WiMax Operator's Manual

104 CHAPTER 5 ■ STRATEGIES FOR SUCCESSFUL DEPLOYMENT OF PHYSICAL INFRASTRUCTURES
I point out, however, that the cells deployed within broadband wireless networks are usually
larger than those in cellular telephone networks.
A Note on Channel Assignments from One Cell to Another Concerning this matter of frequency reuse,
it should be understood that channels, according to standard engineering practice, cannot be
reused from cell to cell because a signal fades only gradually over distance and will not be sufficiently
attenuated so as not to cause grave interference if another user tries to occupy the
same channel in an adjacent cell. Therefore, a channel would normally be reused only one cell
diameter away at best. Figure 5-1 shows frequency reuse patterns in a cellular network; for
simplicity’s sake, it displays only four channels. The exception would occur when the cell was
divided into sectors, which are discussed next.
Figure 5-1. Frequency reuse within a cellular architecture
Today this standard engineering practice has been subject to some modification because
of the appearance of advanced modulation techniques such as direct sequence, Code-Division
Multiple Access (CDMA), and orthogonal frequency division modulation (OFDM); the introduction
of polarization diversity; and the emergence of smart antenna technology. All these
techniques, described in detail later in this chapter, increase the immunity of a transmission
from interference and allow reuse of a channel at reduced distances—in other words, less than
one cell diameter away. Reuse, at least within certain sectors in adjacent cells, does then in fact
become possible, though certain minimum distances still have to be maintained. Coincidentally,
simple rules of thumb for channel spacing become increasingly difficult to formulate.
As it happens, numerous schemes and formulae have been developed for cellular telephone
networks for optimizing frequency reuse patterns, but all of these, understandably, are
optimized for mobility and must take into account the fact that a subscriber terminal must be
able to “see” two base stations simultaneously at the boundary of a cell in order to initiate a
handoff. As a result, such techniques cannot be imported into the broadband fixed wireless
arena without extensive modification. Considerably less attention has been given to maximizing
frequency reuse in fixed broadband networks through improved calculations for base
station placement or through new protocols for dynamic channel assignment.
Part of the problem has to do with the ad hoc nature of most fixed wireless broadband
networks to date. Typically, wireless broadband networks have been the product of startup
companies with limited resources that have tended to add capacity as needed with no overall
plan of what a fully loaded network would look like and with no staff with either the training,
inclination, or mandate to refine the formulae used by cellular network engineers to perform
cell mapping and base station distribution. The tendency instead has been to rely on relatively
crude cell-splitting techniques to accommodate increased traffic and, ironically, on ultrasophisticated
adaptive modulation and beam steering technologies for the same purpose,
neither of which, incidentally, require much engineering skill on the part of the network