Hi Steve:
2 points, one on your wireless question, the other one
on xDSL.
a) First, when you go from 28 to 38GHz, you need to
overcome 2 effects: greater frequency attenuation,
and greater rain fade. As I indicated earlier, if you
use antennas of the same size at 28Ghz and 38Ghz,
the increased frequency attenuation is more than made up
by the increased transmitter and antenna gains. However,
the increased rain fade effect can be quite severe and is
not quite made up by the combined effect of the increased
antenna gains.
You are asking for a very precise quantification for a
computation I have never liked. As WW pointed out,
the rain fade effect is computed based either on 99.9%
or 99.99% service availability. This means that
in every area, you are trying to find the most intense
rain storms and you are trying to design a system which can
handle the rain fade under these worst case conditions.
But then you are trying to draw BW conclusions which hold
on average. To give you an example, where I live we
don't see rain typically from May to October, but
2 years ago, we had a set of unbelievably intense rain storms
which created massive floods for the Sacramento river and
all its tributaries. I am almost certain that a rain fade
computation would use these storms as benchmark, and based
on the attenuation figures used by rain fade models, would
conclude that the information carrying capacity of 100MHz
at 38Ghz is much less than 100Mhz at 28Ghz. However, this is a
computation based on may be 2 or 3 days out of several years.
Most of the time, the information carrying capacity of 100MHz
at 38Ghz is at least equal to 100MHz at 28Ghz. You are
using worst-case conditions to draw some conclusions
which hold on an average basis. I am sure WW may have a
slightly different opinion, but except for system design
purposes (such as sizing of cells), I have never been
comfortable with drawing very general conclusions out
of a rain fade analysis. Note that even for system design
purposes there are all sorts of contingencies that can be
used for handling large rain storms (increase in power,
fallback in modulation scheme from higher order QAM to QPSK,
etc...)
b) I know you tend to be somewhat pessimistic about xDSL,
but in the residential market, I fail completely to see
what advantage an HFC cable structure would have
over FTTC+VDSL for the last 100 feet. The architectures
are similar (they make a similar use of fiber), but
VDSL will not only carry more information than cable,
it is dedicated to each user. In the long run, I really fail
to see how cable will dominate over xDSL in residential
markets (for business markets, like most posters on this thread,
I must plead guilty to a wireless bias).
Best regards,
Bernard Levy |
