Page 191 - Antennas for Base Stations in Wireless Communications

164 Chapter Four

every sector, which has the advantage of smaller visual impact and a
simpler installation procedure. The resulting measured radiation pat-
tern for the sector beam next to the GSM 900 array multibeams is
included in Figure 4.23. Due to the horizontal beamforming, the array
antenna provides approximately 5 dB higher antenna gain than the
sector antenna, which has a gain of 12.7 dBi. In the plot, the relative
level of the sector beam pattern is adjusted for easier visual comparison
between the sector pattern and the array antenna pattern envelope.
Three GSM 900-MHz base stations comprising fixed multibeam array
34
antennas were installed and evaluated in the live GSM network. One
transceiver was used for transmitting the control channel in the whole
sector; i.e., it was connected to a sector antenna in the downlink and
to the array antenna in the uplink. Another transceiver was connected
to the sector antenna in both uplink and downlink. This transceiver
worked like an ordinary sector transceiver and was used as a reference
during the performance measurements. The remaining three trans-
ceivers were connected to the multibeam array antenna in both uplink
and downlink. The array antenna system is characterized by having
very effective uplink diversity, obtained by the many degrees of freedom
including polarization, angle, and pattern.
The performance test was designed to introduce interference into
the system gradually. This was done in order to have full control over
the quality of the network, as ensuring that customers experience high
quality throughout a test is important. Gradually increasing interfer-
ence was generated by applying different frequency plans. The number
of frequencies used was decreased, meaning that a higher fraction of the
frequencies experienced a re-use corresponding to re-use one. 41
The quality improvement of multibeam array antennas increases with
increasing interference compared to the sector-only antennas, as dis-
played in Figure 4.24. The least dense frequency plan with nine deployed
frequencies, which leads to essentially no internal interference, provides
similar quality for both configurations. When interference is introduced,
the two configurations start to differ. The quality level corresponding to
12 intra-cell handovers/(Erlang*hour) is selected to be a suitable quality
level for comparison, corresponding to a satisfactory network quality.
As shown in the figure, the sector antenna system needs 120% more
frequencies to provide the same quality as the array antenna system at
this quality level. In other words, the fixed multibeam array antenna
system requires 55% fewer frequencies. The released frequencies can
be used to increase capacity by adding an additional frequency plan. It
was also found that the multibeam array antenna functionality could
be used to obtain valuable information about the distribution of traffic
in the network.