Page 106 - Antennas for Base Stations in Wireless Communications

Base Station Antennas for Mobile Radio Systems 79

An example of a TDD system is the Chinese Time Division-Synchronous
Code Division Multiple Access (TD-SCDMA) standard, and this already
employs adaptive beamforming techniques using both omnidirectional
and directional antenna arrays.
An example of a directional smart antenna is shown in Figure 2.19.
This uses the same methods for the control of elevation patterns as
more conventional “dumb” antennas, but the whole antenna array is
formed from multiple subarrays of the more conventional type. The
multiple subarrays, each a column of elements, are closely matched in
performance, and sampling facilities are provided in each column to
allow the whole array to be calibrated. As with a conventional antenna,
it is possible to achieve diversity using dual-polar elements. A high-
band, four-column array is typically around 320 mm wide and allows
beam-steering to ±60° from boresight. The amount by which the beam
can be steered off boresight is limited by the appearance of potentially
high azimuth sidelobe levels as the steering angle increases. To optimize

Figure 2.19 Example of a directional beam-
forming antenna for a TD-SCDMA system
(Photo courtesy of Comba Telecom)