An equalization method for fiber link multipath interference and inter-symbol interference of an optical fiber communication system

Abstract

The application discloses an equalization method for optical fiber link multipath interference and inter-symbol interference of an optical fiber communication system. The method comprises the following steps: obtaining a pulse amplitude modulation signal from the optical transmitter, performing double binary coding on the pulse amplitude modulation signal, and transmitting the coded pulse amplitude modulation signal to an optical receiver after filtering processing; performing equalization training on the feedforward equalizer by using the double binary coded pulse amplitude modulation signal to obtain a trained feedforward equalizer; obtaining the filtered pulse amplitude modulation signal from the optical receiver as a received signal to be equalized, and performing diode coding on the received signal to be equalized; performing adaptive feedforward equalization on the diode coded received signal by using the trained feedforward equalizer, and decoding the equalized signal to recover the pulse amplitude modulation signal transmitted by the optical transmitter. The application can realize equalization for optical fiber link multipath interference and inter-symbol interference.

Description

Technical Field

[0001] This invention relates to the technical field of optical fiber communication, and in particular to a method for equalizing multipath interference and intersymbol interference in optical fiber links of an optical fiber communication system. Background Technology

[0002] With the continuous emergence of new application scenarios such as artificial intelligence, ultra-high-definition video, cloud computing, and big data models, global data traffic is growing exponentially. The rapidly increasing demand for traffic is driving further upgrades in bandwidth for links within and between data centers. Under this trend, the low-cost, simple-structure, and low-power IM-DD (Intensity Modulation and Direct Detection) system solution has become dominant in the field of data communication and is gradually evolving towards single-wavelength 400Gbps and even 800Gbps.

[0003] However, in the process of moving towards this speed, system performance faces severe challenges, the two most important of which are multipath interference (MPI) and inter-symbol interference (ISI) caused by limited device bandwidth. As link topologies become more complex, contaminated fiber optic connectors within the link cause optical signals to be reflected multiple times, eventually reflecting back in the same direction as the fiber propagation, superimposing with the normal signal. These reflected signals are collectively referred to as MPI. MPI severely impairs the stability of IM-DD system communication, posing a significant challenge to effectively mitigating MPI, especially in high bit-rate and high-sensitivity receiver scenarios. Simultaneously, due to limited device bandwidth and enhanced dispersion effects, signals are prone to inter-symbol interference during propagation, leading to symbol overlap, severely degrading signal quality and increasing the bit error rate. The existence of MPI and ISI not only reduces the transmission limit of IM-DD systems but also increases the complexity of equalization and error control. This makes it impossible for existing technologies to effectively suppress the performance degradation caused by MPI and ISI while maintaining system simplicity and low cost advantages. Summary of the Invention

[0004] To overcome the problem that existing technologies are prone to inter-symbol interference (ISI) during signal propagation, leading to symbol overlap, severely deteriorating signal quality, and increasing the bit error rate, the present invention aims to propose a method for equalizing multipath interference and ISI in optical fiber communication systems. This method can effectively equalize multipath interference and ISI during signal propagation, thereby reducing symbol overlap and improving spectral efficiency and signal quality.

[0005] To achieve the objectives of this invention, the following technical solution is adopted:

[0006] A method for equalizing multipath interference and intersymbol interference in an optical fiber communication system, the optical fiber communication system including an optical transmitter and an optical receiver; the optical receiver includes a feedforward equalizer, and the method includes the following steps:

[0007] The pulse amplitude modulation signal is obtained from the optical transmitter, the pulse amplitude modulation signal is encoded in dual binary, and the encoded pulse amplitude modulation signal is filtered and then transmitted to the optical receiver.

[0008] The feedforward equalizer is trained by using the pulse amplitude modulation signal after double binary encoding to obtain a trained feedforward equalizer.

[0009] The filtered pulse amplitude modulation signal is obtained from the optical receiver as the received signal to be equalized, and the received signal to be equalized is diode encoded.

[0010] The received signal after diode encoding is adaptively fed forward equalized using a trained feedforward equalizer, and the equalized signal is then decoded to recover the pulse amplitude modulation signal transmitted by the optical transmitter.

[0011] In the above technical solution, by performing dual binary encoding on the pulse amplitude modulation signal, the high-frequency components of the transmitted signal are effectively reduced, enhancing the tolerance of the pulse amplitude modulation signal to the bandwidth limitation of optoelectronic devices during transmission. This confines more signals to the low-frequency region, thereby reducing the negative impact of bandwidth limitation on the signal during transmission and reducing the decoding complexity of the optical receiver when receiving the signal. Using the dual binary encoded pulse amplitude modulation signal to train the feedforward equalizer enables the trained equalizer to effectively equalize the received signal, thus reducing symbol overlap. Both the dual binary encoding and diode encoding are partial response encodings. Since the signal energy is concentrated in the lower frequency band, partial response encoding can transmit more information within the same bandwidth, thereby improving spectral efficiency. Furthermore, the equalization method based on partial response encoding achieves an optimal balance between signal integrity, spectral efficiency, resistance to nonlinear impairments, and complexity in fiber optic links through deep collaboration between encoding and signal processing.

[0012] Furthermore, the process of acquiring the pulse amplitude modulation signal from the optical transmitter includes:

[0013] The optical transmitter generates a pseudo-random sequence and performs a Gray mapping on the pseudo-random sequence to obtain a pulse amplitude modulation signal.

[0014] Furthermore, the process of performing dual binary encoding on the pulse amplitude modulation signal, filtering the encoded pulse amplitude modulation signal, and then transmitting it to the optical receiver includes:

[0015] The pulse amplitude modulation signal is added to a signal delayed by one signal period to complete the double binary encoding;

[0016] The pulse amplitude modulation signal after completing the dual binary encoding is upsampled, and the upsampled pulse amplitude modulation signal is pulse shaped using a root raised cosine filter. The pulse-shaped pulse amplitude modulation signal is then transmitted to the optical receiver.

[0017] Furthermore, the upsampling is 2x upsampling; the roll-off factor of the root-raised cosine filter is 0.1, and the tap length is 128.

[0018] In the above technical solution, the pulse shaping is to filter the upsampled pulse amplitude modulation signal to achieve higher spectral efficiency. By using a 2x upsampling rate, it provides operating space for the subsequent root-raised cosine filter, enabling it to generate a smooth analog waveform. The root-raised cosine filter at the optical transmitter end and the root-raised cosine filter at the optical receiver end are combined, and the overall response is raised cosine. Therefore, there is no inter-symbol interference when sampling at the symbol center point.

[0019] Furthermore, the process of obtaining the filtered pulse amplitude modulation signal from the optical receiver as the received signal to be equalized includes:

[0020] The optical receiver also includes a single photodetector, which converts the optical signal of the pulse amplitude modulation signal transmitted from the optical transmitter into an electrical signal to obtain the received signal to be equalized.

[0021] Furthermore, the process of diode encoding the received signal to be equalized includes:

[0022] The received signal to be equalized is filtered using a root-raised cosine filter, and the filtered received signal to be equalized is downsampled.

[0023] The received signal to be equalized after downsampling is subtracted from the signal delayed by one signal period to complete diode encoding.

[0024] Furthermore, the downsampling involves reducing the sampling rate to 1; the root-raised cosine filter has a roll-off factor of 0.1 and a tap length of 128.

[0025] In the above technical solution, diode encoding of the received signal to be equalized can suppress the nonlinear damage caused by multipath interference introduced into the signal during transmission, thereby enhancing the robustness against multipath interference.

[0026] Furthermore, the pulse amplitude modulation signal in the optical transmitter is transmitted to the optical receiver via an optical fiber transmission link;

[0027] An improved binary code is constructed based on the aforementioned dual binary code and the aforementioned diode code to limit signal power to the low-frequency region in order to cope with bandwidth limitations;

[0028] The optical fiber used in the optical fiber transmission link is a standard single-mode optical fiber with a loss factor of 0.2 dB / km and a dispersion of 16.0 ps / (nm * km).

[0029] Furthermore, the feedforward equalizer employs the least mean square algorithm for adaptive training and parameter updates.

[0030] Furthermore, the optical receiver uses maximum likelihood sequence detection to decode the equalized signal in order to recover the pulse amplitude modulation signal transmitted by the optical transmitter.

[0031] In the above technical solution, the improved binary encoding based on the dual binary encoding and the diode encoding can limit the signal power to the low frequency region to cope with the bandwidth limitation. The known ISI introduced by the improved dual binary encoding can be processed by MLSD to avoid the error propagation problem of the decision feedback equalizer. In addition, the improved dual binary encoding can further optimize the level distribution and balance power and noise immunity.

[0032] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0033] This invention proposes a method for equalizing multipath interference (MPI) and intersymbol interference (ISI) in optical fiber communication systems. By performing dual binary encoding on the pulse amplitude modulation (PAM) signal, the high-frequency components of the transmitted signal are effectively reduced, enhancing the tolerance of the PAM signal to bandwidth limitations of optoelectronic devices during transmission. This confines more signals to the low-frequency region, reducing the negative impact of bandwidth limitations on the signal during transmission and lowering the decoding complexity of the optical receiver. The dual binary encoded PAM signal is used to train the feedforward equalizer, enabling the trained equalizer to effectively equalize the received signal, thereby reducing intersymbol overlap. Both the dual binary encoding and diode encoding are partial response encodings. Since the signal energy is concentrated in the lower frequency band, partial response encoding can transmit more information within the same bandwidth, thus improving spectral efficiency. Furthermore, the equalization method based on partial response encoding achieves an optimal balance between signal integrity, spectral efficiency, resistance to nonlinear impairments, and complexity in the optical fiber link through deep collaboration between encoding and signal processing. This method effectively suppresses performance degradation caused by MPI and ISI while maintaining system simplicity and low cost. Attached Figure Description

[0034] Figure 1 A flowchart illustrating the steps of a method for equalizing multipath interference and inter-symbol interference in an optical fiber communication system, provided in an embodiment of this application.

[0035] Figure 2 A schematic diagram illustrating the principle of the equalization method for multipath interference and inter-symbol interference in an optical fiber communication system based on partial response coding provided in this application embodiment;

[0036] Figure 3 This is a schematic diagram of the spectrum before and after partial response encoding provided in an embodiment of this application. Detailed Implementation

[0037] To facilitate understanding of the present invention, a more complete description will be given below with reference to the accompanying drawings. Preferred embodiments of the invention are shown in the drawings. However, the invention can be implemented in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided to provide a thorough and complete understanding of the disclosure of the invention.

[0038] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein in the description of the invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0039] Example 1:

[0040] This embodiment provides a method for equalizing multipath interference and intersymbol interference in an optical fiber communication system. The optical fiber communication system includes an optical transmitter and an optical receiver; the optical receiver includes a feedforward equalizer, see [link to documentation]. Figure 1 The method includes the following steps:

[0041] Step S1: Obtain the pulse amplitude modulation signal from the optical transmitter, perform double binary encoding on the pulse amplitude modulation signal, filter the encoded pulse amplitude modulation signal, and transmit it to the optical receiver;

[0042] Step S2: Use the pulse amplitude modulation signal after double binary encoding to perform equalization training on the feedforward equalizer to obtain a trained feedforward equalizer.

[0043] Step S3: Obtain the filtered pulse amplitude modulation signal from the optical receiver as the received signal to be equalized, and perform diode encoding on the received signal to be equalized;

[0044] Step S4: Use the trained feedforward equalizer to perform adaptive feedforward equalization on the diode-encoded received signal, and decode the equalized signal to recover the pulse amplitude modulation signal transmitted by the optical transmitter.

[0045] The optical receiver is of the intensity detection type.

[0046] In a preferred embodiment, step S1, the process of acquiring the pulse amplitude modulation signal from the optical transmitter, includes:

[0047] The optical transmitter generates a pseudo-random sequence and performs a Gray mapping on the pseudo-random sequence to obtain a Pulse Amplitude Modulation (PAM) signal.

[0048] Specifically, the process of performing dual binary encoding on the pulse amplitude modulation signal, filtering the encoded pulse amplitude modulation signal, and then transmitting it to the optical receiver includes:

[0049] The pulse amplitude modulation signal is added to a signal delayed by one signal period to complete the double binary encoding (the double binary encoding is a type of partial response encoding).

[0050] The pulse amplitude modulation signal after completing the dual binary encoding is upsampled by 2 times, and a root raised cosine filter (RRC) with a roll-off factor of 0.1 and a tap length of 128 is used to perform pulse shaping on the upsampled pulse amplitude modulation signal, and the pulse-shaped pulse amplitude modulation signal is transmitted to the optical receiver.

[0051] Understandably, the pulse shaping process involves filtering the upsampled pulse amplitude modulation signal to achieve higher spectral efficiency. By using a 2x upsampling rate, it provides operational space for the subsequent root-raised-cosine filter, enabling it to generate a smooth analog waveform. The root-raised-cosine filter at the optical transmitter end and the root-raised-cosine filter at the optical receiver end combine to produce an overall raised-cosine response. Therefore, there is no inter-symbol interference when sampling at the symbol center point.

[0052] In a preferred embodiment, step S3, the process of obtaining the filtered pulse amplitude modulation signal from the optical receiver as the received signal to be equalized, includes:

[0053] The optical receiver also includes a single photodetector, which converts the optical signal of the pulse amplitude modulation signal transmitted from the optical transmitter into an electrical signal to obtain the received signal to be equalized.

[0054] Specifically, the process of diode encoding the received signal to be equalized includes:

[0055] The received signal to be equalized is filtered using a root-raised cosine filter with a roll-off factor of 0.1 and a tap length of 128, and the filtered received signal to be equalized is sampled at 1 sampling rate.

[0056] The received signal to be equalized after downsampling is subtracted from the signal delayed by one signal period to complete diode encoding (the diode encoding belongs to partial response encoding).

[0057] Understandably, diode encoding of the received signal to be equalized can suppress nonlinear damage caused by multipath interference introduced during signal transmission, thereby enhancing robustness against multipath interference.

[0058] In a preferred embodiment, the pulse amplitude modulation signal in the optical transmitter is transmitted to the optical receiver via an optical fiber transmission link;

[0059] An improved dual-binary encoding is constructed based on the aforementioned dual-binary encoding and diode encoding to limit signal power to the low-frequency region to address bandwidth limitations. In the partial response encoding, dual-binary encoding and diode encoding are selected to construct the improved dual-binary encoding, enabling the optical transmitter to enhance the signal's tolerance to bandwidth limitations of optoelectronic devices during transmission through dual-binary encoding. At the same time, the optical receiver can suppress multipath interference introduced during signal transmission by using diode encoding.

[0060] The optical fiber used in the optical fiber transmission link is a standard single-mode optical fiber with a loss factor of 0.2 dB / km and a dispersion of 16.0 ps / (nm * km).

[0061] In a preferred embodiment, the feedforward equalizer employs the Least Mean Squares (LMS) algorithm for adaptive training and parameter updates. Specifically, in the optical receiver, the digital signal processing section includes RRC matched filtering corresponding to the optical transmitter and an inter-symbol interference compensation method based on the LMS algorithm for adaptive training and parameter updates.

[0062] In a preferred embodiment, the optical receiver employs Maximum Likelihood Sequence Detection (MLSD) to decode the equalized signal to recover the pulse amplitude modulation signal transmitted by the optical transmitter. Specifically, the optical receiver inputs the received signal to be equalized into a trained feedforward equalizer for adaptive feedforward equalization, which significantly shortens the channel memory length. MLSD is then performed, and the equalized symbol is output. The effectiveness is measured by analyzing the bit error rate curves of different factors. The MLSD is a unique scheme designed specifically for partial response coding characteristics.

[0063] Understandably, the improved binary coding constructed based on the dual binary coding and the diode coding can limit the signal power to the low-frequency region to cope with bandwidth limitations. The known ISI introduced by the improved dual binary coding can be processed by MLSD to avoid the error propagation problem of the decision feedback equalizer. In addition, the improved dual binary coding can further optimize the level distribution and balance power and noise immunity.

[0064] In this embodiment, by performing dual binary encoding on the pulse amplitude modulation signal, the high-frequency components of the transmitted signal are effectively reduced, enhancing the tolerance of the pulse amplitude modulation signal to the bandwidth limitation of optoelectronic devices during transmission. This confines more signals to the low-frequency region, thereby reducing the negative impact of bandwidth limitation on the signal during transmission and reducing the decoding complexity of the optical receiver when receiving the signal. Using the dual binary encoded pulse amplitude modulation signal to train the feedforward equalizer enables the trained equalizer to effectively equalize the received signal, thus reducing inter-symbol overlap. Both the dual binary encoding and diode encoding are partial response encodings. Since the signal energy is concentrated in the lower frequency band, partial response encoding can transmit more information within the same bandwidth, thereby improving spectral efficiency. Furthermore, the equalization method based on partial response encoding achieves an optimal balance between signal integrity, spectral efficiency, resistance to nonlinear impairments, and complexity in the optical fiber link through deep collaboration between encoding and signal processing.

[0065] Example 2:

[0066] This embodiment builds a simulated communication system based on the method described in Embodiment 1, such as... Figure 2 As shown in the diagram. There are three parts: an optical transmitter, a transmission link simulating multipath interference, and an optical receiver.

[0067] In the optical transmitter section, digital signal processing was implemented using Matlab, including the generation of pseudo-random binary sequences (PRBS), modulation format mapping, double binary encoding, upsampling, and RRC filtering, which generated the transmission symbols. The modulation format was PAM-4, the laser wavelength was 1550 nm, the transmission rate was 56 GBaud, and the transmission power was 0 dBm.

[0068] In the fiber optic transmission link, the modulated optical signal enters the transmission link for transmission. The fiber type used is standard single-mode fiber (SSMF), with a loss factor of 0.2 dB / km and a dispersion of 16.0 ps / (nm * km). To simulate the impact of MPI while considering the factors of actual fiber fabrication processes and technologies, multiple reflected signal interferences actually exist. Therefore, the cases of reflection paths 1, 2, 3, and 4 were studied. To ensure the effectiveness of the MPI simulation, the delay time and corresponding fiber length of each branch must exceed the optical delay length corresponding to the conditions that generate uncorrelated MPI. The optical signal is split into two branches at a certain ratio (e.g., a splitting ratio of 99:1) using a coupler. The upper branch is used for signal transmission, with a transmission length set to 10 km. The signal in the lower branch is a backup of the signal in the upper branch, but it has different attenuation and delay after passing through different links. The attenuation can be adjusted by the variable optical attenuator (VOA) shown in the figure, and the delay can be changed by setting the length of the SSMF in the lower branch. The signals from the two branches are transmitted through their respective transmission links and combined into a single signal by an optical coupler before entering the optical receiver. At this point, the signal contains nonlinear impairments due to multipath interference.

[0069] In the optical receiver section, the optical signal is converted into an electrical signal, i.e., the received signal to be equalized, by a single photodetector (PD). Then, the received signal to be equalized is subtracted from the current signal and the signal delayed by one symbol period for adaptive feedforward equalization and MLSD, and the output is the equalized symbol.

[0070] The equalization method uses a direct detection optical receiver, and its digital signal processing includes an RRC matched filter corresponding to the optical transmitter and an intersymbol interference (ISI) compensation method based on the LMS algorithm. The RRC filter in the optical transmitter aims to shape the pulse and achieve higher spectral efficiency; therefore, the optical receiver also needs a corresponding RRC matched filter. The LMS algorithm is used to initially reduce the impact of ISI.

[0071] In communication systems, partial response coding (PSC) can compress signal bandwidth by leveraging inter-symbol correlation, thereby increasing data transmission rate within the same frequency band. In channels with MPI (Multi-Level Interference), it significantly reduces the bit error rate. To illustrate its working principle more intuitively... Figure 2 The effects of the improved dual binary encoding at the transmitter and receiver are shown in the diagrams.

[0072] The aforementioned encoding method reduces the negative impact of bandwidth limitations on the signal during transmission by improving the correlation between symbols and confining the signal to the low-frequency region. Dual binary coding enhances the signal's tolerance to bandwidth limitations of optoelectronic devices during transmission. At the receiving end, diode coding is used to suppress nonlinear impairments caused by multipath interference introduced during transmission. The introduced known ISI can be processed using MLSD, avoiding the error propagation problem of decision feedback equalizers, making it suitable for high-speed fiber optic communication systems. The equalization method based on partial response coding achieves an optimal balance between signal integrity, spectral efficiency, resistance to nonlinear impairments, and complexity in fiber optic links through deep collaboration between coding and signal processing. The improved dual binary coding effectively suppresses high-frequency noise components, offering significant advantages in high-speed fiber optic communication systems. In PAM-4 signals, the improved dual binary coding further optimizes the level distribution. After generating a random PAM-4 signal using the Randi function and Gray coding, using Dual binary coding allows the transmitted signal to occupy seven levels, with a Nyquist frequency approximately 1.5 times lower than PAM4, minimizing signal attenuation caused by ISI and reducing crosstalk effects. After passing through a channel with MPI (Mean Differential Indicator), some signal levels change to small differences. Diode encoding can redistribute signal energy to higher frequencies and eliminate DC components, resulting in greater differences between levels and improved discernibility, thus reducing the impact of MPI on the signal. This advantage is particularly helpful in achieving high-channel output targets, balancing power and noise immunity.

[0073] The comparison of the equalization method's effectiveness involves performing adaptive feedforward equalization on the received signal to be equalized, and then using maximum likelihood sequence detection to output the equalized symbol, which is obtained by analyzing the bit error rate curves of different factors.

[0074] The above description is merely an embodiment of the present invention and does not limit the patent scope of the present invention. Any equivalent structural or procedural transformations made based on the content of the present invention's specification and drawings, or direct or indirect applications in other related technical fields, are similarly included within the patent protection scope of the present invention.

Claims

1. A method of equalizing multi-path interference and inter-symbol interference in an optical fiber link of an optical fiber communication system, the optical fiber communication system comprising an optical transmitter and an optical receiver; the optical receiver comprising a feed-forward equalizer, characterized in that, The method comprises the following steps: Obtaining a pulse amplitude modulation signal from the optical transmitter, double-binary encoding the pulse amplitude modulation signal, and transmitting the encoded pulse amplitude modulation signal to the optical receiver after filtering processing; Using the double-binary encoded pulse amplitude modulation signal to perform equalization training on the feedforward equalizer to obtain a trained feedforward equalizer; Obtaining the filtered pulse amplitude modulation signal from the optical receiver as a received signal to be equalized, and diode encoding the received signal to be equalized; Using the trained feedforward equalizer to perform adaptive feedforward equalization on the diode encoded received signal, and decoding the equalized signal to recover the pulse amplitude modulation signal transmitted by the optical transmitter; The process of double-binary encoding the pulse amplitude modulation signal and transmitting the encoded pulse amplitude modulation signal to the optical receiver after filtering processing comprises: Adding the pulse amplitude modulation signal to a signal delayed by one signal period to complete double-binary encoding; Upsampling the double-binary encoded pulse amplitude modulation signal, pulse shaping the upsampled pulse amplitude modulation signal using a root-raised cosine filter, and transmitting the pulse-shaped pulse amplitude modulation signal to the optical receiver; The process of diode encoding the received signal to be equalized comprises: Filtering the received signal to be equalized using a root-raised cosine filter, and downsampling the filtered received signal to be equalized; Subtracting the downsampled received signal to be equalized from a signal delayed by one signal period to complete diode encoding.

2. The method of claim 1, wherein the method further comprises: The process of obtaining a pulse amplitude modulation signal from the optical transmitter comprises: The optical transmitter generates a pseudo-random sequence and performs Gray mapping on the pseudo-random sequence to obtain a pulse amplitude modulation signal.

3. The method of claim 1, wherein the method further comprises: determining a plurality of channel taps for each of the plurality of optical channels; and determining a plurality of channel taps for each of the plurality of optical channels. The upsampling is 2x upsampling; the roll-off factor of the root-raised cosine filter is 0.1, and the tap length is 128.

4. The method of claim 1, wherein the method further comprises: determining a plurality of channel taps for each of the plurality of optical channels; and determining a plurality of channel taps for each of the plurality of optical channels. The process of obtaining the filtered pulse amplitude modulation signal from the optical receiver as a received signal to be equalized comprises: The optical receiver further comprises a single photodetector that converts an optical signal of the pulse amplitude modulation signal transmitted by the optical transmitter into an electrical signal to obtain a received signal to be equalized.

5. The method of claim 1, wherein the method further comprises: determining a plurality of channel taps for each of the plurality of optical channels; and determining a plurality of channel taps for each of the plurality of optical channels based on the plurality of channel taps for each of the plurality of optical channels. The downsampling is reducing the sampling rate to 1x; the roll-off factor of the root-raised cosine filter is 0.1, and the tap length is 128.

6. The method of claim 1-5, wherein, The pulse amplitude modulation signal in the optical transmitter is transmitted to the optical receiver through an optical fiber transmission link; An improved binary encoding is constructed based on the double-binary encoding and the diode encoding to limit signal power in a low-frequency region to cope with bandwidth limitations; The optical fiber transmission link uses a standard single-mode optical fiber with a loss coefficient of 0.2 dB / km and a dispersion of 16.0 ps / (nm*km).

7. The method of claim 1, wherein the method further comprises: determining a plurality of channel taps for each of the plurality of optical channels; and determining a plurality of channel taps for each of the plurality of optical channels. The feedforward equalizer uses a least mean square algorithm for adaptive training and parameter updating.

8. The method of claim 1, wherein, The optical receiver decodes the equalized signal using maximum likelihood sequence detection to recover the pulse amplitude modulated signal transmitted by the optical transmitter.

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