Radio Frequency Identification (RFID): Principles and

Applications

Presented to Prof. Kirk

By Pierre-Olivier Charlebois

#110227622

November 30, 2004 Electromagnetic fields and waves

ECSE 352a McGill University

Abstract RFID is a recent and promising technology that operates around radio frequencies and aims at providing us with an alternative to bar codes. Other companies anticipate using RFID’s tag to locate people and objects as an input to computers. This paper will first distinguish passive and active devices and then discuss the theoretical principles leading to the implementation of miniature RFID’s tags. Finally, a concrete example will depict their impact on tomorrow’s society. (2013 words) Introduction The miniaturization of electrical components widened the range of technological products and permitted the apparition of invisible devices. In recent years, radio frequency identification technology (RFID) has benefited of a growing interest. Numerous companies are forecasting many applications to those tiny devices. Not only could it provide a successor to the barcodes we see on every commercialized product, but it could also be used to track those products through their entire life cycle. However, many technological considerations have to be resolved before their cost is low enough to be commercialized on a large scale basis. Typical RFID devices include a stationary reader and very small tags. While the reader can be built as big as required, the tags must remain small, have a very low cost and yet provide enough storage to uniquely identify any given object. Those criteria impose severe constraints on the range within which a tag can be identified by the reader. The tag’s antenna must fit on an area of 47 x 74 mm2 [3] but be able to significantly modulate the incoming signal. Their implementation remains the biggest technological challenge gapping the concept from its realisation. Two types of RFID’s RFID tags – or transponders – come in two different flavours. The first one is referred as “passive” tags because they don’t have any onboard power supply. They consume power provided by the reader through inductive coupling. They only operate in the presence of a nearby reader. The range of action of such tags is very limited and does not exceed few feet. However, they are very attractive for their small size and low cost which could soon go under the 20 cents threshold that would permit them to compete with conventional barcodes. Inversely, “active tags” - or Smart Active Labels (SAL) - are self-powered. They don’t need to be in a reader’s range to operate. They can continuously broadcast their presence and can even interact with other tags. They are far more efficient to low amplitude signals and can more easily respond by signals above the noise level. Their onboard power supply allows them to support more processing capability which significantly widens the number of possible applications. However, their cost remains very high and

makes them suitable for tracking high value product such as laptops and for sensing applications. Similarly, they will never be as small as passive transponders. Even if both tags differ significantly, they rely on similar basic principles to operate. The following section will describe how systems involving passive tags are achieved. Operating Principles The main components of a RDFI device are the reader and a tag. Regardless of the actual physical implementation, the reader and the tag must always perform a set of simple tasks. C. Steve and V. Thomas [6] nicely decoupled the problem in 6 comprehensive subparts that are addressed here below. The reader’s tasks 1) Energize the tag Faraday’s law stipulates that a time-varying magnetic field flowing through a closed loop will induce a voltage around that loop. Since passive tags rely on the energy provided by the reader, a time-varying magnetic flux must flow trough their antenna. Typically, the reader will generate such a flux by inducing a current around circular loops. The expression for the magnetic field strength [3] along the normal axis is given by:

3

2

2raNI

H⋅⋅= (1)

where I is the current through the wire, N the total number of loops, a the radius of the loop and r this distance from the field. This expression assumes that r >> a.

The tag is powered by the magnetic flux which varies as 3

1r

. Although we can modify

the other parameters to increase the flux, it will always significantly decay with distance. 2) Emit a carrier signal As we would expect from their “radio frequency” appellation, RFID uses frequency modulation to carry data. The reader supplies a carrier signal that will be slightly modulated by the tag which can also be used as a shared clock between the devices. High frequency carrier signals would definitely be more efficient in transmitting power, but is restrained by the available bandwidth. Currently, the highest commercial band that would be available for RFIDs is 13.56 MHz and will probably become the standard. 3) Detect and decode modulated signal The key role of the reader is to interpret the modulated signal. A good transmission method is the “Frequency Shift Keying”. FSK has a good immunity to noise and permits sufficient data rate. Typically, the tag performs backscatter modulation to send its data. It modulates the carrier signal to 1/8 the carrier frequency for logical 0 and 1/10 for logical 1. Fig. 1 shows the block diagram of the reader.

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Figure 1) Block diagram representing the reader and the tag. The reader amplifies the oscillator’s signal and induces a carrier signal in the air. The tag receives the signal and uses that energy to send it’s memory content.

The tag’s task 1) Draw energy from the magnetic field In order to work, passive tags must drain the energy available in the surrounding magnetic field and which couples them to the reader via their antenna coil. This process is very similar to transformers where one coil induces a flux in the iron core that will in turn reinduce a voltage around a second coil. The time-varying flux induces a voltage in a second coil using the relation:

dtd

NVΦ= (2) where � ⋅=Φ dSB (3)

N is the number of turn in the second coil, � is the flux, B the magnetic flux density and S the area of the coil. This can be immediately applied to the reader – tag pair where the air replaces the magnetic core. (1) and (2) immediately impose a maximum operating range. Most tags operate at a DC voltage of 2.5V which can only be reached if the magnetic flux is above some threshold value. The induced voltage will be alternating and will need to be rectified. Note that the angle of the tag’s antenna coil with respect to the normal axis of the reader’s coil will strongly influence the B field through the coil. Those concerns will be addressed in the Antenna section. 2) Resonate to carrier signal The tag’s antenna can be modeled to a typical LC circuit having a capacitor and an inductor in parallel. Such a circuit has a resonance frequency of

LCfo π2

1= (3)

The tag’s parameter are tuned to match the carrier frequency ( LCπ2 )-1 = 13.56 MHz. However, it must be able to resonate to 2 slightly different frequencies to achieve modulation.

3) Modulate carrier signal to transmit data That is probably the most critical step. The whole process relies on the fact that a tag can transmit its content to a reader. Chen [1] describes how to perform amplitude modulation using a tuned/detuned mode. When the tag is perfectly tuned, it resonates back a signal with higher then if it is detuned. Fig. 2 shows how such is a circuit can be implemented. Suppose that tunedCLπ2 is set such that fo = 13.56MHz and that the CMOS is activated by the microcontroller, then Ldetuned is short-circuited and the tag will resonate at exactly fo. However, if the microcontroller disables the CMOS, the coil inductance becomes Ltuned + Ldetunes and the tag will send a signal of weaker amplitude. Since both devices are clocked to fo, the reader can read the data emitted by the tag.

Figure 2) The microcontroller can set the CMOS to an open or short circuit at Ldetuned. It will effectively change the resonance frequency by allowing the current to flow through Ldetuned.

The following section will study more thoroughly how the tag’s antenna impacts the performance of the system.

Tag’s Antenna Issues The above study assumed the circular coil of the tag’s antenna was perfectly perpendicular to the incoming field as depicted in Fig. 3 a). However, this is clearly not the case. A tag placed on an item can take any orientation leading to situation such as Fig. 3b) or even worse in Fig 3c). In those cases, the field flowing through the antenna is scaled down by a factor of cos(�) and may even fall to zero when the tag is parallel to the reader’s field. Hence, the tag could remain inactive even when located within the operating range of the reader. The most straightforward approach to overcome this problem is to place several tags on an object each being differently oriented. Another one would be to build readers with two perpendicular antennas. However, both cases will lead to interferences making the signal significantly harder to interpret and will increase the costs. Fosters study “Antenna Problems in RFID Systems” [4] also state the importance of using signals circularly polarized to reduce the impact of orientation. Fig 4) depicts how an oriented dipole antenna would miss a linearly polarized signal while still receiving a circularly polarized signal.

Another issue is to build miniaturized antennas at low cost. Cichos and his team [3] studied the possibility of using polymer based antennas. The coils would be printed on 47 by 74 mm2 layer. Although this method reduces the cost, it produces antennas having high ohmic losses. However, laminated layers of conductive thick film pastes will provide a low cost alternative for short range applications. They are likely to replace the current metal coils as the polymeric materials lower their resistance. Applications It may not yet appear to us, but RFID will likely revolutionize our society. Roy Want from Intel Research said: “… this technology will help bridge the gap between the digital networked world and the physical world” [7]. This statement is becoming more and more true as researches on RFID’s progress. The following situation will best describe what was meant by Want’s text: ��������� ������ ������� ���������������������������� ������ �������� ���������

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��All this information from the real world can be analysed and stored in computers. However, there are strong security issues that have to be address before. Many papers proposed a variety of encoding scheme (J. Ari [1] and J. Alfonsi [2]) but many concerns remain.

Figure 3) The tag can take any orientation. The situation

depicted in A) optimizes the field flowing through A while the coil in C won’t have any field flowing through it.

Figure 4) A linearly polarized field would not induce any signal in antenna A) but

would excite B)

Figure 5) Commercially available RFID tags

Conclusion This paper showed that RFID devices are based on basic electro-magnetic principles. Passive RFID’s most important application would be to replace actual barcodes while active RFID’s will serve as precise sensors which could be used as an input to external computers. RFID will most likely bring important changes in our everyday life. It will effectively enable us to acquire data from the physical world and process it. However, the miniaturization processes still impose severe technological constraints that are not yet resolved. References [1] J. Ari, “Yoking-Proofs for RFID Tags”, Proceedings of the IEEE Conference on Pervasive Computing, March 2004 [2] J. Alfonsi, “Privacy Debate Centers on Radio Frequency Identification”, IEEE Security and Privacy, page 12, March 2004 [3] S. Cichos, “Performance Analysis of Polymer based Antenna-Coils for RFID”, IEEE Polytronic Conference, 2002 [4] P. R. Foster, “Antenna Problems in RFID Systems”, Institution of Electrical Engineers, Savoy Place, London, UK [5] M. Lionel and L. Yunhao, “LANDMARC: Indoor Location Sensing Using Active RFID”, Pervasive Computing and Communications, pages 407-415, 23-26 March 2003. [6] C. Steve and V. Thomas, “Optimization of Inductive RFID Technology”, IEEE International Symposium on Electronics and the Environment, pages 82-87, 7-9 May 2001. [7] R. Want, “Enabling Ubiquitous Sensing with RFID”, Intel Research, pages 84-86, April 2004