Page 105 - Antennas for Base Stations in Wireless Communications

78 Chapter Two

Link Control (HDLC); the system uses a single primary (control) device,
and all other devices (including RETs and TMAs) are secondary devices
29
that respond to the primary device. The application layer defines a set
of elementary procedures (a command set) that provides all the required
functionality for the connected secondary devices.
The trend toward the increasing complexity of tower-top equipment
and the application of RET techniques to other radio systems has led
to the continuing development of AISG standards, and the availability
of a low-cost control bus on the tower is stimulating the development of
other equipment, including various forms of system monitors that make
use of a remote data connection from the tower top.

2.3.9.3 Azimuth Pattern Control For many years, the prospect of imple-
menting “smart” antennas has provided a continuing incentive to
researchers. It has been demonstrated that such systems can deliver
great benefits in terms of system capacity and range of communica-
tion, but their implementation in current mobile radio systems has
proved difficult and expensive. There is a large gap between what can
be provided by a simple “dumb” antenna and a smart antenna providing
adaptive beamforming, C/N optimization, and capacity maximization.
RET techniques may be seen as a first step in filling that gap. A further
important step is the provision of remote control of the azimuth beam-
width 30,31,32 and the azimuth beam direction. 33,34 Antennas using these
relatively low-cost techniques will operate with any radio air interface,
any modulation technique, at any power level, and with any number of
RF carriers. They also provide symmetrical characteristics on transmit
and receive bands. Sometimes described as “semi-smart,” these methods
provide relatively slow changes in antenna characteristics, typically
taking seconds or tens of seconds to change the beam shape or position,
but they are fast enough to allow adaption to match most changes in
the pattern of network traffic demand. 35

2.3.10 Antennas for TD-SCDMA Systems
Air-interface protocols that employ time-division duplex (TDD) trans-
mission use the same transmission frequency for the up- and downlinks
and are able to make generally more effective use of “smart” antenna
techniques than are protocols that use frequency-division duplex (FDD).
The use of a single frequency allows valid channel state information
(CSI) to be obtained rapidly for transmission in both directions. In
the case of an FDD system, especially one with large frequency spac-
ing between the up- and downlinks, obtaining up-to-date CSI at the
base station for the downlink is challenging; rapid updating requires
significant channel capacity and consequent loss of user capacity.