The Internet Encyclopedia (Volume 3)

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Ugweje WL040/Bidgolio-Vol I WL040-Sample.cls June 19, 2003 17:10 Char Count= 0

178 RADIOFREQUENCY ANDWIRELESSCOMMUNICATIONS

established the first successful and practical radio sys-

tem. His experiment in 1895 demonstrated the transmis-

sion of radio signals a distance of 2 kilometers (Proakis &

Salehi, 2002). He conducted additional experiments lead-

ing to 1901 when his radiotelegraph system transmitted

radio signals across the Atlantic Ocean, from England

to Newfoundland, about 1,700 miles away (Mobile Tele-

phone History, 2002). However, only telegraphic codes

were transmitted. On December 24, 1906, Reginald Fes-

senden accomplished the first radio communication of

human speech over a distance of 11 miles from Brant

Rock, Massachusetts, to ships in the Atlantic Ocean (Mo-

bile Telephone History, 2002). Radio was no longer lim-

ited to telegraph codes; it was no longer just a wireless

telegraph. This was a remarkable milestone highlighting

the beginning of the voice-transmitted age.

In the early years of RF wireless communication, radio

broadcasting was the most deployed wireless communi-

cation technology. The invention of the vacuum tube and

vacuum triode hastened the advancement in radio trans-

mission of voice signals. Radio broadcast by way of am-

plitude modulation and later frequency modulation (FM)

was made possible. Amplitude modulation of the radio

frequency was used to carry information until FM was in-

troduced in the late 1930s (Mark & Zhuang, 2003). After

FM was introduced, many other RF wireless systems such

as television, one- and two-way radio, and radar were in-

troduced between the late 1920s and the mid-1950s. An-

other milestone was witnessed in the late 1970s, which

marked the beginning of the growth in cellular mobile

radios and personal communication services. The first

successful commercial analog cellular mobile telephone

was demonstrated in 1979 (Durgin, 2003). Currently,

wireless communication of all kinds abounds in our

society.

System Architecture

In RF wireless communication systems, radio waves are

used to transfer information between a transmitter (Tx)

and a receiver (Rx). RF systems can be classified as ei-

ther terrestrial-based or space-based systems. Terrestrial-

based systems include microwave point-to-point, wireless

local area networks, and cellular mobile radio, just to

mention a few. Terrestrial microwave systems are limited

in distance and line-of-sight (LOS) propagation may be

required. Relay towers using carefully aligned directional

antennas are often used to provide an unobstructed path

over an extended distance. The data signal is processed,

up- or down-converted, modulated or demodulated, fil-

tered, and amplified at the transceivers. The transmitted

signal propagates through the air and is attenuated by

several propagation mechanisms discussed below.

Space-based systems (e.g., the satellite) are similar to

terrestrial microwave systems except that signals travel

from earth-based ground stations to a satellite (uplink)

and a signal is sent back from the satellite to another

earth-based ground station (downlink). This achieves a

far wider coverage area than the earth-based systems.

The satellite system could be in geostationary earth orbit,

medium earth orbit, or low earth orbit.

A typical wireless communication system is shown in

Figure 1. It consists of a source of information, a hardware

subsystem called the transmitter, the channel or means

by which the signal travels, another hardware subsystem

called the receiver, and a destination of the information

(the sink).

The source supplies the information to the transmit-

ter in the form of audio, video, data, or a combination

of the three. The Tx and Rx combination is used to con-

vert the signal into a form suitable for transmission and

IF RF

LO Filter High Power

Amplifier

Tx

Antenna

Amplifier

Oscillator

Data

in

Transmitter

Processes

RF IF

Low Power Filter LO

Amplifier

Rx

Antenna

Amplifier

Oscillator

Data

out

Receiver

Processes

Filter

PT, GT

Propagation Effects

(reflection, refraction, distortion, loss,

scattering, absorption, etc)

PR, GR

Source

Sink

Transmitter

Receiver

Channel

Figure 1: Simplified model of terrestrial-based RF wireless communication systems.