Page 104 - Antennas for Base Stations in Wireless Communications

Base Station Antennas for Mobile Radio Systems 77

track accurately. The phase shifters are connected to elements at the top
and bottom of the array through cables of similar length, so the natural
location for the phase shifters is close to the center of the array.
Many dual-band antennas with multiple input ports are used on
more than one radio network, perhaps employing different air inter-
face standards, and to facilitate separate optimization on each band,
these antennas are almost invariably provided with RET control. Their
design combines the engineering challenges referred to in the context
of dual-band antennas with those of the control of mutual impedances
and broadband polarization decoupling that are needed to provide sat-
isfactory RET performance. For the same application reason, remote
azimuth steering (RAS) and remote azimuth beamwidth control (RAB)
will also be implemented on these complex antennas.
Direct current (DC) motors or stepper motors are commonly used to
provide remote operation of the phase shifters; they are typically located
close to the bottom of the antenna to allow access for replacement in
case of failure. Early RET antenna designs were fitted with external
motor drives, but with the increasing acceptance of remote control tech-
niques, the complete drive system will be installed within the profile
of the array (some manufacturers are already providing this arrange-
ment). Some means of manual adjustment is usually provided for use
in case of motor failure. A mechanical indicator to show the current tilt
setting provides confidence if there is any doubt that the remotely set
parameter has not been achieved.

2.3.9.2 Remote Control Interface and Protocol Following early proposals
for antennas with RET it became clear that the development of this
technology would progress much more quickly if there were a stan-
dard interface for the control system. Proprietary systems would pres-
ent problems if an antenna manufacturer discontinued a product line
or if a network operator wished to change its supplier for commercial
reasons. Antenna manufacturers, network operators, and infrastruc-
ture vendors formed the Antenna Interface Standards Group (AISG) in
2002, and this body, in conjunction with The 3rd Generation Partnership
Project (3GPP), has developed interface standards whose objective is
to provide interoperability among different vendors’ equipment. The
25
standards define a three-layer protocol stack. On the PHY layer, the
standards 26,27 define alternative connection means between tower-top
equipment and the controller. These are an RS485 bus and a system
using a low-frequency modulated subcarrier introduced into the coax-
ial connection between the base station and the tower-top equipment.
Among other parameters, the standards define relevant supply volt-
ages and currents, subcarrier characteristics, connectors, and pin-out
28
details. The data link layer is based on a subset of High-level Data