Asynchronous Reception of 2 RFID Tags

Commercial radio frequency identification (RFID) readers have to resolve collisions between tags, without sacrificing throughput. This work proposes a Viterbi joint sequence detector as well as a 2-symbol joint tag information detector that can resolve a collision between two tags in the physical layer. In sharp contrast to prior art, the proposed closed-form signal model takes into account the asynchrony level between the two collided tag responses, which is not uncommon with commercial, low-cost RFID tags that follow industry's Gen2 protocol. The asynchrony is considered as the time offset \tau between the beginnings of the two tags' responses and is modeled through a derived shaping matrix that depends on the delayed tag information. Performance evaluation of the proposed detectors with simulated data under Ricean fading, as well as experimental data with software-defined radio, reveals improved performance compared to prior art, under various operating regimes. It is also shown that for different values of the parameter \tau , BER does not present a monotonic behaviour. As a collateral dividend, it is found that clustering techniques on the filtered received signal should explicitly take into account the time offset \tau , since the latter modifies the number of observed clusters.