The Internet Encyclopedia (Volume 3)

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Shankar WL040/Bidgolio-Vol I WL040-Sample.cls June 19, 2003 16:49 Char Count= 0

Propagation Characteristics of Wireless ChannelsPropagation Characteristics of Wireless Channels

P. M. Shankar,Drexel University

Introduction 124

Propagation of Signals 125

Transmission Loss 125

Signal Variability and Fading 129

Optical Wireless Systems 132

Concluding Remarks 132

Appendix: Power Units 133

Glossary 133

Cross References 133

References 133

INTRODUCTION

Wireless systems encompass a wide range of information

transmission mechanisms, including cordless phones,

paging systems, cell phones, satellite communication sys-

tems, maritime-mobile systems, industrial and medical

monitoring systems, infrared (IR) remote controls, and

so on. These systems have unique operating frequency

bands. The choice of the frequencies is often determined

by the range of operation (a few meters for a cordless

phone to thousands of kilometers for satellite commu-

nication systems), the medium through which the signal

traverses (e.g., urban areas with tall structures, vast empty

spaces in rural areas, and multilevel, multistructure envi-

ronments in factories and malls), the amount of data to

be transmitted (low volume of data in maritime mobile

systems and IR-based remote control systems to Gbits/s

in satellite systems). The other factors that are equally

important are the size of the transmitting and receiving

antennas and the cost and maintenance of installations.

A broad classification of various frequencies used in wire-

less communications is shown in Table 1; abbreviation in

table are used subsequently in text.

Two important considerations of communication sys-

tems must be kept in mind:

1. The physical dimensions of an antenna are inversely
   proportional to its frequency. Thus, at low frequencies,
   the size of the antenna may be too large for these fre-
   quencies to be used in mobile systems. For example, at
   30 kHz, the wavelength is 10,000 m:

[

wavelength=

velocity

frequency

=

3 × 108

30 × 103

=10,000 m

]

.

Note that the required sizes of the antennas will be

on the order of a half wavelength. This means that for

transmission at 30 kHz, we would require an antenna

of length 5,000 m.

2. The amount of data that can be transmitted is directly
   proportional to the frequency. The data rates at 30 kHz
   will be less than 30 kbps, whereas the data rates at
   1 GHz can be on the order of 1 Gbps. The actual data
   rates will be determined by a number of factors such as
   the transmission distance, the characteristics of the in-
   tervening medium, and the modulation–demodulation
   schemes.

Once we take these considerations into account, it is

clear that the very low frequencies can be eliminated from

the range of operation of the practical wireless communi-

cation systems, which demand transmission at very high

data rates, portability (i.e., mobile systems), and compact-

size. The large size of antennas and low data rates asso-

ciated with low frequencies eliminate them from consid-

erations in modern wireless communication systems and

networks. Even though the frequencies on the higher end

of the spectrum (Table 1) appear to offer smaller size an-

tennae and higher data rates, their use in wireless systems

is limited by other factors. The primary reason for their

unsuitability is that these frequencies are severely atten-

uated by “obstructions” in their path. A case of a direct

path (line of sight [LOS]) between the transmitter and a

receiver is shown in Figure 1.

In a typical urban environment containing tall build-

ings and other structures, these signals (EHF) will be

unable to penetrate and reach the transmitter (Bertoni,

Honcharenko, Maciel, & Xia, 1994; Jakes, 1974; Pardo,

Cernicharo, & Serabyn, 2001; Parsons, 1996; Saunders,

1999; Steele & Hanzo, 1999). They require a clear line of

sight between the transmitter and receiver, making them

well suited for satellite-to-ground (or ground-to-satellite)

transmission or satellite-to-satellite transmission. They

may also be used in very specific applications involv-

ing short-range transmission of a large volume of data

in confined spaces with no obstructions. Thus, it is pos-

sible to narrow down the choice of the frequencies for

practical wireless systems based on the additional con-

sideration of the manner in which the signals reach the

receiver from the transmitter. Frequencies that can pen-

etrate buildings, be reflected, refracted, diffracted, and

scattered from buildings, trees, and still reach the receiver

must be available. The important considerations for the

choice of frequencies can now be restated as follows:

1. Size of the antenna


2. Ability and convenience to mount the antenna on
   portable units, moving vehicles, and so on


3. Ability to reach the receiver even when a LOS path is
   not available


4. Data rates

These important factors narrow the frequencies to UHF

or typically in the range of 900 MHz to 3 GHz. Additional

factors that affect the performance of the wireless systems

will be discussed later.

124