Research On The Characteristics Of FSO Generalized Channel

Free Space Optics (FSO), is a line-of-sight broadband wireless communication technology that uses light as media to transmit information in free space. It is the hot research subject nowadays in the world. The communication quality of FSO links is greatly affected by weather conditions and link distance. So appropriate modulation and channel coding technologies are required to be applied in FSO system to eliminate bad influences caused by variable channel characteristics and improve system reliability.In this paper, for the purpose of improving reliability and maturity of FSO system, the key technologies of modulation channel, coding channel, channel capacity, channel equalization are researched based on definition of generalized channel. The main work can be summarized as follows:(1) Several typical modulation technologies based on their characteristics are analyzed. Their performances are compared from aspects of symbol format, mathematical model and so on. Their merits and demerits on power efficiency and spectral efficiency are discussed additionally to provide theoretical basis for the selection of modulation method in practical application.(2) Build the proper channel model for the BSC, BEC, QEC and AWGNC to analyze the channel capacity based on the theory of generalized channel. The channel capacity of PPM and MPPM modulation are deduced in detail with QEC model. The selection of modulation parameters is studied to achieve optimizing channel capacity.(3) The key algorithms of several common channel coding technology, including RS code, Turbo code and LDPC code are deduced and proved. The coding and decoding methods are discussed in detail. Then the Galois field is studied thoroughly, the mathematical method of GF expansion from GF(3) to GF(3m) is deduced, which provides the theoretical basis for the hardware realization of BCH code with three states in Galois field. This coding method can map the L-PPM perfectly and improve signal constellation utilization ratio. (4) The maximum likelihood method is researched for PPM, MPPM and DPPM demodulation in AWGN channel. Formula derivation of log-likelihood function is discussed in detail. Then the hardware design scheme is discussed. At last, the hardware realization methods of encoder and decoder for the RS code, Turbo code and LDPC code are proposed, and has been validated in practical system.(5) In order to verify BER performance of various channel coding method in FSO system, the experimental measurement system is designed and many outdoor experiments are carried out under various weather conditions. Experimental data is obtained to do comparative analysis for the engineering reference.(6) Time domain equalization technology is discussed for its advantages of eliminating the bad influence caused by variable channel characteristics. The fixed-step and variable step-size LMS adaptive algorithms are researched and simulated with Matlab.This paper has presented detailed research on modulation channel, coding channel, channel capacity, channel equalization. The research results has shown that the FSO system based on above key technologies is feasible and technically realizable.