Backscattering modulation 101

Backscattering modulation 101: VNA measurements Simon Hemour1, Nicolas Barbot2 2023 IEEE International Conference on RFID Technology and Applications (RFID-TA) | 979-8-3503-3353-4/23/$31.00 ©2023 IEEE | DOI: 10.1109/RFID-TA58140.2023.10290457 1 Université de Bordeaux, IMS, 33405 Talence, France Univ. Grenoble Alpes, Grenoble INP, LCIS, 26000 Valence, France 2 * [email protected] Abstract— Hardware tag characterization is one of the most critical steps in RFID and zeropower IoT development flow. However, the measurement process and its respective setup can be long and cumbersome. This paper describes how to use basic but uncharted VNA modes to yield simple, fast and flexible measurements without any spectrum analyser, SDR, oscilloscope, or anechoic chamber. A simple case study is presented, with transponders that are based on simple rotating scatterers. They are handcrafted from metallic tape, so they can be easily replicated by enthusiastic researchers. The VNA allows to estimate both magnitude and phase of frequency components around the illuminating carrier frequency (i.e. the intermodulations) backscattered by the transponder. It can also measure the modulated backscattered power as a function of the carrier frequency. These modes of operation can eventually be used to realize both identification and sensing of linear time-variant (LTV) transponders at a significant range. Keywords— VNA, frequency offset, LTV transponders, modulation, wireless, zeropower, IQ demodulation. I. INTRODUCTION Measurements are instrumental in the Research and Development process of RadioFrequency Identification transponders (RFID). Hence, tag activation distance is commonly characterized in every scenario, while amplitude (RSSI) and phase of backscattered signals can be estimated using classical RFID readers. To push further measurement capabilities, the RFID reader may sometimes be replaced or augmented by a vector signal generator and a spectrum analyser or a high speed (MS/s) downconverted IQ sampler to observe and characterize signals on a given bandwidth and/or in a given interval. This setup allows for additional characterization, such as Power on Tag Forward (POTF) Power on tag Reverse (POTR), delta RCS, and read range over larger bandwidth [1-6]. Such a setup can also perform chipless tag measurement. For example linear time-variant (LTV) transponders are typically wirelessly characterized using a vector signal generator and a real time spectrum analyzer, both using the same 10MHz reference to achieve coherent detection [4]. However, no instrument can match the Vector Network Analyzers (VNA) in terms of flexibility, wide frequency range, calibration-grade measurements, and optimum use of couplers. VNA are classically used to measure S-parameters in both magnitude and phase. Commercial instruments can sweep over dozens of GHz bandwidth in a few milliseconds. Thus, some VNA-based RFID measurements, such as radar cross section are already known to yield high dynamic range and accuracy. The Common limitation, however, is that the measurement result is jeopardized by the multiple reflections coming from the environment, so that additional 979-8-3503-3353-4/23/$31.00 ©2023 IEEE measurements are necessary to remove the artefacts [7] through multi-step calibration procedure. An alternative way to circumvent the many reflections of the illuminating frequency is to operate at its harmonic. For this scenario, VNA in frequency offset mode has been used previously for measuring harmonic transponders [8-9], but the intermodulations have not yet been investigated. The purpose of this paper is twofold. The first objective is to show how well the vector architecture of the VNA matches the advanced measurements required for the characterization of the modulated signals backscattered by LTV transponders. Two techniques will be highlighted, namely (i) the FFT extraction (frequency domain) of Zero-Span ‘A’ receiver measurements (Time domain) and (ii) the Frequency- Offset ‘A’ receiver measurement (shifted frequency domain). The second objective is to show that robust linear time-variant transponders can be easily built for a few dollars. These rotating scatterers can be easily reproduced by enthusiastic researchers using simple metallic tape. The testbench is able to realize the operation of identification and sensing using the proposed LTV transponders. Identification is performed from a constant side-band measurement of a wideband frequency-swept carrier (measured in frequency offset mode). Sensing function is achieved from the measurement of the magnitude and phase of the modulated power (zero span mode) With the proposed setup, identification and sensing can be done at a read range of several meters while maintaining the read time below a few seconds. II. VECTOR NETWORK ANALYSIS PRINCIPLE As described in fig. 1-a, a VNA is by default assuming that the device under test is a linear time-invariant system. The S11-parameter (reflexion coefficient) is computed as the ratio between one single tone seen as the ‘output’ (reflected wave b1) and one single tone seen as ‘input’ (forward wave a1). From an hardware standpoint, the two waves are coupled from the transmission line connecting the generator to the Device Under Test (DUT), down-converted, and eventually sampled (Receiver A measuring the b1 wave image, and receiver R measuring the the a1 wave image (diagram of fig. 1-b). When a Linear Time Variant (LTV) or a non-linear time invariant device is measured, the reflected b1 wave is no longer a single tone, but contains many intermodulation frequency components around the a1 initial “carrier” frequency, and at its harmonics. If the intermodulation frequencies are smaller than the IF bandwidth filter, they can be captured by the ‘A’ receiver in a zero-span mode (sometimes also named ‘CW mode’). In this mode, the frequency sweep is disabled. The VNA does not plot the frequency response to the swept carrier frequency, but traces the magnitude (or phase) of the received signal as a function of time. Of course, this data can be post-processed to visualize the frequency content (see |A(t)| and FFT(|A(t)|) in fig. 1-b)). Note however that since the carrier passes through the IF bandwidth filter, available resolution is reduced 169 RFID 2023 Authorized licensed use limited to: University Duisburg Essen. Downloaded on November 03,2023 at 09:23:12 UTC from IEEE Xplore. Restrictions apply. Fig. 1 Comparison between classical mode, zero span mode and frequency offset mode of a VNA [10] Since RFID engineers’ objective is to differentiate the modulated reflections of the transponder (around the carrier frequency) from the reflections of the environment (at the very carrier frequency), the frequency offset mode of the VNA can be used. In this configuration (Fig. 1-c) the receiver can operate at a frequency which is different from the carrier frequency. If the IF bandwidth is low enough, reflection by the environment can be separated from the modulated signal and the received signal is only a function of the LTV transponder. The two latter modes will be investigated in this paper. III. TAGS DESIGN Linear time-variant transponders which do not use a chip have been proposed in [11-13]. Note that all these transponders can backscatter new frequency components around the carrier frequency used by the reader when they are in movement. This technique allows one to drastically increase their read ranges compared to Linear Time-Invariant (LTI) transponders. The proposed tags have been designed by cutting an aluminum sheet. Different shapes have been realized such as a rectangle, two triangles and a T-shape and are presented in Fig. 2. Note that these designs can be easily reproduced with simple metallic tape. These scatterers are then placed on an adhesive support which can be rotated by a motor. The rotational speed of the motor is controlled by an Arduino. The microcontroller is programmed to generate a Pulse Width Modulation (PWM) signal on a General Purpose Input Output (GPIO) connected to an Electronic Speed Control (ESC). This setup allows one to easily control the rotational speed of the motor. Note that in all experiments, the rotational speed has been set to approximately 60 tr/s. Fig. 2 Photograph of the proposed transponders. Tags are placed on a rotating support during the measurement. 170 Authorized licensed use limited to: University Duisburg Essen. Downloaded on November 03,2023 at 09:23:12 UTC from IEEE Xplore. Restrictions apply. Fig. 3 Photograph of the measurement bench in a real environment. Fig. 5 Fourier transform of the measured A parameter in zero span mode. Carrier frequency has been set to 2.1 GHz. One can also remark that in this setup, the A parameter is measured as a function of the time and variation of amplitude and/or phase of the received signal will directly impact the received signal in the time domain. Fig. 4 Measured A parameter in CW mode and an IF bandwidth of 3 kHz. Sweep time is equal to 134 ms.. IV. VNA-BASED MEASUREMENT BENCH The measurement bench is built around a VNA PNA N5222A by Agilent. The bench uses a single UWB antenna (A.H. Systems, inc. SAS-571) directly connected to Port 1 (monostatic configuration). The rotating tag is placed in front of the antenna at a distance of 20 cm. The output power of the VNA has been set at -5 dBm for all the measurements. The whole bench is placed in a real environment. A picture of the measurement bench is presented in Fig. 3. Anechoic environment can be used to significantly reduce the multipath propagation. IV. ROTATIONAL SPEED SENSOR For this analysis, the VNA is placed in Zero span mode. In this mode the frequency sweeping is disabled and has been set to 2.1 GHz. Note that any frequency can be used as long as the considered transponders backscatter a power. If the scatterer is resonant, the resonant frequency can be used to maximize the backscattered power. The sampling rate at the receiver has been set to 2 kS/s by setting the IF bandwidth to 3 kHz. Note that with this IF bandwidth all frequency components backscattered by the tag can be captured by the VNA. Number of points has been set to 401 points which corresponds to an acquisition time of 134 ms. Fig. 4 presents the variation in magnitude of the A parameter in the time domain. The acquisition time of 134 ms (sampling time=0.5ms) allows one to capture about 7 periods of the signal (fr=60Hz). Also note that the maximum frequency of the measured signal should not be higher than 1 kHz to satisfy the Nyquist-Shannon theorem. The variation seen in time corresponds to the amplitude modulation of the backscattered signal induced by the rotation of the scatterers. By applying a Fourier transform to the complex signals presented in Fig. 4, one can obtain the spectral content of the backscattered signal around the carrier sent by the VNA. Fig. 5 presents the fourier transform of the (complex) A parameter. Since the signal is periodic due to the rotation movement, the spectrum is composed of peaks located at multiples of the rotational frequency. Note that the frequency components located at ± 2𝑓𝑟 are due to the polarization modulation [13]. The components located at 𝑛 𝑓𝑟 are due to the micro Doppler modulation [14]. The peak located at 0 Hz is due to the reflection of the antenna and the objects present in the environment. Finally the position of the peak corresponding to the modulated signal allows one to realize an accurate wireless sensor. From Fig. 5, the rotational speed of the transponder has been measured at 116/2=58 Hz which is in agreement with the analytic model [13] and the mechanical setup. IV. TAG IDENTIFICATION This section presents a procedure allowing to extract the modulated power as a function of the frequency. Results are similar to the ones presented in [4, Fig. 8], [14, Fig. 12] and [15, Fig. 3] (all obtained with a SVG and a RT SA) but can be now fully realized by a single instrument. For this analysis, the VNA is placed in linear frequency sweep between 500 MHz and 4 GHz using 401 points. A frequency offset of 110 Hz is applied on the receiver A. This offset corresponds approximately to twice the rotational frequency of the tag (i.e., 171 Authorized licensed use limited to: University Duisburg Essen. Downloaded on November 03,2023 at 09:23:12 UTC from IEEE Xplore. Restrictions apply. REFERENCES 2 𝑓𝑟). More importantly, the IF bandwidth has been set to 50 Hz to efficiently filter the static reflections of the antenna and the objects present in the environment (all located at 0 Hz). With these settings the sweep time is equal to 7.1 s. Note that since the rotation frequency and the frequency offset are not perfectly equal, the measurement power seen on receiver ‘A’ beats at a frequency which is equal to the difference of these two frequencies. Note that to obtain the total energy left by the IF bandwidth filter, an averaging should be performed in the time domain (AVERAGING PER POINTS). Similar results can also be obtained using a Max Hold on the measured trace. Fig. 6 shows the resulting magnitude as a function of the offsetted frequency for all transponders and when no tag is rotating (Empty). The energy below 800 MHz is due to the reflection of the generator’s phase noise at the antenna port (the antenna is not matched below this frequency). The peaks located above 1 GHz are due to the modulated power backscattered by the transponders. These variations in modulated power depend on the considered scatterer and can be used to identify the tag. Interestingly, this quantity is linked to the delta RCS [4] of the scatterers. This quantity can also be evaluated theoretically from the scattering matrix of the scatterers [15] and can provide an estimator which is independent from the distance. More importantly, and as opposed to classical chipless measurement, all measurements have been done without subtracting the empty measurement. V. CONCLUSION The paper introduces a flexible measurement bench allowing to characterize the performance of any LTV transponder by using a VNA. Backscattered signals can be extracted in time and frequency domain based on the CW mode of the instrument. Delta RCS can also be extracted over large bandwidth by using the frequency offset of the instrument. Finally we show that the sensitivity of the instrument is sufficient and allow us to design LTV transponders using simple metallic tape. We hope that results presented in the paper can be reproduced by other researchers. [1] Derbek, V., Steger, C., Weiss, R. et al. A UHF RFID measurement and evaluation test system. Elektrotech. Inftech. 124, 384–390 (2007). https://doi.org/10.1007/s00502-007-0482-z [2] P. V. Nikitin and K. V. S. Rao, “LabVIEW-Based UHF RFID Tag Test and Measurement System,” in IEEE Transactions on Industrial Electronics, vol. 56, no. 7, pp. 2374-2381, July 2009, doi: 10.1109/TIE.2009.2018434. [3] R. Colella, L. Catarinucci, and L. Tarricone, “Measurement system for over-the-air evaluation of UHF RFID tags quality,” Wireless Power Transfer, vol. 4, no. 1, pp. 33–41, 2017. https://doi.org/10.1017/wpt.2016.13 [4] N. Barbot, O. Rance, and E. Perret, “Differential RCS of modulated tag,” IEEE Trans. Antennas Propag., vol. 69, no. 9, pp. 6128–6133, Sep. 2021. [5] J. Kimionis, A. Bletsas and J. N. Sahalos, “Design and implementation of RFID systems with software defined radio,” 2012 6th European Conference on Antennas and Propagation (EUCAP), Prague, Czech Republic, 2012, pp. 3464-3468, doi: 10.1109/EuCAP.2012.6206487. [6] N. Kargas, F. Mavromatis and A. Bletsas, “Fully-Coherent Reader With Commodity SDR for Gen2 FM0 and Computational RFID,” in IEEE Wireless Communications Letters, vol. 4, no. 6, pp. 617-620, Dec. 2015, doi: 10.1109/LWC.2015.2475749. [7] P. V. Nikitin and K. V. S. Rao, “Theory and measurement of backscattering from RFID tags,” in IEEE Antennas and Propagation Magazine, vol. 48, no. 6, pp. 212-218, Dec. 2006, doi: 10.1109/MAP.2006.323323. [8] F. Amato and S. Hemour, “The Harmonic Tunneling Tag: a Dual-Band Approach to Backscattering Communications,” 2019 IEEE International Conference on RFID Technology and Applications (RFID-TA), Pisa, Italy, 2019, pp. 244-247, doi: 10.1109/RFID-TA.2019.8891996. [9] K. Gumber, C. Dejous and S. Hemour, “Harmonic Reflection Amplifier for Widespread Backscatter Internet-of-Things,” in IEEE Transactions on Microwave Theory and Techniques, vol. 69, no. 1, pp. 774-785, Jan. 2021, doi: 10.1109/TMTT.2020.3038222. [10] J. P. Dunsmore, “Handbook of microwave component measurements: with advanced VNA techniques”. John Wiley & Sons, 2020. [11] H. Stockman, “Communication by means of reflected power,” Proceedings of the IRE, vol. 36, no. 10, pp. 1196–1204, Oct. 1948. [12] M. S. Reynolds, “A 500°C tolerant ultra-high temperature 2.4 GHz 32 bit chipless RFID tag with a mechanical BPSK modulator,” in 2017 IEEE International Conference on RFID (RFID), 2017, pp. 144–148. [13] N. Barbot and E. Perret, “Linear time-variant chipless RFID sensor,” IEEE RFID J., vol. 6, pp. 104–111, 2022. [14] A. Azarfar, N. Barbot, and E. Perret, “Chipless RFID based on micro-doppler effect,” IEEE Transactions on Microwave Theory and Techniques, vol. 70, no. 1, pp. 766–778, Jan. 2022. [15] N. Barbot, “Delta RCS expression of linear time-variant transponders based on polarization modulation,” in 2022 IEEE 12th International Conference on RFID Technology and Applications (RFID-TA), Cagliari, Italy, 2022, pp. 55–58. Fig. 6 Measured S11 parameter with a frequency offset of 110 Hz and an IF bandwidth of 50 Hz. Sweep time is equal to 7 s. 172 Authorized licensed use limited to: University Duisburg Essen. Downloaded on November 03,2023 at 09:23:12 UTC from IEEE Xplore. Restrictions apply.

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