Switched Beam Antennas versus Adaptive Antennas

Nonadaptive switched beam antennas select from a range of precalculated beam patterns.
Adaptive antennas adjust a weight vector to optimize the beam pattern to the
noise and interference environment.
An adaptive antenna maximizes downlink coverage and minimizes uplink interference
(by nulling out unwanted signal energy). A nonadaptive switched beam antenna
will, by definition, be less efficient than an adaptive antenna but requires considerably
less processing overhead.
Adaptive antennas deliver maximum gain in significant interface environments. In
low-interference environments (for example, rural areas), switched beams perform
almost as well as adaptive.
One practical issue to consider is the impact of these link gain products on other performance
metrics—for example, dropped call rates. If you have a switched beam
antenna on a site near a highway, say, with 6 × 60° beams facing the highway, a closein
user traveling along the highway will pass through six sectors (separate beams) in a
few milliseconds. It is very hard to manage power control and handover, and very easy
to lose the user. If the user is three or four miles away, even if he is traveling fast, his
rate of progress through the beam patterns will be relatively slow. These issues have to
date prevented the wide-scale deployment of smart antennas (in addition to cost and
complexity considerations).
Additionally, smart antennas require a very linear transmit/receive path (linear
HPA). This means phase accuracy is important, because phase offsets are used to
change the beam pattern. If the linearity is needed anyway (for example, in a 3G Node
B), then this is not an applicable additional overhead.
Smart antennas can be used to solve internetwork interference problems. In PHS in
Japan, where three network operators shared spectrum in a nonduplex nonpaired
band allocation, the operators’ networks did not clock together. The internetwork,
interuser, intersymbol interface created capacity problems. (PHS uses an Ethernet-type
MAC access protocol. If a time slot is occupied, the transceiver moves to another time
slot). The ISI was detected as occupied bandwidth (that is, the capacity disappeared).
The problem was alleviated by using smart antennas to null out interference.