Adaptive equalization for underwater wireless communications
Wireless acoustic communications through the underwater channel presents many challenges. On one hand, the channel is often characterized by an extended time-varying multipath structure spanning a hundred or more symbols, limited channel coherence times, and high signal loss due to absorption resulting in limited available bandwidth. On the other hand, many applications for which wireless communications through this channel would be useful impose severe limits on the available power and the cost of the required hardware. These constraints limit not only the available transmit power but also the available computational power. Thus, modulation and demodulation approaches must be developed which can cope with the often difficult channel conditions but which also require only limited resources. This paper reviews the salient features of the underwater acoustic propagation environment, presents several techniques which have been developed to provide reliable communications in this environment, and highlights some of the remaining challenges.