Long-Range Underwater Acoustic Communication Performance Under Warm-Eddy-Related Sound-Speed Variability in a Deep-Water Environment

This study investigates the relationship between warm-eddy-related sound-speed variability and long-range underwater acoustic communication performance in a deep-water environment. A field experiment was conducted in the East Sea off the Korean Peninsula using an omnidirectional acoustic source deployed at a depth of 121–125 m and a near-seafloor hydrophone moored at 2441 m. Binary phase-shift keying (BPSK) and quadrature phase-shift keying (QPSK) signals were transmitted over source–receiver ranges of 5–45 km, while expendable bathythermograph (XBT) observations were conducted at 5 km intervals from 0 to 55 km. XBT observations, complemented by Korea Operational Oceanographic System (KOOS) temperature fields, indicated a relatively uniform warm-core-influenced region over approximately 0–35 km and a stronger-gradient transition region toward and beyond approximately 40 km. The 10–11 °C thermal layer and corresponding 1491–1493 m/s sound-speed layer became substantially compressed in the longer-range region. Acoustic-channel analysis showed stable direct-path propagation over approximately 15–30 km, whereas the direct-path contribution weakened and bottom-reflected propagation became increasingly important beyond 35 km. Both BPSK and QPSK achieved their best performance at approximately 15–20 km and showed substantial degradation beyond 35 km, with reduced output SNR and increased BER. Repeated-transmission statistics confirmed significant range dependence in the principal channel and communication metrics. A controlled fixed-range simulation further showed that changing the sound-speed structure alone, under identical bathymetric and geometric conditions, substantially altered the modeled direct-path propagation. These results indicate that upper-ocean sound-speed variability can contribute to the observed channel and communication changes, while the field observations likely reflect the combined effects of sound-speed variability and range-dependent bathymetry. The findings provide field-based evidence for understanding and predicting long-range underwater acoustic communication reliability under mesoscale oceanographic variability.