Page 187 - Antennas for Base Stations in Wireless Communications

160 Chapter Four

optimized in order to minimize the total base station power required to
guarantee an acceptable quality of service. 29
A more general implementation of a fixed multibeam antenna system
may have more beams than antenna elements or columns, thus pro-
viding non-orthogonal beams, particularly if the beams are formed at
baseband. The number of beams may be arbitrarily large, thus making
a fixed multibeam system and a steered-beam system less distinguish-
able. 30
The principal function in uplink for the fixed multibeam array antenna
system, just as for the reference sector system, is to combine all energy
from the desired signal arriving at the antenna. This process may
comprise the combination of signals from different cells, as in a softer
handover situation in WCDMA, as well as the combination of signals
from the diversity antennas. The same types of receive diversity used
in sector-antenna systems, for example, spatial or polarization diversity,
may also be used in fixed multibeam array antenna systems.
In downlink, the desired action is to transmit the signal to the user
device as efficiently as possible by selecting one of the available beams.
This means that information concerning the angular location of the
user device, such as direction-of-arrival (DOA) information, is needed,
at least on a beam resolution level. Other information such as traffic
load per beam is used in the beam selection process to improve system
performance regarding amount of traffic served. The typical situation is
that the downlink transmission is performed via only one of the beams
within each cell included in the active set. The limitation to one beam
only is imposed to minimize interference spread in the network. In a
soft handover situation in WCDMA, the transmission is performed via
one beam in each cell, for all cells in the active set.
As is shown in Figure 4.19, DOA estimation can be performed using
the uplink received data for each user. The estimated DOA information
and other similar metrics providing sufficient information, such as the
beam with highest received signal power for a given user, can be used
for downlink beam selection.
In contrast to higher order sectorization, the introduction of a fixed
multibeam array antenna system may require changes on the interface
between the radio base station and the radio network controller. For
WCDMA, one example of information that needs to be available in the
radio network controller, to allow efficient use of system resources, is
the spatial distribution of interference. In a sector antenna system, such
as the reference system discussed previously, there exist only cell-based
quality measures. However, for a fixed multibeam antenna system mea-
sures are needed on a beam-by-beam basis since, due to the spatial
resolution, some beams might carry a high traffic load whereas others
carry less. In a beam carrying heavy traffic, it may be necessary to block