Page 184 - Antennas for Base Stations in Wireless Communications

Advanced Antennas for Radio Base Stations 157

1.8
1.6
1.4

1.2
1
Active sites
0.8 H.O.S. sites
Other sites
0.6
0.4
0.2

0
Ref H20 H10
Figure 4.18 Scenario comparison of speech capacity per site in a real net-
work when a limited number of sites with higher order sectorization are
introduced.

provides a feasible option for increasing the capacity of third-generation
networks without finding new sites, a possibly expensive and time-
consuming process, provided that the practical installation issues are
solved cost effectively.

4.10 Fixed Multibeam Array Antenna
As an alternative to increasing the number of sites or cells per site as
the means to enhance capacity in cellular networks, multibeam array
antennas may be introduced. Fixed multibeam systems are character-
ized by the use of a set of fixed azimuth beams in each cell for transmis-
sion and reception. 7,23−27 Each beam in the set has predefined properties
such as pointing direction and beam shape and covers only part of the
cell. The beams are generated either by radio frequency (RF) beam-
former or at baseband or by a combination thereof. In the case when the
fixed beams are formed at baseband, signal coherency is required from
baseband to the antenna aperture. If the fixed beams are formed at RF,
then coherency is required from the beamformer to the antenna aper-
ture. In such an implementation, the need for calibration of the radio
chains is eliminated because the antenna unit, including the beamform-
ing network, is manufactured with required coherency. A block diagram
showing a fixed multibeam antenna system for one cell is presented in
Figure 4.19.