Page 188 - Antennas for Base Stations in Wireless Communications

Advanced Antennas for Radio Base Stations 161

new users whereas in other beams adding users will not be a problem.
Another issue for the radio network algorithms in WCDMA networks
is the handling of multiple scrambling codes. As downlink transmission
with different codes is not orthogonal, it is necessary to allocate scram-
bling codes based on the location of the user within the cell. Thus, the
radio network controller needs spatial information for each user device
such as an identification of the beam showing the lowest path loss.
Fixed multibeam array antennas have also been shown to enhance the
quality of services and the network capacity of Enhanced GPRS (EGPRS)
to provide packet-switched mobile services. For the 1/3 reuse scheme, a
capacity gain of over 200% is realized in a homogeneous downlink system
employing a relatively low complexity array antenna system compared
31
to a three-sector configuration. The investigated array antenna system
comprises eight interleaved beams in azimuth generated by two four-
by-four Butler matrices. The matrices are connected to four antenna
columns with orthogonally polarized radiating elements.
An alternative to planar multibeam array antennas is to use a cylin-
drical array antenna. Combined with a Butler matrix feeding network,
the antenna can produce a fixed set of radiation patterns with narrow
beams in directions around the cylinder axis. With two Butler matrices
back-to-back, simultaneous fixed multibeam patterns and cell-defining
omnidirectional patterns can be provided. 32

4.10.1 Field Trials
The performance of fixed multibeam antenna systems have been evalu-
ated in a number of field trial activities in GSM and TDMA (IS-136)
systems. 33−39 The results show considerably increased capacity when
using fixed multibeam array antenna systems.
Fixed multibeam array antennas were tested as an option to enhance
33
capacity in a GSM network. A two-dimensional antenna array, a
planar array antenna, Figure 4.21, was developed for the field-trials,
which were performed in a live network operating in the 900 MHz fre-
quency band. Evaluations of the field trial results indicate that adaptive
antenna systems are able to give considerably increased performance.
Using dual-polarized antennas makes polarization diversity schemes
possible and only a single array antenna is needed in each direction
from a base station, i.e., for each cell, which minimizes installation and
aesthetical issues on site.
Within the scope of the trial, the array antenna was used both in trans-
mit and receive mode. Therefore, the antenna was required to work over
the full 900-MHz frequency band, with a total bandwidth of about 10%,
which was achieved with a design using aperture-coupled microstrip
patches. A principle view of the array antenna is shown in Figure 4.22.