Adaptive coding and modulation (ACM) technique for performance enhancement of FSO Link

One of the major factors which degrades the Free Space Optics (FSO) link performance is the atmospheric turbulence. Turbulence manifests in increased Bit Error Rate (BER) leading to degradation in the link performance. Adaptive Coding and Modulation (ACM) technique is used to mitigate this effect. This technique is an adaptive transmission scheme wherein transmission parameters such as modulation scheme, code rate, etc., are adjusted based on Channel State Information (CSI). In this paper, we have considered lognormal channel model in low turbulence conditions. Three adaptive transmission schemes viz., variable data rate, variable modulation schemes and variable coding rate are considered. The entire work has been carried out in MATLAB. We see improvement in FSO link performance in terms of average transmitted data rate when variable data rate adaptation scheme is used. Improvement in spectral efficiency is observed when the other two adaptation schemes are used. Design of adaptive transmitter for all the three schemes is presented and data rate of RF feedback channel is determined.

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