Uwe Dettmar

New measurement results for the localization of UHF RFID transponders using an Angle of Arrival (AoA) approach

In this paper we present new measurement results for an Angle of Arrival (AoA) approach to localize RFID tags at 868 MHz. Self-designed three element antenna arrays, off-the-shelf IDS R901G RFID reader ICs, and UPM Raflatec DogBone RFID tags are used to generate and detect ISO 18000–6C compliant signals. The experimental setup comprises three antenna arrays and a test environment of size 3 times 3 square meters with 25 test points. The AoA is estimated using the phase differences in the complex baseband signals of adjacent antenna elements within one array. A mean positioning error of 0.21 m was achieved for the considered test grid.

Efficient differentially coherent code/Doppler acquisition of weak GPS signals

Differentially coherent combining of correlation results is proposed for the acquisition of weak GPS signals, i.e. signals down to -147 dBm received power. It is shown that, with a combined coherent/differentially coherent acquisition scheme, GPS signals can be acquired in 200 msec without any assistance data. By complementing the proposed scheme with methods to estimate the fractional delay, the acquisition unit can provide high quality Doppler and delay estimates such that additional tracking units seem obsolete. Since the need for a complex FFT post-processing of correlation results is avoided, the proposed scheme offers an efficient way of implementing a rapid acquisition GPS receiver in hardware.

Improved AoA based localization of UHF RFID tags using spatial diversity

In a recent paper we presented new measurement results for an angle of arrival based approach to localize UHF RFID transponders in an indoor environment with off-the-shelf IDS R901G RFID reader ICs and UPM Raflatac DogBone RFID transponders. We were able to localize the transponders with a mean accuracy of 0.21 m using three antenna array positions for triangulation in a 3 m × 3 m room. In this paper we present an RSS based algorithm to select a special subset of antenna array positions for the estimation of the transponder position in a measurement setup with five different antenna array positions. With this algorithm a reduction of the mean measurement error by 50% was achieved for the considered test grid compared to the original approach in [1].

UHF RFID localization system based on a phased array antenna

This paper presents new experimental results of angle of arrival (AoA) measurements for localizing passive RFID tags in the UHF frequency range. The localization system is based on the principle of a phased array with electronic beam steering mechanism. This approach has been successfully applied within a UHF RFID system and it allows the precise determination of the angle and the position of small passive RFID tags. The paper explains the basic principle, the experimental setup with the phased array and shows results of the measurements.

Modulation for HIPERLAN type 2

Currently, standards for wireless multimedia communication with high bit rate in the 5 GHz band are being developed in the US, Europe, and Japan. The available user data rate for these systems is supposed to be significantly greater than 10 Mbps. The high data rate and the channel characteristics are the main challenges for the design of the physical layer and in particular the selection of modulation technique for such a system. In this paper two modulation techniques, single-carrier and multicarrier modulation (implemented by OFDM), are considered as candidates. Both techniques are introduced, individually optimized to some extent and finally compared with regard to link performance and other aspects like coverage and power consumption. It is shown that for the assumed conditions the addressed coherent OFDM scheme provides about 2-3 dB performance gain compared to single-carrier modulation as well as non-coherent OFDM.

Improved UHF RFID localization accuracy using circularly polarized antennas

We present the design of two new antennas for an indoor angle-of-arrival based passive UHF RFID tag localization method. First, a circularly polarized patch antenna that can be used as the readers transmit antenna or as the tag antenna. Second, a circularly polarized three-element patch antenna array for the reader. We use measurements in a 3 m × 3 m test grid with 25 equally spaced measurement points, placed into a standard office room, to show that the new antennas improve localization accuracy by 53% compared to a system equipped with linearly polarized antennas. The new circularly polarized system is able to localize a tag in our setup with a measured mean error of 9 cm and a measured mean standard deviation of 7 cm.

Localization of Passive UHF RFID Tags Using the AoAct Transmitter Beamforming Technique

The localization of passive ultra-high frequ- ency (UHF) radio frequency identification (RFID) tags is an emerging technology that has reached a high level of interest in industry and research. Most solutions use classical techniques, such as the angle-of-arrival or the received-signal-strength methods. As an alternative, we present a new approach for estimating the position of a passive UHF RFID tag by using a transmitter beamforming technique that we call angle-of-activation (AoAct). It is based on manipulating the shape of the radiation pattern of the interrogators transmitter antenna array. By pivoting the main lobe of the pattern in the azimuth plane of the array and by utilizing the very sensitive response threshold of passive UHF RFID tags, we are able to estimate the tag direction. We use three-channel digital beamforming, where the phases and amplitudes of the antenna arrays feeding radio frequency signals are controlled by the baseband I and Q components of the interrogators transmitter signal. Our self-developed hardware pivots the main lobe of a three-element antenna array in a range of ±52° with a resolution of 0.02°. It senses the AoAct with a mean measured accuracy of 1.2° in the anechoic chamber, 1.9° in a basic multipath environment, and 2.5° in a real-world multipath environment. After estimating the AoAct of a tag from different locations, the tag position is triangulated. Experiments show that our AoAct system delivers a localization accuracy of 7 cm in a typical multipath environment.

Combined channel estimation and frequency correction for packet oriented mobile communication systems

We show how the well known least square (LS) algorithm can be used for combined channel estimation and frequency correction in packet oriented mobile communication systems. The method is derived by investigation of the effect of a frequency offset, e.g., due to the instability of the local oscillators, on the LS estimates. The algorithm corrects frequency offsets resulting in phase shifts per symbol that are in the range of several degrees. Imperfect correlation properties of the applied synchronisation sequence are automatically compensated by the algorithm.

An improved channel model for passive UHF RFID systems

For preliminary performance evaluation of an UHF RFID system, it is important to have a simple and fast simulation environment available. The calculation of the receive signal power at arbitrary positions of a predefined environment should be fast and accurate to find dead zones where tags cannot be read although they are well within the readers communication range. Changes in the system configuration or the simulated environment must be easy to implement. In this paper we use the simple and fast calculable UHF RFID channel model from [1] and improve it in different aspects to better fit the real indoor multipath signal propagation. A comparison between measurements and simulations shows that our model delivers 1.8 dB better performance in average than the original model, while adding only little complexity and computing time. Using our simulation environment, it is easy and fast to find dead zones for arbitrary system setups.

Using a circularly polarized patch antenna to optimize the passive UHF RFID indoor channel

In this paper we describe the design and performance of a new circularly polarized broadband patch antenna for RFID applications. It works both in the current European UHF range (865 MHz - 868 MHz) and the new European UHF range according to ETSI TS 102 902 (915 MHz - 921 MHz). Simulations show that circular patch antennas improve the multipath behavior of the channel significantly. In [2] we show that switching the system frequency strongly improves the channels fading characteristic. Because an RFID system can be equipped with the new antenna for readers and tags we can make use of both beneficial effects to increase tag read probabilities and read ranges in typical indoor environments. RSS measurements are used to verify the simulation results and to quantify the performance of the new antenna.