WiMax Operator's Manual

CHAPTER 5 ■ STRATEGIES FOR SUCCESSFUL DEPLOYMENT OF PHYSICAL INFRASTRUCTURES 113
of the industry has long sought some means of getting around or through obstructions. The
collective term for the technologies for doing so is non–line of sight (NLOS).
NLOS is a term used rather loosely in the industry—too loosely in my opinion. The term
applies to a number of distinct technologies of varying capabilities and maturity, and the performance
of so-called NLOS equipment in the field is far from uniform. While manufacturers
that have appropriated the NLOS nomenclature are fond of claiming that their respective
products will double the effective coverage of a network, and in some cases can frame fairly
plausible arguments based on enhanced link budgets, such claims must always rest upon certain
assumptions regarding the distribution of obstructions within the locale in question,
assumptions that may not necessarily hold true in individual instances. Thus, no claim should
be taken at face value, and any radio that has to be used in a NLOS setting should be thoroughly
tested in the environment in which it is to be used before it is purchased.
NLOS, as I will use the term, refers to any technique for lessening the effects of physical
obstructions, and the emphasis is on the word lessening, because no NLOS technique can
entirely eliminate the effects of blockage. The success of the technique is directly measurable
in terms of the signal strength at the radio front end, the block of circuitry where actual demodulation
of the signal begins. NLOS is never an all-or-nothing proposition. It is a matter of
greater or lesser signal strength—it is that simple.
Most of the manufacturers that have survived in the public broadband wireless arena now
claim to manufacture NLOS equipment. Within the industry overall, a near consensus has
been reached on the matter, and that consensus is that networks of any size operating in the
lower microwave regions are not viable without NLOS equipment. I am inclined to accept this
consensus on purely empirical grounds, leaving aside precise definitions of just what NLOS is,
because, simply stated, the attrition rate among network operators attempting to use strict
line-of-sight first-generation equipment has been horrendous. Obviously, other factors are at
work as well, but the inability of operators to reach customers without line of sight to a base
station limits absolutely the size of the subscriber market. And that this limitation may be such
as to rule out half of the potential customers in a network of one kilometer radius cells does
much to explain the failure of so many of the pioneers.
What NLOS Means in Terms of Wave Propagation
The object of this book is to go light on RF theory and heavy on practicality. However, to understand
even the rudiments of NLOS, one must know something about how physical objects
affect radio wave propagation.
When a radio wave encounters a physical object—and by that I mean anything from an air
molecule to a mountain—it can behave in one of three ways. It can give up some of its energy
to the object in the form of heat, a process known as absorption. It can rebound from the object
without surrendering appreciable energy to it, a process called reflection. Or it can bend
around the object, a process known as diffraction. These three processes, by the way, are
not mutually exclusive. A reflected signal, for instance, may immediately be diffracted as it
encounters a different contour of the object reflecting it, and in every case where reflection or
diffraction occurs, some energy will be absorbed as well.
Absorption
Pure absorption does not change the direction of the radio wave but robs it of energy and
reduces the fade margin. Since the signal steadily loses energy simply by being propagated