LC-OSTAR-IRS: Tunable Liquid Crystal Surfaces for High-Performance UWOC

This letter presents a liquid-crystal-based optical simultaneously transmitting and reflecting intelligent surface (LC-OSTAR-IRS) for underwater wireless optical communication (UWOC). Unlike conventional hybrid or metasurface designs, the proposed architecture exploits the refractive-index contrast between water and tunable liquid crystals (LCs), enabling a single element to dynamically support both reflection and transmission. This provides dual region coverage while eliminating the need for separate components. We integrate LC-OSTAR-IRS with non-orthogonal multiple access (NOMA) and rate-split multiple access (RSMA) schemes, evaluating performance under realistic underwater turbulence and pointing errors. Results demonstrate that LC-OSTAR-IRS with RSMA achieves significantly higher aggregate spectral efficiency (ASE) and superior user fairness compared to hybrid OSTAR-IRS (H-OSTAR-IRS) benchmarks, particularly in geometric configurations satisfying total internal reflection (TIR) conditions.