  Radio frequency identification (RFID) has been widely has broad applications.
One such application is to use RFID to track inventory in warehouses and retail
stores. In this application, timely identifying the missing items is an ongoing
engineering problem. A feasible solution to this problem is to map each tag to
a time slot and verify the presence of a tag by comparing the status of the
predicted time slot and the actual time slot. However, existing works are time
inefficient because they only verify tags one by one in singleton slots but
ignore the collision slots mapped by multiple tags. To accelerate the
identification process, we use bit tracking to verify tags in collision slots
and design two protocols accordingly. We first propose the Sequential String
based Missing Tag Identification (SSMTI) protocol, which converts all time
slots to collision slots and enables tags in each slot to reply to a designed
string simultaneously. By using bit tracking to decode the combined string, the
reader can verify multiple tags together. To improve the performance of SSMTI
when most tags are missing, we further propose the Interactive String based
Missing Tag Identification (ISMTI) protocol. ISMTI improves the strategies of
designing strings for each collided tag so that the reader can verify more tags
using shorter strings than SSMTI.Besides, ISMTI can dynamically adjust the
verification mechanism according to the proportion of missing tags to maintain
time efficiency. We also provide theoretical analysis for proposed protocols to
minimize execution time and evaluate their performance through extensive
simulations. Compared with state-of-the-art solutions, the proposed SSMTI and
ISMTI can reduce the time cost by as much as 39.74% and 68.87%.
