Page 89 - Antennas for Base Stations in Wireless Communications

62 Chapter Two

With no steps taken to enhance the isolation, a typical achieved result
is of the order of 26 dB for an antenna with a 2° beamtilt and typical
beam-shaping, but is lower for an antenna with 0° tilt. The achievement
of an isolation of 30 dB or more usually requires additional polarization
compensation.
The simplest way to minimize both the unwanted coupling between
alternate cross-polarized elements and the asymmetry of the array envi-
15
ronment is to place each radiating element in the center of a cavity.
This cavity has a side length of a little less than a wavelength, deter-
mined by the interelement spacing chosen as described in 2.3.3. Its
depth must be sufficient to create the required environmental symme-
try, but may also be determined by the target front-to-back ratio (F/b),
deeper cavities generally providing higher values. The depth necessary
to provide an enhanced F/b ratio usually ensures that the intertier
coupling is much reduced and an optimally dimensioned cavity can also
reduce the variability of the azimuth beamwidth with frequency.

2.3.4 Multiband and Wideband Arrays
In many countries, an operator commonly has access to two frequency
bands for GSM services and an additional band for 3G services. To mini-
mize the costs of antenna systems and tower space rental, as well as to
respond to public pressure for less obtrusive antenna systems, operators
may use antennas providing service on several frequency bands. The dif-
ferent histories and current environments of operators have created a
variety of requirements for these antennas, so there is a correspondingly
wide variety of specifications and solutions. Replacing existing single-
band antennas with new multiband antennas is always challenging,
because the quality of current network coverage must be maintained. Any
compromises in performance must be made on the added frequency band,
so as a consequence alternative solutions exist for networks approaching
dual-band operation from different directions.
In general, low-band GSM operators have added high-band coverage to
create additional capacity in dense networks. The use of frequency bands
is transparent to users, all of whom now have multiband handsets, and it
is not of primary importance that the high-band coverage is contiguous
as long as sufficient total capacity is available at any location. Dual-band
antennas are attractive for this purpose because they allow extra network
capacity to be added without additional antennas, avoiding requirements
for added windload and increased visual profile. The advent of remote
electrical tilt (RET) has allowed independent control of the beamtilt on
each band, greatly adding to the functionality of these antennas.
Where permitted by spectrum availability, many networks that pre-
viously had only high-band GSM frequency allocations have added