Abstract
Free-space optical (FSO) communication enables high rate data transmission
over the atmospheric channel. However, turbulence-induced fading poses severe
challenges for operating these systems. Employing spatial diversity has been
proposed as an effective measure to improve system performance. In this paper,
we first develop a comprehensive model for multiple-input multiple-output
(MIMO) coherent and differential FSO systems with heterodyne detection taking
into account all relevant signal and noise terms. For the sake of comparison,
a similar model is also developed for direct detection. An asymptotic performance
analysis is presented for MIMO FSO systems employing coherent and differential
space-time codes (STCs) over Gamma-Gamma fading channels. We consider the
practically important case of two transmit and an arbitrary number of receive
apertures and provide a simple STC design criterion for coherent and differential
FSO systems. Our results reveal that, in contrast to FSO systems with intensity
modulation and direct detection (IM/DD), for coherent and differential FSO
systems, orthogonal space-time block codes previously introduced in the RF
literature, are preferable over repetition STCs, which do not yield full diversity.
We quantify the asymptotic performance gain of coherent space-time coded FSO
systems compared to IM/DD systems and show that this gain is mainly caused
by the superiority of heterodyne detection compared to direct detection rather
than the different STC and modulation designs.
© 2012 IEEE
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