Free-Space Optical Communication: A Diversity-Multiplexing Tradeoff Perspective

Due to the rapid growth of free-space optical (FSO) communication over the last few years, there is an exigency to develop diversity-multiplexing tradeoff (DMT) metric in order to compare and design new multiple-input multiple-output (MIMO) FSO schemes. This paper introduces the DMT for MIMO-FSO systems that utilize the intensity modulation and direct detection technique for which the channel gains are real and positive, contrary to MIMO radio-frequency (MIMO-RF) systems where the channel gains are complex. Log-normal, gamma-gamma, and negative exponential channel models are used in this paper to cover weak to saturated turbulence environments. It is shown that at zero multiplexing gain ( r ), the log-normal channel offers maximum but finite diversity order that depends on the block length within which the channel matrix is constant. On the other hand, negative exponential channel offers minimum diversity order. However, for r \geq 1 , DMT curves under all channel models are of the same nature; and for a given r , the diversity is half as that observed in the MIMO-RF channels. To gain additional insights into the DMT of an MIMO-FSO system, we also develop upper and lower bound expressions of DMT and determine the minimum block length at which these bounds are identical.