Path loss characteristics of green light non-line-of-sight communication in turbid seawater

This study proposes a laser underwater non-line-of-sight (NLOS) multiple scattering simulation model incorporating the pointing probability method. Using MATLAB software, the transmission characteristics of a 532 nm laser in different seawater environments were simulated, and the effects of seawater type, transceiver elevation angle, and transmission distance on the path loss were systematically analyzed. The simulation results indicate that over longer transmission distances, path loss in NLOS transmission increases significantly with rising seawater turbidity, which is consistent with the traditional line-of-sight transmission pattern. However, during short-distance transmission, NLOS transmission exhibits a phenomenon distinctly different from line-of-sight transmission: path loss decreases as seawater turbidity increases. This "turbidity gain" effect, which contradicts traditional theory, is fundamentally attributed to the following reasons under close-range NLOS transmission conditions: higher turbidity enhances the multiple scattering effect in water, while, simultaneously, the probability of backward scattering in turbid water is lower compared to that in clear seawater. These two factors together enable more forward-scattered light energy to reach the receiver via non-direct paths, thereby partially compensating for the signal loss caused by absorption and scattering. Short-distance experimental studies have further validated the above simulation results. This discovery not only challenges conventional understanding of underwater optical transmission but also provides a novel theoretical foundation, to our knowledge, and technical support for optimizing receiver-side signal acquisition strategies, designing underwater optical communication systems, and evaluating the performance of marine optical detection technologies.