Revisiting reading rate with mobility: Rate-adaptive reading in COTS RFID systems

Radio-frequency identification (RFID) systems, as major enablers of automatic identification, are currently supplemented with various interesting sensing functions, e.g., motion tracking. All these sensing applications forcedly require much higher reading rate (i.e., sampling rate) such that any fast movement of tagged objects can be accurately captured in a timely manner through tag readings. However, COTS RFID systems suffer from an extremely low individual reading rate when multiple tags are present, due to their intense channel contention in the link layer. In this work, we present a holistic system, called Tagwatch, a rate-adaptive reading system for COTS RFID devices. This work revisits the reading rate from a distinctive perspective: mobility. We observe that the reading demands of mobile tags are considerably more urgent than those of stationary tags because the states of the latter nearly remain unchanged; meanwhile, only a few tags (e.g., < 20%) are actually in motion despite the existence of a massive amount of tags in practice. Thus, Tagwatch adaptively improves the reading rates for mobile tags by cutting down the readings of stationary tags. Our main contribution is a two-phase reading design, wherein the mobile tags are discriminated in the Phase I and exclusively read in the Phase II. We built a prototype of Tagwatch with COTS RFID readers and tags. Results from our microbenchmark analysis demonstrate that the new design outperforms the reading rate by 3.2x when 5% of tags are moving.