Performance analysis of stacked RFID tags

Most RFID tag performance analysis is based on the assumption that the tag is stand-alone when interacting with the readers. However, in the real world, RFID tags are often stacked together when deployed, i.e., RFID-labeled carton boxes stacked on the pallet. When multiple RFID tag antennas are placed close to each other, each antenna behaves like a shielding and reflecting object to others and the radiation pattern and other EM properties will be detuned. The stacking impact often leads to performance degradation and in many cases, read and write failure. In this paper, we first derive an RCS model to generalize a loaded antenna as a shielding and reflecting object. We then introduce a normalized mutual impedance matrix of multiple antennas to formulate the coupling effects among stacked antennas. To quantify and visualize the stacking impact in a real world scenario, we also construct a 3-D EM model in Agilent® AMDS consisting of 27 (3 × 3 × 3) stacked carton boxes, each with Alien® tags attached on the surface. Our theoretical calculation agrees with the simulation and shows that both tag radiation pattern and other EM properties will be greatly altered by neighboring tags. Compared with tags in stand-alone scenarios, the degradation at different positions could be very significant (more than 6dB in some directions).