Does Probabilistic Constellation Shaping Benefit IM-DD Systems without Optical Amplifiers?

Probabilistic constellation shaping (PCS) has been widely deployed to coherent optical communications owing to its shaping gain over the uniform signaling and fine-grained rate adaptation to the underlying fiber channel condition. These merits stimulate the study of applying PCS to short-reach applications dominated by intensity modulation (IM) and direct detection (DD). Different from coherent systems, commercial IM-DD systems typically do not employ optical amplification to save the cost and power consumption. This brings a few unique challenges to take advantage of PCS, because the system is no longer subject to an average power constraint (APC) but a peak power constraint (PPC). This paper provides a comprehensive investigation of PCS in IM-DD systems without optical amplifiers. In particular, we reveal that a PPC system can be partially or even fully converted to an APC system in which the conventional PCS offers its merits, when the electronic transmitter uses digital signal processing that enhances the peak-to-average power ratio of the signal. The findings are experimentally verified in an IM-DD system using 4- and 8-ary pulse amplitude modulations.