WiMax Operator's Manual

88 CHAPTER 4 ■ SETTING UP PHYSICAL INFRASTRUCTURE
provide the network operator with baseline information, however, and as such it is an indispensable
part of the planning process.
A network operator can elect to use internal staff to perform the site survey or use outside
consultants. In either case prior experience is highly desirable. At a minimum the individuals
performing the survey must be familiar with the basic concepts discussed in the next section.
While the survey is under way, all observations and readings should be carefully and completely
recorded, and the final report should be kept in a secure place where it can be consulted
by responsible individuals in the organization as the need arises. The report should indicate
the exact locations of all network nodes, including elevations. Using the Global Positioning
System (GPS) coordinates to record positions is an excellent idea.
Determining Line of Sight and Computing Fresnel Zones
The site survey begins with visual observations, and the first task is to attempt to establish clear
line of sight from the subscriber location to the base station; the assumption here is that right
of way has already been granted for antenna placement in both locations. The process is quite
straightforward and involves nothing more than standing at the level of the transceiver
antenna in one of the locations and training a pair of binoculars on the other antenna site. If
a clear path lies between the two points with several yards separating the nearest obstruction
from the imaginary line connecting the two points, then one can assume that line-of-sight preconditions
have been met.
Another method available in the United States, though not always elsewhere, is to purchase
three-dimensional aerial maps of the area one wants to serve. These provide elevations
of all buildings and natural obstructions such as hills and bluffs. Arizona-based AirPhotoUSA is
a vendor that has served many wireless operators.
If obstructions directly cross the imaginary line, then one obviously lacks clear line of
sight. If obstructions nearly cross that line without quite impinging upon it, then one must proceed
to the next phase, the calculation of what is known as the Fresnel zone.
By strict definition, Fresnel zones consist of an infinite series of concentric rings surrounding
the nodal point of transmission, with each ring defined by the phase relationship between
the main beam of the transmitter and the two dominant side lobes (these terms are explained
shortly). The phase fluctuates from one zone to the next so that side lobe reflections are sometimes
in phase with the main beam and sometimes antiphase to it.
So how do such side lobes arise? Directional antennas of the sort generally employed in
broadband wireless networks tend to focus energy into narrow beams, but they never produce
just one beam. Rather, they produce a dominant central beam and two or more side lobes that
contain less energy than the main beam. They also produce low-energy rear beams. Depending
on the polarization of the transmitter, the so-called side lobes may be above or below the
main beam or on either side of it.
Now, even when the direct signal has a clear path to the receiver antenna, these side lobes
may encounter obstructions since they are offset from the main beam and describe different
propagation paths. When they do strike obstructions, they will be at least partially reflected,
and the reflections may impinge upon the main lobe, either reinforcing or canceling the signal
depending on the phase relationship of the two.
Here an element of confusion enters the discussion because field technicians tend to
employ a different definition of Fresnel zones. By this second definition, a Fresnel zone refers
not to a concentric ring but to an elliptical volume surrounding the imaginary line of sight and