Bit error rate of M-pulse position modulated laser beams for vertical links operating in weak oceanic turbulence

Abstract

Abstract The on-axis scintillation index of laser beams is investigated by employing the Rytov method in weakly turbulent oceanic medium for up/downlink coupling of laser communication between any underwater vehicles or divers. For vertical link, the formulation of the on-axis scintillation index of laser beams is derived analytically and evaluated for plane, collimated Gaussian and spherical beams in specific medium, including the Atlantic Ocean at mid and low latitudes associating temperature and salinity changes at low latitude, at mid latitude-summer and at mid latitude-winter. Using the scintillation index, bit error rate (BER) performances of M-pulse position modulation (M-PPM) are investigated for these types of laser beam. The variations of the scintillation index against the uplink/downlink propagation distances, source size and zenith angle are examined, and BER variations versus the Kolmogorov microscale and the symbol orders, and results are compared. It is noted that the behavior of the scintillation index that depends on the relative strength of temperature and salinity fluctuations which changes in depth, is different for uplink/downlink, and for each latitude due to its distinct characteristics. Source size that minimizes the scintillation index values is in the range of about 0.1 cm-0.2 cm for all latitudes.