In strong oceanic turbulence, we investigate the bit error rate (BER) performance of underwater wireless optical communication links by employing phase shift keying subcarrier intensity modulated Gaussian laser beam at the transmitter and positive-intrinsic-negative photodetector having finite sized aperture at the receiver. Using the extended Huygens–Fresnel principle, which is conventionally used to analyze the optical beam propagation through turbulence, we evaluate the optical intensity and corresponding signal power over the receiver aperture. Gamma–gamma statistical model for the received intensity is adopted due to strong oceanic turbulence and the required aperture averaged scintillation for this model is obtained by the use of asymptotic Rytov theory. In our performance investigation, we consider the effects of various oceanic turbulences, modulation, receiver noise type, and the photodetector parameters on the BER performance.
The average bit-error-rate, ⟨ BER ⟩ , of optical wireless system using higher order mode laser beam is investigated when atmospheric turbulence shows anisotropic and non-Kolmogorov characteristics. Results reveal that increase in anisotropy in both x- and y-directions positively affects the optical wireless systems performance. Increase in the beam order results in an increase in ⟨ BER ⟩ for any anisotropy level, and thus, higher order beams adversely affect the optical wireless systems performance. Larger structure constant, beam source size, and propagation distance result in larger ⟨ BER ⟩ , but larger wavelength, inner scale length, and signal-to-noise ratio tend to reduce ⟨ BER ⟩ . Increase in the power-law exponent of non-Kolmogorov turbulent spectrum first increases the ⟨ BER ⟩ until a certain value, and then ⟨ BER ⟩ starts to decrease when the power-law exponent is further increased. Adverse effect of higher order laser beam holds to be valid for any power-law exponent of non-Kolmogorov turbulence.
Turbulence affects optical propagation, and, as a result, the intensity is attenuated along the path of propagation. The attenuation becomes significant when the turbulence becomes stronger. Transmittance is a measure indicating how much power is collected at the receiver after the optical wave propagates in the turbulent medium. The on-axis transmittance is formulated when a flat-topped optical beam propagates in a marine atmosphere experiencing anisotropic non-Kolmogorov turbulence. Variations in the transmittance are evaluated versus the beam source size, beam number, link distance, power law exponent, anisotropy factor, and structure constant. It is found that larger beam source sizes and beam numbers yield higher transmittance values; however, as the link distance, power law exponent, anisotropy factor, or structure constant increase, transmittance values are lowered. Our results will help in the performance evaluations of optical wireless communication and optical imaging systems operating in a marine atmosphere.