Time domain zero-padding based adaptive-PAM signal transmission with high spectral efficiency in IMDD optical communication system

In this paper, an adaptive pulse amplitude modulation (APAM) scheme is proposed and experimentally demonstrated in the intensity-modulation and direct-detection (IMDD) optical communications system. In the proposed scheme, the channel is divided into two sub-channels, and different PAM mapping can be chosen for different sub-channel according to the fading conditions. In addition, the 20-km standard single mode fiber (SSMF) transmission of 24 Gbit/s 16/4-APAM signal with the spectral efficiency (SE) up to 6 bit/s/Hz is experimentally demonstrated. The experiment results show that the bit error rate (BER) of the 16/4-APAM signal can be achieved less than 2.4e−2.