An Optimal Q-Algorithm for the ISO 18000-6C RFID Protocol

Abstract

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...

Efficient Tag Identification in Mobile RFID Systems

“…Hence rt f (r) = (p i (r)) 3 · (p t (r)) 2 . Therefore we can derive that p i (r) = rt(f ) rt (1) and p t (r) = (rt(1)) 3 (rt(f )) 2 . We can store the values of p i (r) and p t (r) at various distance r. In our probabilistic model, for simplification of the problem we do not consider the impact This full text paper was peer reviewed at the direction of IEEE Communications Society subject matter experts for publication in the IEEE INFOCOM 2010 proceedings This paper was presented as part of the main Technical Program at IEEE INFOCOM 2010.…”

“…The ALOHA protocol was first developed for random access in packet radio networks. To improve efficiency for RFID systems, the slotted ALOHA is developed [1] [2]. In [9], Schoute analyzes the performance of the dynamic frame-sized ALOHA.…”

“…This standard is based on a slotted ALOHA process to regulate tag responses. Prior research [1][2] on improving performance have focused on optimizing the parameters used in this slotted ALOHA model. However, these research only considered optimizing the parameters when reading static RFID tags under ideal environmental conditions.…”

Architectures and protocols for fast identification in large-scale RFID systems

“…There is a lively research interest on MAC solutions for RFID systems based on the ISO/IEC 18000-6 standard (i.e, [6] - [14]). In [6], the authors propose a tree structured collision resolution scheme.…”

“…In [13], the optimal contention window is derived according to estimations of the tag population. In [14], an adaptive algorithm for the contention window is proposed to maximize the number of reads per second. The presence of multiple access interference and the impact on the MAC performance has been studied in [15].…”