Daniel Arnitz

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.

Analysis of an indoor UWB channel for multipath-aided localization

We present a detailed analysis of an indoor UWB channel measurement campaign. The focus is on the modeling of the deterministic part of the multipath channel using a-priori known relevant reflections and scatterers, found from an available floor plan. Our approach uses virtual signal sources, whose locations and visibilities can be calculated using simple ray-launching techniques. The channel analysis steps exploit these results, using an effective multipath cancellation method that introduces virtually no artifacts. We show that the corresponding multipath-components can explain up to 90% of the UWB channel impulse responses in terms of energy capture. This is important for multipath-aided indoor localization, which provides robust position fixes using a single base station only.

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.

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.

Experimental characterization of ranging in IEEE802.15.4a using a coherent reference receiver

Real time locating systems (RTLS) and wireless sensor networks are currently a hot and challenging research topic. Ultra wideband (UWB) shows promising properties for these systems, such as low-power transmitter designs and highly accurate ranging, where cm-level accuracy is achievable even in multipath intensive environments. A high-performance reference receiver architecture based on coherent processing is presented in this paper. It is used to define quality metrics of the received signals, as for example peak signal to noise ratio (PSNR) and line-of-sight signal to noise ratio (LSNR) for ranging, and Eb/N0 for communication. These metrics are used to evaluate the experimental data in their quality and their ranging performance. Furthermore, performance trade-offs w.r.t. system parameter and receiver architecture choices, in particular for non-coherent receivers, can be analyzed. In this work, the reference receiver is verified experimentally using IEEE 802.15.4a compliant signals. The ranging performance is correlated to the signal-to-noise ratios for an indoor line-of-sight measurement campaign.

Simultaneous Imaging, Sensor Tag Localization, and Backscatter Uplink via Synthetic Aperture Radar

This paper presents an extension of synthetic aperture radar (SAR) techniques to enable simultaneous radar imaging, sensor tag localization, and backscatter-based data uplink from multiple sensor tags in a cluttered environment. A unified system model is presented that leverages coherent processing of backscattered signals gathered over the synthetic aperture for all three of these purposes. The proposed approach, using balanced orthogonal codes for SAR-based localization as well as the backscatter data uplink, is shown to have several favorable properties, including straightforward tag-vs-clutter discrimination, straightforward multiple access among tags, and improved signal-to-noise ratio during localization. A proof-of-principle indoor experiment is presented in the X-band (10–13 GHz) using two custom-designed backscatter tags interrogated by a vector network analyzer functioning as an FMCW radar. The proposed system model is validated by simultaneous imaging of a cluttered scene, tag localization with a maximum range error of 9 mm, and data demodulation from both tags telemetering temperature changes at a rate of 1 bit/s at ranges of 4.4 m and 4.7 m. The resulting point-spread functions of tags demonstrate a range resolution of 4.7 cm and a cross-range resolution of 9.1 cm.