the demodulated decimal bit information is converted to binary to obtain the demodulated bit information.

2. The chirp spread spectrum signal modulation and demodulation method based on index modulation according to claim 1, characterized in that, The creating index set in step 1 refers to that all bit information in the set is arranged in pairs as {0,0}, {0,1}, {1,1}, …, {2 SF -2, 2 SF -1},{2 SF , 2 SF }; wherein, represents a spreading factor of the linear frequency modulation spread spectrum signal, which is a value preset by the sending end in a range of 0 to 2.

3. The chirp spread spectrum signal modulation and demodulation method based on index modulation according to claim 1, characterized in that, as described in step 2 is obtained according to the following scheme: ； wherein, represents an interval value, represents a floor operation, represents a logarithm operation with base 2, represents the number of permutations and combinations of any two different numbers taken from represents the number of permutations and combinations of any two different numbers taken from represents the number of permutations and combinations of any two different numbers taken from represents the number of permutations and combinations of any two different numbers taken from 4. The chirp spread spectrum signal modulation and demodulation method based on index modulation according to claim 1, characterized in that, The converting the divided bit information into a decimal value in step 2 means that it is determined whether the number of bits of the random bit information can be divided by If not, a zero padding operation is performed at the end of the bit information until the zero-padded random bit information can be divided by one bit.

5. The chirp spread spectrum signal modulation and demodulation method based on index modulation according to claim 1, characterized in that, The linear frequency spread spectrum signal in step 3 is generated by the following formula: ； wherein represents a linear frequency modulated spread spectrum signal generated for the th sampling point, has the values 0, 1,... , represents an exponential operation with the natural constant as base, represents the imaginary unit symbol, represents the circle constant.

6. The chirp spread spectrum signal modulation and demodulation method based on index modulation according to claim 1, characterized in that, The preamble signal in step 3 is composed of 8 upchirp signals in series, and each upchirp signal is obtained by the following formula: ； wherein, represents a generated upchirp signal of the sample points.

7. The chirp spread spectrum signal modulation and demodulation method based on index modulation according to claim 1, characterized in that, The non-coherent demodulation in step 4 is obtained by the following formula: ； wherein, represents the demodulated signal, , represents an absolute value operation, represents a fast Fourier operation, represents the linear frequency modulation spread spectrum signal of the received frame signal, represents a conjugate operation.

8. The chirp spread spectrum signal modulation and demodulation method based on index modulation according to claim 1, characterized in that, The estimated parameter in step 5 is obtained by the following formula: ； wherein represents an estimate parameter related to the signal-to-noise ratio, represents the first sample value of the demodulated frame signal of the first frame signal, represents the first sample value of the demodulated frame signal of the first frame signal, represents the first sample value of the demodulated frame signal of the 9. The chirp spread spectrum signal modulation and demodulation method based on index modulation according to claim 1, characterized in that, The step of obtaining the index combination of the demodulated bit information using the amplitude in step 6 is as follows: First, the difference between the first and second amplitudes is calculated , and the difference between the second and third amplitudes ; Second step, when , extract the first amplitude corresponding to the frequency unit value , get the initial index combination of demodulation bit information ; Third step, when , the first and second amplitude corresponding to the frequency unit value and , get the initial index combination of demodulation bit information , Indicates the sorting operation from small to large.

10. The chirp spread spectrum signal modulation and demodulation method based on index modulation according to claim 1, characterized in that, The step of updating the initial index combination using the index matching demodulation algorithm in step 7 is as follows: First, using the following index matching formula, the initial index combination corresponding to the decimal demodulated bit information is calculated: ； wherein, denotes the decimal demodulation bit information corresponding to the initial index combination, and denote two index values in the initial index combination, respectively; The second step, when When the initial index combination is In the sorted amplitudes, extract the frequency unit values ​​corresponding to the amplitudes sequentially after the first amplitude, and combine the indices. As a new initial index combination, where Indicates the extracted sorting order of the first... For each amplitude value, calculate the decimal demodulation bit information corresponding to the initial index combination, until the condition is not met. Until then, the demodulated decimal bit information is obtained; The third step, when When the initial index combination is In the sorted amplitudes, extract the frequency unit values ​​corresponding to the amplitudes after the second amplitude, and combine the indices. As a new initial index combination, where Indicates the extracted sorting order of the first... For each amplitude value, calculate the decimal demodulation bit information corresponding to the initial index combination, until the condition is not met. Until then, the demodulated decimal bit information is obtained.

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