A Study of Modulation Formats for the Blue Ray Underwater Optical Modem

The transition from acoustic to underwater optical communication links poses many constraints in terms of the maximum link distance, since the latter can only be used in very short-range communications (<;0.1 km). Despite this fact, visible light communication systems represent a disruptive change for Autonomous Underwater Vehicles and/or underwater sensor networks, allowing for much higher data rates than those achievable with acoustic links. Blue Ray is a compact, low power, inexpensive, and lightweight optical modem designed for fast data transmission between MEDUSA underwater vehicles at ranges of up to 15 m in clear water. The original design uses On Off Keying (OOK) at the physical level, with data rates of 20-200 kb/s over short-range, line-of-sight, configurations. The present work examines alternative modulation formats - Pulse Amplitude Modulation (PAM), Pulse Position Modulation (PPM), and Pulse Width Modulation (PWM) -, which can be implemented in the existing hardware of Blue Ray and potentially yield improved rates or robustness. The results of simulations and real-world testing under controlled conditions show that, from all the studied modulation techniques, PPM is generally more power efficient, maximizes the signal-to-noise ratio, and has a higher average bitrate than OOK (for the same achievable Bit Error Rate), despite the lower spectral modulation efficiency. Very good agreement is found between theoretical predictions and the optical modem's experimental performance.