Many-to-One Mapping Based Truncated Probabilistic Shaped 64-QAM for Metro Transmissions Enabled by Polar Code

Metro transmissions requiring high capacity with low complexity are becoming more appealing. Aiming to cope with the increasing traffic of metro transmissions, high-order quadrature amplitude modulation (QAM) formats requiring high optical signal-to-noise ratio (OSNR) are candidates owing to the high spectral efficiency. Although the distribution matcher (DM) based probabilistic shaping (PS) technology with suitable shaping depth could improve the OSNR sensitivity for high order QAM, the DM-based PS schemes are suboptimal for metro transmissions due to the high complexity. Besides, if a PS scheme with large shaping depth is employed for high order QAM, some difficulties on the receiver-side digital signal processing (DSP) chain would occur and further deteriorate the system sensitivity. Therefore, to achieve probabilistic shaping with low complexity and balance shaping depth and OSNR sensitivity simultaneously, a polar coded many-to-one (MTO) mapping based truncated probabilistic shaped 64-ary quadrature amplitude modulation (PTPS-64QAM) is proposed and experimentally investigated, which could decrease the system complexity by utilizing specific-designed MTO mapper and polar code with short code length simultaneously. Experimental results show that about 2-dB and 1-dB improvements in terms of OSNR sensitivity and optimal launch power could be obtained under optical back-to-back (OBTB) case and transmission case compared to uniform distributed 64QAM (UD-64QAM) respectively. And an 800-km transmission of 60-GBaud PTPS-64QAM with a post bit error ratio (post-BER) lower than 1E-5 could be achieved, which implies that PTPS-64QAM scheme might be a powerful candidate for future metro transmissions.