Page 172 - Antennas for Base Stations in Wireless Communications

Advanced Antennas for Radio Base Stations 145

that the beam shape is tailored to the beamtilt angle. For example, a non-
linear phase-shifter can provide high gain at the cell edge for small tilt
angles to maximize area coverage and null-fill near the base station for
larger downtilt angles, as shown in Figure 4.10, in order to avoid service
areas inside the cell with poor path gain, which could be detrimental,
especially for high data-rate users. Such an antenna can, therefore, be
deployed for both coverage and capacity scenarios, even if the resulting
system operating point is uncertain when the network is rolled out.
The optimal beamtilt angle depends on site location, antenna instal-
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lation height, cell size (inter-site distance, ISD), and traffic load.
Basically, the smaller the cell size, the larger the beam downtilt angle
should be. Beam downtilt has the most impact for high gain, narrow
vertical beamwidth antennas. Highest impact is achieved in areas with
small cells and/or high antenna installations. In larger cells, antenna
downtilt is still useful in order to reduce local interference problems, to
decrease the cell size, or to improve coverage (signal-to-noise ratio or
SNR) inside the cell away from the cell border. However, this is at the
cost of reduced coverage at the cell border.
For efficient use of the radio access network, antenna beamtilt is con-
trolled adaptively from an operation center, the so-called remote (elec-
trical) tilt. The purpose of a control system is to perform the antenna
beamtilt setting, as requested from the operation center. The beamtilt is
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set per antenna or beam independently for optimal system performance.
Optimal tilt angle settings may differ with frequency band for multiband
antennas with independent tilt control possibilities on a band-by-band
basis. The tilt angle of each antenna can also be set locally at the base
station by connecting a portable computer to the control system or manu-
ally on site without using a control system. Systems using antennas with
remote tilt control may be adapted to the existing traffic situation or
changes in the propagation environment on a short- or long-term basis.
Although the discussion has been focused on single beam antennas,
beamtilt, which makes use of the elevation dimension, can be used in

Figure 4.10 Elevation radiation patterns of an antenna with
a tilt unit containing nonlinear phase shifter for two different
tilt settings: untilted high-gain pencil beam (solid line) for
maximum cell edge coverage and downtilted shaped beam
(dashed line) for uniform cell coverage and, hence, capacity,
with interference suppression toward neighbor cells accom-
plished by low sidelobe levels above the main beam.