In this paper, we propose two adaptive frame size Aloha algorithms, namely adaptive frame size Aloha 1 (AFSA1) and\nadaptive frame size Aloha 2 (AFSA2), for solving radio frequency identification (RFID) multiple-tag anti-collision\nproblem. In AFSA1 and AFSA2, the frame size in the next frame is adaptively changed according to the real-time\ncollision rate measured in the current frame. It is shown that AFSA1 and AFSA2 can significantly improve the\ntransmission efficiency of RFID systems compared to the static Aloha, and AFSA2 produces transmission efficiency\nsimilar to that of the electronic product code (EPC) Q-selection algorithm (Variant II), while the mean identification\ndelay of AFSA2 is much smaller than that of EPC Q-selection algorithm (Variant II). It is also shown that the transmission\nefficiency of AFSA2 and EPC Variant II is very close to its upper bound which is obtained by assuming that the reader\nknows the number of unidentified tags. It is worth noting that when the threshold of the collision rate is chosen to be\n0.5 or 0.6, AFSA2 can maintain the transmission efficiency well above 0.65 for the case of a typical EPC code length of\n96 bits and for the investigated range of tag population, i.e., from 2 to 1000, while keeping the mean identification\ndelay below ten transmit contentions. Very light computational burden at the reader is needed: the reader needs only\nto measure the collision rate in the current frame and then to double or halve the frame size accordingly. No\nadditional computational burden is required at the tag side.
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