WiMax Operator's Manual

108 CHAPTER 5 ■ STRATEGIES FOR SUCCESSFUL DEPLOYMENT OF PHYSICAL INFRASTRUCTURES
throughput rates are apt to be half or less the theoretical maxima at the farthest transmission
distances.
In practice, only about a gigabit of capacity is likely to be reliably available, even assuming
that generous 200MHz spectrum allocation. If you settle on an oversubscription rate of four to
one, a fairly conservative figure, then the network can accommodate 400 subscribers at
a 10Mbps throughput rate for each albeit with no service guarantees. If one can charge appropriately—say,
$500 to $1,000 per month—that may be the basis of a sustainable business, at
least in the short run, though an annual revenue in the $2 million to $4 million range may not
permit the network operator to capitalize the expansion of the network.
Furthermore, in such a macrocellular architecture—absent the newer NLOS technologies—obstructions
will put many potential subscribers entirely out of reach. What percentage
of potential subscribers cannot be served is a matter of dispute among authorities, and in
fact the particular topography in which the network is being deployed can give rise to enormous
variations in this regard. By most accounts, at least 40 percent—and perhaps as many
as 80 percent—of potential subscribers are unlikely to be served because of the presence of
obstructions.
Faced with such constraints, network operators can resort to a number of ploys. They can
use repeaters to reach certain subscribers, a sort of partial cell-splitting approach. They can
employ sectoral antennas to achieve frequency use within the cell. They can employ dual
polarization to reuse spectrum. They can resort to full-on cell splitting and begin to create a
microcellular network. Or they can use NLOS technologies to access out-of-reach subscribers,
though only in the immediate vicinity of the central base station and not at the outer periphery
of the macrocell. They can also do all of these. But what they must do in all cases is to increase
spectral efficiency, that is, overall carrying capacity of the networks based on available spectral
resources.
Principles of Frequency Reuse and the
Technologies for Achieving It
In every wireless network with multiple users and limited spectrum, the network operator
is forced to confront two conflicting demands: the need for aggressive frequency reuse and
the need to mitigate interference. Put another way, a channel can only be reused when interference
is reduced, but reuse of a channel inevitably increases interference levels! Frequency
reuse conduces to an expanded customer base and maximum exploitation of available spectrum,
but unless the mitigation of interference can be achieved in some measure as well,
the quality of service available to the subscriber will be unacceptable. The pleasant irony
here is that the technologies that improve frequency reuse generally reduce the effects of
interference as well, but trade-offs are always involved; in other words, very pronounced frequency
reuse will increase interference, and minimizing interference will limit the degree of
frequency reuse possible.
Use of Repeaters
A repeater consists of an antenna and a simple radio transceiver without much intelligence
behind it. A repeater normally uses a simple point-to-point connection back to a remote base
station, which in turn communicates with a central office base station. A repeater has no