Abstract
Free-space optical communications (FSO) propagated over a clear atmosphere
suffers from irradiance fluctuation caused by small but random atmospheric
temperature fluctuations. This results in decreased signal-to-noise ratio
(${\hbox {SNR}}$)
and consequently impaired performance. In this paper, the error performance
of the FSO using a subcarrier intensity modulation (SIM) based on a binary
phase shift keying (BPSK) scheme in a clear but turbulent atmosphere is presented.
To evaluate the system error performance in turbulence regimes from weak to
strong, the probability density function (pdf) of the received irradiance
after traversing the atmosphere is modelled using the gamma-gamma distribution
while the negative exponential distribution is used to model turbulence in
the saturation region and beyond. The effect of turbulence induced irradiance
fluctuation is mitigated using spatial diversity at the receiver. With reference
to the single photodetector case, up to 12 dB gain in the electrical SNR is
predicted with two direct detection PIN photodetectors in strong atmospheric
turbulence.
© 2009 IEEE
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