Adaptive Probabilistic Shaping Using Polar Codes for FSO Communication

Abstract

An adaptive polar coded probabilistic shaping scheme for free-space optical (FSO) communication is proposed and demonstrated to improve transmission performance. The input distribution can be adaptively adjusted by maximizing the achievable rate under different FSO turbulence channels. Furthermore, the transmission rate is less than the achievable rate to guarantee reliable transmission. In different FSO turbulent channels, the feasible regions of the optimization problem are explored with the aid of the exponential distribution, and the complexity of the single optimization is decreased to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$O( {\log ( {{\varepsilon ^{ - 1}}} )} )$</tex-math></inline-formula> . The systematic polar codes are adopted to combat the fading, and a sparse-dense encoder for polar codes is designed to maintain the target distribution. The achievable rate and block error rate (BLER) performance of this adaptive probabilistic shaping scheme are evaluated. The results indicate that the adaptive non-uniform distribution generated by the proposed scheme outperforms the uniform distribution under the same channel conditions. The code rate of 0.8 is more suitable than 0.9 in coping with various turbulence channels. The proposed scheme can obtain a range of 0.50∼0.90 dB shaping gains over the uniform distribution from weak to moderate turbulence channels for both the achievable rate and the BLER performance at 1.5 bits per channel use.