Cardinality Estimation in RFID Systems with Multiple Readers

Radio frequency identification (RFID) is an emerging technology for automatic object identification. An RFID system consists of a set of readers and several objects, with each object equipped with a small chip, called a tag. In this paper, we consider the anonymous cardinality estimation problem in an RFID system consisting of several readers. To achieve complete system coverage and increase the accuracy of measurement, multiple readers with overlapping interrogation zones are deployed. We study the problem under two different circumstances. First, we assume that the readers cannot perform interrogations synchronously. This models the case when the readers are not equipped with accurate clocks or synchronization imposes a high overhead. Under such condition, we propose an asynchronous exclusive estimator to estimate the number of tags that are exclusively located in the zone of a selected reader. By using this estimator, we propose an asynchronous multiple-reader cardinality estimation (A-MRCE) algorithm. In the second scenario, we assume that readers can perform interrogations synchronously. We propose a synchronous exclusive estimator and a synchronous multiple-reader cardinality estimation (S-MRCE) algorithm to estimate the total number of tags. For the exclusive estimators, we show that they are asymptotically unbiased and we derive upper bounds on the variance of error. We validate our analytical model via simulations. Results show that although the A-MRCE algorithm enjoys the asynchronous operation of the readers, it performs worse than the S-MRCE algorithm in terms of estimation error. Compared to the enhanced zero-based (EZB) and lottery frame (LoF) algorithms, the variance of the estimation error for both A-MRCE and S-MRCE algorithms increases linearly with the number of readers, while it increases exponentially for EZB and LoF algorithms.