Experimental demonstration of visual locating the underwater wireless optical communication device by using the communication signal as a guiding light

Underwater wireless optical communication (UWOC) is suited to transmit large volumes of data for ocean engineering. In practical applications, due to the quite significant attenuation of light in water, the transmission distance of UWOC is shorter. Furthermore, because light travels along straight lines, the two UWOC devices in a system should stay in the communication range and point to each other to establish an optical link. To make this process easier, it is feasible and pragmatic to apply a UWOC system between a mobile robot and an immobile underwater platform, where the robot could locate, move, and point to the immobile device. In such an application, the visual locating method is preferred to provide position information to establish a communication link since it has quite high accuracy. While using this method, several light-emitting diodes (LEDs) as guiding lights need to be installed in the immobile platform, and a camera on the robot is used to photograph them. Then, the position of the immobile platform could be estimated from the center coordinates of the light spots in the image. To reduce the complexity of this system, this work directly shares the transmitting signal in a half-duplex UWOC system as the guiding light. The transmitter of UWOC on the immobile terminal is based on four separated and distributed LEDs, which are modulated simultaneously with on–off keying (OOK) format. The performance of such a method is demonstrated by a simulation experiment in an artificial pool. The results show that, when the UWOC system works normally with a 12.5 Mbps rate, visual locating can be realized effectively. When the distance between the two terminals is within 390 cm, the estimation error is no more than 2.15 cm. When the yaw angle is within 10 deg, the estimation error is no more than 2.04 deg. This accuracy is sufficient for the short-range location of many UWOC links. The proposed method is simple and useful for UWOC systems between underwater robots and immobile platforms.