Sound transmission in underwater acoustic communication channels and signal-processing systems has been described. A basic purpose in this research is to develop digital underwater acoustic communication equipment suitable for practical applications.
Transducers play an important role in underwater acoustic communication equipment, thus their structures and operating characteristics will be the first topic introduced in this chapter, which will help us to design suitable and feasible underwater acoustic communication equipment. Then, three types of civil digital underwater acoustic communication equipment being developed by authors will be discussed: (1) Underwater acoustic telecontrol communications equipment in which a digital time correlation accumulation decision system has been employed, (2) underwater acoustic multimedia communication equipment in which an improved FH-SS system has been employed, and (3) a digital underwater acoustic communication prototype using the innovative APNFM system. Using these systems or prototypes, some expected experimental test results have been obtained in shallow-water acoustic channels.