Design and development of an LED-based optical communication system with active alignment control

While acoustic modems have long been the dominant wireless communication method for underwater applications, they incur high cost and large power, and can only deliver low data rates. Recently, light-emitting diode (LED)-based optical communication has emerged as a promising approach to low-power, high-rate data transfer underwater over short-to-medium distances. However, LED-based communication relies on a close-to-line-of-sight link between the transmitter and the receiver, which presents significant challenges in its underwater robotic applications, where the underlying robots undergo constant motions due to propulsion and/or disturbances. In this paper we propose a novel and compact LED-based communication system with active alignment control, which maintains the communication link despite the underlying platform movement. Details on the system design and implementation are provided. The prototype is able to communicate at 115,200 bps over at least 23 m in swimming pool tests. Experiments involving a stationary transmitter and a mobile receiver mounted on a terrestrial robot are conducted to demonstrate the performance of the alignment maintenance system. In particular, it is shown that, for a communication distance of 3 m, the data loss under active alignment control is 9.9% at the transmission rate of 115,200 bps, when the robot orientation varies ±60°, while in comparison, the data loss is 56.2% in the absence of alignment control.