Rules of Thumb in Planning

Macrocell performance can be improved by using adaptive downtilt to reduce interference
visibility, which in turn will reduce noise rise. However, the downtilt also
reduces the coverage footprint of the cell site. The other useful rule of thumb is to try
and position Node B sites close to the offered traffic to limit uplink and downlink code
power consumption. The effect is to increase cell range. Unfortunately, most sites are
chosen pragmatically by real estate site acquisition specialists and are not really in the
right place for a 3G network to deliver optimum performance.
Figure 11.17 shows how user geometry (how close users are to the Node B) determines
cell footprint. As users move closer to the cell center they absorb less downlink
code domain power. This means more code domain power is available for newcomers
so the cell footprint grows (right-hand circle). If existing users are relatively distant
from the Node B, they absorb more of the Node B’s code domain power and the cell
radius shrinks (left-hand circle).
Interference and noise rise can be reduced by using sectored antennas and arranging
receive nulls in a cloverleaf pattern. Typical Node B configuration might therefore
include, say, a single RF carrier omnidirectional antenna site for a lightly populated
rural area, a three-sector site for a semi-rural area (using 1 × RF carrier per sector), a
three-sector site configuration with two RF carriers per sector for urban coverage, or
alternatively, an eight-sector configuration with either one or two RF carriers per sector
for dense urban applications.

These configurations are very dependent on whether or not network operators are
allowed to share Node B transceivers, in which case four or more 5-MHz RF channels
would need to be made available per sector to give one RF channel per operator.