Spatial Reuse in Underwater Acoustic Networks using RTS/CTS MAC Protocols

Using analytic models and simulation results, we examine spatial reuse and the effectiveness of RTS/CTS MAC protocols in underwater acoustic networks. We are not looking at the question of network throughput, which is affected mostly by propagation delay, but rather the protocol’s ability to prevent collisions. RTS/CTS-based collision-avoidance protocols require successful detection of the handshake packets in order to be effective. This criterion leads to an RTS/CTS effectiveness metric which depends upon channel characteristics such as spreading loss, attenuation, and ambient noise, as well as communication parameters such as the detection threshold and type of modulation. The relatively low spreading losses in underwater acoustic channels allows increased interference from distant interferers, which reduces the effectiveness of collision-avoidance protocols. This in turn reduces spatial reuse and network goodput, and increases power consumption. A competing channel effect, however, is the frequency-dependent attenuation. The attenuation strongly suppresses distant interferers, improving the effectiveness of collision-avoidance protocols. A third channel effect is frequency-dependent ambient noise, which in turn reduces effectiveness for links consisting of widely separated nodes. The RTS/CTS effectiveness generally decreases with decreasing acoustic frequency. In terms of communication parameters, RTS/CTS effectiveness and spatial reuse decrease with increasing detection threshold. This implies that modulation techniques which work with low-SINR packet detectors, such as low-bitrate FH-FSK, will have good spatial reuse. Modulation techniques such as high-bitrate PSK, however, require higher detection thresholds, and will have reduced collision-avoidance effectiveness. 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.