Characterization and network consequences of low spreading loss in underwater acoustic networks

This thesis presents several related pieces of work on underwater acoustic networks (UANs). Underwater wireless networks generally use acoustics, as radio is heavily attenuated and light is strongly scattered underwater. UANs are becoming more widely deployed for scientific, environmental, industrial, and military applications. The network nodes can be stationary sensor nodes, underwater vehicles, surface buoys or vehicles providing a gateway to radio networks, or bottom nodes providing a gateway to cabled undersea networks. Packet detection in interference in UANs, and its critical role in the effectiveness of collision-avoidance medium-access control (MAC) protocols, is a primary focus of this thesis. Spreading loss measures the decrease in received energy as a function of range, and determines the level of long-range interference. We present a new spreading model, the mixed-exponent spreading model, for UAN nodes using a matched-filter detector as a low-power wakeup detector. Under this model, there are distinct spreading-loss exponents for packet detection and interference, due to the matched-filter detector\u27s signal processing. We validate this spreading model numerically, and with direct measurements of the spreading exponents from shallow-water experimental data. The widely used, but poorly grounded, practical spreading model is inconsistent with our experimental measurements. Our results suggest caution for its continued use to model performance of UANs. Building on our spreading analysis, we analyze the effectiveness of collision-avoidance MAC protocols in UANs, namely what fraction of collisions are avoided when using the protocol, independent of propagation delay. The low spreading loss in UANs, in particular with the mixed-exponent spreading model, can lead to low collision-avoidance effectiveness compared with radio networks. In addition, we argue that many UANs will be relatively mobile and sparse relative to terrestrial sensor networks, reducing the importance of network energy consumption and throughput of medium-access control protocols as metrics. This survey challenges some of the assumptions made in past UAN research, with a goal of aiding researchers entering the area of UANs from terrestrial sensor networks. Finally, we document the design decisions for a new underwater acoustic modem, as a contribution to researchers entering the field from terrestrial sensor networks