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Abstract-Passive radio-frequency identification (RFID) systems based on the ISO/IEC 18000-6C (aka EPC Gen2) protocol have typical read rates of up to 1200 unique 96-bit tags per second. This performance is achieved in part through the use of a medium access control algorithm, christened the Q-algorithm, that is a variant of the Slotted Aloha multiuser channel access algorithm. We analyze the medium access control algorithm employed by the ISO/IEC 18000-6C RFID air interface protocol and provide a procedure to achieve optimal read rates. We also show that theoretical performance can be exceeded in many practical use cases and provide a model to incorporate real-world data in read-rate estimation.Note to Practitioners-Estimating read-rates in RFID has always been something of a black art. At one end of the spectrum, in the pure-theory approach, rates are estimated by taking the duration per bit and calculating the total number of bits that can be decoded per second. This approach does not take any of the protocol overheads or real-world conditions into account. In the pure-experimental approach, a standard test case is used to relatively compare read-rates as several factors-tags, readers, firmware, protocols, etc., are varied. Neither of these approaches really provides any insight into the problem of estimating read rates for the general case.In this paper, we take on this problem by developing a first-principles model of collision probability in the Gen2 medium access control layer. Collisions of tag responses are a dominant factor in determining read rates in Gen2 systems. Using this model, we show that the worst case efficiency of the protocol can be no less than 36.8%, i.e., it should be possible to see more than 36.8% of a given population of tags per unit time. We them develop a dynamic Q-algorithm that performs much better than the worst case, and show its performance relative to a static Q-algorithm.We then relax the assumptions underlying the above algorithm so as to be able to incorporate real-world situations and provide a framework wherein practitioners can make some measurements of a particular situation and use our model to estimate expected read rates. Three important factors that need to be considered are: (i) the different decoding times for different types of slot-occupancy; (ii) the capture effect, wherein a two-occupancy slot is decoded as a valid tag because the backscatter powers are sufficiently different; and (iii) the distribution of backscatter powers. We develop a model to account for these three factors.Although our models make several assumptions, we have designed and deployed readers that justify almost all of them. We are currently working on developing a deeper characterization of the backscatter power distribution of a population of tags. This will allow us to use the signal processing capability of our readers to Manuscript received July 29, 2007 disambiguate two-occupancy slots and boost read rates well-above those predicted by our model. This is the focus of our current resear...

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