| Currently, free space optical communication has attracted more and more attentions. A lot of research has proved that the effect of turbulence atmosphere to the free space optical communication is very significant and is the key restrict factor of performance improvement.In this work, exponentiated Weibull distribution(EW) has been used as the fading channel model, and the expressions of symbol error rate(SER) for MPPM of free space optical system have been calculated with hard and soft decisions. In hard decision method, the SER expressions with fixed, optimized and dynamic decision threshold scheme(FDT, ODT, DDT) have been derived with Gauss-Laguerre quadrature rule. In soft decision method, the SER expression is derived based on curve fitting method. This work is helpful for the performance improvement of the free space optical communication in turbulence atmosphere and system design.The main contents are as follows:1. The exponentiated Weibull distribution has been used to describe the probability density function of the received intensity of free space optical communication systems. The closed forms of the bit error rate for BPSK and MPSK modulation have been obtained using the Gauss-Laguerre quadrature rule. Monte Carlo simulation has been used to verify the correctness of the analytical result. The SERs in weak-to-strong turbulence atmosphere is plotted against the average signal to noise ratio(SNR), respectively. The study shows that FDT will result in the error floor, which can be eliminated by ODT and DDT. In addition, the SER performance of DDT scheme is better than that of ODT.2. The SER expressions for MPPM FSO system over EW distribution with soft decision method are derived. And the approximate SER expressions are derived based on the curve fitting method with Gaussian fitting function. In addition, the comparison have been made between the accurate expressions and approximate expressions. Monte Carlo simulation has also been used to verify the correctness of the analytical result. It is found that soft decision method performs better than hard decision method under the same turbulence conditions and receiver apertures. |
