  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.
