Low spreading loss in underwater acoustic networks reduces RTS/CTS effectiveness

The relatively low spreading losses in underwater acoustic channels allows increased interference from distant interferers, which reduces the effectiveness of collision-avoidance MAC protocols. These collisions in turn reduce spatial reuse and network goodput, and increase power consumption. A competing channel effect, however, is the frequency-dependent absorption. The absorption can strongly suppress distant interferers, improving the effectiveness of collision-avoidance protocols. A third channel effect is frequency-dependent ambient noise, which reduces effectiveness for links consisting of widely separated nodes. RTS/CTS effectiveness generally decreases with decreasing acoustic frequency. We present analytic, numerical, and simulated results detailing how each of the major characteristics of the physical channel and physical layer affect the RTS/CTS effectiveness. We find that RTS/CTS effectiveness can drop to between 50%--90% for source and receiver separated by more than about two-thirds of the maximum packet range. The effect depends heavily on the acoustic frequency. We also present as a hypothesis an alternative physically based spreading model that distinguishes between desired signals, which are typically coherently detected via the largest of their multipath arrivals, and interfering signals whose effect on detection performance is related to the total energy in all of their multipath arrivals.