Sectorization

I have already touched upon sectorizing slightly. In this section I will attempt a more complete
explanation.
Sectorizing is using an array of highly directional antennas to direct intense radio frequency
(RF) energy into a designated area of the cell and little energy elsewhere. The sector
defined by the antenna array appears like a pie slice when depicted on a diagram. Sectorization
itself is a species of spatial diversity, of which adaptive beam steering is another. In both
cases the operator is able to define subchannels in three-dimensional space rather than by frequency
division or the use of sequential time slots. Figure 5-3 shows how a directional antenna
defines a sector by sweeping a narrow arc. The figure shows a directivity polar plot for such
an antenna. Sectorizing is somewhat akin to cell splitting and indeed may be viewed as a sort of poor
man’s cell splitting inasmuch as it allows the operator to reuse spectrum aggressively without
installing a new base station and defining a new cell. It is a standard tactic in the majority of
broadband wireless networks.
Sectoral antenna arrays vary according to the number of sectors they form, with three to
eight sectors being the usual range and four and six being the most common numbers. Obviously,
the more sectors, the narrower the beam width of each antenna in the array. Some arrays
are configurable to cover the whole range from three to eight, that is, the number of sectors
that can be made to vary by adjustments in the antenna itself. Since each antenna in a sectoral
array will be provided with a separate radio, sectorization definitely entails higher costs for the
network operator.
Sectors are akin to cells in that they ordinarily permit a channel to be reused one sector
away but not in intervening sectors. This means that when four sectors are present, the reuse 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.