Research On Subcarrier Multiplexing Based On Probabilistic Shaping For Transoceanic Fiber Transmission System

The optical fiber transmission system,which is the core of the information network,is under huge pressure to expand its capacity due to the increasing expansion of Internet traffic.As the cornerstone of the global broadband network and a key communication technique connecting countries throughout the world,the transoceanic fiber transmission system has set the highest demand for both transmission capacity and transmission distance.According to Shannon’s theorem,increasing transmission capacity reduces the signal-to-noise ratio and limits the increase in transmission distance.As a result,achieving ultra-long-distance and ultra-high-capacity transoceanic fiber transmission has become a research challenge in optical fiber communication.Multi-span fibers and optical amplifiers are cascaded in the transoceanic transmission system,and the accumulation of amplifier spontaneous emission(ASE)noise and fiber nonlinear effect has a serious impact on the system’s transmission performance.Researchers are focusing their efforts on how to better decrease the link damage effect to increase the capacity and the transmission distance of the transoceanic transmission system.This paper focuses on the combination of probabilistic shaping and subcarrier multiplexing to mitigate channel noise and improve nonlinearity tolerance,and the main research content is as follows:First,this thesis begins by reviewing the fundamental theory of coherent optical communication systems and briefly analyzing the system transmitter,transmission link,and receiver;After that,the principles of signal generation techniques such as probabilistic shaping and subcarrier multiplexing are discussed in depth;Finally,the principles of damage equalization algorithms such as dispersion,nonlinearity,frequency shifting,and laser phase noise are also discussed.Second,this thesis focuses on signal generation techniques based on a combination of probabilistic shaping and subcarrier multiplexing to improve the performance of transoceanic transmission system.The generation strategy and transmission performance of probabilistic shaping subcarrier multiplexing signals are investigated using simulated transoceanic fiber transmission system links.The simulation results show that the transmission distance of the probabilistic shaped quad-subcarrier PDM-16 QAM signal(H=3.5)is respectively 33.5% and 15.9% longer than that of the uniform quad-subcarrier and the probabilistic shaped single-carrier PDM-16 QAM signal with 50GHz-bandwidth filter.When the filter’s bandwidth is 37.5GHz,the probabilistic shaped dual and quadsubcarrier signal(H=3.7)have equal transmission performance,and the transmission distance of the probabilistic shaped dual-subcarrier PDM-16 QAM signal(H=3.7)is respectively 11.9% and 6.7% longer than that of the uniform dual-subcarrier and the probabilistic shaped single-carrier PDM-16 QAM signal(H=3.7)at 20% FEC threshold.Furthermore,we built an optical fiber loop transmission experimental platform to verify the feasibility of the scheme.Finally,to further improve the performance of the transoceanic transmission system,this paper focuses on multi-subcarrier optimization methods based on probabilistic shaping multi-subcarrier multiplexing signals.For the bandwidth-limiting effect of the multisubcarrier multiplexing signal,the entropy loading method and the pre-emphasis method are studied and compared.The simulation results demonstrate that the transmission performance of the two methods is respectively 10.1% and 7% higher than the unoptimized system at20% FEC threshold,so the pre-emphasis method is more suitable for this system.The transmission performance of the pre-emphasized probabilistic shaped quad-subcarrier PDM-16 QAM system is 3.2% higher than the previous better system with 37.5GHz-bandwidth filter at 20% FEC threshold,which achieves further improvement for the transoceanic transmission system.