Page 101 - Antennas for Base Stations in Wireless Communications

74 Chapter Two

performance but it consumes resources at both base stations and also
in the backhaul link joining them. System optimization requires care-
ful control of coverage overlap, and the adjustment of electrical tilt is a
powerful tool to enable this. In a situation in which one cell is overloaded
with traffic but an adjacent cell has spare capacity, the use of adjust-
able tilt can allow the overloaded station to reduce its footprint (and
therefore its traffic load) while, at the same time, the adjacent station
can expand its footprint by reducing its antenna tilt angle. The ability
21
to remotely control the beamtilt of base station antennas is a powerful
technique that can be extended to optimize network capacity by includ-
ing real-time adaptation to traffic patterns. 22

2.3.9.1 General Design Issues The usual method for producing electri-
cal tilt is to apply a linear phase shift across the array aperture. To
maintain the levels of sidelobes and null fill, no change must occur in
the amplitudes of the currents, and their relative phases must remain
unchanged apart from the added linear incremental shifts.
When designing phase shifters, we could consider the use of PIN-
diodes or varactor diodes or the use of more mechanical methods
such as dielectrically loaded lines, trombone lines, or tapped lines. The
severe constraints on PIM generation and the relatively high power
ratings required currently rule out the use of electronic phase shifters
for BS antennas, so mechanical techniques must be used. Variable
transmission-line structures must avoid the use of sliding mechanical
contacts, so electrical connections usually rely on capacitance and not
on physical contact.
Lines with variable velocity ratio, loaded by sliding dielectric slugs,
appear attractive because no conductor contact is needed, but designing
a phase shifter with a large range of adjustable phase and low VSWR
over a wide frequency band is not easy. Some sliding dielectric arrange-
23
ments are in use, but many successful designs have used tapped-line
phase shifters. 24
Figure 2.16 shows an 8l array with a linear phase shift introduced,
retarding the phase of successive elements by a variable phase j°. To
drive an 8-element array, this method requires four phase shifters deliv-
ering a phase shift in the range 0°–j°, together with two delivering
0°–2j° and one delivering 0°–4j°. One simplification that is generally
adopted, at the cost of some loss in gain and in the control of sidelobe
levels, is to use a fixed phase between immediately adjacent elements,
providing a phase corresponding to the mean downtilt that the antenna
will provide; for an antenna with 0°–10° downtilt, this would typically be
set at s(sin5°) where s is the interelement spacing in electrical degrees
at band center. The arrangement now requires only three phase shifters,
two providing 0°–2j° and the other 0°–4j°.