High-Frequency RFID: Applications and Challenges

Transcription

1 Work Sponsored By High-Frequency RFID: Applications and Challenges Georgia Tech Foundation Center for Organic Photonics and Electronics Georgia Research Alliance NSF CAREER Grant ECS Prof. Gregory D. Durgin For Further Reading H. Stockman, Communication by Means of Reflected Power, in Proceedings of the IRE, October 1948, p K. Finkenzeller, RFID Handbook: Fundamentals and Applications in Contactless Smart Cards and Identification, 2 nd ed., Wiley Inc., J.D. Griffin, G.D. Durgin, A. Haldi, and B. Kippelen, Radio Link Budgets for 915 MHz RFID Antennas Placed on Various Objects, in WCNG Wireless Symposium 05, Austin TX, October D. Kim, M. A. Ingram, and W.W. Smith Jr., Measurements of Small-Scale Fading and Path Loss for Long Range RF Tags, IEEE Transactions on Antennas and Propagation, vol. 51, no. 8, pp , Aug J.D. Griffin, A Radio Assay for the Study of Radio Frequency Tag Antenna Performance, Tech. Report PG-TR JDG, Georgia Tech Propagation Group MS Thesis, D.M. Dobkin and S.M. Weigand, UHF RFID and Tag Antenna Scattering, Part I: Experimental Results and Part II: Tag Array Scattering Theory, Microwave Journal of Theory and Techniques, 2006 Presentation Overview Background Overview of RFID Challenges for RFID Radio Links Challenges for RFID Tags Challenges for RFID Readers Cross-Disciplinary Challenges The Future of RFID What This Talk Does NOT Address Detailed Signal-Processing Issues Software and Backhaul Issues Database and Inventory Software Privacy and Security Issues Integrated Circuit Design Background 1

2 Prof. Durgin Background The Propagation Group at GT Prof. Durgin has been teaching for 3 years at Georgia Tech Virginia Tech PhD from MPRG in Dec Post Doc at Osaka University Director of The Propagation Group at Georgia Tech Authored first Space- Time/MIMO textbook Frequent consultant to industry The Propagation Group at GT Faculty Collaborators Radio Wave Propagation RFID Radiolocation (E911) Propagation Modeling Direction Finding and Array Technology Space-Time/MIMO Radio Channels Applied Electromagnetics Emag/ RF Chem/ COPE Steffes Peterson Kenney Kippelen Perry Marder Specific Contributors to this Talk Joshua Griffin Radio Assay, MIMO, Multi-Antenna Interrogation (PhD) Albert Lu Antennas, RFID Receiver design, Applied Emag (Eng./LumoFlex) Joel Prothro Organic Dielectrics for RF, On-Metal Antenna Performance (MS) Anil Rohatgi Spread Spectrum and Anti-Collision (MS) What is Far-Field RFID? 2

3 Historical Perspective RF Terminology in This Presentation 1935 Watson-Watt invents RADAR 1948 Stockman publishes seminal work on backscatter radio in Proceedings of the IRE 1973 Watson s keylesss doorway entry patent 1980s First deployments of electronic article surveillance (EAS) 1993 First highway toll tags deployed in NY City (EZ Pass) 1999 AutoID center established at MIT by Proctor and Gamble 2003 Walmart mandate to suppliers for providing RFID tags on inventory palettes Reader (Interrogator) a device that retrieves information from an RF tag Beacon an RF device that periodically transmits information without reception Transponder an RF device that retransmits without any waveform reconstruction (no modulation or demodulation capability) Transceiver an RF device capable of modulation and demodulation What is an RF Tag? RFID is just one application of RF Tags Our Definition of an RF Tag: A low-profile, low-power transponder Actually, there are 3 types of RF Tags Near-field transponder Far-field transponder Full Transceiver RF Tag Power Supply Differentiators Purely-Passive Tags Powered solely from received RF signals Requires rectification of incoming waves Battery Assisted Tags Battery is used instead of in-coming RF power Finite lifetime Energy Scavenging Tags Super capacitors Solar cells Thermocouples Piezoelectrics Near vs. Far-field RFID Trade-offs Example Applications Far-field RFID Long range (1-10m) High-frequency electronics Malevolent, high-loss RF channel High bandwidth available Compact, multiple antennas on a tag Near-field RFID Range less than 1m Conventional circuitry effective Resistance to on-object degradations Small bandwidth Only one loop antenna possible Passive Data Exchange Sensors Measure glucose levels in the human body Pace maker monitor Environmental monitoring Radio Frequency Identification (RFID) Toll collection Access control Livestock tracking Inventory management Airport luggage security Medical ID bracelets Self checkout 3

4 The RF Tag System (Near-Field) Near-Field RFID Attributes Inductive Coupling between reader & RF tag Low Frequency Short Range (always less than 1m) Direct Influence of RFID Tag on Reader Physically limited in range and orientation. Hertzian Dipole (Simplest Radiator) Tiny current element Radiates at the Origin Points up (z-direction) Simplest possible radiation scenario All radiation theory is based on this simple result Radiation Equations Field solution for a radiating current I z Near-fields fall off 1/r 2 and 1/r 3 Far-fields fall off 1/r (1/r 2 in power) The RF Tag System (Far-Field) Far-Field RFID Attributes Transmitter antenna radiates Tag antenna scatters wave with modulation Receiver antenna captures information In some architectures, one antenna for RX & TX Higher Frequency Potentially Longer Range (more than 1m) RFID Tag Effects Sensed via Traveling Wave 4

5 Classes of RFID Emitters EPC Class Characteristic Programmability Class 0 "Read Only" passive tags Programmed as part of the semiconductor manufacturing process Class 0+ "Write-Once, Read-Many" passive tags Extension of standards allows 0+ to be rewriteable Class 1 "Write-Once, Read-Many" passive Programmed once by the customer then Class 2 Rewritable passive tags Can be reprogrammed many times Class 3 Semi-passive tags Can be reprogrammed many times Class 4 Active tags Can be reprogrammed many times Class 5 Readers N/A Table 1. RFID tag classifications. (ImpInj) The RF Tag System RF Tag Components Antennas: Far field Backscatter Printed Dipole Printed Folded Dipole Printed Inverted F Antenna (PIFA) Patch Antenna The RF Tag System Modulation Circuitry Load Modulation -- the carrier signal is modulated by switching an impedance from a matched condition to an un-matched condition to alter the reflection coefficient PIN Diode Modulation Forward DC Bias => RF Short Reverse DC Bias => RF Open Forward Bias (RF short) Reverse Bias (RF open) PIN Diode Modulation Measurement of Backscatter Modulation Example PIN diode modulator built by Propagation Group Researcher Joshua Griffin Connects to antenna units and modulation waveforms Sample Output 5

6 Challenges for RFID Links Challenges for RFID Links Power-Up Link Budget Backscatter Link Budget Radiation Safety Levels Polarization Mismatch Small-Scale Fading Multi-Band Operation Conventional RF Link Budget P = P + G R T TX _ reader Reader Transmit Antenna Gain + G Tag Free-Space Tag Antenna Gain 4π 20log d 20log Object Penalty λ Tag-Reader Separation Distance Measured In the Radio Assay Highlights of Forward Link Budget Power loss is to the square of the distance (d 2 ) Losses increase with higher frequency Directional antennas are easier to make at higher frequency Limits power-up on a passive tag RF Tag Backscatter Link Budget RF Tag Backscatter Link Budget Backscattered Power 4π PR = GTX _ reader + GRX _ reader + PT 40log d 40log λ + 2G + 20log Γ Γ 2 Object Penalty Tag Antenna Gains At Reader 10 A B Transmit Power Tag-Reader Separation Distance Tag Antenna Gain Reflection Change Between Switched Loads Γ A, B Z = Z A, B A, B * 0 Z + Z 0 Adjustment For on-object Degradations 6

7 Highlights of Backscatter Link Budget Radiation Safety Levels Power loss is to the fourth power of the distance (d 4 ) in line-of-sight channel Additional losses due to material attachments Directional antennas at the reader make a big difference in the link budget Must consider RF tag loading effects Limits the actual information exchange between tag and reader High Transmit Power Required Forward Link Power-Up Weak Modulated Backscatter (1/r 4 losses) Limiting Factor is often Radiation Safety Readers and People share space Repeated exposure in the work place Microwave safety levels must be strictly observed Polarization Issues Analogy to a Famous Physics Problem Example has vertical polarization on all three antennas Optimum for backscatter link budget Two polarizers allow light through if they are aligned If one polarizer is set at 90 degrees, there is complete blockage Analogy between the reader antennas in the previous slide near-zero power transfer. Analogy to a Famous Physics Problem Polarization Issues Insertion of a 45-degree polarizer allows some light to pass through the entire setup. Final polarization has been rotated by 90 degrees. Analogous to the slanted RFID tag setup in previous slide Complete polarization mismatch on the backscatter link However, excellent isolation between reader antennas 7

8 Power Level Polarization Issues First Encounter With Wave Fading 25% Total Power Loss Extremely Good Carrierto-Interference Ratio Constructive and destructive interference causes voltage to vary along different points in space on the transmission line. Source Voltage Power Level Mobile Power Meter Signal Forward Propagating Wave Position Backward Propagating Wave Load Small-Scale Fading in Radio Links Small-Scale Fading in Radio Links Received Signal Propagating Waves Mobile Receiver Antenna Position The principle is the same as constructive/destructive interference on the transmission line. Example measurement at 5.85 GHz made by moving receiver antenna over 1m area. Double Fading in Backscatter Links Tag Antenna Diversity Use multiple antennas on the same tag Switch antennas when one experiences a fade A deep fade prohibits power-up of a tag A fade can be experienced twice on a backscatter link 8

9 Multi-Band Issues 865 MHz, 915 MHz, 955 MHz used in different countries Individual signal may be narrowband, but range of operability is broadband Challenges for RFID Tags Challenges for RFID Tags Powering Up a Passive Tag Battery Issues Antenna Design Issues Multi-Tag Coupling On-Metal Degradations On-Dielectric Degradations Tag Diversity Manufacturing Issues Powering Up a Passive Tag Passive RF chip must convert incoming RF to DC voltage Basic rectifier shown above Half-wave rectifier circuit shown above Ideal Voltage Conversion: V DC = 0.5 x (V AC ) pp V TO is turn-on Voltage for Diode V TO Powering Up a Passive Tag Powering Up a Passive Tag Voltage Doubler circuit capable of producing twice the DC ouput with a few more capacitors and diodes Ideal Voltage Conversion: V DC = (V AC ) pp 2V TO Ideal Voltage Conversion: V DC = 2 (V AC ) pp 4V TO 9

10 Powering Up a Passive Tag Full RF Tag Hardware Schematic Charge pump rectifier Diodes and capacitors AC voltage converted to DC Voltage stepped up (current stepped down) Ideal Voltage Conversion: V DC = N(V AC ) pp 2NV TO Drawbacks Higher complexity Diminishing returns for added stages Longer charging transient A: Antenna B: Modulator C: Charge Pump D: Signaling Electronics Note potential self-modulation problem Battery Issues Flexible Batteries? Cost Lifetime Disposability Form-factor Flexibility Battery specifications are heavily influenced by propagation issues Fledgling technology Difficult to manufacture batteries > 1.5V Zinc-Carbon structure: non-rechargeable Supply issues Cell reliability is poor Bending/exposure limits Antenna Design Issues Multi-Tag Coupling Wire designs Area antennas Patch antennas Unorthodox designs Each tag has a radar cross-section (RCS) RCS ~ electromagnetic area Overlapping antennas steal each other s power 10

11 Multi-Tag Coupling On-Object Degradations Realistic scenarios many involve many RFID tags Higher-gain antennas have larger electromagnetic area Trade-off: an efficient individual RFID tag is a neighbor to other RFID tags Coplanar strip has most fringing fields Environment will affect operation Antenna Design and Manufacture Key Fabrication Issue: Skin Depth Conductor Materials Stamped Metal Electroless Cu, Ag Silver Inks Exotics Substrates Papers PET plastic Liquid Crystal Polymer (LCP) Key Fabrication Issue: Skin Depth Goals: Low Temperature Process Minimal Metal Deposition but Stay Thicker than the Skin Depth RF Tag Antennas Planar folded dipole for 915 MHz Flexible PET * Substrate Silver Electroless Silver Copper Electroless Copper Rigid FR4 Substrate Baseline 1 oz. milled copper on FR4 substrate * Polyethylene terephthalate 11

12 On-Dielectric Degradations RF Tag Antenna Material Attachment RFID tag pattern changes when placed on dielectric media Radio Assay Tag Pattern on a Wooden Slab Picture shows equipment at Georgia Tech for measurement RF Tag dipole placed on 1-inch plywood J.D. Griffin, A Radio Assay for the Study of Radio Frequency Tag Antenna Performance, Master s thesis, Georgia Institute of Technology, Balun-Transformer Tag Pattern on a Wooden Slab Back Side Front Side Balun + Transformer allows an RF tag antenna, connected with co-planar strip feeds, to be read by 50- Ohm coaxial cable Allows emulation of a chip on arbitrary antenna designs Question: On which side of the antenna is the dielectric wooden slab? 12

13 On-Dielectric Degradations Dielectric material actually draws radiated power into medium Usually this is opposite the desired direction of operation Penalty is assessed twice on the backscatter link budget On-Dielectric Degradations Simple equivalent circuit for explaining dielectric degradation Lower Impedance medium draws power Virtually all non-magnetic materials have higher medium impedance than free space η µ ε ε 0 = Ohms r 0 On-Metal Degradations Ideal signaling occurs when switch between open and short circuit Formula depends on radiation resistance of antenna, Z 0 Γ A, B Z = Z A, B A, B Z + Z Maximum difference between reflections: Γ open = +1, Γ short = RF Isolation with Engineered Polymers Air Incident Wave Antenna Ground Plane Dielectric Incident Wave Antenna Ground Plane Find practical, low cost techniques for making thin, flexible substrates for RF tags. Substrates have low velocity of propagation that retard phase progression and isolate printed antennas. On-Metal Degradation On-Metal Degradations Formula depends on radiation resistance of antenna, Z 0 Γ short Z = Z short short Z + Z 0 >> 0 1 Difference between states diminished: Γ open = +1, Γ short ~ +0.9 on metal impedance drop (Z 0 drops below Z short ) 13

14 Tag Pattern for Foil-Coated Wood Tunable Impedance Matching Network Pi Network Back Side Front Side Loss of average gain (about 10 db) and significant shielding on metal side of the pattern. J. H. Sinksy and C.R. Westgate, Design of an Electronically Tunable Microwave Impedance Transformer, in Proc. of the 1997 IEEE MTT-S Int. Microwave Symposium. Part 2 (of 3), ser. IEEE MTT-S Int. Microwave Symposium Digest, vol. 2 Denver, CO USA : IEEE, pp , Measured E-Plane Antenna Patterns Measured E-Plane Antenna Patterns G.S. Smith, Directive Properties of Antennas for Transmission into a Material Half-Space, IEEE Transactions on Antennas and Propagation, vol. 32, pp , Gain Penalty Gain Penalty Results AGP Average gain penalty due to each material * These values were interpolated to 915 MHz from data of similar materials given by : A.R.V. Hippel, Dielectric Materials and Applications. New York : The Technology Press of M.I.T. and John Wiley and Sons, Inc., ** Undiluted antifreeze *** At approximately room temperature 14

15 Gain Penalty Example Backscatter Communication Radio Link Budget Power Radio Link Budget Challenges for RFID Readers Challenges for RFID Readers Antenna Selection Self-Interference Interrogation through Portals Mechanical Spinning Single-Antenna Readers Directional Antennas When used to transmit, they focus energy in a particular direction When used to receive, they reject radiation from sources outside their major lobe Directional Antennas Industry Portal Design Tagged object moves through portal via conveyor belt Time-varying, space-varying radio channel 15

16 Spinning Through Portals Single-Antenna Readers Object can be spun through the portal to increase successful read rate Spinning provides both space and polarization diversity Using one antenna for transmit and receive Inexpensive and Compact, but Need a good match on the antenna Requires excellent, high-isolation coupler Dynamic range issues Single Antenna Architecture Circulator/coupler allows dual-use antenna Design is extremely sensitive to mismatched antenna Coupler must provide extremely high isolation between ports The Future of RFID The Future of RFID Energy Scavenging and Power Storage Energy Scavenging Low-Cost Fabrication Procedures Organic Electronics Localization Capability Array Interrogation Mechanical Vibrations Thermocouples (body heat?) Solar Power Supercapacitors Thin, flexible batteries 16

17 Low-Cost Fabrication Organic Electronics Silver Inks Electroless Metal Depositions Exotic Materials Organic Substrates Screen Printing Any good ideas? Pros Low-temperature processing Inexpensive fabrication Non-toxic Cons High turn-on voltage Barrier technology issues Low frequency cut-off Temporal degradation Exotic antenna: see-through Indium-Tin Oxide (ITO) dipole Localization and Array Interrogation Spatio-Temporal Backscatter Signaling Arrays for reading RFID tags (transmitter and receiver) Localization with arrays and direction finding Limitless signal-processing possibilities Key Questions Spread Spectrum for RFID and Anti-Collision How can spread-spectrum be used in an anticollision scheme? How can this scheme lower complexity and power consumption? How many simultaneous reads are ultimately possible? 17

18 What is Anti-Collision? Multiple tag backscatter simultaneously How to separate one tag s information from the sea of backscatter? Spread Spectrum Theory-Encoding Each tag generates a unique high-frequency pseudo-random chipping sequence The sequence is multiplied by low-frequency data and then transmitted Signal a(t) represents the data and c(t) represents the chipping sequence. The transmitted waveform is thus x(t) = a(t) * c(t) Spread Spectrum Theory-Decoding The received signal is a superposition of multiple tag backscatter y(t)=x1(t) + x2(t) high frequency We wish to recover x1(t) low frequency If both chipping sequences were of the range 1 : 1, then: y(t)*c1(t) = x1(t) + x2(t) * c1(t) (low freq) + (high freq) Low pass filtering the result will leave behind only x1(t): the desired data. M-Sequence Generation Easy to generate and decode N shift registers result in 2 N -1 length codes Low complexity, low power consumption No need for reception and decoding Differential-Offset Sequences Each chip is equipped with a set of two PN generators tapped to create maximal length sequences of 255 bits. The two sequences are XORed together with a set phase shift between them. This phase shift determines the ID of the tag Tag Design Supports Anti-Collision for up to 255 tags ID is set by delaying the clock to the second PN generator Tag IDs are reset able on the fly 18

19 Receiver Software The chipping sequences are easily simulated for a desired tag using Matlab The received signal is then captured, sampled, and processed in Labview. Sequence Convergence Measured RF Tag Data a.) x1(t) = y(t) * c(t) LPF Received Signal (Single Tag) Frequency Spectrum b.) Low frequency component c.) Low freq Data (RSS in this figure) 19