Outage Analysis in MIMO Free-Space Optical Channels with Pulse-Position Modulation

Free space optical communication is an attractive alternative to radio frequency for the purpose of transmitting data on the order of gigabits per second. The main drawback in communicating via the free space optical channel is the detrimental effect the atmosphere has on a propagating laser beam. Atmospheric turbulence causes random fluctuations in the irradiance of the received optical laser beam, commonly referred to as scintillation. The scintillation process is slow compared to the large data rates typical of optical transmission. As such, we adopt a quasi-static block fading model and study the outage probability of the channel under the assumption of orthogonal pulse-position modulation. We investigate the mitigation of scintillation through the use of multiple lasers and multiple apertures, thereby creating a multiple-input multiple output (MIMO) channel. Non-ideal photodetection is also assumed such that the combined shot noise and thermal noise are considered as signal-independent additive Gaussian white noise. Assuming perfect receiver channel state information (CSI), we compute the signal-to-noise ratio exponents for the cases when the scintillation is lognormal, exponential, gamma-gamma and lognormal-Rice distributed, which cover a wide range of atmospheric turbulence conditions. Furthermore, we illustrate very large gains, in some cases larger than 20 dB, when transmitter CSI is also available by adapting the transmitted electrical power. N. Letzepis is with Institute for Telecommunications Research, University of South Australia, SPRI Building Mawson Lakes Blvd., Mawson Lakes SA 5095, Australia, e-mail: nick.letzepis@unisa.edu.au. A. Guillen i Fabregas is with the Department of Engineering, University of Cambridge, Cambridge CB2 1PZ, UK, e-mail: guillen@ieee.org.