I. Introduction
Underwater wireless networks have applications in many areas such as oceanographic research, oil industry, underwater monitoring, and military systems and are mostly implemented by means of sensors and underwater autonomous vehicles [1]. Operating under the water, these networks are subject to the detrimental effects of acoustic channels that are far more challenging than those of the radio channels. Acoustic propagation is possible only at low frequencies due to the frequency-dependent absorption loss which limits the operating frequency of most underwater systems to below 30 kHz [2]. For example a typical system may have a carrier frequency of 12 kHz modulated by a signal with bandwidth not exceeding 5 kHz [3]. Such a system is considered an ultra-wideband system in the sense that the signal bandwidth is not negligible compared to the carrier frequency. Low propagation speed (1500 m/s) is another issue in underwater communications which leads to channels with delay spreads as long as tens or hundreds of milliseconds that cause strong frequency selectivity. Motion induced Doppler distortions are very extreme in underwater channels due to the low propagation speed, even when the transmitter/receiver are not moving fast. Doppler shifts of tens of Hertz are common and cause great distortions due to the limited available bandwidth. Time-varying multipath fading observed in the radio channels is present also in the acoustic channels and is intensified in shallow waters due to the strong surface and bottom reflections. In spite of these challenges, acoustic waves are the only option for underwater communications since electromagnetic waves cannot propagate over long distances under the water.