An Introduction to Wireless Technology, Second Edition

Chapter 2. Wireless Methodologies

This chapter discusses some of the basic physics associated with wireless

transmission techniques and goes on to describe how the technology is

implemented in various applications. This description includes a more in-depth

look at some of the networking technologies mentioned in Chapter 1,

“Introduction” on page 1.

2.1 Radio Frequency Characteristics

The characteristics of radio waves in any frequency band determine how useful

those frequencies are for the service required. The main characteristic of

interest is how signals are changed or distorted, by absorption and reflection, by

the air and other physical media before reaching the receiver. Since a radio

signal is a particular form of energy, it is useful to consider the different forms of

energy and how they can be converted and transmitted.

2.1.1 General Aspects

In order to understand some of the properties of radio and infrared propagation

it is important that the underlying principles or the physics of electromagnetic

waves be defined. A good starting point is a discussion of energy and the ways

it can be transmitted. Energy is defined in physics as the result of multiplying

power by time and may take a number of different forms (such as sound,

electrical, kinetic, potential, chemical, heat, and nuclear energy). A fundamental

principle of physics is that of the conservation of energy - energy can be neither

gained nor lost, only converted from one form to another. When we talk about

generating a particular form of energy, say electricity, what we are really doing

is changing one form of energy (heat to electricity) in a power generating station.

Some forms of energy are mechanical in nature, for example, kinetic energy. An

object is said to have kinetic energy by virtue of its motion. A moving vehicle

has kinetic energy and this has been converted from chemical energy in the

gasoline and air used to power the engine. If the vehicle is driven to the top of a

hill, it is said to have gained potential energy. The energy is stored in the

vehicle and can be released by coasting down the other side of the hill thus

becoming kinetic energy again. If the vehicle is brought to a halt, it will lose its

kinetic energy, which will be converted to heat by raising the temperature of the

brakes. Whenever one form of energy is converted to another, the process may

not be very efficient in changing the energy into the form desired. The internal

combustion engine moving our vehicle will convert some of the chemical energy

into kinetic energy, but quite a lot will be converted to heat. This energy is not

lost, it is just not in a form which is useful in moving the vehicle. Even this

wasted heat is welcome on a cold day!

A special form of potential energy is called elastic potential energy. An example

of this is a clock spring which can store the kinetic energy used to wind it and

release it again as kinetic energy to drive the clock.

A special form of kinetic energy is heat. The temperature of an object is due to

minute vibrations of its molecules. As an object cools, it will transmit its heat

energy to its surroundings, air and other objects, by means of infrared radiation

and conduction. Eventually the object and its surroundings will all have the

same temperature and no energy will be transferred. This is known as entropy,

 Copyright IBM Corp. 1994, 1995 21