The core functionality of an RFID system is the communication between a reader and a tag. The communication is carried out using RF waves, which are basically the EM waves with frequencies from the subspectrum of EM frequency spectrum called radio frequencies. The propagation of these waves is governed by the underlying physics principles. The goal of this chapter is to help you understand some physics concepts related to this communication. To accomplish this goal, we will explore three avenues: generation and propagation of the RF wave carrying the data signal from the source to the antenna, emission of the RF wave by the antenna into the free space, and propagation of the RF wave traveling through the space. Pay attention to the characteristics that affect the performance of an RFID system during this journey of the RF wave.

Understanding Radio Frequency Communication
Generally speaking, RFID is a means to identify an object using radio frequency transmission, which suggests that communication is involved in the identification process. The communication takes place between two devices: a reader that needs the information and a tag that has the information. Before we dive into the physics of communication, let's get on the same page about some concepts that are at the heart of this communication.

Elements of Radio Frequency Communication
Radio frequency communication uses the EM waves with frequencies from a specific part of the EM frequency spectrum. Therefore, the underlying physics behind RF communication is the same as for any communication that uses electromagnetic waves to carry information. The four major players that make this communication happen are the following:

• Data signal This is the wave that actually contains the information that needs to be sent to the receiver.
• Carrier signal This is the wave that carries the data signal.
• Modulation This is the process that encodes the data signal into the carrier signal and creates the radio wave that is actually transmitted by the antenna to propagate.
• Antenna This is a device used to transmit and receive signals such as radio waves.

Here is how these four players work together to make the communication happen.

First, understand that the information is communicated through changes (such as vibrations) in the carrier signal. The carrier signal itself is a constant signal unchanging in frequency and voltage--for example, a sine wave. It represents no information. As an analogy, I would not convey much information if I merely produced a constant sound out of my mouth, such as:

To convey some information, I would need to speak different sentences and different words in a sentence. In radio frequency communication, the information is encoded into the carrier signal using a technique called modulation, which means variation or change. You take the data signal that represent the information and impress it on a constant radio wave called a carrier. The data signal, as a result, varies (or modulates) the carrier wave. Once transmitted through an antenna, the two go together dancing over the air in the form of a modulated signal. The process of encoding the data signal into the carrier wave is called modulation. The transmitted modulated signal is received by the antenna on the receiving end and is demodulated to obtain the data signal. The process is depicted in Figure 1.

NOTE: In an RFID system, both the reader and the tag have their own antennas through which they communicate with each other. A tag is also called a transponder because it responds to the reader's attempt to read it, and the reader is also called a transceiver because it receives information from the tag.

That all sounds good. But note that the original data signal itself has information in it, which is represented by the changes inherent in the signal. So the question is: Why don't we transmit the original data signal, or why do we need modulation in the first place?

Modulation: Don't Leave Antenna Without It
There are several reasons for the use of modulation in communication. Discussing the following two will be sufficient for the scope of this book.

The Propagation Problem
A data signal generally comprises a whole range of different frequencies together. The problem with the low-frequency components of the signal is that few communication media will allow the propagation of low frequencies without distortion. Modulation presents the solution to this problem by copying these low-frequency components to high-frequency carrier waves.

The Transmission Problem
The low-frequency data signal will have a high wavelength and as a result will require very large antennas for transmission and reception. Here is the rule of thumb: To achieve a useful amount of radiation, the antenna length should be at least one quarter of the wavelength of the wave to be propagated. For example, consider a signal component with frequency of 1 KHz. The wavelength for this wave will be:

A 75-kilometer-high antenna (the Tower of Babylon)? You get the point. Modulation solves this problem by sending the low-frequency signal inside a high-frequency carrier wave.