Adaptive Modulation for FSO IM/DD Systems With Multiple Transmitters and Receivers

Atmospheric turbulence is a major impairment in free space optical (FSO) communication systems. Since turbulence induced fading exhibits quasi-static characteristics, reliable feedback of channel state information is possible allowing the implementation of adaptive transmission. In this letter, we consider multiple-input single-output (MISO) and single-input multiple-output (SIMO) FSO systems with pulse position modulation (PPM) and pulse amplitude modulation (PAM). We propose an adaptive algorithm where the modulation size/type is adjusted according to the channel fading conditions. We formulate the design of adaptive algorithm to maximize the spectral efficiency under peak and average power constraints while maintaining a targeted value of outage probability. We present extensive Monte Carlo simulations to demonstrate the performance of proposed adaptive scheme over turbulence channels.