A Measurement Strategy for Test and Characterization of UHF RFID Systems

Alírio Soares Boaventura and Nuno Borges Carvalho Dep. Electrónica, Telecomunicações e Informática,

Instituto de Telecomunicações Universidade de Aveiro

Aveiro, Portugal

Abstract – In this paper a new approach to RFID system characterization is presented. This new characterization system uses three different antennas and a multi-channel high frequency oscilloscope to gather the full behavior of the envelope as also of the RF signal in the air interface. It is expected that by using this measurement framework, information of tag RF efficiency and complete UHF RFID channel can be obtained, which can be useful in RFID system design and characterization . Index Terms — RFID, Measurement, Tag, Channel.

I. INTRODUCTION

As the radio frequency identification (RFID) is becoming a widespread technology, also a growing need for RFID measurement, test and characterization systems are rising. RFID reader and tag measurements are needed both for physical (PHY) layer test and analysis and for conformance, interoperability and protocol validation. Common tag PHY metrics include:

1. Communication range (at a given transmitted power), 2. Tag sensitivity (minimum power to activate the tag), 3. Tag backscattering efficiency (ratio between the

reflected power and the incident power in the tag), 4. Radar Cross Section (RCS) of the tag, 5. Frequency dependent sensitivity (and optimal

operating frequency), 6. Tag consumption and dissipation.

Different setup configurations can be used to direct or indirectly obtain these metrics. The simplest one consists in using a “sniffer” (e.g. a Vector Signal Analyzer, VSA) to listen the reader-tag air interface and to record the data communication for further analysis. The “sniffer” has to be configured with a proper trigger setting, for instance, trigger on power level detected or on frequency component detected. The metrics can be later extracted from the recorded data. More advanced topologies can emulate an RFID reader: a transmitter (which can be implemented, for instance, with a Vector Signal Generator, VSA) is used to interrogate the tag with a valid command and a receiver (for instance, a VSA) captures the tag response. These topologies are very common [1]-[5], since they provide much more flexibility than the first approach, for instance, several PHY characteristics (e.g. timing and codification parameters, modulation depth,

frequency channel, transmitted power level) can be modified. Nevertheless the use of a VSA only allows gathering information on the envelope signal, and not on the RF waveform. Moreover the sniffer is positioned in between the reader and the tag, and it is difficult to separate the tag and transmitter/reader behavior. For instance one very important problem when dealing with RFID system optimization is to know the tag backscattering efficiency, and the sensitivity of the reader, since both will impose the maximum system coverage range. So, it would be important to achieve a measurement at the transmitter/reader and at the tag simultaneously. In this paper we propose a multiple-antenna UHF RFID measurement system based on a high frequency real-time scope with 3 RF input channels, that, beyond the aforementioned metrics, will allow to perform the following RFID measurements:

1. Forward RFID UHF channel characterization 2. Reverse RFID UHF channel characterization 3. Tag conversion efficiency

II. RELATED WORKS

Several RFID measurement systems are found in the literature. Also commercial equipments specially conceived for RFID measurements started to be released. In [1] and [2] a method for measuring signal backscattering from a passive RFID tag and a formula for determining the tag RCS have been presented. By using a network analyzer connected to an antenna (in an anechoic chamber with a tag inside) the authors were able to measure the return loss of the antenna, and based on the measurement, extract information of reflected power from the tag and determine the tag RCS. In [3] the authors studied the problems related to tag performance degradation (caused by absorption, reflection and detuning) when attached to objects/products. To do so, an UHF RFID measurement system was implemented in order to qualify the tag performance in various application environments. Tag sensitivity and reading range measurements were done for different mounting materials, reader power level, operating frequency and antenna orientation.

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The work presented in [4] dealt with passive UHF tag characterization focused on the energy collection (tag sensitivity) and backscattering (RCS) aspects. For this purpose, these parameters have been measured for dipole antennas with different widths. The energy collection and backscatter are greatly influenced by the material to which the tag is attached and it has been shown that the antenna geometry affects both of them and that small changes can lead to large impact on the tag performance. Recently, a measurement system has been implemented in [5]. This system is based on the LabVIEW-controlled PXI (PCI eXtensions for Instrumentation) platform [6]. It is able to perform fully automated measurements on Gen2 tags (and other protocols) over a wide range of frequencies (800-1000MHz) and power levels up to 30dBm. The characteristics of reader-to-tag commands can be defined by the user and a LabVIEW based user interface displays the measurements (e.g. frequency dependent reading range, sensitivity and backscattered signal). Some commercial solutions also exist: for instance, the modular PXI platform, an open industry standard [6][7], designed for measurement and automation applications, allows the implementation of customized RFID measurement systems. This platform [6] has been used, for example, in the works presented in [3] and [5]. Another example is the measurement system provided by [7], that is also based on the PXI platform. This system covers various RFID standards and frequency bands (13.56MHz, 860-960MHz, 2.4GHz) and allows operation in three different configurations: “sniffer” mode, reader emulation mode and tag emulation mode (able to act as a tag and challenge an RFID reader). Nonetheless, none of the previously mentioned measurement systems perform full channel characterization, or includes a simultaneous characterization of reader sensitivity and tag sensitivity. Moreover it is not clear what is happening at the RF level, since all the methods operate at the envelope of the RF signal.

II. PROPOSED CHARACTERIZATION SYSTEM

In this paper we propose a multiple antenna UHF RFID measurement system capable of fully characterize both forward (reader-to-tag) UHF channel and reverse (tag-to-reader) channel, including the tag behavior. For this purpose an RFID reader is used to interrogate the tag through antenna A1 and to receive the response from the tag (Fig. 1), antenna A3; a third antenna, A2, is placed at the tag location to capture the tag response at the tag position and antenna, A3, is used to measure the tag backscattered signal at the reader location. The RFID reader is also used to have qualitative information of whether the tag does or does not respond. A high speed oscilloscope (50 GS/s) is used to measure the UHF RF signals. This allows a complete channel characterization including the tag RF operation and efficiency. Fig. 1 depicts the proposed measurement test bed. With this setup it is possible to measure the transmitted signal from the RFID reader in CH1, the received signal in CH3 and

a sample of the signal received and backscattered by the tag in CH2. Since they are all received simultaneously they can be synchronized with a common reference, and thus allow measurement of several important figures of merit. For instance if we consider the signal in A1, A2 and A3, we can define the following relationships:

Alien RFID Reader

High FrequencyOscilloscope

A1

SplitterD

PassiveTAGA3

A2

CH3

CH1

CH2

TX

RX

Splitter

Fig. 1 - Proposed Measurement Bench: a commercial EPC-Global

C1 G2 RFID reader [9-10] and tag were used. A fixed distance D and fixed transmitted power of 27dBm where used in the measurements

�����= 10log (

����

��) (1)

���������= 10log (

������

����) (2)

������ = 10log (�

������) (3)

�����

is the gain between the reader transmitted power, CH1, and the received power at the tag, CH2, and it represents the path loss from the RFID reader to the tag. This can be calculated and carefully studied, and if the time delay is also considered, then a fully air interface impulse response can also be considered. In ���������

, equation (2), the signal at antenna 2 must be measured in two different instants. This is a fundamental need, since in this case the objective is to measure the difference between the RF input signal and the RF response signal from the tag, allowing this way to clearly identify the overall tag RF behavior and efficiency. In order to do so, baseband information as is presented in Fig. 2, will be used, that is the envelope signal (received signal power) at the tag is represented in Fig. 2 as point A, ��� , and the response is identified in Fig. 2 as point B, which corresponds to the transmitted signal back to the reader by the tag, ��� �� . It should be stressed that this information is gathered as close as possible to the tag, in order to evaluate each RF component in Antenna A2. Finally the received signal at the reader is measured and evaluated using ������ , which accounts for the reverse channel. Again, it should be stressed that different signals will be measured at different time instants, that is the forward path is measured in slot time A as in Fig. 2, while the reverse path will be measured in slot time B, since it is mainly the response from the tag that is important.

Fig. 2 – Measured Envelope Signal at Antenna A2.

II. MEASURED RESULTS

In order to confirm and to gather information on the now presented characterization scheme a real commercial tag was evaluated in the laboratory using the arrangement presented in Fig.1. Fig. 3 and 4 present the envelope signals transmitted by the RFID reader as in antenna A1 and at the receiver, measured by antenna A3. As can be seen in Fig. 3 only the downlink information is visible, while in Fig. 2 both the downlink and uplink signal is visible. In all cases the signal attenuations caused by power splitters and cables should be considered in final metric calculations.

Fig. 3 – Measured Envelope Signal at Antenna A1.

Fig. 5, presents a zoom version in each time instance, point A and B in Fig. 2, this zoomed version allows a correct RF evaluation of each path as described. Table 1, presents the obtained gains of this scheme. It should be stressed here that this preliminary measurement were done only considering the amplitude patterns, but the same thing could be done using the time delay at the RF signal, and thus the amplitude gains could be interchanged with real impulse responses including amplitude and time delay information.

II. CONCLUSIONS

In this paper a new test bench is proposed for UHF RFID evaluation. Preliminary results state that this approach is viable for commercial tag evaluations. Further work is needed to include phase delay and full impulse response characterization.

Fig. 4 – Measured Envelope Signal at Antenna A3.

Fig. 5 – Measured RF Signal at each antenna. GTTag GTagTagTx GTagTxR -21 dB -20.4 dB -19 dB Table 1 – Gain Calculations

REFERENCES

[1] P. V. Nikitin and K. V. S. Rao, “Measurement of backscattering from RFID tags”, Proceedings of Antennas Measurement Techniques Association Symposium, Oct. 2005

[2] Nikitin, P. V. and K. V. S. Rao, “Theory and measurement of backscattering from RFID tags”, IEEE Antennas and Propagation Magazine, vol. 48, no. 6, pp. 212-218, December 2006.

[3] V. Derbek, C. Steger OVE, R. Weiss OVE, J. Preishuber-Pflu¨ gl, M. Pistauer, “A UHF RFID measurement and evaluation test System”, Springer-Verlag, 2007

[4] Lauri Sydanheimo, Jussi Nummela, Leena Ukkonen, John McVay , Ahmad Hoorfar, and Markku Kivikoski, “Characterization of Passive UHF RFID Tag Performance”, IEEE Antennas and Propagation Magazine, Vol. 50, No. 3, June 2008

[5] Pavel V. Nikitin, Senior Member, IEEE, and K. V. Seshagiri Rao, Senior Member, “LabVIEW-Based UHF RFID Tag Test and Measurement System”, IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 56, NO. 7, JULY 2009

[6] National Instruments: PCI eXtensions for Instrumentation [7] PXI System Alliance: PXI hardware specification [8] CISC, PXI and LabVIEW based, RFID MeETS – Measurement

and Evaluation Test System [9] Alien technology, Hardware Setup Guide ALR-8800 [10] Alien technology, Reader Interface Guide, September 2007

A | B

A | B

Antenna A1

Antenna A2 Antenna A3

Amplitude