An Adaptive TDMA-based MAC Protocol for Underwater Acoustic Sensor Networks

Acoustic propagation through water suffers from attenuation that increases with both signal frequency and transmission range, with time-varying long propagation delays. The power spectral density of underwater ambient noise also changes with frequency. As a result, the usable channel bandwidth is heavily constrained and Medium Access Control (MAC) protocol design for Underwater Acoustic Sensor Networks (UASNs) is challenging. Striking a balance between channel utilisation and network end-to-end delay is particularly difficult. The Combined Free/Demand Assignment Multiple Access (CFDAMA) protocol has been shown to effectively minimise end-to-end delay and maximise channel utilisation, but existing approaches are reliant on synchronisation which is hard to achieve underwater. This paper introduces a novel robust MAC protocol, based on CFDAMA, exclusively designed for UASNs and called CFDAMA-NoClock. It is capable of providing an adaptive MAC solution without the need for synchronisation amongst independent node clocks. The protocol demonstrates a high level of practicality and simplicity. Both analytical models and comprehensive event-driven simulation of several underwater scenarios show that CFDAMA-NoClock can offer excellent delay/utilisation performance under two distinct traffic types and with various network parameters selected based on practical UASN technologies.