Evaluation of the scintillation loss for optical communication systems with direct detection

In optical communications through the atmosphere, the evaluation of a link feasibility often requires the quantification of the scintillation penalty in terms of power loss. To find how much additional optical power is needed to reach the bit-error-rate (BER) requirements, the optical-power fluctuations must be characterized as well as the response of the receiver to those fluctuations. In the present analysis, the direct-detected optical power is assumed to be either lognormal or gamma-gamma distributed. To account for the dynamics of the atmospheric channel, a distinction is made between short-term and long-term BERs. For a simple On-Off Keying (OOK) modulation, expressions of scintillation losses are given for different system requirements. Specifically, an upper bound is set to any of the three following quantities: the long-term BER, the probability of having a too-high short-term BER, or the mean time during which the short-term BER is too high. Results show that, without any fade mitigation, losses under moderate scintillation are considerable. Finally, a simple code-word approach shows how scintillation losses can be reduced by channel coding. (c) 2007 Society of Photo-Optical Instrumentation Engineers.