Fast Composite Counting in RFID Systems

Counting the number of tags is a fundamental issue and has a wide range of applications in RFID systems. Most existing protocols, however, only apply to the scenario where a single reader counts the number of tags covered by its radio, or at most the union of tags covered by multiple readers. They are unable to achieve more complex counting objectives, i.e., counting the number of tags in a composite set expression such as $S_{1} \bigcup S_{2} - S_{3} \bigcap S_{4}$. This type of counting has realistic significance as it provides more diversity than existing counting scenario, and can be applied in various applications. We formally introduce the RFID composite counting problem, which aims at counting the tags in an arbitrary set expression and obtain its strong lower bounds on the communication cost. We then propose a generic Composite Counting Framework CCF that provides estimates for any set expression with desired accuracy. The communication cost of CCF is proved to be within a small factor from the optimal. We build a prototype system for CCF using USRP software defined radio and Intel WISP computational tags. Also, extensive simulations are conducted to evaluate the performance of CCF. The experimental results show that CCF is generic, accurate and time-efficient.