Page 159 - Antennas for Base Stations in Wireless Communications

132 Chapter Four

One fundamental assumption for all concepts, except transmit diver-
sity, is that no additional feedback is needed from the user device to the
radio base station due only to the employment of any of the advanced
antenna system concepts. This means that any user device can be served
via a conventional sectorized system or an advanced antenna system,
without being informed about the type of system it is connected to.

4.3 Three-Sector Reference System
The reference system is defined as a three-sector antenna system
equipped in each sector with one downlink branch, and two uplink
branches for space or polarization diversity reception. The reason for
selecting this as a reference system is that it is the most common cel-
lular base station antenna configuration in macro cell applications. The
three-sector antenna system is used as a reference case when imple-
mentations and performance are discussed for the different advanced
antenna concepts.
Receive diversity is a means to improve base station receiver sensitiv-
ity (signal-to-noise ratio) and thereby coverage of wireless communica-
tion systems. The sensitivity improvement comes from receiving signals
using multiple antennas (receiver diversity branches), the principle
being that the signals received on different antennas will fade more
or less independently in relation to their fading correlation. Since the
occurrence of two independent signals experiencing deep fades simulta-
neously is statistically less likely than the occurrence of correspondingly
deep fades for a single signal, the receiver system will provide diversity
gain. Assuming maximum ratio combining, there is an inherent gain, or
power gain, from using multiple antennas that stems from the increased
total antenna area (when going from one to two antennas). This area
gain exists regardless of the statistical behavior of the received signals
on the diversity branches. Two-way receive diversity is implemented in
most cellular standards. To further improve the sensitivity in uplink,
low-noise amplifiers are mounted close to the antenna, so-called tower-
mounted amplifiers.
A fundamental aspect of antenna-based solutions is the type of inter-
ference distribution over a cell that results from the antenna radia-
tion pattern properties in a sectorized system. A cell (also a sector when
discussing macro sites) is a region that is serviced by the same radio
resource, for example, as identified by the strongest broadcast/pilot sig-
nals in GSM and WCDMA. Two major antenna design parameters that
influence the cell shape and the structure of the cell plan are the azimuth
half-power beamwidth (HPBW) and the azimuth pointing direction.
Two different cell plans are commonly employed—Ericsson cell plan
and Bell cell plan, as shown in Figure 4.1. The idealized cell shapes of