A multi-user transmission method suitable for LFM system

Abstract

The present invention discloses a multi-user transmission method applicable to an LFM system, comprising: obtaining original user information of each of a plurality of users and a frequency modulation slope of each user; different users have different frequency modulation slopes; for each user, passing the original user information of the user through an encoder and an interleaver to obtain the user's codeword; using an SSK-CSS modulator corresponding to the user to modulate the user's codeword to obtain the user's modulation sequence; the SSK-CSS modulator has a preset spreading factor and a frequency modulation slope equal to the user's frequency modulation slope; the SSK-CSS modulators corresponding to different users have the same spreading factor; superimposing the modulation sequences of the plurality of users to obtain a mixed modulation sequence; performing noise processing on the mixed modulation sequence to obtain a signal to be transmitted and transmitting the signal. The present invention can support concurrent transmission by multiple users, and each user has a consistent transmission rate without transmission rate loss, making it suitable for use in scenarios where a large number of users are transmitting concurrently.

Description

Technical Field

[0001] The present invention belongs to the technical field of wireless communications, and in particular relates to a multi-user transmission method applicable to an LFM system. Background Art

[0002] In terrestrial IoT transmission, in addition to classic multiple access technologies such as time division multiple access (TDMA), frequency division multiple access (FDMA), and code division multiple access (CDMA), multi-user transmission solutions using linear spread spectrum (LFM) technology have also received considerable attention and research. According to Shanmuga SJ Pet al., "A survey on LoRa networking: Research problems, current solutions, and open issues," multi-user transmission can be achieved at the LoRa physical layer by controlling the spreading factor, transmission power, bandwidth, and channel. Regarding the LoRaWAN protocol, the LoRa Alliance's "LoRaWAN-Specification V1.1" states that ALOHA and TDMA can be used to achieve concurrent multi-user transmission on the transmission link.

[0003] C. Zhang, L. Wang, L. Jiao, S. Wang, J. Shi and J. Yue, “A novel orthogonal LoRa multiple access algorithm for satellite Internet of Things” (in China Communications, vol. 19, no. 3, pp. 279-289, March 2022), proposed a multi-user transmission scheme based on frequency division multiplexing. However, as the number of users increases, the transmission data rate decreases exponentially. Emim MA et al., “An enhanced receiver to decode superposed LoRa-like signals,” proposed a multi-user transmission scheme based on LoRa modulation with different large spreading factors. This scheme is highly complex and consumes a lot of storage resources. Summary of the Invention

[0004] In order to solve the above problems existing in the related art, the present invention provides a multi-user transmission method applicable to the LFM system. The technical problem to be solved by the present invention is achieved by the following technical solutions:

[0005] The present invention provides a multi-user transmission method applicable to an LFM system, comprising:

[0006] Obtaining original user information of each user among multiple users, as well as the frequency modulation slope of each user; the frequency modulation slopes of different users are different;

[0007] For each user, the original user information of the user is passed through an encoder and an interleaver to obtain the codeword of the user;

[0008] Using an SSK-CSS modulator corresponding to the user, modulating the user's codeword to obtain a modulation sequence for the user; the SSK-CSS modulator has a preset spreading factor, and a frequency modulation slope is the frequency modulation slope of the user; the SSK-CSS modulators corresponding to different users have the same spreading factor;

[0009] Superimposing the modulation sequences of the multiple users to obtain a mixed modulation sequence;

[0010] Noise processing is performed on the mixed modulation sequence to obtain a signal to be sent, and the signal to be sent is sent.

[0011] In some embodiments, the step of passing the original user information of the user through an encoder and an interleaver to obtain the codeword of the user includes:

[0012] After the original user information of the user passes through the encoder, the encoding result of the user is obtained;

[0013] After the encoding result passes through an interleaver, the codeword of the user is obtained.

[0014] In some embodiments, the codeword is a binary bit stream; and the step of using an SSK-CSS modulator corresponding to the user to modulate the user's codeword to obtain the user's modulation sequence includes:

[0015] Convert the codeword of the i-th user into a decimal number to obtain the transmission data symbol of the i-th user;

[0016] The transmission data symbol in each symbol period of the i-th user is input into the i-th SSK-CSS modulator to obtain the modulation sequence of the i-th user. The modulation sequence of the i-th user is expressed as follows:

[0017]

[0018] Among them, s i (l) is the modulation sequence of the i-th user, i is 1, 2, ..., U, U is the number of users, M = 2 SF is the number of sampling points in each symbol period, SF is the preset spreading factor, m iis the transmitted data symbol in each symbol period of the i-th user, which is used to represent the data symbol transmitted in one symbol period of the original user information of the i-th user, μ i is the frequency modulation slope of the i-th SSK-CSS modulator, l = 0, 1, ..., M-1, j is the imaginary unit, and exp(.) is the exponential function with the natural constant e as the base.

[0019] In some embodiments, superimposing the modulation sequences of the multiple users to obtain a mixed modulation sequence includes:

[0020] The U modulation sequences are superimposed to obtain a mixed modulation sequence. The formula of the mixed modulation sequence is as follows:

[0021] s(l)=s1(l)+s2(l)+,...,+s U-1 (l)+s U (l);

[0022] Wherein, s(l) represents the mixed modulation sequence, U is the number of users of the multiple users, and s U (l) represents the modulation sequence of the Uth user.

[0023] In some embodiments, performing noise processing on the mixed modulation sequence to obtain a signal to be transmitted includes:

[0024] The mixed modulation sequence is passed through an additive white Gaussian noise channel to obtain a signal to be transmitted. The formula of the signal to be transmitted is as follows:

[0025] r(l)=s(l)+n(l);

[0026] Where r(l) is the signal to be sent, s(l) is the mixed modulation sequence, n(l) is the mean of 0 and the variance of σ 2 Gaussian noise.

[0027] The present invention also provides a multi-user transmission method applicable to the LFM system, comprising:

[0028] Obtaining a received signal and a frequency modulation slope of each user among a plurality of users corresponding to the received signal; the frequency modulation slopes of different users are different;

[0029] For each user, dechirping the received signal according to the frequency modulation slope of the user to obtain a dechirped signal of the user;

[0030] performing a discrete Fourier transform on the dechirped signal of the user to obtain a transformation result;

[0031] Performing soft decision non-coherent demodulation processing on the transformation result to obtain a demodulation log-likelihood ratio of the user;

[0032] The demodulated log-likelihood ratio of the user is passed through a deinterleaver and a decoder to obtain the original user information of the user.

[0033] In some embodiments, performing soft decision non-coherent demodulation processing on the transformation result to obtain the demodulation log-likelihood ratio of the user includes:

[0034] Performing modulo processing on the transformation result to obtain a modulo result;

[0035] Squaring the modulo result to obtain a processing result;

[0036] Perform maximum processing and logarithmic processing on the processing result to obtain a demodulated log-likelihood ratio of the user.

[0037] In some embodiments, obtaining the original user information of the user after passing the demodulated log-likelihood ratio of the user through a deinterleaver and a decoder includes:

[0038] Passing the demodulated log-likelihood ratio of the user through a deinterleaver to obtain a deinterleaved demodulated log-likelihood ratio;

[0039] The deinterleaved demodulated log-likelihood ratio is passed through a decoder to obtain the original user information of the user.

[0040] In some embodiments, the dechirped signal of the user is expressed by the following formula:

[0041]

[0042] Among them, r i (l) is the dechirped signal of the i-th user, i is 1, 2, ..., U, U is the number of users, M = 2 SF is the number of sampling points in each symbol period, SF is the preset spreading factor of the SSK-CSS modulator used to generate the received signal, r(l) is the received signal, j is the imaginary unit, and exp(.) is an exponential function with the natural constant e as the base.

[0043] In some embodiments, the transformation result is expressed by the following formula:

[0044]

[0045] Among them, R i (q) is the transformation result of the i-th user, i is 1, 2, ..., U, U is the number of users, r i(l) is the dechirped signal of the i-th user, q = 0, 1, ..., M-1, M = 2 SF is the number of sampling points in each symbol period, SF is the preset spreading factor of the SSK-CSS modulator used to generate the received signal, and l=0, 1, ..., M-1.

[0046] The present invention has the following beneficial technical effects:

[0047] It can support concurrent transmission by multiple users, and the transmission rate of each user is consistent without any transmission rate loss, making it suitable for scenarios with massive concurrent transmission by users.

[0048] An improved DFT-based soft-decision non-coherent demodulation method with lower implementation complexity and better demodulation performance can be used for signal demodulation, thereby reducing the occupation of storage resources and improving demodulation efficiency.

[0049] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] Figure 1 A flow chart of a multi-user transmission method applicable to an LFM system and applied to a transmitting end provided in an embodiment of the present invention;

[0051] Figure 2 An exemplary flowchart of processing original user information corresponding to three different users by a transmitting end provided in an embodiment of the present invention;

[0052] Figure 3 A flowchart of an exemplary multi-user transmission method applicable to an LFM system and applied to a receiving end provided by an embodiment of the present invention;

[0053] Figure 4 An exemplary flowchart of a receiving end processing received signals corresponding to three different users provided in an embodiment of the present invention;

[0054] Figure 5 A schematic diagram of exemplary simulation results provided for an embodiment of the present invention. DETAILED DESCRIPTION

[0055] The present invention will be further described in detail below with reference to specific examples, but the embodiments of the present invention are not limited thereto.

[0056] In the description of the present invention, the terms "first" and "second" are used for descriptive purposes only and should not be understood to indicate or imply relative importance or implicitly specify the number of the technical features indicated. Therefore, a feature specified as "first" or "second" may explicitly or implicitly include one or more of the features. In the description of the present invention, "plurality" means two or more, unless otherwise specifically defined.

[0057] In the description of this specification, the reference terms "one embodiment", "some embodiments", "example", "specific example", or "some examples" mean that the specific features, structures, materials, or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials, or characteristics described can be combined in any appropriate manner in any one or more embodiments or examples. In addition, those skilled in the art can combine and combine different embodiments or examples described in this specification.

[0058] Although the present invention is described herein in conjunction with various embodiments, in the process of implementing the claimed invention, those skilled in the art can understand and implement other variations of the disclosed embodiments by reviewing the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other components or steps, and "a" or "an" does not exclude multiple situations. A single processor or other unit can implement several functions listed in the claims. Certain measures are recorded in different dependent claims, but this does not mean that these measures cannot be combined to produce good results.

[0059] Figure 1 This is a flow chart of a multi-user transmission method applicable to an LFM system provided by an embodiment of the present invention, which is applied to a transmitting end, such as Figure 1 As shown, the method includes the following steps:

[0060] S101. Obtain original user information of each user among a plurality of users, and a frequency modulation slope of each user; the frequency modulation slopes of different users are different.

[0061] Here, the original user information is any data that the user needs to transmit, such as audio data, text, etc.

[0062] Here, a corresponding frequency modulation slope can be pre-assigned to each user, and the frequency modulation slopes of different users are different. For example, when there are three users, the frequency modulation slope assigned to the first user is m1; the frequency modulation slope assigned to the second user is m2; and the frequency modulation slope assigned to the third user is m3.

[0063] S102: For each user, pass the original user information of the user through an encoder and an interleaver to obtain the user's codeword.

[0064] Here, for each user, the original user information of the user can be passed through an encoder to obtain the encoding result of the user, and then the encoding result can be passed through an interleaver to obtain the codeword of the user; wherein, the encoders and interleavers corresponding to different users are the same.

[0065] Exemplarily, the encoder can be an encoder with an input of K bits and an output of N bits, so that the original user information d1, d2 and d3 corresponding to user 1, user 2, and user 3 can first pass through the same (N, K) encoder and then pass through the same interleaver to obtain the corresponding codewords c1, c2 and c3.

[0066] S103. Use the SSK-CSS modulator corresponding to the user to modulate the user's codeword to obtain the user's modulation sequence; the SSK-CSS (Skle Shift Keying-Chirp Spread Spectrum) modulator has a preset spreading factor, and the frequency modulation slope is the frequency modulation slope of the user; the SSK-CSS modulators corresponding to different users have the same spreading factor.

[0067] Here, the codeword is a binary bit stream. Specifically, the codeword of the i-th user can be converted into a decimal number to obtain the i-th user's transmission data symbol. Then, the transmission data symbol of the i-th user in each symbol period is sent to the i-th SSK-CSS modulator to obtain the i-th user's modulation sequence. The formula for the i-th user's modulation sequence is as follows:

[0068]

[0069] in, T represents each symbol period, B is the system bandwidth of the transmitter, s i (l) is the modulation sequence of the i-th user, i is 1, 2, ..., U, U is the number of users, M = 2 SF is the number of sampling points in each symbol period, SF is the preset spreading factor, m i is the data symbol transmitted in each symbol period of the i-th user (i.e., a decimal number converted from the SF-bit binary bit stream), which is used to represent the data symbol transmitted in one symbol period of the original user information of the i-th user, μ i is the frequency modulation slope of the i-th SSK-CSS modulator, l = 0, 1, ..., M-1, j is the imaginary unit, and exp(.) is the exponential function with the natural constant e as the base.

[0070] S104: Superimpose modulation sequences of multiple users to obtain a mixed modulation sequence.

[0071] Specifically, U modulation sequences are superimposed to obtain a mixed modulation sequence. The formula of the mixed modulation sequence is as follows:

[0072] s(l)=s1(l)+s2(l)+,...,+s U-1 (l)+s U (l);

[0073] Where s(l) represents the mixed modulation sequence, s U (l) represents the modulation sequence of the Uth user.

[0074] S105: Perform noise processing on the mixed modulation sequence to obtain a signal to be sent, and send the signal to be sent.

[0075] Specifically, the mixed modulation sequence can be passed through an additive white Gaussian noise channel to obtain a signal to be transmitted. The formula of the signal to be transmitted is as follows:

[0076] r(l)=s(l)+n(l);

[0077] Where r(l) is the signal to be sent, s(l) is the mixed modulation sequence, and n(l) is the sequence with a mean of 0 and a variance of σ. 2 Gaussian noise.

[0078] Here, the transmitting end may transmit the signal to be sent to the corresponding receiving end.

[0079] For example, Figure 2 This is a flowchart of how the transmitter processes the original user information corresponding to three different users. Figure 2 As shown in the figure, the original user information d1, d2, and d3 corresponding to users 1, 2, and 3 are first passed through the same encoder and interleaver in sequence to obtain the codewords c1, c2, and c3 corresponding to users 1, 2, and 3; then, using the same spreading factor (SF) and frequency modulation slope μ i The SSK-CSS modulator, the code word c i Modulation is performed, and the symbol length can be obtained as M=2 SF The modulation sequence s i , that is, we get s1, s2 and s3; superimpose these three modulation sequences together to get the mixed modulation sequence s c ; By making the mixed modulation sequence s c Through the additive white Gaussian noise (AWGN) channel, the mixed modulation sequence s c Add noise n(l) to obtain the signal to be sent r(l). For example, Figure 2As shown, the SF of the three SSK-CSS modulators is 13, and the frequency modulation slopes of the three SSK-CSS modulators corresponding to users 1, 2, and 3 are 1, 1.05, and 1.1, respectively.

[0080] In an embodiment of the present invention, SSK-CSS technology is used as a modulation technology. This technology leverages the separable nature of signals with different frequency modulation slopes to achieve multi-user transmission. The user information sequence generated by the signal source is encoded and interleaved, and a different frequency modulation slope is assigned to each user. This is then modulated using an SSK-CSS modulator. Consequently, this technology supports concurrent transmission by multiple users, with consistent transmission rates for each user and no loss of transmission rate. This makes it suitable for scenarios with massive concurrent transmissions from a large number of users.

[0081] Figure 3 is another flow chart of a multi-user transmission method applicable to an LFM system provided by an embodiment of the present invention, which is applied to a receiving end, such as Figure 3 As shown, the method includes the following steps:

[0082] S201: Acquire a received signal and a frequency modulation slope of each user among a plurality of users corresponding to the received signal; the frequency modulation slopes of different users are different.

[0083] Here, the receiving end obtains the received signal by receiving the to-be-sent signal sent by the corresponding transmitting end.

[0084] S202: For each user, dechirp the received signal according to the user's frequency modulation slope to obtain the user's dechirped signal.

[0085] Here, the user's dechirped signal is expressed by the following formula:

[0086]

[0087] Among them, r i (l) is the dechirped signal of the i-th user, i is 1, 2, ..., U, U is the number of users, M = 2 SF is the number of sampling points in each symbol period, SF is the preset spreading factor of the SSK-CSS modulator used to generate the received signal, and r(l) is the received signal.

[0088] Here, the formula for the dechirped signal of the i-th user is expanded as follows:

[0089]

[0090] Where n′(l) is the result of the superposition of the noise term n(l) and the interference signals of other U-1 users.

[0091] S203: Perform discrete Fourier transform on the dechirped signal of the user to obtain a transformation result.

[0092] S204: Perform soft decision non-coherent demodulation processing on the transformation result to obtain the user's demodulation log-likelihood ratio.

[0093] Specifically, the specific process of S203 to S204 includes:

[0094] 1) The dechirped signal r of the i-th user i (l) Perform discrete Fourier transform (DFT) at M points to obtain R i (q), the principle is as follows:

[0095]

[0096] Where q = 0, 1, ..., M-1;

[0097] 2) R i (q) Through the modulus operation, we get Here’s how it works:

[0098]

[0099] 3) Yes Perform the square operation to get Here’s how it works:

[0100]

[0101] 4) Yes Perform maximum and logarithmic operations to obtain the transmitted data symbol m i The tth binary bit b in i,t The demodulation log-likelihood ratio (LLR) of is calculated as follows:

[0102]

[0103] Where, t=0,...,SF-1,b i,t is the t-th binary bit in a symbol period of the i-th user, Λ(b i,t ) is b i,t The corresponding demodulated log-likelihood ratio, and They respectively represent the index set whose lth position is 0 and the index set whose lth position is 1 in the binary bit representation corresponding to the decimal numbers 0 to M-1 according to the natural mapping.

[0104] S205 : The user's demodulated log-likelihood ratio passes through a deinterleaver and a decoder to obtain the user's original user information.

[0105] Here, the demodulated LLR of the i-th user can be deinterleaved by the deinterleaver, and then the deinterleaved demodulated LLR is sent to the decoder to obtain the original user information of the i-th user, that is, the original data bit stream of the original user information of the i-th user

[0106] For example, Figure 4 The following is a flowchart of the receiving end processing the received signals corresponding to three different users. Figure 4 As shown, after the received signal r(l) is dechirped by 1, 2, and 3, the dechirped signals corresponding to users 1, 2, and 3 are obtained. By passing the dechirped signals of users 1, 2, and 3 through the same non-coherent demodulator, the dechirped signals of users 1, 2, and 3 are respectively subjected to soft decision non-coherent demodulation processing, and the demodulation log-likelihood ratios corresponding to users 1, 2, and 3 are obtained. Figure 4 The demodulated log-likelihood ratios corresponding to users 1, 2, and 3 are sequentially passed through the same deinterleaver and the same decoder to obtain the original user information corresponding to users 1, 2, and 3.

[0107] In an embodiment of the present invention, at a receiving end, a received signal is first dechirped using down-chirped signals with different slopes. The dechirped signal is then subjected to a DFT transform. The DFT transform result is then demodulated using a soft-decision non-coherent demodulation method to obtain demodulated soft information for each user. The obtained soft information is then sent to a deinterleaver and a decoder to recover the original data bit stream of each user's information. In this manner, an improved DFT-based soft-decision non-coherent demodulation method with low implementation complexity and good demodulation performance is used for signal demodulation, thereby reducing storage resource usage and improving demodulation efficiency.

[0108] In order to further illustrate the beneficial effects of the present invention, a simulation experiment is provided below.

[0109] 1.1 Simulation conditions

[0110] Table 1 lists the simulation parameters used in this transmission method.

[0111]

[0112] Table 1

[0113] 1.2 Simulation results and analysis

[0114] Figure 5 The error performance of the transmission method is given. Figure 5The curve marked with a star represents the bit error performance of user information 1 under the AWGN channel; the curve marked with a triangle represents the bit error performance of user information 2 under the AWGN channel; and the curve marked with a cross represents the bit error performance of user information 3 under the AWGN channel. Figure 5 From the simulation results, we can see that since the three user information are assigned the same spreading factor, their performance is similar. Specifically, considering BER = 1×10 -5 The actual signal-to-noise ratio corresponding to user information 1, user information 2, and user information 3 is approximately -27.4dB, which is 1.4dB lower than the target signal-to-noise ratio (i.e., -26dB). The simulation results show that the transmission method has achieved the established performance indicators.

[0115] The above is a further detailed description of the present invention in conjunction with specific preferred embodiments, and the specific implementation of the present invention should not be considered to be limited to these descriptions. For those skilled in the art of the present invention, without departing from the concept of the present invention, several simple deductions or substitutions can be made, which should be considered to fall within the scope of protection of the present invention.

Claims

1. A multi-user transmission method applicable to an LFM system, characterized in that: include: Obtaining original user information of each user among multiple users and a frequency modulation slope of each user; Different users have different FM slopes; For each user, the original user information of the user is passed through an encoder and an interleaver to obtain the codeword of the user; Using an SSK-CSS modulator corresponding to the user, modulating the user's codeword to obtain a modulation sequence for the user; the SSK-CSS modulator has a preset spreading factor, and a frequency modulation slope is the frequency modulation slope of the user; the SSK-CSS modulators corresponding to different users have the same spreading factor; Superimposing the modulation sequences of the multiple users to obtain a mixed modulation sequence; Performing noise processing on the mixed modulation sequence to obtain a signal to be sent, and sending the signal to be sent; The codeword is a binary bit stream; and the SSK-CSS modulator corresponding to the user is used to modulate the user's codeword to obtain the user's modulation sequence, specifically including: Convert the codeword of the i-th user into a decimal number to obtain the transmission data symbol of the i-th user; The transmission data symbol in each symbol period of the i-th user is sent to the i-th SSK-CSS modulator to obtain the modulation sequence of the i-th user. The modulation sequence of the i-th user is expressed as follows: ； in, is the modulation sequence of the i-th user, i is , is the number of users of the plurality of users, is the number of sampling points in each symbol period, is the preset spreading factor, is the data symbol transmitted in each symbol period of the i-th user, and is used to represent the data symbol transmitted in one symbol period of the original user information of the i-th user. is the frequency modulation slope of the i-th SSK-CSS modulator, , is the imaginary unit, It is an exponential function with the natural constant e as its base.

2. The multi-user transmission method applicable to the LFM system according to claim 1, characterized in that: The step of passing the original user information of the user through an encoder and an interleaver to obtain a codeword of the user includes: After the original user information of the user passes through the encoder, the encoding result of the user is obtained; After the encoding result passes through an interleaver, the codeword of the user is obtained.

3. The multi-user transmission method applicable to the LFM system according to claim 1, characterized in that: The superimposing the modulation sequences of the multiple users to obtain a mixed modulation sequence includes: The U modulation sequences are superimposed to obtain a mixed modulation sequence. The formula of the mixed modulation sequence is as follows: ； in, represents the mixed modulation sequence, is the number of users of the plurality of users, Indicates the The modulation sequence of each user.

4. The multi-user transmission method applicable to the LFM system according to claim 1, characterized in that: The performing noise processing on the mixed modulation sequence to obtain a signal to be sent includes: The mixed modulation sequence is passed through an additive white Gaussian noise channel to obtain a signal to be transmitted. The formula of the signal to be transmitted is as follows: ； in, is the signal to be sent, is the mixed modulation sequence, The mean is 0 and the variance is Gaussian noise.

5. A multi-user transmission method applicable to an LFM system, characterized in that: include: Obtaining a received signal and a frequency modulation slope of each user among a plurality of users corresponding to the received signal; the frequency modulation slopes of different users are different; wherein the received signal is the signal to be transmitted in the method according to any one of claims 1 to 4 above; For each user, dechirping the received signal according to the frequency modulation slope of the user to obtain a dechirped signal of the user; performing a discrete Fourier transform on the dechirped signal of the user to obtain a transformation result; Performing soft decision non-coherent demodulation processing on the transformation result to obtain a demodulation log-likelihood ratio of the user; The demodulated log-likelihood ratio of the user is passed through a deinterleaver and a decoder to obtain the original user information of the user.

6. The multi-user transmission method applicable to the LFM system according to claim 5, characterized in that: The performing soft decision non-coherent demodulation processing on the transformation result to obtain the demodulation log-likelihood ratio of the user includes: Performing modulo processing on the transformation result to obtain a modulo result; Squaring the modulo result to obtain a processing result; Perform maximum processing and logarithmic processing on the processing result to obtain a demodulated log-likelihood ratio of the user.

7. The multi-user transmission method applicable to the LFM system according to claim 5, characterized in that: The method of obtaining the original user information of the user after passing the demodulated log-likelihood ratio of the user through a deinterleaver and a decoder includes: Passing the demodulated log-likelihood ratio of the user through a deinterleaver to obtain a deinterleaved demodulated log-likelihood ratio; The deinterleaved demodulated log-likelihood ratio is passed through a decoder to obtain the original user information of the user.

8. The multi-user transmission method applicable to the LFM system according to claim 5, characterized in that: The dechirped signal of the user is expressed by the following formula: ； in, is the dechirped signal of the i-th user, i is , is the number of users of the plurality of users, is the number of sampling points in each symbol period, is a preset spreading factor of an SSK-CSS modulator used to generate the received signal, is the received signal, is the imaginary unit, It is an exponential function with the natural constant e as its base.

9. The multi-user transmission method applicable to the LFM system according to claim 5, characterized in that: The transformation result is expressed by the following formula: ； in, is the transformation result of the i-th user, i is , is the number of users of the plurality of users, is the dechirped signal of the i-th user, , is the number of sampling points in each symbol period, is a preset spreading factor of an SSK-CSS modulator used to generate the received signal, .

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