Underwater optical link range extension via real-time statistical method for symbol combining

Abstract

Underwater optical communication systems currently face several unique challenges, such as attenuation, scattering, and changes in background noise due to ambient light levels or disturbances. These impairments result in varying link quality, leading to limited link distance and difficulty in reliably decoding transmitted information. In order to enable longer links and increased sensitivity in noisy environments, we employ dynamic data rate adaptation based on forward error correction with symbol repetition. The receiver carries out decoding using the sum of log-likelihood ratios (LLRs) for each set of repeated symbols as soft inputs. However, misalignment caused by environmental disturbances or relative movement between underwater communication terminals, such as in vehicle-to-vehicle communication, requires phase detection to determine the correct alignment for sets of repeated symbols prior to computing the LLR sums. We present a novel heuristic, motivated by Kullback-Leibler divergence, using the distribution of LLRs to carry out phase alignment. The receiver continuously applies phase detection in real time to account for varying alignments. Results show that our proposed approach is effective in maintaining reliable optical links for critical communications in underwater environments with varying conditions. In the open ocean, where the extinction length can be 20 meters, this technology can be used to achieve a 2.76x increase in link distances.