Wireless optical transmission method based on asymmetric limiting optical simulation radio frequency division multiplexing

Abstract

The invention discloses a wireless optical transmission method based on asymmetric limiting optical imitation radio frequency division multiplexing, and relates to the technical field of wireless communication. The method comprises the following steps: S1, preprocessing input signal data, including mapping, design of modulation symbols and symmetric construction, and generating a frequency domain signal; s2, mapping the frequency domain signal to a time domain based on real-number discrete affine Fourier transform, and generating an AFDM (Alternating Frequency Division Multiplexing) signal; s3, performing asymmetric clipping on the AFDM signal, transmitting the AFDM signal to a receiving end through an optical communication channel, and performing reverse restoration on the AFDM signal, including restoration of the clipped signal and demodulation of data; and S4, based on the design of a real DAFT kernel function, deducing a new c1 parameter range, realizing complete separation of different delayed signal paths in a DAFT domain, and achieving full diversity. According to the invention, the problem of signal distortion caused by a multipath effect in optical communication is solved.

Description

Technical Field

[0001] The invention relates to the technical field of wireless communication, and in particular to a wireless optical transmission method based on asymmetric amplitude-limited optical analog radio frequency division multiplexing. Background Art

[0002] With the rapid development of modern communication technology, optical communication has become an important part of the communication field with its advantages of high speed, large capacity and low loss. However, due to the limitations of physical properties, optical communication systems require that the signals transmitted in the channel must be non-negative real numbers. This unique requirement puts higher standards on modulation and waveform design technology. The existing orthogonal frequency division multiplexing technology (OFDM) has been widely used in wireless communications due to its high spectral efficiency and robustness to frequency selective fading. However, there are challenges when directly applying traditional OFDM to optical communication systems, such as the complex form of the signal and the high peak-to-average power ratio (PAPR) do not meet the requirements of signal non-negativity. To this end, researchers proposed asymmetric clipping optical orthogonal frequency division multiplexing (ACO-OFDM) technology, which generates non-negative real signals through asymmetric clipping to meet the transmission requirements of optical communication. However, ACO-OFDM has problems such as reduced spectral efficiency and insufficient anti-interference ability in multipath channels, and shows certain limitations in scenarios with obvious multipath effects such as multimode fiber communication and free space optical communication.

[0003] As an emerging multi-carrier modulation technology based on discrete affine Fourier transform (DAFT), AFDM has shown unique advantages in wireless communications. AFDM can effectively separate the delay of multipath channels, reduce interference, and achieve high diversity gain by utilizing the time-frequency sparsity of the channel, thereby improving anti-fading capability and system reliability. In addition, the AFDM receiver can use a simple linear detection algorithm with low implementation complexity. However, the successful experience of AFDM in wireless communications cannot be directly transferred to the field of optical communications. The main reason is that the signal it generates is in complex form, which does not meet the requirements of optical communications for non-negative real signals. To solve this problem, additional signal processing methods are required, such as adding bias or nonlinear transformation, but this will significantly increase the complexity and power consumption of the system, limiting its practical application.

[0004] Existing optical communication modulation technologies, such as ACO-OFDM, can meet the non-negativity requirements, but their anti-interference capabilities are limited in complex multipath channel scenarios; while AFDM has technical advantages in multipath environments, it is difficult to apply to optical communication systems due to the signal non-negativity problem. This technical bottleneck limits the performance improvement of optical communication systems in complex multipath environments. How to introduce the advantages of AFDM technology into the field of optical communications and solve the signal non-negativity constraint problem is a key problem that needs to be overcome urgently. Summary of the invention

[0005] The purpose of the present invention is to propose a wireless optical transmission method based on asymmetric limited optical frequency division multiplexing, which solves the signal non-negativity constraint and multipath interference problems in optical communication systems by combining the multipath component separation capability of AFDM with the non-negativity processing method of ACO-OFDM, and realizes efficient and reliable signal transmission. It is particularly suitable for complex scenarios with significant multipath effects such as multimode optical fiber communication and free space optical communication.

[0006] To achieve the above object, the present invention proposes a wireless optical transmission method based on asymmetric limited optical frequency division multiplexing, and the specific steps are as follows:

[0007] Step S1, preprocessing the input optical communication signal data at the transmitting end, including mapping of frequency domain data, design of modulation symbols and symmetry construction of data, to generate a frequency domain signal that meets specific conditions;

[0008] Step S2, mapping the frequency domain signal generated in step S1 to the time domain based on the real discrete affine Fourier transform DAFT to generate an AFDM time domain signal;

[0009] Step S3, using the odd half-wave symmetry characteristic to asymmetrically clip the AFDM time domain signal generated in step S2, setting the negative value part of the AFDM time domain signal to zero, and transmitting it to the receiving end through the optical communication channel; the receiving end reversely restores the signal, including recovering the clipped signal and demodulating the data;

[0010] Step S4: Based on the design of the real DAFT kernel function, derive the new c 1 The parameter range enables signal paths with different delays to be completely separated in the DAFT domain, achieving full diversity.

[0011] Preferably, in step S1, the input optical communication signal data is mapped to the frequency domain subcarrier to form a frequency domain symbol. In order to ensure that the frequency domain signal is a real number, the frequency domain symbol is set to a conjugate symmetric form, and the formula is as follows:

[0012] X[Nk]=X[k] * ;

[0013] Where X[k] represents the symbol of the kth subcarrier, X[k] * is the conjugate complex number of X[k], and X[Nk] is the symbol of the Nk-th subcarrier.

[0014] Preferably, to meet the requirements of asymmetric clipping, data is loaded only on odd-numbered subcarriers, and even-numbered subcarrier symbols are set to zero, and the formula is as follows:

[0015] X[2k]=0;

[0016] X[2k+1] = frequency domain symbol.

[0017] Preferably, in step S2, the frequency domain signal generated in step S1 is mapped to the time domain using a real discrete affine Fourier transform DAFT to generate an AFDM time domain signal, and the specific steps are as follows:

[0018] Step S21, the calculation formula of the kernel function of discrete affine Fourier transform DAFT is as follows:

[0019]

[0020] The calculation formula for its inverse transform is as follows:

[0021]

[0022] Where x[n] is the input signal, S[m] is the output signal, N is the number of subcarriers, M is the length of the DAFT transform, and c 1 、c 2 is a free parameter in the modulation design, m is the output signal index, n is the input signal index, and j is an imaginary number;

[0023] Step S22: Adjust the kernel function of the discrete affine Fourier transform DAFT to remove the complex exponential kernel function. Item, will Replace with cos(2πc 1 n 2 ), the calculation formula of the adjusted discrete affine Fourier transform DAFT kernel function is as follows:

[0024]

[0025] The time domain signal generated by the adjusted kernel function is real and retains the multipath separation capability of discrete affine Fourier transform DAFT.

[0026] Preferably, cos(2πc 1 n 2 ) is calculated as follows:

[0027]

[0028] Preferably, in S3, the odd half-wave symmetry characteristic is utilized to perform asymmetric clipping ACO on the AFDM time domain signal generated in step S2, and the specific steps are as follows:

[0029] Step S31: Since the time domain signal x(t) has odd half-wave symmetry, its negative half does not carry valid information. The negative part is forced to be zero through asymmetric clipping. The formula is as follows:

[0030] xclipped (t) = max(x(t), 0);

[0031] Among them, x clipped (t) is the clipped signal, t is the continuous time variable of the signal;

[0032] Step S32, demodulate and restore the received optical signal to achieve accurate data recovery; the receiving end restores the received optical signal to an electrical signal through a photoelectric converter Photodetector, and uses the known odd half-wave symmetry characteristics to restore the clipped negative half signal, the formula is as follows:

[0033]

[0034] Among them, x recovered (t) is the recovered signal, T is the signal period;

[0035] Perform an inverse transform on the real DAFT kernel function to restore the frequency domain sign. The calculation formula is as follows:

[0036]

[0037] The original input data is extracted by utilizing the conjugate symmetry between frequency domain symbols and the odd subcarrier allocation rule.

[0038] Preferably, in step S4, the left shift logic and the offset distance of the non-zero elements in the channel matrix are derived, and the mapping relationship of the signal path in the DAFT domain is described in combination with the right shift logic. The specific steps are as follows:

[0039] Step S41, left shift and right shift brought by real number kernel function;

[0040] Step S42: derive new c 1 Parameter range, to achieve full diversity, to ensure that there is no overlap between the right and left shift paths and between the left and right shift paths.

[0041] Preferably, in step S41, the left shift and right shift brought by the real number kernel function are specifically performed as follows:

[0042] Step S411: In conventional AFDM, c 1 The range is derived as follows:

[0043] The elements of the channel matrix are:

[0044]

[0045] in,

[0046]

[0047] When v i = 0,

[0048]

[0049] Right shift logic is +c 1 Imported, its offset is:

[0050] loc i =(2Nc 1 l i ) N ;

[0051] Among them, l i is the delay of the ith path, H i [p,q] is the channel matrix element of the i-th path, p and q are frequency domain indices, is the frequency domain contribution function, v i is the Doppler shift of the i-th path, loc i is the offset of the i-th path;

[0052] Step S412: Realization of the kernel function introduces left-shift logic. In ACO-AFDM, the kernel function is replaced by:

[0053]

[0054] Left shift logic by Introduced, its channel matrix elements become:

[0055]

[0056] in,

[0057]

[0058] When v i =0:

[0059]

[0060] The new offset is:

[0061] loc′ i =(-2Nc 1 l i ) N ;

[0062] Left shift logic is done by -c 1 The non-zero elements of the channel matrix are shifted from the diagonal to the left by loc′ i locations.

[0063] Preferably, in step S42, the new c 1 The steps for deriving the parameter range are as follows:

[0064] Step S421: right-shift the non-overlapping paths 1 Range deduction:

[0065] From the derivation of the original AFDM, the right shift is defined by the following formula:

[0066] loc i =(2Nc 1 l i ) N ;

[0067] In order to avoid overlap between right-shift paths, the following conditions must be met:

[0068]

[0069] Let l i+1 -l i = 1, to ensure that the paths do not overlap, the right-shifted c 1 The scope must meet:

[0070] c 1 >0;

[0071] Step S422: Left shift the non-overlapping paths 1 The range is derived as follows:

[0072] The offset range for left shift is:

[0073] loc′ i =(-2Nc 1 l i ) N ;

[0074] In order to avoid overlap between left-shift paths,

[0075]

[0076] Similarly, assuming that the minimum delay difference between the paths is 1, we can obtain the left shift c to ensure that the paths do not overlap. 1 The range is:

[0077] c 1 >0;

[0078] Step S423: The leftward movement path and the rightward movement path do not overlap. 1 Range derivation: To avoid overlap between left-shift paths and right-shift paths, the maximum value of all right-shift paths must be less than N / 2, and the minimum value of all left-shift paths must also be less than N / 2.

[0079] Step S424: The paths of rightward and leftward movement do not overlap and need to satisfy c 1>0, the left-shift path and the right-shift path do not overlap and must satisfy the following conditions:

[0080]

[0081] Final c 1 The range is:

[0082]

[0083] The ACO-AFDM channel matrix is ​​completely separated in the DAFT domain to achieve full diversity.

[0084] Preferably, in step S423, the maximum value of the rightward shift is:

[0085] 2N 1 l max ;

[0086] Let it be less than N / 2:

[0087]

[0088] After finishing, we get:

[0089]

[0090] The left offset is:

[0091] -2Nc 1 l max ;

[0092] make get:

[0093]

[0094] After finishing, we get:

[0095]

[0096] Therefore, the present invention proposes a wireless optical transmission method based on asymmetric limited optical analog frequency division multiplexing, and its beneficial effects are as follows:

[0097] (1) The present invention provides a wireless optical transmission method based on asymmetric amplitude-limited AFDM, which realizes efficient signal transmission that is resistant to multipath interference in optical communication channels by combining the multipath component separation capability of AFDM (AFDM) and the non-negative signal generation mechanism of asymmetric clipping (ACO).

[0098] (2) The present invention provides a wireless optical transmission method based on asymmetric limited optical analog frequency division multiplexing, which can significantly improve the system performance in complex multipath scenarios such as multimode optical fiber communication and free space optical communication while meeting the non-negative real signal transmission requirements of the optical communication channel, solves the signal distortion problem caused by multipath effect in optical communication, and provides an effective solution to the stability and reliability of optical communication systems in complex scenarios.

[0099] The technical solution of the present invention is further described in detail below through the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0100] Figure 1 It is an overall flow chart of a wireless optical transmission method based on asymmetric limited optical analog radio frequency division multiplexing of the present invention;

[0101] Figure 2 It is the curve of bit error rate changing with signal-to-noise ratio under multipath condition of ACO-AFDM and ACO-OFDM of the present invention. DETAILED DESCRIPTION

[0102] In order to make the technical solutions, advantages and purposes of the present invention clearer, the technical solutions of the embodiments of the present invention are clearly and completely described below. The described embodiments are part of the embodiments of the present invention, rather than all the embodiments. Based on the described embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work belong to the protection scope of this application.

[0103] Unless otherwise defined, technical or scientific terms used in the present invention shall have the common meanings understood by one having ordinary skills in the field to which the present invention belongs.

[0104] like Figure 1 As shown, the present invention provides a wireless optical transmission method based on asymmetric limited optical analog frequency division multiplexing to simultaneously meet the non-negative requirements of the optical channel for signal transmission and the anti-interference ability in a complex multipath environment. The specific process is as follows:

[0105] Step S1: preprocessing the input optical communication signal data at the transmitting end, including mapping of frequency domain data, design of modulation symbols and symmetry construction of data, to generate a frequency domain signal that meets specific conditions.

[0106] Step S11, transmission data processing part: In view of the real number and non-negative requirements of optical communication signals, the transmitter needs to process the input data. The input data is mapped to the frequency domain subcarrier to form a frequency domain symbol for subsequent modulation. In order to ensure that the time domain signal is a real number, the frequency domain symbol is set to a conjugate symmetric form:

[0107] X[Nk]=X[k]* ;

[0108] Where X[k] represents the symbol of the kth subcarrier, X[k] * is the conjugate complex number of X[k], and X[Nk] is the symbol of the Nk-th subcarrier.

[0109] Step S12: To meet the requirement of asymmetric clipping, data is only loaded on odd-numbered subcarriers, and symbols of even-numbered subcarriers are set to zero. The formula is as follows:

[0110] X[2k]=0;

[0111] X[2k+1] = frequency domain symbol;

[0112] Through this design, the time domain signal is ensured to have odd half-wave symmetry, providing a basis for subsequent asymmetric trimming.

[0113] Step S2: Map the frequency domain signal generated in step S1 to the time domain based on the real discrete affine Fourier transform DAFT to generate an AFDM time domain signal. The specific steps are as follows:

[0114] Step S21, the calculation formula of the kernel function of discrete affine Fourier transform DAFT is as follows:

[0115]

[0116] The calculation formula for its inverse transform is as follows:

[0117]

[0118] Where x[n] is the input signal, S[m] is the output signal, N is the number of subcarriers, M is the length of the DAFT transform, and c 1 、c 2 is a free parameter in the modulation design, m is the output signal index, n is the input signal index, and j is an imaginary number;

[0119] Compared with DFT, the kernel function of DAFT is a complex form with quadratic phase modulation, including the term and These parameters adjust how the signal is transformed in the time and frequency domains, providing greater flexibility.

[0120] Step S22: Adjust the kernel function of the discrete affine Fourier transform DAFT to remove the complex exponential kernel function. Item, will Replace with cos(2πc 1 n 2 ), the calculation formula of the adjusted discrete affine Fourier transform DAFT kernel function is as follows:

[0121]

[0122]

[0123] The time domain signal generated by the adjusted kernel function is real and retains the multipath separation capability of discrete affine Fourier transform DAFT.

[0124] Step S3, using the odd half-wave symmetry characteristic to asymmetrically clip the AFDM time domain signal generated in step 2 ACO, set the negative value part of the AFDM time domain signal to zero, and transmit it to the receiving end through the optical communication channel; the receiving end reversely restores the signal, including recovery of the clipped signal and demodulation of the data, and the specific steps are as follows:

[0125] Step S31: Since the time domain signal x(t) has odd half-wave symmetry, its negative half does not carry valid information. The negative part is forced to be zero through asymmetric clipping. The formula is as follows:

[0126] x clipped (t) = max(x(t), 0);

[0127] Where t is the continuous time variable of the signal, x clipped (t) is the clipped signal, which meets the non-negative requirement of the optical communication channel, and clipping does not cause information loss. After asymmetric clipping, the energy of the signal is concentrated in the non-negative part, which effectively improves the energy efficiency of the optical communication system.

[0128] Step S32, demodulate and restore the received optical signal to achieve accurate data recovery; the receiving end restores the received optical signal to an electrical signal through a photoelectric converter Photodetector, and uses the known odd half-wave symmetry characteristics to restore the clipped negative half signal, the formula is as follows:

[0129]

[0130] Among them, x recovered (t) is the restored signal, and T is the signal period.

[0131] Then, the real DAFT kernel function is inversely transformed to restore the frequency domain symbol. The calculation formula is as follows:

[0132]

[0133] The original input data is extracted by utilizing the conjugate symmetry between frequency domain symbols and the odd subcarrier allocation rule.

[0134] Step S4: Based on the design of the real DAFT kernel function, derive the new c 1The parameter range enables signal paths with different delays to be completely separated in the DAFT domain, achieving full diversity.

[0135] First, the left-shift logic and the offset distance of the non-zero elements in the channel matrix are derived, and the right-shift logic is combined to describe the mapping relationship of the signal path in the DAFT domain. The specific steps are as follows:

[0136] Step S41, the left shift and right shift brought by the real kernel function, the specific steps are as follows:

[0137] Step S411: In conventional AFDM, c 1 The range is derived as follows:

[0138] The elements of the channel matrix are:

[0139]

[0140] in,

[0141]

[0142] When v i = 0,

[0143]

[0144] Right shift logic is +c 1 The non-zero elements of the channel matrix are offset from the diagonal to the right by loc i positions, whose offset is:

[0145] loc i =(2Nc 1 l i ) N ;

[0146] Among them, l i is the delay of the ith path, H i [p,q] is the channel matrix element of the i-th path, p and q are frequency domain indices, is the frequency domain contribution function, v i is the Doppler shift of the i-th path, loc i is the offset of the i-th path, c 1 is a free parameter in the modulation design;

[0147] Step S412: Realization of the kernel function introduces left-shift logic. In ACO-AFDM, the kernel function is replaced by:

[0148]

[0149] Left shift logic by Introduced, its channel matrix elements become:

[0150]

[0151] in,

[0152]

[0153] When v i =0:

[0154]

[0155] The new offset is:

[0156] loc′ i =(-2Nc 1 l i ) N ;

[0157] Left shift logic is done by -c 1 The non-zero elements of the channel matrix are shifted from the diagonal to the left by loc′ i locations.

[0158] Step S42: derive new c 1 Parameter range, to achieve full set, to ensure that the right and left shift paths do not overlap, and the left shift path does not overlap with the right shift path, the new c 1 The steps for deriving the parameter range are as follows:

[0159] Step S421: right-shift the non-overlapping paths 1 Range deduction:

[0160] From the derivation of the original AFDM, the right shift is defined by the following formula:

[0161] loc i =(2Nc 1 l i ) N ;

[0162] In order to avoid overlap between right-shift paths, the following conditions must be met:

[0163]

[0164] Let l i+1 -l i = 1, to ensure that the paths do not overlap, the right-shifted c 1 The scope must meet:

[0165] c 1 >0;

[0166] Step S422: Left-shift the non-overlapping c paths1 The range is derived as follows:

[0167] The offset range for left shift is:

[0168] loc′ i =(-2Nc 1 l i ) N ;

[0169] In order to avoid overlap between left-shift paths, it is necessary to satisfy

[0170]

[0171] Similarly, assuming that the minimum delay difference between the paths is 1, we can obtain the left shift c to ensure that the paths do not overlap. 1 The range is:

[0172] c 1 >0;

[0173] Step S423: The leftward movement path and the rightward movement path do not overlap. 1 Range deduction: In order to avoid overlap between the left-shift path and the right-shift path, it is necessary to satisfy that the maximum value of all right-shift paths is less than N / 2 and the minimum value of all left-shift paths is also less than N / 2.

[0174] The maximum right offset is:

[0175] 2N 1 l max ;

[0176] Let it be less than N / 2:

[0177]

[0178] After finishing, we get:

[0179]

[0180] The left offset is:

[0181] -2Nc 1 l max ;

[0182] make get:

[0183]

[0184] After finishing, we get:

[0185]

[0186] Step S424: From steps S421, S422, and S423, it can be concluded that the paths of rightward and leftward shifts do not overlap and need to satisfy c 1 >0, the left-shift path and the right-shift path do not overlap and must satisfy the following conditions:

[0187]

[0188] Final c 1 The range is:

[0189]

[0190] The ACO-AFDM channel matrix is ​​completely separated in the DAFT domain to achieve full diversity.

[0191] like Figure 2 The figure shows the bit error rate (BER) performance comparison between the ACO-AFDM method and the traditional ACO-OFDM method under different signal-to-noise ratios (SNRs). The analysis shows that under multipath channel conditions, the BER of the ACO-AFDM method is significantly better than that of the ACO-OFDM method, with a lower bit error rate; as the SNR increases, the BER of ACO-AFDM decreases faster, especially in the high SNR area, its performance advantage is more significant; in addition, ACO-AFDM shows stronger anti-interference ability under multipath channel conditions, effectively suppresses the inter-symbol interference (ISI) caused by multipath, and significantly enhances its anti-multipath interference ability.

[0192] Therefore, the present invention provides a wireless optical transmission method based on asymmetric limited optical AFDM, which realizes efficient signal transmission with resistance to multipath interference in optical communication channels by combining the multipath component separation capability of AFDM and the non-negative signal generation mechanism of asymmetric clipping (ACO). While meeting the non-negative real signal transmission requirements of optical communication channels, the present invention can significantly improve the system performance in complex multipath scenarios such as multimode optical fiber communication and free space optical communication, solves the signal distortion problem caused by multipath effect in optical communication, and provides an effective solution for the stability and reliability of optical communication systems in complex scenarios.

[0193] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that they can still modify or replace the technical solution of the present invention with equivalents, and these modifications or equivalent replacements cannot cause the modified technical solution to deviate from the spirit and scope of the technical solution of the present invention.

Claims

1. A wireless optical transmission method based on asymmetric limited optical frequency division multiplexing, characterized in that: The specific steps are as follows: Step S1, preprocessing the input optical communication signal data at the transmitting end, including mapping of frequency domain data, design of modulation symbols and symmetry construction of data, to generate a frequency domain signal that meets specific conditions; Step S2, mapping the frequency domain signal generated in step S1 to the time domain based on the real discrete affine Fourier transform DAFT to generate an AFDM time domain signal; Step S3, using the odd half-wave symmetry characteristic to asymmetrically clip the AFDM time domain signal generated in step S2, setting the negative value part of the AFDM time domain signal to zero, and transmitting it to the receiving end through the optical communication channel; the receiving end reversely restores the signal, including recovering the clipped signal and demodulating the data; Step S4: Based on the design of the real DAFT kernel function, a new c1 parameter range is derived so that signal paths with different delays can be completely separated in the DAFT domain to achieve full diversity.

2. According to claim 1, a wireless optical transmission method based on asymmetric limited optical frequency division multiplexing is characterized in that: In step S1, the input optical communication signal data is mapped to the frequency domain subcarrier to form a frequency domain symbol. In order to ensure that the frequency domain signal is a real number, the frequency domain symbol is set to a conjugate symmetric form, and the formula is as follows: X[N-k]=X[k] * ; Where X[k] represents the symbol of the kth subcarrier, X[k] * is the conjugate complex number of X[k], and X[Nk] is the symbol of the Nk-th subcarrier.

3. According to claim 2, a wireless optical transmission method based on asymmetric limited optical frequency division multiplexing is characterized in that: To meet the requirements of asymmetric clipping, data is only loaded on odd-numbered subcarriers, and the even-numbered subcarrier symbols are set to zero. The formula is as follows: X[2k]=0; X[2k+1] = frequency domain symbol.

4. The wireless optical transmission method based on asymmetric limited optical frequency division multiplexing according to claim 1, characterized in that: In step S2, the frequency domain signal generated in step S1 is mapped to the time domain using the real discrete affine Fourier transform DAFT to generate an AFDM time domain signal. The specific steps are as follows: Step S21, the calculation formula of the kernel function of discrete affine Fourier transform DAFT is as follows: The calculation formula for its inverse transform is as follows: Where x[n] is the input signal, S[m] is the output signal, N is the number of subcarriers, M is the length of the DAFT transform, c1 and c2 are free parameters in the modulation design, m is the output signal index, n is the input signal index, and j is an imaginary number; Step S22: Adjust the kernel function of the discrete affine Fourier transform DAFT to remove the complex exponential kernel function. Item, will Replace with cos(2πc1n 2 ), the calculation formula of the adjusted discrete affine Fourier transform DAFT kernel function is as follows: The time domain signal generated by the adjusted kernel function is real and retains the multipath separation capability of discrete affine Fourier transform DAFT.

5. The wireless optical transmission method based on asymmetric limited optical frequency division multiplexing according to claim 4 is characterized in that: cos(2πc1n 2 ) is calculated as follows:

6. The wireless optical transmission method based on asymmetric limited optical frequency division multiplexing according to claim 4, characterized in that: In S3, the odd half-wave symmetry characteristic is used to perform asymmetric trimming ACO on the AFDM time domain signal generated in step S2. The specific steps are as follows: Step S31: Since the time domain signal x(t) has odd half-wave symmetry, its negative half does not carry valid information. The negative part is forced to be zero through asymmetric clipping. The formula is as follows: x clipped (t)=max(x(t),0); Among them, x clipped (t) is the clipped signal, t is the continuous time variable of the signal; Step S32, demodulate and restore the received optical signal to achieve accurate data recovery; the receiving end restores the received optical signal to an electrical signal through a photoelectric converter Photodetector, and uses the known odd half-wave symmetry characteristics to restore the clipped negative half signal, the formula is as follows: Among them, x recovered (t) is the recovered signal, T is the signal period; Perform an inverse transform on the real DAFT kernel function to restore the frequency domain sign. The calculation formula is as follows: The original input data is extracted by utilizing the conjugate symmetry between frequency domain symbols and the odd subcarrier allocation rule.

7. The wireless optical transmission method based on asymmetric limited optical frequency division multiplexing according to claim 6, characterized in that: In step S4, the left shift logic and the offset distance of the non-zero elements in the channel matrix are derived, and the mapping relationship of the signal path in the DAFT domain is described in combination with the right shift logic. The specific steps are as follows: Step S41, left shift and right shift brought by real number kernel function; Step S42: derive a new c1 parameter range to achieve full diversity and ensure that the right-shift and left-shift paths do not overlap with each other and that the left-shift path and the right-shift path do not overlap with each other.

8. The wireless optical transmission method based on asymmetric limited optical frequency division multiplexing according to claim 7, characterized in that: In step S41, the left shift and right shift brought by the real kernel function are specifically performed as follows: Step S411: In conventional AFDM, the range of c1 is derived as follows: The elements of the channel matrix are: in, When v i = 0, The right shift logic is introduced by +c1, and its offset is: place i =(2Nc1l i ) N ; Among them, l i is the delay of the ith path, H i [p,q] is the channel matrix element of the i-th path, p and q are frequency domain indices, is the frequency domain contribution function, v i is the Doppler shift of the i-th path, loc i is the offset of the i-th path; Step S412: Realization of the kernel function introduces left-shift logic. In ACO-AFDM, the kernel function is replaced by: Left shift logic by Introduced, its channel matrix elements become: in, When v i =0: The new offset is: place' i =(-2Nc1l i ) N ; The left shift logic is introduced by -c1, and the non-zero elements of the channel matrix are shifted from the diagonal to the left by loc′ i locations.

9. The wireless optical transmission method based on asymmetric limited optical frequency division multiplexing according to claim 7, characterized in that: In step S42, the steps for deriving a new c1 parameter range are as follows: Step S421, derivation of c1 ranges that do not overlap between right shift paths: From the derivation of the original AFDM, the right shift is defined by the following formula: place i =(2Nc1l i ) N ; In order to avoid overlap between right-shift paths, the following conditions must be met: Let l i+1 -l i =1, to ensure that the paths do not overlap, the range of c1 shifted right must satisfy: c1>0； Step S422: The c1 range that does not overlap between left shift paths is derived as follows: The offset range for left shift is: place' i =(-2Nc1l i ) N ; In order to avoid overlap between left-shift paths, Similarly, assuming that the minimum delay difference between paths is 1, the range of c1 shifted left to ensure that paths do not overlap is: c1>0； Step S423, deriving a c1 range where the left-shift path and the right-shift path do not overlap. To avoid overlap between the left-shift path and the right-shift path, it is necessary to satisfy that the maximum value of all right-shift paths is less than N / 2 and the minimum value of all left-shift paths is also less than N / 2; Step S424: The non-overlap between the rightward and leftward paths needs to satisfy c1>0, and the non-overlap between the leftward and rightward paths needs to satisfy: The final range of c1 is: The ACO-AFDM channel matrix is ​​completely separated in the DAFT domain to achieve full diversity.

10. The wireless optical transmission method based on asymmetric limited optical frequency division multiplexing according to claim 9, characterized in that: In step S423, the maximum value of the rightward shift is: 2Nc1l max ; Let it be less than N / 2: After finishing, we get: The left offset is: -2Nc1l max ; make get: After finishing, we get:

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