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. The property of polarization can be exploited to reuse spectrum within the same space by
setting up airlinks of opposing polarity, a tactic known as polarization diversity. Reuse, however,
will reach only a factor of two by this means, so polarization must rank among the weaker
methods for enhancing spectrum reuse.
Dual polarization can be used in tandem with spatial diversity via sectorization or adaptive
arrays, but I know of no commercial product with such capability, though an experimental
system developed by Lucent was so enabled. In most systems one or another strategy is
adopted, that is, polarization diversity or spatial diversity.
In sum, polarization diversity is part of the network operator’s bag of tricks for extracting
the best performance from a particular radio in a particular RF environment. The aforementioned
EDX software has subprograms for plotting the effects of polarization diversity on
reception.