Optical OFDM implementation method and system based on high-dimensional index modulation

Abstract

The invention belongs to the field of optical communication, and particularly discloses an optical OFDM (Orthogonal Frequency Division Multiplexing) implementation method and system based on high-dimensional index modulation, serial bit streams are converted into parallel bit streams, and each sub-bit stream generates a frequency domain OFDM signal through an in-phase orthogonal index modulation technology and a high-dimensional signal mapper; a frequency domain OFDM signal is converted to a time domain through methods of direct current bias, asymmetric amplitude limiting or polarity activation index and the like, a non-negative real signal suitable for intensity modulation / direct detection optical communication is generated, and the non-negative real signal is sent to a channel to be sent after being subjected to digital-to-analog conversion, low-pass filtering and electro-optical conversion. Compared with a traditional two-dimensional signal, a high-dimensional signal constellation diagram has a larger minimum Euclidean distance, larger noise margin can be provided, and the energy efficiency and the spectrum efficiency of the system can be effectively improved through the adopted high-dimensional in-phase orthogonal index modulation technology. Compared with a traditional intensity modulation / direct detection optical OFDM system based on two-dimensional signals, the system provided by the invention has better system performance.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of optical communication, and more particularly, relates to an optical OFDM implementation method and system based on high-dimensional index modulation. BACKGROUND

[0002] Orthogonal frequency division multiplexing (OFDM) technology has become one of the key technologies of modern optical communication systems due to its high spectral efficiency, strong anti-dispersion ability and flexible resource allocation ability. Compared with traditional single-carrier modulation, OFDM technology decomposes a high-speed serial data stream into multiple parallel low-speed sub-data streams, thereby converting a frequency-selective fading wideband channel into a series of flat-fading narrowband sub-channels, greatly simplifying the complexity of channel equalization. In optical communication, especially in intensity modulation / direct detection (IM / DD) systems, OFDM can effectively overcome the inter-symbol interference caused by fiber dispersion, significantly improving the transmission capacity and distance of the system.

[0003] Index modulation OFDM (OFDM-IM) technology has advantages in spectral efficiency, energy efficiency and reliability compared with traditional OFDM. However, the traditional index modulation OFDM technology only considers two-dimensional signal constellation, such as multi-phase shift keying (MPSK) and multi-quadrature amplitude modulation (MQAM) constellation, whose minimum Euclidean distance is limited, and the noise tolerance is limited, which limits the performance of the system.

[0004] Therefore, how to effectively improve the reliability of the system is a problem to be solved at present. SUMMARY

[0005] In view of the defects of the prior art, the purpose of the present application is to provide an optical OFDM implementation method and system based on high-dimensional index modulation, which can effectively improve the reliability of the system.

[0006] To achieve the above-mentioned purpose, in a first aspect, the present application provides an optical OFDM implementation method based on high-dimensional index modulation, comprising the following steps: S10, transforming a serial bit stream to be sent into parallel sub-bit streams, and generating a frequency domain OFDM signal through high-dimensional index modulation, wherein the high-dimensional index modulation uses a high-dimensional signal constellation with a dimension D≥3; S20, converting the frequency domain OFDM signal into a non-negative real signal; S30, sending the non-negative real signal into an optical channel for transmission after digital-to-analog conversion, low-pass filtering and electro-optical conversion; S40, at the receiving end, obtaining a received signal by photoelectric conversion, low-pass filtering and analog-to-digital conversion on the received optical signal, and converting the received signal into a bipolar OFDM signal; S50, perform high-dimensional index demodulation on the bipolar OFDM signal to recover the original transmitted bit stream.

[0007] The optical OFDM implementation method based on high-dimensional index modulation provided in this application has the following advantages: By employing a high-dimensional signal constellation diagram with dimension D≥3 for index modulation, compared to a traditional two-dimensional signal constellation diagram, the high-dimensional signal has a larger minimum Euclidean distance at the same average power and modulation order, providing greater noise margin and thus directly improving the system's noise immunity and reliability. High-dimensional index modulation maps the coordinate components of high-dimensional symbols to the in-phase and quadrature components of subcarriers, and combines the conversion of non-negative real signals with optical channel transmission, ensuring the compatibility and integrity of the signal in intensity modulation / direct detection systems. The entire method, through the synergy of the advantages of high-dimensional signals and index modulation technology, maintains signal robustness during modulation, transmission, and demodulation, effectively improving system reliability.

[0008] As a further preferred embodiment, in step S10, the high-dimensional index modulation specifically includes: For each sub-bit stream branch g, 1 ≤ g ≤ G, divide p bits into p Re Bit, p Im Bit and p s Bit, i.e., p = p Re + p Im + p s A high-dimensional signal constellation diagram is used to map into a subframe of the OFDM signal. Each OFDM subframe contains n subcarriers and is mapped to a high-dimensional symbol. A D-dimensional symbol can be represented as a column vector S. D = [W1W2… W d …W D ] T T represents the transpose operation, W d Let d represent the d-th dimension coordinate component of a symbol in a high-dimensional signal constellation diagram of size M, where 1 ≤ d ≤ D; p Re Bits select k active indices from the in-phase components of n subcarriers to map the first k coordinate components of the high-dimensional symbol. The position of the active index is denoted as I. Re,g = [I Re1 I Re2 … I Rek The mapped subcarrier in-phase component is represented as S. Re,g = [W1(I Re1 W2(I) Re2 … W k (I Rek )] T The remaining Dk coordinate components are mapped to the orthogonal components of the subcarriers, i.e., S. Im,g= [W k+1 (I Im1 W k+2 (I Im2 … W D (I Im(D-k) )] T The activation position of the orthogonal component is denoted as I. Im,g The formula for calculating p bits in each OFDM subframe is: Where C(n,k) represents the binomial coefficient, that is, the number of combinations of taking k from n; This indicates a platform function, i.e., rounding down; Combining the in-phase and quadrature components, the mapped OFDM signal subframe is represented as X. g =S Re,g + jS Im,g , j 2 = -1; G OFDM signal subframes are combined into one OFDM signal, that is , where N=nG represents the number of subcarriers of the OFDM signal. As a further preferred embodiment, the conversion of the frequency domain OFDM signal into a non-negative real signal employs a DC bias method, wherein the DC bias method specifically comprises: The frequency domain OFDM signal is conjugate symmetrically extended into 2N subcarriers, and the extended subcarriers satisfy complex conjugate relations. The extended frequency domain OFDM signal is subjected to inverse fast Fourier transform to obtain the time domain OFDM signal; A positive DC bias signal is added to the time-domain OFDM signal to obtain a non-negative time-domain OFDM signal.

[0009] As a further preferred embodiment, the conversion of the frequency domain OFDM signal into a non-negative real signal employs an asymmetric limiting method, wherein the asymmetric limiting method specifically comprises: The frequency domain OFDM signal is extended by odd-number subcarriers to 2N subcarriers, where only the odd-number subcarriers carry complex signals. The extended signal is then subjected to conjugate symmetric extension to form an OFDM signal containing 4N subcarriers; The extended frequency domain OFDM signal is subjected to inverse fast Fourier transform to obtain the time domain OFDM signal; By clipping the time-domain OFDM signal and setting the negative values ​​to zero, a non-negative time-domain OFDM signal is obtained.

[0010] As a further preferred embodiment, the conversion of the frequency domain OFDM signal into a non-negative real signal employs a polarity activation index transformation method, which specifically comprises: The frequency domain OFDM signal is conjugate symmetrically extended into 2N subcarriers, and the extended subcarriers satisfy complex conjugate relations. The extended frequency domain OFDM signal is subjected to inverse fast Fourier transform to obtain the time domain OFDM signal; A polarity activation index transformation is performed on the time-domain OFDM signal, expanding each sampling point in the time-domain OFDM signal into two sampling points according to the signal polarity, thus obtaining a non-negative time-domain OFDM signal.

[0011] As a further preferred embodiment, when the non-negative real signal is generated using a DC bias method, the conversion of the received signal into a bipolar OFDM signal includes: The received signal is de-DC biased to obtain a bipolar time-domain signal; A fast Fourier transform is performed on the bipolar time-domain signal to obtain a frequency-domain OFDM signal; The original frequency domain OFDM signal is recovered based on the complex conjugate relationship of the transmitted signal.

[0012] As a further preferred embodiment, when the non-negative real signal is generated using an asymmetric limiting method, the conversion of the received signal into a bipolar OFDM signal includes: The received signal is subjected to a fast Fourier transform to obtain a frequency domain OFDM signal; Take the first N odd-numbered subcarriers from the frequency domain OFDM signal to recover the original frequency domain OFDM signal.

[0013] As a further preferred embodiment, when the non-negative real signal is generated using the polarity activation index transformation method, the conversion of the received signal into a bipolar OFDM signal includes: The received signal is subtracted from two adjacent time-domain sampled signals to obtain a bipolar time-domain OFDM signal. A fast Fourier transform is performed on the bipolar time-domain OFDM signal to obtain a frequency-domain OFDM signal; The original frequency domain OFDM signal is recovered based on the complex conjugate relationship of the transmitted signal.

[0014] As a further preferred embodiment, in step S50, the high-dimensional index demodulation specifically includes: Maximum likelihood detection is performed on each frequency domain OFDM signal subframe to obtain estimates of the in-phase index, orthogonal index, and high-dimensional symbol at the transmitter. Based on the estimation results, the original transmitted bit stream is obtained through bit recovery.

[0015] Secondly, this application provides an optical OFDM system based on high-dimensional index modulation for implementing the method described in any one of the above, including a transmitter and a receiver; The sending end includes: The bit blocker module is used to transform the input serial bit stream into a parallel sub-bit stream; A high-dimensional indexed OFDM modulator module is used to generate frequency-domain OFDM signals through high-dimensional indexed modulation, wherein the high-dimensional indexed modulation uses a high-dimensional signal constellation diagram with dimension D≥3. A unipolar signal generator module is used to convert the frequency domain OFDM signal into a non-negative real signal; A digital-to-analog converter module is used to convert the non-negative real signal into an analog signal; A low-pass filter module is used to perform low-pass filtering on the analog signal; The electro-optic conversion module is used to convert the filtered analog signal into an optical signal and send it into the optical channel; The receiving end includes: A photoelectric conversion module is used to convert received optical signals into electrical signals; A low-pass filter module is used to perform low-pass filtering on the electrical signal; The analog-to-digital converter module is used to convert the filtered electrical signal into a digital signal; A bipolar signal generator module is used to convert the digital signal into a bipolar OFDM signal; A high-dimensional indexed OFDM demodulator module is used to perform high-dimensional indexed demodulation on the bipolar OFDM signal; The bit recovery module is used to recover the original transmitted bit stream.

[0016] It is understandable that the beneficial effects of the second aspect mentioned above can be found in the relevant descriptions in the first aspect mentioned above, and will not be repeated here. Attached Figure Description

[0017] Figure 1 This is a flowchart of the optical OFDM implementation method based on high-dimensional index modulation provided in this application; Figure 2 This is a structural block diagram of the transmitter in an optical OFDM system based on high-dimensional index modulation provided in this application embodiment; Figure 3 This is a structural block diagram of the high-dimensional indexed OFDM modulator provided in the embodiments of this application; Figure 4 This is a block diagram of the receiver in an optical OFDM system based on high-dimensional index modulation provided in this application embodiment; Figure 5 This is a flowchart illustrating the implementation of Method 1 provided in this application embodiment; Figure 6 This is a flowchart illustrating the implementation of Method 2 provided in this application embodiment; Figure 7This is a flowchart illustrating the implementation of Method 3 provided in this application embodiment; Figure 8 This is a comparison chart of simulation results of the method provided in the embodiments of this application. Detailed Implementation

[0018] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application.

[0019] like Figure 1 As shown, this application provides a method for implementing optical OFDM based on high-dimensional index modulation, including steps S10 to S50, which are detailed below: Step S10: The serial bit stream to be transmitted is transformed into a parallel sub-bit stream, and a frequency domain OFDM signal is generated by high-dimensional index modulation, wherein the high-dimensional index modulation uses a high-dimensional signal constellation diagram with dimension D≥3.

[0020] This step uses a high-dimensional signal constellation diagram with dimension D≥3 for high-dimensional index modulation. Since the high-dimensional signal constellation diagram has a larger minimum Euclidean distance than the traditional two-dimensional signal constellation diagram under the same conditions of average power and modulation order, it can provide a larger noise margin and thus directly and effectively improve the reliability of the traditional optical OFDM system.

[0021] Step S20: Convert the frequency domain OFDM signal into a non-negative real signal.

[0022] This step converts the frequency-domain OFDM signal into a non-negative real signal, making it suitable for intensity-modulated / direct-detection optical communication systems and generating a non-negative real signal suitable for optical channel transmission. Specifically, this can be achieved using methods such as DC bias, asymmetric limiting, or polarity activation indexing.

[0023] Step S30: The non-negative real signal is sent into the optical channel for transmission after digital-to-analog conversion, low-pass filtering and electro-optic conversion.

[0024] This step uses digital-to-analog conversion and low-pass filtering to obtain a smooth analog OFDM signal waveform. Then, electro-optical conversion converts the electrical signal into an optical signal, which is then intensity modulated to achieve reliable transmission in wired or wireless optical channels.

[0025] Step S40: At the receiving end, the received optical signal is converted into a received signal through photoelectric conversion, low-pass filtering and analog-to-digital conversion, and the received signal is converted into a bipolar OFDM signal.

[0026] This step recovers the electrical domain signal through photoelectric conversion and signal processing, and restores the unipolar received signal to a bipolar OFDM signal through conversion operations, providing a basis for subsequent demodulation and ensuring correct signal processing at the receiving end.

[0027] Step S50: Perform high-dimensional index demodulation on the bipolar OFDM signal to recover the original transmitted bit stream.

[0028] This step uses high-dimensional index demodulation, such as maximum likelihood detection, to estimate the in-phase index, orthogonal index, and high-dimensional symbols at the sending end, thereby accurately recovering the original bit stream and completing the entire communication process.

[0029] The optical OFDM implementation method based on high-dimensional index modulation provided in this application has the following advantages: By employing a high-dimensional signal constellation diagram with dimension D≥3 for index modulation, compared to a traditional two-dimensional signal constellation diagram, the high-dimensional signal has a larger minimum Euclidean distance at the same average power and modulation order, providing greater noise margin and thus directly improving the system's noise immunity and reliability. High-dimensional index modulation maps the coordinate components of high-dimensional symbols to the in-phase and quadrature components of subcarriers, and combines the conversion of non-negative real signals with optical channel transmission, ensuring the compatibility and integrity of the signal in intensity modulation / direct detection systems. The entire method, through the synergy of the advantages of high-dimensional signals and index modulation technology, maintains signal robustness during modulation, transmission, and demodulation, effectively improving system reliability.

[0030] In one embodiment, the technical solution to achieve the above objective can be as follows: In order to solve the application problem of high-dimensional index modulation OFDM technology in the field of optical communication, this embodiment provides an optical OFDM implementation method and system based on high-dimensional index modulation. The main purpose is to improve the energy efficiency, spectral efficiency and reliability of traditional two-dimensional optical OFDM systems based on intensity modulation / direct detection.

[0031] like Figure 2 As shown, the transmitting end of the optical OFDM system based on high-dimensional index modulation in this embodiment includes: a bit blocker module, a high-dimensional index OFDM modulator module, a unipolar signal generator module, a digital-to-analog converter and low-pass filter module, and an electro-optic converter module.

[0032] Among them, such as Figure 3 As shown, the high-dimensional indexed OFDM modulator module includes: an in-phase index selector module, an orthogonal index selector module, a high-dimensional signal mapper module, and a frequency-domain OFDM signal generator module. In the unipolar signal generator module, three methods can be used to generate non-negative real signals in optical communication: DC bias method, asymmetric limiting method, and polarity activation index transformation method.

[0033] like Figure 4As shown in the figure, the receiving end of the optical OFDM system based on high-dimensional index modulation in this embodiment includes: an optical-electric converter module, a low-pass filtering and analog-to-digital conversion module, a bipolar signal generator module, a high-dimensional index OFDM demodulator module, and a bit recovery module.

[0034] The optical OFDM system based on high-dimensional index modulation in this embodiment includes the following steps: S1: The input high-speed serial bit stream is transformed into a low-speed parallel sub-bit stream by the bit grouper module. The number of bits p contained in each sub-bit stream is p = c / G, where c represents the number of bits transmitted by an OFDM signal, and G represents the number of sub-bit streams or the number of groups of c bits. The G groups of sub-bit streams are input into the high-dimensional index OFDM modulator module.

[0035] S2: For any group of sub-bit stream branches g input, 1 ≤ g ≤ G, the p bits are divided into p Re bits, p Im bits and p s bits, that is, p = p Re + p Im + p s . Through high-dimensional signal constellation mapping, it enters a sub-frame of the OFDM signal. Each sub-frame of the OFDM signal contains n sub-carriers, mapping a high-dimensional symbol. A D-dimensional symbol can be represented as a column vector S D = [W1W2… W d … W D T , T represents the transpose operation, and W d represents the d-th dimensional coordinate component of the symbol in the high-dimensional signal constellation with size M, 1 ≤ d ≤ D, D ≥ 3.

[0036] The p Re bits are input into the in-phase index selector module, and k activations are selected from the in-phase components of the n sub-carriers to map the first k (1 ≤ k < D) coordinate components of the high-dimensional symbol. The activated index positions are represented as I Re,g = [I Re1 I Re2 …I Rek . The in-phase component of the mapped sub-carrier is represented as S Re,g = [W1(I Re1 ) W2(I Re2 ) … W k (I Rek )] T , and the remaining (D – k) coordinate components are mapped to the orthogonal components of the sub-carrier, that is, S Im,g = [W k+1 (I Im1 ) W k+2 ​(I Im2 … W D (I Im(D-k) )] T The activation position of the orthogonal component is denoted as I. Im,g = [I Im1 I Im2 … I Im(D-k) The p-bit energy in each OFDM subframe can be calculated as:

[0037] Where C(n, k) represents the binomial coefficient, that is, the number of combinations of choosing k from n. This represents the platform function, i.e., rounding down. Combining the in-phase and quadrature components, the mapped OFDM signal subframe is represented as X. g =S Re,g + jS Im,g , j 2 = -1. In the frequency domain OFDM signal generator module, G OFDM signal subframes constitute one OFDM signal, i.e.

[0038] Where N = nG represents the number of subcarriers of the OFDM signal.

[0039] For example, when D = 4, n = 4, k = 2, M = 16, p Re = p Im = 2, p s = 4. The relationship between the index positions of the in-phase and quadrature components of the subcarrier in the frequency domain OFDM signal subframe and the mapped high-dimensional signal is shown in Table 1: Table 1

[0040] S3: In order to generate a non-negative real signal suitable for intensity modulation / direct detection optical communication, three methods can be used in the unipolar signal generator module.

[0041] S31: As Figure 5 Method 1 shown is the DC bias method, which consists of a conjugate symmetry module, an inverse fast Fourier transform (IFFT) module, and a DC bias module.

[0042] In the conjugate symmetric module, the input frequency-domain OFDM signal containing N subcarriers is expanded into 2N subcarriers. The expanded subcarriers satisfy the following complex conjugate relationship:

[0043] Where Re(·) and Im(·) represent the operations of retrieving the real and imaginary parts of complex numbers, respectively, and the superscript... This indicates the complex conjugate operation.

[0044] The OFDM signal in the frequency domain is converted to the time domain using the IFFT module.

[0045] ifft(·) represents the inverse fast Fourier transform operation. Since the frequency domain OFDM signal satisfies the complex conjugate relationship, the resulting time domain signal is a real vector.

[0046] Add an appropriate positive DC bias signal b to the DC bias module. DC Given a time-domain OFDM signal, obtain x' 方法一 =x 方法一 + b DC This yields a non-negative time-domain OFDM signal with a length of 2N.

[0047] For example, when N = 4, the frequency domain OFDM signal and the time domain OFDM signal after conjugate symmetry can be represented as follows:

[0048] S32: As Figure 6 Method 2 shown is the asymmetric clipping method, which consists of an odd-numbered subcarrier conjugate symmetric module, an IFFT module, and a clipping module.

[0049] In the odd-subcarrier conjugate symmetric module, the input frequency-domain OFDM signal containing N subcarriers will be expanded to 2N subcarriers, where only the odd-numbered subcarriers carry complex signals, i.e.

[0050] Then, using the conjugate symmetry operation in method S31, X is... 方法二 Expanded to an OFDM signal containing 4N subcarriers, i.e.

[0051] The OFDM signal in the frequency domain is converted to the time domain using the IFFT module.

[0052] To obtain a non-negative time-domain OFDM signal, the clipping module operates as follows:

[0053] By clipping, a non-negative time-domain OFDM signal x' with a length of 4N can be obtained. 方法二 .

[0054] S33: As Figure 7 Method 3, shown, is the polar activation index transformation method, which consists of a conjugate symmetric module, an IFFT module, and a polar activation index transformer module.

[0055] Similar to Method S31, in the conjugate symmetric module, the input frequency-domain OFDM signal containing N subcarriers is expanded into 2N subcarriers. The expanded subcarriers satisfy the following complex conjugate relationship:

[0056] The OFDM signal in the frequency domain is converted to the time domain using the IFFT module.

[0057] Since the frequency domain OFDM signal satisfies the complex conjugate relationship, the resulting time domain signal is a real vector.

[0058] In the polarity-activated index converter module, the time-domain OFDM signal x is converted... 方法三 Each sampling point in the index is expanded into two based on the signal polarity, and the index transformation rule is as follows:

[0059] Where |·| represents taking the absolute value. Through polarity activation index transformation, a non-negative time-domain OFDM signal x' of length 4N can be obtained. 方法三 .

[0060] For example, when N = 2, assume the generated time-domain OFDM signal x 方法三 = [positive-negative-positive-positive], then the time-domain signal after polarity extension is represented as x' 方法三 = [positive 00|negative|positive 0 positive 0].

[0061] S4: The unipolar, non-negative time-domain OFDM signal x (=x') generated in step S3 is... 方法一 or x' 方法二 or x' 方法三 The input digital-to-analog converter and low-pass filter module produce a smooth analog OFDM signal waveform x. e (t).

[0062] S5: Convert the OFDM signal waveform generated by S4 to x e (t) is converted into an optical domain OFDM signal x via an electro-optical converter module. o (t), intensity modulation is performed, and the output signal is sent to a wired or wireless optical channel for transmission. It can be assumed that the signal is only affected by additive white Gaussian noise.

[0063] S6: As Figure 4As shown, the photoelectric converter module performs direct detection and converts the received analog optical domain OFDM signal y o (t) is converted into an electronic domain OFDM signal y e (t).

[0064] S7: Convert the signal y output in step S6... e (t) Input low-pass filter and analog-to-digital converter modules to obtain discrete unipolar sampled signal y = x + n AWGN n AWGN This represents additive white Gaussian noise.

[0065] S8: In order to recover the transmitted frequency domain OFDM signal, the received unipolar OFDM signal needs to be converted into a bipolar OFDM signal. The conversion method for the unipolar OFDM signal generated in step S3 is as follows: S81: Based on the unipolar signal method generated by DC bias in step S31, first perform the DC bias removal operation to obtain the bipolar time-domain signal y'=y -b DC Then, a Fast Fourier Transform is performed to convert the generated bipolar time-domain OFDM signal y' to the frequency domain, resulting in a frequency-domain OFDM signal Y' of length 2N. 方法一 =fft(y') = [Y0Y1… Y 2N-1 ], fft(·) represents the Fast Fourier Transform operation. Based on the complex conjugate relationship of the transmitted signal, the original transmitted signal X' is recovered. 方法一 ,Right now

[0066] S82: Based on the unipolar signal generated by asymmetric limiting in step S32, a fast Fourier transform is directly applied to convert the time-domain OFDM signal y into a frequency-domain OFDM signal, resulting in a frequency-domain OFDM signal Y' of length 4N. 方法二 =fft(y) =[Y0Y1… Y 4N-1 ], then take Y' 方法二 The original transmitted signal X' is recovered from the first N odd-numbered subcarriers. 方法二 ,Right now

[0067] S83: Based on the unipolar signal method generated by the polarity activation index transformation in step S33, first perform the subtraction operation between two adjacent time-domain sampled signals, that is, the time-domain OFDM signal y = [y0y1… y] of length 4N. 4N-1 Subtracting the odd-numbered signals from adjacent even-numbered signals in the equation yields a bipolar time-domain OFDM signal of length 2N, i.e.

[0068] Perform a Fast Fourier Transform (FFT) to generate a bipolar time-domain OFDM signal y' 方法三 Converting to the frequency domain yields a frequency domain OFDM signal Y' of length 2N. 方法三 =fft(y') = [Y0Y1… Y 2N-1 Based on the complex conjugate relationship of the transmitted signal, the original transmitted signal X' is recovered. 方法三 ,Right now

[0069] S9: Recover the frequency domain OFDM signal X' of length N from step S8. 方法一 、X' 方法二 、or X' 方法三 The input high-dimensional indexed OFDM demodulator, the input frequency domain OFDM signal is represented as X' = [X'1] T X'2 T …X' g T …X' G T Then, for each frequency domain OFDM signal subframe X' g Maximum likelihood detection is performed to obtain estimates of the in-phase index, orthogonal index, and high-dimensional symbol at the sending end, i.e., I'. Re,g , I' Im,g and S' D .

[0070] S10: Based on the estimation result of step S9, the original transmitted bit stream is obtained through the bit recovery module.

[0071] The beneficial effects of the technical solution provided in this embodiment: (1) Applying high-dimensional signal constellation diagrams to intensity modulation / direct detection optical OFDM systems can provide greater noise margins because, compared to traditional two-dimensional signal constellation diagrams, high-dimensional signal constellation diagrams have a larger minimum Euclidean distance under the same conditions of average power and modulation order. Therefore, they can directly and effectively improve the reliability of traditional optical OFDM systems.

[0072] (2) For high-dimensional signals, in-phase quadrature index modulation technology is used. Only one high-dimensional signal is transmitted in each optical OFDM signal subframe, which can effectively improve the energy efficiency and spectral efficiency of the optical communication system because in-phase quadrature index modulation carries more index bits and does not occupy transmission power.

[0073] (3) For intensity-modulated / direct-detection optical OFDM systems, three methods are used to convert bipolar high-dimensional index-modulated optical OFDM signals into unipolar signals: ① Method 1, the DC bias method, has high spectral efficiency, is easy to implement, and has strong compatibility; ② Method 2, the asymmetric limiting method, has high energy efficiency and high system reliability; ③ Method 3, the polarity activation index transformation method, is easy to implement and has high system reliability. The high-dimensional index-modulated optical OFDM system provided in this embodiment can achieve a trade-off between energy efficiency, spectral efficiency, and system reliability according to different implementation methods and design requirements.

[0074] The following is a specific implementation example of this application: This specific embodiment provides an optical OFDM system based on high-dimensional index modulation. Assuming a four-dimensional hexadecimal signal constellation diagram is used, i.e., M = 16, D = 4, the coordinates of all symbols are shown in Table 2. Other parameters are as follows: N = 128, G = 32, n = 4. The mapping relationship of the high-dimensional symbols is shown in Table 1. The transmitter sends 10... 6 The frame OFDM signal is used to statistically analyze the system's bit error rate (BER). Assuming the provided high-dimensional indexed modulation-based optical OFDM system is tested in a visible light channel, the simulation environment parameters are shown in Table 3. Furthermore, channel estimation and received signal channel equalization are assumed to be under ideal conditions.

[0075] Table 2

[0076] Table 3

[0077] Simulation results are as follows Figure 8 As shown, the high-dimensional indexed modulation optical OFDM system provided in this embodiment outperforms the traditional optical OFDM system using a two-dimensional signal constellation diagram in visible light channels.

[0078] The key technical point of this embodiment is: (1) In order to apply high-dimensional signal constellation diagrams to intensity modulation / direct detection optical OFDM systems, this embodiment provides an optical OFDM implementation method and system based on high-dimensional index modulation. Since high-dimensional signal constellation diagrams have a larger minimum Euclidean distance than traditional two-dimensional signal constellation diagrams under the same average power and modulation order, they can provide greater noise margin and thus directly and effectively improve the reliability of traditional optical OFDM systems. Because a high-dimensional symbol has D coordinate components, this embodiment uses in-phase orthogonal index modulation technology for high-dimensional signals, and only one high-dimensional signal is transmitted in each optical OFDM signal subframe, which can effectively improve the energy efficiency and spectral efficiency of the optical communication system, because in-phase orthogonal index modulation carries more index bits and does not occupy transmission power.

[0079] (2) For intensity modulation / direct detection optical OFDM systems, three methods are used to convert bipolar high-dimensional index modulated optical OFDM signals into unipolar signals. These methods are: ① Method 1, the DC bias method, has high spectral efficiency, directly adding an appropriate DC component to the generated bipolar OFDM signal, and is easy to implement and highly compatible; ② Method 2, the asymmetric limiting method, reduces the power of the transmitted signal through clipping, resulting in high system energy efficiency and high system reliability; ③ Method 3, the polarity activation index transformation method, directly generates unipolar optical OFDM signals through the polarity index of the time-domain signal, and is easy to implement and highly reliable. The high-dimensional index modulation-based optical OFDM system provided in this embodiment can achieve a trade-off between energy efficiency, spectral efficiency, and system reliability according to different implementation methods and design requirements.

[0080] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A method for implementing optical OFDM based on high-dimensional index modulation, characterized in that, Includes the following steps: S10, the serial bit stream to be transmitted is transformed into a parallel sub-bit stream, and a frequency domain OFDM signal is generated by high-dimensional index modulation, wherein the high-dimensional index modulation uses a high-dimensional signal constellation diagram with dimension D≥3. S20, convert the frequency domain OFDM signal into a non-negative real signal; S30, the non-negative real signal is sent into the optical channel for transmission after digital-to-analog conversion, low-pass filtering and electro-optical conversion; S40, at the receiving end, the received optical signal is converted into a received signal through photoelectric conversion, low-pass filtering and analog-to-digital conversion, and then the received signal is converted into a bipolar OFDM signal; S50, perform high-dimensional index demodulation on the bipolar OFDM signal to recover the original transmitted bit stream.

2. The optical OFDM implementation method based on high-dimensional index modulation as described in claim 1, characterized in that, In step S10, the high-dimensional index modulation specifically includes: For each sub-bit stream branch g, 1 ≤ g ≤ G, divide p bits into p Re Bit, p Im Bit and p s bits, i.e., p = p Re + p Im + p s A high-dimensional signal constellation diagram is used to map into a subframe of the OFDM signal. Each OFDM subframe contains n subcarriers and is mapped to a high-dimensional symbol. A D-dimensional symbol can be represented as a column vector S. D = [W1 W2… W d … W D ] T T represents the transpose operation, W d Let d represent the d-th coordinate component of a symbol in a high-dimensional signal constellation diagram of size M, where 1 ≤ d ≤ D; p Re Bits select k active indices from the in-phase components of n subcarriers to map the first k coordinate components of the high-dimensional symbol. The position of the active index is denoted as I. Re,g = [I Re1 I Re2 … I Rek The mapped subcarrier in-phase component is represented as S. Re,g = [W1(I Re1 W2(I) Re2 … W k (I Rek )] T The remaining Dk coordinate components are mapped to the orthogonal components of the subcarriers, i.e., S. Im,g = [W k+1 (I Im1 W k+2 (I Im2 … W D (I Im(D-k) )] T The activation position of the orthogonal component is denoted as I. Im,g The formula for calculating p bits in each OFDM subframe is: Where C(n,k) represents the binomial coefficient, that is, the number of combinations of taking k from n; This indicates a platform function, i.e., rounding down; Combining the in-phase and quadrature components, the mapped OFDM signal subframe is represented as X. g = S Re,g + jS Im,g , j 2 = -1; G OFDM signal subframes are combined into one OFDM signal, that is , where N=nG represents the number of subcarriers of the OFDM signal.

3. The optical OFDM implementation method based on high-dimensional index modulation as described in claim 1, characterized in that, The conversion of the frequency domain OFDM signal into a non-negative real signal employs a DC bias method, which specifically involves: The frequency domain OFDM signal is conjugate symmetrically extended into 2N subcarriers, and the extended subcarriers satisfy complex conjugate relations. The extended frequency domain OFDM signal is subjected to inverse fast Fourier transform to obtain the time domain OFDM signal; A positive DC bias signal is added to the time-domain OFDM signal to obtain a non-negative time-domain OFDM signal.

4. The optical OFDM implementation method based on high-dimensional index modulation as described in claim 1, characterized in that, The conversion of the frequency domain OFDM signal into a non-negative real signal employs an asymmetric limiting method, which specifically involves: The frequency domain OFDM signal is extended by odd-number subcarriers to 2N subcarriers, where only the odd-number subcarriers carry complex signals. The extended signal is then subjected to conjugate symmetric extension to form an OFDM signal containing 4N subcarriers; The extended frequency domain OFDM signal is subjected to inverse fast Fourier transform to obtain the time domain OFDM signal; By clipping the time-domain OFDM signal and setting the negative values ​​to zero, a non-negative time-domain OFDM signal is obtained.

5. The optical OFDM implementation method based on high-dimensional index modulation as described in claim 1, characterized in that, The conversion of the frequency domain OFDM signal into a non-negative real signal employs the polarity activation index transformation method, which specifically involves: The frequency domain OFDM signal is conjugate symmetrically extended into 2N subcarriers, and the extended subcarriers satisfy complex conjugate relations. The extended frequency domain OFDM signal is subjected to inverse fast Fourier transform to obtain the time domain OFDM signal; A polarity activation index transformation is performed on the time-domain OFDM signal, expanding each sampling point in the time-domain OFDM signal into two sampling points according to the signal polarity, thus obtaining a non-negative time-domain OFDM signal.

6. The optical OFDM implementation method based on high-dimensional index modulation as described in claim 1, characterized in that, When the non-negative real signal is generated using the DC bias method, the conversion of the received signal into a bipolar OFDM signal includes: The received signal is de-DC biased to obtain a bipolar time-domain signal; A fast Fourier transform is performed on the bipolar time-domain signal to obtain a frequency-domain OFDM signal; The original frequency domain OFDM signal is recovered based on the complex conjugate relationship of the transmitted signal.

7. The optical OFDM implementation method based on high-dimensional index modulation as described in claim 1, characterized in that, When the non-negative real signal is generated using an asymmetric limiting method, the conversion of the received signal into a bipolar OFDM signal includes: The received signal is subjected to a fast Fourier transform to obtain a frequency domain OFDM signal; Take the first N odd-numbered subcarriers from the frequency domain OFDM signal to recover the original frequency domain OFDM signal.

8. The optical OFDM implementation method based on high-dimensional index modulation as described in claim 1, characterized in that, When the non-negative real signal is generated using the polarity activation index transformation method, the conversion of the received signal into a bipolar OFDM signal includes: The received signal is subtracted from two adjacent time-domain sampled signals to obtain a bipolar time-domain OFDM signal. A fast Fourier transform is performed on the bipolar time-domain OFDM signal to obtain a frequency-domain OFDM signal; The original frequency domain OFDM signal is recovered based on the complex conjugate relationship of the transmitted signal.

9. The optical OFDM implementation method based on high-dimensional index modulation as described in claim 1, characterized in that, In step S50, the high-dimensional index demodulation specifically involves: Maximum likelihood detection is performed on each frequency domain OFDM signal subframe to obtain estimates of the in-phase index, orthogonal index, and high-dimensional symbol at the transmitter. Based on the estimation results, the original transmitted bit stream is obtained through bit recovery.

10. An optical OFDM system based on high-dimensional index modulation for implementing the method of any one of claims 1 to 9, characterized in that, Includes the sending end and the receiving end; The sending end includes: The bit blocker module is used to transform the input serial bit stream into a parallel sub-bit stream; A high-dimensional indexed OFDM modulator module is used to generate frequency-domain OFDM signals through high-dimensional indexed modulation, wherein the high-dimensional indexed modulation uses a high-dimensional signal constellation diagram with dimension D≥3. A unipolar signal generator module is used to convert the frequency domain OFDM signal into a non-negative real signal; A digital-to-analog converter module is used to convert the non-negative real signal into an analog signal; A low-pass filter module is used to perform low-pass filtering on the analog signal; The electro-optic conversion module is used to convert the filtered analog signal into an optical signal and send it into the optical channel; The receiving end includes: A photoelectric conversion module is used to convert received optical signals into electrical signals; A low-pass filter module is used to perform low-pass filtering on the electrical signal; The analog-to-digital converter module is used to convert the filtered electrical signal into a digital signal; A bipolar signal generator module is used to convert the digital signal into a bipolar OFDM signal; A high-dimensional indexed OFDM demodulator module is used to perform high-dimensional indexed demodulation on the bipolar OFDM signal; The bit recovery module is used to recover the original transmitted bit stream.

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