Page 68 - Antennas for Base Stations in Wireless Communications

Base Station Antennas for Mobile Radio Systems 41

uses only one antenna per sector, and three antennas can be mounted
closely together on a single pole. This gives an advantage in cost (one
dual-polar antenna replaces two space-diversity antennas), and the
lower windload and absence of a head-frame reduce the cost of the
supporting structure. The reduced visual profile of a polarization diver-
sity base station reduces the difficulty of obtaining planning consent
in urban areas.
Further useful diversity gain can be obtained in some environments
by the use of both spatial and polarization diversity; the use of 4-branch
diversity can be expected to increase, especially for 3G and later systems.
5
Diversity gain can also be achieved on the downlink; it requires more
advanced signal processing techniques than receive-path diversity, but
the use of high-state modulation formats—for example for High-Speed
Downlink Packet Access (HSDPA)—is making its use necessary.
The future migration of mobile radio systems to higher data rates
and the quest for increased spectral efficiency is expected to result in
more techniques such as multiple-input, multiple-output (MIMO) and
space-time block coding as well as other techniques that further exploit
the use of multiple antennas.
Slant linear polarization has the unusual characteristic that the
signal transmitted by a +45° transmitting antenna is received without
polarization loss on an antenna with –45° polarization. The labeling of
ports on a dual-polar base station antenna should therefore be regarded
as being purely for identification unless it has been agreed whether the
designation refers to transmitted or received signals. Fortunately this
has no consequence in operation.

2.2.1.9 Effects of Imperfect Antennas on Diversity Performance To arrive
at appropriate specifications, we need to understand how practical
antenna performance defects may impact system performance.
Radiation Pattern Defects The implementation of spatial diversity using
identical vertically polarized antennas creates no new problems, as the
radiation patterns should match closely in both planes and the gain
from both antennas in any specified direction will be almost equal. The
only significant risk will be of some elevation pattern mismatch caused
by manufacturing tolerances or mismatched mechanical tilts.
A dual-polar antenna creates a much greater challenge in ensur-
ing accurate pattern matching for the two polarizations. A dual-polar
base station antenna is an unusual device in that it is a linear array
in which the elements are arrayed in a plane at 45° to their own prin-
cipal plane. This has the consequence that some properties of the azi-
muth and elevation patterns are dependent on one another in unusual
ways. Figure 2.2 shows the two copolar azimuth radiation patterns of