Reliable underwater acoustic communication is constrained by challenging signal propagation conditions. The low speed of sound, long multipath delay spreads, scattering from sea surface and bottom, acoustic noise, and platform motion can produce channels that are simultaneously frequency- and time-selective. These effects are particularly pronounced in long-range and mobile undersea scenarios, where channel estimation and equalisation become difficult, and where small improvements in waveform robustness can translate into substantial gains in link reliability.

This paper examines Orthogonal Time Frequency Space (OTFS) modulation as a promising approach for resilient underwater acoustic communications. OTFS sends data symbols in the delay-Doppler domain, rather than standard communication approaches that send data directly in time-frequency, such as time-division quadrature amplitude modulation (QAM) and orthogonal frequency division modulation (OFDM). This enables OTFS to manage propagation paths with distinct delays and Doppler shifts in a structured form. Underwater acoustic channels often exhibit long-delay multipath, path-dependent Doppler, and rapid time variations, making the delay-Doppler domain a natural way to represent and analyse them. We identify the operating regimes in which delay-Doppler signalling may provide practical advantages in underwater scenarios, including improved robustness to mobility, diversity across delay and Doppler dimensions, and tractable equalisation in doubly dispersive channels.

We present a high-level technical assessment of OTFS principles in the context of underwater acoustic propagation and outline an initial simulation framework for evaluating waveform performance under representative channel conditions. The proposed assessment considers static and time-varying multipath, and additive acoustic noise, with performance evaluated against conventional multicarrier baselines using metrics such as symbol error rate, packet reliability and required signal-to-noise ratio for target link performance. The discussion highlights underwater-specific issues that must be addressed before RF-derived OTFS assumptions can be transferred directly. The overarching objective is to support the development of robust, long-range underwater acoustic communication systems for future maritime and defence applications.