To reduce time of re-identifying tags, we propose a novel adaptive framed slotted aloha (AFSA) protocol for radio frequency identification (RFID) tag anti-collision in this paper. The AFSA protocol can allocate each tag a slot number after successfully identifying the tag. The slot number signifies when the tag could be identified during an identification cycle. If some tags which have been identified in the last identification cycle need to be re-identified, a reader adopting the AFSA protocol can orderly identify these tags according to the tags' slot number. And, to further reduce re-identifying time, we also improve the ADFA protocol in the second half of this paper. Both the theoretical analysis and the simulation show that, the AFSA protocol and the improved AFSA protocol can reduce read time when repeatedly identifying tags.
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