Page 157 - Antennas for Base Stations in Wireless Communications

130 Chapter Four

well as multistream MIMO (Multiple-Input Multiple-Output) systems.
Several other frequently used terms for these antennas include advanced
antenna systems, adaptive antenna base stations, smart antennas, smart
base station antennas, MIMO antennas, and multi-antenna systems.
In this chapter, antenna technologies ranging from a conventional
three-sector antenna system to a fully coherent adaptive antenna system
for radio base stations are discussed, with a focus on macro base station
antennas for second generation (GSM) and non-MIMO third-generation
(UMTS) systems.

4.1 Benefits of Advanced Antennas
In both second- and third-generation mobile systems, interference is the
main limiting factor with respect to achieving high traffic capacity. In
second-generation systems, a standard procedure to increase capacity
is cell splitting. Cell splitting typically means that new base stations
are introduced in the network, and the area served by each base sta-
tion is reduced. The maximum capacity per base station (site or cell) is
basically constant in a well-planned mobile cellular network. By adding
more base stations, i.e., cells, for serving a given area, the total system
capacity per unit area is increased.
Advanced antennas have been suggested and deployed as an effi-
cient means to meet the rapidly increasing traffic volume. 1−8 The move
toward advanced antennas is driven by the shortcomings of the stan-
dard method, the cell split, which has become more and more difficult
to implement in urban areas, both due to lack of locations that are
suitable from a system perspective (propagation, transmission, and so
on) and also due to aesthetical reasons (zoning regulations) and site
cost aspects.
Advanced antennas in the setting discussed here offer a means for
increasing system capacity or coverage while making use of existing
sites and without affecting the user device requirements. Increased
capacity is obtained mainly by reducing interference in the network
using improved spatial filtering. This means that directive beams,
narrower in azimuth or elevation than in the conventional three-sector
system, are used for communicating with a user. Examples of capacity
solutions are antenna beamtilt, higher order sectorization, multibeam
array antennas, and steered-beam array antennas. The beamwidth
in multibeam and steered-beam array antennas is narrower than the
cell or the sector, i.e., only part of the cell where users are located is
covered. This requires information regarding the user’s location within
the cell, either explicitly in terms of actual location of the user device or
implicitly in terms of which base station antenna beam arrangement is
most beneficial. In a second-generation system, like GSM, the improved