Constellation shaping OFDM (Orthogonal Frequency Division Multiplexing) device and method for coherent optical transmission

Abstract

The invention relates to the technical field of optical communication, and particularly discloses a constellation shaping OFDM (Orthogonal Frequency Division Multiplexing) device and method for coherent optical transmission. The device comprises a coherent transmitter, an optical fiber transmitter and a CS-OFMD receiver, the coherent transmitter comprises two driving lasers, an MZM (Mach Zehnder Mach-Zehnder Modulator) and two independent filters; the CS-OFMD receiver includes a balanced photodetector. The method comprises the following steps: S1, processing an input signal; s2, performing coherent transmission; s3, clipping and quantizing the real part and the imaginary part of the QAM signal; s4, performing external modulation and generating and transmitting a modulation signal; s5, transmitting the modulated optical signal, amplifying the optical signal, and filtering to remove out-of-band noise; s6, converting the CS-OFDM signal back to an electric signal in a receiving end, and performing coherent detection; s7, performing clock phase recovery; and S8, the received signal is subjected to digital processing, and digital CS-OFDM demodulation is carried out. According to the method, the performance can be improved under the condition that the calculation efficiency and the energy efficiency are not sacrificed, and the cost and the implementation complexity are remarkably reduced.

Description

Technical Field

[0001] The present invention relates to the field of optical communication technology, and in particular to a constellation shaping OFDM device and method for coherent optical transmission. Background Art

[0002] In recent years, Orthogonal Frequency Division Multiplexing (OFDM) technology has rapidly emerged in the field of wireless communications and has been widely used in standards such as IEEE 802.11. As an innovative modulation technology, OFDM has significant advantages over traditional single-carrier systems, especially in resisting frequency selective fading. By dividing the overall communication channel into multiple narrowband sub-channels (such as Figure 1 As shown in the figure), and each sub-channel can withstand flat fading independently, OFDM significantly improves the reliability and efficiency of data transmission in complex environments.

[0003] One of the most notable features of OFDM is its spectral efficiency. Compared to single-carrier systems that typically exhibit a wide spectrum with residual bands (typically characterized by a spectrum characteristic with a raised cosine shape), OFDM is designed to occupy a more compact spectral space. The rectangular spectrum characteristic of OFDM ensures that almost all of the transmitted energy is concentrated within the specified bandwidth, thereby achieving minimal spectral leakage. This feature not only optimizes the utilization of the available spectrum, but also reduces interference to adjacent channels, making it an ideal choice in modern communication systems.

[0004] The emergence of coherent optical orthogonal frequency division multiplexing (CO-OFDM) marks an important evolution of this technology in fiber-optic communication systems. While retaining the advantages of traditional OFDM technology, CO-OFDM introduces new features to meet the needs of optical networks. A prominent feature of CO-OFDM is its flexibility in optical signal management, which enables efficient "add-drop" functionality. This flexibility allows for dynamic allocation of bandwidth and resources, which is particularly suitable for optical networks that require high capacity, scalability, and adaptability.

[0005] As the demand for higher data rates and more efficient spectrum utilization continues to grow, the integration of OFDM technology in optical communications has become particularly important. The popularity of data-intensive applications such as cloud computing, video streaming and the Internet of Things (IoT) is increasing the pressure on the existing communication infrastructure. CO-OFDM successfully addresses these challenges by providing a powerful framework to maximize spectral efficiency and improve system performance.

[0006] OFDM uses a single-tap equalizer, eliminating the need for more complex multi-tap equalizers, such as time-division equalizers with multiple inputs and multiple outputs, which also require specific adaptation speeds. In single-carrier modulation, the finite impulse response taps must be continuously updated for each symbol received due to the randomness of polarization mode dispersion. OFDM achieves parallel processing through fast Fourier transforms, which makes it particularly suitable for implementation on field programmable gate arrays. In addition, PMD and partial dispersion can be effectively compensated through a simple cyclic prefix.

[0007] OFDM also supports monitoring of the signal-to-noise ratio within the spectral granularity of MHz or KHz, thereby improving performance. Compared with single-carrier systems, OFDM provides about 1dB improvement in receiving sensitivity while significantly improving bandwidth utilization.

[0008] One of the main disadvantages of OFDM is its high peak-to-average power ratio, which requires the use of better RF components, such as amplifiers, at the transmitter. In addition, due to the relatively long symbol duration, the system is more sensitive to laser phase noise. At the same time, the use of cyclic prefixes limits the maximum capacity of the system to a certain extent.

[0009] Adaptive OFDM is a technology that optimizes data transmission by dynamically adjusting the bit and power allocation on subcarriers based on channel conditions. By evaluating the changing quality of the communication channel, adaptive OFDM can allocate more bits to subcarriers with better signal quality while reducing the bit rate on subcarriers with poor signal quality. In addition, the technology is able to adjust the power level allocated to each subcarrier, thereby improving the overall system performance and efficiency. This flexibility makes adaptive OFDM particularly good in environments with fluctuating channel conditions, which can increase data rates and reliability.

[0010] However, the disadvantage of this technology is that it requires negotiation between the transmitter and the receiver. In addition, the algorithm required to optimize the signal-to-noise ratio of each subcarrier is relatively complex, such as the water level filling algorithm. Studies have shown that adaptive OFDM can mitigate nonlinear crosstalk effects including four-wave mixing and cross-phase modulation, and has a significant effect on multi-optical OFDM channels.

[0011] In PS-QAM systems, the symbol distribution is optimized to match the Gaussian-like characteristics of the fiber channel by assigning higher probabilities to specific symbols, thereby maximizing capacity. This "gold standard" technology enables higher data rates without additional bandwidth, marking a significant advancement in modern optical networks. However, the implementation of this technology requires complex modulation format generation and associated digital signal processing, and is therefore not energy efficient. Summary of the invention

[0012] In view of the above-mentioned problems in the prior art, the present invention provides a constellation shaping OFDM device and method for coherent optical transmission, which can achieve performance improvement without sacrificing computing efficiency and energy efficiency, significantly reduce costs and implementation complexity, and is suitable for real-time optical network applications with high energy efficiency requirements.

[0013] To achieve the above-mentioned purpose, the present invention proposes a constellation shaping OFDM device for coherent optical transmission, which includes: a coherent transmitter, an optical fiber transmitter and a coherent optical orthogonal frequency division multiplexing CS-OFMD receiver; the coherent transmitter includes two driving lasers and MZM, and two independent filters; the coherent optical orthogonal frequency division multiplexing CS-OFMD receiver includes a balanced photodetector.

[0014] A constellation shaping OFDM method for coherent optical transmission as described in the device, the method comprising:

[0015] S1. Process the input signal. At the transmitting end, a pseudo-random binary sequence generator generates the input signal required for modulation. The transmission process includes CS subcarrier encoding and cyclic prefix insertion.

[0016] S2, perform coherent transmission, insert navigation signals to ensure channel estimation and synchronization, and use input electrical pulses to generate Nyquist pulse shaping responses;

[0017] S3, clipping and quantizing the real and imaginary parts of the QAM signal;

[0018] S4, perform external modulation, generate and transmit modulated signals;

[0019] S5, transmitting the modulated optical signal, amplifying the optical signal and filtering to remove out-of-band noise;

[0020] S6, at the receiving end, converting the coherent optical orthogonal frequency division multiplexing CS-OFDM signal back into an electrical signal and performing coherent reception and detection;

[0021] S7, performing clock phase recovery;

[0022] S8. The received signal is digitally processed and the digital coherent optical orthogonal frequency division multiplexing CS-OFDM is demodulated; the demodulation process is opposite to the process at the transmitting end, and the cyclic prefix is ​​removed.

[0023] Preferably, in S1, the specific process of CS subcarrier encoding is to divide the input PRBS into n c data streams and performs subcarrier OFDM modulation on each data stream using the CS method; in CS-OFDM, the intermediate frequency subcarrier adopts a higher-order modulation format, while the low-frequency and high-frequency subcarriers adopt a lower-order modulation format.

[0024] Preferably, in S1, the cyclic prefix insertion is to add a cyclic prefix to the modulated signal and integrate the cyclic prefix CP into the symbol stream; the cyclic prefix insertion process includes appending the end segment of the inverse fast Fourier transform iFFT packet to the start part of the CS-OFDM symbol; the number of symbols in the cyclic prefix CP is determined by 1 = nc × CP and rounded to the nearest integer.

[0025] Preferably, in S2, the additional navigation signal is used to compensate for the frequency offset caused by the mismatch between the transmitter and receiver lasers; the minimum overlap of the pulse-shaped pulses is determined by the pulse raised cosine filter.

[0026] Preferably, in S3, the specific steps for clipping and quantizing the real and imaginary parts of the QAM signal are as follows:

[0027] S31. Signal sampling: Sample the input QAM signal to obtain the continuous values of the real and imaginary parts, and the sampling rate satisfies the Nyquist sampling theorem to avoid distortion;

[0028] S32. Dynamic range analysis: Perform dynamic range analysis on the sampled real and imaginary part signals to determine the maximum and minimum values of the signals;

[0029] S33. Clipping processing: Set the clipping threshold, clip the signal values outside the threshold range to within the threshold value. The clipping threshold is set according to the peak-to-average power ratio PAPR requirement, and the clipped signal values are normalized to the range [-1, 1] for subsequent processing;

[0030] S34. Discrete quantization: According to the target modulation format, discretize the normalized real and imaginary part signals according to the discrete quantization level 2 N , where N is the number of quantization bits;

[0031] S35. Constellation point mapping: Remap the quantized signal to the discrete points in the standard QAM constellation diagram to generate a signal that conforms to the target modulation format, which is used as the input of the subsequent OFDM module.

[0032] Preferably, in S4, the external modulation is performed by a coherent transmitter using two drive lasers and an MZM; the specific process is that the drive lasers generate continuous optical signals at frequency f o and input the signals into two MZMs respectively, and perform in-phase I-channel modulation of the OFDM signal and quadrature-phase Q-channel modulation of the OFDM signal.

[0033] Preferably, in S5, the modulated optical signal is transmitted through an optical fiber link; the optical signal is amplified by an erbium-doped fiber amplifier; the operation of filtering and removing out-of-band noise is processed by an optical bandpass filter.

[0034] Preferably, in S6, the process of converting the CS-OFDM signal back into an electrical signal is performed using a balanced photodetector; the coherent reception process uses a driving laser and includes a local oscillator signal; the transmission signal and the local oscillator signal maintain phase coherence; and the coherent detection technology mixes the received signal with the local oscillator signal.

[0035] Preferably, in S7, the phase and frequency of the clock information between the transmitting end and the receiving end do not match, and are contained in the "laser-local oscillator" beat frequency item; the step of performing clock phase recovery is to extract the phase through an electrical filter at the receiving end, and to achieve zero-fill coding by turning off one subcarrier or turning off multiple subcarriers in other CS-OFDM implementations, thereby generating a CS-OFDM signal so that the clock signal and the modulation signal do not overlap in frequency.

[0036] Therefore, the present invention proposes a constellation shaping OFDM device and method for coherent optical transmission, which has the following beneficial effects:

[0037] (1) The present invention reduces the impact of nonlinear crosstalk such as cross-phase modulation (XPM) and four-wave mixing (FWM) by using low-order modulation formats for low-frequency and high-frequency subcarriers. It can solve the roll-off effect (filtering effect) on high-frequency subcarriers while reducing the nonlinear crosstalk effect between channels. The unique configuration is closely related to geometric shaping and can enhance performance while maintaining low complexity, which has obvious advantages over benchmark adaptive OFDM and single-carrier PS-QAM.

[0038] (2) The present invention provides significant performance advantages in coherent optical systems, ensuring energy and cost efficiency, as well as computational effectiveness.

[0039] (3) The present invention has the advantages of achieving minimum delay, dynamic bandwidth allocation, and SNR monitoring at MHz / KHz granularity, achieving performance improvement without sacrificing computing efficiency and energy efficiency, significantly reducing costs and implementation complexity, and is suitable for real-time optical network applications with high energy efficiency requirements.

[0040] The technical solution of the present invention is further described in detail below through the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 It is a spectrum diagram of the existing overall OFMD communication channel divided into multiple narrowband sub-channels;

[0042] Figure 2is the existing overall OFMD communication channel composition diagram, (a) is a schematic diagram of a 5-channel adaptive OFDM system operating over a transmission distance of 40 km, (b) is the receive color mapping constellation diagram generated by applying different modulation levels to the test channel 3;

[0043] Figure 3 The invention is in VPIphotonics TM Transmission link diagram of the proposed technique for coherent optical transmission evaluated in simulation software;

[0044] Figure 4 is a cyclic prefix insertion diagram of the present invention;

[0045] Figure 5 1 is a diagram showing a specific embodiment of the present invention, (a) is a schematic diagram of subcarrier modulation selection in CS-OFDM, (b) is a comparison diagram of the CS-32OFDM constellation diagram and the traditional 16QAM OFDM constellation diagram when the signal-to-noise ratio is 5dB;

[0046] Figure 6 It is a comparison chart of symbol error rate and signal-to-noise ratio between CS-32QAM (40GBaud, 3.25 bits / symbol), 16QAM OFDM (32.5GBaud, 4 bits / symbol) and 16QAM single carrier (32.5GBaud, 4 bits / symbol). DETAILED DESCRIPTION

[0047] 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.

[0048] 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.

[0049] like Figure 1 As shown in the figure, OFDM has significant advantages over traditional single-carrier systems, especially in resisting frequency selective fading. By dividing the overall communication channel into multiple narrowband sub-channels and allowing each sub-channel to withstand flat fading independently, OFDM significantly improves the reliability and efficiency of data transmission in complex environments. However, its high peak-to-average power ratio requires the use of better performing RF components, such as amplifiers, at the transmitter. In addition, due to the relatively long symbol duration, the system is more sensitive to laser phase noise. At the same time, the use of cyclic prefixes limits the maximum capacity of the system to a certain extent.

[0050] like Figure 2 As shown in Figure 1, although adaptive OFDM can mitigate nonlinear crosstalk effects including four-wave mixing and cross-phase modulation, and has a significant effect on multi-optical OFDM channels, its disadvantage is that it requires negotiation between the transmitter and the receiver. Figure 2 As shown in (a), the system is a 5-channel adaptive OFDM system operating over a transmission distance of 40 km, with a rate of more than 20 Gb / s for each channel. Figure 2 (b) shows the received color mapping constellation generated by applying different modulation levels to test channel 3. It can be seen that the constellation does not exhibit the typical Gaussian distribution characteristics in the single-carrier probability-shaped QAM system.

[0051] The present invention proposes a constellation shaping OFDM device for coherent optical transmission, which includes: a coherent transmitter, an optical fiber transmitter and a coherent optical orthogonal frequency division multiplexing (CS-OFMD) receiver; the coherent transmitter includes two driving lasers and an MZM, and two independent filters; the coherent optical orthogonal frequency division multiplexing (CS-OFMD) receiver includes a balanced photodetector.

[0052] like Figure 3 As shown, the present invention proposes a constellation shaping OFDM method for coherent optical transmission, the method comprising:

[0053] S1. Process the input signal. At the transmitting end, a pseudo-random binary sequence generator generates the input signal required for modulation. The transmission process includes CS subcarrier encoding and cyclic prefix insertion.

[0054] The specific process of CS subcarrier encoding is to divide the input PRBS into n c data streams, and uses the CS method to perform subcarrier OFDM modulation on each data stream; in CS-OFDM, the intermediate frequency subcarrier uses a higher-order modulation format (such as 16QAM and 32QAM), while the low-frequency and high-frequency subcarriers use a lower-order modulation format (such as 4QAM). This method can solve the roll-off effect (filtering effect) on the high-frequency subcarrier and reduce the nonlinear crosstalk effect between channels.

[0055] like Figure 4 As shown, cyclic prefix insertion is to add a cyclic prefix to the modulated signal and integrate the cyclic prefix CP into the symbol stream; the cyclic prefix insertion process includes appending the end segment of the inverse fast Fourier transform iFFT packet to the beginning of the CS-OFDM symbol; the number of symbols in the cyclic prefix CP is 1 = n c ×CP and rounded to the nearest integer.

[0056] S2, perform coherent transmission, insert navigation signals to ensure channel estimation and synchronization, and use input electrical pulses to generate Nyquist pulse shaping responses;

[0057] In coherent transmission, an additional navigation signal is used to compensate for the frequency offset caused by the mismatch between the transmitter and receiver lasers, and to mitigate the phase noise caused by the transceiver laser linewidth; the minimum pulse overlap for pulse shaping is determined by a pulse raised cosine filter, thereby reducing the possibility of inter-symbol interference (ISI) and using two independent filters for the in-phase ('I') and quadrature-phase ('Q') components of the OFDM signal in the VPIphotonicsTM software design.

[0058] S3, clipping and quantizing the real and imaginary parts of the QAM signal;

[0059] Clipping can limit the real and imaginary parts of the QAM signal to a specific level range. This is very useful for reducing the peak-to-average power ratio in OFDM systems, because excessive PAPR will have an adverse effect on the power amplifier. Typically, a clipping ratio of 13dB can be applied to modulation formats up to 256QAM.

[0060] The specific steps for clipping and quantizing the real and imaginary parts of the QAM signal are:

[0061] S31, signal sampling: sampling the input QAM signal to obtain continuous values ​​of the real part and the imaginary part, and the sampling rate satisfies the Nyquist sampling theorem to avoid distortion;

[0062] S32, dynamic range analysis: performing dynamic range analysis on the real and imaginary signals after sampling to determine the maximum and minimum values ​​of the signals;

[0063] S33, Clipping processing: Set the clipping threshold, clip the signal value that exceeds the threshold range to within the threshold value, the clipping threshold is set according to the peak-to-average power ratio PAPR requirement, and the clipped signal value is normalized to the range of [-1,1] for subsequent processing;

[0064] S34, Discrete quantization: According to the target modulation format, the normalized real and imaginary signals are quantized at discrete quantization level 2 N , where N is the number of quantization bits for discretization;

[0065] S35, constellation point mapping: remap the quantized signal to discrete points in the standard QAM constellation diagram to generate a signal that conforms to the target modulation format as the input of the subsequent OFDM module.

[0066] S4, perform external modulation, generate and transmit modulated signals;

[0067] External modulation is performed by a coherent transmitter using two driving lasers and an MZM; the specific process is that the driving laser is at a frequency f o A continuous wave optical signal is generated under the condition of 100nm and 100nm, which is input into two MZMs respectively and modulated into the in-phase I path of the OFDM signal and the orthogonal Q path of the OFDM signal. Here, the modulated CS-OFDM signal is used to change the intensity and phase of the laser output, thereby effectively encoding the data onto the optical carrier.

[0068] S5, transmitting the modulated optical signal, amplifying the optical signal and filtering to remove out-of-band noise;

[0069] The modulated optical signal is transmitted through an optical fiber link, using an erbium-doped fiber amplifier and removing out-of-band noise through an optical bandpass filter in order to achieve the required optical power.

[0070] S6, converting the CS-OFDM signal back into an electrical signal at the receiving end and performing coherent reception and detection;

[0071] The process of converting the CS-OFDM signal back to an electrical signal is performed using a balanced photodetector; the coherent reception process uses a driving laser and includes a local oscillator; the transmission signal and the local oscillator maintain phase coherence; and the coherent detection technology mixes the received signal with the local oscillator signal.

[0072] This method can extract amplitude and phase information, significantly improving sensitivity and performance, especially in high-capacity, long-distance communications, which has greater advantages than direct detection. Coherent heterodyne detection technology is specifically designed for high-bit-rate CS-OFDM signals and does not use complex electrical or optical phase-locked loops.

[0073] S7, performing clock phase recovery;

[0074] The clock information is the phase and frequency mismatch between the transmitter and the receiver, and is contained in the "laser-local oscillator" beat frequency term; the clock phase recovery step is to extract it through an electrical filter at the receiver and implement zero padding coding by turning off one subcarrier or multiple subcarriers in other CS-OFDM implementations to generate a CS-OFDM signal so that the clock signal does not overlap with the modulated signal in frequency. The performance of the proposed method depends on the strength of the extracted clock signal, which can be adjusted by driving the I and Q MZMs slightly asymmetrically (bias adjustment).

[0075] S8. The received signal is digitally processed and the digital CS-OFDM is demodulated; the demodulation process is opposite to the process at the transmitting end, and the cyclic prefix is ​​removed.

[0076] This process relies heavily on using only the pilot signal to effectively recover the signal and perform channel compensation.

[0077] Figure 3 The VPI software is shown in TM Schematic diagram of a coherent optical transmission link containing CS-OFDM obtained by simulation. In this embodiment, for simplicity, four subcarriers are used (and one additional subcarrier is turned off for clock recovery), and 5, 4, 2 and 2 bits are allocated to each symbol, corresponding to 32QAM, 16QAM, 4QAM and 4QAM, respectively. The highest modulation format is allocated to the subcarrier at the center frequency to avoid potential filter roll-off (this design may have advantages for DWDM transmission). More importantly, the lowest modulation format should always be allocated to more subcarriers, located in the center of the constellation diagram. This arrangement concentrates more bits in the center of the constellation diagram, while fewer bits are allocated to the outer ring constellation points.

[0078] like Figure 5 (a) shows the result graph for CS-32QAM. Figure 5 (b) compares the CS-32 OFDM constellation diagram with the traditional 16QAM OFDM constellation diagram, and the transmitter signal-to-noise ratio of both is 5dB.

[0079] In the simulation, the bit rate of CS-32QAM is set to 130Gb / s, the baud rate is 40Gbaud, and the total average is 3.25 bits per symbol. For comparison, the 16QAMOFDM system and the single-carrier 16QAM system are also simulated. These two systems have the same bit rate (130Gb / s), the baud rate is 32.5Gbaud, and 4 bits per symbol.

[0080] Figure 6 The relationship between symbol error rate and signal-to-noise ratio for the three above-mentioned technologies is shown. It is obvious from the figure that CS-32QAM outperforms 16QAM OFDM and single-carrier OFDM over the entire selected SNR range.

[0081] Experimental results show that the innovative CS-OFDM framework of the proposed method provides significant performance advantages in coherent optical systems, ensuring energy and cost efficiency, as well as computational effectiveness, which is crucial for modern optical communication networks.

[0082] Therefore, the present invention provides a constellation shaping OFDM device and method for coherent optical transmission, which adopts a simple bit loading strategy and only uses 2- or 3-order QAM. This approach allows for flexible arrangement of low-order QAM subcarriers across the entire spectrum, addressing inter-channel non-linear effects and device / filter roll-off, thereby ensuring stronger robustness under various conditions. The present invention can achieve performance improvement without sacrificing computational efficiency and energy efficiency, significantly reducing costs and implementation complexity, and is suitable for real-time optical network applications with high energy efficiency requirements.

[0083] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit them. Although the present invention has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that they can still modify or equivalently replace the technical solutions of the present invention, and these modifications or equivalent replacements cannot make the modified technical solutions deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A constellation shaping OFDM device for coherent optical transmission, characterized in that: The device comprises: a coherent transmitter, an optical fiber transmitter and a coherent optical orthogonal frequency division multiplexing (CS-OFMD) receiver; the coherent transmitter comprises two driving lasers and an MZM, and two independent filters; the coherent optical orthogonal frequency division multiplexing (CS-OFMD) receiver comprises a balanced photoelectric detector.

2. A constellation shaping OFDM method for coherent optical transmission according to the apparatus of claim 1, characterized in that: The method includes: S1. Process the input signal. At the transmitting end, a pseudo-random binary sequence generator generates the input signal required for modulation. The transmission process includes CS subcarrier encoding and cyclic prefix insertion. S2, perform coherent transmission, insert navigation signals to ensure channel estimation and synchronization, and use input electrical pulses to generate Nyquist pulse shaping responses; S3, clipping and quantizing the real and imaginary parts of the QAM signal; S4, perform external modulation, generate and transmit modulated signals; S5, transmitting the modulated optical signal, amplifying the optical signal and filtering to remove out-of-band noise; S6, at the receiving end, converting the coherent optical orthogonal frequency division multiplexing CS-OFDM signal back into an electrical signal and performing coherent reception and detection; S7, performing clock phase recovery; S8. The received signal is digitally processed and the digital coherent optical orthogonal frequency division multiplexing CS-OFDM is demodulated; the demodulation process is opposite to the process at the transmitting end, and the cyclic prefix is ​​removed.

3. The constellation shaping OFDM method for coherent optical transmission according to claim 2, characterized in that: In S1, the specific process of CS subcarrier encoding is to divide the input PRBS into n c data streams and performs subcarrier OFDM modulation on each data stream using the CS method; in CS-OFDM, the intermediate frequency subcarrier adopts a higher-order modulation format, while the low-frequency and high-frequency subcarriers adopt a lower-order modulation format.

4. The constellation shaping OFDM method for coherent optical transmission according to claim 2, characterized in that: In S1, the cyclic prefix insertion is to add a cyclic prefix to the modulated signal and integrate the cyclic prefix CP into the symbol stream; The cyclic prefix insertion process includes appending the end segment of the inverse fast Fourier transform iFFT packet to the beginning of the CS-OFDM symbol; the number of symbols in the cyclic prefix CP is determined by 1=nc×CP and rounded to the nearest integer.

5. The constellation shaping OFDM method for coherent optical transmission according to claim 2, characterized in that: In S2, the additional navigation signal is used to compensate for the frequency offset caused by the mismatch between the transmitter and receiver lasers; the minimum pulse overlap of the pulse shaping is determined by the pulse raised cosine filter.

6. The constellation shaping OFDM method for coherent optical transmission according to claim 2, characterized in that: In S3, the specific steps of clipping and quantizing the real and imaginary parts of the QAM signal are: S31, signal sampling: sampling the input QAM signal to obtain continuous values ​​of the real part and the imaginary part, and the sampling rate satisfies the Nyquist sampling theorem to avoid distortion; S32, dynamic range analysis: performing dynamic range analysis on the real and imaginary signals after sampling to determine the maximum and minimum values ​​of the signals; S33, Clipping processing: Set the clipping threshold, clip the signal value that exceeds the threshold range to within the threshold value, the clipping threshold is set according to the peak-to-average power ratio PAPR requirement, and the clipped signal value is normalized to the range of [-1,1] for subsequent processing; S34, Discrete quantization: According to the target modulation format, the normalized real and imaginary signals are quantized at discrete quantization level 2 N , where N is the number of quantization bits for discretization; S35, constellation point mapping: remap the quantized signal to discrete points in the standard QAM constellation diagram to generate a signal that conforms to the target modulation format as the input of the subsequent OFDM module.

7. The constellation shaping OFDM method for coherent optical transmission according to claim 2, characterized in that: In S4, the external modulation is performed by a coherent transmitter using two driving lasers and an MZM; the specific process is that the driving laser is at a frequency f o A continuous wave optical signal is generated under the condition, and the signal is input into two MZMs respectively, and the in-phase I-path modulation of the OFDM signal and the orthogonal phase Q-path modulation of the OFDM signal are performed.

8. The constellation shaping OFDM method for coherent optical transmission according to claim 2, characterized in that: In S5, the modulated optical signal is transmitted through an optical fiber link; the optical signal is amplified by an erbium-doped optical fiber amplifier; and the filtering and out-of-band noise removal operations are processed by an optical bandpass filter.

9. The method for coherent optical transmission using constellation shaping OFDM according to claim 2, characterized in that: In S6, the process of converting the CS-OFDM signal back into an electrical signal is performed using a balanced photodetector; the coherent reception process uses a driving laser and includes a local oscillator signal; the transmission signal and the local oscillator signal maintain phase coherence; and the coherent detection technology mixes the received signal with the local oscillator signal.

10. The method for coherent optical transmission using constellation shaping OFDM according to claim 2, characterized in that: In S7, the phase and frequency of the clock information between the transmitting end and the receiving end do not match, and are contained in the "laser-local oscillator" beat frequency term; the step of clock phase recovery is to extract the phase through an electrical filter at the receiving end, and to achieve zero padding coding by turning off one subcarrier or turning off multiple subcarriers in other CS-OFDM implementations, thereby generating a CS-OFDM signal so that the clock signal and the modulated signal do not overlap in frequency.

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