Doctor's Theses (authored and supervised):

N. Gvozdenovic:
"Enhancing efficiency and throughput of nearfield coupled RFID systems";
Supervisor, Reviewer: C. Mecklenbräuker, M. Beach; Institute of Telecommunications, 2015; oral examination: 09-03-2015.

English abstract:
Radio frequency identification (RFID) tags are massively used for contactless identification and manufacturing logistics. Many companies and universities are actively involved in the research and development of this technology. Nevertheless, many open questions remain, which was the motivation for the work documented in this thesis. In my research I investigated two closely related topics. In the first I investigated existing high frequency radio frequency identification (HF RFID) systems, with special focus on the optimization and automatization of the production, as well as spurious emission that poses a danger for the eavesdropping. In the second part we developed prototypes for millimeter sized ultra-highfrequency ultra-wideband radio frequency identification
(UHF UWB RFID) tag with on-chip antennas (OCA). These tags are powered by magnetic coupling on UHF, which is also used for dowlink communications. On the uplink UWB is used. My special interest was synchronization problem on the uplink.
In the first part of the thesis I was dealing with efficient power and data transfer for contactless chip-cards. Working frequency is 13.56 MHz and the information is transferred between a card and a reader through magnetic coupling. Notable implementations of
these cards are in electronic ticketing systems, biometric passports, for identification as HF RFID (high-frequency radio frequency identification), in smart phones as NFC(near-field communication), etc. A card consists of an integrated circuit (IC) and an antenna. As antenna a planar spiral inductor (coil) is used. For designing a card, a good model of both the coil and the chip is essential.
In the chapter 2 I analyze planar spiral coils which are used as antennas. I developed a numerical model for spiral coil based on the partial element equivalent circuit (PEEC) method. In combination with nonlinear optimization algorithm it optimizes and synthesizes
coils, i.e. generates physical dimensions of an HF RFID antenna according to the specified equivalent circuit parameters. The main application is an HF RFID standard antenna for 13.56 MHz. Additionally I present a procedure for accurate measurement of
impedance of coils, and simple fitted formulas for quick calculation of coil parameters in order to achieve required resonance frequency.
In the chapter 3 I analyze non-linearity of the integrated circuit of the HF RFID tag.
I model the non-linearity both in time and frequency domain versus input power. A method for accurate measurement of impedance and harmonics produced by the chip is presented, and the archived resonance frequency and bandwidth of HF RFID tag is evaluated.
Spurious emissions from contactless chip-cards pose important challenges from the point of security and privacy due to danger of eavesdropping. In the chapter 4 I investigate the out-of-band unwanted emission produced by the HF RFID caused by nonlinearity of the integrated circuitry and the radiation properties of the antenna on these harmonic frequencies. These emissions are evaluated by measurements in both lab environment and in a Gigahertz Transverse Electromagnetic (GTEM) cell. I propose the optimization of the antenna shape so that it also influences radiation on the harmonics
for two different purposes. On one hand it is used for spurious emission suppression techniques - where taking into the account the strength of the harmonics produced by the chip, and the radiation properties of the antenna I finely tune the antenna so it does
not radiate on the most dangerous harmonics. One the other hand I propose the opposite: use the strongest harmonics produced by the chip and tune the antenna so that it radiates well on these harmonics. On that way it is feasible to create a second link on a
harmonic frequency and the reading distance from the coil can be increased.
In the second part of my thesis presented in chapter 5 I report on my work done on the development of UHF UWB RFID tag with OCA. For RFID and sensor applications, the use of ultra-wideband can reduce power consumption. Integrated entities with OCA due to itīs low efficiency are limited to the communication to a very short operational
range. Implementation of OCA in RFID tags reduce overall cost, but pose few challenges. When antenna is electrically small,with increase of bandwidth, available capacity saturates, and for high values of the Q-factor even rapidly decreases. For a passive RF driven grain, the required power for driving a transmitter frontend is delivered
by an asymmetric transmission scheme. In this scheme the downlink (from the reader to the tag) is implemented by inductive coupling using a narrow-band UHF carrier. In the opposite direction from the tag to the reader (uplink), we designed different ultrawideband
transmission schemes. The first prototype was actively powered and we used it to perform channel measurements of a millimeter sized UWB RFID tag with OCA.
The manufactured prototype transmits carrier-based UWB signal, with adjustable pulse duration and carrier frequency. A measurement setup has been developed to emulate relevant office scenario and based on our measurements we proposed a near-field UWB channel model. In the next prototype full asymmetric scheme is implemented in UHF
UWB RFID tag. 4-PPM UWB is used on the uplink and UHF communication on the downlink to solve the problem of synchronization. To investigate feasibility of MIMO implementation in this scheme, a set of near-filed MIMO UWB channel measurements was performed.

HF RFID, contactless chip cards, PEEC

Created from the Publication Database of the Vienna University of Technology.