Research On Future-oriented Coherent PON And Key Algorithms Of Coherent DSP

By 2020,fiber optic access network has been deployed countrywide covering more than400 million users.With the explosive development of network services,the bandwidth demand of access network is increasing.The future passive optical network system(PON)need to meet the needs of higher speed,longer transmission distance,more users and different scenarios.Coherent detection,polarization multiplexing and digital signal processing technologies make coherent PON become the key technical scheme to meet the upgrade and expansion of optical access system in the future.This paper mainly studies the future-oriented coherent PON system,explores its system architecture,and focuses on some key algorithms involved in coherent digital signal processing.The main innovations and technical contributions are as follows:1)In order to meet the requirment of larger capacity and more users of PON system in the future,an architecture of UDWDM-PON system is proposed.A real-time coherent UDWDM-PON system is experimental demonstrated.The downlink 1000×10 Gb/s PDMQPSK signals are transmitted in 5 GHz interval on 40 km SSMF,and uplink 200×10 Gb/s PDM-QPSK signals are transmitted in 12.5 GHz interval.In order to further improve the sensitivity of the ONU receiver,the downstream signal can also choose the PS-QPSK modulation format to provide each user with data rate of 7.5 Gb/s.By using the method of rotating disturbance on PS-QPSK,the real-time digital signal processing of PDM-QPSK formats and PS-QPSK formats are realized in FPGA at the same time.The results show that the proposed PDM-QPSK and PS-QPSK coherent UDWDM-PON downstream system can achieve the power budget of 28 d B and 30 d B respectively.The proposed architecture and real-time experimental results can provide a reference for the implementation of future optical access network systems with super multi-user and large capacity requirements.2)According to the characteristics of TDM-PON upstream burst mode transmission,a new design scheme of preamble and corresponding digital signal processing algorithm are proposed.By sharing preamble among several algorithm modules and pre-storage of equalization coefficient,the length of preamble is shortened,the time of signal recovery is reduced,and the transmission efficiency is improved.The designed preamble and DSP algorithm are verified and demonstrated in the 112 Gb/s PDM-QPSK coherent TDM-PON system.The experimental results show that the time length of the preamble is shortened by nearly 26% and the transmission performance is not reduced.3)Aiming at some problems and situations of the coherent PON application,the key algorithms of digital signal processing in coherent receiver are studied and optimized.Firstly,for some cases with fast polarization rotation transition in aerial optical cable,a novel LMS equalization algorithm is proposed.The algorithm uses the relationship between the two polarizations to calculate the error in order to eliminate the influence of frequency and phase deviation in the calculation process.This new LMS algorithm can track polarization rotating speeds of 12 Mrad/s and 3.3 Mrad/s in 100 Gb/s PDM-QPSK and 400Gb/s PDM-16 QAM simulation systems,and track 2.5 Mrad/s polarization rotating speeds in 10 Gb/s PDM-QPSK real-time experiments.Compared with the tracking speed of several hundred krad/s achieved by the common algorithm,the polarization state tracking ability by the novel algorithm is greatly improved.Secondly,in order to solve the problem of amplitude and phase imbalance between inphase(I)and quadrature(Q)channels in coherent receiver,an improved equalizer with real valued structure is realized in real-time experimental system.The implementation complexity and resource consumption are analyzed.It is proved that reception sensitivity can be greatly improved at the cost of few resources.In extreme cases,the performance can be improved by 2.5 d B in 10 Gb/s PDM-QPSK real-time experiments,and the impact of IQ imbalance can be compensated effectively.Finally,in order to meet the demand of low delay and low power consumption requirements of the ONU in PON system,the DSP algorithm of ONU side is optimized.By grading multiple modules and adaptively enabling the part of the hierarchical module,the power consumption on the ONU side can be reduced,the delay and power consumption of the whole PON system is effectively saved.