Weather Attenuation Peaks

There are also some attenuation peaks caused by water vapor and oxygen resonance
effects. This is called resonant absorption. Attenuation peaks for water occur at 22 and 183
GHz. Attenuation peaks for oxygen occur at 60 and 119 GHz. Figure 15.3 shows these
attenuation characteristics. Note that the 60-GHz oxygen line exhibits a propagation loss
of 15 dB per kilometer. This is good news and bad news. You can get very high reuse
ratios at 60 GHz, but you have to accommodate the propagation loss in the link budget.
Broadband fixed-access networks avoid these resonant absorption lines, but they do
all suffer from weather effects. The wetter the climate, the more fading margin needs to
be taken into account in the link budget.

The impact of weather can be observed on wideband fixed-access performance
specifications, as demonstrated in Table 15.4. Zone K is rather dry (Arizona); Zone N is
rather wet. Note also in the table, which is based on a Lucent product implemented at
10 GHz, how the range reduces with the wider band channel (a wider noise floor).
Availability is specified as 99.99 percent (four nines availability) rather than the five
nines availability of wireline copper access. To deliver five nines availability would
require a higher link budget. Range would reduce further.
As frequency increases, weather effects become more pronounced. In Table 15.5—a
Lucent product implemented at 26 GHz—some additional gain is provided by a separately
mounted (nonintegral) antenna.

Range is also dependent on the modulation used. (Higher-level modulation techniques
absorb more power for the same demodulator bit error performance.) Tables
15.6 and 15.7—a Lucent product implemented at 38 GHz—shows how moving from 4-
level QAM to 16-level QAM requires an additional 8 dB of link budget, a doubling of
the power and some implementation loss. The benefit here is taken as capacity gain
rather than range gain.
As frequency increases, coverage becomes increasingly line of site dependent. The
benefits of a nonintegral antenna (more directivity) become more significant. Note also
the antennas become more compact as frequency increases.
Anumber of vendors are also promoting OFDM/OFDM solutions for broadband fixed
access. Cisco, Broadcom, and Pace have a product that uses OFDM within a 6 MHz channel
to give a duplex 20 Mbps traffic stream or 40 Mbps in a 12 MHz channel. Note the
commonality with digital TV multicarrier OFDM implementation in Europe and Asia.