Conventional versus Smart Antennas

Conventional passive antenna design continues to improve—better materials, better
matching and coupling techniques, provision of polarization diversity and space
diversity to provide uplink gain, and careful sectorization to deliver downlink gain.
Electronic downtilt antennas are semi-smart in that they can be made adaptive to
changing interference conditions or can shift loading (if you shrink the cell radius,
you support fewer users). Truly adaptive beam forming antennas are, to date, not in
widespread deployment; however, the IMT2000 5 MHz allocations, particularly the
allocation of adjacent nonpaired bands, provide plenty of opportunity for co-channel
internetwork interference, which may make smart antennas a far more plausible economic
proposition.
For the moment, conventional passive antennas answer the majority of deployment
requirements. The example shown in Table 13.1 is a typical dual polarized antenna
with some downtilt (not dynamically adaptable). The dual polarization is on the
receive path. Beamwidth is 90° and the antenna has a 20 dB front-to-back ratio, which
means signals from the back of the antenna will be attenuated by 20 dB (relative to the
incoming signals from the front) when received by the antenna.
Front-to-back ratio
is also better for the 1900 MHz antenna. Power handling is less (higher losses at 1900
MHz create more heat gain). Wind loading for the 800 and 1900 MHz is quoted as the
same, although the 1900 Hz antenna is, as you would expect, smaller and lighter. The
intermodulation figure (<152 dB) is also quoted for the 1900 MHz antenna
Companies such as Raytheon and Paratek have gained considerable experience
with smart antennas in the satellite space sector (including Iridium and Globalstar).
The trick is to bring the technology down to earth at a down-to-earth price. The example
shown in Figure 13.8 is from Paratek (www.paratek.com) and is part of its DRWIN
(Dynamically Reconfigurable Wireless Network) proposition. Lucent has a similar
product proposition. 309