  We propose an adaptive transmission technique for free space optical (FSO)
systems, operating in atmospheric turbulence and employing subcarrier phase
shift keying (S-PSK) intensity modulation. Exploiting the constant envelope
characteristics of S-PSK, the proposed technique offers efficient utilization
of the FSO channel capacity by adapting the modulation order of S-PSK,
according to the instantaneous state of turbulence induced fading and a
pre-defined bit error rate (BER) requirement. Novel expressions for the
spectral efficiency and average BER of the proposed adaptive FSO system are
presented and performance investigations under various turbulence conditions
and target BER requirements are carried out. Numerical results indicate that
significant spectral efficiency gains are offered without increasing the
transmitted average optical power or sacrificing BER requirements, in
moderate-to-strong turbulence conditions. Furthermore, the proposed variable
rate transmission technique is applied to multiple input multiple output (MIMO)
FSO systems, providing additional improvement in the achieved spectral
efficiency as the number of the transmit and/or receive apertures increases.
