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

TL;DR

This paper investigates the benefits of probabilistic constellation shaping (PCS) in short-reach intensity modulation direct detection systems without optical amplifiers, revealing conditions under which PCS can be advantageous despite peak power constraints.

Contribution

It provides a comprehensive analysis of PCS in IM-DD systems without optical amplifiers, highlighting how peak-to-average power ratio enhancement can enable classical PCS benefits under peak power constraints.

Findings

PCS can be beneficial if peak-to-average power ratio is increased.

Experimental validation with 4- and 8-ary PAM demonstrates the potential advantages.

Peak power constraints can be mitigated through signal enhancement techniques.

Abstract

Probabilistic constellation shaping (PCS) has been widely applied to amplified coherent optical transmissions 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) direct detection (DD) systems. As commercial IM-DD systems typically do not employ optical amplification to save the cost and power consumption, they are no longer subject to an average power constraint (APC) but a peak power constraint (PPC), which poses unique challenges to take full advantages of PCS. This paper provides a comprehensive investigation of PCS in IM-DD systems without optical amplifiers. In particular, we reveal that if the transmitter enhances the peak-to-average power ratio of the signal, a PPC system can be partially or even fully converted to an APC system in which the classical PCS offers its merits. The findings are verified through an IM-DD experiment using 4- and 8-ary pulse amplitude modulations.