Page 209 - Antennas for Base Stations in Wireless Communications

182 Chapter Five

(a) Sector division angle = HPBW (b) Sector division angle > HPBW
Figure 5.5 HPBW in horizontal planes

from 50°–80° for the three-sector per cell scheme and from 30°–45° for
the six-sector per cell scheme, respectively. These observations suggest
that using beamwidths that are narrower than those of the sectors
can increase system capacity. For example, in a three-sector per cell
scheme with an HPBW of 120°, which equals a sector division angle,
system capacity in the uplink is just 70% of the system capacity with
an HPBW of around 75°. Because HPBW is designed to be equal to the
sector division angle, as shown in Figure 5.5a, the advantage is that
a uniform power-level distribution will be achieved across the sector.
However, unfortunately, such an area design usually leads to many
large overlapping portions between the adjacent sectors, which causes
severe interference, reducing system capacity. For an HPBW narrower
than the sector division angle, as shown in Figure 5.5b, the size of the
overlapping areas is significantly reduced. Subsequently, system capac-
ity can be increased by suppressing the interference among the adjacent
sectors. The drawback of such an area design is, however, that some por-
tions will suffer low-power levels. Therefore, the optimal HPBW of the
base station antennas will increase system capacity significantly, but a
tradeoff between the size of the overlapping areas and the uniformity
of power-level distribution is required.

5.2. Design Considerations for Antennas
from a Systems Point of View

All mobile cellular communication networks require increased system
capacity, improved coverage, better QoS, and reduced transmitted power.
The base station antennas in these networks can be designed to enhance
network performance. For example, when diversity effect is used in the net-
works for better QoS, the spatial isolation between sectors is vital for per-
formance and can be achieved by using the base station antennas as spatial
filters. It is well-known that the HPBW and sidelobe levels will strongly
influence the spatial filtering performance of base station antennas.
Thus, the radiation performance of a directive base station antenna in