Page 61 - Antennas for Base Stations in Wireless Communications

34 Chapter Two

shadowing problems. Severe shadowing problems are better resolved by
changing the location or height of the antenna; where a higher antenna
causes problems with frequency re-use, adding a microcell to cover the
unserved coverage hole may be a better option.
Reducing the elevation beamwidth and selecting the elevation angle
of the beam maximum provide further methods for controlling the field
strength over the intended service area of a cell. If a base station antenna
has the maximum of its elevation radiation pattern aligned precisely in
the horizontal plane, in many situations the elevation of the antenna
over the surrounding area—and at longer distances the curvature of the
earth—will ensure the maximum signal will pass over the heads of most
users. By directing the beam slightly downward, the field strength for
most users within the intended coverage area will be increased and at
the same time the power radiated into neighboring cells will be reduced,
significantly improving the C/I ratio in neighboring cells that share the
same frequency. The effect may be compared with dipping the headlights
of an automobile to avoid dazzling approaching drivers.
The elevation pattern of a column of uniformly excited radiating ele-
ments is characterized by a succession of minor lobes and nulls both
above and below the main beam. Nulls below the main beam—especially
the null closest to it—can cause areas of poor coverage close to the BS;
side lobes immediately above the main beam can cause interference
with neighboring cells if the main beam is downtilted or if the terrain
rises between one BS and another. Elevation pattern shaping is com-
monly used to fill at least the first null below the main beam and to
suppress the level of sidelobes for some chosen range of elevation angles
above the main beam.
In order to serve street-level users from antennas mounted on the
roofs of nearby high buildings, antennas with large elevation beam-
widths (and therefore with low gain) may be desirable. In this situa-
tion many users are typically located well below the horizontal as seen
from the BS antenna, and large beamtilts may be used both to correctly
illuminate the intended users as well as to reduce interference levels
in surrounding cells.
Figure 2.1 shows one way in which we can envisage the frequency re-
use situation. Cell A uses frequency f 1 and provides a usable signal as
far as distance d 1 in some azimuth direction. Beyond this distance, the
signal provided from Cell A is too low to provide a reliable link, but is
too high to allow the frequency to be re-used, whereas at some distance
d 2 the level of interference from Cell A has fallen to a level that allows
f 1 to be re-used by Cell B. In order to provide the maximum network
capacity, we need to ensure that signal intensity falls as rapidly as pos-
sible with distance and we assist this both by site placement and by
downtilting the antenna at Cell A. 1