Polar coded probabilistic shaping PAM8 based on many-to-one mapping for short-reach optical interconnection.

In this paper, a polar coded probabilistic shaping (PS) 8-ary pulse amplitude modulation (PAM8) based on many-to-one (MTO) mapping is investigated for short-reach optical interconnection. By ingeniously assigning parity bits to ambiguities positions, no extra PS redundancy and no complex distribution matcher are required in the scheme comparing to traditional probabilistic amplitude shaping (PAS). The noise distributions after different transmission distances are studied and an optimal clock recovery method for PS signal is proposed to degrade the impact of severe eye skew effect on BER performance. The experimental results show that up to 1.2 dB and 0.8 dB shaping gains are respectively achieved over back-to-back (BTB) and 2-km standard single mode fiber (SSMF) transmission. With the help of the proposed optimal clock recovery method in the PS scheme, the shaping gain is improved from 0.15 dB to 0.4 dB after 10-km transmission. Moreover, compared to low-density parity-check (LDPC) code, the polar coded PS-PAM8 can provide an additional coding gain of 2.2 dB with code length of 256, which proves the performance superiority of polar code in short code length. Therefore, the proposed polar coded PS-PAM8 with low complexity and satisfactory BER performance is believed to be an alternative solution for the cost-sensitive short-reach optical interconnection.