Page 175 - Antennas for Base Stations in Wireless Communications

148 Chapter Four

angle significantly improves pole capacity compared to no beamtilt, with a
tilt angle of about one half-power beamwidth below the midpoint between
two sites being close to optimal. A second observation is that the wider the
elevation beamwidth, the less variation in capacity over beamtilt angles.
A third observation is that the elevation and azimuth beamwidths have
to be selected carefully to reduce variations in illumination of the cell
(i.e., path gain), since these variations are of utmost importance for the
pole capacity. All antennas within each category, high gain and low gain,
respectively, have the same gain values, and the elevation beamwidths
are chosen to fulfill that.
Electrical beamtilt and azimuth and elevation beamwidths are key
factors for improving downlink performance in mobile networks. To
achieve the potential gains, which depend on antenna parameters as
well as cell characteristics, the settings of the beamtilt angles need to
be optimized in each real network separately. This study focused on
pole capacity performance on downlink. Considering that the uplink
and lower traffic load will typically result in smaller tilt angles being
optimal, a compromise between uplink and downlink performance, as
well as between capacity and coverage, is necessary.

4.8 Modular High-Gain Antenna
The modular high-gain antenna concept is used for maximizing cover-
age (cell range) in environments with low traffic intensity. This antenna
is characterized by offering an improved link budget in terms of high
antenna gain in both uplink and downlink. A vertical combination of
multiple sector antennas results in higher gain and enables the possi-
20
bility of extended coverage without altering the azimuth beamwidth.
Mechanical as well as electrical downtilt of the antenna main beam
are incorporated to reduce interference in neighboring cells and/or to
maximize coverage area. Modularity allows for simplified logistics, with
relaxed requirements on infrastructure and transportation means, with
site-based antenna assembly using high-precision self-aligning mount-
ing frames to ensure excellent electrical performance.
By combining commercially available hardware, such as stan-
dard sector antennas, combiners, and coaxial cables, into a high-gain
antenna, a modular and robust solution is achieved that is suitable
for link budget improvement in coverage-limited scenarios. The sector
antennas used as building blocks, referred to as subpanels, are fed by a
feed network, which includes power dividers, delay lines (phase shift-
ers), and coaxial cables. The subpanels may have different radiation
characteristics, and they are installed to get a desired radiation pat-
tern behavior. Amplitude taper, potentially nonuniform, is provided by
the divider/combiner, whereas phase taper is handled by delay lines or