During the 400 days and nights before we finalized this book on January 1, 2012,
we saw the sunset decorated by the raindrop, and were accompanied by the flowers
blooming to withering. All those PAST memories are engraved on our hearts and will
last eternally, cementing the most profound friendship in the world.
This book is exclusively dedicated to WiMAX. The focus of the book is
on the applications of WiMAX networks, with delivery of multimedia
content to wireless and mobile devices being the area of greatest atten-
tion. WiMAX has crossed a number of major milestones in the recent
PAST.
Mobile and wireless application development has come a long way in the PAST few
years. It has progressed beyond the hype of wireless Web applications for consumers
to the reality of high-value mobile applications for corporate users. Opportunities
abound for creating new mobile and wireless applications that provide vital benefits to
any business. A sampling of these benefits includes increased worker productivity,
reduced processing costs, heightened accuracy, and competitive advantage. In contrast
is the concern that developing mobile and wireless applications will involve many new
technologies and concepts that many corporate developers are still learning to use.
Multi-carrier modulation? Orthogonal Frequency Division Multi-
plexing (OFDM) particularly? has been successfully applied to
a wide variety of digital communications applications over the PAST
several years. Although OFDM has been chosen as the physical layer
standard for a diversity of important systems? the theory? algorithms?
and implementation techniques remain subjects of current interest.
This is clear from the high volume of papers appearing in technical
journals and conferences.
Many wireless communications channels consist of multiple signal paths from the
transmitter to receiver. This multiplicity of paths leads to a phenomenon known
as multipath fading. The multiple paths are caused by the presence of objects in the
physical environment that, through the mechanisms of propagation, alter the path of
radiated energy. These objects are referred to as scatterers. In the PAST, researchers
often looked at ways to mitigate multipath scattering, such as in diversity systems.
Multiple-input, multiple-output (MIMO) systems, on the other hand, use multipath
diversity to their advantage; a MIMO system has the ability to translate increased
spatial diversity into increased channel capacity.
In the PAST few decades, a technological revolution has occurred that has changed
the way we live in dramatic ways. This technological revolution is the result of
the emergence and evolution of a wide variety of new wireless networking tech-
nologies. Now people using these technologies are able to access the network and
control many applications at will with their handheld devices anywhere, anytime.
Although these technologies have made a long lasting impact in the revolution, it
has also opened up various challenging issues which are yet to be resolved to make
them more efficient and cost-effective.
This book is a result of the recent rapid advances in two related technologies: com-
munications and computers. Over the PAST few decades, communication systems
have increased in complexity to the point where system design and performance
analysis can no longer be conducted without a significant level of computer sup-
port. Many of the communication systems of fifty years ago were either power or
noise limited. A significant degrading effect in many of these systems was thermal
noise, which was modeled using the additive Gaussian noise channel.
Over the PAST ten years there has been a revolution in the devel-
opment and acceptance of mobile products. In that period, cel-
lular telephony and consumer electronics have moved from the
realm of science fiction to everyday reality. Much of that revolu-
tion is unremarkable – we use wireless, in its broadest sense, for
TV remote controls, car keyfobs, travel tickets and credit card
transactions every day.
Driven by the desire to boost the quality of service of wireless systems closer to that afforded
by wireline systems, space-time processing for multiple-input multiple-output (MIMO)
wireless communications research has drawn remarkable interest in recent years. Excit-
ing theoretical advances, complemented by rapid transition of research results to industry
products and services, have created a vibrant and growing area that is already established
by all counts. This offers a good opportunity to reflect on key developments in the area
during the PAST decade and also outline emerging trends.
The serious study of the practice of how to determine the appropriate content of a
specification is a seldom-appreciated PASTime. Those who have the responsibility to
design a product would prefer a greater degree of freedom than permitted by the con-
tent of a specification. Many of those who would manage those who would design
a product would prefer to allocate all of the project funding and schedule to what
they consider more productive labor. These are the attitudes, of course, that doom a
project to defeat but they are hard to counter no matter how many times repeated by
design engineers and managers. A system engineer who has survived a few of these
experiences over a long career may retire and forget the PAST but we have an endur-
ing obligation to work toward changing these attitudes while trying to offer younger
system engineers a pathway toward a more sure success in requirements analysis and
specification publishing.
