Dual local oscillator based time and frequency domain power equalization to simplify coherent upstream optical access systems and methods

Abstract

This application relates to the field of optical communication technology and discloses a simplified coherent uplink optical access system and method based on dual local oscillators and time-frequency domain power equalization. This system employs time-frequency division multiplexing, significantly increasing the flexibility of optical network resource allocation. In the optical network unit (ONU), the IQ modulator is simplified to a single electro-optic modulator, reducing costs. At the optical line terminal (OLT), two local oscillators with different powers are used to heterodyne-receive two subcarriers. In the time domain, the received electrical signal power can be controlled at an appropriate level, improving the overall dynamic range of the system. In the frequency domain, the power of the two subcarriers of the received electrical signal can be made similar, improving the system's tolerance to subcarrier power differences. Optimizing the receiver bandwidth in the OLT improves the system's tolerance to DC leakage, enabling flexible and high-speed fiber optic access networks. The system performance has been comprehensively evaluated and optimized, and it has broad application prospects in future fiber optic access networks.

Description

Technical Field

[0001] This application relates to the field of optical communication technology, and in particular to a simplified coherent uplink optical access technology based on time-frequency domain power equalization using dual local oscillators. Background Technology

[0002] With the rapid development of 5G-A and 6G mobile internet, virtual reality and augmented reality, and 8K / 16K ultra-high-definition video services, access-side traffic is experiencing explosive growth, accelerating the development of optical access networks. As a cost-effective and scalable solution, Passive Optical Networks (PONs) have proven to be a successful broadband network solution connecting service providers and end users, capable of meeting ever-increasing bandwidth demands. It is foreseeable that future PON systems will need to achieve net user transmission rates of 200 Gb / s or even higher. Traditional PONs often employ Intensity Modulation / Direct Detection (IMDD) systems, which have limited receiver sensitivity and supported modulation orders, hindering further improvements in system capacity and transmission distance within given optical power budgets and spectral resources. In contrast, coherent detection can simultaneously acquire amplitude and phase information of the optical field, exhibiting higher receiver sensitivity and spectral efficiency in high-order modulation and multi-dimensional modulation scenarios, and significantly expanding the system power budget margin. Therefore, coherent detection-based passive optical networks (CPON) have been proposed and have become one of the important candidate solutions for Very High Speed ​​PON (VHSP) being studied by the International Telecommunication Union (ITU).

[0003] The existing evolution paths of PON mainly include TDM-PON based on single-wavelength time division multiplexing and WDM-PON based on multi-wavelength multiplexing. The former performs time slot multiplexing for multiple users on the same wavelength, which is simple in structure and low in cost, but has limited flexibility in terms of service rate upgrades and bandwidth allocation fineness. The latter improves the total capacity and anti-interference capability by allocating independent wavelengths to different users or user groups, but requires more wavelength division multiplexing devices and light source resources, leading to increased system cost and operation and maintenance complexity. Against this background, in order to further improve the elasticity of access bandwidth and the flexibility of resource scheduling under the premise of controllable cost, the Time-Frequency Division Multiplexed CPON (TFDM-CPON) architecture based on time-frequency multiplexing has been proposed in recent years to provide users with configurable access rates in both time and frequency dimensions.

[0004] However, the first challenge facing coherent PON in its adoption by optical access networks is the cost of the transmitters on the optical network unit (ONU) side. Traditional coherent transmission schemes typically require IQ modulators to simultaneously modulate the in-phase and quadrature components of the optical signal to carry the complete information of the complex modulated signal. IQ modulators are complex and expensive, while optical access networks have a large number of cost-sensitive ONUs. If each ONU uses an IQ modulator, the overall deployment cost of the system will increase significantly, hindering the large-scale commercialization of coherent PON. Therefore, how to reduce the hardware complexity and cost of the ONU transmitter while maintaining the advantages of coherent detection is one of the important issues facing uplink coherent PON.

[0005] Furthermore, uplink TFDM-CPON faces a critical technical challenge in power balancing during practical engineering applications. Due to differences in optical path loss (OPL) between different Optical Network Units (ONUs) and Optical Line Terminals (OLTs), the received optical power (ROP) from different ONUs at the OLT varies significantly, reaching up to 20 dB. In traditional uplink time-division multiplexing PON systems, this requires a large dynamic range in the receiver; however, in TFDM-CPON, power imbalance between subcarriers introduces additional problems. The presence of multiple subcarriers, the delay deviation between receiver channels, and the limited quantization accuracy of the analog-to-digital converter (ADC) result in non-negligible sensitivity losses on weak subcarriers, significantly degrading their bit error rate performance. Therefore, in uplink TFDM-CPON, effective power balancing must be achieved simultaneously in both the time and frequency domains to improve overall system performance and reduce performance differences between different ONUs and subcarriers.

[0006] Meanwhile, due to the finite extinction ratio and bias point drift of the Mach-Zehnder modulator (MZM), DC leakage in uplink PON is also a significant problem. For PON systems with multiple ONUs, even ONUs in non-transmitting states may generate some optical DC leakage. This leakage optical power is superimposed on the uplink signal from active ONUs at the OLT, significantly reducing the signal-to-noise ratio (SNR) of the effective signal. In extreme cases, when the uplink signal originates from an ONU far from the OLT with high link loss, while a large number of inactive ONUs are located near the OLT in low-loss positions, the accumulated DC leakage power may even exceed the effective uplink signal power, leading to a severe degradation in system performance and making it difficult to meet the requirements of high-order modulation and high-bit-rate transmission for receiver sensitivity and bit error rate.

[0007] Therefore, in the existing technology, uplink TFDM-CPON faces multiple challenges such as high cost of optical network unit transmitters, large dynamic range power equalization, and suppression of cumulative DC leakage from multiple ONUs. There is an urgent need to propose a new system architecture and receiver structure to effectively improve the transmission performance and stability of uplink coherent PON systems under complex power distribution conditions while ensuring that system complexity and cost are controllable. Summary of the Invention

[0008] The purpose of this application is to provide a simplified coherent uplink optical access system and method based on time-frequency domain power equalization using dual local oscillators, in order to solve the problems mentioned in the background art.

[0009] This application discloses a simplified coherent optical communication system based on dual local oscillators and time-frequency domain power equalization, applied to uplink fiber optic access networks, including: Multiple optical network units (ONUs) are provided, each containing an optical transmitter for generating and transmitting a first subcarrier optical signal or a second subcarrier optical signal. Different ONUs are assigned different optical subcarriers, and the ONUs transmit the first subcarrier optical signal and the second subcarrier optical signal according to the assigned optical subcarriers. An optical transmission link is used to couple optical signals from different optical network units through an optical coupler, perform carrier aggregation on the optical signals of the first subcarrier and the second subcarrier, and transmit them through a standard single-mode optical fiber to transmit the optical signals of each optical network unit to the optical line terminal. An optical line terminal includes an integrated coherent receiver and a subsequent digital signal demodulation module for receiving and demodulating signals received from different optical network units.

[0010] In a preferred embodiment, the optical network unit includes: The digital signal generation module is used to generate subcarrier modulated (SCM) signals through digital signal processing (DSP) technology. A digital-to-analog converter module is used to convert the subcarrier modulation signal output by the digital signal generation module into an analog electrical signal; The first laser is used to generate the optical carrier required to transmit the time-frequency division multiplexing signal. For different optical network units, the laser frequency of the first laser may be different, and the frequency is determined by the allocation of the access network as either the first subcarrier frequency or the second subcarrier frequency. The electro-optic modulation module is used to modulate the radio frequency carrier of the analog electrical signal output by the digital-to-analog converter and load the modulated radio frequency signal onto the optical carrier to realize the conversion of electrical signal to optical signal.

[0011] In a preferred embodiment, the optical transmission link includes: An optical coupling module is used to couple optical signals from different optical network units to achieve carrier aggregation of the first subcarrier optical signal and the second subcarrier optical signal; The optical fiber module is used to transmit the carrier-aggregated optical signal to the optical line terminal in a low-loss manner through a single-mode optical fiber.

[0012] In a preferred embodiment, the optical line terminal includes: The second laser is used to generate the first local oscillator light, which, together with the signal light after passing through the optical transmission link, is input into the coherent receiver to complete the reception of the first optical subcarrier of the optical signal. The third laser is used to generate the second local oscillator light, which is then input into the coherent receiver along with the signal light to complete the reception of the second optical subcarrier of the optical signal. The first optical attenuation module is used to adjust the power of the first local oscillator light, thereby adjusting the power of the first subcarrier in the received analog electrical signal; The second optical attenuation module is used to adjust the power of the second local oscillator light, thereby adjusting the power of the second subcarrier in the received analog electrical signal; The local oscillator coupling module is used to couple the first local oscillator light and the second local oscillator light after power adjustment through an optical coupler, and simultaneously input them into the integrated coherent receiver module. An integrated coherent receiving module is used to work with the first local oscillator and the second local oscillator to complete the photoelectric conversion of optical signals, converting the first photonic carrier and the second photonic carrier optical signals into corresponding electrical signals; An analog-to-digital converter module is used to convert the analog electrical signal output by the integrated coherent receiving module into a digital signal for subsequent digital signal processing. The digital signal demodulation module is used to demodulate the digital signal received by the optical line terminal at the current moment through digital signal processing technology.

[0013] In a preferred embodiment, the digital signal generation module includes: The quadrature amplitude modulation unit is used to map the input bit stream into constellation point symbols; Pilot insertion unit, used to insert pilot symbols before the mapped constellation point symbols, so as to enable the digital signal processing algorithm at the receiving end to converge quickly; The subcarrier modulation unit is used to convert the signal after the pilot is inserted into an SCM signal and to convert the phase information into intensity information, so that it can be transmitted through the electro-optic modulator. The upsampling unit is used to resample the signal from the sampling rate corresponding to its baud rate to the sampling rate corresponding to the digital-to-analog converter, so as to match the sampling rate of the digital-to-analog converter module.

[0014] In a preferred embodiment, the first laser serves as the light source for each optical network unit, and the first laser frequencies of different optical network units may be different. Specifically, the optical access network divides the optical network units into two groups, each group corresponding to a photonic carrier, and the center frequency of the photonic carrier is the first laser frequency of that optical network unit.

[0015] In a preferred embodiment, the two sets of optical network units correspond to a first optical carrier and a second optical carrier, respectively, and the frequency interval between the two optical carriers must satisfy the following: The frequency of the first photonic carrier is less than that of the second photonic carrier; The frequency difference between the first and second optical carriers must be greater than the bandwidth of the transmitted signal. Considering the frequency of the first subcarrier is... The frequency of the second subcarrier is The signal baud rate is The roll-off factor of the molding filter is , must meet ,and .

[0016] In a preferred embodiment, the electro-optic modulation module includes an electro-optic modulator, which may be a Mach-Zehnder modulator (MZM) or an electroabsorption modulator (EAM).

[0017] In a preferred embodiment, the second laser, in order to achieve heterodyne reception of the first subcarrier, generates a first local oscillator with a frequency of . .

[0018] In a preferred embodiment, the third laser, in order to achieve heterodyne reception of the second subcarrier, generates a second local oscillator with a frequency of [missing value]. .

[0019] In a preferred embodiment, the first optical attenuation module includes an optical attenuator. The value of the optical attenuator is adjusted according to the power of the first subcarrier. The greater the power of the first subcarrier, the greater the attenuation of the optical attenuator. The first optical attenuation module and the second optical attenuation module are adjusted simultaneously to ensure that the power of the two subcarriers in the received radio frequency signal is close.

[0020] In a preferred embodiment, the second optical attenuation module includes an optical attenuator. The value of the optical attenuator is adjusted according to the power of the second subcarrier. The greater the power of the second subcarrier, the greater the attenuation of the optical attenuator. The first and second optical attenuation modules are adjusted simultaneously to ensure that the power of the two subcarriers in the received radio frequency signal is close.

[0021] In a preferred embodiment, the integrated coherent receiving module has a bandwidth of It should be slightly larger than To ensure: An integrated coherent receiver module can receive signals from both subcarriers completely; DC leakage from other unsignaled optical network units, with a frequency close to or It can be filtered out by the low-pass effect of the bandwidth of the integrated coherent receiver module.

[0022] In a preferred embodiment, the digital signal demodulation module includes: The burst frame detection unit is used to determine the start time of the current frame signal and extract the signal of the current frame; The down-conversion unit is used to shift the digital signal of the first subcarrier or the second subcarrier from the intermediate frequency to the baseband for subsequent digital signal processing. The burst-mode coherent digital signal processing unit is used to perform digital signal processing on the baseband signal to recover signal impairments, including orthogonalization, dispersion compensation, clock recovery, channel equalization, frequency offset estimation, and phase recovery.

[0023] An equalizer is used to adaptively equalize baseband signals using a decision-guided least mean square (DD-LMS) algorithm to suppress inter-symbol interference introduced by the channel. The DD-LMS algorithm iteratively finds the optimal weight coefficients to minimize the mean square error.

[0024] In a preferred embodiment, the system further includes a bit error rate testing module for performing bit error rate testing on the complex signal output by the digital signal demodulation module to evaluate system performance.

[0025] This application also discloses an uplink optical access method based on time-frequency domain power equalization using dual local oscillators, comprising the following steps: In an optical network unit, the bit stream is converted into a complex quadrature amplitude modulation signal through constellation point mapping, and then the complex information of the complex signal is converted into intensity information through subcarrier modulation, so that the signal can be modulated by a single electro-optic modulator. In an optical network unit, the generated signal is converted into an electrical signal by a digital-to-analog converter and then modulated into an optical signal by an electro-optic modulator. Different optical network units may use one of two different frequency photonic carriers. In an optical transmission link, optical signals from various optical network units are coupled together to achieve carrier aggregation, and then transmitted to the optical line terminal via single-mode fiber. In an optical line terminal, two local oscillators are used to receive two subcarriers using the same integrated coherent receiver module, and the power of the two local oscillators is adjusted by an optical attenuation module. The higher the power of the subcarrier, the lower the power of its corresponding local oscillator. In an optical line terminal, optical signals are received by an integrated coherent receiver module and converted into electrical signals by an analog-to-digital converter. In the optical line terminal, the received signal is digitally processed to recover the signal, and the final received signal is then analyzed for bit errors.

[0026] The preceding text provides a general overview of the system architecture and method of this application for easy understanding. To further clarify the specific scope of protection of this application and to more systematically and rigorously define the core hardware modules and control logic in the aforementioned system and method, the following section, in conjunction with this application, will provide a more detailed and standardized description of the specific technical solution of the simplified coherent uplink optical access system and method based on dual local oscillators with time-frequency domain power equalization: This application discloses a simplified coherent uplink optical access system based on time-frequency domain power equalization using dual local oscillators, applicable to fiber optic access networks, comprising: Multiple optical network units, each of which includes an optical transmitter, is used to convert complex information of a complex signal into intensity information through subcarrier modulation, and to load the intensity information onto an optical carrier through a single electro-optic modulator to generate a subcarrier modulated optical signal; wherein different optical network units are assigned a first subcarrier frequency or a second subcarrier frequency, and transmit the first subcarrier optical signal or the second subcarrier optical signal according to the assigned subcarrier frequency; An optical transmission link is used to perform carrier aggregation of the first subcarrier optical signal and the second subcarrier optical signal from different optical network units through an optical coupler, and to transmit the carrier-aggregated optical signal to the optical line terminal through a single-mode optical fiber. Optical line terminals, including: The second and third lasers generate the first and second local oscillators, respectively; The first optical attenuation module and the second optical attenuation module independently adjust the power of the first local oscillator and the second local oscillator, respectively. The greater the power of the first subcarrier, the greater the attenuation of the first optical attenuation module; the greater the power of the second subcarrier, the greater the attenuation of the second optical attenuation module. The first optical attenuation module and the second optical attenuation module are adjusted simultaneously so that the power of the first subcarrier and the second subcarrier in the received electrical signal is similar, so as to achieve frequency domain power balance. The local oscillator coupling module is used to couple the first local oscillator light and the second local oscillator light after power adjustment; An integrated coherent receiving module is used to perform photoelectric conversion on the coupled local oscillator light and the carrier-aggregated optical signal, and to perform heterodyne reception on the first subcarrier optical signal and the second subcarrier optical signal respectively, converting them into corresponding electrical signals. An analog-to-digital converter module is used to convert the analog electrical signal output by the integrated coherent receiving module into a digital signal; The digital signal demodulation module is used to demodulate the digital signal and recover the signal transmitted by each optical network unit.

[0027] In a preferred embodiment, the optical network unit includes: A digital signal generation module is used to generate subcarrier modulated signals using digital signal processing technology; A digital-to-analog converter module is used to convert the subcarrier modulation signal output by the digital signal generation module into an analog electrical signal; A first laser is used to generate an optical carrier, the laser frequency of which is allocated to either the first subcarrier frequency or the second subcarrier frequency according to the optical access network. An electro-optic modulation module is used to load the analog electrical signal output by the digital-to-analog converter onto the optical carrier generated by the first laser, thereby realizing the conversion of electrical signal to optical signal and outputting the subcarrier modulated optical signal.

[0028] In a preferred embodiment, the optical transmission link includes: An optical coupling module is used to couple the first subcarrier optical signals and the second subcarrier optical signals from different optical network units to achieve carrier aggregation; An optical fiber module is used to transmit the carrier-aggregated optical signal to the optical line terminal via a single-mode optical fiber.

[0029] In a preferred embodiment, the digital signal generation module includes: The quadrature amplitude modulation unit is used to map the input bit stream into constellation point symbols; Pilot insertion unit, used to insert pilot symbols before the mapped constellation point symbols, so as to enable the fast convergence of the digital signal processing algorithm at the receiving end; The subcarrier modulation unit is used to convert the signal after the pilot is inserted into a subcarrier modulation signal, convert phase information into intensity information, so that the signal can be modulated and transmitted through the single electro-optic modulator; The upsampling unit is used to resample the signal from the sampling rate corresponding to its baud rate to the sampling rate corresponding to the digital-to-analog conversion module.

[0030] In a preferred embodiment, the optical access network divides the plurality of optical network units into two groups, with the first group of optical network units corresponding to a first subcarrier frequency. The second group of optical network units corresponds to the second subcarrier frequency. ;in, ,and , For the signal baud rate, This is the roll-off factor for the shaped filter.

[0031] In a preferred embodiment, the frequency of the first local oscillator light generated by the second laser is: The frequency of the second local oscillator light generated by the third laser is This results in heterodyne reception relationships being formed between the first local oscillator and the first subcarrier optical signal, and between the second local oscillator and the second subcarrier optical signal.

[0032] In a preferred embodiment, the bandwidth of the integrated coherent receiving module is... Slightly larger , so that: The integrated coherent receiving module is capable of completely receiving the signals of the first subcarrier and the second subcarrier. Optical DC leakage from an optical network unit in a non-transmitting state is shifted to a higher frequency after being received by the heterodyne receiver, falling outside the bandwidth of the integrated coherent receiver module, and is filtered out by the low-pass effect of the integrated coherent receiver module.

[0033] In a preferred embodiment, the first optical attenuation module and the second optical attenuation module are further configured to adjust the power of the first local oscillator and the second local oscillator simultaneously according to the overall power of the current frame signal, so as to control the power of the electrical signal output by the integrated coherent receiving module within the effective quantization range of the analog-to-digital conversion module, thereby achieving time-domain power equalization.

[0034] In a preferred embodiment, the electro-optic modulator in the optical network unit is a Mach-Zehnder modulator or an electroabsorption modulator.

[0035] In a preferred embodiment, the digital signal demodulation module includes: The burst frame detection unit is used to determine the start time of the current frame signal and extract the signal of the current frame; The downconversion unit is used to shift the digital signal of the first subcarrier or the second subcarrier from the intermediate frequency to the baseband. The burst-mode coherent digital signal processing unit is used to perform digital signal processing on the baseband signal to recover signal impairments, including orthogonalization, dispersion compensation, clock recovery, channel equalization, frequency offset estimation, and phase recovery. An equalizer is used to adaptively equalize baseband signals using a decision-guided least mean square algorithm to suppress inter-symbol interference introduced by the channel.

[0036] In a preferred embodiment, the system further includes a bit error rate testing module for performing bit error rate testing on the signal output by the digital signal demodulation module to evaluate system performance.

[0037] This application also discloses a simplified coherent uplink optical access method based on time-frequency domain power equalization using dual local oscillators, comprising the following steps: S1: In an optical network unit, the bit stream is converted into an orthogonal amplitude modulation signal through constellation point mapping, and then the complex information of the orthogonal amplitude modulation signal is converted into intensity information through subcarrier modulation to generate a subcarrier modulation signal; wherein, different optical network units use a first subcarrier frequency or a second subcarrier frequency according to the allocated subcarrier frequency; S2: In the optical network unit, the subcarrier modulation signal generated in step S1 is converted into an analog electrical signal by a digital-to-analog converter, and the analog electrical signal is loaded onto the optical carrier by a single electro-optic modulator to generate a subcarrier modulated optical signal; S3: In the optical transmission link, the subcarrier modulated optical signals from each optical network unit are carrier aggregated through an optical coupler, and the carrier aggregated optical signals are transmitted to the optical line terminal through a single-mode optical fiber. S4: In the optical line terminal, a second laser and a third laser are used to generate a first local oscillator (LOO) beam and a second LEO beam, respectively, which are used to perform heterodyne reception of the first subcarrier optical signal and the second subcarrier optical signal transmitted in step S3. According to the power of the first subcarrier and the second subcarrier, the power of the first LEO beam and the second LEO beam are independently adjusted by the first optical attenuation module and the second optical attenuation module, respectively. The higher the subcarrier power, the greater the attenuation of the corresponding LEO beam, so that the power of the two subcarriers in the received electrical signal is similar, thereby achieving frequency domain power equalization. The first LEO beam and the second LEO beam with adjusted power are coupled through a local oscillator coupling module. S5: In the optical line terminal, the local oscillator light coupled in step S4 and the optical signal after carrier aggregation in step S3 are input together into the integrated coherent receiving module for photoelectric conversion. The first subcarrier optical signal and the second subcarrier optical signal are heterodyne received respectively to obtain an electrical signal containing the first subcarrier and the second subcarrier. The electrical signal is then converted into a digital signal by an analog-to-digital converter. S6: In the optical line terminal, the digital signal obtained in step S5 is subjected to burst mode digital signal processing, including burst frame detection, downconversion, signal impairment recovery and adaptive equalization, and the signal transmitted by each optical network unit is demodulated and recovered.

[0038] In a preferred embodiment, in step S4, the power of the first local oscillator and the second local oscillator are adjusted simultaneously according to the overall power of the current frame signal, so as to control the power of the electrical signal output by the integrated coherent receiving module within the effective quantization range of the analog-to-digital converter, thereby achieving time-domain power equalization.

[0039] In a preferred embodiment, the frequency of the first subcarrier is: The frequency of the second subcarrier is The signal baud rate is The roll-off factor of the molding filter is ,in ,and In step S4, the frequency of the first local oscillator light generated by the second laser is... The frequency of the second local oscillator light generated by the third laser is This results in heterodyne reception relationships being formed between the first local oscillator and the first subcarrier optical signal, and between the second local oscillator and the second subcarrier optical signal.

[0040] In a preferred embodiment, in step S5, the bandwidth of the integrated coherent receiving module is slightly greater than... This causes optical DC leakage from optical network units in a non-transmitting state to be shifted to a higher frequency after being received by the heterodyne receiver, falling outside the bandwidth of the integrated coherent receiver module, and thus filtered out by the low-pass effect of the integrated coherent receiver module.

[0041] The embodiments of this application have the following technical effects: 1. Time-frequency division multiplexing enables the rational allocation of time and subcarrier frequency. This application employs a time-frequency division multiplexing (TFD) scheme. By dividing time and frequency domain resources into multiple sub-channels for multiplexing, the spectrum utilization efficiency is improved. Unlike traditional frequency division multiplexing and time division multiplexing, TFD can achieve more flexible resource scheduling within the same time period through precise control of time and frequency allocation, adapting to higher data transmission demands.

[0042] 2. Simplify optical network units and reduce costs. This application proposes a method in optical network units (ONUs) to convert complex signals into intensity information using subcarrier modulation (SCM), which can then be converted into optical signals by a single electro-optic modulator. Compared to traditional coherent transmission schemes, each ONU can simplify the IQ modulator to a single electro-optic modulator. Since there are many ONUs and they are cost-sensitive, this simplification helps to reduce costs.

[0043] In the proposed solution, compared to the traditional coherent receiving solution, an additional laser and two optical attenuators are added at the optical line terminal. Considering the low cost of optical attenuators and the fact that the optical line terminal is not cost-sensitive, this complexity is acceptable.

[0044] 3. Improve the overall dynamic range of the system. This application proposes adding an optical attenuation module after the local oscillator light at the optical line terminal to control the local oscillator power input to the integrated coherent receiver module. Since each frame of signal may come from different optical network units and the power may be different, too low signal power may cause excessive quantization noise in the subsequent analog-to-digital conversion module, while too high signal power may cause clipping in the subsequent digital-to-analog conversion module. By controlling the local oscillator power, the electrical signal power output by the integrated coherent receiver module can be controlled within a suitable range, so that signals with a wider range of received optical power can be received well, thereby improving the overall dynamic range of the system.

[0045] 4. Improve the system's tolerance to subcarrier power differences. This application proposes using two local oscillators with different powers in the optical line terminal to receive two subcarriers respectively. The two local oscillators control the power of the electrical signals of the two subcarriers respectively, so that the power of the two subcarriers in the electrical signal output by the integrated coherent receiver module is comparable. In the analog-to-digital conversion module, if the power difference between the subcarriers of the electrical signal is large, the lower-power subcarrier will exhibit greater quantization noise, and the effect of the slight delay of the IQ channel will be amplified. By using two local oscillators to perform heterodyne reception on the two subcarriers respectively and adjusting the optical power of the two local oscillators respectively, the power of the two subcarriers can be made similar, thereby improving the system's tolerance to subcarrier power differences.

[0046] 5. Improve the system's tolerance to DC leakage. This application proposes using a bandwidth slightly greater than [missing information] in the optical line terminal. The coherent receiving module; optical network units in non-transmitting state may generate a certain amount of optical DC leakage. This leakage optical power will be superimposed on the uplink signal at the optical line terminal, significantly reducing the signal-to-noise ratio of the effective signal; in this application, the optical DC leakage is shifted to a high frequency by using two subcarriers for heterodyne reception, and then the DC leakage is filtered out by the low-pass effect of the limited bandwidth of the coherent receiving module through the heterodyne reception method, thereby improving the system's tolerance to DC leakage.

[0047] In summary, this application significantly improves the performance and practicality of fiber optic access networks by using time-frequency division multiplexing, simplifying optical network units, increasing the overall dynamic range of the system, improving the system's tolerance to subcarrier power differences, and improving the system's tolerance to DC leakage. It has significant application value and broad market prospects.

[0048] The aforementioned five advantages summarize the direct gains brought by this application from a system-level macroscopic application perspective. Building upon this, to further clarify and support the specific limitations in this application and reveal the inherent logic of each microscopic technical feature in actual operation, the following will delve into the core physical nodes of the system (i.e., optical network units, optical transmission links, and optical line terminals) to provide a more specific supplementary explanation of the deeper technical effects resulting from the combination of specific hardware configurations and signal processing steps in the above technical solutions: The technical effects brought by the above technical solutions can be understood from the following levels. First, on the optical network unit 101 side, the subcarrier modulation unit in the digital signal generation module converts the complex information of the quadrature amplitude modulation signal into intensity information, enabling the electro-optic modulation module to complete the conversion from electrical signal to optical signal using only a single electro-optic modulator (such as a Mach-Zehnder modulator or an electroabsorption modulator), without the need for the complex and costly IQ modulators used in traditional coherent transmission schemes. In application scenarios with a large number of optical network units in optical access networks, the cumulative hardware cost savings at the system level from eliminating one IQ modulator per optical network unit are considerable. Meanwhile, the pilot symbols inserted by the pilot insertion unit before the constellation point symbols provide a reference benchmark for various algorithms in the burst mode coherent digital signal processing unit at the receiver, which helps to achieve rapid convergence of channel estimation and carrier recovery algorithms in the burst initiation stage of the frame signal.

[0049] In the optical transmission link 102, the optical coupling modules will be carried on the first subcarrier frequency respectively. Second subcarrier frequency The optical signals are carrier aggregated, enabling uplink signals from multiple optical network units to share the same standard single-mode fiber for transmission to the optical line terminal 103. The two subcarrier frequencies satisfy... The spacing condition ensures that the two subcarriers do not overlap in the frequency domain after carrier aggregation, providing a frequency domain basis for the separate reception and independent processing of the two subcarriers in the subsequent optical line terminal.

[0050] In the optical line terminal 103, the second laser and the third laser generate frequencies of [frequency values ​​to be filled in]. and The first and second local oscillators are configured such that they form heterodyne reception relationships with the corresponding subcarriers from the low-frequency and high-frequency sides of the signal spectrum, respectively. The direct technical effect of this frequency configuration is that, after photoelectric conversion by the integrated coherent receiver module, the first and second subcarrier signals are respectively shifted to the corresponding frequencies. The positive and negative intermediate frequency positions of the center (e.g.) Figure 3 As shown in the spectrum of the 303 electrical signal, in the digital signal demodulation module, the signal can be moved to the baseband for independent processing through the downconversion unit.

[0051] The technical effect of the independent power adjustment of the first and second local oscillator beams by the first and second optical attenuation modules is reflected in frequency domain power equalization. Since the beat frequency current amplitude in coherent reception is proportional to the square root of the product of the signal optical power and the local oscillator beam power, when the power of the first subcarrier optical signal... Power of the second subcarrier optical signal When the optical path losses are unequal, the corresponding local oscillator power is adjusted in the opposite direction. and This allows the power of the two subcarriers in the electrical signal output by the integrated coherent receiver module to tend to be consistent. This frequency domain power equalization mechanism effectively alleviates the problem of relatively increased quantization noise of weak subcarriers caused by excessive differences in the power of the factor carriers in the analog-to-digital conversion module, thereby improving the system's tolerance to subcarrier power differences.

[0052] In the time domain, the first and second optical attenuation modules are also used to simultaneously adjust the power of the first and second local oscillators based on the overall power level of the current frame signal, so as to control the power of the electrical signal output by the integrated coherent receiver module within the effective quantization range of the analog-to-digital converter. When the frame signal power from the near-end low-loss optical network unit is high, the attenuation of the two optical attenuation modules is increased to prevent clipping distortion in the analog-to-digital converter; when the frame signal power from the far-end high-loss optical network unit is low, the attenuation of the two optical attenuation modules is decreased to reduce the relative impact of quantization noise. This time-domain power equalization mechanism shares the same set of dual local oscillator and dual optical attenuation module hardware with the frequency-domain power equalization mechanism, and the two work together to significantly extend the overall dynamic range of the receiver.

[0053] Bandwidth of integrated coherent receiver module Designed to be slightly larger This bandwidth design, combined with the aforementioned dual-local oscillator outer heterodyne receiver configuration, effectively suppresses optical DC leakage. Optical DC leakage in non-transmitting optical network units, caused by the finite extinction ratio of the electro-optic modulator or bias point drift, has a frequency close to the subcarrier frequency. or After being received by heterodyne, the beat frequency was moved to a position much higher than that of the target frequency. The frequency position falls outside the passband of the integrated coherent receiver module and is filtered out by its own low-pass characteristics (such as...). Figure 3 (The electrical signal spectrum of the 303 is shown in the figure). This DC leakage suppression mechanism does not require the introduction of additional filtering hardware or digital signal processing algorithms, and effectively improves the system's tolerance to cumulative optical DC leakage of multi-optical network units without increasing system complexity.

[0054] In the digital signal demodulation module, the burst frame detection unit enables the optical line terminal to correctly identify the boundaries of frame signals from different optical network units, adapting to the time-discontinuous arrival of frame signals in uplink burst mode transmission. The burst mode coherent digital signal processing unit sequentially performs operations such as orthogonalization, dispersion compensation, clock recovery, channel equalization, frequency offset estimation, and phase recovery, gradually recovering the various damages suffered by the signal during fiber optic transmission and photoelectric conversion. The decision-guided least mean square algorithm-driven equalizer iteratively adjusts the weight coefficients to minimize the mean square error, adaptively compensating for residual inter-symbol interference. An optional bit error rate (BER) testing module provides quantitative data for evaluating the system's performance under different operating conditions.

[0055] In summary, this application simplifies the optical network unit structure by employing subcarrier modulation at the transmitter, achieves time-frequency domain dual-dimensional power equalization through independent power control of dual local oscillators at the receiver, and suppresses optical DC leakage by utilizing the synergistic cooperation of heterodyne reception and a coherent receiver module with limited bandwidth. These three elements are organically combined in a unified system architecture, enabling the uplink time-frequency division multiplexing coherent optical access system to simultaneously achieve a large dynamic range, high subcarrier power difference tolerance, and high optical DC leakage tolerance under controlled transmitter costs. This effectively addresses the uplink optical access transmission requirements under complex power distribution conditions.

[0056] The specification of this application contains numerous technical features distributed across various technical solutions. Listing all possible combinations of these technical features (i.e., technical solutions) would make the specification excessively lengthy. To avoid this problem, the various technical features disclosed in the above-described invention, the various technical features disclosed in the following embodiments and examples, and the various technical features disclosed in the accompanying drawings can be freely combined to form various new technical solutions (all of which are considered to have been described in this specification), unless such a combination of technical features is technically infeasible. For example, one example discloses feature A+B+C, and another example discloses feature A+B+D+E. Features C and D are equivalent technical means that serve the same function, and technically only one needs to be used; they cannot be used simultaneously. Feature E can technically be combined with feature C. Therefore, the solution A+B+C+D should not be considered as described because it is technically infeasible, while the solution A+B+C+E should be considered as described. Attached Figure Description

[0057] Figure 1 This is a block diagram of a simplified coherent uplink optical access system based on time-frequency domain power equalization using dual local oscillators, according to the first embodiment of this application.

[0058] Figure 2 This is a flowchart illustrating a simplified coherent uplink optical access method based on time-frequency domain power equalization using dual local oscillators, according to a second embodiment of this application.

[0059] Figure 3 This is a schematic diagram of the spectral distribution in each module of the time-frequency domain power equalization simplified coherent uplink optical access system based on dual local oscillators according to the first embodiment of this application.

[0060] Figure 4 It is the signal spectrum of the first subcarrier and the second subcarrier transmitted according to the first embodiment of this application.

[0061] Figure 5 a and 5b are the spectra of the electrical signals received using one local oscillator and two power-adjustable local oscillator receivers, respectively, according to the first embodiment of this application.

[0062] Figure 6 According to the first embodiment of this application, when using one local oscillator and two local oscillators, the time-frequency domain power equalization based on dual local oscillators simplifies the receiver sensitivity performance of the coherent uplink optical access system.

[0063] Figure 7This is a graph showing the relationship between receiver sensitivity performance and local oscillator power in a simplified coherent uplink optical access system based on time-frequency domain power equalization using one or two local oscillators, according to the first embodiment of this application.

[0064] Figure 8 According to the first embodiment of this application, when using one local oscillator and two power-tunable local oscillators, the time-frequency domain power equalization based on dual local oscillators simplifies the receiver dynamic range of the coherent uplink optical access system.

[0065] Figure 9a According to the first embodiment of this application, when using one local oscillator and two power-tunable local oscillators, the time-frequency domain power equalization based on dual local oscillators simplifies the tolerance of the coherent uplink optical access system to subcarrier power difference. Figure 9b yes Figure 9a When two power-adjustable local oscillators are used, the local oscillator power used is the difference in power between different subcarriers.

[0066] Figure 10 This refers to the tolerance of optical DC leakage in the simplified coherent uplink optical access system based on time-frequency domain power equalization using dual local oscillators according to the first embodiment of this application. Detailed Implementation

[0067] In the following description, many technical details are presented to help the reader better understand this application. However, those skilled in the art will understand that the technical solutions claimed in this application can be implemented even without these technical details and various variations and modifications based on the following embodiments.

[0068] Explanation of some concepts: Time-Frequency Division Multiplexing (TFDM) is a signal multiplexing technique that improves signal transmission efficiency by simultaneously utilizing both time and frequency dimensions to divide resources. In TFDM, signal transmission is divided into multiple sub-channels, each with specific resource allocations in time and frequency. This method effectively utilizes spectrum and time resources in communication systems, increasing system capacity and interference immunity.

[0069] Carrier aggregation: In the uplink time-frequency division multiplexing (TFDM) access scenario described in this application, it refers to the process of optically coupling single-carrier optical signals from multiple optical network units through an optical coupler, combining uplink signals carried on different optical subcarriers (or different subcarrier frequencies) in the optical domain. Through carrier aggregation, multiple independent uplink optical signals can be transmitted simultaneously in the same optical fiber, achieving the sharing and multiplexing of optical fiber transmission resources while maintaining the frequency domain differentiation of each carrier. This facilitates the use of coherent reception and digital signal processing technologies by the optical line terminal to separately recover and demodulate the uplink signals on different carriers.

[0070] Receiver sensitivity refers to the minimum power of the input signal that a receiver can effectively receive and correctly demodulate. In other words, receiver sensitivity represents the power level of the weakest signal that a receiver can successfully receive and process under conditions of a sufficiently high signal-to-noise ratio (SNR). Receiver sensitivity is usually expressed in decibels (dBmW), representing the lowest signal power that the receiver can receive.

[0071] Dynamic range refers to the range of input signal power that a receiver can operate normally while meeting predetermined performance specifications (such as bit error rate, block error rate, or signal-to-noise ratio requirements). Dynamic range is determined by the minimum input signal power the receiver can correctly receive and the maximum input signal power it can withstand without saturation, clipping, or severe distortion. It is usually expressed as the logarithm of the ratio of these two power levels (decibels, dB). A larger dynamic range indicates that the receiver can simultaneously handle weaker and stronger signals within the same system.

[0072] Optical DC leakage refers to a non-negligible constant optical power component that exists in optical channels that do not carry effective data signals or should be in an "off" state during optical intensity modulation and optical signal transmission, due to factors such as the modulator's limited extinction ratio, bias point drift, or device imperfections. This DC component typically manifests as optical "background light" or bias power unrelated to the useful signal. In multi-user shared passive optical networks, optical DC leakage from different optical network units can superimpose at the optical line termination port, raising the equivalent optical power baseline at the receiver and reducing the optical signal-to-noise ratio of the effective signal.

[0073] Single-mode fiber (SSMF): This typically refers to fiber optic cable conforming to the ITU-T G.652 standard. This standard defines the geometry and optical properties of single-mode fiber, aiming to ensure its compatibility and performance in different communication systems. Its core and cladding have a small refractive index difference, guaranteeing propagation in a single mode. It exhibits low optical attenuation and dispersion, supporting high-speed, long-distance fiber optic transmission.

[0074] An integrated coherent receiver (ICR) is a device used in optical communication systems to receive and demodulate coherent signals. The core principle of an ICR is based on coherent detection, which involves mixing the received signal with a local oscillator (LO) signal to generate in-phase and quadrature-phase components. By detecting these two components, the amplitude and phase information of the transmitted signal can be effectively recovered, thereby demodulating the original data. This technique can recover both amplitude and phase information simultaneously, thus offering higher receiver sensitivity and better noise immunity.

[0075] An equalizer is a signal processing device or algorithm used to compensate for distortion and inter-symbol interference (ISI) introduced into a channel. In this application, the equalizer adaptively adjusts in conjunction with a decision-guided least mean square (DD-LMS) algorithm to minimize errors and restore the amplitude and phase information of the signal.

[0076] The following is a brief summary of some of the innovative aspects of this application: Passive Optical Networks (PONs) are a cost-effective and scalable optical access network solution that meets the ever-increasing bandwidth demands of modern access networks. Coherent PONs stand out in high-capacity applications due to their superior receiver sensitivity and higher power budget. Time-frequency division multiplexing (TFD) coherent PONs not only provide low-latency connections but also support more dynamic bandwidth allocation, making them a promising candidate for next-generation optical access networks. However, in the uplink, power equalization remains a challenge. Differences in signal power in the time domain can test the receiver's dynamic range, while differences in subcarrier power in the frequency domain can degrade the system. Furthermore, optical DC leakage in the optical network units carrying the signal reduces the effective signal-to-noise ratio, which is another issue that needs to be addressed.

[0077] To address the aforementioned problems, the inventors of this application, after in-depth research, propose a simplified coherent uplink optical access system and method based on time-frequency domain power equalization using dual local oscillators. Specifically, subcarrier modulation (SCM) is used in the optical network unit transmitter, allowing optical signals to be transmitted via a single electro-optic modulator. In the optical line terminal, two local oscillators with different powers are used to receive the two subcarriers respectively. Each local oscillator controls the power of the two subcarrier electrical signals, ensuring that the power of the two subcarriers in the output electrical signal of the integrated coherent receiver module is equivalent. Simultaneously, by using heterodyne reception of the two subcarriers, optical DC leakage is shifted to a higher frequency. Then, through the limited bandwidth of the coherent receiver module, a low-pass effect is exhibited, filtering out DC leakage and thus improving the system's tolerance to DC leakage.

[0078] Furthermore, a key feature of this application is the use of two locally oscillating optical fibers with adjustable power to receive the two subcarriers of the time-frequency division multiplexed signal. By controlling the power of the local oscillators, the power of the electrical signal output by the integrated coherent receiver module can be controlled within a suitable range in the time domain, ensuring good reception of signals with a wider range of optical power and improving the overall dynamic range of the system. Furthermore, controlling the power of the two local oscillators separately allows for similar power levels of the two subcarriers, increasing the system's tolerance to power differences between subcarriers.

[0079] Another key innovation lies in proposing the use of a bandwidth slightly greater than [missing information] in optical line terminals. The coherent receiver module uses a heterodyne reception method with two subcarriers to shift the optical DC leakage to a high frequency. Then, through the limited bandwidth of the coherent receiver module, it exhibits a low-pass effect to filter out the DC leakage, thereby improving the system's tolerance to DC leakage.

[0080] Furthermore, this application represents an improvement to uplink time-frequency division multiplexing coherent access systems. SCM modulation simplifies the transmitter and prevents optical DC leakage from falling into the signal band. Two adjustable-power local oscillators heterodyne-receive subcarrier signals, improving the receiver's dynamic range and tolerance to subcarrier power differences. Simultaneously, the optical DC leakage is flipped to a higher frequency, filtered out by the low-pass effect of the receiver's bandwidth. The resulting time-frequency division multiplexing uplink optical access network has a large dynamic range and can tolerate higher subcarrier power differences and higher optical DC leakage.

[0081] To make the objectives, technical solutions, and advantages of this application clearer, the embodiments of this application will be described in further detail below with reference to the accompanying drawings.

[0082] Firstly, this application proposes a simplified coherent uplink optical access system based on time-frequency domain power equalization using dual local oscillators, the structure of which is as follows: Figure 1As shown. The system includes: multiple optical network units, optical transmission links, and an optical line terminal. A detailed description follows: Multiple optical network units 101 are provided, each of which includes an optical transmitter for generating and transmitting a first subcarrier optical signal or a second subcarrier optical signal. Different optical network units are assigned different optical subcarriers, and the optical network units transmit the first subcarrier optical signal and the second subcarrier optical signal according to the assigned optical subcarriers. Optical transmission link 102 is used to couple optical signals from different optical network units through an optical coupler, perform carrier aggregation on the optical signals of the first subcarrier and the second subcarrier, and transmit them through a standard single-mode optical fiber, so as to transmit the optical signals of each optical network unit to the optical line terminal. An optical line terminal 103 includes an integrated coherent receiver and a subsequent digital signal demodulation module for receiving and demodulating signals received from different optical network units.

[0083] Through the above system, this application realizes a simplified coherent uplink optical access network with time-frequency domain power equalization.

[0084] Secondly, in this application, the optical network unit, such as Figure 1 As shown in 101, it includes: The digital signal generation module is used to generate subcarrier modulated (SCM) signals through digital signal processing (DSP) technology. A digital-to-analog converter module is used to convert the subcarrier modulation signal output by the digital signal generation module into an analog electrical signal; The first laser is used to generate the optical carrier required to transmit the time-frequency division multiplexing signal. For different optical network units, the laser frequency of the first laser may be different, and the frequency is determined by the allocation of the access network as either the first subcarrier frequency or the second subcarrier frequency. The electro-optic modulation module is used to modulate the radio frequency carrier of the analog electrical signal output by the digital-to-analog converter, and load the modulated radio frequency signal onto the optical carrier to realize the conversion of electrical signal to optical signal. The spectrum diagram of the modulated optical signal is shown in the figure below. Figure 3 As shown in Figure 301.

[0085] Thirdly, in this application, the optical transmission link, such as Figure 1 As shown in Figure 102, it includes: The optical coupling module is used to couple optical signals from different optical network units to achieve carrier aggregation of the first subcarrier optical signal and the second subcarrier optical signal. The spectrum after carrier aggregation is as follows: Figure 3 As shown in Figure 302; The optical fiber module is used to transmit the carrier-aggregated optical signal to the optical line terminal in a low-loss manner through a single-mode optical fiber.

[0086] Fourthly, in this application, the optical line terminal, such as... Figure 1 As shown in Figure 103, it includes: The second laser is used to generate the first local oscillator light, which, together with the signal light after passing through the optical transmission link, is input into the coherent receiver to complete the reception of the first optical subcarrier of the optical signal. The third laser is used to generate the second local oscillator light, which is then input into the coherent receiver along with the signal light to complete the reception of the second optical subcarrier of the optical signal. The first optical attenuation module is used to adjust the power of the first local oscillator light, thereby adjusting the power of the first subcarrier in the received analog electrical signal; The second optical attenuation module is used to adjust the power of the second local oscillator light, thereby adjusting the power of the second subcarrier in the received analog electrical signal; The local oscillator coupling module is used to couple the power-adjusted first and second local oscillator beams via an optical coupler, and simultaneously inputs them to the integrated coherent receiver module. A schematic diagram illustrating the relationship between the coupled local oscillator beam and the signal beam is shown below. Figure 3 The spectrum of the 303 optical signal is shown below; An integrated coherent receiver module, used together with the first and second local oscillators, performs photoelectric conversion of optical signals, converting the first and second photonic carrier optical signals into corresponding electrical signals. The spectrum of the electrical signals is as follows: Figure 3 The electrical signal spectrum of the 303 is shown below; An analog-to-digital converter module is used to convert the analog electrical signal output by the integrated coherent receiving module into a digital signal for subsequent digital signal processing. The digital signal demodulation module is used to demodulate the digital signal received by the optical line terminal at the current moment through digital signal processing technology.

[0087] Fifthly, in this application, Figure 1 The digital signal generation module described in 101 includes: The quadrature amplitude modulation unit is used to map the input bit stream into constellation point symbols; Pilot insertion unit, used to insert pilot symbols before the mapped constellation point symbols, so as to enable the digital signal processing algorithm at the receiving end to converge quickly; The subcarrier modulation unit is used to convert the signal after the pilot is inserted into an SCM signal and to convert the phase information into intensity information, so that it can be transmitted through the electro-optic modulator. The upsampling unit is used to resample the signal from the sampling rate corresponding to its baud rate to the sampling rate corresponding to the digital-to-analog converter, so as to match the sampling rate of the digital-to-analog converter module.

[0088] Sixthly, in this application, Figure 1 The first laser described in section 101 serves as the light source for each optical network unit, and the frequencies of the first lasers in different optical network units may differ. Specifically, the optical access network divides the optical network units into two groups, each group corresponding to one optical subcarrier. The center frequency of the optical subcarrier is the frequency of the first laser of that optical network unit. The relationship between the two subcarriers must satisfy: The frequency of the first photonic carrier is less than that of the second photonic carrier, such as Figure 3 As shown in Figure 301; The frequency difference between the first and second optical carriers must be greater than the bandwidth of the transmitted signal. Considering the frequency of the first subcarrier is... The frequency of the second subcarrier is The signal baud rate is The roll-off factor of the molding filter is , must meet ,and ,by Figure 3 Taking the 301 as an example, the frequency difference between the two subcarriers is approximately .

[0089] Seventhly, in the aforementioned optical line terminal, the frequencies and powers of the second and third lasers must satisfy a certain relationship, such as... Figure 3 As shown in 303: The second laser, in order to achieve heterodyne reception of the first subcarrier, generates a first local oscillator with a frequency of [frequency to be filled in]. The third laser, in order to achieve heterodyne reception of the second subcarrier, generates a second local oscillator with a frequency of [frequency missing]. ; like Figure 3 The spectrum of the 303 optical signal is shown, where the first local oscillator power and the second local oscillator power represent the power of the laser after passing through the optical attenuation module. The value of the first optical attenuator is adjusted according to the power of the first subcarrier; the higher the power of the first subcarrier, the greater the attenuation of the first optical attenuator, and the lower the power of the first local oscillator. Similarly, the value of the second optical attenuator is adjusted according to the power of the second subcarrier; the higher the power of the second subcarrier, the greater the attenuation of the second optical attenuator. The first and second optical attenuation modules are adjusted simultaneously to ensure that the powers of the two subcarriers in the received RF signal are close. Figure 3 Taking the 303 optical signal spectrum as an example, the first subcarrier has a high power and the second subcarrier has a low power, which corresponds to the first local oscillator having a low power and the second local oscillator having a high power.

[0090] Eighth aspect, Figure 1 The integrated coherent receiver module in the 101 has a bandwidth of It should be slightly larger than To ensure: An integrated coherent receiver module can receive signals from both subcarriers completely; DC leakage from other unsignaled optical network units, with a frequency close to or It can be filtered out by the low-pass effect of the bandwidth of the integrated coherent receiver module.

[0091] Ninth aspect, Figure 1 The integrated coherent receiver module in the 101 series has a local oscillator input consisting of the local oscillator light coupled from the first and second local oscillator lights. Considering... for Then the optical field of the local oscillator after coupling can be expressed as: in, and These represent the power of the first and second local oscillators, respectively. Represents the imaginary unit, satisfying ; It is a time variable; and These represent the phases of the first and second local oscillators, respectively. After integrating the coherent receiving module, the photocurrents of the in-phase and quadrature components can be expressed as: in, , These are the powers of the first subcarrier and the second subcarrier, respectively. The responsivity of a photodetector in an integrated coherent receiver module is usually expressed in amperes per watt (A / W). Responsivity indicates how much current a photodetector can generate under a given incident light power, and it depends on factors such as detector material, wavelength, and temperature. , , , These represent the phase differences between the first subcarrier and the first local oscillator, the second subcarrier and the second local oscillator, the first subcarrier and the second local oscillator, and the second subcarrier and the first local oscillator, respectively.

[0092] Considering that the bandwidth of the integrated coherent receiver module is only slightly greater than ,frequency It will fall outside the receiver bandwidth, therefore the photocurrent can be simplified to: This formula shows that the scheme can achieve power equalization. For example, when the instantaneous subcarrier power satisfies... When selecting local oscillator power This makes the received subcarrier power and This balances the power distribution, thereby mitigating the sensitivity loss caused by subcarrier power differences and achieving frequency domain power equalization. When both subcarrier powers are simultaneously high, the power is simultaneously reduced... and This prevents transimpedance amplifier saturation and analog-to-digital converter clipping, thus achieving time-domain power equalization. Furthermore, heterodyne detection converts the DC component to a high-frequency intermediate frequency (IF), which is attenuated by the limited receiver bandwidth, thus providing high tolerance for DC leakage.

[0093] In a tenth aspect, the digital signal demodulation module 103 includes: The down-conversion unit is used to shift the digital signal of the first subcarrier or the second subcarrier from the intermediate frequency to the baseband for subsequent digital signal processing. The burst-mode coherent digital signal processing unit is used to perform digital signal processing on the baseband signal to recover signal impairments, including orthogonalization, dispersion compensation, clock recovery, channel equalization, frequency offset estimation, and phase recovery.

[0094] An equalizer is used to adaptively equalize baseband signals using a decision-guided least mean square (DD-LMS) algorithm to suppress inter-symbol interference introduced by the channel. The DD-LMS algorithm iteratively finds the optimal weight coefficients to minimize the mean square error.

[0095] Eleventhly, this application also discloses a simplified coherent uplink optical access method based on time-frequency domain power equalization using dual local oscillators, such as... Figure 2 As shown, it includes the following steps: Step 201: In the optical network unit, the bit stream is constellation point mapping and subcarrier modulation is performed, and it is resampled to the same sampling rate as the digital-to-analog converter. Step 202: In the optical network unit, the generated signal is converted into an electrical signal by a digital-to-analog converter and then modulated into an optical signal by an electro-optic modulator. Step 203: In the optical transmission link, optical signals from different optical network units are coupled to achieve carrier aggregation, and then transmitted through the optical fiber link; Step 204: In the optical line terminal, two local oscillator beams are used. The power of the two local oscillator beams is adjusted according to the power of the two subcarriers, and the two local oscillator beams are coupled together. Step 205: In the optical line terminal, the optical signal is received by an integrated coherent receiver, converted into an electrical signal, and then converted into a digital signal by an analog-to-digital converter. Step 206: In the optical line terminal, the signal is demodulated and recovered through burst mode digital signal processing to obtain the final signal and perform bit error analysis.

[0096] This application employs time-frequency division multiplexing technology, combined with the proposed dual local oscillator time-frequency domain power equalization technology, and frequency-flipping optical DC leakage suppression technology to achieve uplink optical access communication with high dynamic range, high subcarrier power difference tolerance, and high optical DC leakage tolerance, possessing the following advantages: (1) Time-frequency division multiplexing (TFD) enables reasonable allocation of time and subcarrier frequency. This application adopts a time-frequency division multiplexing scheme. The signal is multiplexed by dividing the resources in the time and frequency domains into multiple sub-channels, thereby improving spectrum utilization efficiency. Unlike traditional frequency division multiplexing and time division multiplexing, TFD can achieve more flexible resource scheduling and adapt to higher data transmission requirements by precisely controlling the time and frequency allocation within the same time period.

[0097] (2) Simplifying optical network units and reducing costs. This application proposes that in an optical network unit, complex signals are converted into intensity information through subcarrier modulation (SCM), and then converted into optical signals by a single electro-optic modulator. Compared with the traditional coherent transmission scheme, each optical network unit can simplify the IQ modulator to a single electro-optic modulator. Since there are many optical network units and they are cost-sensitive, this simplification helps to reduce costs. In the scheme proposed in this application, compared with the traditional coherent reception scheme, an additional laser and two optical attenuators are added at the optical line terminal. Considering that the cost of optical attenuators is low and the optical line terminal is not cost-sensitive, this complexity is acceptable.

[0098] (3) Improve the overall dynamic range of the system. This application proposes to add an optical attenuation module after the local oscillator light at the optical line terminal, thereby controlling the local oscillator power input to the integrated coherent receiver module. Since each frame of signal may come from different optical network units and the power may be different, too low signal power may cause too much quantization noise in the subsequent analog-to-digital conversion module, while too high signal power may cause clipping in the subsequent digital-to-analog conversion module. By controlling the local oscillator power, the electrical signal power output by the integrated coherent receiver module can be controlled within a suitable range, so that signals with a wider range of received optical power can be received well, thereby improving the overall dynamic range of the system.

[0099] (4) Improve the system's tolerance to subcarrier power difference. This application proposes to use two local oscillators with different powers in the optical line terminal to receive two subcarriers respectively. The two local oscillators control the power of the electrical signals of the two subcarriers respectively, so that the power of the two subcarriers in the electrical signal output by the integrated coherent receiver module is comparable. In the analog-to-digital conversion module, if the power difference between the subcarriers of the electrical signal is large, the lower-power subcarrier will have greater quantization noise, and the effect of the weak delay of the IQ channel will be amplified. By using two local oscillators to perform heterodyne reception on the two subcarriers respectively, and adjusting the optical power of the two local oscillators respectively, the power of the two subcarriers can be made similar, thereby improving the system's tolerance to subcarrier power difference.

[0100] (5) Improve the system's tolerance to optical DC leakage. This application proposes using a bandwidth slightly greater than [missing information] in the optical line terminal. The coherent receiving module; optical network units in non-transmitting state may generate a certain amount of optical DC leakage. This leakage optical power will be superimposed on the uplink signal at the optical line terminal, significantly reducing the signal-to-noise ratio of the effective signal; in this application, the optical DC leakage is shifted to a high frequency by using two subcarriers for heterodyne reception, and then the DC leakage is filtered out by the low-pass effect of the limited bandwidth of the coherent receiving module through the heterodyne reception method, thereby improving the system's tolerance to DC leakage.

[0101] In summary, this application significantly improves the performance and practicality of uplink fiber optic access networks by using time-frequency division multiplexing, simplifying the optical network unit transmitter, increasing the overall dynamic range of the system, improving the system's tolerance to subcarrier power differences, and improving the system's tolerance to optical DC leakage. It has significant application value and broad market prospects.

[0102] This concludes the implementation method for a simplified coherent uplink optical access system based on time-frequency domain power equalization using dual local oscillators.

[0103] The following section introduces the verification steps and results of the experimental system for a simplified coherent uplink optical access system based on time-frequency domain power equalization using dual local oscillators, as presented in this example.

[0104] The specific steps of a simplified coherent uplink optical access system based on time-frequency domain power equalization using dual local oscillators in the experimental system are as follows: In this example, the experimental framework can be... Figure 1 Obtained from Figure 1 In the diagram, number 101 is the optical network unit, number 102 is the optical transmission link, and number 103 is the optical line terminal.

[0105] Figure 2This paper demonstrates the specific implementation steps of a simplified coherent uplink optical access system based on time-frequency domain power equalization using dual local oscillators. The details are as follows: Step 201: In the optical network unit, the bit stream is constellation point mapping and subcarrier modulation is performed, and it is resampled to the same sampling rate as the digital-to-analog converter. Step 202: In the optical network unit, the generated signal is converted into an electrical signal by a digital-to-analog converter and then modulated into an optical signal by an electro-optic modulator. Step 203: In the optical transmission link, optical signals from different optical network units are coupled to achieve carrier aggregation, and then transmitted through the optical fiber link; Step 204: In the optical line terminal, two local oscillator beams are used. The power of the two local oscillator beams is adjusted according to the power of the two subcarriers, and the two local oscillator beams are coupled together. Step 205: In the optical line terminal, the optical signal is received by an integrated coherent receiver, converted into an electrical signal, and then converted into a digital signal by an analog-to-digital converter. Step 206: In the optical line terminal, the signal is demodulated and recovered through burst mode digital signal processing to obtain the final signal and perform bit error analysis.

[0106] Figure 3 The diagram illustrates the spectral distribution of each module in a simplified coherent uplink optical access system based on time-frequency domain power equalization using dual local oscillators. Details are as follows: In the spectrum diagram of optical network unit 301, different optical network units may be assigned a first subcarrier (frequency: ) or the second subcarrier (frequency is The system transmits a subcarrier modulated (SCM) signal. The first and second subcarriers must meet two conditions: first, the frequency of the first optical subcarrier must be lower than that of the second optical subcarrier; second, the frequency difference between the first and second optical subcarriers must be greater than the bandwidth of the transmitted signal. Considering the frequency of the first subcarrier is... The frequency of the second subcarrier is The signal baud rate is The roll-off factor of the molding filter is , must meet ,and . Figure 3 Article 301 provides a preferred example where the frequency difference between the two subcarriers is approximately ; In the 302 optical transmission link spectrum diagram, optical signals from various optical network units are coupled to achieve carrier aggregation. At this point, the coupled light simultaneously possesses a first subcarrier and a second subcarrier. Because the first and second subcarriers traveled different distances before coupling, they experienced different losses, resulting in different powers for the coupled first and second subcarriers. Furthermore, optical network units in a non-transmitting state may generate some optical DC leakage, which will appear at the first subcarrier frequency. Second subcarrier frequency The surrounding area affects the effective signal-to-noise ratio of the signal. Figure 3 Reference 302 provides a preferred example in which the second subcarrier transmits a longer distance before coupling, therefore the power of the second subcarrier is lower and the power of the first subcarrier is higher, and the first subcarrier frequency... Second subcarrier frequency Optical DC leakage was detected nearby; In the spectrum diagram of the 303 optical line terminal, the received signal is received by two subcarriers through two local oscillators with different powers, as shown in the optical signal spectrum of 303. The first subcarrier has high power, so the first local oscillator uses low power; the second subcarrier has even lower power, so the second local oscillator uses high power. This application also proposes using a bandwidth slightly larger than [missing information] in the optical line terminal. The coherent receiving module, wherein the bandwidth of the integrated coherent receiving module is shown by the dashed line in the optical signal spectrum of 303, can completely receive the signal and can filter out optical DC leakage by means of the low-pass effect, as shown in the electrical signal spectrum of 303. Finally, in the preferred embodiment given in 303, the power of the first subcarrier and the second subcarrier of the received electrical signal are comparable, and DC leakage is greatly suppressed.

[0107] Figure 4 The signal spectrum of the first and second subcarriers after aggregation is presented. The two subcarriers are clearly visible in the spectrum. The different subcarriers come from different optical network units, and the power of the subcarriers after aggregation may be different.

[0108] Figure 5 The spectra of the received electrical signals are shown using a single local oscillator and two locally tunable local oscillators. The results indicate that: Using a local oscillator optical receiver, the received electrical signal may have subcarrier power differences, which will affect the quantization of low-power subcarriers and amplify the impact of IQ channel delay. Using two local oscillator optical receivers with adjustable power ensures that the received electrical signal subcarrier power is comparable, which improves the system's tolerance to subcarrier power differences.

[0109] Figure 6The comparison of receiver sensitivity performance using one local oscillator and two local oscillator receivers is presented, and the results show that: When using a single local oscillator receiver, with 2E-2 (15.3% soft-decision forward error correction) as the bit error threshold, the receiver sensitivity can reach -26.3 dBm; When using two local oscillator optical receivers, with 2E-2 (15.3% soft-decision forward error correction) as the bit error threshold, the receiver sensitivity can reach -25 dBm; When using two local oscillators for reception, if both local oscillators are set to maximum power (13 dBm in the experiment), using two local oscillators will result in a sensitivity penalty of about 1.3 dB.

[0110] Figure 7 The relationship between receiver sensitivity performance and local oscillator power is presented for using one and two local oscillator receivers. The results show that: The higher the local oscillator power, the greater the sensitivity cost of using two local oscillators for receiving light. This is because an additional local oscillator increases the total power of the local oscillators, thereby introducing greater noise.

[0111] Figure 8 A comparison of the dynamic range of the receivers using one local oscillator and two locally adjustable local oscillators is presented, and the results show that: When using a single local oscillator optical receiver, with 2E-2 as the bit error threshold, the receiver's dynamic range can reach approximately 16.3 dB; When using two power-adjustable local oscillator optical receivers, with 2E-2 as the bit error threshold, the receiver dynamic range can reach >25 dB; Although using two power-adjustable local oscillator optical receivers comes at the cost of sensitivity, it can significantly improve the receiver's dynamic range.

[0112] Figure 9a The tolerance of the system to subcarrier power difference is given when using a single local oscillator receiver and when using two locally adjustable local oscillator receivers. Figure 9b Then it was given Figure 9a When using two adjustable local oscillators, the local oscillator power used by the power difference of different subcarriers is shown in the results: When using a single local oscillator optical receiver, the system can only tolerate a subcarrier power difference of <4 dB at the cost of 1 dB sensitivity. When using two power-tunable local oscillator optical receivers, the system can tolerate a subcarrier power difference of more than 16 dB at the cost of 1 dB sensitivity. Although using two power-tunable local oscillator optical receivers comes at the cost of sensitivity, it can significantly improve the system's tolerance to subcarrier power differences.

[0113] Figure 10 The proposed time-frequency domain power equalization simplified coherent uplink optical access system based on dual local oscillators demonstrates its tolerance to optical DC leakage. Results show that... Increasing the frequency spacing between the local oscillator and the subcarrier helps improve tolerance to optical DC leakage; This solution is an effective way to address the problem of optical DC leakage.

[0114] After completing the experimental verification of the above key indicators and confirming their macroscopic superiority, in order to more comprehensively demonstrate the overall technical solution of this application and to deeply reveal how the various hardware modules and signal processing steps in the system are precisely coupled and cooperate with each other to achieve the above-mentioned superior performance, a more detailed specific embodiment will be provided below in conjunction with a complete system communication process, so as to provide further technical support for this application.

[0115] This embodiment provides a specific implementation scheme for a simplified coherent uplink optical access system based on time-frequency domain power equalization using dual local oscillators. This system is applied to uplink fiber optic access networks, and its overall architecture remains the same. Figure 1 As shown, it includes multiple optical network units 101, optical transmission links 102, and an optical line terminal 103. Unlike the previous embodiments, this embodiment will combine the complete signal transmission process to provide a more in-depth explanation of the signal processing details within each module, the collaborative methods between modules, and the resulting technical effects, in order to fully support all the technical features defined in the claims of this application.

[0116] Specifically, in this embodiment, the optical access network divides multiple optical network units into two groups. The first group of optical network units corresponds to the first subcarrier frequency. The second group of optical network units corresponds to the second subcarrier frequency. It should be noted that different groups of optical network units are assigned different optical subcarrier frequencies, and each optical network unit transmits either a first subcarrier optical signal or a second subcarrier optical signal according to its group. The relationship between the two subcarrier frequencies needs to satisfy the following constraints: Greater than ,and and The difference is greater than (Formula 1). Wherein, The signal baud rate represents the number of symbols transmitted per second. The roll-off factor of the shaping filter typically ranges from 0 to 1 and is used to control the transition band width of the signal spectrum. This represents the actual spectral bandwidth occupied by a single subcarrier modulated signal. The physical meaning of Formula 1 is that the frequency interval between two subcarriers must be greater than the bandwidth of a single subcarrier signal to ensure that the two subcarriers do not overlap in the frequency domain, thereby ensuring that the optical line terminal can effectively receive and demodulate each subcarrier separately. For example, as... Figure 3 As shown in Figure 301, the frequency difference between the two subcarriers can be selected as approximately This provides a sufficient frequency protection interval between the two subcarriers.

[0117] Furthermore, each optical network unit (ONU) includes an optical transmitter, whose core feature is the use of subcarrier modulation technology to convert the complex information of a complex signal into intensity information. This allows the signal to be modulated by a single electro-optic modulator, eliminating the need for the expensive IQ modulators used in traditional coherent transmission schemes. Given the large number of ONUs and their cost-sensitivity, simplifying the IQ modulator to a single electro-optic modulator significantly reduces the hardware cost and system complexity of each ONU. More specifically, the optical transmitter includes a digital signal generation module that generates subcarrier modulated signals using digital signal processing technology. The digital signal generation module internally includes a quadrature amplitude modulation (QAM) unit, a pilot insertion unit, a subcarrier modulation unit, and an upsampling unit, which process the signal sequentially. The QAM unit first maps the input bitstream into constellation point symbols, converting the binary bit sequence into constellation points on the complex plane. Each constellation point carries both amplitude and phase information. The pilot insertion unit then inserts a pre-defined pilot symbol before the mapped constellation point symbol. This pilot symbol provides known reference information to the receiver's digital signal processing algorithms, enabling rapid convergence of algorithms such as channel estimation and carrier recovery. This is particularly important for burst-mode uplink access systems. The subcarrier modulation unit converts the pilot-inserted signal into a subcarrier-modulated signal. Its core operation is converting the signal's phase information into intensity information. After subcarrier modulation, the signal only contains intensity variations, thus allowing modulation and transmission via a single electro-optic modulator. The upsampling unit resamples the signal from its baud rate to the sampling rate required by the digital-to-analog converter module to achieve rate matching between the two.

[0118] The optical network unit also includes a digital-to-analog converter (DAC) module and an electro-optic modulation module. The DAC module converts the subcarrier modulation signal output from the digital signal generation module into an analog electrical signal. A first laser is used to generate the optical carrier, and its laser frequency is set as the first subcarrier frequency according to the allocation of the optical access network. Or the second subcarrier frequency That is, the frequency of the first laser corresponds one-to-one with the group to which the optical network unit belongs. The electro-optic modulation module loads the analog electrical signal output from the digital-to-analog converter onto the optical carrier generated by the first laser, realizing the conversion of electrical signal to optical signal and outputting a subcarrier modulated optical signal. Optionally, the electro-optic modulator in the electro-optic modulation module can be a Mach-Zehnder modulator or an electroabsorption modulator; this application does not limit this. The spectrum of the modulated optical signal is as follows: Figure 3 As shown in Figure 301, each optical network unit generates a subcarrier modulated optical signal at its corresponding subcarrier frequency.

[0119] Regarding optical transmission links, such as Figure 1 As shown in Figure 102, the optical transmission link includes an optical coupling module and an optical fiber module. The optical coupling module couples the first subcarrier optical signals and the second subcarrier optical signals from different optical network units to achieve carrier aggregation. The carrier-aggregated optical signal contains both the first and second subcarriers, and its spectrum is as follows: Figure 3 As shown in Figure 302. It should be noted that because the optical path transmission distances between different optical network units and optical line terminals are different, the optical path losses experienced by the optical signals of each optical network unit vary. Therefore, the power of the first and second subcarriers after carrier aggregation is usually not equal. For example, as... Figure 3 As shown in Figure 302, the second subcarrier has experienced a longer transmission distance before coupling and has lower power, while the first subcarrier has higher power. Furthermore, optical network units in non-transmitting states may still experience optical DC leakage due to the limited extinction ratio of the electro-optic modulator and bias point drift. This leakage optical power occurs near the frequencies of the first and second subcarriers. The fiber optic module transmits the carrier-aggregated optical signal to the optical line terminal in a low-loss manner through standard single-mode fiber. The aforementioned subcarrier power difference and DC leakage problems are precisely the core issues that this application aims to address with its dual-local oscillator receiver architecture.

[0120] The core innovation of this application lies in the dual-local oscillator receiver and power equalization architecture of the optical line terminal. Figure 1 As shown in Figure 103, the optical line terminal includes a second laser, a third laser, a first optical attenuation module, a second optical attenuation module, a local oscillator coupling module, an integrated coherent receiver module, an analog-to-digital converter module, and a digital signal demodulation module.

[0121] The second laser generates the first local oscillator beam, and the third laser generates the second local oscillator beam. To achieve heterodyne reception of the first subcarrier, the frequency of the first local oscillator beam is set to... (Formula 2); To achieve heterodyne reception of the second subcarrier, the frequency of the second local oscillator is set to... (Formula 3). The design intent of Formulas 2 and 3 is to maintain a frequency interval exactly equal to the signal bandwidth between the frequency of each local oscillator and the center frequency of its corresponding subcarrier, so that at the output of the integrated coherent receiver module, the signal of each subcarrier is shifted to a frequency that is equal to the signal bandwidth. The intermediate frequency position is the center frequency. It is worth noting that the frequency of the first local oscillator is lower than the frequency of the first subcarrier, while the frequency of the second local oscillator is higher than the frequency of the second subcarrier. The two local oscillators are located on opposite sides of the signal spectrum. This "outer" arrangement provides a key condition for subsequent DC leakage suppression.

[0122] The first and second optical attenuation modules independently adjust the power of the first and second local oscillator (LOO) beams, respectively. This is a key method for achieving frequency domain power equalization. The adjustment strategy is as follows: the higher the power of the first subcarrier, the greater the attenuation of the first optical attenuation module, resulting in lower power for the first LOO beam; similarly, the higher the power of the second subcarrier, the greater the attenuation of the second optical attenuation module, resulting in lower power for the second LOO beam. The first and second optical attenuation modules are adjusted simultaneously, ensuring that the power of the first and second subcarriers in the electrical signal output by the integrated coherent receiver module is similar. The physical principle behind this adjustment mechanism is that in coherent reception, the amplitude of the received electrical signal is proportional to the product of the square root of the signal optical power and the square root of the LOO beam power. Therefore, when the signal optical power of a subcarrier is high, reducing its corresponding LOO beam power can decrease the amplitude of the electrical signal for that subcarrier; conversely, when the signal optical power of a subcarrier is low, increasing its corresponding LOO beam power can increase the amplitude of the electrical signal for that subcarrier. By adjusting the local oscillator power in this way, the power imbalance caused by the difference in optical path loss between the two subcarriers can be effectively compensated, thereby achieving frequency domain power balance.

[0123] Furthermore, the first and second optical attenuation modules are also used to achieve time-domain power equalization. In uplink time-division multiplexing access scenarios, each frame signal may come from different optical network units, and the received optical power between different frames may vary significantly. When the received optical power is too high, the amplitude of the electrical signal output by the integrated coherent receiver module is too high, which may cause clipping distortion in the subsequent analog-to-digital converter (ADC). When the received optical power is too low, the amplitude of the electrical signal is too low, which may cause excessive quantization noise in the ADC. To solve this problem, the first and second optical attenuation modules simultaneously adjust the power of the first and second local oscillators based on the overall power of the current frame signal, controlling the power of the electrical signal output by the integrated coherent receiver module within the effective quantization range of the ADC, thereby achieving power equalization in the time domain and significantly expanding the dynamic range of the receiver. For example, the optical line terminal (OLT) can adjust the attenuation value of the optical attenuation module in the local oscillator optical path based on the real-time detected received signal power or the power estimate in the digital signal output by the analog-to-digital converter (ADC); alternatively, it can perform feedforward settings on the optical attenuation module based on the optical path loss information and received power information of each optical network unit known in advance by the OLT. This application does not limit the specific control method of the optical attenuation module. This time-domain power equalization mechanism works in conjunction with the aforementioned frequency-domain power equalization mechanism, enabling the system to achieve effective power equalization simultaneously in both the time and frequency domains.

[0124] The local oscillator coupling module couples the first and second local oscillator beams after power adjustment via an optical coupler. The coupled local oscillator beams, along with the carrier aggregation signal light arriving via the optical transmission link, are input into the integrated coherent receiver module. For example... Figure 3 As shown in the spectrum of the 303 optical signal, the coupled local oscillator light includes a first local oscillator light located on the low-frequency side of the first subcarrier and a second local oscillator light located on the high-frequency side of the second subcarrier. The power of the two local oscillators is adjusted in reverse according to the subcarrier power.

[0125] The integrated coherent receiver module completes the conversion from optical signal to electrical signal, and performs heterodyne reception on the first subcarrier optical signal and the second subcarrier optical signal, respectively. Another core innovation of this embodiment lies in the bandwidth design of the integrated coherent receiver module and the resulting DC leakage suppression function. The bandwidth of the integrated coherent receiver module... Set to slightly larger This bandwidth design has a dual technical effect. First, the bandwidth is sufficient to completely receive the signals of the first and second subcarriers because, after heterodyne reception, each subcarrier signal is shifted to a position where... The spectral bandwidth occupied by the centered mid-frequency position is exactly [missing information]. Therefore, slightly larger The receiving bandwidth is sufficient to meet the complete signal reception requirements. Secondly, the optical DC leakage from optical network units in non-transmitting state has a frequency close to... or After being received by heterodyne receivers, the signal is moved to a high-frequency position. Since the first and second local oscillators are located outside the signal spectrum, the frequency of the component generated by the optical DC leakage after beat frequency is much higher than that of the signal. Therefore, it falls outside the bandwidth of the integrated coherent receiver module and is naturally filtered out by the low-pass effect of the integrated coherent receiver module. For example... Figure 3 As shown in the electrical signal spectrum of the 303, the power of the two subcarriers in the received electrical signal is comparable, and DC leakage is significantly suppressed.

[0126] To further clarify the working principles of power equalization and DC leakage suppression, a mathematical description of the integrated coherent receiver module is given below. Let... equal Then the optical field of the local oscillator after coupling can be expressed as: (Formula 4) in, and These represent the power of the first and second local oscillators after adjustment by the optical attenuation module, respectively. Represents the imaginary unit, satisfying ; It is a time variable; and These represent the initial phases of the first and second local oscillators, respectively. This represents the natural exponential function.

[0127] After integrating the coherent receiving module, the photocurrents of the in-phase and quadrature components can be expanded into an expression containing four beat frequency terms. The frequency of the beat frequency term between the first local oscillator and the first subcarrier is... The frequency of the beat frequency term between the second local oscillator and the second subcarrier is (i.e., in the negative frequency direction) These two items are the target signal components. The beat frequency terms of the first subcarrier and the second local oscillator, and the beat frequency terms of the second subcarrier and the first local oscillator, respectively contain frequencies including… The factor, due to Much larger The frequencies of these cross-beat frequency terms are far beyond the bandwidth range of the integrated coherent receiver module, and are therefore filtered out by the low-pass effect.

[0128] After bandwidth filtering, the remaining effective photocurrent can be simplified as follows: (Formula 5) (Formula 6) in, and These are the signal optical powers of the first subcarrier and the second subcarrier, respectively. The responsivity of a photodetector in an integrated coherent receiver module is usually expressed in amperes per watt, representing the amount of current that the photodetector can generate under a given incident light power. and These represent the phase differences between the first subcarrier and the first local oscillator, and between the second subcarrier and the second local oscillator, respectively. and These represent the photocurrents of the same-direction component and the orthogonal component, respectively; and These represent the cosine function and the sine function, respectively.

[0129] The mechanism for frequency domain power equalization can be clearly seen from Equations 5 and 6. The amplitude of the received electrical signal of the first subcarrier is proportional to... The amplitude of the received electrical signal of the second subcarrier is proportional to... When the signal optical power of the two subcarriers is unequal, for example... Less than When this is the case, the local oscillator power can be selected. Greater than So that the two products and By approximating the power of each subcarrier, the sensitivity loss caused by subcarrier power differences is mitigated, achieving frequency domain power equalization. Simultaneously, when the overall power of the two subcarriers is too high or too low, it can be simultaneously reduced or increased. and This is to control the electrical signal amplitude within the optimal quantization range of the analog-to-digital conversion module, achieving time-domain power equalization. Furthermore, for optical DC leakage, its optical frequency is close to... or The frequency of the component generated after beating the first oscillator is Including the DC leakage offset, the frequency of the component generated after the second local oscillator beats is also far from zero. Due to the use of heterodyne reception, the DC leakage component is flipped to a frequency region higher than the receiving bandwidth, thus being suppressed by the limited bandwidth of the integrated coherent receiving module. This mechanism gives the system a high tolerance for cumulative DC leakage from multiple inactive optical network units.

[0130] The analog-to-digital converter (ADC) module converts the analog electrical signal output from the integrated coherent receiver module into a digital signal for subsequent digital signal processing. Due to the combined effect of the aforementioned frequency-domain power equalization and time-domain power equalization mechanisms, the power of the electrical signal input to the ADC module is controlled within a suitable range, and the powers of the two subcarriers are similar. This allows the ADC module to effectively quantize the signal with a smaller quantization error, thereby improving the overall signal quality of the system.

[0131] The digital signal demodulation module demodulates the received digital signals to recover the signals transmitted by each optical network unit. Specifically, the digital signal demodulation module includes the following processing units: a burst frame detection unit to determine the start time of the current frame signal and extract the valid signal of the current frame from the continuously received digital signal stream. This is crucial for uplink transmission in burst mode because the frame signals transmitted by different optical network units may be discontinuous in time; and a down-conversion unit to shift the digital signal of the first subcarrier or the second subcarrier from the intermediate frequency (IF) to the baseband. Specifically, for the first subcarrier, the down-conversion frequency is positive. For the second subcarrier, the down-conversion frequency is negative. After down-conversion, the baseband signal can be processed by standard digital signal processing. The burst-mode coherent digital signal processing unit performs a series of digital signal processing operations on the baseband signal to recover the damage suffered by the signal during transmission. Specifically, this includes orthogonalization to compensate for the non-orthogonality between in-phase and quadrature components in the integrated coherent receiver module, dispersion compensation to eliminate signal broadening caused by fiber dispersion, clock recovery to align to the optimal sampling time, channel equalization to compensate for channel frequency response unevenness, frequency offset estimation to track and compensate for the frequency offset between the signal light and the local oscillator, and phase recovery to track and compensate for the phase change between the signal light and the local oscillator. The equalizer uses a decision-guided least mean square algorithm to adaptively equalize the baseband signal to suppress inter-symbol interference introduced by the channel. The decision-guided least mean square algorithm obtains the error signal by comparing the signal output by the equalizer with the decision value of the nearest constellation point, and then iteratively adjusts the weight coefficients of the equalizer based on the error signal to minimize the mean square error.

[0132] Optionally, the system in this embodiment further includes a bit error rate (BER) testing module, used to perform BER testing on the signal output by the digital signal demodulation module to evaluate the overall system performance. By comparing the demodulated bit sequence with the original transmitted bit sequence bit by bit, the BER of the system can be calculated, thereby quantitatively evaluating the actual effectiveness of the aforementioned power equalization mechanism and DC leakage suppression mechanism.

[0133] Based on the above system architecture, this embodiment also provides a corresponding simplified coherent uplink optical access method based on time-frequency domain power equalization using dual local oscillators, combined with... Figure 2As shown, the specific steps are as follows.

[0134] Step 100: Transmitter signal processing is performed in the optical network unit. The bitstream is mapped to constellation point symbols by the quadrature amplitude modulation (QAM) unit, forming a QAM signal. Subsequently, a pilot insertion unit inserts pilot symbols before the constellation point symbols to assist in fast signal recovery at the receiver. The subcarrier modulation unit converts the complex information of the signal after pilot insertion into intensity information, generating a subcarrier modulated signal. It should be noted that different optical network units use the first subcarrier frequency according to their assigned subcarrier frequencies. Or the second subcarrier frequency The upsampling unit resamples the signal to the operating sampling rate of the digital-to-analog converter module.

[0135] Step 200 involves electro-optical conversion within the optical network unit. The subcarrier modulation signal generated in step 100 is converted into an analog electrical signal via a digital-to-analog converter. This analog electrical signal is then loaded onto the optical carrier generated by the first laser using a single electro-optic modulator, generating a subcarrier modulated optical signal. The use of a single electro-optic modulator instead of an IQ modulator is due to the simplification of converting complex information into intensity information in step 100 during subcarrier modulation. This simplification is key to achieving low cost at the transmitter in this application.

[0136] Step 300: Carrier aggregation and transmission are performed in the optical transmission link. Subcarrier modulated optical signals from each optical network unit are carrier aggregated through an optical coupler, so that the first subcarrier optical signal and the second subcarrier optical signal are combined into the same optical fiber, and the carrier-aggregated optical signal is transmitted to the optical line terminal through a standard single-mode optical fiber.

[0137] Step 400: Adjust and couple the local oscillator power at the optical line terminal. This step further includes: Step 410: Using a second laser and a third laser to generate a first local oscillator beam and a second local oscillator beam, respectively, with frequencies set according to Formulas 2 and 3, to be used for heterodyne reception of the first subcarrier optical signal and the second subcarrier optical signal, respectively; Step 420: Adjusting the power of the first local oscillator beam and the second local oscillator beam independently through the first optical attenuation module and the second optical attenuation module, respectively, according to the power of the first subcarrier and the second subcarrier. The higher the subcarrier power, the greater the attenuation of the local oscillator beam, so that the power of the two subcarriers in the received electrical signal is similar, thereby achieving frequency domain power balance; At the same time, adjusting the power of the two local oscillator beams simultaneously according to the overall power level of the current frame signal, controlling the power of the electrical signal output by the subsequent integrated coherent receiving module within the effective quantization range of the analog-to-digital converter, thereby achieving time domain power balance; Step 430: Coupling the first local oscillator beam and the second local oscillator beam after power adjustment through the local oscillator coupling module.

[0138] Step 500: Photoelectric conversion and analog-to-digital conversion are performed at the optical line terminal. The local oscillator light coupled in step 430 and the optical signal aggregated in step 300 are input together into the integrated coherent receiver module for photoelectric conversion. The integrated coherent receiver module performs heterodyne reception on the first subcarrier optical signal and the second subcarrier optical signal respectively to obtain an electrical signal containing the first subcarrier and the second subcarrier. The bandwidth of the integrated coherent receiver module is slightly larger than... This causes optical DC leakage from optical network units in a non-transmitting state to be shifted to a higher frequency after heterodyne reception, falling outside the receiving bandwidth and being filtered out by the low-pass effect. Subsequently, the analog-to-digital converter converts the analog electrical signal into a digital signal.

[0139] Step 600: Digital signal demodulation and recovery are performed in the optical line terminal. The digital signal obtained in step 500 undergoes burst-mode digital signal processing. First, the start position of the current frame is determined and the frame signal is extracted by the burst frame detection unit; then, the target subcarrier signal is shifted from the intermediate frequency (IF) to the baseband by the down-conversion unit; subsequently, signal impairment recovery processing is performed sequentially, including orthogonalization, dispersion compensation, clock recovery, channel equalization, frequency offset estimation, and phase recovery; finally, an adaptive equalizer is driven by a decision-guided least mean square algorithm to suppress inter-symbol interference, completing the demodulation and recovery of the signal. Optionally, the performance of the demodulated signal can also be evaluated using a bit error rate (BER) testing module.

[0140] To further clarify the quantitative basis of the key parameters of this application, the core working principle of the system, and the automatic control logic of power balance, the following supplementary explanations of the above technical features are provided in conjunction with specific numerical values ​​and control schemes.

[0141] I. Quantitative Explanation of Key Parameters In this application, the bandwidth BR of the integrated coherent receiver module is designed to be slightly greater than [value missing]. Specifically, BR should satisfy: The lower limit of this range ensures that the spectrum of both subcarrier signals can be completely received; the upper limit ensures that the signals of the two subcarriers shifted to the intermediate frequency after heterodyne reception do not alias in the frequency domain. In a specific embodiment, when the signal baud rate GBaud, roll-off factor hour, GHz, if the frequency difference between the two subcarriers is GHz If 80 GHz is selected, then the reasonable range for BR is 35.2 GHz to 44.8 GHz. GHz, meaning that, assuming complete signal reception, it is only slightly greater than 35.2 GHz (approximately 2% to 5% higher). The above quantization range also applies to "BR slightly greater than" in this application. The specific value of BR is not limited by the word "BR". Those skilled in the art can flexibly determine it according to the subcarrier frequency spacing and signal baud rate in the actual system.

[0142] II. Further Explanation of Local Oscillator Frequency Configuration and Heterodyne Reception Principle make The first local oscillator frequency generated by the second laser is The second local oscillator frequency generated by the third laser is That is, the two local oscillator beams are located outside the corresponding subcarrier spectrum. After photoelectric conversion by the integrated coherent receiver module, the photocurrents of the in-phase and quadrature components (after filtering out cross-beat frequency terms due to the receiver's low-pass effect) can be simplified as follows: Where Ps1 and Ps2 are the signal optical powers of the first and second subcarriers, respectively; P1 and P2 are the powers of the first and second local oscillator beams after adjustment by the optical attenuation module, respectively; R is the responsivity (unit: A / W) of the photodetector in the integrated coherent receiver module; Δθ1 and Δθ2 are the phase differences between the first subcarrier and the first local oscillator beam, and between the second subcarrier and the second local oscillator beam, respectively; t is the time variable. As can be seen from the above equation, the amplitude of the received electrical signal of the first subcarrier is proportional to... The amplitude of the received electrical signal of the second subcarrier is proportional to... When the optical power of the two subcarrier signals is unequal (e.g. When selecting ,make Frequency domain power equalization can be achieved by adjusting P1 and P2 simultaneously when the overall power of the two subcarriers is too high or too low, keeping the electrical signal amplitude within the effective quantization range of the analog-to-digital conversion module. This achieves time domain power equalization. Therefore, the two equalization mechanisms described above share the same set of dual local oscillator and dual optical attenuation module hardware, working together through different control strategies without requiring additional hardware resources.

[0143] After the above heterodyne reception, the first subcarrier signal is shifted to... The second subcarrier signal is shifted to the positive intermediate frequency position centered on the center. The two local oscillators are located at the negative intermediate frequency position centered on zero frequency, and can be down-converted and moved to baseband for independent processing in the digital signal demodulation module. For optical DC leakage from the non-transmitting optical network unit, its optical frequency is close to ω1 or ω2; since the two local oscillators are located on the outer side of the signal spectrum ( , The frequency of the component generated by the DC leakage after beating the local oscillator on the opposite side is approximately... The DC leakage is much higher than the bandwidth BR of the integrated coherent receiver module, and is therefore naturally filtered out by its low-pass effect. This DC leakage suppression mechanism does not require the introduction of additional filtering hardware or dedicated digital signal processing algorithms, and effectively improves the system's tolerance to accumulated optical DC leakage from multi-optical network units without increasing system complexity.

[0144] III. Specific Implementation Methods of Power Monitoring and Feedback Control In a preferred embodiment provided in this application, to achieve precise dynamic adjustment of the first optical attenuation module and the second optical attenuation module, the optical line terminal 103 also embeds a power monitoring and feedback control module (not shown separately in the accompanying drawings). Specifically, after completing the analog-to-digital conversion, the digital signal demodulation module estimates the average digital domain power of the currently received first and second subcarriers in real time through its internal digital signal processing algorithm, and sends the calculated subcarrier power difference and the overall power level of the current frame to the feedback control module. The feedback control module has a built-in closed-loop control algorithm (e.g., PID control), which generates a first adjustment signal and a second adjustment signal according to the received subcarrier power difference, respectively, to drive the first optical attenuation module and the second optical attenuation module (e.g., an electrically controlled variable optical attenuator) to adjust independently: when the power of the first subcarrier is detected to be higher than that of the second subcarrier, the attenuation of the first optical attenuation module is increased, and the power of the first local oscillator is reduced; otherwise, the attenuation is reduced; the two optical attenuation modules continue to adjust until the power difference of the received electrical signals of the two subcarriers converges to a preset tolerance threshold (e.g., 0.5 dB), forming a complete physical closed loop. Simultaneously, the feedback control module also synchronously adjusts the two optical attenuation modules according to the overall power level of the current frame, controlling the total electrical signal power output by the integrated coherent receiver module within the effective quantization range of the analog-to-digital converter module, thereby simultaneously achieving frequency domain power equalization and time domain power equalization within the same closed-loop framework. Furthermore, the optical line terminal can also pre-set the attenuation value of the optical attenuation module using a feedforward method based on pre-known optical path loss information between each optical network unit and the optical line terminal, and then perform fine correction using the aforementioned closed-loop feedback; this application does not limit the specific control method for power equalization.

[0145] In summary, the synergistic relationship between the various technical features in this embodiment can be summarized as follows: At the transmitting end, subcarrier modulation technology converts complex signals into intensity signals, enabling each optical network unit to complete optical modulation with only a single electro-optic modulator. This reduces the cost and complexity of the transmitting end and provides the prerequisites for subsequent heterodyne reception and DC leakage suppression. At the receiving end, independent power control of the dual local oscillators simultaneously achieves frequency-domain power equalization and time-domain power equalization: in the frequency domain, the differentiated adjustment of the two local oscillator powers compensates for the power difference between subcarriers, significantly improving the system's tolerance to subcarrier power differences; in the time domain, the overall adjustment of the two local oscillator powers controls the electrical signal power within the effective quantization range of the analog-to-digital converter module, significantly expanding the receiver's dynamic range. The frequency design of arranging the two local oscillators outside the signal spectrum, combined with the integrated coherent receiver module's limited receiving bandwidth slightly larger than the signal bandwidth, flips optical DC leakage to a higher frequency and filters it out through a low-pass effect, effectively improving the system's tolerance to DC leakage. The three technical effects mentioned above—reducing transmitter costs, achieving time-frequency domain power equalization, and suppressing optical DC leakage—work together to realize a simplified coherent uplink optical access system with high dynamic range, high subcarrier power difference tolerance, and high DC leakage tolerance, ensuring the transmission performance and stability of the uplink time-frequency division multiplexing coherent optical access system under complex power distribution conditions.

[0146] It is important to note that the above technical solution is not a simple superposition of several known technologies, but rather an organic whole solution formed by in-depth analysis of three interdependent technical contradictions in uplink time-frequency division multiplexing coherent optical access systems: dynamic changes in time-domain power, frequency-domain subcarrier power imbalance, and accumulated optical DC leakage from multiple optical network units. Specifically, the subcarrier modulation unit in optical network unit 101 converts complex signals into intensity information, which not only allows a single electro-optic modulator to replace an IQ modulator, thus reducing transmitter costs, but also ensures that the optical DC leakage of the subcarrier-modulated optical signal and the effective signal have frequency separation in the optical domain—this frequency domain characteristic is precisely the frequency of the first local oscillator in optical line terminal 103. ) and second oscillator (frequency) This provides the conditions for heterodyne reception from the outside of the signal spectrum, and this configuration of external heterodyne reception makes the frequencies close to each other. or The optical DC leakage is shifted to a position much higher than the bandwidth of the integrated coherent receiver module after beat frequency. Frequency position (e.g.) Figure 3As shown in the spectrum of the 303 electrical signal, it is filtered out by its low-pass effect. In other words, there is a strict technical dependency between the subcarrier modulation method at the transmitting end, the frequency configuration of the dual local oscillator at the receiving end, and the bandwidth design of the integrated coherent receiver module. Without any one of these components, it is impossible to simultaneously achieve complete signal reception and effective suppression of optical DC leakage without adding extra hardware.

[0147] Meanwhile, the frequency domain power equalization achieved by the independent adjustment of the two local oscillator optical powers by the first and second optical attenuation modules—even the beat frequency current amplitude—is achieved. and The approach is similar to that of a dual-LO heterodyne receiver architecture, because only when each subcarrier is received heterodyneally by an independent LO can the amplitude of the received electrical signal on the corresponding subcarrier be controlled without mutual interference by adjusting the power of a single LO. Furthermore, the time-domain power equalization achieved when the two optical attenuation modules simultaneously adjust the LO power according to the overall power of the current frame shares the same set of optical attenuation modules with the frequency-domain power equalization, and the two are collaboratively implemented through different control strategies of the same hardware. This scheme, which achieves power leveling simultaneously in the time and frequency domains through unified dual-LO power control, and is structurally completely shared with the heterodyne receiver and the DC leakage suppression of the integrated coherent receiver module with limited bandwidth, exhibits significant integrity and inherent consistency in its technical conception. It is not a solution readily conceived by those skilled in the art when faced with the aforementioned three types of technical contradictions.

[0148] The technical effects of the above-mentioned technical solution can be understood from the following aspects. First, on the optical network unit 101 side, the subcarrier modulation unit in the digital signal generation module converts the complex information of the quadrature amplitude modulation signal into intensity information, so that the electro-optic modulation module only needs to use a single electro-optic modulator (such as a Mach-Zehnder modulator or an electroabsorption modulator) to complete the conversion of electrical signals to optical signals, without the need to use the complex and costly IQ modulators in traditional coherent transmission schemes. In application scenarios with a large number of optical network units in optical access networks, the cumulative hardware cost savings of eliminating one IQ modulator per optical network unit at the system level are considerable. At the same time, the pilot symbols inserted by the pilot insertion unit before the constellation point symbols provide a reference benchmark for various algorithms in the burst mode coherent digital signal processing unit at the receiver, which helps to achieve rapid convergence of channel estimation and carrier recovery algorithms in the burst initiation stage of the frame signal.

[0149] In the optical transmission link 102, the optical coupling modules will be carried on the first subcarrier frequency respectively. Second subcarrier frequency The optical signals are carrier aggregated, enabling uplink signals from multiple optical network units to share the same standard single-mode fiber for transmission to the optical line terminal 103. The two subcarrier frequencies satisfy... The spacing condition ensures that the two subcarriers do not overlap in the frequency domain after carrier aggregation, providing a frequency domain basis for the separate reception and independent processing of the two subcarriers in the subsequent optical line terminal.

[0150] In the optical line terminal 103, the second laser and the third laser generate frequencies of [frequency values ​​to be filled in]. and The first and second local oscillators are configured such that they form heterodyne reception relationships with the corresponding subcarriers from the low-frequency and high-frequency sides of the signal spectrum, respectively. The direct technical effect of this frequency configuration is that, after photoelectric conversion by the integrated coherent receiver module, the first and second subcarrier signals are respectively shifted to the corresponding frequencies. The positive and negative intermediate frequency positions of the center (e.g.) Figure 3 As shown in the spectrum of the 303 electrical signal, in the digital signal demodulation module, the signal can be moved to the baseband for independent processing through the downconversion unit.

[0151] The technical effect of the independent power adjustment of the first and second local oscillator beams by the first and second optical attenuation modules is reflected in frequency domain power equalization. Since the beat frequency current amplitude in coherent reception is proportional to the square root of the product of the signal optical power and the local oscillator beam power, when the power of the first subcarrier optical signal... Power of the second subcarrier optical signal When the optical path losses are unequal, the corresponding local oscillator power is adjusted in the opposite direction. and This allows the power of the two subcarriers in the electrical signal output by the integrated coherent receiver module to tend to be consistent. This frequency domain power equalization mechanism effectively alleviates the problem of relatively increased quantization noise of weak subcarriers caused by excessive differences in the power of the factor carriers in the analog-to-digital conversion module, thereby improving the system's tolerance to subcarrier power differences.

[0152] In the time domain, the first and second optical attenuation modules are also used to simultaneously adjust the power of the first and second local oscillators based on the overall power level of the current frame signal, so as to control the power of the electrical signal output by the integrated coherent receiver module within the effective quantization range of the analog-to-digital converter. When the frame signal power from the near-end low-loss optical network unit is high, the attenuation of the two optical attenuation modules is increased to prevent clipping distortion in the analog-to-digital converter; when the frame signal power from the far-end high-loss optical network unit is low, the attenuation of the two optical attenuation modules is decreased to reduce the relative impact of quantization noise. This time-domain power equalization mechanism shares the same set of dual local oscillator and dual optical attenuation module hardware with the frequency-domain power equalization mechanism, and the two work together to significantly extend the overall dynamic range of the receiver.

[0153] Bandwidth of integrated coherent receiver module Designed to be slightly larger This bandwidth design, combined with the aforementioned dual-local oscillator outer heterodyne receiver configuration, effectively suppresses optical DC leakage. Optical DC leakage in non-transmitting optical network units, caused by the finite extinction ratio of the electro-optic modulator or bias point drift, has a frequency close to the subcarrier frequency. or After being received by heterodyne, the beat frequency was moved to a position much higher than that of the target frequency. The frequency position falls outside the passband of the integrated coherent receiver module and is filtered out by its own low-pass characteristics (such as...). Figure 3 (The electrical signal spectrum of the 303 is shown in the figure). This DC leakage suppression mechanism does not require the introduction of additional filtering hardware or digital signal processing algorithms, and effectively improves the system's tolerance to cumulative optical DC leakage of multi-optical network units without increasing system complexity.

[0154] In the digital signal demodulation module, the burst frame detection unit enables the optical line terminal to correctly identify the boundaries of frame signals from different optical network units, adapting to the time-discontinuous arrival of frame signals in uplink burst mode transmission. The burst mode coherent digital signal processing unit sequentially performs operations such as orthogonalization, dispersion compensation, clock recovery, channel equalization, frequency offset estimation, and phase recovery, gradually recovering the various damages suffered by the signal during fiber optic transmission and photoelectric conversion. The decision-guided least mean square algorithm-driven equalizer iteratively adjusts the weight coefficients to minimize the mean square error, adaptively compensating for residual inter-symbol interference. An optional bit error rate (BER) testing module provides quantitative data for evaluating the system's performance under different operating conditions.

[0155] In summary, this application simplifies the optical network unit structure by employing subcarrier modulation at the transmitter, achieves time-frequency domain dual-dimensional power equalization through independent power control of dual local oscillators at the receiver, and suppresses optical DC leakage by utilizing the synergistic cooperation of heterodyne reception and a coherent receiver module with limited bandwidth. These three elements are organically combined in a unified system architecture, enabling the uplink time-frequency division multiplexing coherent optical access system to simultaneously achieve a large dynamic range, high subcarrier power difference tolerance, and high optical DC leakage tolerance under controlled transmitter costs. This effectively addresses the uplink optical access transmission requirements under complex power distribution conditions.

[0156] It should be noted that in this patent application, relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising one" does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element. In this patent application, if it refers to performing an action according to an element, it means performing the action at least according to that element, including two cases: performing the action only according to that element, and performing the action according to that element and other elements. Expressions such as "multiple," "repeatedly," and "various" include two, two times, two kinds, and more than two, more than two times, and more than two kinds.

[0157] All documents mentioned in this application are considered to be incorporated in their entirety into the disclosure of this application so that they can serve as a basis for modifications if necessary. Furthermore, it should be understood that after reading the foregoing disclosure of this application, those skilled in the art can make various alterations or modifications to this application, and these equivalent forms also fall within the scope of protection claimed in this application.

Claims

1. A dual local oscillator based time and frequency domain power equalized simplified coherent upstream optical access system applied to fiber access network, characterized in that, include: Multiple optical network units, each of which includes an optical transmitter, is used to convert complex information of a complex signal into intensity information through subcarrier modulation, and to load the intensity information onto an optical carrier through a single electro-optic modulator to generate a subcarrier modulated optical signal; wherein different optical network units are assigned a first subcarrier frequency or a second subcarrier frequency, and transmit the first subcarrier optical signal or the second subcarrier optical signal according to the assigned subcarrier frequency; An optical transmission link is used to perform carrier aggregation of the first subcarrier optical signal and the second subcarrier optical signal from different optical network units through an optical coupler, and to transmit the carrier-aggregated optical signal to the optical line terminal through a single-mode optical fiber. Optical line terminals, including: The second and third lasers generate the first and second local oscillators, respectively; The first optical attenuation module and the second optical attenuation module independently adjust the power of the first local oscillator and the second local oscillator, respectively. The greater the power of the first subcarrier, the greater the attenuation of the first optical attenuation module; the greater the power of the second subcarrier, the greater the attenuation of the second optical attenuation module. The first optical attenuation module and the second optical attenuation module are adjusted simultaneously so that the power of the first subcarrier and the second subcarrier in the received electrical signal is similar, so as to achieve frequency domain power balance. The local oscillator coupling module is used to couple the first local oscillator light and the second local oscillator light after power adjustment; An integrated coherent receiving module is used to perform photoelectric conversion on the coupled local oscillator light and the carrier-aggregated optical signal, and to perform heterodyne reception on the first subcarrier optical signal and the second subcarrier optical signal respectively, converting them into corresponding electrical signals. An analog-to-digital converter module is used to convert the analog electrical signal output by the integrated coherent receiving module into a digital signal; The digital signal demodulation module is used to demodulate the digital signal and recover the signal transmitted by each optical network unit.

2. The optical access system of claim 1, wherein, The optical network unit includes: A digital signal generation module is used to generate subcarrier modulated signals using digital signal processing technology; A digital-to-analog converter module is used to convert the subcarrier modulation signal output by the digital signal generation module into an analog electrical signal; A first laser is used to generate an optical carrier, the laser frequency of which is allocated to either the first subcarrier frequency or the second subcarrier frequency according to the optical access network. An electro-optic modulation module is used to load the analog electrical signal output by the digital-to-analog converter onto the optical carrier generated by the first laser, thereby realizing the conversion of electrical signal to optical signal and outputting the subcarrier modulated optical signal.

3. The optical access system of claim 2, wherein, The digital signal generation module includes: The quadrature amplitude modulation unit is used to map the input bit stream into constellation point symbols; Pilot insertion unit, used to insert pilot symbols before the mapped constellation point symbols, so as to enable the fast convergence of the digital signal processing algorithm at the receiving end; The subcarrier modulation unit is used to convert the signal after the pilot is inserted into a subcarrier modulation signal, convert phase information into intensity information, so that the signal can be modulated and transmitted through the single electro-optic modulator; The upsampling unit is used to resample the signal from the sampling rate corresponding to its baud rate to the sampling rate corresponding to the digital-to-analog conversion module.

4. The optical access system according to claim 1, characterized in that, The optical access network divides the plurality of optical network units into two groups, a first group of optical network units corresponding to a first sub-carrier frequency , and a second group of optical network units corresponding to a second sub-carrier frequency ; wherein , and , is a signal baud rate, is a shaping filter roll-off factor; And, the first local light generated by the second laser has a frequency of , and the second local light generated by the third laser has a frequency of ; so that the first local light and the first sub-carrier optical signal, the second local light and the second sub-carrier optical signal form a heterodyne receiving relationship respectively.

5. The optical access system according to claim 4, characterized in that, Bandwidth of the integrated coherent reception module slightly greater than such that: The integrated coherent receiving module is capable of completely receiving the signals of the first subcarrier and the second subcarrier. Optical DC leakage from an optical network unit in a non-transmitting state is shifted to a higher frequency after being received by the heterodyne receiver, falling outside the bandwidth of the integrated coherent receiver module, and is filtered out by the low-pass effect of the integrated coherent receiver module.

6. The optical access system of claim 1, wherein, The first optical attenuation module and the second optical attenuation module are also used to adjust the power of the first local oscillator and the second local oscillator simultaneously according to the overall power of the current frame signal, so as to control the power of the electrical signal output by the integrated coherent receiving module within the effective quantization range of the analog-to-digital conversion module, so as to achieve time-domain power equalization.

7. The optical access system of claim 1, wherein, The digital signal demodulation module includes: The burst frame detection unit is used to determine the start time of the current frame signal and extract the signal of the current frame; The downconversion unit is used to shift the digital signal of the first subcarrier or the second subcarrier from the intermediate frequency to the baseband. The burst-mode coherent digital signal processing unit is used to perform digital signal processing on the baseband signal to recover signal impairments, including orthogonalization, dispersion compensation, clock recovery, channel equalization, frequency offset estimation, and phase recovery. An equalizer is used to adaptively equalize baseband signals using a decision-guided least mean square algorithm to suppress inter-symbol interference introduced by the channel.

8. A dual local oscillator based time and frequency domain power equalized simplified coherent upstream optical access method, characterized in that, Includes the following steps: S1: In an optical network unit, the bit stream is converted into an orthogonal amplitude modulation signal through constellation point mapping, and then the complex information of the orthogonal amplitude modulation signal is converted into intensity information through subcarrier modulation to generate a subcarrier modulation signal; wherein, different optical network units use a first subcarrier frequency or a second subcarrier frequency according to the allocated subcarrier frequency; S2: In the optical network unit, the subcarrier modulation signal generated in step S1 is converted into an analog electrical signal by a digital-to-analog converter, and the analog electrical signal is loaded onto the optical carrier by a single electro-optic modulator to generate a subcarrier modulated optical signal; S3: In the optical transmission link, the subcarrier modulated optical signals from each optical network unit are carrier aggregated through an optical coupler, and the carrier aggregated optical signals are transmitted to the optical line terminal through a single-mode optical fiber. S4: In the optical line terminal, a second laser and a third laser are used to generate a first local oscillator (LOO) beam and a second LEO beam, respectively, which are used to perform heterodyne reception of the first subcarrier optical signal and the second subcarrier optical signal transmitted in step S3. According to the power of the first subcarrier and the second subcarrier, the power of the first LEO beam and the second LEO beam are independently adjusted by the first optical attenuation module and the second optical attenuation module, respectively. The higher the subcarrier power, the greater the attenuation of the corresponding LEO beam, so that the power of the two subcarriers in the received electrical signal is similar, thereby achieving frequency domain power equalization. The first LEO beam and the second LEO beam with adjusted power are coupled through a local oscillator coupling module. S5: In the optical line terminal, the local oscillator light coupled in step S4 and the optical signal after carrier aggregation in step S3 are input together into the integrated coherent receiving module for photoelectric conversion. The first subcarrier optical signal and the second subcarrier optical signal are heterodyne received respectively to obtain an electrical signal containing the first subcarrier and the second subcarrier. The electrical signal is then converted into a digital signal by an analog-to-digital converter. S6: In the optical line terminal, the digital signal obtained in step S5 is subjected to burst mode digital signal processing, including burst frame detection, downconversion, signal impairment recovery and adaptive equalization, and the signal transmitted by each optical network unit is demodulated and recovered.

9. The method of claim 8, wherein, In step S4, the power of the first local oscillator and the second local oscillator is adjusted simultaneously according to the overall power of the current frame signal, so as to control the power of the electrical signal output by the integrated coherent receiving module within the effective quantization range of the analog-to-digital converter, thereby achieving time-domain power equalization.

10. The method according to claim 8, characterized in that, The first subcarrier frequency is , the second subcarrier frequency is , the signal baud rate is , the shaping filter roll-off factor is , wherein , and ; in step S4, the first local light generated by the second laser has a frequency of , and the second local light generated by the third laser has a frequency of , so that the first local light and the first subcarrier optical signal, and the second local light and the second subcarrier optical signal form heterodyne receiving relationship respectively.

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