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 the harsh conditions of industrial environments where the communication channel is prone to signal fading. We describe the impact of periodically changing environment on small-scale fading effects in industrial indoor wireless networks. We use a site-specific ray tracer for predicting signal propagation based on a building blueprint representation to simulate the signal propagation paths between a receiver and transmitter in a periodically changing indoor environment. 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, hence varying the resultant waveform perceived by a receiver. We present the results of two scenarios, each with different node placements and moving object velocities and we show that signal fading is approximately periodic. We then 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 School of Computer Science & Software Engineering, University of Western Australia, Perth, WA 6009, Australia. email:{valance, datta }@csse.uwa.edu.au