Field Demonstrations of Wide-Beam Optical Communications Through Water–Air Interface

Abstract

The connectivity of undersea sensors and airborne nodes across the water-air interface has been long sought. This study designs a free-space wireless laser communications system that yields a high net data rate of 850 Mbit/s when perfectly aligned. This system can also be used for an extended coverage of 1963 cm 2 at the receiver while sustaining a net data rate of 9 Mbit/s over 10 m. The utility of this system was verified for direct communications across the water-air interface in a canal of the Red Sea based on a pre-aligned link as well as a diving pool under a mobile signal-searching mode. The canal deployment measured a real-time data rate of 87 Mbit/s when pre-aligned in turbid water over 50 min, which confirms the system robustness in harsh water environments. In the pool deployment, a drone configured with a photodetector flew over the surface of the water and recorded the underwater signals without a structureassisted alignment. Using a four-quadrature amplitude-modulated orthogonal frequency-division multiplexing (4-QAM-OFDM) modulation scheme provided a net data rate of 44 Mbit/s over a 2.3-m underwater and 3.5-m air link. The results validated the link stability and mitigated problems that arise from misalignment and mobility in harsh environments, which paves the way for future field applications. INDEX TERMS Underwater communication, optical modulation, wireless communications, water-to-air communications, cross-medium communications, mobility I. INTRODUCTION The concept of the Internet of Underwater Things (IoUT) was proposed in 2012 to satisfy the demands of underwater communication networks [1]. The wide, license-free bandwidth and low energy consumption in such environments promote the consideration of underwater wireless optical communication (UWOC) as a transformative technology compared with conventional marine acoustic and radio-frequency (RF) technologies for high-speed communication activities in the IoUT. Verifications of the UWOC physical layer are progressing rapidly [2], with multiple Gbit/s-level UWOC links reported in laboratory studies [3]-[8]. Beyond investigations under ideal laboratory environments, researchers have considered the effects of various natural underwater processes on the performance of UWOC links. Bubbles [9], waves [10], aquatic life [11], water turbidity [12], and oceanic turbulence [13] all degrade communication performance in UWOC by altering the path of light propagation and by inducing misalignment.

Security of Underwater and Air-Water Wireless Communication

“…2 (b), where the airborne node receives and decodes information from the backscattered RF waves after striking the ripples produced by the acoustic waves on the surface. Similarly, the use of direct light from air to water has been explored in [8] and [61], followed by various works attempting to enhance the performance [60,[62][63][64][65][66][67][68][69][70][71][72].…”

Diffused-Line-of-Sight Communication for Mobile and Fixed Underwater Nodes

“…3(b), the laser's frequency response was measured by using a vector network analyzer (Agilent, E8361C), which was pre-calibrated prior to the measurements by using an electronic-calibration module (Agilent, 85093-60010). Since identical laser and measurement equipment with the work of [38] were used and tested at the same time, the optical spectrum and frequency response of the laser were adapted from [38]. The -3-dB bandwidth is up to 1 GHz with the shortpass filter, compared with the 14 MHz measured without the shortpass filter.…”

“…(a) Spectrum of a white-light laser generated by having a blue laser illuminating a phosphor plate, mounted with and without a short-pass filter; (b) Frequency response of the laser mounted with and without a short-pass filter. [38]…”

Security enhancement for OFDM-UWOC system using three-layer chaotic encryption and chaotic DFT precoding

“…Besides, eavesdroppers can place a mirror or diffraction grating [8] somewhere between the transmitter and receiver to tap light from the line-of-sight beam. To reduce the difficulty of alignment, an omni-directional light source [9] and a widebeam laser diode (LD) [10] are also widely used in UWOC, which undoubtedly increase the probability of information leakage. Simultaneously, potential attackers may also use the non-lineof-sight channel formed by the scattered light [11,12] with increased concealment.…”