The efficiency of RFID applications is severely degraded when multiple tags transmit their IDs simultaneously. As a result, many anticollision algorithms have been proposed for reducing collisions at the reader. However, these algorithms focus only on the tags within communication range of the reader in the previous or current frames. That is, they ignore tags which move out of range of the reader in one frame, but then move back within range several frames later. As a result, the identification performance of the reader is seriously degraded. The present study analyzes the effect of re-entering tags on the performance of three common anticollision algorithms and proposes a new reblocking algorithm (RBA) to alleviate the problem. The lower and upper bounds of the collision probability for re-entering tags are derived using a multinomial coefficients approach. The performance evaluation results show that the proposed algorithm consumes fewer cycles and provides a more robust identification performance than existing blocking algorithms in real-world RFID systems characterized by re-entering tags.
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