Page 60 - Antennas for Base Stations in Wireless Communications

Base Station Antennas for Mobile Radio Systems 33

with the general principles of network and cell planning. Special atten-
tion is given to the ways in which antenna specifications are chosen and
achieved to optimize network coverage and capacity.

2.2 Antenna Performance Parameters
In order to provide the required azimuth beamwidth and gain, a BS
antenna typically comprises a vertical array of radiating elements. The
design of the individual elements provides the required azimuth radia-
tion pattern characteristics, whereas the vertical extent of the array
and the number of radiating elements in the array is chosen to provide
the necessary gain.
The azimuth beamwidth of a base station antenna is chosen to suit
the frequency re-use plan chosen for the network. Base stations typi-
cally support three cells spaced 120° apart in azimuth, although this
plan is by no means universal especially where coverage may be limited
by buildings or hills or where usage patterns are nonuniform, as in the
case of a BS alongside a major highway. Code division multiple access
(CDMA) systems require careful control of the overlap between cells to
avoid excessive loss of capacity in soft/softer handoff modes, so a 3-dB
beamwidth of 65° is the norm. Global System for Mobile Communication
(GSM) systems typically use antennas with 65° beamwidth in urban
areas but often use wider beamwidths in rural areas. The apparent gap
between 65° sectors is filled from an adjacent BS. In dense urban areas
the planning of most networks is determined by the need to provide
sufficient capacity rather than to cover the largest possible area, so dif-
ferent considerations may apply to the design of BS antennas.
For azimuth angles beyond the edges of the served cell, allowing some
overlap for handoff control and optimization, the signal radiated by a
BS antenna has no functional use and serves only to impair the C/I
ratio in other cells. For this reason, it is desirable that beyond ±60° from
boresight the azimuth pattern falls monotonically at as high a rate as
practicable and that rearward radiation is suppressed.
Although we are accustomed to seeing rooftop BS antennas in many
cities, a substantial number of cells use antennas mounted at lower
levels; these make use of the built environment to limit the area they
cover—allowing more intensive frequency re-use—and they can more
easily be disguised to reduce problems with planning consent.
The use of omnidirectional antennas is usually limited to small in-fill
base stations because their ability to support only a single cell limits
their capacity and frequency re-use capabilities.
The main BS network is usually provided with antennas having the
highest gain economically possible; this reduces the number of stations
needed, improves in-building penetration, and helps to resolve some