Performance Evaluation of LOS and NLOS Vertical Inhomogeneous Links in Underwater Visible Light Communications

In this paper, underwater visible light communication (UWVLC) vertical links are modeled and evaluated, taking account of the inhomogeneous nature of underwater (UW) environment. An equivalent simplified model of stratified $N$ layers is employed in which variations in refractive index and attenuation profiles across UW depth are considered. A generalized path loss expression is deduced which allows estimation of the vertical link loss prior to link design. Mathematical expressions of the received power for line-of-sight (LOS) and non-line-of-sight (NLOS) links between transmitters (Tx) and receivers are deduced. We evaluate the performance by computing the received power and bit error rate for inhomogeneous underwater links. Numerical examples are used to illustrate the proposed models. Deviations from expected results when considering a homogeneous underwater model are discussed. A simple underwater bilayer model is then introduced, which is considered a rough approximate model in comparison to the stratified $N$ layers model. This model assists in estimating the UW link behavior without extensive calculations. The conditions necessary for applying this model are discussed and justified. Moreover, the effects of Tx orientation along with narrowing the transmitted light cone on LOS and NLOS vertical links are examined. A Tx power saving of 30%-50% in UWVLC links is proved to be achieved by rotating the Tx and narrowing its emitted light cone.