Section 1.1. Fundamental Concepts

1.1. Fundamental Concepts

A wave is a disturbance that transports energy from one point to another.

Electromagnetic waves are created by electrons in motion and consist of oscillating electric and magnetic fields. These waves can pass through a number of different material types.

The highest point of a wave is called a crest, and the lowest point is called a trough.

The distance between two consecutive crests or two consecutive troughs is called the wavelength.

One complete wavelength of oscillation of a wave is called a cycle.

The time taken by a wave to complete one cycle is called its period of oscillation.

The number of cycles in a second is called the frequency of the wave. The frequency of a wave is measured in hertz (abbreviated as Hz) and named in honor of the German physicist Heinrich Rudolf Hertz. If the frequency of a wave is 1 Hz, it means that the wave is oscillating at the rate of one cycle per second. It is common to express frequency in KHz (or kilohertz = 1,000 Hz), MHz (or megahertz = 1,000,000 Hz), or GHz (or gigahertz = 1,000,000,000 Hz).

Amplitude is the height of a crest or the depth of a trough from the undisturbed position. The former is also called the positive amplitude, and the latter the negative amplitude. In general, the amplitude at a certain point of a wave is its height or depth from the undisturbed position, and is called positive or negative accordingly.

Figure 1-1 shows several parts of a wave.

Figure 1-1. Different parts of a wave.

Radio or radio frequency (RF) waves are electromagnetic waves with wavelengths between 0.1 cm and 1,000 km. Another equivalent definition in terms of frequency is radio waves are electromagnetic waves whose frequencies lie between 30 Hz and 300 GHz. Other electromagnetic wave types are infrared, visible light wave, ultraviolet, gamma-ray, x-ray, and cosmic-ray.

RFID uses radio waves that are generally between the frequencies of 30 KHz and 5.8 GHz.

A continuous wave (CW) is a radio wave with constant frequency and amplitude. From a communications vantage, a CW does not have any embedded information in it but can be modulated to transmit a signal.

Modulation refers to the process of changing the characteristics of a radio wave to encode some information-bearing signal. Modulation can also refer to the result of applying the modulation process to a radio wave.

Radio waves can be affected by the material through which they propagate. A material is called RF-lucent or RF-friendly for a certain frequency if it lets radio waves at this frequency pass through it without any substantial loss of energy. A material is called RF-opaque if it blocks, reflects, and scatters RF waves. A material can allow the radio waves to propagate through it but with substantial loss of energy. These types of materials are referred to as RF-absorbent. The RF-absorbent or RF-opaque property of a material is relative, because it depends on the frequency. That is, a material that is RF-opaque at a certain frequency could be RF-lucent at a different frequency. The RF properties of some example materials are provided in Table 1-2, following a discussion of RFID frequency types.

Table 1-2. RF Properties of Example Material Types
Material	LF	HF	UHF	Microwave
Clothing	RF-lucent	RF-lucent	RF-lucent	RF-lucent
Dry wood	RF-lucent	RF-lucent	RF-lucent	RF-absorbent
Graphite	RF-lucent	RF-lucent	RF-opaque	RF-opaque
Liquids (some types)	RF-lucent	RF-lucent	RF-absorbent	RF-absorbent
Metals	RF-lucent	RF-lucent	RF-opaque	RF-opaque
Motor oil	RF-lucent	RF-lucent	RF-lucent	RF-lucent
Paper products	RF-lucent	RF-lucent	RF-lucent	RF-lucent
Plastics (some types)	RF-lucent	RF-lucent	RF-lucent	RF-lucent
Shampoo	RF-lucent	RF-lucent	RF-absorbent	RF-absorbent
Water	RF-lucent	RF-lucent	RF-absorbent	RF-absorbent
Wet wood	RF-lucent	RF-lucent	RF-absorbent	RF-absorbent

Classes of RFID frequency types include the following:

Low frequency (LF)
High frequency (HF)
Ultra high frequency (UHF)
Microwave frequency

The following subsections discuss these frequency types.

1.1.1. Low Frequency (LF)

Frequencies between 30 KHz and 300 KHz are considered low, and RFID systems commonly use the 125 KHz to 134 KHz frequency range. A typical LF RFID system operates at 125 KHz or 134.2 KHz. RFID systems operating at LF generally use passive tags (discussed in Section 1.2.1), have low data-transfer rates from the tag to the reader, and are especially good if the operating environment contains metals, liquids, dirt, snow, or mud (a very important characteristic of LF systems). Active LF tags (discussed in Section 1.2.1) are also available from vendors. Because of the maturity of this type of tag, LF tag systems probably have the largest installed base. The LF range is accepted worldwide.

1.1.2. High Frequency (HF)

HF ranges from 3 MHz to 30 MHz, with 13.56 MHz being the typical frequency used for HF RFID systems. A typical HF RFID system uses passive tags, has a slow data-transfer rate from the tag to the reader, and offers fair performance in the presence of metals and liquids. HF systems are also widely used, especially in hospitals (where it does not interfere with the existing equipment). The HF frequency range is accepted worldwide.

The next frequency range is called very high frequency (VHF) and lies between 30 and 300 MHz. Unfortunately, none of the current RFID systems operate in this range. Therefore, this frequency type is not discussed any further.

1.1.3. Ultra High Frequency (UHF)

UHF ranges from 300 MHz to 1 GHz. A typical passive UHF RFID system operates at 915 MHz in the United States and at 868 MHz in Europe. A typical active UHF RFID system operates at 315 MHz and 433 MHz. A UHF system can therefore use both active and passive tags and has a fast data-transfer rate between the tag and the reader, but performs poorly in the presence of metals and liquids (not true, however, in the cases of low UHF frequencies such as 315 MHz and 433 MHz). UHF RFID systems have started being deployed widely because of the recent RFID mandates of several large private and public enterprises, such as several international and national retailers, the U.S. Department of Defense, and so on (see Chapter 10, "Standards"). The UHF range is not accepted worldwide.

1.1.4. Microwave Frequency

Microwave frequency ranges upward from 1 GHz. A typical microwave RFID system operates either at 2.45 GHz or 5.8 GHz, although the former is more common, can use both semi-active and passive tags, has the fastest data-transfer rate between the tag and the reader, and performs very poorly in the presence of metals and liquids. Because antenna length is inversely proportional to the frequency (see Section 1.2.1.1.2), the antenna of a passive tag operating in the microwave range has the smallest length (which results in a small tag size because the tag microchip can also be made very small). The 2.4 GHz frequency range is called Industry, Scientific, and Medical (ISM) band and is accepted worldwide.

International restrictions apply to the frequencies that RFID can use. Therefore, some of the previously discussed frequencies might not be valid worldwide. Table 1-1 lists some example frequency-use restrictions for RFID together with the maximum allowable power and duty cycle (explained later in this chapter).

Table 1-1. International RFID Frequency Regulations
Country/Region	LF	HF	UHF	Microwave
United States	125134 KHz	13.56 MHz 10 watts effective radiated power (ERP)	902-928 MHz, 1 watt ERP or 4 watts ERP with a directional antenna with at least 50-channel hopping.	24002483.5 MHz, 4 watts, ERP 57255850 MHz, 4 watts ERP
Europe	125134 KHz	13.56 MHz	865865.5 MHz, 0.1 watts ERP, Listen Before Talk (LBT). 865.6867.6 MHz, 2 watts ERP, LBT. 867.6868 MHz, 0.5 watts ERP, LBT.	2.45 GHz
Japan	125134 KHz	13.56 MHz	Not allowed. MPHPT (Ministry of Public Management, Home Affairs, Posts and Telecommunications) has opened up 950956 MHz band for experimentation.	2.45 GHz
Singapore	125134 KHz	13.56 MHz	923925 MHz. 2 watts ERP.	2.45 GHz
China	125134 KHz	13.56 MHz	Not allowed. Future possibility: 840843 MHz and/or 917-925 MHz. SAC (Standardization Administration of China) is entrusted to formulate the RFID regulations.	24462454 MHz, 0.5 watts ERP

Table 1-2 lists RF properties of some example materials.

Radio waves are susceptible to interference from various sources, such as the following:

Weather conditions such as rain, snow, and other types of precipitation. However, as mentioned before, these are not an issue at LF and HF.
The presence of other radio sources such as cell phones, mobile radios, and so on.
Electrostatic discharge (ESD). ESD is a sudden flow of electrical current through a material that is an insulator under normal circumstances. If a large potential difference exists between the two points on the material, the atoms between these two points can become charged and conduct electric current.

The discussion now turns to how RFID technology works.

A radio device called a tag is attached to the object that needs to be identified. Unique identification data about this tagged object is stored on this tag. When such a tagged object is presented in front of a suitable RFID reader, the tag transmits this data to the reader (via the reader antenna). The reader then reads the data and has the capability to forward it over suitable communication channels, such as a network or a serial connection, to a software application running on a computer. This application can then use this unique data to identify the object presented to the reader. It can then perform a variety of actions such as updating the location information of this object in the database, sending an alert to the floor personnel, or completely ignoring it (if a duplicate read, for example).

As you can understand from this description, RFID is also a data-collection technology. However, this technology has some unique characteristics that enable users to apply it in areas beyond the reach of traditional data-collection technologies, such as bar codes.

An RFID application is implemented by an RFID system, which constitutes the entire technology end-to-end.