Page 125 - Antennas for Base Stations in Wireless Communications

98 Chapter Three

mitigating the coverage issue for subscribers closer to the base station
antenna. The antennas should also provide upper sidelobe suppression
and high front-to-back ratio for minimizing interference with neighbor-
ing cells. To reduce interference among different mobile phone channels,
the intermodulation distortion (IMD) must be less than –103 dBm.
In the past, a space diversity technology with one transmitting array
and two receiving arrays was usually realized. Nowadays, a dual-polar-
ized antenna with two independently operating slanted arrays, one at
+45° degree and the other at −45° polarization, is implemented predomi-
nantly. For good diversity performance, the decoupling between the two
arrays is required to be at least 30 dB.
For reducing installation effort and costs, antenna arrays are needed
that are able to operate in all mobile communication bands, which can
considerably reduce the number of antennas required by network opera-
tors. To provide more flexibility in network design, the gain in each
frequency band can be different from other bands. Again, decoupling
between output ports for different frequency bands is a critical issue in
the design process.
Commercially available base station antenna arrays are based on
dipole antenna technology. Although different brand names are used,
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3
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such as the vector dipole, butterfly dipole, and directed dipole, they
all use a reflector to produce a directional radiation pattern from electric
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dipoles. Other companies employ aperture-coupled patch antennas as
basic elements to develop their outdoor base station antenna series. In
general, the performances of these antennas is excellent.
In the following section, we present five novel designs of base station
antennas for outdoor and indoor coverage, all based on L-probe fed patch
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antenna technology. We hope these provide practicing engineers with
alternative design solutions and new insights when designing their
base station antennas.

3.2 Case Studies

3.2.1 Case 1: An Eight-Element-Shaped
Beam Antenna Array
In this section, the design of a linearly polarized, shaped beam antenna
array using a genetic algorithm (GA) is presented. Having a base station
antenna with null fill to avoid possible areas of reducing field strength
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in the service area is desirable. This antenna is usually comprised of
a number of identical elements mounted along a vertical line to form
an antenna array. If these elements are arranged with appropriate ele-
ment spacing and excited with currents of appropriate amplitudes and
phases, a characteristic-shaped radiation pattern in the vertical plane