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Compared to on-off keying (OOK), M-ary pulse amplitude modulation (M-PAM,M>2) is more spectrally efficient. However, to detect M-PAM signals reliably,the requirement of accurate channel state information is more stringent.Previously, for OOK systems, we have developed a receiver that requires fewpilot symbols and can jointly detect the data sequence and estimate the unknownchannel gain implicitly. In this paper, using the same approach, we extend ourprevious work and derive a generalized receiver for M-PAM systems. AViterbi-type trellis-search algorithm coupled with a selective-store strategyis adopted, resulting in a low implementation complexity and a low memoryrequirement. Therefore, the receiver is efficient in terms of energy, spectra,implementation complexity and memory. Using theoretical analysis, we show thatits error performance approaches that of maximum likelihood detection withperfect knowledge of the channel gain, as the observation window lengthincreases. Also, simulation results are presented to justify the theoreticalanalysis.

Efficient Direct Detection of M-PAM Sequences with Implicit CSI Acquisition for The FSO System