Equalization Algorithms In High-rate And Short-reach Optical Communication System Based On DML

With the development of social information and emerging services,higher requirements are put forward for the access bandwidth and transmission capacity at the user side.In this case,it would increase the pressure for the optical transmission systems,especially for the short-reach ones.The advancement of capacity and transmission rate would inevitably lead to a rapid increase in the cost of construction.While,the short-reach optical links are the closest to user terminals,which are very sensitive to the cost and power consumption.To achieve the upgrade of the short-reach communication system at a low cost,the intensity modulation direct detection transmission system is the preferred solution,which is based on the directly modulated laser and limited-bandwidth optoelectronic devices(DML-IMDD).However,the transmission performance of this system is severely affected by the inter-symbol interference(ISI)and nonlinear damage caused by bandwidth limitation,fiber dispersion,and DML chirp.For this reason,it is one of the research hotspots that studying effective channel equalization schemes to improve the performance of the high-rate limitedbandwidth DML-IMDD system.To improve the performance of the limited-bandwidth DML-IMDD system with low cost,there are some researches on the equalization algorithm in this article.Firstly,the system modeling analysis of high-rate DML-IMDD system with bandwidth limitation is involved.Then,a suitable feature construction is designed and the corresponding equalization schemes are proposed.Finally,based on DML and low-cost transceivers,a simulation system and an experimental system are built to prove the effectiveness of the proposed equalization schemes.The research contents are specified as follows.1)The author sketches out the significance and background of the equalization schemes in limitedbandwidth DML-IMDD system,as well as some key technologies.Also,the author analyzes and summarizes the research status of equalization algorithms for dealing with the performance impairment of optical transmission systems at home and abroad.2)By theoretical derivation and system modeling,the critical factors affecting the transmission performance of the limited-bandwidth DML-IMDD system are analyzed.These factors include the bandwidth limitation of the devices,square-law detection at receiver,fiber dispersion,and nonlinear chirp signals,etc.In this case,the system modeling provides certain theoretical support for the design of effective channel compensation schemes.3)Aiming at the deficiency of the traditional Volterra nonlinear equalizer,namely the excessive high-order features,slow convergence speed,and instability of the coefficient update algorithms,an improved second-order feature is constructed based on the channel damage characteristics.According to this feature construction,the author designs the improved Volterra equalizer,whose tap coefficients are obtained by support vector machine(SVM)training.In this way,it would accelerate the convergence speed,reduce the overhead in training phase and improve the accurate of the equalization algorithm.And then,it is designed and achieved that only a single SVM model is required to achieve multi-level signals' equalization,which reduces the computational complexity of multiple classifications.Finally,a limited-bandwidth DML-IMDD system with 80Gb/s PAM-4/8signal is set up to prove the effectiveness of the proposed equalizer.4)The common fully-connected deep neural network(DNN)has difficulties in complex structure and actual deployment used in the low-cost short-reach communication system.For this purpose,the author designs a simple DNN equalizer with the Adam algorithm and dropout layer.And on this basis,the “pruning operation” of redundant connections is used to further simplify the structure of this DNN equalizer.In the end,the DML-IMDD simulation system based on 80Gbs/s PAM-4 signal transmission is built to verify the proposed simplified DNN equalization scheme.