Introduction

Abstract

Digital underwater communications are becoming increasingly important, with numerous applications emerging in environmental monitoring, exploration of the oceans, and military missions. Until the mid-nineties, the research was focused on hardware and on communication transmitters and receivers for the transmission of raw bits. In network terminology, this is known as the physical layer. A breakthrough was achieved in the mid-nineties by Stojanovic et al. [1], who showed that phase-coherent communication is feasible by integrating a phase-locked loop into a decision-feedback equalizer [2]. Such a receiver can be applied to a single hydrophone, although robust operation at high data rates, say >1 kbit/s, generally requires the presence of a (vertical) hydrophone array for reception. Indeed, multichannel adaptive equalizers have proven to be versatile and powerful tools. If the use of a receive array is impractical, as in multinode networks, then frequency-shift keying (FSK) is often used as a fairly robust modulation for single-receiver systems [3–5]. However, the corresponding data rates are of the order of 100 bit/s. Although progress is still reported on the physical layer, for example on multicarrier modulations or covert communications, a basic set of modulations and receiver algorithms is now available to support research on higher levels in network architectures.