DSP-assisted 56 Gbps I/Q 16-QAM underwater optical wireless communication link: impact of water turbidity and receiver aperture on BER, EVM, and range

This paper describes the design and simulations of a 16-QAM in-phase/quadrature (I/Q) modulated underwater optical wireless communication (UOWC) link operating at 56 Gbps via coherent receiver architecture using adaptive equalizer, frequency offset estimation, and carrier phase estimation for minimizing turbulence-induced impairments. The trade-off between optical collection efficiency and achievable range is quantified by evaluating system performance for five representative water types (pure sea, clear ocean, coastal ocean, Harbor I, and Harbor II) over a range of distances for three different receiver aperture diameters (12 cm, 16 cm, and 20 cm): log(BER), error vector magnitude (EVM%), and received optical power (ROP). With all the water types, the 20 cm aperture provides maximum link ranges of 124 m, 46 m, 21.75 m, 9 m, and 5.25 m for pure sea, clear ocean, coastal ocean, Harbor I, and Harbor II water, respectively, and EVM is always contained within a narrow range of 16.6–17.3 % at the threshold ranges across the various water types. Constellation diagrams were taken at each FEC-limited range, which show increasingly closer symbol clustering with the increasing aperture size, ranging from 12 cm to 20 cm, supporting the quantitative measured BER and EVM trends.