Page 177 - Antennas for Base Stations in Wireless Communications

150 Chapter Four

tilt implemented on an antenna element basis over the full length of a
high-gain antenna. Of course, the combination of electrical tilt in the
feed network with electrically tiltable subpanels provides full electrical
tilt, with the added benefit of independent control of the tilt settings.
The polarization of subpanels using the same corporate feed network
is typically the same. However, combining subpanels of different polar-
izations is also possible, for example, to avoid deep pattern nulls. Still,
the main part of the power is fed to subpanels with identical polarization
in order to achieve the desired improvement in antenna gain and, as a
consequence, coverage. Similarly, subpanels that differ with respect to
other properties, for example, size and gain, can be combined. Null-fill-
ing for identical copolarized subpanels can be achieved by nonuniform
phase and amplitude weighting, depending on the number of subpanels.
Filling the first pattern nulls to about –20 dB will only incur a small
reduction, a few tenths of a dB, in peak antenna gain.
Based on the gain improvements for the configurations with multiple
subpanels, an increase in coverage is achieved. To estimate the general
behavior, a basic range-dependent model for the relative coverage area
2/a
is written as ∆A = ∆G , where ∆G is the relative antenna gain and a is
the path-loss exponent. The relative coverage area with different high-
gain antenna configurations for a equal to 3.5 is shown in Table 4.2.
Introducing modular high-gain antennas, Figure 4.13, into a network
improves the link budget and increases coverage up to 70%. It is ideal
for wide-area coverage in rural areas with low traffic density, over flat
ground, along highways, and over sea. The antenna concept improves
the link budget 2.5 dB to 4 dB equally in both uplink and downlink.
The number of sites may be reduced with this concept, which reduces
overall deployment cost. Existing sites may be retrofitted with modular
high-gain antennas without any changes in base station equipment or
feeder arrangement.

4.8.1 Case Study
In rural deployments and other low traffic scenarios, coverage is a key
performance indicator and solutions that minimize the required number
of base station sites are highly desirable to reduce capital expenditure

TABLE 4.2 Relative Coverage Area With Different High-Gain Antenna Configurations
(Subpanel with 65ç and 8ç azimuth and elevation half-power beamwidth, respectively,
and path-loss exponent ` = 3.5)
Antenna Gain (dBi) Relative coverage area (%)
Single subpanel 18 100
Dual-subpanel 20.5 140
Tri-subpanel 22 170