BPSK Subcarrier Intensity Modulated Free-Space Optical Communications in Atmospheric Turbulence

TLDR

Free‑space optical links experience irradiance fluctuations from atmospheric turbulence, which degrade signal‑to‑noise ratio and overall performance. This study evaluates the error performance of a binary phase‑shift keying subcarrier intensity‑modulated FSO system operating in turbulent air. The authors model received irradiance with a gamma‑gamma distribution for weak to strong turbulence and a negative exponential for saturation, and they mitigate turbulence effects using spatial diversity at the receiver. Compared to a single photodetector, employing two direct‑detection PIN photodetectors yields up to a 12 dB electrical SNR gain under strong turbulence.

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 (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.