The Internet Encyclopedia (Volume 3)

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RADIOWAVEPROPAGATION 183

Internet standards such as the Internet protocol (IP),

extensible markup language (XML), hypertext markup

language (HTML), and the hypertext transfer protocol

(HTTP) and is designed to work with all wireless network

technology. More information can be obtained from the

WAP Forum(Wireless Application Protocol, 2000) and in

the chapter on WAP in this encyclopedia.

Bluetooth is a wireless technology that makes possi-

ble connectivity to the Internet from mobile computers,

mobile phones, and portable handheld devices without

the need for cable connections. It facilitates fast and se-

cure transmission of both voice and data, without LOS

propagation. Some characteristics of Bluetooth technol-

ogy are summarized in Table 2. Detailed information on

Bluetooth can be found in another chapter in this ency-

clopedia.

Satellite communication is one of the traditional RF

wireless communication systems. Signals can be trans-

mitted directly from a ground station (GS) or gateway

on earth to a satellite, and back to another GS. Some-

times, the signal can be routed through another satellite

(intersatellite) before it is transmitted back to the GS. We

can identify a satellite system by how far the satellite is

from the earth. The closer the satellite is to the earth, the

shorter the time it takes to send signals to the satellite.

There are three satellite orbits, namely, low earth orbit

(LEO), medium earth orbit (MEO), and geosynchronous

earth orbit (GEO).

LEO satellites are closest to the earth, beginning about

100 miles above the surface, and only take a couple of

hours to circle the earth. Because LEO systems are orbit-

ing so quickly, multiple satellites are required to provide

constant coverage in one location. LEO systems have the

capability to receive calls from the earth and pass them

to an earth-based switching system in much shorter time

than other satellites. However, because of the speed of the

satellite, it is frequently necessary to handoff a particular

call to a second satellite just rising over the horizon. This

is similar to a cellular mobile radio system (discussed be-

low), except that in this case it is the cell site (the satellite)

that is moving rather than the user. The lower orbit has

the advantage of allowing access to very low-power de-

vices (Printchard, 1993). LEO satellites are used mainly

for wireless transfer of electronic mail, pager systems,

worldwide mobile telephony, spying, remote sensing, and

video conferencing.

GEO satellites circle the earth at a height of 22,300

miles, orbiting at the same rate as the earth rotates so

that they appear stationary from the earth’s perspective.

Most GEO satellites rely on passive bent-pipe architec-

ture so that they receive signals from transceivers on

earth, amplify them, and send them back to specific re-

gions on earth. GEO systems are used for a wide array

of services including television broadcasts, long-distance

telecommunications, and various scientific and military

applications. GEO satellites are well suited to transmit-

ting data, but may be undesirable for voice communi-

cations because of the long propagation delay. It takes

about one-fourth of a second for a signal to travel from

a terrestrial GS to the satellite and back. If the receiver

GS replies, it takes another one-fourth of a second, re-

sulting in a total of half a second (Printchard, 1993). This

is an unacceptably long delay for voice communication.

Hence, voice communications are seldom carried via GEO

satellites.

MEO satellites can be found between 1,000 and 22,300

miles and are mainly used for global positioning and nav-

igation systems. MEO satellites are not as popular as the

LEO or GEO for reasons beyond the scope of this paper.

New wireless or cellular mobile radio technologies are

classified under emerging wireless technologies. These are

technologies currently under research and development

or technologies that are undergoing field tests. In short,

these technologies are not widely deployed. These include

the third generation (3G) technologies and the forth gen-

eration (4G) technologies. The goal of these technologies

is to seamlessly integrate a wide variety of communication

services such as high-speed data, video, and multimedia

traffic as well as voice signals. Some of these technologies

can be realized by combining existing technologies. For

example, one of the most promising approaches to 3G is

to combine a wideband code division multiple access air

interface with the fixed network of a global system for

mobile communications (GSM). It is expected that these

new technologies will increase the performance of the ex-

isting wireless systems. These technologies will provide

multimedia capability at much higher rates with Internet

connectivity.

RADIO WAVE PROPAGATION

Propagation is the process of wave motion, which is very

important in the design and operation of RF systems.

Because the received signal is always different from the

transmitted signal, due to various propagation impair-

ments, and because of the nature of the propagation it-

self, it is necessary to understand the properties of radio

wave propagation. This is most important in telecommu-

nication applications in predicting the transmission char-

acteristics of the channel. When radio waves are radiated

from an antenna, propagation is governed by the follow-

ing mechanisms.

Free Space Propagation

This is the ideal propagation mechanism when the Tx and

the Rx have direct LOS and are separated by a distanced

between the Tx and the Rx. IfPtis the transmitted power,

the received powerPr, a function of distanced, is given by

Pr(d)=PtGtGr

λ^2

(^4 πd)^2 L

=PtAetAer

1

(λd)^2 L

(1)

whereAe,G, andLare the effective area of antenna, an-

tenna gain, and system loss factor, respectively. The sub-

scripts “t” and “r” refer to the transmitter and receiver

respectively. From this relationship, we observe that the

received power diminishes at the rate of 20 dB/decade

as the distance increases. The productPtGtis defined as

EIRP, introduced earlier; i.e.,EIRP=PtGt.

Reflection

When a radio wave strikes an object with dimensions

very large compared to its wavelength, reflection occurs.