Abstract
In this paper, spatial pulse position modulation (SPPM) is used as a case study to investigate the performance of the optical spatial modulation (SM) technique in outdoor atmospheric turbulence (AT). A closed-form expression for the upper bound on the asymptotic symbol error rate (SER) of SPPM in AT is derived and validated by closely-matching simulation results. The error performance is evaluated in weak to strong AT conditions. As the AT strength increases from weak to strong, the channel fading coefficients become more dispersed and differentiable. Thus, a better error performance is observed under moderate-to-strong AT compared to weak AT. The performance in weak AT can be improved by applying unequal power allocation to make free-space optical communication (FSO) links more distinguishable at the receiver. Receive diversity is considered to mitigate irradiance fluctuation and improve the robustness of the system to turbulence-induced channel fading. The diversity order is computed as half of the number of detectors. Performance comparisons, in terms of energy and spectral efficiencies, are drawn between the SPPM scheme and conventional MIMO schemes such as repetition coding and spatial multiplexing.
Highlights
Free-space optical communication (FSO) technology is a promising complement to existing radio frequency communications
Our paper differs from these previous works in that we have considered a full-fledged optical SM (OSM) scheme, which entails using both the spatial index of the sources and the transmitted digital signal modulation to convey the information bits
The theoretical upper bound on the asymptotic symbol error rate (SER) for spatial pulse position modulation (SPPM) based an FSO system has been presented in different atmospheric turbulence regimes from weak to strong
Summary
Free-space optical communication (FSO) technology is a promising complement to existing radio frequency communications. We consider the use of a low-complexity MIMO technique, known as spatial modulation (SM) [8,9], to enhance the spectral efficiency of FSO systems. In the SSK scheme, no digital signal modulation is used, and the information bits are encoded solely on the spatial index of the optical sources. Our paper differs from these previous works in that we have considered a full-fledged OSM scheme, which entails using both the spatial index of the sources and the transmitted digital signal modulation to convey the information bits. In order to enhance the spectral efficiency of PPM, a variant of the OSM technique termed spatial pulse position modulation (SPPM) [17] is explored in this paper.
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