Abstract-We address several inter-related aspects of underwater network design within the context of a cross-layer approach. We first highlight the impact of key characteristics of the acoustic propagation medium on the choice of link layer parameters; in turn, the consequences of these choices on design of a suitable MAC protocol and its performance are investigated.Specifically, the paper makes contributions on the following fronts: a) Based on accepted acoustic channel models, the pointto-point (link) capacity is numerically calculated, quantifying sensitivities to factors such as the sound speed profile, power spectral density of the (colored) additive background noise and the impact of boundary (surface) conditions for the acoustic channel; b) It provides an analysis of the Micromodem-like linklayer based on FH-FSK modulation; and finally c) it undertakes performance evaluation of a simple MAC protocol based on ALOHA with Random Backoff, that is shown to be particularly suitable for small underwater networks.

Abstract-We propose an OFDM receiver capable of estimating and correcting, on a symbol-by-symbol basis, the subcarrier dependent Doppler shifting due to the movement of source and receiver in an underwater acoustic network. We propose two methods of estimation: one of which is based upon the marginal maximum likelihood principle, and one of which is ad-hoc. We compare the performance of both estimators to the Cramer-Rao lower bound. We show through simulation that the proposed receiver design performs well for a source that is accelerating at 0.29 m/s 2 .

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Signals transmitted through underwater channels experience attenuation due to dissipation of acoustic energy by spreading as well as by absorption. The path loss due to absorption is found to be highly dependent upon the frequency. Ambient noise, which also greatly affects accurate reception of the signal, is also highly dependent upon frequency. For these reasons, the received SNR cannot be assumed to be constant over wideband acoustic signaling schemes. In this paper we determine the (signaling) rate vs. range curves for a FH-FSK modem considering the frequency selective nature of signal attenuation in an acoustic medium.

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