Master Thesis

Efficient Multiple-Access Scheme in Dense RFID Systems using Pseudo-Random Sequences

In radio frequency identification (RFID) systems, the proliferation of readers and tags introduces a number of technical problems. While traditional RFID applications deal with the identification of a sigle tag, one of the main challenges is the identification of multiple tags. If the number of tags is relatively large and two or more tags occupy the same RF channel simultaneously, we have collisions between tags, and the reader can not identify the data of the tags. The identification efficiency in RFID systems generally decreases for an increasing number of tag collisions. Advanced readers have the capability to cope with this problem by providing anti-collision techniques. So far, the most widely proposed schemes are variants of ALOHA and tree algorithms [1], where framed slotted ALOHA (FSA) is used in several RFID protocols [2, 3] due to its simple implementation.

In FSA, the reader broadcasts a request signal to the tags including a frame size. Upon receiving the message, each tag in the read range randomly selects one slot and sends the response back to the reader. There are different variants of anti-collision techniques based on FSA [4], but most of them assume known the number of tags a priori, which is not practical. Since the number of tags present in the read range of a reader is usually not known, the performance of a transmission control algorithm depends on the estimation of the number of tags n, and the choice of the corresponding frame size N. In [5], a lower bound on the number of tags is proposed based on the fact that in every collision at least two tags are involved, and each tag chooses only one slot for sending its message. A dynamic FSA (DFSA) where the frame size is dynamically allocated according to the estimated number of tags. These schemes work well if the number of tags is relatively small compare to the frame size N. However, if the number of tags becomes large, and the frame size N is limited to for example 256 slots, both methods are not advisable in practice.

In scenarios where n >> N, the probability of tag collision increases rapidly, and since the frame size N can not be increased indefinitely, we have to somehow restrict the number of responding tags. In [4], an enhanced DFSA (EDFSA) method is presented, where the problem has been solved by restricting the number of tag responses. The idea behind this is to create groups of tags, and to allow only one group to respond at each read cycle. Although, the proposed scheme outperforms the conventional algorithms, it does not propose an optimum grouping strategy. Furthermore, the number of tags is assumed to be known in advance which is a non practical assumption.

In this project, we study a multiple access scheme using pseudo-random sequences. Using spread spectrum signal modulations we would like to read multiple tags simultaneously in a time slot and thus reduce the number of collisions. This is beneficial as it speeds-up the reading process by reducing the communication between the reader and the tags, and also to improve the security of the system.

[1] K. Finkenzeller, RFID Handbook, John Wiley and Sons, 2003.
[2] ISO 18000 Part 6, Information technology automatic identification and data capture - radio frequency identification for item management air interface, 2003.
[3] EPCglobal, Class 1 Generation 2 UHF Air interface Protocol Standard V1.0.9, 2005.
[4] Su-Ryun Lee and Chae-Woo Lee, An Enhanced Dynamic Framed Slotted ALOHA Anti-collision Algorithm, EUC Workshops, pp. 403-412, 2006.
[5] Harald Vogt, Multiple Object Identification with Passive RFID Tags, IEEE International Conference on Systems, Man and Cybernetics, 2002.

Type Master Thesis
Subject area Multiple Access Schemes, RFID Networks
Type of work 60% Theory, 40% Software (Matlab)
Student Christopher Maltby
Supervisor Dr. Carlo Mutti
Professor Prof. Dr. Armin Wittneben