FFT Based Rate Estimation for UHF RFID Systems Hamed Salah, Hazem A. Ahmed, Joerg Robert, Albert Heuberger Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Information Technology (Communication Electronics) {hamed.kenawy, hazem.a.elsaid, joerg.robert, albert.heuberger}@fau.de Abstract—This paper focuses on the rate estimation in radio frequency identification (RFID) reader receivers, which plays a significant role for the stability and the efficiency of RFID systems. RFID readers have to estimate the rate of the tag reply, i.e. the so-called backscatter link frequency (BLF). According to the focused EPCglobal Class-1 Gen-2 standard, the BLF can have a variation of up to +/- 22%. Most literature uses time domain algorithms to estimate this BLF. However, the proposed techniques are not suitable to estimate the rate when multiple tags reply simultaneously to the reader. In case of such tag collision we utilized the frequency domain representation of the replies to estimate the BLF. The performance is tested with different rates and the both supported modulation techniques, i.e. FM0 and Miller. Simulation results show the high performance of utilizing the FFT for estimating the BLF. This especially holds in case of collisions that cannot be easily estimated by means of time domain algorithms. I. I NTRODUCTION In the recent years, as Radio-Frequency IDentification (RFID) technology became more popular, more challenges have been met by designers of RFID readers. In typical applications RFID readers have to communicate with multiple tags to be identified in a short time. RFID tags transmit their IDs in different time slots, while diversities of transmitted signals from different tags, mainly on power and data rates, may affect the stability and efficiency of RFID reader receivers severely [1]. In UHF RFID systems, the data rate variations are much more crucial than in other communication systems. These data rate variations arise from two reasons that can not be avoided in practical systems. Firstly, low cost RFID tags use a slow system clock for the digital baseband operation, resulting in a measurement error for the parameters that are transmitted by the reader to determine the tag data rate. In the EPCglobal Class-1 Gen-2 RFID protocol, this tag reply rate is called Backscatter Link Frequency (BLF) [2]. Secondly, the tag clock frequency is usually affected by manufacturing process variations, which also contribute to the BLF variations. Based on the standard, a maximum BLF tolerance of ±22% is allowed with a BLF range from 40 kHz to 640 kHz. Time limitation adds an extra challenge for the design of the synchronization module. After receiving the reply from the tag, the RFID reader is required to give a response within 20 T pri where T pri = 1 BLF [2]. At the highest BLF, the operation time left for the whole reader is about 31.25 μs, and only a part of it is left for the synchronization. This puts a limit on the use of some complicated rate estimation algorithms. Several rate estimation algorithms have been proposed to compensate the BLF variations in RFID systems. All these algorithms are used to estimate the BLF during a single tag reply. However, nobody tried to address the rate estimation if more than one tag replies in the same slot, i.e. in case of a collision. Many algorithms have been presented to recover at Figure 1. FM0 encoding scheme and tag extended preamble least one tag from a collision. However, all these algorithms assume that the BLF of all tags are perfectly synchronized. Liu and Huang et al. [3] presented a rate estimation algorithm based on 12 preamble correlation banks, followed by an early late technique. The BLF range is specified by the correlator that returns the maximum value. Their implementation performs well in applications that are using high rates. For slow rates, the bad performance is compensated using the early late algorithm. For this reason, this technique is not suitable for estimating the rate of a tag in a collided slot. Angerer et al. [4] and Wang et al. [5] proposed new algorithms with lower complexity than [3]. However, also they are not suitable for the rate estimation of collided slot as they depend on the rate estimation in the time domain. In this paper, we present a rate estimation technique based on the Fast Fourier Transform (FFT) to estimate the rate of a single tag reply, or of the strongest tag from multiple simultaneous tag replies. The technique is tested for a single tag reply and for up to four collided tags. A comparison between the performance of the algorithm at low BLF (i.e. 40 kHz), and high BLF (i.e. 640 kHz) is presented. Furthermore, we study the effect of the modulation technique of the tag on the rate estimation accuracy. Finally, the Percentage of Failure (PoF) is used as a performance metric to compare the results. This paper is organized as follows. Section 2 explains the most important basics of the EPCglobal standard. In Section 3, the rate estimation techniques is explained and tested for successful slots (single tag reply) and collided slots. In Section 4, the simulation results are discussed. Finally, the conclusion is drawn in Section 5.