Introduction

Underwater acoustic sensor networks (UWASN) have been attracting growing research interests in recent decades due to the emerging applications [1, 2]. For the environmental monitoring purpose, UWASN can perform real-time pollution monitoring of streams, lakes, ocean bays, drinking water reservoirs, local ponds, etc. For the undersea and offshore oil exploration, UWASN can perform oil leakage detection, equipment monitoring, and seismic imaging. For the military application, UWASN can be used for tactical target identification and intrusion detection. In addition, UWASN can help understand biological behaviors under different water conditions. To realize UWASN, underwater acoustic communications (UAC) is one of the key communication techniques. In this chapter, we first briefly review UAC and its challenges. Then, cooperative orthogonal frequency-division multiplexing (OFDM) communications is introduced to address these challenges, and we focus on the design of the dual-hop relay-aided (RA-)UAC from three aspects, namely, power allocation, decomposed fountain codes design, and packet transmission reliability. Finally, the organization of this monograph is given at the end of this chapter.