Long Range and Low Powered RFID Tags with Tunnel Diode

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1 Long Range and Low Powered RFID Tags with Tunnel Diode Francesco Amato, Christopher W. Peterson, Muhammad B. Akbar and Gregory D. Durgin School of Electrical and Computer Engineering Georgia Institute of Technology, Atlanta, Georgia Abstract In this paper, we present a 5.8 GHz RFID tag equipped with a high gain, low power reflection amplifier based on a tunnel diode. Experimental results show that the realized prototype achieves gains above 40 db and requires only 9 µw of biasing power. The tag detects very low RF signals (< -90 dbm) and provides read ranges up to km. Long communication ranges and Manchester encoding are achieved by biasing on and off the tunnel diode. Index Terms RFID; modulation; Manchester encoding; long range; reflection amplifier; quantum tunneling; tunnel diode; low powered RFID; backscatter radio; radio propagation; modulation factor. I. INTRODUCTION Technologies that enable RFID systems to communicate at long ranges (100 m and beyond) will open up countless applications in sensing, telemetry, and personal communications. No longer limited by range, RFID transceivers located on the ground will communicate with flying objects (drones) and monitor the sky in real time. A reader mounted on a car will receive real time information about the surrounding environment: local business; attractions; advertisements; and intelligent road signs will enrich and support applications for self-driving cars (Fig. 1). To improve the RFID communication range, Amato et al. [1] and Smida et al. [] have suggested the integration of tunnel diodes as a substitute of traditional passive loads in the tag circuitry. In [3], the authors demonstrated the feasibility of the idea. In this article, we present a complete proof-of-concept design, fabrication, and measurement of a 5.8 GHz RFID tag equipped with a high gain, low power reflection amplifier based on a tunnel diode (Fig. ). By exploiting the quantum tunneling effect, the realized reflection amplifier detects very low RF signals (< -90 dbm); reflects the impinging power; and amplifies it with a gain above 40 db compared to a conventional RF tag. With a modulated DC power of only 9 µw the prototype achieves backscatter modulation and Manchester encoding for distances as high as 3 m and demonstrates how this distance can be extended to read ranges of km. This paper is organized as follows: a general overview of a RFID system and the characterization of the realized reflection amplifier are outlined in Section II and III. In Section IV, the possibility of using a tunnel diode to implement amplitude modulation and Manchester encoding is demonstrated, while Section V highlights the experimental tag reader Fig. 1: An RFID reader mounted on a car communicates with long range tags to obtain real time information from the environment: local business, tourist attractions, billboards, etc. Power RFin RFout fc Frequency DC block tunnel diode Power RF choke Demodulator Charge Pump or Battery modulated DC bias a) b) Microcontroller fls fm fc Frequency Fig. : Block diagram of the RFID tag with tunnel diode and continuous wave (CW) modulation. a) forward link; b) backward link. results of long range communications achieved by using the proposed tag. Section VI summarizes the results and suggests future research directions. fus

2 II. R ATIONALE Bias Tee A monostatic RFID system is defined by the well known forward (1) and the backward () link equations [4]: λ Pt = PT GT Gt, (1) 4πr 4 λ PRmin = PT GT GR Gt M; () 4πr with PT the power transmitted by the reader; GT the gain of the transmitter; GR the gain of the receiver; Gt the gain of the tag antenna; λ the wavelength of the carrier; r the reader-to-tag distance; M the modulation factor; and PRmin the minimum power level that can be detected by the reader. The maximum amount of power (Pmatch ) passed from the antenna to the integrated circuit (IC) of the tag is defined as C1 VBias RFin Tunnel Diode Fig. 3: Fabricated reflection amplifier tuned at 5.8 GHz. The stub is 100 mil (.54 mm) long and 10 mil (0.54 mm) wide; a capacitor C1 of 1.5 pf has been used as DC block. A Pmatch = III. T UNNEL DIODE REFLECTION AMPLIFIER CHARACTERIZATION In [3], the authors outlined a method to design and test a microwave reflection amplifier with tunnel diode. In this section, the updated results of a reflection amplifier tuned at 5.8 GHz are shown. To tune the reflection amplifier at the desired frequency, a stub of 100 mil (.54 mm) in length has been added to the original design (Fig. 3). To characterize the prototype, two sets of measurements are useful: first, by measuring the s11 (Fig. 4a), both the resonant frequency and the optimal bias power are identified; second, Vector V (3) 8Rant with Rant being the tag antenna resistance. To increase the communication distance r of an RFID system at a specific frequency, without changing the transmitted power PT and the gain of the transmitting and receiving antennas (GT and GR ), an improvement of the modulation factor M is required. The modulation factor depends on the reflection coefficients of the tag modulating loads. Passive tags have M as high as 0.5; while in semi-passive tags, the maximum modulation factor is 1 at most [4]. At the cost of more power requirements, values of M > 1 can be achieved by introducing active loads (such as biased tunnel diodes) onto the antenna. Such loads allow an increase in the power of the backscatter modulated waveform. The simplest modulation of RFID systems is on-off keying (OOK); in this case, the tag circuitry is turned on to generate binary 1 s and turned off to generate binary 0 s: in Fig., a reader generating a continuous wave (CW) reaches a tag equipped with a tunnel diode; a square wave biases the tunnel diode on and off implementing the OOK modulation. The reflection amplifier scheme discussed in this paper resembles an active RFID tag as it adds gain to the modulated waveform. Nevertheless, there are still substantial power savings over an actual active tag because of the unique lowvoltage characteristics of the tunnel diode coupled with the absence of an RF oscillator on the tag. Tuning stub Vbias A Generator Pin 1 a)1 3 Vbias b) Fig. 4: Setups used for the characterization of the reflection amplifier. a) Measurements on VNA E5071B with avg. 16 and b) measurements with circulator, signal generator E847C, and signal analyzer CXA-N9000A. analyzer setup: Resolution BW 3 khz, Video BW 100 khz, avg. 10, point 1001, span 1 MHz. the return gains (Fig. 4b) are measured as function of the injected RF input. The complex impedances of the tunnel diode biased both at optimal voltage (ZON ), and not biased (ZOF F ) are measured. As shown in Fig. 5, and as expected by the non-linearity of the tunnel diode, ZON is a function of the impinging microwave power: for RF power inputs below -50 dbm, a better matching is observed and higher return gains are expected as the resistance -R approaches 50 Ω and the reactance tends to 0 Ω. On the other hand, RF power inputs above -40 dbm decrease the matching and no or little amplification is expected. Fig. 6 confirms these results: at very low RF input levels, the reflection amplifier has extremely high return gains (above 40 db for Pin -90 dbm) and low or no amplifications for Pin >-40 dbm. During the measurements, an ammeter read the current drawn by the reflection amplifier; Fig. 7 shows its characteristic curve when a -70 dbm RF input is applied: a negative resistance region with biasing voltages ranging from 60 to 60 mv, for a total biasing range of 00 mv is observed. As predicted by Eq. 3 and confirmed by Fig. 6, an input power of -40 dbm or below can be amplified when a matched antenna is used. IV. M ODULATION The device in Fig. 3 can reflect, amplify and modulate the backscattered signal of an RFID system. Modulation takes

3 Generator Pin 1 3 A Square wave generator Fig. 8: Testing amplitude modulation with tunnel diode. A square waveform is generated with V low = 0 V and V high = 69 mv. Fig. 5: Measured impedance, Z ON, of the reflection amplifier, for variable RF input powers at 5.8 GHz when a voltage V bias = 69 mv is applied. I bias = 418 µa, P bias = 9 µw. Return Gain [db] P in [dbm] Fig. 6: Measured return gains as function of the RF power at the reflection amplifier input: input frequency 5.8 GHz, V bias = 69 mv, I bias = 418 µa, P bias = 9 µw. M = 38 P in = -83 dbm. Fig. 7: Measured characteristic curve (at 300 K) of the reflection amplifier in Fig. 3. Fig. 9: Reflected modulated signals generated by a semipassive circuitry and a reflection-amplifier circuitry configured as in Fig. 8. The RF input signal level is -60 dbm, the modulating signal is a 50 khz square wave. place by turning on and off the biasing voltage of the tunnel diode. To verify this possibility, a wave generator has been used to bias the tunnel diode with a square wave of amplitude ranging between 0 and 69 mv at 50 khz and a signal analyzer has been used to measure the reflected signal (Fig. 8). The obtained results are shown in Fig. 9 and compared with those of semi-passive tag circuitry: for an input microwave signal of -60 dbm, a modulated output of -41 dbm is obtained (corresponding to a gain of 0 db); moreover, a difference of 35.7 db between the reflection amplifier and the semi-passive circuitry is observed. Two demodulated signals in the time domain are shown in Figg. 10 and 11 for modulating square waves of 50 khz and 1.5 MHz respectively, suggesting the possibility of high speed modulations. Nevertheless, a continuous square wave is not enough to transmit wireless data in an RFID link. Therefore, to demonstrate the possibility of transmitting data using the reflection amplifier, a simple word (0xA4) has been coded using Manchester encoding (Tab. I) and reproduced by the wave generator used to bias the tunnel diode. The reflected signal has been demodulated in the time domain, the results obtained are illustrated in Fig. 1 and compared with an ideal signal. V. WIRELESS BACKSCATTER TEST To test the wireless capabilities of our prototype, the realized reflection amplifier has been connected to a patch antenna

4 5.8 I channel Q channel TABLE I: Transmitted word. Hex word binary sequence Manchester encoding 0xA Amplitude [mv] t [s] Fig. 10: 50 khz reflected square wave in the time domain. I and Q channels. Amplitude [mv] I channel Q channel t [s] Fig. 11: 1.5 MHz reflected square wave in the time domain. I and Q channels. to form a 5.8 GHz tag (Fig. 14). Both the forward and the backward links have been tested, and the results have confirmed achievable high communication ranges by using a properly biased tunnel diode. All the antennas used for the following tests were matched at 5.8 GHz (Fig. 13). A. Forward link The forward link has been studied first. The realized reflection amplifier has shown a very high sensitivity (Fig. 6): it can backscatter RF signals as low as -90 dbm; for P t = -90 dbm allowing a forward link r F of km at 5.8 GHz s 11 [db] Tx antenna Rx antenna Tag antenna f [GHz] Fig. 13: coefficients of the 5.8 GHz antennas used for wireless measurements. for an EIRP (P T G T ) of 4 W. An experiment has been set to validate this prediction (Fig. 15); for practical reasons, a distance r F = 7 m has been preferred and the transmitted power has been adjusted accordingly (P T = -0 dbm, EIRP = -14 dbm) to have -73 dbm impinging on the tag. Fig. 17 shows the results: without biasing, the tag reflects an RF signal of -80 dbm; conversely, when a modulating square wave is applied, a reflected modulated signal of -55 dbm is observed. B. Backward Link The complete backscattering link has been finally tested. The reflection amplifier tag has been placed at 3.3 m distance from the transceiver and biased with a square wave of V pp = 69 mv (Fig. 18). At this distance, the transmitted power level, P T, has been set to -0 dbm to have a power level impinging on the reflection amplifier of -83 dbm (M = 38 db from Fig. 6). A receiving antenna gain G R = 1 db and an amplifier of G amp = 30 db have been used to detect the backscattered signal level P Rmin. Fig. 19 shows the obtained results: a modulated signal of 78 dbm appears when the biasing square wave of the tunnel diode is turned on. Fig. 1: The 0xA4 word modulated with Manchester encoding, reflected by the reflection amplifier and decoded in the time domain. Fig. 14: amplifier tag consisting of a 5.8 GHz patch antenna and the tunnel diode circuitry. The leads are used to bias the tunnel diode with a modulating square wave.

5 Square wave generator Generator PT 1 Square wave generator Tag 3 Fig. 15: Forward link measurements: PT = -0 dbm, GT = 6 db, Gt = 6 db, rf = 7 m, fm = 50 khz. 3.3 m Tx/Rx Ant Sig. Gen. + Spec. Fig. 18: Backward link measurements: PT = -0 dbm, GT = Gt = 6 db, GR = 1 db, Gamp = 30 db, r = 3.3 m. Fig. 16: Photo of the forward link measurement setup. The reflection amplifier is modulated with a 50 khz square wave oscillating between 0 and 69 mv. By using Eq. (PT = -0 dbm, f = 5.8 GHz, GR = db), the PRmin vs distance r have been plotted to compare the performances between the reflection amplifier tag (M = 38 db) and an ideal semi-passive tag (M = 0 db). As it is shown in Fig. 0, for a transceiver noise floor of -95 dbm [5], the tag with tunnel diode allows a greater communication distance respect to an ideal semi-passive tag. VI. C ONCLUSIONS Fig. 19: Backward link results: signal backscattered by the reflection amplifier tag and observed at the transceiver. When the tunnel diode is biased, peaks at 50 khz above and below the carrier frequency are observed. In this paper, a microwave RFID tag capable of long distance communication (up to km) for very low biasing powers (9 µw ) has been presented. The high sensitivity (< -90 dbm) makes the tunnel diode based reflection amplifier a 50 No bias 69 mv modulating square wave Pout [dbm] f [GHz] Fig. 17: Forward link results: modulated signal reflected by the tag and measured on the signal analyzer with bias and no bias. Fig. 0: Estimated link budget of a 5.8 GHz RFID system. At 3.3 m, the tag with reflection amplifier (M = 38 db) gives a higher backscattered signal when compared to an ideal semipassive tag (M = 0 db). The estimated power level (-76 dbm) is in line with the measured values (-78 dbm in Fig. 19).

6 very good candidate for long range RFID links. In Tab. II, the obtained results are compared with the performances of other wireless technologies, i.e. WiFi and Bluetooth Low Energy (BLE): the presented tag offers both lower power consumption and higher ranges, while keeping the hardware free of local oscillators and mixers. TABLE II: Wireless technologies compared Technology Power Consumption Distances Tdiode tag (semi-passive) 9 µw km BLE (active) 33.3 µw [6] 150 m [7] 80.11n (active) 100 mw [8] 00 m [9] The tunnel diode-based RFID tag can be easily improved. Amplitude modulation and Manchester encoding have already been illustrated by appropriately biasing on and off the tunnel diode; but sophisticated coding techniques ( [10], [11]) can increase the communication distances. These results suggest better requirements in the sensitivity of next generation readers and encourage to implement the described prototype as a single chip component. VII. ACKNOWLEDGEMENTS This work was supported, in part, by NSF Grant ECCS # REFERENCES [1] F. Amato and G. D. Durgin, A tunnel diode reflection amplifier for RFID antennas, Apr 013, poster presentation, Orlando FL. [] B. Smida and S. Islam, Full-duplex wireless communication based on backscatter amplifier, in Communications Workshops (ICC), 014 IEEE International Conference on, June 014, pp [3] F. Amato, C. W. Peterson, B. P. Degnan, and G. D. Durgin, A 45 uw bias power, 34 db gain reflection amplifier exploiting the tunneling effect for RFID applications, in RFID (RFID), 015 IEEE International Conference on, Apr [4] J. Griffin and G. Durgin, Complete link budgets for backscatter-radio and RFID systems, Antennas and Propagation Magazine, IEEE, vol. 51, no., pp. 11 5, Apr [5] Motorola. (01). [Online]. Available: com/web/business/products/rfid/rfid\%0readers/fx9500/ Documents/FX9500 Specifications.pdf [6] A. Dementyev, S. Hodges, S. Taylor, and J. Smith, Power consumption analysis of bluetooth low energy, zigbee and ant sensor nodes in a cyclic sleep scenario, in Wireless Symposium (IWS), 013 IEEE International, April 013, pp [7] Joe Decuir. (015, Jul.) Bluetooth 4.0: Low energy. [Online]. Available: [8] S. J. Thomas, Modulated backscatter for low-power high-bandwidth communication, Master s thesis, Duke University, Durham, 013. [9] P. Gaonkar, D. Tandur, and G. Rafiq, Range performance evaluation of ieee 80.11n devices, in Industrial Technology (ICIT), 015 IEEE International Conference on, March 015, pp [10] G. Durgin, C. Valenta, M. Akbar, M. Morys, B. Marshall, and Y. Lu, Modulation and sensitivity limits for backscatter receivers, in RFID (RFID), 013 IEEE International Conference on, Apr. 013, pp [11] G. Durgin, Balanced codes for more throughput in RFID and backscatter links, in IEEE RFID-TA, Sep. 015.