WiMax Operator's Manual

110 CHAPTER 5 ■ STRATEGIES FOR SUCCESSFUL DEPLOYMENT OF PHYSICAL INFRASTRUCTURES
factor is two, and for six the factor is three, and so on. As indicated earlier, advanced modulation
techniques loosen such reuse constraints to some extent.
In very large cells, as you have seen, the narrow beams formed by sectoral antennas spread
out over distance, so actual frequency reuse capabilities will be reduced at the outer periphery.
Naturally the bigger the cell, the greater the spreading and the more the sectors will overlap.
Sectoral antennas can be used in both the lower microwave and millimeter microwave
regions, though the physical form of the antenna will differ with frequency, with horns and
waveguides being employed at the highest frequencies and arrays of spaced omnidirectional
pole antennas at the lower frequencies. Sectoral antennas of whatever type are considerably
more expensive than the omnis used in WLAN applications, but, given the vastly increased
spectral efficiency that they confer upon the network, the cost is trifling. Indeed, using such
devices has almost no downside, though they do concentrate the radiated energy, somewhat
increasing the potential for interference outside the cell. The cure for that is to polarize the
antennas in the horizontal plane so that only horizontally disposed magnetic fields are propagated.
The whole array is then tilted downward so that beyond a certain distance the radiation
will simply be absorbed into the ground. The following section explains polarization itself.
Polarization Diversity
The term polarization refers to a property common to all radio waves, namely that the magnetic
waves emanating from the antenna tend to propagate outward in a shallow ellipse. If
the antenna element is upright, the wave propagation will be in the horizontal plane, and if the
antenna itself is horizontal, a vertical propagation pattern will occur. In either case the wave
front is said to be polarized in one or another dimension—or linearly polarized, to use the technical
term. The third dimension, through which the airlinks extend, is occupied by wave fronts
in either state of polarization as they make their way toward the receiver site.
Both transmit and receive antennas are generally polarized in the same dimension, and if
they encounter signals of the opposite polarization, that is, offset by 90 percent, they will interact
with relatively few magnetic lines of force and will not develop signals of much strength.
The result is that signals of opposing polarization will not interfere with one another even if
they occupy the same channel. Signals of opposite polarization, incidentally, are said to be
orthogonal to one another.
In self-installs, particularly those involving indoor antennas, polarization is apt to be haphazard,
which argues against the use of an airlink based on simple linear polarization. One
solution is circular polarization, described next.
Now it is also possible to tilt antenna elements at intermediate angles and thus create a
multitude of polarization states, but in such cases, the various states will not be orthogonal
to one another and will interfere. Nevertheless, radios have been created that could resolve
several nonorthogonal polarization states and reuse spectrum very aggressively in this manner.
At this time, however, no such radio is available for broadband operators. What is available
are radios that offset two signals 45 degrees from the vertical so that both antenna elements
are tilted. The total separation is still 90 degrees and thus fully orthogonal, but the propagation
patterns tend to be more useful, though dual vertical and horizontal polarizations are
employed as well in broadband networks, such an arrangement being the aforementioned circular
polarization. Circular polarization is usually provided to a single radio, and its purpose,
as indicated previously, is to afford the best reception in a random polarization environment.
Figure 5-4 shows the various polarization states.