Efficient Symbol Detection for the FSO IM/DD System with Automatic and Adaptive Threshold Adjustment: The Multi-level PAM Case

Abstract: To detect M-ary pulse amplitude modulation signals reliably in an FSO communication system, the receiver requires accurate knowledge about the instantaneous channel attenuation on the signal. We derive here an optimum, symbol-by-symbol receiver that jointly estimates the attenuation with the help of past detected data symbols and detects the data symbols accordingly. Few pilot symbols are required, resulting in high spectral efficiency. Detection can be performed with a very low complexity. From both theoretical analysis and simulation, we show that as the number of the detected data symbols used for estimating the channel attenuation increases, the bit error probability of our receiver approaches that of detection with perfect channel knowledge.