Two-Stage Self-Interference Cancellation for Full-Duplex Underwater Acoustic Systems

Underwater acoustic (UWA) communication suffers from the limited available bandwidth of acoustic links. We consider in-band full-duplex (FD) communication in UWA channels, which has the potential of doubling the channel capacity. The key challenge of FD operation is the strong self-interference (SI) introduced by the near-end transmission. For a time-invariant UWA channel, the SI can be efficiently cancelled using an adaptive filter. In practice, the time-varying reflections particularly introduced by the moving sea surface limit the cancellation performance. In this paper, we propose a two-stage SI cancellation scheme with a transceiver structure comprising one projector and two hydrophones. In the first stage, the stable SI path from each hydrophone is cancelled using a digital canceller based on the recursive least-squares (RLS) algorithm with dichotomous coordinate descent (DCD) iterations. In the second stage, the scheme tries to cancel the residual SI using an RLS-DCD-based adaptive beamformer. The beamformer combines the residual signals at the hydrophones in such a way to minimize the final residual SI. The cancellation performance of the proposed scheme is investigated using the Waymark simulator and with lake experiments. As demonstrated by the simulation results, the SI can be efficiently cancelled with the proposed two-stage SI cancellation scheme. The experimental results also demonstrate the capability of the adaptive beamformer in reducing the residual SI after digital cancellation.