Information bit logarithm likelihood ratio calculation method of M-PPM communication system based on APD

Abstract

The invention discloses an information bit logarithm likelihood ratio calculation method of an M-PPM communication system based on an APD. The information bit logarithm likelihood ratio calculation method is characterized by including the following steps that first, according to the characteristics of M-PPM signals in optical communication in free space, the probability density of the number of electrons output by the APD when the M-PPM signals have optical pulses in every time slot and the probability density of the number of electrons output by the APD when the M-PPM signals do not have optical pulses in every time slot are respectively calculated with an accurate statistic model of the number of electrons output by the APD; second, according to the probability density of the number of electrons output by the APD in every time slot of the M-PPM signals, the likelihood ratio in every time slot of the M-PPM signals is calculated; third, according to the likelihood ratio in every time slot of the M-PPM signals, the logarithm likelihood ratio of every bit of the M-PPM signals is calculated. The logarithm likelihood ratio determined with the method is high in accuracy and can be used for correcting error generated in the decoding operation process in the prior art as an initial value of error control coding iterative decoding, the decoding accuracy is improved, and thus the performance of the communication system is promoted.

Description

technical field [0001] The invention relates to a signal processing technology in an optical communication system, in particular to a calculation method of an information bit logarithmic likelihood ratio of an APD-based M-PPM communication system. Background technique [0002] Multi-ary pulse position modulation (M-PPM) is an intensity modulation / direct detection quadrature signaling scheme. Under the complex peak-average power constraints of long-distance free-space optical links, M-PPM is still the best modulation method closest to the channel capacity limit. Due to the existence of unfavorable factors such as light attenuation caused by absorption and scattering effects of various gas molecules and aerosol particles in free space, and light intensity fluctuations caused by atmospheric turbulent motion, the channel conditions deteriorate sharply. This is extremely unfavorable for M-PPM communication systems that require a large signal-to-noise ratio to obtain ideal system...

AI Technical Summary

Problems solved by technology

Although this approximation method using an asymmetric Gaussian model reduces the computational complexity to a certain extent, the accuracy of the log-likelihood ratio obtained is low, and this log-likelihood ratio is used as the initial value of iterative decoding It will cause error accumulation in the subsequent decoding operation process, affect the decoding result, cause decoding errors, and eventually lead to the deterioration of the performance of the communication system

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