Page 166 - Antennas for Base Stations in Wireless Communications

Advanced Antennas for Radio Base Stations 139

4.6 Transmit Diversity
Transmit diversity is a concept used for improved downlink capacity
in which multiple antennas are used. Radio base stations are already
equipped with multiple antennas, which are used for receive diversity.
The idea is to use these antennas to mitigate the effects of fast fading also
in downlink. A number of transmit diversity schemes exist, such as space-
time coding, delay diversity, antenna hopping, and phase hopping. 12
Alamouti space-time coding is an open-loop transmit diversity scheme
that uses two antennas to simultaneously transmit two signals to the
13
user device. This scheme, however, requires user devices with special
space-time decoding algorithms. On the other hand, delay diversity,
antenna hopping, and phase hopping are transparent to user devices
and can be employed for existing devices. The delay diversity scheme
transmits delayed replicas of the signal by utilizing two antennas at the
base station. The user device needs to receive an uncorrelated signal
from each transmit antenna to take advantage of the technique. The
radio base station antennas need to be spaced sufficiently far apart
or transmit with orthogonal polarizations. Delay diversity artificially
creates increased time dispersion in the propagation channel, which is
resolved by the equalizer in the user device. In the antenna hopping
scheme, the transmitted signal is switched among the available base
station antennas. The signal then passes through a propagation channel
that varies even if the user device is stationary. With antenna hopping,
no additional hardware, such as transceivers or combiners, is required
in the base station, since only one antenna is transmitting at a time.
The phase-hopping scheme also aims at creating a propagation channel
that is nonstationary. This scheme transmits identical signals that are
phase-offset across two antennas at the base station.
Most transmit diversity schemes use only two antenna branches.
Each of these antennas uses only half of the power of a system with
a single antenna in order to maintain constant transmit power. In the
case when full transmit power is used on each antenna simultaneously,
the transmit diversity schemes can be used to increase coverage. All
schemes can be extended to more than two antennas.

4.7 Antenna Beamtilt

Beamtilt or adjustment of the main beam direction in elevation, typically
between 0° to 10° downward, is used for many purposes. One main pur-
pose is to obtain good cell-border coverage. The beam direction is adjusted
to follow the surface of the ground to radiate as much power as possible
toward the cell border. Another application is to increase capacity by
reducing inter-cell interference, which is achieved by beam downtilt and
the corresponding increased isolation between the cells. The main beam
is directed such that the upper slope of the main beam radiation pattern