7-M/130-Mbps LED-to-LED Underwater Wireless Optical Communication Based on Arrays of Series-Connected LEDs and a Coaxial Lens Group

In this paper, the arrays consist of series-connected monochromatic light-emitting diodes (LEDs) are proposed as the transmitter and receiver to implement an LED-to-LED underwater wireless optical communication (UWOC) system. An array of green LEDs is used as the transmitter to enhance the emitted optical power, and the high-frequency attenuation of the LEDs is compensated by a pre-equalization circuit. Red LEDs are connected in series, forming an array as the detector to extend the bandwidth of the receiver. Furthermore, a lens group for homogenizing and focusing the light spot is designed to increase optical power-density at the plane of detector, thus significantly enhancing the signal-to-noise ratio (SNR) of the captured signal. The proposed array with 3 series-connected red LEDs can increase the data rate by 22.2 % compared with a single red LED. An LED-to-LED UWOC system with a data rate of 130 Mbps over a 7-m underwater channel is realized with a BER lower than the forward error correction (FEC) threshold of
$3.8\times 10^{-3}$
. To the best of our knowledge, this is the first time that the series-connected LEDs as the detector, combining with a lens group for homogenization and focusing, are applied in an LED-to-LED UWOC system, which achieve the highest rate-distance product among the reported LED detection-based wireless optical communication (WOC) systems.