The Fundamentals of Backscatter Radio and RFID Systems Joshua Griffin [email protected] www.disneyresearch.com

Disney Research, Pittsburgh 4615 Forbes Ave. Pittsburgh, PA 15213

About Myself • Post-doctoral Associate with Disney Research, Pittsburgh – Work closely with researchers at Carnegie Mellon University • Professor Dan Stancil • Darmindra Arumugam, PhD student

Prof. Stancil

Darmindra Copyright 2009

2

About Myself • BS in Engineering from LeTourneau University in 2003. • MS in Electrical and Computer Engineering (ECE) from Georgia Tech in 2005. • PhD in ECE from Georgia Tech in January, 2009. • The Propagation Group – Founded and directed by Professor Gregory Durgin – http://www.propagation.gatech.edu

Prof. Durgin Copyright 2009

3

Part I 1. Introduction to Backscatter Radio and RFID 2. A Short History of RFID and Backscatter Radio Devices 3. The Fundamentals of Backscatter RFID Propagation 4. Radio Link Budgets for Backscatter RFID 5. Typical UHF RFID Performance Shown Through Example 6. Q & A 7. Break

Copyright 2009

4

Part II 8. 9. 10. 11.

RFID Modulation and Coding RFID System Communication Protocols Spread Spectrum Backscatter RFID Backscatter Radio and RFID Systems Using Multiple Antennas 12. Backscatter RFID: A Look to the Future 13. Q & A

Copyright 2009

5

The RFID Handbook  

by Klaus Finkenzeller ISBN 978-0470844021 The “grand-daddy” of RFID reference books. Lots of material on all types of RFID tags (electronic article surveillance, inductive, and far-field).

7

Copyright 2006-2009

The RF in RFID  

by Daniel Dobkin ISBN 978-0750682091 Good soft-cover reference of RF issues for UHF/farfield RFID tags. Written by a physicist who writes well without watering down concepts. Good engineering reference.

8

Copyright 2006-2009

INTRODUCTION TO BACKSCATTER RADIO AND RFID Copyright 2009

9

What is RFID? • RFID – radio frequency identification • RFID is simply one application of an RF tag. RF tags are used as – RFID devices – Sensors

• An RF tag system usually consists of an interrogator (or reader) and a low-profile transponder or transceiver (or tag).

Copyright 2009

10

RF Tags – Why you should care! • RF tags are used in a many compelling applications: – RFID: toll collection, item/inventory tracking, parcel tracking, baggage handling, healthcare systems, building security, parking passes, animal tracking, etc… – Sensors: position location, motion sensing, corrosivity sensors, temperature sensors, etc… http://www.flickr.com/photos/midnightcomm /171587228/ Copyright 2009

11

RF Tags – Why you should care! – Imagination is the limit: RF tags are useful in many situations where wireless communication is needed between a small, low-power tag and a more complex reader.

Copyright 2009

12

The World of RF Tags

Increasing range, complexity, and cost Copyright 2009

13

The World of RF Tags • Active RF Tags – Transceivers – i.e., they can transmit and receive signals to/from the reader. – Always contain a power source on the RF tag. Power is used to transmit signals. – Have the longest range and highest complexity of any RF tag.

Copyright 2009

14

The World of RF Tags

Ubisense Active Tag www.ubisense.net

Q-track Active Tag http://www.q-track.com/

Copyright 2009

15

The World of RF Tags • Semi-passive RF Tags – Transponders – they can receive signals from the reader, but do not transmit signals to the reader. – Instead, a signal is provided by the reader and altered by the RF tag for wireless communication. – Always contain a power source on the RF tag. Power is not used to transmit signals. – Have the second longest range of all RF tags. Copyright 2009

16

The World of RF Tags • Passive RF Tags – Transponders – they can receive signals from the reader, but do not transmit signals to the reader. – Instead, a signal is provided by the reader and altered by the RF tag for wireless communication. – Do not contain a power source on the RF tag. Power is rectified from the signal from the reader. – Have the shortest range of all RF tags. Copyright 2009

17

The World of RF Tags

Increasing range, complexity, and cost Copyright 2009

18

Inductive RF Tags • Inductive RF tags operate in the reader antenna’s near field.

Copyright 2009

19

Fields of a Small Current Loop • Fields from an electrically small current loop in the (x,y) plane (i.e., a z oriented magnetic dipole)

Section 5.4 from R. C. Johnson, ed., Antenna engineering handbook, New York : McGraw-Hill, 1993. Copyright 2009

20

Near-field Power Decay

Copyright 2009

21

Inductive RF Tags • Inductive RF tags operate much like transformers – Near field magnetic fields couple the RF tag and reader loop antennas – An impedance change at the RF tag is referred to the reader. This is called load modulation.

Copyright 2009

22

Charge Pumps • Often, 1 to 2 volts is required to turn on the RFIC; however, the received signal usually provides much less. • Charge pumps are used to increase the voltage of the signal received by the tag’s antenna.

J. D. Griffin and G. D. Durgin, “Complete Link Budgets for Backscatter Radio and RFID Systems,” IEEE Antennas and Propagation Magazine, April, 2009. Copyright 2009

23

Charge Pumps • The basic charge pump used in RF tag integrated circuits is based on the Dickson Charge Pump. • When the capacitors are uncharged and the input waveform is negative:

Copyright 2009

24

Charge Pumps • When the waveform swings positive:

Copyright 2009

25

Charge Pumps • Once the capacitors have charges and the input waveform is positive:

Copyright 2009

26

Charge Pumps

Copyright 2009

27

Inductive RF Tags • Although most are passive, it is possible for inductive RF tags to be semi-passive.

Copyright 2009

28

Surface Acoustic Wave Tags  

More expensive than passive RFICs Requires a piezoelectric substrate   



Lithum Niobate or Quartz RF-to-SAW transducer SAW has low velocity of propagation

SAW tags are not limited by power-up   

Long-range ID applications (shipyards, warehouses) On-metal applications (metal shipping containers) Low-power applications (munitions, sensitive electronics) 29

Copyright 2006-2009

SAW Transducer 

Interdigitated Transducer converts a UHF/microwave EM wave into a surface acoustic wave   

Transducer is electrically small, acoustically large Digits are half-wavelength apart for acoustic wave in medium Alternating polarities of tension and compression launch wave

− +

+ −

31

Copyright 2006-2009

Simple Equivalent SAW Circuit Model 



Approximate SAW on piezoelectric as an electrical signal on an electrically long transmission line. Reflector bars represent mismatches that absorb, transmit, and reflect power at different amounts.

Antenna

Equivalent Transmission Line

+ -

Zbar

32

Copyright 2006-2009

Zbar

SAW Response

Copyright 2006-2009

33

The World of RF Tags

Increasing range, complexity, and cost Copyright 2009

34

Backscatter RF Tags • Backscatter RF tags fall the heading of backscatter radio – Backscatter radio is the broad class of systems that communicate using scattered electromagnetic waves – Backscatter RF tags are designed to operate in the reader antenna’s far field (1/r2 power loss)

Copyright 2009

35

Backscatter RF Tags • A backscatter RF tag scatters electromagnetic waves using load modulation

J. D. Griffin and G. D. Durgin, “Gains for RF Tags Using Multiple Antennas,” IEEE Transactions on Antennas and Propagation, vol. 56, no. 2, pp. 563–570, 2008. Copyright 2009

36

Backscatter RF Tags and Radar

Radar Pulse

Scattered Signal

Copyright 2009

37

Backscatter RF Tags

Copyright 2009

38

Backscatter Reflection Coefficient

J. D. Griffin and G. D. Durgin, “Complete Link Budgets for Backscatter Radio and RFID Systems,” IEEE Antennas and Propagation Magazine, April, 2009. Copyright 2009

39

Backscatter RF Tag Readers • Readers come in two flavors: – Monostatic – Bistatic

• Unique Challenges of Backscatter Readers – Two signals are received • Self-interference • Modulated backscatter signal

– High power and dynamic range

Copyright 2009

40

Understanding the Backscattered Signal

Copyright 2009

41

Understanding the Backscattered Signal

Copyright 2009

42

Backscatter RF Tag Readers

Copyright 2009

43

Solutions for Self-Interference • Direct-conversion receiver with baseband DC blocking capacitors. • Active carrier cancellation at the receiver input. • Use cross-polar reader transmitter and receiver antennas.

Copyright 2009

44

Backscatter RF Tag Readers

J. D. Griffin and G. D. Durgin, “Complete Link Budgets for Backscatter Radio and RFID Systems,” IEEE Antennas and Propagation Magazine, April, 2009. Copyright 2009

45

A SHORT HISTORY OF RFID AND BACKSCATTER RADIO DEVICES Copyright 2009

46

Then… • Backscatter RF tags have their origins in radar – Radar was developed during WWII – There was a need to identify the aircraft. The solution was the Identify Friend or Foe (IFF) system. • Most of these systems were active transmitter/receiver systems.

• Groundbreaking paper published in 1948: H. Stockman, “Communication by Means of Reflected Power,” Proceedings of the I.R.E., vol. 36, no. 10, pp. 1196—1204, 1948. Copyright 2009

47

Then… • 1940s and 1950s – the foundation for RFID laid • 1960s – development of electronic article surveillance (EAS) systems – 1-bit tags

• 1970s – development work • 1980s – RFID began finding mainstream commercial applications, in particular electronic toll collection. UHF and microwave tags still using discrete components. • 1990s – development of useful Schottky diodes allow the entire tag to be integrated on a single chip. • 2000s – establishment of widely accepted protocols – e.g. the Electronic Product Code protocols. Copyright 2009

48

Now… • Current issues for RF tags: – Communication range – Communication reliability – RF tag cost – RF tag footprint size

Copyright 2009

49

The Great Embassy Seal Bug  



Given as “gift” to US by USSR in 1946 Passive transduction of sound, interrogated from across the street in the Soviet Embassy Undiscovered until 1952

http://www.spybusters.com/Great_Seal_Bug.html

50

Copyright 2006-2009

Anatomy of the Passive Seal Bug 







Invented by Leon Theremin Vibrating diaphragm changes capacitive load seen by antenna Analog speech modulates the backscattered information Reflected signal looks like small-carrier AM

51

Copyright 2006-2009

THE FUNDAMENTALS OF BACKSCATTER RFID PROPAGATION Copyright 2009

52

Basic Backscatter Channel

Copyright 2006 - 2009

53

The M × L × N Backscatter Channel

J. D. Griffin and G. D. Durgin, “Gains for RF Tags Using Multiple Antennas,” IEEE Transactions on Antennas and Propagation, vol. 56, no. 2, pp. 563–570, 2008. Copyright 2006 - 2009

54

Pinhole Channel

Copyright 2006 - 2009

55

Link Correlation

J. D. Griffin and G. D. Durgin, “Link Envelope Correlation in the Backscatter Channel,” IEEE Communications Letters, vol. 11, no. 9, pp. 735–737, 2007. Copyright 2006 - 2009

56

Channel Summary • The M x L x N Backscatter – a general channel in which a backscatter radio system with M transmitter, L RF tag, and N receiver antennas operates. • It is a pinhole channel which implies two sub-links – Forward Link – Backscatter Link

• Link correlation describes the relationship between the fading in the forward and backscatter links. Copyright 2009

57

RADIO LINK BUDGETS FOR BACKSCATTER RFID Copyright 2009

58

Link Budgets • Fundamentally, power is the critical factor that limits wireless system performance. • Link budgets are used by wireless engineers to account for all of the power gains and losses in their system. • Conventional Radio – one link budget • Backscatter Radio – two link budgets are needed: – The power-up link budget – The backscatter link budget Copyright 2009

59

Power-Up Link Budget

Copyright 2006 - 2009

60

Polarization Mismatch Loss

• Accounts for the power loss due to polarization mismatch • Circular polarization is often used resulting in:

Copyright 2006 - 2009

61

Power Transmission Coefficient

Copyright 2006 - 2009

62

On-Object Gain Penalty

On-Object Gain Penalties in dB Cardboard Acrylic Pine De-ionized Ethylene Ground Aluminum Glycol Beef Slab Sheet Slab Plywood Water 0.9

1.1

4.7

5.8

7.6

10.2

10.4

J. D. Griffin, G. D. Durgin, A. Haldi, and B. Kippelen, “RF Tag Antenna Performance on Various Materials Using Radio Link Budgets,” IEEE Antennas and Wireless Propagation Letters, vol. 5, pp. 247-250, 2006.

Copyright 2006 - 2009

63

Blockage Loss

• Loss caused by blockage to the line-of-sight. • In conventional transmitter-to-receiver links, the lognormal distribution is used to account for shadowing.

• No studies of blockage losses in backscatter radio systems have been reported; therefore, it usually assumed that B = 1 (in the linear scale) Copyright 2006 - 2009

64

Fade Margin

• The fade margin is the additional power that must be transmitted in a fading channel to maintain a given outage probability compared to a channel that does not fade. • Multipath fading results in pockets of adequate power and pockets of very little power. • RF tag range and reliability are significantly affected by multipath fading. Copyright 2006 - 2009

65

Fade Margin • Multipath Fading: – Waves are often scattered off objects in the environment before they reach the RF tag. – These scattered – or multipath – waves arrive at the tag with different amplitudes, phases, and angles of arrival – When the multipath waves add destructively, a fade occurs. Copyright 2009

Example measurement at 5.85 GHz made by moving receiver antenna over 1m area. G. D. Durgin, Space-Time Wireless Channels. Upper 66 Saddle River, NJ, USA: Prentice Hall, 2003.

Fade Margin • Multipath fading is usually characterized using a probability distribution function – Rayleigh distribution – if the line-of-sight is blocked – Rician – if the line-of-sight is unobstructed

Copyright 2009

67

Fade Margin

Outage Probability

K = -∞ dB

K = 0 dB

K = 3 dB

K = 10 dB

Fp

F2

Fβ

Fp

F2

Fβ

Fp

F2

Fβ

Fp

F2

Fβ

0.5

2

6

4

1

5

3

1

3

2

0

1

1

0.1

10

22

15

9

20

14

7

16

11

3

7

5

0.05

13

29

19

12

26

17

10

21

15

4

9

6

0.01

20

43

28

19

40

26

16

34

22

6

13

9

0.005

23

49

32

22

46

29

19

40

26

7

15

10

0.001

30

63

40

29

60

37

26

54

33

9

20

13

Fade margins are in dB. J. D. Griffin and G. D. Durgin, “Complete Link Budgets for Backscatter Radio and RFID Systems,” IEEE Antennas and Propagation Magazine, April, 2009.

Copyright 2009

68

Backscatter Link Budgets • The three backscatter link budgets describe the modulated backscatter power received at the reader – Monostatic Backscatter Link Budget – Bistatic Dislocated Backscatter Link Budget – Bistatic Collocated Backscatter Link Budget

J. D. Griffin and G. D. Durgin, “Complete Link Budgets for Backscatter Radio and RFID Systems,” IEEE Antennas and Propagation Magazine, April, 2009.

Copyright 2006 - 2009

69

Backscatter Link Budgets Monostatic

Bistatic Dislocated

Copyright 2006 - 2009

70

Product-Rayleigh PDFs

Copyright 2009

71

Backscatter Fade Margins Outage Probability 0.5 0.1 0.05 0.01 0.005 0.001

K = -∞ dB

K = 0 dB

K = 3 dB

K = 10 dB

Fp

F2

Fβ

Fp

F2

Fβ

Fp

F2

Fβ

Fp

F2

Fβ

2

6

4

1

5

3

1

3

2

0

1

1

10

22

15

9

20

14

7

16

11

3

7

5

13

29

19

12

26

17

10

21

15

4

9

6

20

43

28

19

40

26

16

34

22

6

13

9

23

49

32

22

46

29

19

40

26

7

15

10

30

63

40

29

60

37

26

54

33

9

20

13

Fade margins are in dB. J. D. Griffin and G. D. Durgin, “Complete Link Budgets for Backscatter Radio and RFID Systems,” IEEE Antennas and Propagation Magazine, April, 2009. Copyright 2009

72

Modulation Factor

J. T. Prothro and G. D. Durgin, “Improved Performance of a Radio Frequency Identification Tag Antenna on a Metal Ground Plane,” Master’s thesis, The Georgia Institute of Technology, 2007. [Online]. Available: http://www.propagation.gatech.edu/Archive/PG_TR_070515 _JTP/PG_TR_070515_JTP.pdf

Copyright 2006 - 2009

73

Differential Radar Cross Section • The radar cross section (RCS) of an RF tag is defined as:

• As the tag switches between states A and B, the differential RCS is

P. V. Nikitin, K. V. S. Rao, and R. D. Martinez, “Differential RCS of RFID Tag,” Electronics Letters, vol. 43, no. 8, pp. 431–432, 2007. 74

Copyright 2009

TYPICAL UHF RFID PERFORMANCE SHOWN THROUGH EXAMPLE Copyright 2009

75

915 MHz RFID System Example

Copyright 2009

76

System Specifications • Reader – Reader antennas: right-hand circularly polarized with gain of 7 dBi (5 in the linear scale) at 915 MHz. – Can operate in a monostatic or bistatic dislocated mode – Reader has a sensitivity of -80 dBm

Copyright 2009

77

System Specifications • RFID Tag – Tag antenna: a single folded dipole that is linearlypolarized with a free-space, load-matched gain of 2.1 dBi (1.6 in the linear scale). – Antenna’s free-space impedance is 300+j100 Ω. – The tag uses ASK modulation and switches between two loads: ZA,RFIC = 20 – j350Ω and ZB,RFIC = 2-j0.1 Ω. – The tag sensitivity is -13 dBm at 915 MHz.

Copyright 2009

78

System Specifications • RFID Tag – Impedance matching network transforms the antenna impedance to match that of the RFIC in impedance state A.

J. D. Griffin and G. D. Durgin, “Complete Link Budgets for Backscatter Radio and RFID Systems,” IEEE Antennas and Propagation Magazine, April, 2009.

Copyright 2009

79

System Specifications • Propagation Environment – A LOS exists between the reader and the RFID tag. – Clutter causes multipath propagation resulting in a Rician K factor of 3 dB. – 5% outage probability is desired – RFID tags are attached to both cardboard and aluminum objects.

Copyright 2009

80

Link Budget Calculations • Transmit Power, PT: FCC limits transmit power in the 902-928 MHz ISM band to 4 W EIRP. (in the linear scale)

GT = 5 (in the linear scale), PT = 29 dBm (or 800 mW) • On-object gain penalty, Θ: Gain Penalty in dB Cardboard Acrylic Pine De-ionized Ethylene Ground Aluminum Glycol Beef Slab Sheet Slab Plywood Water 0.9

1.1

4.7

5.8

7.6

10.2

10.4

J. D. Griffin, G. D. Durgin, A. Haldi, and B. Kippelen, “RF Tag Antenna Performance on Various Materials Using Radio Link Budgets,” IEEE Antennas and Wireless Propagation Letters, vol. 5, pp. 247-250, 2006.

Copyright 2009

81

Link Budget Calculations • Fade Margins: – Power-up fade margin, Fp: 10 dB (10 in the linear scale) – Monostatic fade margin, F2: 21 dB (126 in the linear scale) – Bistatic, dislocated fade margin, Fβ: 15 dB (32 in the linear scale) Outage Probability

K = -∞ dB

K = 0 dB

K = 3 dB

K = 10 dB

Fp

F2

Fβ

Fp

F2

Fβ

Fp

F2

Fβ

Fp

F2

Fβ

0.5

2

6

4

1

5

3

1

3

2

0

1

1

0.1

10

22

15

9

20

14

7

16

11

3

7

5

0.05

13

29

19

12

26

17

10

21

15

4

9

6

0.01

20

43

28

19

40

26

16

34

22

6

13

9

0.005

23

49

32

22

46

29

19

40

26

7

15

10

0.001

30

63

40

29

60

37

26

54

33

9

20

13

Fade margins are in dB. J. D. Griffin and G. D. Durgin, “Complete Link Budgets for Backscatter Radio and RFID Systems,” IEEE Antennas and Propagation Magazine, April, 2009.

Copyright 2009

82

Link Budget Calculations • Polarization mismatch, X: Since the reader antennas are circularly-polarized and the RFID tag is linearly-polarized:

Copyright 2009

83

Link Budget Calculations • Modulation factor, M, and Transmission Coefficient, τ:

Cardboard Attachment

Aluminum Attachment

Copyright 2009

84

Power-Up Links

J. D. Griffin and G. D. Durgin, “Complete Link Budgets for Backscatter Radio and RFID Systems," IEEE Antennas and Propagation Magazine, April, 2009.

Copyright 2006 - 2009

85

Backscatter Links

J. D. Griffin and G. D. Durgin, “Complete Link Budgets for Backscatter Radio and RFID Systems," IEEE Antennas and Propagation Magazine, April, 2009.

Copyright 2006 - 2009

86

Conclusion • RF tags are usually limited by the power-up link, not by the backscatter link. • Material attachment can have a significant effect on RF tag performance • Link correlation reduces RF tag performance – Monostatic configuration – Bistatic configuration

Copyright 2009

87

Further Reading General RFID D. M. Dobkin, The RF in RFID: Passive UHF RFID in Practice. Burlington, MA: Newnes, 2008. K. Finkenzeller, RFID Handbook: Fundamentals and Applications in Contactless Smart Cards and Identification, 2nd ed. New York: John Wiley and Son LTD, 2003.

General Antenna and Propagation W. L. Stutzman and G. A. Thiele, Antenna Theory and Design, 2nd ed. Hoboken, NJ: John Wiley and Sons, 1998. G. D. Durgin, Space-Time Wireless Channels. Upper Saddle River, NJ, USA: Prentice Hall, 2003.

Introduction to RFID L. Reindl, G. Scholl, T. Ostertag, C.C.W. Ruppel W. –E. Bulst, and F. Seifert, “SAW Devices as Wireless Passive Sensors,” Proceedings of the 1996 IEEE Ultrasonics Symposium, vol. 1, San Antonio, TX, November, 1996, pp. 363-637. K. V. S. Rao, P. V. Nikitin, and S. F. Lam, “Antenna Design for UHF RFID Tags: A Review and a Practical Application,” IEEE Transactions on Antennas and Propagation, vol. 53, no. 12, pp. 3870–3876, 2005. P. Nikitin, K. V. S. Rao, and S. Lazar, “An Overview of Near Field UHF RFID,” in IEEE International Conference on RFID, Grapevine, TX, 2007, pp. 167–174. Copyright 2009

88

Further Reading RFID History H. Stockman, “Communication by Means of Reflected Power,” Proceedings of the I.R.E, vol. 36, no. 10, pp. 1196–1204, 1948. J. Landt, “The History of RFID,” IEEE Potentials, vol. 24, no. 4, pp. 8–11, 2005.

Backscatter RFID Propagation J. D. Griffin and G. D. Durgin, “Gains for RF Tags Using Multiple Antennas,” IEEE Transactions on Antennas and Propagation, vol. 56, no. 2, pp. 563–570, 2008. P. Nikitin and K. V. S. Rao, “Performance Limitations of Passive UHF RFID Systems,” in Proceedings of IEEE Antenna and Propagation Society International Symposium, Albuquerque, New Mexico, 2006, pp. 1011–1014. J. D. Griffin and G. D. Durgin, “Link Envelope Correlation in the Backscatter Channel,” IEEE Communications Letters, vol. 11, no. 9, pp. 735–737, 2007. P. Nikitin and K. V. S. Rao, “Antennas and Propagation in UHF RFID Systems,” in IEEE International Conference on RFID, Las Vegas, TX, 2008, pp. 277–288. M. Ingram, M. Demirkol, and D. Kim, “Transmit Diversity and Spatial Multiplexing for RF Links Using Modulated Backscatter,” in Proceedings of the International Symposium on Signals, Systems, and Electronics, Tokyo, Japan, July 2001.

Copyright 2009

89

Further Reading S. R. Banerjee, R. Jesme, and R. A. Sainati, “Performance Analysis of Short Range UHF Propagation as Applicable to Passive RFID,” in 2007 IEEE International Conference on RFID, Gaylord Texan Resort, Grapevine, TX, USA, March 2007, pp. 30–36. S. R. Banerjee, R. Jesme, and R. A. Sainati, “Investigation of Spatial and Frequency Diversity for Long Range UHF RFID,” in IEEE Antennas and Propagation Society International Symposium, San Diego, CA, USA, July 2008, pp. 1–4.

Radio Link Budgets for Backscatter RFID J. D. Griffin and G. D. Durgin, “Complete Link Budgets for Backscatter Radio and RFID Systems,” IEEE Antennas and Propagation Magazine, April, 2009. J. D. Griffin, G. D. Durgin, A. Haldi, and B. Kippelen, “RF Tag Antenna Performance on Various Materials Using Radio Link Budgets,” IEEE Antennas and Wireless Propagation Letters, vol. 5, pp. 247–250, 2006. D. M. Dobkin and S. M. Weigand, “Environmental Effects on RFID Tag Antennas,” in IEEE MTT-S International Microwave Symposium Digest, 2005, pp. 135–138. J. T. Prothro and G. D. Durgin, “Improved Performance of a Radio Frequency Identification Tag Antenna on a Metal Ground Plane,” Master’s thesis, The Georgia Institute of Technology, 2007. [Online]. Available: http://www.propagation.gatech.edu/Archive/PG_TR_070515_JTP/PG_TR_070515_ JTP.pdf

Copyright 2009

90

Further Reading 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. 1740–1749, 2003. P. V. Nikitin, K. V. S. Rao, and R. D. Martinez, “Differential RCS of RFID Tag,” Electronics Letters, vol. 43, no. 8, pp. 431–432, 2007.

Copyright 2009

91