Feasibility analysis of modulation formats in different seawater types and practical implementation on underwater optical communication testbed

Abstract

In this paper, we investigate on feasibility analysis of various modulation formats in different seawater types. We have simulated various baseband modulation schemes like Pulse Position Modulation (PPM), Pulse Interval Modulation (PIM) and Dual Header—Pulse Interval modulation (DHPIM) in addition to conventional On–Off Keying (OOK) with data speeds 200Mbps. We have tested these modulation schemes under different seawater types with different downwelling diffuse attenuation coefficients (k_d). These simulation results show the performance of the modulation formats in terms of Probability of Error (PoE) and Signal-to-Noise Ratio (SNR). Different seawater types considered in the simulations. All the simulations were carried out up to the range of 30 m to match the practical possibilities. SNR shows variation from 10⁵ to 10 for the range of 1–30 m. Moreover, the difference in SNR values is higher for different seawater types and is significant at longer ranges. The Probability of Error, the indicator for reliability of communication has more dependence on the type of modulation scheme, and DHPIM has shown significant improvement even for long ranges more than 20 m whereas other modulation schemes offer better than 10^–6 PoE but are limited to distances less than 15 m. Further, to validate the simulations and to practically verify, a real-time experiment with a 532nm green laser was performed on a UWOC testbed with various modulation schemes, and both the simulations and UWOC testbed experiment results were compared. To correct additional effects of window losses that are inevitable in real-time implementation, additional simulations were carried out and PoE was presented.