Performance Enhancement of FEC-Coded 32QAM DMT Signals With Rate-Flexible HCS in FSO Systems

A rate-flexible hybrid constellation shaping (HCS) is proposed and demonstrated in polar-coded discrete multi-tone (DMT) free-space optical (FSO) communications. This joint design considers the constraints of geometry and probabilities in coded modulations to maximize the channel capacity and flexibly adjusts the parameters according to the data rates. Compared to the regular scheme over free-space links, the capacity of the proposed scheme at different data rates is improved by about 0.25 bits/ symbol when the launch power is -6dBm. And the power budgets of our approach with polar codes are saved more than 1.0 dB at the bit error rate of
${10}^{-4}$
. Even in weak turbulence conditions, the Q2 factor of the proposed maintains the maximum and is improved by
$1.84\sim 3.40$
dB than the regular scheme, indicating its better ability to combat turbulence.