A Link State Dependent TDMA Protocol for Industrial Wireless Sensor Network Applications in Periodically Changing Environments

Existing TDMA-based MAC protocols for wireless sensor networks are not specifically built to consider communication channels that are prone to fading. We describe the impact of periodically changing environment on small-scale fading effects in industrial indoor wireless networks. Using a site-specific ray tracer, we show that the position of nodes and the periodic movements of objects with constant velocities in the environment have significant impact on signal fading. Finding that fading is approximately periodic, we propose a TDMA-based MAC protocol for wireless sensor networks built for industrial applications that uses link state dependent scheduling. In our approach, nodes gather samples of the channel quality and generate prediction sets from the sample sets in independent slots. Using the prediction sets, nodes only wake up to transmit/receive during scheduled slots that are predicted to be clear and sleep during scheduled slots that may potentially cause a transmitted signal to fade. We simulate our proposed protocol and compare its performance with the well published Z-MAC protocol. We found that our protocol significantly improves packet throughput and energy consumption as compared to Z-MAC. We also found that in conditions which are not perfect under our assumptions, the performance of our protocol degrades gracefully.