Constellation Shaping for WDM systems using 256QAM/1024QAM with Probabilistic Optimization

In this paper, probabilistic shaping is numerically and experimentallyinvestigated for increasing the transmission reach of wavelength divisionmultiplexed (WDM) optical communication system employing quadrature amplitudemodulation (QAM). An optimized probability mass function (PMF) of the QAMsymbols is first found from a modified Blahut-Arimoto algorithm for the opticalchannel. A turbo coded bit interleaved coded modulation system is then applied,which relies on many-to-one labeling to achieve the desired PMF, therebyachieving shaping gain. Pilot symbols at rate at most 2% are used forsynchronization and equalization, making it possible to receive inputconstellations as large as 1024QAM. The system is evaluated experimentally on a10 GBaud, 5 channels WDM setup. The maximum system reach is increased w.r.t.standard 1024QAM by 20% at input data rate of 4.65 bits/symbol and up to 75% at5.46 bits/symbol. It is shown that rate adaptation does not require changing ofthe modulation format. The performance of the proposed 1024QAM shaped system isvalidated on all 5 channels of the WDM signal for selected distances and rates.Finally, it was shown via EXIT charts and BER analysis that iterativedemapping, while generally beneficial to the system, is not a requirement forachieving the shaping gain.