Sea-Trial Characterization of the Spatial Self-Interference in In-Band Full-Duplex Underwater Acoustic Systems: A Validated Finite-Element Model and Receive-Array Design

In this paper, we develop an in-band full-duplex underwater acoustic modem as a measurement platform and report sea-trial measurements to characterize spatial self-interference (SI) and to validate SI suppression performance under practical deployment constraints. Due to the strong SI power and the ultra-short SI propagation path, SI can severely degrade the detection and characterization of weak signals in practical deployments; therefore, accurate SI modeling and field measurements are essential for reliable detection and measurement-informed suppression. Thus, we develop a finite-element-method based numerical SI model that incorporates complex mechanical structures and realistic transducer radiation characteristics. To mitigate the performance degradation of adaptive beamformers in detection caused by strong power SI, we propose a quasi-nested receive array. Numerical simulations and sea-trial measurements show that, for a seven-element array, the proposed quasi-nested array achieves the highest output signal-to-interference-plus-noise ratio (SINR) among the compared arrays, with gains of up to 11 dB in simulations and 8.7 dB in sea trials over the uniform linear array (ULA). In addition, simulations under SI-field uncertainty confirm that the quasi-nested array maintains the highest output SINR among the compared array configurations.