Experimental and numerical comparison of probabilistically shaped 4096 QAM and a uniformly shaped 1024 QAM in all-Raman amplified 160 km transmission

• Published in: Optics express 2018-02, Vol.26 (3), p.3535-3543
• Main Authors: Okamoto, Seiji, Terayama, Masaki, Yoshida, Masato, Kasai, Keisuke, Hirooka, Toshihiko, Nakazawa, Masataka
• Format: Article
• Language: English
• ISSN: 1094-4087; 1094-4087
• DOI: 10.1364/OE.26.003535

We describe an experimental and numerical comparison of a probabilistically shaped (PS) 4096 QAM signal and a uniformly shaped 1024 QAM signal. Both modulation formats have the same transmission rate and a spectral efficiency of 15.3 bit/s/Hz. In the computational simulation, we compared the generalized mutual information (GMI) of both modulation formats with bit-wise soft decision decoding and bit-wise hard decision decoding. For bit-wise hard decision decoding with an overhead of 7%, a shaping gain of 1.8 dB was attained. Then we successfully transmitted a single-channel PS-4096 QAM signal for the first time in an all-Raman amplified 160-km link, in which the transmission performance was improved compared with a uniformly shaped 1024 QAM with the same transmission rate. Transmissions with a high QAM multiplicity were achieved by using an optical phase-locked loop (OPLL) and a frequency stabilized fiber laser locked to an acetylene absorption line. Thanks to a shaping gain based on a bit-wise hard decision decoder, the 1.9-dB power margin, which agreed with the simulation result to within 0.1 dB, was increased after transmission.