A novel, highly sensitive scheme to detect the resonance peaks in the spectrum of chipless RFID signals is presented. The detection is based on finding the zeros in the derivative of the group delay of the received signal. In order to be able to accurately detect these zeros in the presence of noise, the received signal is filtered using a prolate spheroidal wave function-based model. This allows great increases in the distance at which chipless RFIDs can be accurately read. The detection method can be used standalone or in addition to traditional amplitude-based detection schemes

Dullaert, Wouter

Reichardt, Lars

Rogier, Hendrik

English

Wouter Dullaert

Lars Reichardt

Hendrik Rogier

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Improved Detection Scheme for Chipless RFIDs Using Prolate Spheroidal Wave Function-Based Noise Filtering

Ghent University Academic Bibliography

ANTENNAS AND WIRELESS PROPAGATION LETTERS 1Improved Detection Scheme for Chipless RFIDsUsing Prolate Spheroidal Wave Function-basedNoise FilteringWouter Dullaert, Lars Reichardt, Student Member IEEE Hendrik Rogier, Senior Member IEEEAbstract—A novel, highly sensitive scheme to detect the reso-nance peaks in the spectrum of chipless RFID signal is presented.The detection is based on finding the zeros in the derivative ofthe group delay of the received signal. In order to be able toaccurately detect these zeros in the presence of noise the receivedsignal is filtered using a Prolate Spheroidal Wave Function basedmodel. This allows great increases in the distance at whichchipless RFIDs can be accurately read. The detection methodcan be used standalone or in addition to traditional amplitudebased detection schemes.Index Terms—UWB, PSWF, RFIDI. INTRODUCTIONRadio Frequency Identification (RFID) technology can beroughly divided into two categories: RFIDs with an IntegratedCircuit (IC) on the tag, and so-called chipless RFIDs. In recentyears, RFID tags with ICs have seen major adoption in variousindustries, because of their robust performance, decent readingrange and relatively low price. However battery life (if the IChas a power source) and the cost of the IC prevent the RFIDfrom becoming the new barcode.In order to overcome these issues recent research activityhas shifted towards fully passive, chipless RFIDs. However,due to the lack of an internal power supply the operationrange is drastically reduced. Furthermore the chipless RFIDhas no means to adapt itself to a more difficult propagationchannel. These difficulties have made Ultra Wideband (UWB)techniques a popular choice for these kinds of RFID systems:1) The low spectral energy density allows the system tocoexist with other wireless systems.2) The aggregated energy over the bandwidth results in anacceptable overall signal level.3) The large bandwidth of pulse based systems makes thesystem almost immune to multipath fading.Because the RFID tag itself has no power source, it mustuse the power of the incident field to transmit its data. Variousways of encoding data into the tag have been presented.For instance, [1], [2] encode the data into the phase of thebackscattered signal. The tag emits two signals using fre-quency or polarisation diversity: a signal providing a referencephase and a signal with a phase difference that depends on thetag. The amount of bits that can be stored by this sort of tagdepends on the resolution at which the phase difference canbe reliably resolved at the tag reader. In [3] a pulse positionmodulation technique is used. Based on which pulses fromthe emitted signal are delayed, a bitstring can be decoded.Others, such as [4], [5], operate on the amplitude of thefrequency spectrum by notching out certain frequencies in theinterrogation signal.In this letter a new method is proposed to detect the datatransmitted by chipless RFIDs that rely on this last frequency-domain method, by detecting the zeros in the derivative of thegroup delay. Using a model based on Prolate Spheroidal WaveFunctions (PSWFs) the noise in the group delay is reduced,making this a viable approach. It allows to read the tags upto a distance of 10 cm, which was previously impossible.In Section II the RFID tags under consideration are brieflydescribed. The new detection method and PSWF-based modelare presented in Section III. Results of the new method arecompared to the traditional method in Section IV. Finally, theconclusions are presented in Section V.II. CHIPLESS RFID TAGThis letter relies on measured data from RFID tags pre-sented in [4]. These tags are fully passive without ICs orinternal power supply. A schematic of the entire system isshown in Fig. 1. The tag reader transmits an interrogationsignal onto which the tag encodes its data using a series ofresonators to notch out certain frequencies in the spectrum ofthe interrogation signal. The coded signal is transmitted alonga polarisation orthogonal to the polarisation of the receiveantenna in order to minimize interference. The signal is thenprocessed by the tag reader, where notched out frequenciesrepresent a logic zero, the other frequencies a logic one.Measurements of the passive tag can be easily performed bydoing a frequency sweep with a network analyser at variousdistances, [4].The RFID tag consists of orthogonally polarised UWBtransmit and receive antennas connected to a microstrip line.Resonators are placed besides this microstrip line, suppressingcertain frequencies in the signal that propagates along the mi-crostrip line, as shown schematically in Fig. 2. The resonancefrequency of the resonators can be changed by manipulatingthe overall length l of the spiral (the dotted part of the spiralon Fig. 2). The other dimensions shown in Fig. 2 are constantfor all resonators. The amount of information encoded intosuch a tag is determined by the bandwidth of the signal andthe quality of the resonators. The tag itself operates between3.9 GHz and 4.4 GHz over a bandwidth of 500 MHz and istherefore compliant with the UWB specification.In this band,the UWB antenna has a flat response, but the spiral resonatorsANTENNAS AND WIRELESS PROPAGATION LETTERS 2Tag ReaderTagInterrogatorAntennaTag InputAntenna Multi-resonatorsTag OutputAntennaReceiveAntennaFig. 1. RFID System SchematicRXMicrostrip line Z0 = 50 ΩTotal spiral length = l ≈ 23 mm at 3.92 GHzb = 6 mma = 6 mmd1 = 0.2 mmd2 = 0.3 mmw = 1.2 mmTXFig. 2. Schematic of a spiral resonator, from [4]make the tag resonate at 3.92, 3.96, 4.06, 4.11, 4.2, 4.28 and4.36 GHz, storing 7 bits. The response of the 0000000 tag,resonating at all specified frequencies, measured at 5 cm, isshown in Fig. 3.The tag information is decoded by comparing the nor-malised spectrum of the received pulse to a reference signal,in this case the normalised spectrum of a 0000000 tag. If thedifference between the received and reference signal exceedsa threshold of 5 dB at a resonance frequency, the bit isconsidered to be a logic one. This method of detection isshown in Fig. 4 for a 0110001 tag at 5 cm distance. The blacklines correspond to logic zeros, the dashed red lines to logicones. The signal has already been processed by a time gatingalgorithm, [4] to improve detection and overcome couplingbetween the receiver antennas. Bit 2 and 7 are correctlydetected as logic ones, whereas bit 3 is not. Adjusting thethreshold will not solve the problem because bit 4 will thenbe wrongly decoded as a logic one.It is obvious that this decoding technique has poor perfor-mance. It only uses the amplitude of the signal, making it verysensitive to interference with other signals near the resonancefrequencies of the tag. Also path loss caused by the increaseddistance between the tag and reader makes it hard to determinea good value for the detection threshold. The performance canbe improved by also considering the phase of the signal. Sucha detection scheme is presented in the following section.III. PHASE-BASED DETECTIONA. Detection SchemeIn addition to relying on the amplitude characteristic of thereceived signal, a technique that uses the phase of the signalto detect the resonances in the tag is proposed. The spiral3.9 4 4.1 4.2 4.3 4.4−32−30−28−26−240 0 0 0 0 0 0Frequency (GHz)|S21|(dB)Fig. 3. Amplitude of the response of a 0000000 tag3.9 4 4.1 4.2 4.3 4.4−100100 0 0 01 1 1Frequency (GHz)|S21(f)|(dB)-|S21,ref(f)|(dB)SignalThresholdFig. 4. Classic tag decoding result for a 0110001 tagresonators that encode the bits into the tag spectrum are notchfilters that cause a spike in the phase response. The largest rateof change is found at the resonance frequency of the filter: thegroup delay of the signal exhibits a maximum at the resonancefrequency. This means that by finding the zeros on a fallingedge of the second-order derivative of the phase of the receivedsignal, the resonance frequencies can be detected. It is assumedthat the detector knows the location of the potential resonancesin the tags.Due to various factors, such as production variability, theresonance frequency can deviate a bit from the specifiedfrequency. This can be seen in Fig. 3: the minima of theamplitude do not perfectly correspond to the predefined res-onance frequency (the vertical lines). In order to compensatefor this, a small band around the resonance frequency fres:fres −  < f < fres +  is investigated. The choice of is critical for the accuracy of the system and depends on theminimum spacing between the resonance frequencies and totalbandwidth available. As a rule of thumb  = 0.003×fc is used,with fc the center frequency of the used frequency band.Unfortunately the noise present in any measured signalwill increase drastically in its derivatives. This can be seenin Fig. 5, which shows the second-order derivative of thephase response of a 0000000 tag. However, we propose tocombine this detection method with a filtering technique thatANTENNAS AND WIRELESS PROPAGATION LETTERS 33.9 4 4.1 4.2 4.3 4.4−101·10−20 0 0 0 0 0 0Frequency (GHz)∂2df6S21(rad/Hz2)Fig. 5. Second derivative of the phase-response of the 0000000 tagresults in signals that are smooth enough to allow accuratedetection of the zeros in the second-order derivative of theirphase characteristic.B. PSWF-based FilterIn order to reduce the noise contribution of the measuredphase response, the received phase signal is modeled usinga series of Prolate Spheroidal Wave Functions (PSWFs), [6],[7]. These functions arise from the following maximisationproblem, over all bandlimited functions:α2(T ) =∫ T/2−T/2 f2(t)dt∫∞−∞ f2(t)dt(1)This means that the PSWFs are the bandlimited functionswith the most energy concentration in a given time interval[−T/2, T/2]. This makes these PSWFs extremely suited tomodel bandlimited signals. The received phase Φ(f), being afunction of frequency, is expanded into a truncated series ofPSWFs ψ(f), as in [8] but for the continuous case:Φ(f) =K∑k=0Akψ(f) (2)As PSWFs are specifically tailored to describe bandlimitedsignals, only a very limited amount of coefficients is neededto accurately describe the actual signal. The measurementnoise, however, is typically not bandlimited: it requires alarge amount of coefficients. By truncating the series in anappropriate manner the noise is filtered out and a smooth signalremains. Once the coefficients for the model are obtained, thefirst and second-order derivative of the signal are calculated:∂2∂fΦ(f) =K∑k=0Ak∂2∂fψ(f) (3)In Fig. 6 the coefficients of the phase response of the0000000 tag are shown. It is seen that the amplitude of the0 10 20 30 40−10−5051015PSWF orderAkFig. 6. Model coefficients of the phase-response of the 0000000 tag3.9 4 4.1 4.2 4.3 4.4−22−20−18−16 0 0 0 0 0 0 0Frequency (GHz)6S21(rad)OriginalReconstructionFig. 7. Reconstruction of the phase response of the 0000000 tagcoefficients is almost zero above order 25: the model providesa very compact representation of the signal. In Fig. 7 both theoriginal and reconstructed phase response are shown. A verygood agreement is obtained. However the reconstructed signalis much smoother than the original. This is verified on Fig.8, showing the second-order derivative of the reconstructedsignal. The excessive noise from Fig. 5 is removed and thezeros on falling edges can now be detected. Furthermore thereceiver does not need a reference signal. Only the location ofthe potential resonances is needed to obtain accurate detection.IV. RESULTSThis new detection scheme is tested on 2 different tags:a 0000000 tag and a 0110001 tag. In all relevant figures,the following convention is used: the full black vertical linesmark frequencies at which the tag resonates. The dashedred lines indicate an absence of a resonance. The dottedlines indicate the guard inteval with a 2 width around aresonance frequency. Using the rule of thumb,  becomes: = 0.003× 4.15 GHz ≈ 12.45 MHz.A. 0000000 TagThe filtered second-order derivative of the phase for the0000000 tag, measured at 5 cm distance, has already beenshown in Fig. 8. All resonances, except the first one, haveANTENNAS AND WIRELESS PROPAGATION LETTERS 43.9 4 4.1 4.2 4.3 4.4−1−0.500.51·10−30 0 0 0 0 0 0Frequency (GHz)∂2df6S21(rad/Hz2)Fig. 8. Filtered second derivative of the phase-response of the 0000000 taga zero on a falling edge within the guard interval. This taghighlights the most important condition for this detectionscheme to work: the spacing between the resonances mustbe large enough so that they can be individually recognised.When the resonances are too close together, such as thefirst and second or third and fourth, they start to appear asone resonance and cannot be accurately distinguished: neitherby this method, nor by the the traditional amplitude basedmethods. This is an issue that needs to be considered duringthe tag design.B. 0110001 TagThe filtered second-order derivative of the phase responsefor the 0110001 tag is shown in Figs. 9 and 10, measured at5 cm and 10 cm respectively. As this tag does not containany resonances too close to each other, all falling edges of thesecond-order derivate of the phase response are clearly visibleand located at the right frequencies. At a measurement distanceabove 10 cm decoding errors start to occur. This result is agreat improvement over the traditional method that failed todecode this tag, even at 5 cm.V. CONCLUSIONA new method for detecting the resonances in spectralencoded chipless RFIDs has been presented. The new methodrelies on the second-order derivative of the phase of the signalinstead of its amplitude and does not require a reference signal.In order to be able to calculate the second-order derivative aPSWF-based filter is applied to the phase of the signal. Thenew method allows accurate detection of the resonances inthe received signal, for distances of up to 10 cm, whereas thetraditional method cannot detect the presence or absence of allresonances, even when measured at 5 cm. In future work thePSWF-based filtering technique will be compared to similarapproaches such as wavelet based denoising.3.9 4 4.1 4.2 4.3 4.4−1−0.500.51·10−30 0 0 01 1 1Frequency (GHz)∂2df6S21(rad/Hz2)Fig. 9. Second-order derivative of the 0110001 tag phase-response at 5 cm3.9 4 4.1 4.2 4.3 4.4−1−0.500.51·10−30 0 0 01 1 1Frequency (GHz)∂2df6S21(rad/Hz2)Fig. 10. Second-order derivative of the 0110001 tag phase-response at 10cmREFERENCES[1] S. Mukherjee, “Chipless RFID using stacked multilayer patches,” 2009Applied Electromagnetics Conference (AEMC), pp. 1–4, Dec. 2009.[2] I. Balbin and N. C. Karmakar, “Phase-Encoded Chipless RFID Transpon-der for Large-Scale Low-Cost Applications,” IEEE Microwave and Wire-less Components Letters, vol. 19, no. 8, pp. 509–511, Aug. 2009.[3] S. Gupta, B. Nikfal, and C. Caloz, “RFID System based on Pulse-PositionModulation using Group Delay Engineered Microwave C-Sections,” inAsia Pacific Microwave Conference 2010, Yokohoma, Japan, 2010.[4] L. Reichardt, G. Adamiuk, G. Jereczek, and T. Zwick, “Car-to-infrastructure communication using chip-less, passive RFID tags,” in2010 IEEE Antennas and Propagation Society International Symposium.Toronto, Ontario, Canada: IEEE, Jul. 2010, pp. 1–4.[5] S. Preradovic and N. Karmakar, “Design of fully printable planar chiplessRFID transponder with 35-bit data capacity,” in European MicrowaveConference, 2009., no. October, 2009, pp. 13–16.[6] D. Slepian and H.O. Pollak, “Prolate Spheroidal Wave Functions, FourierAnalysis, and Uncertainty - I,” Bell Systems Technology Journal, vol. 40,no. 1, pp. 43–64, Jan. 1961.[7] M. Abramowitz and I. Stegun, Handbook of Mathematical Functions.Dover Publications, 1972, pp. 751–759.[8] W. Dullaert and H. Rogier, “Novel Compact Model for the RadiationPattern of UWB Antennas Using Vector Spherical and Slepian Decompo-sition,” IEEE Transactions on Antennas and Propagation, vol. 58, no. 2,pp. 287–299, 2010.

Improved detection scheme for chipless RFIDs using prolate spheroidal wave function-based noise filtering

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Improved detection scheme for chipless RFIDs using prolate spheroidal wave function-based noise filtering

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