Weakly dispersive modal pulses in long-range underwater acoustic communications

Abstract

Weakly dispersive modal pulses are contributions to acoustic wave field from individual modes that are characterized by weak dispersion- and scattering-induced broadening. These pulses experience little propagation-induced distortions and are suitable for communications applications. This work investigates, using numerical simulations, how to design a communication system that takes advantage of the physics of weakly dispersive modal pulses. Two groups of weakly dispersive modal pulses are identified in typical mid-latitude ocean environments: the lowest order modes, and mode numbers with the waveguide invariant being near-zero (often around mode 20 at 75 Hz). This work analyzes the source and receiving array requirements for achieving low bit error rates (BERs) in a binary communication without performing channel equalization. It is shown that low BERs are achieved with only 3 hydrophones for mode 1 processing at 500 km and with 30 hydrophones for mode 20 at 400 km range with good signal-to-noise ratio (SNR). It is demonstrated that if depths of hydrophones are allowed to vary with the source-receiver distance, 2 hydrophones are often sufficient to achieve low BERs even with intermediate mode numbers. Thus, full modal resolution is often unnecessary to achieve low BERs. The effects of variable SNR are also studied. [Work supported by ONR.]