Smart Antennas

We have shown that you can take a passive antenna and change its coverage footprint
by changing the antenna aperture (its size), or by using multiple antennas and physically
moving them further apart or closer together. Adaptive antennas effectively take
multiple antennas and move them further apart or closer together electrically—by
changing the phase and amplitude relationship between the antenna elements—rather
than physically. These are called smart antennas, because they can be made smart
enough to deliver specific downlink coverage footprints. On the uplink, smart antennas
can be used to null out unwanted interference, improving sensitivity.
We said earlier that the difference in free space loss between 900 and 1800 MHz is 6
dB, and the actual loss is typically 1 or 2 dB higher because of increased refraction and
reflection at the higher frequency. Increasing our operating frequency to 2 GHz (for
IMT2000) adds another dB or so to the loss, which needs to be accommodated in the
link budget. By providing additional selective gain, smart antennas can recover some
of this lost link budget but at a price.
Smart antennas come in two flavors—switched beam and adaptive. The product
from Nortel (see Figure 13.7) is a switched beam smart antenna giving 7 dB gain from
a three-sector, four-element array (voting on a slot-by-slot basis between antennas).
The HPA and LNA are both mast mounted.
The 7 dB gain effectively means, in theory, that the same coverage is available at
1800 MHz as was available from a 0 dB antenna at 900 MHz. In practice, many 900-
MHz cellular installations already use quite substantial gain, for instance, using two
physically spaced receive antennas to give diversity gain on the uplink. Typically,
about 17 dB of gain is available on the uplink and about 9 dB on the downlink to give
a balanced uplink/downlink link budget (because mobiles have less power than base
stations). These antenna arrays are typically 65° beamwidth or 90° beamwidth. 304