Low Power, Accurate Time Synchronization MAC Protocol for Real-time Wireless Data Acquisition

The issues of real-time wireless data acquisition, how to design and manage thousands of sensor nodes located over a vast geographical area, have received more and more attention during the last years. This paper proposes a real-time wireless data acquisition MAC protocol meeting the requirements of high throughput, low end-to-end latency, low energy consumption and accurate time synchronization. A hybrid approach, combining the advantages of Time Division Multiple Access (TDMA) and Frequency Hopping Spread Spectrum (FHSS), is adopted for anti-jamming and collision prevention. The packets of commands and data are delivered in a "bucket brigade"-like manner for optimum bandwidth utilization and low end-to-end latency. Experiments show that the bandwidth utilization exceeds 25%, and the mean latency per hop is kept to 8 to 9 milliseconds. Correspondingly, we propose a novel time synchronization approach which tightly coupled with TDMA scheme. As the non-determinism of time-stamp is removed from the transmission path, the frequency difference between local clock and parent's clock is estimated by periodically receiving beacons. The average per-hop synchronization error is in the 0.6 mu s range, which is markedly better than those of widely used algorithms, such as Reference-Broadcast Synchronization (RBS) and Timing-sync Protocol for Sensor Networks (TPSN).