Underwater optical wireless communication (UOWC) is an emerging technology designed to enable advanced high data rate applications, such as environmental monitoring, underwater exploration, and secure communication for military and defense purposes. Accordingly, a high-capacity UOWC system utilizing dual-polarization states and 16-quadrature amplitude modulation with orthogonal frequency-division multiplexing is proposed. The system leverages a single laser diode operating at 532 nm to generate orthogonally polarized signals that are transmitted along the X and Y polarization axes, achieving an independent data rate of 40 Gbps per polarization state with a total capacity of 80 Gbps. The performance of the proposed system is evaluated through the bit error rate (BER), error vector magnitude (EVM), and constellation diagrams for 10 distinct waterbodies. The findings demonstrate that water types with lower attenuation, such as pure water, Jerlov I, clear ocean, and Jerlov IA, enable longer transmission distances of 13.93, 13, 10, and 10 m, respectively, at a BER below the threshold limit (3.8×10−3) and an EVM of less than 16%. Conversely, higher attenuation in Harbor I and Jerlov III waters leads to shorter achievable ranges of 3 m at the same BER and EVM values.
A high-capacity spectral-efficient dual-polarization quadrature phase-shift keying (DP-QPSK)-polarization shift-keying (PolSK) hybrid modulation scheme for terrestrial free-space optics (FSO) transmission link is proposed and investigated. A DP-QPSK signal modulated at 300 Gbps and a PolSK signal modulated at 40 Gbps are simultaneously transmitted using a single optical carrier over the FSO link. The proposed link performance is investigated under different weather conditions, where the bit error rate metric is used to evaluate the performance of the PolSK modulated signal and the error vector magnitude parameter is used for the DP-QPSK signal. The FSO link range and the required received power are carefully explored. The conducted numerical simulations of the proposed system showed reliable 340-Gbps data transmission over link ranges varying from 1.6125 to 50 km depending on the weather conditions. The impact of the channel scintillation due to atmospheric turbulence is also investigated. The proposed high-speed FSO transmission system offers a promising solution for high-bandwidth hungry systems used for the internet of things, 5G, and smart cities. It can also be used in developing fronthaul/backhaul links for future wireless networks and optical access networks. The performance of the proposed transmission system is compared with recently published work in the literature.
We report the performance comparison of mode division multiplexing (MDM)-based radio over free space optics (RoFSO) transmission system using different modulation formats viz. alternate mark inversion, nonreturn to zero (NRZ), return to zero-differential phase shift keying (RZ-DPSK), and NRZ-DPSK under varying atmospheric turbulence conditions and data transmission rates. The results show that the NRZ-DPSK modulation format performs considerably better as compared to other modulation formats. Furthermore, the enhanced performance of the proposed NRZ-DPSK modulation-based MDM-RoFSO transmission system is investigated under different weather conditions and the results are compared with previously reported works.
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