Exploiting polarization state for beyond 10 Gbps underwater optical wireless data transmission in hostile channel conditions

Abstract

This paper experimentally demonstrates the performance of subcarrier intensity modulation with polarisation division multiplexing (SIM-PDM) in a range of different water conditions. Underwater optical wireless communication (UOWC) is an emerging technology that offers high speed, low latency links over link distances in the order of metres. However, the effects of the UOWC channel present a challenge when designing a reliable link. These include: turbulence induced fading, which causes fluctuations in the received signal amplitude; particulate absorption, which causes an attenuation in the received optical power; and scattering, which causes spatial and temporal dispersion in the received signal. The SIM technique offers a resilience to turbulence compared to the state of the art on-off keying scheme, whilst additionally offering the potential for multi-level modulation orders – and therefore increased data rates – by encoding data on the signal phase as well as amplitude. In this work, PDM is used in conjunction with SIM to increase the spectral efficiency by separately modulating data across two orthogonal polarisation states. As long as these signals propagate identical channels, the polarisation states are maintained. Here, two orthogonally polarised laser beams are independently modulated with quadrature amplitude modulation (QAM), implemented via SIM to form the QAM-SIM-PDM technique. The performance of this technique is evaluated in terms of bit error rate and the maximum achievable data rate in clear, turbulent, and turbid water conditions. It is shown that data rates in excess of 10 Gbps are achievable using the QAM-SIM-PDM technique.