Page 182 - Antennas for Base Stations in Wireless Communications

Advanced Antennas for Radio Base Stations 155

narrower than in the reference three-sector case when the number of
cells is increased, the azimuth beamwidth of the antenna pattern needs
to be reoptimized. The beamwidth shall be narrow to give good spatial
filtering effect in azimuth but wide enough to offer sufficient gain over
the entire sector. The optimization leads to the use of antennas with a
decreased half-power beamwidth. Whereas in a three-sector WCDMA
system, the beamwidth that maximizes the capacity in downlink is
about 65° (Ericsson cell plan) and about 75° (Bell cell plan), in a six-
sector system the optimum beamwidth is about 35°. 9,22 The results for
the uplink are similar to the downlink case, and the same optimal beam-
widths apply.
The main application of higher order sectorization is for increasing
system capacity in interference-limited radio network scenarios, i.e.,
scenarios where the signal-to-interference ratio (SIR) is limiting perfor-
mance. The increase in capacity comes from the improved spatial filter-
ing offered by antennas with narrower azimuth beamwidth that, in turn,
allows more cells per unit area. From a capacity performance point of
view, and based on a first-order model, a higher order sectorized system
is able to serve N/3 times as many users as the reference system in both
uplink and downlink due to the improved spatial filtering in azimuth.
In practice, the performance gain is somewhat lower than this due to,
for example, angular spread in the propagation environment. Angular
spread also leads to an increased fraction of users being in handover,
thus requiring resources in terms of hardware and output power.
Spatial filtering has a negligible effect in a noise-limited scenario,
unless it is accompanied by a change in the effective SNR. If the vertical
dimension of the antennas used in the reference, three-sector system is
the same as that of the antennas in the higher order sectorized system,
the latter will have higher gain. Also, the total power resource may
increase with higher order sectorization. This occurs if identical radio
chains, including power amplifiers, are used in each sector for both types
of sectorization. Increases in antenna gain or available power trans-
late into increased signal-to-noise ratio and, hence, potential coverage
improvements.
Increased antenna gain is useful not only for improved coverage but
also, to some extent, for improved capacity. In WCDMA, increased antenna
gain reduces the output power required for fulfilling network quality
requirements on downlink, and thus, as power is a limited resource in
the base station, the reduction can be used for increasing the traffic.
Resources other than the number of radio chains are affected by
increased traffic load. These resources are, for example, baseband pro-
cessing, bus capacity, and back-haul communication capacity between
the base station and the higher level control equipment, such as radio
network controller units.