Page 90 - Antennas for Base Stations in Wireless Communications

Base Station Antennas for Mobile Radio Systems 63

low-band capabilities to enhance their coverage, especially in rural
areas. This usually required additional large antennas, but where
adequate existing high-band coverage existed, the use of dual-band
antennas was not necessary.
The advent of 3G UMTS services added further antenna require-
ments, and again the optimum solutions depended on what systems
and services each network already provided. The preferred network
engineering solution is that independent antennas are provided for each
band, because this facilitates easy independent optimization, upgrade,
and maintenance, but practical constraints have driven the creation
of a variety of hybrid solutions. These range from completely separate
antennas with independent tower-mounted amplifiers (TMAs) and sep-
arate feeder systems, to integrated antennas with common TMAs and
single feeder systems to the equipment cubicle. Most solutions fall into
one of the following categories.

2.3.4.1 Independent Antennas Mounted Side-by-Side Under a Single
Radome These antennas have many arrangements. They generally
comprise two arrays of the same overall physical length and are typi-
cally comprised of one of the following combinations:
Array 1 Array 2
a 850 and/or 900 MHz 1800 and/or 1900 MHz
b 1800 and/or 1900 MHz 1900–2170 MHz (UMTS)
c 1700–2170 MHz 1700–2170 MHz
Although both arrays generally have the same physical length, they
may have different azimuth beamwidths and polarizations. If equipped
with RET facilities, they can also have independent fixed or variable
electrical elevation beamtilt. The only constraint on design is that the
two arrays have a common physical azimuth pointing direction—though
that constraint is being addressed by the introduction of azimuth beam-
width control and beam-steering as described in 2.3.9. Example a is
typical of antennas used by a GSM network adding extra coverage on an
alternative frequency band. Example b would suit a high-band network
adding UMTS coverage, whose requirements for azimuth beamwidth or
polarization may differ between bands and the parameters on the lower
band must be preserved while 3G capabilities are added. Example c is
probably a better solution if the constraints of the older system allow
it to be used. The use of broadband arrays enables antennas with a
common design to be deployed in 2G and 3G networks, providing a great
benefit in terms of logistics. A dual multiband array is less flexible than
using separate arrays for each service, although the availability of RET
and other remote control techniques is addressing this.