WiMax Operator's Manual

CHAPTER 5 ■ STRATEGIES FOR SUCCESSFUL DEPLOYMENT OF PHYSICAL INFRASTRUCTURES 107
network. If, for instance, a large business park full of 20 likely high-usage customers is situated
two miles from the nearest base station in a 5.8GHz network, one may want to consider building
a base station nearer to those businesses or at least securing location rights that will enable
one to do so in the future. One may be able to serve the single subscriber that one currently has
in that business park perfectly adequately from two miles away, but one may not be able to
serve five or ten. In other words, the mapping of the network must extend into the future and
must anticipate the eventual maturity of the network with the full complement of base stations
that will eventually have to be in place. Such projections will never be entirely accurate
because subscriber take rates, equipment advances, and changes in urban topography are
never completely predictable. One can make informed guesses as to growth and change in the
network, however, and then model a distribution of base stations based on sound engineering
principles.
A final general design principle is to provide oneself with choices in terms of base station
placement. In planning for the future one can never be absolutely certain that a desired site
will be available when one is ready to occupy it. A building where the owner has agreed to provide
roof rights may be sold. An antenna tower that has space today may be filled tomorrow.
If one does not have options at the time when expansion is indicated, the network may not be
able to reach potential customers.
Macrocells and Their Limitations
Because a cell is the basic constituent unit of a point-to-multipoint network, the distribution of
cells determines the coverage and capacity of the network and of course has a major bearing on
capital costs. Thus, cell size matters very much.
Network engineers tend to think of radio cells in terms of the areas they encompass. A
large cell with a radius of miles is known as a macrocell, while cells of a kilometer or two in
radius constitute microcells. Cells with radii measured in the hundreds of meters are picocells.
The usual pattern of broadband wireless operators, as indicated in Chapter 2, has been to
launch the network with a single base station defining a single cell and with sufficient transmitting
power to reach all or most of the potential customers within the metropolitan market
addressed by the network operator. Both low-frequency microwave and millimeter microwave
operators have used this type of single-cell architecture.
Lower-frequency microwave operators have been able build macrocells with radii exceeding
15 miles, and if they utilize sectoral antennas—actually clusters of high-gain antennas
distributed around a central mounting pole—they can reuse the available spectrum as much
as four times over in theory, eight being the maximum number of sectors that is practical with
conventional nonadaptive antennas and four being the maximum reuse factor within a single
cell. It would appear to follow, then, that if one starts with a generous frequency allocation—
say, the nearly 200 megahertz (MHz) available in the Multichannel Multipoint Distribution
Service (MMDS) bands—and multiplies that by four and assumes a three-bit-per-hertz ratio,
then one is looking at minimally a couple of gigabits per second total system throughput, a rate
that translates into quite a respectable system capacity.
None of these theoretical maxima can be realized in practice, however, and they cannot
even be approached. The narrow beams that define each sector will spread out over distance
and begin to interfere with one another at the outer edge of the macrocell so that all eight
sectors cannot be extended the full width of the cell. Then too, bit rates drop off with distance
as the signal attenuates, the fade margin declines, and the error rate ascends. In fact, real