Achieving reliable communication over underwater acoustic (UWA) channels is a challenging problem owing to the scarce bandwidth available and the double spreading phenomenon, i.e., spreading in both time (multipath delay spread) and frequency domains (Doppler spread) [Kilfoyle & Baggeroer 2000]. Delay and Doppler spreading is inherent to many practical communication channels, but are considerably amplified in UWA environments [Stojanovic et al. 1994]. Double spreading complicates the receiver structure and makes it difficult to extract the desired symbols from the received signals.

Telemetry systems adopting direct-sequence spread-spectrum (DSSS) based modulation techniques are conventionally referred to as operating at low data rates. Existing literature regarding low data rate UWA communications include [Palmese et al. 2007][Stojanovic et al. 1998][Hursky et al. 2006][Yang & Yang 2008][Stojanovic & Freitag 2000][Blackmon et al. 2002][Ritcey & Griep 1995][Stojanovic & Freitag 2004][Sozer et al. 1999][Tsimenidis et al. 2001][Iltis & Fuxjaeger 1991]. By sacrificing the data rate, DSSS techniques exploit frequency diversity in the frequency-selective UWA channel and benefit from spreading gain to allow many co-channel users. At the receiver side, decentralized reception schemes encompass nonlinear equalization including hypothesis-feedback equalization [Stojanovic & Freitag 2000], and linear equalization including RAKE receivers [Tse & Viswanath 2005]. Performance comparisons of hypothesis-feedback equalization and RAKE reception are presented in [Blackmon et al. 2002].

In this chapter, we consider a single user scenario with a coherent RAKE reception scheme (the noncoherent schemes, which do not require a channel estimation, will be discussed in Chapter 19).