Ulrich Muehlmann

Bandwidth dependence of CW ranging to UHF RFID tags in severe multipath environments

In this paper the impact of the signal bandwidth on the performance of frequency modulated continuous wave (FMCW) radar based ranging to ultra high frequency (UHF) radio frequency identification (RFID) tags is investigated. The analyses are based on ultra-wideband (UWB) channel measurements performed in a warehouse portal, which is a severe multipath environment. It is illustrated that the available bandwidth of the usual ISM bands at 900 MHz, 2.5 GHz and 5.8 GHz is only sufficient for a precise RFID tag localization if moderate or low multipath conditions are given. However, in severe multipath channels the ISM bands are unsuited and UWB signals are needed. The results can be considered a lower bound for signal time of flight (TOF) based localization approaches that utilize Fourier or correlation methods for the signal travel time estimation.

Multifrequency Continuous-Wave Radar Approach to Ranging in Passive UHF RFID

In this paper, we present the extension of a recently published two-frequency continuous-wave (CW) ultra-high-frequency RF identification ranging technique to multiple carriers. The proposed system concept relies on exact phase information; hence, the passive tag cannot be accurately modeled as a frequency-flat linear device. A linearized model of the tags reflection coefficient is devised to bridge the gap between the nonlinear reality and the linear CW radar theory. Estimation error bounds are derived and effects caused by noise and multipath propagation are analyzed in detail. It has been found that systematic errors introduced by the tags reflection characteristic cannot be compensated by using multiple carriers due to large variations caused by detuning. Nonetheless the system, while being vulnerable to multipath propagation effects, still performs well under line-of-sight conditions; mean average errors below 15% of the true distance are possible in typical fading environments..

Characterization and Modeling of UHF RFID Channels for Ranging and Localization

A comprehensive characterization and model of the UHF RFID channel is presented for narrowband through ultra-wideband tag localization systems. The analyses are based on ultra-wideband channel measurements in a warehouse portal, centered around 900 MHz. Measured scenarios include an electromagnetically transparent pallet and a pallet containing liquids, each for a portal shielded by metal backplanes and for a portal shielded by absorbing material. The presented analyses cover the individual channels to and from the tag, the feedback channel, and the backscatter channel, for bi- and monostatic reader setups. We find that the direct path is rarely dominant on the backscatter link despite clear line-of-sight conditions and directive reader antennas. The power ratio between the direct and all indirect paths ranges from -20 through 5 dB for the more common metal portal, and RMS delay spreads are in the range of 10-80 ns. Since only the direct (line-of-sight) path carries the correct distance/direction information, tag localization in such portals requires high robustness with respect to weak line-of-sight components. We also show that classical channel models in UHF RFID, despite predicting the incident power level at the tag accurately, produce far too optimistic estimates of channel parameters relevant to ranging and localization.

Wideband Characterization of Backscatter Channels: Derivations and Theoretical Background

The wireless channel of backscatter radio systems is a two-way pinhole channel, created by the concatenation of two standard wireless channels. We present a method to calculate wideband channel parameters of backscatter channels based on the parameters of the constituent one-way channels. The focus is on characteristics that are vital for narrowband and wideband ranging, such as the K-factor w.r.t. the direct (line-of-sight) path and the RMS delay spread. The presented analyses include uncorrelated as well as correlated channel pairs and are thus valid for bistatic and monostatic antenna setups. We also show that the uncorrelated scattering (US) assumption holds for the backscatter channel provided that the constituent channels are US.

Modeling and Performance Characterization of UHF RFID Portal Applications

UHF RF indentification systems integration into existing processes is a major challenge for trading companies and retailers. The installation of antennas and interrogators and the portal configuration for achieving a required read rate is difficult, and in most cases, sub-optimally implemented. Therefore, it is essential to understand the relationship between achievable read rates and portal setup demands. In this paper, we describe a novel interrogator-to-tag channel model that can be used to determine the required components prior to system installations and tuning processes. Several model parameters are derived from typical portal setups by means of interrogation zone investigations with a custom-made highly sophisticated mobile field strength recorder. Results have shown that the field coverage of the interrogation zone is independent of the antenna height and we have proven that the three-ray LOS-model fits well with real life conditions. The TAG-model parameters mainly depend on the statistical field strength distribution obtained from two independent interrogation zones. Additional measurements will be required in order to find appropriate or even general values for the proposed bimodal field distribution function of the TAG-model for further measurement independent portal characterizations.

Physical limits of batteryless HF RFID transponders defined by system properties

This paper motivates the usage of RF system properties of an ISO/IEC 14443 HF RFID communication link to describe defined properties at the air interface. An analytic derivation of the transponder loading effect and an empirical equation to estimate load modulation are presented. Based on this system model a lower bound for the physical limit of the transponder antenna size is proposed. This specifies the smallest antenna size that is capable of ISO/IEC 10373-6 compliant side-band amplitudes with passive load modulation. Measurement data is used to verify the presented system level descriptions and to extract typical quality factor values of batteryless RFID transponders. The outcome of this work can be applied to other HF RFID communication standards like EMVCo or NFC Forum.

Wideband system-level simulator for passive UHF RFID

A chip manufacturing process requires extensive support of CAD-tools in order to predict the behavior of the embedded circuitry and to ensure the intended system functionality. Past experience shows that the overall performance of UHF RFID systems is mainly limited by multipath propagation and detuning. In this context, system-level simulations are vital to assess the overall performance and improve the embedded circuit. We present a simulator framework capable of handling chip-level tag models, fading MIMO radio channels, and interrogator building blocks on signal level. It is based on highly flexible behavioral tag-models instead of highly accurate but static ASIC models. In contrast to other UHF RFID simulators, it is explicitly designed to handle wideband signals, fading channels, nonlinearities, and detuning effects. The simulator is currently used to develop and evaluate the performance of ranging and realtime channel estimation systems. The presented results emphasize the feasibility of our framework in the evaluation of a range estimation approach between a standard UHF RFID transponder and an interrogator.

Mutual coupling modeling of NFC antennas by using open-source CAD/FEM tools

Near Field Communication (NFC) antennas are not comparable to standard communication antennas used for traditional transmission services. The geometries are complex, narrow conductor cross-sections and long loops located close to materials sensitive to electromagnetic interaction, when integrated, make the application of analytical coupling formulations inapplicable, even when it comes down to non-linear relations forced by ferrite shielding. Commercial finite element (FEM) solvers are powerful, but they do not serve the needs of NFC applications due to complexity, inadequateness and code obscuration. An intensive open source code investigation has identified one alternative solver which is capable to deal with all NFC RFID related coupling effects one can imagine. Automated scripts, the tool-chain, and geometry macros have been developed for rapid prototyping of such. Amongst other open source tools listed, the ElmerFEM solver is the most promising solver for linear and non-linear quasi-static electro-magnetic (EM) NFC problems. The solver has a powerful interface and delivers results close to reality even when computational complexity is a trade of physical resolution and memory capacity of standard commercial workstations.

SDR based RFID reader for passive tag localization using phase difference of arrival techniques

This paper presents a passive radio frequency identification (RFID) reader for a two dimensional localization of tagged objects in the ultra high frequency (UHF) band. The presented prototype is capable of simultaneously estimating the direction and the range of an object. This capability is provided by a single input multiple output (SIMO) antenna configuration in combination with phase difference of arrival (PDOA) techniques. The core of the prototype is a flexible software defined radio (SDR) that allows rapid prototyping and thus a fast system verification. The reader operates at three frequencies that makes the tag localization more robust, especially in multipath environments. The reader verification is shown by measurements in an anechoic chamber. An initial angular accuracy of 3 degrees for the tag direction and 23 cm for the tag range is accomplished in this first stage of the prototype development.

SIMO RFID system performance in an engine test bed

This paper presents a performance evaluation of a radio frequency identification (RFID) system in an indoor multipath environment at 890 MHz. A single input multiple output RFID system is investigated with respect to the feasibility of localizing tagged measurement equipment in an engine test bed. Based on the available signal power within the test bed, the system performance is evaluated with respect to state-of-the-art RFID system components. The evaluations show that an exemplary RFID system experiences no outages when using a passive RFID transponder (tag) with a chip sensitivity of -17:5 dBm and an RFID reader with a transmit power of 30 dBm and a receiver sensitivity of -95 dBm. Additionally, the evaluation allows to deduce system requirement parameters for the further realization of such an RFID reader, e.g., the minimum transmit power for the reliable operation in the engine test bed is 24:5 dBm.