Page 67 - Antennas for Base Stations in Wireless Communications

40 Chapter Two

2.2.1.8 Diversity Systems When a mobile user moves through the envi-
ronment, the signal received at a base station antenna fluctuates widely
in amplitude because of the interaction of direct and reflected signals
from the MS. If another antenna is erected some distance away from the
first, the signal fluctuations on the two antennas will be uncorrelated.
A diversity system uses two antennas and combines their outputs in
some way that exploits this lack of correlation. The result of the use
of diversity is that the availability of the received signal is increased.
For a given signal availability, this increase could alternatively have
been achieved by increasing the transmitted power; so diversity gain
is defined as the equivalent increase in the power required to achieve
some stated reliability.
The multipath nature of mobile radio propagation results in the
strong frequency dependence of propagation loss. This is mitigated by
†
the use of slow frequency hopping (SFH) in GSM systems and the use
of spread-spectrum techniques such as wideband code division multiple
access (W-CDMA) and OFDM. The effect of very short losses of usable
signal is reduced by digital signal coding.
Spatial Diversity Spatial diversity was used on the uplink from the
inception of many mobile radio systems, and most readers will be
familiar with the sight of pairs of antennas firing in each of the three
directions of the sectors supported from a base station. These systems
normally use vertically polarized antennas; both antennas are also used
for transmitting and the availability of two antennas reduces the loss,
complexity, and cost of the required combining arrangements.

Polarization Diversity The signal radiated by a handset is strongly polar-
ized in the direction of its long axis. Seen from the front or rear of a hand-
set user, the polarization will be dominated by the vertical component, but
in the lateral direction a handset is typically held at a large angle to the
vertical, between the mouth and the ear of the standing or seated user,
typically at least 45°. Because of the typical dominance of the vertically
polarized component, the diversity gain obtained from a pair of antennas
with horizontal and vertical polarization is exceeded by one receiving
linear polarization at ±45° from the vertical, so dual slant-polarized
antennas are now in almost universal use for polarization diversity
systems. (These are often called dual-polar or cross-polar arrays.)
The use of polarization diversity is well established in urban areas,
where propagation paths are characterized by extensive reflections and
4
scattering. A base station employing polarization diversity antennas

† Slow implies that the hopping period is at least one bit long.