Underwater optical wireless channel capacity under oceanic turbulence using spatial diversity techniques

Conventional spatial diversity techniques do not reduce the severe impact of the main impairments in underwater optical wireless communication (UOWC) such as absorption and scattering. This paper addresses a complete comparison of Multiple-Input/Single-Output (MISO) UOWC systems by using spatial repetition coding (SRC) and transmit laser selection (TLS) strategies over salinity-induced oceanic turbulence in clear ocean and coastal waters. We consider an SRC system with sufficient space between sources to efficiently increase the total transmitted power while satisfying eye-safety requirements by assuming a maximum optical power per source in order to obtain the same ergodic capacity performance of a TLS system with the same number of sources. Novel closed-form expressions and asymptotic results are derived to compute such a performance and optimize the UOWC system design. The presented results demonstrate that MISO UOWC systems achieve a greater ergodic capacity as the number of laser sources increases in all kinds of waters. This improvement is greater when SRC scheme is adopted for all the presented UOWC scenarios except when extreme oceanic turbulence conditions are considered. Monte Carlo simulations verify analytical and asymptotic results.