Accurate decoding arithmetic of LDPC based on APD receivers

Proceed from the detected optical field's quantum statistics model of the Avalanche Photo Diode (APD) photoelectric detection receivers, an accurate realize method of Intensity-Modulation/Direct-Detection (IM/DD) Free Space Optical Communication (FSOC) fit Low Density Parity Check(LDPC)Codes decoding algorithm was investigated. This scheme, which was based on APD receivers' output statistical model(Webb-Gaussian Model), derivated the accurate calculating method of the original value of the likelihood ratio in LDPC decoding arithmetic over M-PPM demodulation. A detailed decoding process was given at the same time. By taking theoretical analysis and simulations, a result has been proved that:comparing with the most frequent used asymmetric Gaussian approximation model of APD receivers, the scheme introduced in this paper could get a gain of 1-2 dB in the decoding performance while in the same SNR. This gives the theoretical support in order to improve the performance of FSOC system further more.