Communication, a word that many associate with modern technology, actually
has nothing to do with technology. At its core, communication involves nothing
more than the spoken or written word, and symbolic languages like art and music.
Technology has become synonymous with communication because technology
has historically been the method by which communication to or by the general
population takes place. From the printing press to the telephone to radio and TV
broadcasting, technology has touched our lives by providing convenient ways for
a large population to communicate. Because the intertwining of technology and
communication is fundamental to our culture, the technology of communication
in a way defines our culture.
As we enter the early years of the 21st century, humanity is awash in instant communication
based upon the radio technology that makes it possible. As a society
we have near real time access to world events occurring in any corner of the
globe, and as individuals we have instant voice communication with each other by
virtue of the telephone in its many shapes and forms.
Over the last decade of the 20th century, the cell phone redefined our cultural
expectations of communication. The advent of the portable phone along with price
competition among hardware and service providers has brought true personal
voice communication service to a large segment of humanity.
But what of symbolic communication? Computing power and flexibility have
allowed us to digitize these communications to make their dissemination more
convenient, and the portable phone has set an expectation that all communication
requires portability, convenience, and cost effectivity. Unfortunately, these
voice-centric systems have only marginally addressed the more complex nature
of symbolic communications. Moreover, as a society we are no longer content
with simplistic communication. The advent and adoption of the computer and the
myriad software packages available for it has offered the ability to generate a new
wave of symbolic communication combining art, pictures, music, and words into
a targeted multimedia presentation. No longer is a generic presentation enough. It
is now so easy to tailor the presentation of information to an individual or group
that audience-tailored and targeted multimedia presentations are now expected.
By their nature, these presentations are large and require high bandwidth transmission
facilities to accommodate their rapid dissemination. Such facilities are
available within a wired office LAN or to some extent in the wired telephony network
(and by extension the Internet), but these facilities only serve a segmented
local community. The user who is not connected to a wired broadband facility
cannot gain access to this communication.
While there are several low speed portable data systems operating today, their
speed makes them useful for only the most rudimentary of communication: short
written messages like email, or small low resolution images.
The growing volume of targeted multimedia presentation material requires a high
bandwidth delivery facility. Couple this with our society’s need for mobility, and
you quickly realize that currently available ubiquitous coverage wireless data
delivery solutions fall far short of the bandwidth required by this emerging communication
Enter Wi-Fi (Wireless Fidelity). Late 2002 through 2003 has seen a remarkable
interest in 802.11 (a.k.a.: Wi-Fi) network deployment. The 802.11 standard is a
wireless Ethernet standard that was designed to simplify office LAN deployment
by eliminating wiring requirements. Interestingly, with the advent of Broadband
Internet connectivity in the home, this technology has found its niche not in the
office, but in the home. Wi-Fi capability can be added to a computer or PDA by
simply plugging in a card. Laptop computers now come with Wi-Fi functionality
preinstalled, and consumer quality Wi-Fi base stations cost less than $100, and are
becoming easy to install. Wi-Fi has become the equivalent of a cordless phone for
your computer, and just as the advent of the cordless phone presaged the development
and consumer acceptance of cellular and PCS services, the adoption of
Wi-Fi may be giving us the early glimpse of the needs and expectations of the
next generation of wireless data consumers.
The deployment of Wi-Fi and other wireless data delivery technologies have not
stopped at the home and the office. The fact that these devices operate in unlicensed
spectrum is allowing individuals and companies to take the next steps in
deploying area wide wireless data networks. Today more and more systems are
being deployed to provide public Internet access in public areas as small as a coffee
shop (the “hotspot model”) or as large as a community (the “WISP” model).
Some companies are implementing these systems in order to provide ”for sale”
service, while other networks are being implemented by individuals, companies,
or groups to offer free Internet access to those who enter the covered area.
By late 2003, the first 802.16-based equipment began to enter the marketplace.
The 802.16 standard is designed as a next generation broadband data delivery
system for Metropolitan Area Networks (MANs). 802.16 overcomes many of the
shortcomings of 802.11 when used in a MAN environment, and can operate in
licensed and unlicensed bands from 2 GHz to 60 GHz. The additional spectrum,
bandwidth and throughput capability of 802.16 will markedly improve wireless
data delivery, and should allow even more wireless data service areas to be deployed
The initial 802.16 specification effectively offers a solution for providing high
speed wireless communication to fixed locations, but it still does not offer true
mobility. The 802.16e and 802.20 standards are being developed as next generation
solutions that can offer true high speed (over 2 Mbps) connectivity to vehicles
traveling at over 90 MPH.
The successful development and growth of any high-speed wireless data network,
be it standards-based or a proprietary solution, requires not only an understanding
of the data network requirements of the system but also, maybe more importantly,
an understanding of RF propagation and interference management. There
is a significant volume of published information that describes the fundamentals
of wireless LANs and 802.11 technology and/or the networking needs of such a
system, but little information is available regarding the radio propagation issues surrounding
the deployment and management of any of these RF-based technologies.
This book is intended to correct this information gap by providing a basic understanding
of the issues surrounding the implementation of RF-based technologies.
It will be useful to anyone who plans to implement a wireless network. The
concepts in the book are equally valid for systems using licensed or unlicensed
spectrum, and apply equally to any technology selected for implementing the
wireless network. It is written for an audience that has limited or no RF experience,
and will offer the reader a basic but practical understanding of the concepts
behind RF, such as transmitting and receiving, antennas and their effect and use,
RF propagation characteristics, interference management, and regulatory issues.
These concepts will provide the underpinnings for the later chapters, which will
focus on the real-world issues of designing, implementing, and optimizing a wireless
This book will provide a solid general understanding of the issues encountered in
designing and deploying a wireless network, and should help you gain the ability to
effectively plan and construct a network either by yourself or with the help of others.
The primary audience for this book is the technical professional responsible for
deploying a wireless data technology. This audience can be as diverse as the IT
professional who now has to cope with adding wireless connectivity to an office
LAN; the manufacturer, or Value Added Reseller (VAR) who is selling wireless
hardware solutions; the individual, group or community that wishes to learn
enough to be able to effectively deploy this technology to improve the services
available to an area or community, or the engineering professionals that will eventually
lead the development of the ubiquitous carrier class wireless data services
that will someday become to data what cellular and PCS have become to voice.
The secondary audience is the management, sales, marketing, planning, or accounting
professional and the investment community that supports development
of new technologies. The content of this book will be valuable to anyone who is
responsible for the decision to adopt these new wireless technologies as part of a
business plan, and is now facing the problem of not having the appropriate level
of knowledge to “ask the right questions.”
It is my hope that this book will educate and inspire a new generation of wireless
pioneers, and in doing so assure a bright future for wireless data services. I look
forward to the day when our cell phone evolves into a pocket-sized device that
will fulfill all of our communication needs, and allow our words and visions to
instantly reach the farthest corners of the globe.
Chapter 3 Propagation, Path Loss,
Fading and Link Budgets
Understanding how radio waves propagate through space is critical to the design
of any radio-based network. Radio is an electromagnetic wave whose propagation
is affected by many variables. Frequency, distance, terrain, objects in the wave’s
path, and reflections all have an effect on the power of the wave at any point in
space. Because of the myriad of variables affecting the wave, it is impossible to
know with certainty the exact signal strength the wave will have at a particular
point in space. Statistics plays a big role in understanding and defining the “average”
behavior of the wave in an environment.
The frequencies we are interested in are those above 700 MHz, since this is where
adequate amounts of spectrum have been assigned to support high bandwidth
systems. The upper limit to spectrum useful for a non-line of sight communication
path is around 6 GHz. Above this frequency, radio waves behave more like light,
and no longer refract around objects in the path. Frequencies in the 10 to 70 GHz
bands are very useful for building point-to-point communication links that can be
used to connect communications sites back to a hub location for traffic aggregation.
They are also useful for extending high bandwidth connections from one
location to another.
Radio waves, like light waves, get weaker with distance. The attenuation associated
with distance in an unobstructed path is called Free Space Loss (FSL). FSL is
mathematically calculable by the formula 20Log10(Frequency in MHz) + 20Log10
(Distance in Miles) + 36.6, because it is the result of the spreading of the wave as
it propagates away from its source. The attenuation is also frequency dependent.
The higher the frequency, the more attenuation will occur over a given distance.
As you see in Figure 3-1, the loss change follows a 6 dB per octave (a doubling
C H A P T E R 3