E2MAC : An energy efficient MAC for RFID-enhanced wireless sensor networks

The primary aim of any anti-collision protocols is to identify tags quickly, as doing so ensures that a Radio Frequency IDentification (RFID) reader incurs minimal energy wastage and achieves high identification rate. To date, researchers have proposed various protocols to minimize tag collisions and idle slots–key factors that determine a reader’s read rate and energy expenditure. Most of these protocols, however, are designed for single reader systems. To this end, we propose E2MAC, an energy efficient, distributed Medium Access Control (MAC) protocol for identifying and monitoring tags in RFID-enhanced wireless sensor networks. E2MAC exploits the low power capability of a ultra-wideband transceiver and distinct pulses to address the reader collision problem. In addition, it uses ResMon, an enhanced dynamic frame slotted Aloha protocol to read and monitor tags. Lastly, E2MAC uses a novel load balancing algorithm to amortize the cost of reading and monitoring tags to multiple readers. These E2MAC features ensure that the contention level at each reader is kept at a minimum and distributed fairly. As a result, E2MAC has a high reading rate and low energy consumption. In addition, E2MAC helps in minimizing the impact of the tag orientation problem, where a tag becomes unreadable if its antenna is parallel to a reader’s field lines. In particular, the use of multiple readers increases spatial diversity and hence increases the likelihood that a tag is readable by at least one reader. Our simulation results show E2MAC to have very low energy consumption, reading delay and per-reader collision. More importantly, system designers have the flexibility to lower these metrics further with additional readers, bigger frame sizes, or by dividing tags into small groups.