200-Mbps/7-m LED-to-LED Underwater Wireless Optical Communication With Dynamic Topology Reconfiguration

In this paper, a light-emitting diode (LED)-to-LED underwater wireless optical communication (UWOC) system based on a commercial 4 × 4 red LED array is proposed. To the best of our knowledge, this is the first work to utilize dynamic topology reconfiguration for realizing transceiver multiplexing with the same LED array. Specifically, the 16-LED array is configured in series during transmission to achieve high optical power, and reconfigured into four branches, each consisting of 4 LEDs, during reception to improve the signal-to-noise ratio (SNR) via diversity reception. Equivalent circuit models are established to describe the frequency response behavior of the LED in both transmission and reception modes. Based on the proposed model, a pre-equalizer is designed to flatten the system frequency response, extending the 3-dB bandwidth from 3.29 MHz to 23.23 MHz. Experimental results in a 7-m water tank demonstrate a 200-Mbps on-off keying (OOK) transmission, achieving the highest rate-distance product among LED-to-LED wireless optical communication (WOC) systems to date. Furthermore, diversity reception based on maximal ratio combining (MRC) yields a 17.6% improvement in data rate compared to the single-channel reception. In addition, the system demonstrates robust performance under harsh link conditions. It maintains reliable operation in turbid water with an attenuation coefficient of 2.4 dB/m and bubble-induced turbulence, while accommodating a wide misalignment tolerance of 24 cm. Given its high integration, cost-effectiveness, and robustness, the proposed scheme offers a promising solution for resource-constrained nodes in distributed underwater sensor networks and the Internet of Underwater Things (IoUT).