Extended Kalman Filter-Based Active Alignment Control for LED Optical Communication

Light-emitting diode (LED)-based optical communication is emerging as a low-power, low-cost, and high-data rate alternative to acoustic communication for mobile applications underwater. However, it requires a close-to-line-of-sight (LOS) link between the transmitter and the receiver. Alignment for maintaining LOS is challenging due to the constant movement of underlying mobile platforms caused by propulsion and unwanted disturbances. In this paper, we present a novel, compact LED-based communication system with active alignment control, in a two-dimensional setting, that maintains the LOS despite the underlying platform movement. An extended Kalman filter-based algorithm is proposed to estimate the angle between the receiver orientation and the receiver-transmitter line, which is used subsequently to adjust the receiver orientation. The algorithm uses only the measured light intensity from a single photodiode, where successive measurements are obtained via a scanning technique. A simple proportional controller is designed for alignment that also ensures the observability of the system. The effectiveness of the proposed active alignment algorithm is verified in simulation and experiments. In particular, its robustness in the presence of measurement noise is demonstrated via comparison with two alternative algorithms that are based on hill-climbing and three-point-averaging.