Orthogonal index modulation OTFS communication method based on spread spectrum

Abstract

A spread spectrum-based orthogonal index modulation (OTFS) communication method belongs to the field of information and communication technology. The present invention addresses the problem of low spectrum utilization in existing S-OFDM-IM systems. It comprises: at the transmitting end, dividing the bit stream into multiple groups; each group of bit signals is divided into two parts, one part of the bits is used for constellation modulation, and the other part of the bits is used to control the index of the subcarrier; each group of modulated signals generates a DD domain signal through an OTFS block generator, and then performs spread spectrum, inverse sigmoid Fourier transform, Heisenberg transform, adds a cyclic prefix, and performs parallel-to-serial conversion and digital-to-analog conversion to obtain a transmitting signal; at the receiving end, the received signal is subjected to analog-to-digital conversion and serial-to-parallel conversion, and then the cyclic prefix is ​​removed to obtain a one-dimensional time domain received signal, and then subjected to Wigner transform, sigmoid Fourier transform, despread spectrum, demapping and demodulation, and the receiving end bit stream is obtained through a bit synthesizer. The present invention is used for modulation of communication signals.

Description

Technical Field

[0001] The invention relates to an orthogonal index modulation (OTFS) communication method based on spread spectrum, and belongs to the technical field of information and communication. Background Art

[0002] Low-Earth orbit satellite communications offer advantages such as global coverage and flexible deployment. However, the high mobility of satellites can lead to severe Doppler effects, making traditional modulation technologies inadequate. OTFS (Orthogonal Time-Frequency-Space) is a modulation technique that processes signals in the delay-Doppler domain. This effectively mitigates the time- and frequency-varying characteristics of the channel, improving the performance of communication systems in high-speed mobile scenarios.

[0003] The newly proposed precoded OFDM-IM (S-OFDM-IM) uses precoding matrices such as WH and ZC to spread the non-zero data symbols and their indices on the active subcarriers, then compresses them onto all available subcarriers. This leverages multipath and index diversity to increase transmit diversity, better cope with multipath fading channels, and improve system reliability. While the introduction of index modulation and spread spectrum improves system reliability and anti-interference capabilities to a certain extent, it also reduces spectral efficiency.

[0004] In time-varying channel environments such as high-speed mobile communications, OTFS can transform rapidly time-varying channels into two-dimensional, time-invariant channels in the Delay-Doppler (DD) domain, ensuring that all symbols in a transmitted frame receive the same channel gain. Index-based OTFS modulation inherits these advantages of OTFS and further enhances its resistance to Doppler frequency deviation. This makes the transmission of index and data information more stable and reliable in highly dynamic scenarios, reducing index errors and data errors caused by frequency deviation. OTFS-IM (OTFS-index modulation) is a new modulation technology developed based on OTFS.

[0005] In OTFS technology, traditional modulation schemes such as PSK (Phase Shift Keying) and QAM (Quadrature Amplitude Modulation) primarily perform symbol mapping in the time-frequency domain. OTFS-IM, however, leverages the concept of index modulation. Besides traditional symbol-carrying information, it also uses a bit splitter and sub-block divider to flexibly adjust the length of index and symbol bits based on their characteristics, enabling dynamic configuration of index and symbol bits. Compared to the relatively fixed bit allocation of index-based OFDM modulation, it can better utilize limited resources to transmit more information, improving spectrum utilization. Furthermore, it can utilize index information to convey partial data, reducing the energy required for symbol transmission while maintaining the information transmission rate and improving energy efficiency.

[0006] OTFS-IM introduces an additional information dimension through index modulation, making it more difficult for interfering signals to destroy all useful information. When interference is present in the channel, even if some symbols are affected, the receiver can still use the index information and unaffected symbols to recover the data, enhancing anti-interference capabilities. Summary of the Invention

[0007] Aiming at the problem of low spectrum utilization in the existing S-OFDM-IM system, the present invention provides an Orthogonal Index Modulation (OTFS) communication method based on spread spectrum.

[0008] The present invention provides an orthogonal index modulation (OTFS) communication method based on spread spectrum, comprising:

[0009] At the transmitter, the bit stream is divided into multiple groups. Each group of bit signals is divided into two parts: one part is used for constellation modulation, and the other part is used to control the subcarrier index. Each group of modulated signals is generated into a DD domain signal by an OTFS block generator. The DD domain signal is spread based on the Walsh-Hadamard matrix or the Zadoff-Chu matrix to obtain a spread DD domain signal. The spread DD domain signal is converted into a time-frequency domain signal through an inverse sigmoid Fourier transform, and then converted into a time domain signal through a Heisenberg transform. A cyclic prefix is ​​added and parallel-to-serial conversion and digital-to-analog conversion are performed to obtain the transmit signal.

[0010] At the receiving end, the received signal is converted from analog to digital and serial to parallel, and the cyclic prefix is ​​removed to obtain a one-dimensional time domain received signal. The one-dimensional time domain received signal is subjected to a Wigner transform to obtain a two-dimensional time-frequency domain signal, and then a sigmoid Fourier transform is performed to obtain a DD domain received signal. The DD domain received signal is despread and then redistributed into multiple subgroups. The part of each subgroup received signal corresponding to the transmitting end is demapped and demodulated, and then passed through a bit synthesizer to obtain the receiving end bit stream.

[0011] According to the orthogonal index modulation OTFS communication method based on spread spectrum of the present invention, at the transmitting end, the bit stream is divided into g groups by a bit splitter, and the number of bits of each group of bit signals is p a , where the number of bits used to control the subcarrier index is p1, and the number of bits used for constellation modulation is p2;

[0012] Assume that the OTFS frame size output by the OTFS block generator is M×N, where M is the number of subcarriers and N is the number of symbols; divide the total grid MN into g subblocks, each of which is m×n in size, where m is the number of subcarriers in the subblock and n is the number of symbols in the subblock; then:

[0013]

[0014] Where k is the number of sub-block subcarrier activations, C(mn,k) is the number of activation index bit combinations, and M mod is the modulation order.

[0015] According to the orthogonal index modulation OTFS communication method based on spread spectrum of the present invention, for the β-th sub-block, β=1, 2, 3, ..., g, the activation index I of the β-th sub-block is β Expressed as:

[0016] I β ={i β,1 ,…,i β,k},

[0017] where i β,γ ∈{1,2,…,n},β={1,2,…,g},γ={1,…,k};

[0018] The constellation mapping S of the βth sub-block β Expressed as:

[0019] S β ={s β (1),…,s β (k)},

[0020] where s β (γ)∈Λ,β={1,2,…,g},γ={1,2,…,k}, Λ represents the set of constellation modulation symbols;

[0021] Then the bit signal x of the βth sub-block is β for:

[0022] x β ={I β ,S β},β={0,…,g}.

[0023] According to the orthogonal index modulation OTFS communication method based on spread spectrum of the present invention, the DD domain signal generated by the OTFS block generator is represented by X DD :

[0024]

[0025] In the formula κ=0,...,N-1, ι=1,...,M-1, M mod Constellation diagram of 1-order constellation modulation.

[0026] According to the orthogonal index modulation OTFS communication method based on spread spectrum of the present invention, the DD domain signal is spread spectrum based on the Walsh-Hadamard matrix or the Zadoff-Chu matrix, and the DD domain signal after spread spectrum is expressed as sDD :

[0027]

[0028] Where τ is the time delay, v is the Doppler frequency shift, and w ij is the index information set w of the i-th row of the spread spectrum matrix W i The j-th index information, τ0 is the reference delay, v0 is the reference Doppler frequency shift, {x j} is a DD domain signal, indicating X DD modulation symbol sequence.

[0029] According to the orthogonal index modulation OTFS communication method based on spread spectrum of the present invention, the DD domain signal s after spread spectrum DD Convert to time-frequency domain signal X through inverse sigmoid Fourier transform FT :

[0030]

[0031] Where F M is the M-dimensional discrete Fourier matrix, is the N-dimensional discrete Fourier matrix F N The conjugate transposed matrix of ;

[0032] Time-frequency domain signal X FT Converted into time domain signal X through Heisenberg transform T :

[0033]

[0034] In the formula is the M-dimensional discrete Fourier matrix F M The conjugate transposed matrix of .

[0035] According to the orthogonal index modulation OTFS communication method based on spread spectrum of the present invention, the time domain signal X T After adding a cyclic prefix and performing parallel-to-serial conversion and digital-to-analog conversion, the transmitted signal X is obtained.

[0036] According to the orthogonal index modulation (OTFS) communication method based on spread spectrum of the present invention, the received signal X transmitted through the channel to the receiving end is represented as Y, the received signal Y is converted into digital and serial to parallel, and the cyclic prefix is ​​removed to obtain a one-dimensional time domain received signal Y. T ;

[0037] The signal Y is received in one-dimensional time domain T The two-dimensional signal in the time-frequency domain obtained after Wigner transform is expressed as Y FT :

[0038] Y FT =FM Y T .

[0039] According to the orthogonal index modulation OTFS communication method based on spread spectrum of the present invention, the two-dimensional signal Y in the time-frequency domain is FT The DD domain received signal is represented by Y DD :

[0040]

[0041] According to the orthogonal index modulation OTFS communication method based on spread spectrum of the present invention, the DD domain received signal Y is quantized by using the MMSE algorithm. DD Conduct testing and obtain

[0042]

[0043] Where H is the channel matrix, I MN is the identity matrix, ρ snr is the average signal-to-noise ratio;

[0044] Will Rearrange and transform into a two-dimensional signal in the delay-Doppler domain

[0045]

[0046] In the formula Represents a collection;

[0047] For two-dimensional signals After despreading, the signals are divided into multiple subgroups again. The part of the received signal of each subgroup corresponding to the transmitting end is demapped and demodulated, and then the receiving end bit stream is obtained through the bit synthesizer.

[0048] The present invention's beneficial effects include: The method employs orthogonal index modulation (OFDM) based on spread spectrum, addressing the issues of low spectrum utilization and high bit error rate in S-OFDM-IM systems. To further enhance system diversity gain and fading resistance in complex channel environments, waveform modulation is performed based on the precoding matrix of OTFS-IM.

[0049] Based on OTFS-IM, the proposed method uses different spreading matrices to effectively mitigate multipath interference and evenly distribute signal energy across the entire frequency domain, enabling the system to transmit more information within the same bandwidth and thus improving the system's spectral efficiency. A more uniform power spectral density means better utilization of frequency band resources.

[0050] The method of the present invention is based on OTFS-IM. After passing through the ZC or WH matrix, the signal performance is improved, which is better than the channel performance of the S-OFDM-IM system and the bit error rate is reduced. BRIEF DESCRIPTION OF THE DRAWINGS

[0051] Figure 1 Schematic diagram of the transmitting end principle of the orthogonal index modulation OTFS communication method based on spread spectrum according to the present invention; W in the figure represents spread spectrum;

[0052] Figure 2 Schematic diagram of the frame structure design of the method of the present invention based on OTFS-IM;

[0053] Figure 3 Schematic diagram of the receiving end principle of the spread spectrum-based orthogonal index modulation OTFS communication method of the present invention;

[0054] Figure 4 1 is a schematic diagram comparing the WH and ZC precoding channel performances of the S-OTFS-IM system based on the method of the present invention and the existing S-OFDM-IM. In the figure, BER represents the bit error rate and Es / No represents the signal-to-noise ratio.

[0055] Figure 5 Schematic diagram of channel performance comparison of the S-OTFS-IM system based on the method of the present invention. DETAILED DESCRIPTION

[0056] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts shall fall within the scope of protection of the present invention.

[0057] It should be noted that, in the absence of conflict, the embodiments of the present invention and the features in the embodiments may be combined with each other.

[0058] The present invention will be further described below with reference to the accompanying drawings, but is not intended to limit the present invention.

[0059] Specific implementation method 1. Combination Figures 1 to 3 As shown, the present invention provides an orthogonal index modulation OTFS communication method based on spread spectrum, comprising:

[0060] At the transmitter, the bit stream is divided into multiple groups; each group of bit signals is divided into two parts, one part of the bits is used for constellation modulation, and the other part of the bits is used to control the subcarrier index; each group of modulated signals is generated into a DD domain signal by an OTFS block generator; the DD domain signal is spread based on the Walsh-Hadamard matrix or the Zadoff-Chu matrix to obtain a spread DD domain signal, which is converted into a time-frequency domain signal through an inverse sigmoid Fourier transform and then converted into a time domain signal through a Heisenberg transform. A cyclic prefix (CP) is added and parallel-to-serial conversion and digital-to-analog (P / S module) conversion are performed to obtain the transmit signal and send it to the channel for transmission;

[0061] At the receiving end, the received signal is converted from analog to digital (S / P module) and serial to parallel, and the cyclic prefix (CP) is removed to obtain a one-dimensional time domain received signal. The one-dimensional time domain received signal is subjected to a Wigner transform to obtain a two-dimensional time-frequency domain signal, and then a sigmoid Fourier transform is performed to obtain a DD domain received signal. The DD domain received signal is despread and then re-divided into multiple subgroups. The part of each subgroup received signal corresponding to the transmitting end is demapped and demodulated, and then passed through a bit synthesizer to obtain the receiving end bit stream.

[0062] After the DD domain signal in the transmitting end is expanded by the Zadoff-Chu sequence, the sequences generated by different root sequence numbers have an ideal cross-correlation characteristic during cyclic shift, thereby effectively resisting multipath interference.

[0063] After the DD domain signal in the transmitting end is expanded by the Walsh-Hadamard matrix, the signal energy is evenly distributed in the entire frequency domain, so that the system can transmit more information within the same bandwidth, thereby improving the spectrum efficiency of the system.

[0064] In high-speed mobility scenarios, the OTFS-IM system is susceptible to the Doppler effect, causing the frequency of the received signal to shift. This frequency shift destroys signal orthogonality and generates inter-carrier interference (ICI). Multipath propagation is a ubiquitous phenomenon, causing multiple delays in the received signal, resulting in inter-symbol interference (ISI). Furthermore, signal energy may be concentrated in specific delay-Doppler regions, resulting in some wasted frequency bandwidth. However, after Zadoff-Chu sequence expansion, sequences generated by different root numbers exhibit ideal cross-correlation characteristics when cyclically shifted. Specifically, the cross-correlation between two different Zadoff-Chu sequences is zero or near-zero, effectively mitigating multipath interference. After signal expansion using the Walsh-Hadamard matrix, signal energy is evenly distributed across the frequency domain, enabling the system to transmit more information within the same bandwidth, thereby improving system spectral efficiency. More uniform power spectral density means more efficient utilization of frequency bandwidth resources.

[0065] Walsh-Hadamard and Zadoff-Chu sequences are square and reversible, and have the following two key features: (i) the complex numbers of the two matrices have the same size. Therefore, each non-zero data symbol is evenly distributed across N subcarriers, which enables S-OTFS-IM to achieve significant diversity gain over existing OTFS-IM schemes; (ii) the two matrices are orthogonal, i.e., G -1 =G H , which aims to achieve interference-free transmission and keep the Euclidean distance between any pair of signals before and after spreading unchanged.

[0066] The present invention uses the Walsh-Hadamard matrix to spread the OTFS-IM system to improve the spectrum efficiency of the system, and uses the Zadoff-Chu sequence to spread the OTFS-IM system to resist multipath interference.

[0067] A.Walsh-Hadamard Matrix Theory:

[0068] The WH matrix recursively becomes:

[0069]

[0070] The size of the WH matrix is ​​N=2 k Since all elements are real-valued, this matrix is ​​particularly suitable for low-complexity implementation.

[0071] B.Zadoff-Chu (ZC) matrix theory:

[0072] Construct the ZC root sequence as follows:

[0073]

[0074] Where q is an integer, m is any integer relative to N, and n = 1, 2, ..., N. The ZC matrix is ​​represented by f1 = [c1, ..., c N ] T , as its first column, and the other columns are cyclic shifts of f1. For example, when N=4, m=1 and q=0. f1=[-j,-1,-j,1] T . Construct an orthogonal matrix and let -j be a, then the above formula can be obtained as f1=[a,-1,a,1] T .calculate Here Then the ZC matrix is ​​as follows:

[0075]

[0076] The parameters are We can get ||W||2 =1.

[0077] The signal transmitted by the S-OTFS-IM system consists of two parts: one is the multivariate constellation modulation information carried by the activated grid points in the delay-Doppler domain, and the other is the index information implicitly transmitted by the combination of the activated grid points.

[0078] Furthermore, at the transmitting end, the bit stream is divided into g groups by a bit splitter, and the number of bits in each group of bit signals is p a , according to its own characteristics, each group of bit signals is divided into two parts, where the number of bits used to control the subcarrier index is p1, and the number of bits used for constellation modulation is p2;

[0079] Combine Figure 2 As shown in , it is assumed that the OTFS frame size output by the OTFS block generator is M×N, where M is the number of subcarriers and N is the number of symbols; Figure 1 As shown, the total number of grids MN is divided into g sub-blocks, each sub-block is m×n in size, and the number of grids is mn=M×N / g, where m is the number of subcarriers in the sub-block and n is the number of symbols in the sub-block. The total number of bits p in the bit stream is then:

[0080]

[0081] Where k is the number of sub-block subcarrier activations, C(mn,k) is the number of activation index bit combinations, and M mod is the modulation order.

[0082] The total number of activated subcarriers is K = k × g. The p-bit information is divided into g groups, each containing p a =p / g bits, which are divided into two parts, one part of which is used for index selection and the other part is used for constellation mapping.

[0083] In this embodiment, for the βth sub-block, the number of bits p1 is used for index selection, where β = 1, 2, 3, ..., g, for each sub-block, select k activations from the mn delay-Doppler domain grid points for transmitting data symbols, and set the activation index I of the βth sub-block to β Expressed as:

[0084] I β ={i β,1 ,…,i β,k},

[0085] where i β,γ ∈{1,2,…,n},β={1,2,…,g},γ={1,…,k};

[0086] In addition, p2=k(log2(M mod)) Binary bits are used for constellation mapping, and the constellation mapping S of the βth sub-block β Expressed as:

[0087] S β ={s β (1),…,s β (k)},

[0088] where s β (γ)∈Λ,β={1,2,…,g},γ={1,2,…,k}, Λ represents the set of constellation modulation symbols;

[0089] Thus, the data symbol x of the βth sub-block is β The index I can be activated by it β and constellation mapping combination S, then the bit signal x of the βth sub-block is β for:

[0090] x β ={I β ,S β},β={0,…,g}.

[0091] The signals of each group are combined into an M×N OTFS block in the DD domain through the OTFS block generator to obtain the output signal X after the DD domain index modulation is completed. DD .

[0092] Similarly, after completing the index selection and constellation mapping synthesis of all sub-blocks, the OTFS-IM signal of one frame can be expressed as:

[0093]

[0094] This embodiment adopts the combination number method. When the number of subcarrier blocks is large, if the number of index resources in each subcarrier block is mn and the number of activation indexes in each subcarrier block is k, then all activation index combinations have a total of C(mn,k). Then the maximum number of binary bits b that can be used for index mapping should satisfy:

[0095] 2 b ≤C(mn,k).

[0096] The combinatorial number theory method determines the activation index sequence J={c k ,c k-1 ,…,c i ,…,c2,c1}, where c i Represents the index of the i-th activated index resource. Each element in J satisfies the following equation:

[0097] Z=C(c k ,k)+…+C(c k,2)+C(c k ,1),

[0098] Where n>c k >…>c1>0; Z is a decimal number whose value is in the interval [0,C(mn,k)-1].

[0099] The specific steps are summarized as follows:

[0100] Step 1: For each sub-block, convert the binary bits used for index selection into a decimal number Z;

[0101] Step 2: First determine ck, maximize ck so that C(c k ,k)≤Z, update Z, Z=ZC(c k ,k);

[0102] Step 3: Determine ck-1 so that C(c k ,k)≤Z, update Z, Z=ZC(c k ,k) and so on to get each ci;

[0103] Step 4: Iterate to the end to get the activation index sequence:

[0104] J={c k ,c k-1 ,…,c i ,…,c2,c1}+1.

[0105] After all sub-blocks have completed index mapping and multi-constellation modulation, the modulated information will flow into the OTFS signal frame generator. Figure 1 The frame format in the OTFS-IM system is used to obtain the two-dimensional signal in the delay-Doppler domain. The DD domain signal generated by the OTFS block generator is expressed as X DD :

[0106]

[0107] In the formula κ=0,...,N-1, ι=1,...,M-1, M mod Constellation diagram of 1-order constellation modulation.

[0108] Furthermore, the DD domain signal is spread based on the Walsh-Hadamard matrix or the Zadoff-Chu matrix, and the DD domain signal after spread is expressed as s DD :

[0109]

[0110] Where τ is the time delay, v is the Doppler frequency shift, and w ij is the index information set w of the i-th row of the spread spectrum matrix W i The j-th index information, τ0 is the reference delay, v0 is the reference Doppler frequency shift, {x j} is a DD domain signal, indicating X DD The spreading matrix W has a dimension of N×N.

[0111] DD domain signal after spread spectrum DD Convert to the time-frequency domain signal X through inverse sigmoid Fourier transform (ISFFT) FT :

[0112]

[0113] Where F M is the M-dimensional discrete Fourier matrix, is the N-dimensional discrete Fourier matrix F N The conjugate transposed matrix of ;

[0114] In the time-frequency domain, the time dimension is sampled with T, and the frequency dimension is sampled with Δf to obtain a two-dimensional grid of size N×M in the time-frequency domain. The duration of a single S-OTFS-IM signal frame is NT, and the frequency bandwidth occupied is MΔf. Next, the two-dimensional time-frequency signal X is transformed using the M-point IFFT transform. FT Convert it into a time domain signal to obtain the time domain expression of the system signal frame.

[0115] Time-frequency domain signal X FT Converted into time domain signal X through Heisenberg transform T :

[0116]

[0117] In the formula is the M-dimensional discrete Fourier matrix F M The conjugate transposed matrix of .

[0118] To avoid interference between signal frames, each time domain signal X T Add the cyclic prefix to get X CP After parallel-to-serial conversion and digital-to-analog conversion, the transmission signal X is obtained and sent to the wireless channel.

[0119] Going further, combined Figure 3 As shown, the received signal X after the transmission signal reaches the receiving end through the channel is represented as Y. The received signal Y is converted into digital and serial to parallel, and then the cyclic prefix is ​​removed to obtain the one-dimensional time domain received signal Y T ;

[0120] The signal Y is received in one-dimensional time domainT The two-dimensional signal in the time-frequency domain obtained after Wigner transform is expressed as Y FT :

[0121] Y FT =F M Y T .

[0122] In this embodiment, the two-dimensional signal Y in the time-frequency domain is FT The DD domain received signal obtained by performing symplectic Fourier transform is expressed as Y DD , let Y DD (τ,v) is the DD domain signal, which is obtained by sigmoid Fourier transform:

[0123]

[0124] In the S-OTFS-IM system, only a portion of the grid points are activated for information transmission in each signal frame, while the inactivated grid points remain silent. To recover the transmitted information, the receiver first detects the location of the activated grid points in each sub-block and recovers the sub-block index bits. It then performs multi-element constellation demodulation on the activated grid points.

[0125] MMSE algorithm is used to calculate the received signal Y in the DD domain DD Conduct testing and obtain

[0126]

[0127] Where H is the channel matrix, I MN is the identity matrix, ρ snr is the average signal-to-noise ratio;

[0128] Will Rearrange and transform into a two-dimensional signal in the delay-Doppler domain

[0129]

[0130] In the formula Represents a collection;

[0131] For two-dimensional signals After despreading, the signals are divided into multiple subgroups again. The part of the received signal of each subgroup corresponding to the transmitting end is demapped and demodulated, and then the receiving end bit stream is obtained through the bit synthesizer.

[0132] according to Figure 2 The signal frame format of the system in the delay-Doppler domain will be Divide into g sub-blocks. Use the proposed index detection algorithm to obtain the location information of the activation grid points from each word block. In order to maintain generality, take the βth sub-block For example:

[0133] First, the index detection algorithm uses the input received sub-block information and the parameters m and n of the sub-block sub-part size to express each sub-part as a formula based on the system's signal frame format in the delay-Doppler domain, where:

[0134]

[0135] Each sub-section is then vectorized, converting it into an mn×1 one-dimensional vector. Next, the grid energy of the sub-block is calculated and averaged. The calculated average energy information is then used to find the locations with the highest average energy. Specifically, a function is used to find the location corresponding to the maximum value among the mn average energies.

[0136] Finally, based on the acquired position information of the activated grid points, the index mapper in the S-OTFS-IM system receiver recovers the sub-block index bits. Simultaneously, based on the position information of the activated grid points, multivariate constellation demodulation is performed on the grid points at the corresponding positions in the two sub-parts to recover the corresponding modulation bits. Once the index bits and modulation bits for all sub-blocks are recovered, the transmitted information is restored based on the one-to-one correspondence between the sub-blocks and the bit blocks.

[0137] The DD domain receives the signal Y DD After despreading, the signals are divided into g groups again. Then, the part of each subgroup corresponding to the transmitter is demapped and demodulated. The received signal is correlated using the same spreading code as the transmitter to restore the original modulated data.

[0138] Finally, Matlab is used to perform simulation analysis to verify its correctness.

[0139] Combine Figure 4 As shown in the figure, based on the precoding matrix and the OTFS-IM system, the signal system performance is improved. It is superior to the channel performance of the S-OFDM-IM system and the bit error rate is reduced.

[0140] Combine Figure 5 As shown in Figure 2, based on OTFS-IM, after the signal is expanded using the Walsh-Hadamard matrix, the signal energy is evenly distributed across the entire frequency domain, enabling the system to transmit more information within the same bandwidth, thereby improving the system's spectral efficiency. A more uniform power spectral density means that frequency band resources are more fully utilized.

[0141] Based on OTFS-IM, after the signal is expanded through the Zadoff-Chu matrix, the cross-correlation of the sequence is zero or close to zero. This effectively resists multipath interference and improves channel performance compared to the unexpanded matrix.

[0142] This confirms the correctness of the method of the present invention in improving the bit error rate performance by improving spectrum utilization and resisting multipath effects.

[0143] Although the present invention is described herein with reference to specific embodiments, it should be understood that these embodiments are merely illustrative of the principles and applications of the invention. It should be understood that many modifications may be made to the illustrative embodiments, and that other arrangements may be devised, without departing from the spirit and scope of the invention as defined by the appended claims. It should be understood that the various dependent claims and features described herein may be combined in ways other than those described in the original claims. It should also be understood that features described in conjunction with individual embodiments may be used in conjunction with other described embodiments.

Claims

1. A spread spectrum-based orthogonal index modulation (OTFS) communication method, characterized in that: include, At the transmitter, the bit stream is divided into multiple groups. Each group of bit signals is divided into two parts: one part is used for constellation modulation, and the other part is used to control the subcarrier index. Each group of modulated signals is generated into a DD domain signal by an OTFS block generator. The DD domain signal is spread based on the Walsh-Hadamard matrix or the Zadoff-Chu matrix to obtain a spread DD domain signal. The spread DD domain signal is converted into a time-frequency domain signal through an inverse sigmoid Fourier transform, and then converted into a time domain signal through a Heisenberg transform. A cyclic prefix is ​​added and parallel-to-serial conversion and digital-to-analog conversion are performed to obtain the transmit signal. At the receiving end, the received signal is converted from analog to digital and serial to parallel, and the cyclic prefix is ​​removed to obtain a one-dimensional time domain received signal. The one-dimensional time domain received signal is then subjected to a Wigner transform to obtain a two-dimensional time-frequency domain signal, and then subjected to a symplectic Fourier transform to obtain a DD domain received signal. After despreading the DD domain received signal, it is re-divided into multiple subgroups. The part of each subgroup received signal corresponding to the transmitting end is demapped and demodulated, and then the receiving end bit stream is obtained through the bit combiner. Assume that the OTFS frame size output by the OTFS block generator is M×N, where M is the number of subcarriers and N is the number of symbols; The DD domain signal is spread based on the Walsh-Hadamard matrix or the Zadoff-Chu matrix, and the DD domain signal after spread is expressed as s DD : Where τ is the time delay, v is the Doppler frequency shift, and w ij is the index information set w of the i-th row of the spread spectrum matrix W i The j-th index information, τ0 is the reference delay, v0 is the reference Doppler frequency shift, {x j } is a DD domain signal, indicating X DD modulation symbol sequence.

2. The orthogonal index modulation (OTFS) communication method based on spread spectrum according to claim 1, characterized in that: At the transmitting end, the bit stream is divided into g groups by a bit splitter, and the number of bits in each group is p. a , where the number of bits used to control the subcarrier index is p1, and the number of bits used for constellation modulation is p2; Divide the total number of grids MN into g sub-blocks, each sub-block is m×n in size, where m is the number of subcarriers in the sub-block and n is the number of symbols in the sub-block; then: Where k is the number of sub-block subcarrier activations, C(mn,k) is the number of activation index bit combinations, and M mod is the modulation order.

3. The orthogonal index modulation (OTFS) communication method based on spread spectrum according to claim 2, characterized in that: For the βth sub-block, β = 1, 2, 3, ..., g, the activation index I of the βth sub-block is β Expressed as: I β ={i β,1 ,…,i β,k }, among them β,γ ∈{1,2,…,n},β={1,2,…,g},γ={1,…,k}; The constellation mapping S of the βth sub-block β Expressed as: S β ={s β (1),…,s β (k)}, where s β (γ)∈Λ,β={1,2,…,g},γ={1,2,…,k},Λ represents the set of constellation modulation symbols; then the bit signal x of the βth sub-block is β for: x β ={I β ,S β },β={0,…,g}.

4. The orthogonal index modulation (OTFS) communication method based on spread spectrum according to claim 3, characterized in that: The DD domain signal generated by the OTFS block generator is represented as X DD : In the formula M mod Constellation diagram of 1-order constellation modulation.

5. The orthogonal index modulation (OTFS) communication method based on spread spectrum according to claim 4, characterized in that: DD domain signal after spread spectrum DD Convert to time-frequency domain signal X through inverse sigmoid Fourier transform FT : Where F M is the M-dimensional discrete Fourier matrix, is the N-dimensional discrete Fourier matrix F N The conjugate transposed matrix of ; Time-frequency domain signal X FT Converted into time domain signal X through Heisenberg transform T : In the formula is the M-dimensional discrete Fourier matrix F M The conjugate transposed matrix of .

6. The orthogonal index modulation (OTFS) communication method based on spread spectrum according to claim 5, characterized in that: Time domain signal X T After adding a cyclic prefix and performing parallel-to-serial conversion and digital-to-analog conversion, the transmitted signal X is obtained.

7. The orthogonal index modulation (OTFS) communication method based on spread spectrum according to claim 6, characterized in that: The received signal X after the transmission signal passes through the channel and reaches the receiving end is represented as Y. The received signal Y is converted into digital and serial to parallel, and then the cyclic prefix is ​​removed to obtain the one-dimensional time domain received signal Y. T ; The signal Y is received in one-dimensional time domain T The two-dimensional signal in the time-frequency domain obtained after Wigner transform is expressed as Y FT : Y FT =F M Y T 。 8. The orthogonal index modulation (OTFS) communication method based on spread spectrum according to claim 7, characterized in that: The two-dimensional signal Y in the time-frequency domain FT The DD domain received signal is represented by Y DD :

9. The orthogonal index modulation (OTFS) communication method based on spread spectrum according to claim 8, characterized in that: MMSE algorithm is used to calculate the received signal Y in the DD domain DD Conduct testing and obtain Where H is the channel matrix, I MN is the identity matrix, ρ snr is the average signal-to-noise ratio; Will Rearrange and transform into a two-dimensional signal in the delay-Doppler domain In the formula Represents a collection; For two-dimensional signals After despreading, the signals are divided into multiple subgroups again. The part of the received signal of each subgroup corresponding to the transmitting end is demapped and demodulated, and then the receiving end bit stream is obtained through the bit synthesizer.

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