Page 161 - Antennas for Base Stations in Wireless Communications

134 Chapter Four

these cell plans are based on the assumption that identical base stations
and antennas are placed on a regular hexagonal grid, with the three
antennas oriented 120° apart in azimuth (in the horizontal plane), and
that the propagation can be modeled by a uniform path loss model with
no fading. In the case of the Bell cell plan, each antenna is directed toward
a symmetry point among three base stations placed at the vertices of an
equilateral triangle and the cell shape becomes independent of the half-
power beamwidth. Each site in the Bell cell plan covers a hexagon. The
cell shape of the Ericsson cell plan, on the other hand, is strongly depen-
dent on the azimuth half-power beamwidth. Assuming a wave propaga-
tion attenuation proportional to the distance raised to 3.5, a half-power
beamwidth of 65° will produce a hexagonally shaped cell rather than site.
9
In a three-sector WCDMA system, the beamwidth that maximizes the
capacity in downlink is about 65° when each beam is pointing directly to
the closest site (Ericsson cell plan), and about 75° when the beams from
neighboring sites are directed toward the symmetry point in-between
these sites (Bell cell plan) and similar results hold for a GSM system. The
elevation properties of the radiation pattern will also have an impact on
the cell shape, as discussed in the beam-tilt section.

4.4 Three-Sector Omnidirectional Antenna
The three-sector omnidirectional antenna configuration offers a low-cost
solution for low-capacity requirements because it reduces the required
number of power amplifiers by a factor of three, compared to the con-
ventional radio base station arrangement. Therefore, this antenna is
a candidate configuration for early and rapid deployment. In general,
configurations with two or more sector antenna panels may be used to
form combined radiation patterns with fractional or full 360° horizontal
coverage.
A conventional three-sector antenna configuration for downlink
transmission is shown in Figure 4.2a. Three identical sector antennas
are used, sequentially rotated 120° about a vertical axis, with the sector
antenna radiation patterns having identical polarization states over all
azimuth angles in the main beam region. Each antenna is individually
fed and has associated with it a separate signal path (radio chain),
including a separate (downlink) power amplifier module.
The corresponding three-sector omnidirectional configuration is
shown in Figure 4.2b. The antennas are fed (on downlink) from a
common radio, with the power distributed from a dividing/combining
network via dedicated feeder cables. In general, the lengths of the feed-
ers are not identical, which results in differences in phase values. The
same antenna arrangements are used on receive with the introduction
of duplex filters or switches to separate transmit and receive signals.