Key Equalization Technologies For 100G Passive Optical Networks

With the emergency of new bandwidth consuming services such as 4K/8K highdefinition video,virtual reality,and cloud computing,users’ demand for higher bandwidth in the access network increase continuously.Fiber-based optical access networks with larger capacity,lower power loss,and lower cost have gradually replaced traditional copper-based low-speed access networks.Among different architectures,the point-to-multipoint passive optical network(PON)has become the main optical access network architecture due to its low cost and easy maintenance.Over the years,the PON standard has undergone several iterations,and the rate has been upgraded from 1.25 Gb/s to 10 Gb/s.However,with the advent of 5G era,PON needs to support both fixed broadband services and high-speed mobile x Haul applications,therefore its network capacity needs to be upgraded urgently.The international standards organization IEEE began to standardize the next-generation NG-EPON in 2015,hoping to increase the single-channel capacity from 10 Gb/s to 25 Gb/s,and then further increase to 50/100 Gb/s through wavelength division multiplexing(WDM).ITU-T also started 50 Gb/s per wavelength standard discussion from 2018.In addition,discussions are also underway in academia around single-wavelength 100 Gb/s PON.To achieve the rate upgrade,many challenges in the physical link need to be addressed,including bandwidth limitation from optics,fiber dispersion,and system nonlinear distortions.Moreover,since the access network is a network layer that is closest to the users,it is extremely cost-sensitive.In the process of rate upgrade,how to meet the performance requirements while ensuring the low cost of the solution is the research focus.This article focuses on the key issues involved in the high-speed and low-cost process of next-generation optical access networks.The main academic contributions and innovations are as follows:1.Optical domain equalization based on dispersion supported equalization technologyNG-EPON standard requires upgrading the capacity from a single channel 10 Gb/s to25 Gb/s.Operators and equipment vendors hope to reuse the legacy 10 G optics to achieve modulation and demodulation of 25 Gb/s non-return-to-zero(NRZ)signals.In this dissertation,we propose the dispersion supported equalization(DSE)technology,which utilizes the interaction between directly modulated laser(DML)chirp and the negative fiber dispersion to increase the system bandwidth directly from the optical domain.Thanks to bandwidth improvement,25 Gb/s NRZ signals real-time transmission with 10 G DML and10 G avalanche photodiodes(APD)without using additional off-line digital signal processing(DSP)can be realized.By further utilizing the multi-channel characteristics of the dispersion module,a single module can be used to equalize all upstream and downstream channels simultaneously.Then symmetric 4×25 Gb/s time division and wavelength division multiplexing PON systems are achieved in both C-band and O-band.2.Hybrid photoelectric equalization based on optics simplified DSPITU-T hopes to upgrade the single line rate directly from 10 Gb/s to 50 Gb/s.In order to achieve a smooth upgrade,it is hoped that commercially mature 10 G optoelectronic devices will continue to be used.In this case,DSP combined with high-order modulation formats is the only solution.How to reduce the complexity of DSP has become a key issue.We propose a new concept of photoelectric hybrid equalization,and two optical equalization schemes are employed to reduce the complexity of the DSP required at the receiver side.Scheme one uses an external injection locking technique to increase the photon density inside the DML cavity,which can enhance the DML bandwidth while suppressing the DML chirp,Based on this technology,C-band 50 Gb/s discrete multitone signal transmission over20 km single-mode fiber can be supported with only 10 G DML.Scheme two is to use DSE at the transmitter side to enhance the bandwidth first,and then use a simple FFE at the receiver side to further enhance the signal quality,thus successfully achieving 50 Gb/s PAM-4 signal transmission based on 10 G DML and 10 G APD at O-band with a power budget of 26 d B.3.Nonlinear pattern effect mitigation based on neural network pre-equalizationWhen the single-wavelength data rate of the PON comes to 50 Gb/s and 100 Gb/s,receivers with large bandwidth and high sensitivity are required.Commercial APDs have low bandwidth,and PIN-photodetector(PIN-PD)combined with a semiconductor optical amplifier(SOA)is the alternative solution.However,gain saturation will produce a nonlinear pattern effect when SOA input power is too large.Due to this effect,the receiving dynamic range is limited in the point-to-multipoint PON system.In this dissertation,a theoretical analysis of the mechanism of the SOA pattern effect and its influence on the high-speed PAM-4 signal is presented.We proposed to use the powerful nonlinear equalization ability of the neural network(NN)to eliminate this pattern effect.In order to reduce the cost introduced by NN,it is deployed in the optical line terminal(OLT)as a preequalizer so that its cost can be shared by multiple optical network units(ONUs),which increases the feasibility of real deployment.4.Electrical domain equalization based on joint signal processing in both transmitter and receiver sideWith the maturity of 25 Gb/s and 50 Gb/s PON standards,research on a single wavelength of 100 Gb/s is gradually unfolding.The use of advanced DSP to compensate for linear and nonlinear impairments in the system has become the major solution.Two algorithms are proposed in this dissertation for joint equalization at the transmitter and receiver side,avoiding the excessive concentration of DSP complexity at the cost-sensitive ONU side.Algorithm one firstly uses nonlinear Tomlinson-Harashima precoding at the OLT side,and then a second-order Volterra filter is used in the ONU for further equalization.Based on this algorithm,100 Gb/s PAM-4 transmission based on 20 G DML and PD can be achieved with a system power budget of 34 d B.On the other hand,algorithm two employs NN to introduce controllable inter-symbol crosstalk at the transmitter side,making the system response a partial response channel.Then the maximum likelihood estimation is used at the receiver side for recovery.Based on this scheme,a power budget of 32 d B can be achieved.In summary,a series of technologies such as optical domain equalization,hybrid photoelectric equalization,pre-equalization at the transmitter side,and joint equalization at both transmitter and receiver sides are proposed in this dissertation.Based on these technologies,we can use the low-speed optoelectronic components to achieve high-speed signals transmission,which addresses the high complexity and cost problems faced in the process of 100G-PON speed upgrade.While ensuring the system’s overall performance,the required processing difficulty is reduced as much as possible,providing new possibilities for the development of high-speed optical access networks,which will play an irreplaceable role in the standardization of optical access networks and in future commercial deployment.