Convergence Analysis of Iterative Demodulation and Decoding in Free Space Optical Communication Based on EXIT Chart

The joint iterative structure of demodulation and decoding based on Soft Input Soft Output (SISO) algorithm is studied with product code and Pulse position modulation (PPM) in the Free Space Optical (FSO) communication system. To analyze the convergence behavior of iterative processors in the decoder and demodulator, the Extrinsic information transfer (EXIT) chart is introduced and studied. With iterative model suitable for decoding and demodulation being constructed, the iterative convergence of code blocks under different optical powers are simulated. Simulations show that, EXIT chart describes the convergence of the iterative process of soft demodulation and decoding intuitively, and also, it helps to predict the error performance waterfall descent region of the system.