Adaptive probabilistic shaping of free-space optical systems based on an APD detector

This paper proposes an adaptive probabilistic shaping technology based on avalanche photodiode (APD) gain control in the atmospheric turbulence channel. The scintillation index calculated by APD in real-time is used as the channel state information, and the mapping relationship between the optimal gain of APD and the scintillation index is derived. At the transmitting end, based on the genetic algorithm, the optimal distribution of the transmitted signal is solved with the generalized mutual information (GMI) as the objective function. This paper uses the Gamma-Gamma distribution as the turbulence channel model, conducts a simulation experiment of multi-level pulse amplitude modulation (M-PAM) signal transmission, and uses the Monte Carlo method to calculate the GMI and normalized generalized mutual information (NGMI). The results show that compared with the uniform distribution, the adaptive probability shaping technology can effectively improve the achievable information rate, and under the same channel conditions, the GMI has a maximum value at the optimal APD gain.