In recent years RFID technology has been largely adopted by the industry to speed up handling of manufactured goods. Unlike barcodes, the technology has enabled the manufacturers and retailers to keep track of their inventories without human assistance. The biggest USP of RFID technology is its identification capability from a distance, and it does so without requiring line of sight, which can really help remove the highly annoying checkout line part from your shopping experience. The Universal Product Code (UPC) on each of your shopping needs to be manually scanned at the checkout, due to which the customers have to wait in long queues.

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RFID tags support more unique IDs than a barcode system and can even incorporate additional data such as manufacturer details, expiry date, overall weight and storage temperature. Moreover, RFID technology can even identify many different tags located in the same area, so one by one scanning is not required any more.

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Various businesses are moving towards adopting this technology in order to make their processes more efficient and add to the customer delight. One interesting example is a proposal from Air New Zealand to offer its regular customers the opportunity to avoid the baggage-drop counter. Instead, an RFID tag can be attached to their bag and the bag put directly onto the conveyer belt. RFID technology will match the bag to its owner’s destination, sending it into the cargo channel of the right plane.

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Another successful implementation is in meat processing industry. The plant uses a network of RFID readers, with tags attached to carcasses. When carcasses are cut, tags are fitted to the plastic trays holding the various meat products. Thus the product can be tracked within the company and the process is automated to reduce the amount of human contact with products, which is a primary requirement in such industries.

Why not implemented ever since
RFID technology has been around for a long time but one of the major roadblocks to its quick adoption by the industry was the high cost of system implementation and tags. RFID is still not cheap enough to compete with traditional labeling technologies, but it offers immense added value and is now at a critical price point that could enable its large-scale adoption for managing consumer retail goods. Now people see this as an investment that can be recovered elsewhere, such as reduction in labour cost.

How RFID works
There are a variety of RFID systems based on different operating principles, the most important being ‘inductive coupling’ and ‘backscatter coupling.’ These designs take advantage of near-field and far-field electromagnetic properties associated with an RF antenna, respectively. In both cases, enough power can be transferred to the tag for its required operation.

Fig. 1: Near-field and far field
Fig. 1: Near-field and far field

Near-field RFID system. This type of system is sometimes also called inductive coupled system as it works on Faraday’s principle of magnetic induction.

The RFID reader produces an alternating magnetic field in the locality by passing a large alternating current through its coil. Now if a tag with integrated smaller coil comes under the range of this varying magnetic field, an alternating voltage induces across it. This induced voltage is rectified and coupled to a capacitor as shown in Fig. 3. The capacitor gets charged and powers the semiconductor chip for its operation.

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Fig. 2: Inductive coupled system
Fig. 2: Inductive coupled system

Near-field RFID systems send the data back using load modulation technique. As voltage is induced by inductive coupling in the tag, if there is a load a current will flow through the coil, giving rise to its own small magnetic field. This small magnetic field will oppose the reader’s field. Due to this change in field, the reader’s coil will have a small amount of additional current flowing through it, which can easily be detected. The additional current in the reader’s coil is actually proportional to the load across the tag’s coil and hence the load modulation.

This means that if the electronics in the tag applies a variable load to its own antenna, it can actually send an encoded signal representing the ID which is detected at the RFID reader’s side as variation in the current through its coil (refer Fig 3). The modulation scheme can be chosen based on the number of bits to be transmitted, data rate and required redundancy.

Fig. 3: Near-field RFID system
Fig. 3: Near-field RFID system

The range of operation for such a system majorly depends on two things: frequency of operation and radiated energy.

The range over which we can use magnetic induction is approximately equal to c/2Πf, where ‘c’ is the speed of light and ‘f’ is the frequency of operation. This means that if the frequency of operation increases, the distance over which near-field coupling can operate decreases proportionally.

The energy available for induction from the RFID reader’s coil as a function of distance is another limiting factor. The magnetic field is directly proportional to 1/r3, where ‘r’ is the distance between the reader’s coil and the tag. As the distance increase, there is a possibility that the magnetic field cannot induce enough energy to get the tag operative.

Inductive coupling is the most straightforward and foremost approach that was taken up for implementing an RFID system. Such wide adoption has resulted in subsequent standards. Readers can refer to ISO 15693 and 14443 for complete understanding.

Such a system works well for simple applications but when the application requires discrimination between multiple tags in the same common area for a fixed read time, each tag is required to have a higher data rate and thus higher operating frequency, which makes the design complicated. Such complication led to RFID system designs based on far-field communication.

Far-field RFID system. RFID tags based on far-field emissions capture electromagnetic waves propagating from the dipole antenna of the reader as shown in Fig. 4.

Fig. 4: Far-field RFID system
Fig. 4: Far-field RFID system

Another smaller dipole antenna in the tag receives this energy as an alternating potential difference that appears across the arms of the dipole, which is further rectified by a diode and coupled to a capacitor. This stored energy in the capacitor powers the electronics. In this case, the tags are beyond the range of the near field and the ID cannot be transmitted back by load modulation. The technique used for communication in far field is called back scattering.

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The antenna of the tag is designed precisely to tune it to a particular frequency so that it absorbs the maximum of energy that reaches it at that frequency. Now if an impedance mismatch occurs at this frequency, the antenna of the tag will reflect back some energy. This reflected energy can be detected by sensitive radio receivers. The electronics of the tag varies the impedance over time to send back the encoded ID of the tag. This is generally done using a transistor across the dipole antenna and then tuning it partially on/off.

Normally, tags using far-field principles operate at a frequency greater than 100 MHz. Below this frequency, systems work using near-field principles.

Two key factors that limit the range of a far-field RFID system are energy reaching the tag and sensitivity of the receiver. The actual backscattered signal is very small because of the attenuations caused when electromagnetic waves are radiated by the reader to the tag and when reflected back by the tag to the reader. These attenuations make the returning energy proportional to 1/r4. Now as the distance increases, the returning energy becomes smaller and smaller.

Such a communication has become possible due to development of highly sensitive and inexpensive radio receivers these days. These receivers have power levels of the order of -100 dBM in the 2.4GHz range, allowing communication up to 6 metres (as claimed by some manufacturers).

Also, due to shrinking feature size of semiconductor manufacturing, the energy required to power a tag is typically in microwatts. So the tags can be read from a greater distance now.

Active vs passive tags

Fig. 5: RFID tags
Fig. 5: RFID tags

There are several types of RFID systems but broadly they can be divided into active and passive categories.

Active tags. Active tags require a power source or integrated battery to operate, due to which their lifetime is limited. These tags are generally used on large assets such as cargo containers and rail cars that need to be tracked over long distances.

Passive tags. Passive tags, on the other hand, don’t require any battery to operate, which makes these tags suitable for retail trade. These tags have infinite operational life and can be implemented in a very small package.

Generally, passive tags have three parts: an antenna, a semiconductor chip and some kind of encapsulation. In passive tags system tag-reader is responsible for powering up the tag and communicating with it. The tag reader radiates energy, which is captured by the passive tag’s antenna. This energy is used to transmit back the tag’s unique ID. Encapsulation protects the antenna and the chip from environmental conditions.

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Frequency ranges for RFID systems
Care has to be exercised with regard to other radio services, which restrict the range of operating frequencies available for an RFID system. Usually, it is possible to use only those frequency ranges which have been reserved specifically for industrial, scientific and medical applications or for short-range devices (see Table I). These are the frequencies classified around the world as ISM (Industrial-Scientific-Medical) frequency ranges or SRD frequency ranges, and they can also be used for RFID applications.

Standards
RFID systems, like all other air interface systems, work in accordance with some rules. Otherwise, the wireless communication will be a complete mess, with one device interfering with another. There are some sets of physical rules that define how the electromagnetic signals will work and communicate the unique ID bits.

Below are some standards evolved for different RFID systems:
1. ISO TC 23: Animal Identification
2. ISO TC 104: Freight containers
3. ISO TC 204: Road telematics
4. ISO TC 122: Packaging
5. JTC 1/SC 17: Integrated circuit cards (credit cards with embedded tags)
6. JTC 1/SC 31: Automatic identification and data collection techniques

RFID limitations
Cost. The cost has gone down in recent years but it still needs to decrease further to meet the requirement of both low-end and high-end solutions. Prices of active or semi-passive tags limit their application to scanning of high-value goods over long ranges.

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Standardisation. Standards for various groups of applications have evolved but they are a little sparse and leave much freedom in the choice of communication protocols and the format and amount of information stored in the tag. This can cause disagreement between various companies providing solutions based on RFID.

Collision. Anti-collision algorithms that can avoid signal collision while reading several tags at a time are either patented or patent pending, which adds to the cost of RFID-based system development.

Obstacle for RFID signals. Properties of some conductive materials such as goods containing water, or metal surfaces may be an obstacle to RFID application at a given frequency, as they may corrupt data transmission either by absorption or by ambient reflection of the signals.

Manufacturing not 100 per cent perfect. Manufacturing of tags is not 100 per cent failure-free even today. There is still a high percentage of defective tags in each lot.

Quick technology obsolescence. One of the major concerns of companies making RFID solutions is quick obsolescence of technology. With rapidly growing technology, more fault-tolerant readers outdate the older ones very quickly.

Security and privacy issues. Security and privacy is still a discussion for RFID technology. Proper encryption must be ensured at all interfaces where data could be intercepted or transmitted.

Possible attacks. Though there are no such attacks reported, the current RFID software can be vulnerable as reflected by various studies.


The author is a technical editor at EFY

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