WiMax Operator's Manual

CHAPTER 4 ■ SETTING UP PHYSICAL INFRASTRUCTURE 89
representing a region where the presence of physical obstructions will set up strong side lobe
reflections that may disturb the main beam.
The reflected signal may, as you have seen, be in phase or antiphase to the main beam, and
thus it may reinforce it or partially cancel it. One may think that reinforcements would be preferable
to cancellations, but in fact both are undesirable, especially so within the first Fresnel
zone (following the earlier definition) where strong early reflections of the side lobes occur.
The network operator simply has to plot a path that is free from major obstructions.
The distances involved in Fresnel zones are frequency dependent and are also a function
of the radiation patterns of the antenna used. It is best to consult with the manufacturer of the
subscriber terminals and base station equipment to determine the extent of the area above and
below line of sight that must be kept clear of obstructions. Most commercial programs for cell
site location include subprograms for Fresnel zone calculation, and Proxim, a leading manufacturer
of broadband wireless equipment, makes a well-regarded stand-alone program, so
one need not despair if one’s math skills are wanting.
If obstructions do stand between the base station and a valuable subscriber site such as an
office building or residence with multiple subscribers, one remedy may be simply to raise the
base station antenna on a mast at an elevation where it is well above any obstructions. Such a
tactic must not be regarded as a perfect solution, however, because an antenna that is too high
will not be able to reach subscribers in the immediate area of the antenna.
It should be further noted that not all obstructions are equivalent and that considerable
differences may exist among obstructions of the same general type.
A single tree for instance may impose around 15dB to 20dB of signal loss depending on
type and size. A grove of trees may up that figure to 30dB. A building may represent a total loss
of 30dB while a low hill could exceed 40dB. A truly interesting situation occurs when trees sway
in the wind. Momentary variations in loss may exceed 10dB, and generally one must design
around the worst case.
Bear in mind that the presence of obstructions within the first Fresnel zone does not preclude
the establishment of an airlink. It just means that more transmit power will be needed to
achieve the same signal integrity as in an unobstructed path.
RF Analysis
Once the site surveyor has been able to establish whether an airlink is obstructed, and how
severely, if obstructions exist, the time has come to evaluate the RF environment for that airlink
and determine whether it is ultimately useable. This happens with either a dedicated spectrum
analyzer or a specialized software program run over the actual data radio equipment.
All the major instrumentation companies—including HP/Agilent, Anritsu, Wiltek, and
Rohde & Schwartz—make dedicated spectrum analyzers, and most make handheld models
that can be managed by an individual obliged to mount a radio tower. These are precision
instruments capable of wideband tuning and of detecting activities in nearby bands that may
impact transmissions in the bands selected by the network operator. While not inexpensive,
they are designed for outdoor use in adverse weather conditions and will serve the operator
well over years of installation work. And if operators are unwilling to purchase such a device,
they can rent them, though I strongly recommend purchasing one since in a growing network
it will be in more or less continuous use. Also, if the adaptive array antennas are used, a special
type of spectrum analyzer known as a deep memory waveform generator will be required.
Agilent is the principal manufacturer of such devices.