EINSTEIN'S THEORY OF RFID



RFID is based on a chain of scientific laws, discoveries and principles from intellectual pioneers such as: Faraday, Maxwell, Hertz and Marconi. To truly understand this technology one needs to fathom such fields as radio, physics, software and the internet. All told, a very complicated collaboration of technologies is involved in making this modern wonder come to life. Sadly, a complete understanding of RFID is only attainable for the "Einstein's" in our midst. For the remaining mere mortals we have to suffice with a general understanding or a specialized knowledge of a few facets of this powerful technology. This chapter provides a generalist view with an emphasis on core knowledge in the realm of radio.

Electromagnetic Waves
Radio Frequency Identification is a way of storing and retrieving data through electromagnetic transmission to an RF compatible electronic circuit. A sample abstract from one of the early patents (US PN 3,745,569 circa 1971) hints at the complexity hiding under the surface of RFID:
"A transponder having a circuit for the extraction of power from an incident interrogating beam of electromagnetic energy, the extracted power being utilized to operate a digital coding circuit. The transponder further comprises an oscillator circuit for developing a train of pulses of electromagnetic energy with successive pulses occurring in a coded format in accordance with a digital code imparted by the digital coding circuit. The transponder is of sufficiently small size to be affixed in the form of a tag to automobiles, personnel, containers, and other objects to be identified. The electronic tag communicates with an interrogation system."

To really understand RFID from first principles we must discuss the science of Radio.
All radio transmissions use electromagnetic waves that are created when alternating currents flow through an antenna. The word electromagnetic is the concatenation of electric and magnetic and implies two types of linked phenomenon make up the radio wave. We cannot directly see, taste, touch or hear electromagnetic waves, so it is not surprising that their operation is mystifying.

Let's try and understand this better.

First, electric fields are created by differences in voltage. A simple relationship governs this: the higher the voltage, the stronger the field. Second, magnetic fields are created when current flows. Again a simple relationship governs this: the greater the current, the stronger the magnetic field. To quickly recap: Electric fields exist even when current is not flowing whereas Magnetic fields only exist when current is flowing. When the two exist together they are commonly referred to as Electromagnetic Fields (EMF).
EMF is present everywhere in our lives but they are invisible to the eye. Natural sources like the earth's magnetic field cause a compass needle to orient in a North-South direction. Besides these natural sources the electromagnetic spectrum also includes fields generated by man-made sources such as: X-rays and garage door openers. Various other kinds of higher frequency electromagnetic waves are used for radio transmissions in TV, AM or FM radio, cellular and RFID, to name a few.

Frequency Spectrum
In all these applications the power and variance of the electromagnetic fields are vital to their intended operation. An important concept associated with EMF is frequency. Let us imagine an ocean with a series of very regular waves. The frequency simply describes the number of waves per second that crest at the same point in the ocean. Geeks describe this as the oscillations or cycles per second at a static point of measurement. The term wavelength describes the distance between the crest of one wave and the next. Hence wavelength and frequency are correlated: the higher the frequency, the shorter the wavelength. To translate this to radio waves imagine the ocean waves traveling at an enormous speed, the speed of light which is 186,000 miles per second.

Another simple analogy should help reinforce the concept. Tie a long rope to a door handle and keep hold of the free end. Moving it up and then down slowly will generate a single big wave; more rapid motion will generate a series of smaller waves. Rope length is constant; therefore, as you create more waves you are increasing the frequency while making them shorter in distance (wavelength).
Frequency is commonly known as Hertz in honor of radio pioneer, Heinrich Hertz. One cycle per second is 1 Hertz. The frequency of oscillations ranges from 1 Hertz to infinity and this entire range is known as the Frequency Spectrum. Common units are kiloHertz (which is one thousand Hertz, 1 kHz), megaHertz (one million Hertz, 1 MHz), gigaHertz (one billion Hertz, 1 GHz) or teraHertz (one trillion Hertz, 1 THz). Finally, a new unit which is way more then you know what to do with will be known as GatesHertz in honor of Bill Gates.

Frequency Spectrum is viewed as an important resource. Legal and political governing bodies generate complicated rules and regulations to coordinate its use. Specific frequencies are reserved for RFID use.

Currently they are:
LF (low frequency): 125kHz, 134 kHz
HF (high frequency): 13.56 MHz
UHF (ultra high frequency): 868 MHz-Europe, 902 to 928 MHz - USA
Microwave: 2.45 GHz

Electromagnetic fields are generated and received by the antenna. The antenna is designed to radiate energy out into free space and collect radio energy from space. It is important to recognize that in doing this job the antenna is the most important part of the radio system – without it the system is dead. Since the antenna system is common to both the transmitter and the receiver; any change in the antenna affects both transmission and reception.

Antenna & Wave Propagation
We have learned that the antenna changes radio energy from the transmission line into radiated energy and vice versa. What is remarkable is the efficiency with which an antenna does its job. A light bulb is about 20% efficient in changing electrical energy into light whereas the antenna is nearly 100%. We may break down the antenna's operation into two fundamental modes of wireless communications:

• Near Field Communications aka close proximity electromagnetic aka inductive coupling
• Far Field Communications aka propagating electromagnetic waves

In either mode antennas have optimal sizes that relate to the frequency of the signal. Basically, the higher frequencies require smaller antennas due to their shorter wavelengths.

Because the sizes of wavelengths vary, radio signals propagate differently through free space. Some are well suited to short ranges while others are good for transmissions involving very long distances. Typically, the higher the frequency, the shorter the distances the signal will travel. The strength of the radio signal diminishes rapidly as it moves away from the transmitter antenna.

Far field radiation is distinguished by the fact that the intensity is inversely proportional to the square of the distance. In reality, due to obstructions, absorption, and interference the loss is more severe, approaching the inverse of the 5 or 6th power of the distance. Whereas Near field radiation intensity is inversely proportional to the cube of the distance in the region that is less then 1/6 wavelength from a simple loop antenna. (For additional reference see: Principles of Antenna Theory by Kai Fong Lee page 231 and the ARRL Antenna Book pg 2-8 and TI Literature Number 11-08-26-003). In general, this means that near field signals drop off faster then far field as you move away from the antenna.

Obviously the radio link is extremely complicated and requires considerable engineering to achieve 100% read rates. Realize that the energy level to write a tag is greater than reading, therefore, the write range is shorter than the read range.

Modulation & Handshaking
Once we have the radio engineering link operational we may consider how it transports information from one location to another. In a sense, the waves are like an endless line of UPS trucks capable of moving things; however, they are only valuable when they are filled with stuff. The frequency of the radio wave providing the transport is known as the carrier frequency. The information to be carried is mixed with this frequency by a process known as modulation.

Modulation is necessary because the intelligence of the signal, voice or data, is usually a much lower frequency then the carrier and therefore not effectively radiated into space.

Using what we have learned in this chapter we may now describe a typical transmitting sequence for a generic tag-antenna-reader system. Let us start with the greeting. When you meet someone you usually shake their hand. An analogous situation occurs in electronics with a system handshake. The typical handshake for a passive tag is as follows:

• Reader looks for modulation of its radio frequency sine wave to indicate the presence of a tag.
• When a tag's antenna captures the EMF generated by the reader's antenna it initiates a process to respond with a data stream encoded in the carrier.

With a successful handshake the system begins the transmission of information as follows:

• The tag typically starts clocking its data messages against an output transistor, which is connected across coil inputs. In this case the radio link behaves like a transformer where the tag is the primary coil and the reader is the secondary coil.
• As the tag's output transistor shunts the coil, it effectively modulates the carrier to experience a momentarily voltage drop. This pattern of voltage drops corresponds to the information to be sent from the tag to the reader.
• The reader must detect these small voltage drops which represent the modulation. This requires a reader sensitivity that is able to discern 1/1000 of a change from the original carrier wave's amplitude.

By constantly monitoring these voltage drops the reader detects and decodes a bit stream according to the governing modulation method. To further complicate things, the modulation scheme also uses algorithms to fix errors, reduce bandwidth, perform synchronization and other system needs.

Besides these basic tasks the system needs to handle collision avoidance during the simultaneous reading of several tags in the same radio frequency field. In this case the tag and reader must be intelligent enough to detect that more then one tag is present. Failure to recognize this condition leads to all the tags modulating the carrier frequency at the same time and these multiple waveforms arrive at the reader to create a garbled signal. This is referred to as a collision. No data would be transferred to the reader when this happens.

This is like the problem of having an uncoordinated telephone conversation with three people. If everyone talks at once it is impossible to understand the conversation. Some order to the communications will allow everyone to speak and be understood. Similarly, the RFID radio interface requires arbitration so that only one tag transmits data at a time.

While it is possible to transmit all the data from the tag to the reader via amplitude modulation, the practical modulation of data is enhanced by the following methods:

• FSK - frequency shift keying: two different frequencies are used for data transfer. A zero is transmitted as an amplitude modulated clock signal with a different frequency while a one is sent on another amplitude modulated frequency.
• PSK - phase shift keying: similar to FSK except only one frequency is used and the shift between 1's and 0 is accomplished by shifting the peak and trough of the wave forms.

These factors make the air interface the most complicated component of system design. In other words, the radio channel is the weakest link in the system and typically requires an engineered to fit solution.