Modulation and demodulation techniques for underwater acoustic communications

This paper is focused on the characteristics of underwater acoustic communication channels and their implications on the design of bandwidth efficient modulation and demodulation techniques. Both noncoherent and phase coherent modulation and demodulation techniques are considered, and their performance, bandwidth efficiency and implementation complexity are compared.Single carrier and multicarrier (OFDM) modulation and demodulation techniques are also considered. It is illustrated that the performance of high data rate single carrier systems is severely impacted by intersymbol interference (ISI) due to channel dispersion and that powerful equalization algorithms are required to reduce the performance loss caused by ISI. The use of coding in such single carrier systems makes it possible to perform turbo equalization in order to achieve further performance gains. Multicarrier (OFDM) modulation may be used to eliminate the need for high complexity equalizers in channels where the time variations are sufficiently slow relative to the symbol rate carried by each of the subcarriers in the multicarrier system.Further increases in bandwidth efficiency are possible through spatial multiplexing. To achieve this objective, recent work on underwater acoustic communications has focused on MIMO techniques to increase the data rate and improve performance through signal diversity. This work on MIMO will be discussed.As a final topic in this paper, we consider multiple access techniques suitable for use in underwater acoustic networks. The suitability of FDMA, TDMA and CDMA is assessed in view of the characteristics of underwater acoustic channels.