Research On DSP Algorithms Of 100Gbps PM-(D)QPSK Optical Coherent Transmission System

With the rising popularity of high-speed and high-quality videos, data traffics such as HDTV, IPTV and P2P, the bandwidth requirement for the backbone network has increased dramatically. The implementation of 100Gbps efficient transmission per wavelength channel can provide supporting technologies for the construction of super capacity DWDM transmission network and is significant to the construction of national information infrastructure. The 100 Gbps PM-(D)QPSK optical coherent transimission system combines the polarization multiplexing, multi-level modulation and coherent detection technologies, which not noly has the high OSNR sensitivity and frequency spectrum efficiency for the 50GHz wavelength interval of current DWDM link design, but also can employ digital signal processing (DSP) algorithms to compensate for transmission impairments. Therefore it is a suitable candidate for future high-speed optical transmission system. This paper mainly researches on the key DSP algorithms for 100Gbps PM-(D)QPSK optical coherent transmission system and gains a number of innovative research achievements. The main work and innovative contributions are listed as follows.1?In optical coherent receiver, the feed-forward digital timing recovery has the high computing complexity and requirement for ADC sampling rate, and the hybrid timing recovery loop can not compensate the sampling phase errors for two polarization signal simultaneously because of non-all-digital implementation. To solve these problems, we propose to use a Gardner back-feed digital timing recovery algorithm in the 112Gbps PM-(D)QPSK optical coherent receiver and analyze the influences of the various effects induced in optical transmission processing to the timing recovery algorithm's performance. Furthermore, in order to make it easy to hardware implementation and break the limitation of electronic processing rate, we propose a parallel digital timing recovery algorithm based on the above research. Simulation results show that this new algorithm can support 160 parallel processing units to adjust time synchronization for 320 asynchronous samples at the same time. Therefore, the hardware clock frequency is reduced from 56GHz in serial processing to 175MHz in parallel.2?To solve the problem that the practicability of 112Gbps PM-(D)QPSK optical coherent transmission system is greatly limited by the low dispersion tolerance of Gardner timing error detecting algorithm, we propose a joint scheme that embeds an adaptive equalizer into the all-digital timing recovery loop. The simulation results demonstrate that the joint scheme can not only improve the tolerance of system toward residual dispersion form±200ps/nm to above±800ps/nm, but also accomplish synchronization and compensation linear transmission impairments simultaneously. Furthermore, in order to cope with the speed bottleneck of hardware, we propose an adaptive equalization scheme and design a parallel joint scheme of timing recovery and equalization. Finally, we demonstrate that the proposed parallel scheme allow the hardware to process 112G bit/s POLMUX-DQPSK signal at the hundreds MHz range.3?In order to reduce the probability of phase cycle slip caused by residual frequency offset, we propose a carrier frequency offset estimation algorithm based on training sequence. The new algorithm can use training symbol to remove the modulated phase information of estimated signal, which improves the estimation accuracy and avoids the Mth power operation of the classical algorithm. Therefore, the proposed algorithm reduces the computing complexity and has a constant theoretical estimated range for carrier frequency offset ([-RS/2,+RS/2], RS is symbol rate) which is independent of the signal phase modulation level and format. The simulation results demonstrate that its estimated frequency offset range for 112Gb/s PM-(D)QPSK signal can reach [-13GHz,+13GHz] and the training overhead can less than 0.000011%?4?We build a simulation platform of 112Gbps PM-(D)QPSK optical coherent transmission system by using VPItransmissionMaker 8.5and MATLAB softwares. Base on the feasibilities of the key DSP algorithms demonstrated by simulation, we carry out the back-to-back and 1240km transmission experiments of 112Gbps PM-(D)QPSK singal. In the 1240km long haul transmission experiment, the dispersion compensating fibers (DCFs) are not employed in transmission link. All the accumulated dispersions are compensated by DSP algorithms in electric field. The compensation quantity of CD is large than 20000ps/nm and BER performance of system can superior to 1E-3 at 17dB of OSNR when the launch power of signal is no more than +2dBm.