Strong attenuation of most electromagnetic signals in water is the main reason why underwater exploration, surveillance, and other applications currently rely on sub-sea cables and tethers to send data back to the user. While acoustic modems have long been the default wireless communication method for underwater applications, they incur high cost and can only deliver low data rates. In this work, an LED-based optical communication system is presented for applications requiring high data rates, low power, low complexity, and low-to-median communication range. An LED-based communication system, consisting of a transmitter with a super-bright blue LED and a receiver based on a blue-enhanced photo diode, has been developed and tested with the goal of transmitting data at high rates over distances of at least 20 meters. Test results in a swimming pool show the successful transmission of large data over a distance of 23 meters and at transmission rates of 115200 kbps. This system relies on a line-of-sight connection for communication. Since the change of the relative position and orientation between the transmitter and the receiver will affect the quality of communication link, a mechanism is designed so that the transmitter/receiver modules can be rotated to maintain alignment between the communicating parties, despite the movement of underlying platforms (e.g., robots). A signal strength-based feedback control algorithm is developed to maintain alignment. Experimental results involving a stationary transmitter and a mobile receiver mounted on a terrestrial robot are presented to evaluate the performance of the system.
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