The Internet Encyclopedia (Volume 3)

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OVERVIEW OFRF WIRELESSCOMMUNICATION 181

Reflected

Wave

Direct Wave

Space Waves

Surface Wave

Troposphere

Ionospher

e

Earth Curvature

Figure 3: Common types of radio waves in wireless commu-

nication systems.

portion of the radio wave directly affected by terrain and

objects on the terrain. It is guided along the surface of the

earth, reflecting and scattering off buildings, vegetation,

hills, mountains, and other irregularities on the earth’s

surface. These waves propagate outward from the an-

tenna but undergo refraction due to variation in the den-

sity of the atmosphere (Garg & Wilkes, 1996). The signal

strength decreases as the distance between the Tx and the

Rx increases. This wave affects all frequencies in the MF,

HF, and VHF ranges, and it is the dominant wave in cellu-

lar mobile radio systems. Vertical polarization, the direc-

tion of the electric-field component, is best for this type of

wave. The polarization is determined by the construction

and orientation of the antenna.

Tropospheric and ionospheric waves are commonly re-

ferred to as sky waves. They propagate in outer space but

can return to earth by reflection or scattering either in

the troposphere or in the ionosphere. The tropospheric

wave is that portion of the radio wave close to the earth’s

surface as a result of gradual bending in the lower at-

mosphere (Garg & Wilkes, 1996). The bending action is

due to the changing effective dielectric constant of the

atmosphere through which the wave is passing. Its reflec-

tive index gradually decreases with height, resulting in a

bending path taken by the wave. The troposphere extends

about 10 miles above the surface of the earth and applies

to waves with wavelength shorter than 10 m; i.e.,λ<10 m.

The ionospheric wave is similar to the tropospheric wave

except that it travels farther and the reflection occurs in

the ionosphere, 40–400 miles above the earth. This wave

is highly reliable for telemetry, tracking, weather forecast-

ing, and tactical military applications. Note that different

wavelengths are reflected to dissimilar extents in the tro-

posphere and ionosphere.

Radio-Frequency-Based Systems

Figure 4 shows the different forms of RF-based wireless

communication systems, which we have classified into six

groups: microwave RF systems, fixed and mobile satellite

systems, wireless networks and protocols, personal com-

munication systems, remote sensing systems, and emerg-

ing technologies. No distinction is made between the

communication layers and protocols in this classification.

These systems transmit and receive radio waves tuned to

specific bands of frequencies. Microwave is loosely used

to describe all radio frequencies between 1 and 40 GHz.

This includes the UHF, SHF, and EHF systems. The lower

microwave frequencies, i.e., UHF, are most often used

for terrestrial-based RF systems, whereas the higher mi-

crowave frequencies, i.e., SHF and EHF, are used for

satellite communications. A terrestrial microwave system

transmits carefully focused beams of radio waves from a

transmitting antenna to a receiving antenna. A terrestrial

microwave system uses LOS propagation to communicate

between the Tx and the Rx with a typical distance of

30 miles between relay towers.

Personal communication services (PCS) are a new gen-

eration of wireless-phone technology that introduces a

wide range of features and services greater than those

available in analog and digital cellular phone systems (In-

ternational Engineering Consortium, 2003a). It includes

any system that provides people with access to informa-

tion services, such as cellular telephones, home-based sys-

tems (cordless telephones, remote control, short-range

two-way radio), beepers, pagers, and much more (Good-

man, 1997; Rappaport, 2002). PCS provides the user with

an all-in-one wireless phone, paging, messaging, and data

Wireless

Communication

Systems

Cellular Mobile

Telephone

Wireless

Networks &

Protocols

Home-based

Systems

Emerging Wireless

Technologies

Microwave

RF Systems

Remote

Sensing

Fixed & Mobile

Satellite

Personal

Communication

Systems

Wireless

LAN

Wireless Local

Loop

Wireless Application

Protocol

Bluetooth

Pagers

Beepers

Telemetry

Tracking

Weather

Forecast

Figure 4: Different forms of RF-based wireless communication systems.