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Technology Stocks : Winstar Comm. (WCII)
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To: SteveG who wrote (4876)4/1/1998 12:08:00 PM
From: Bernard Levy  Read Replies (2) of 12468
 
Dear SteveG:

You have to keep in mind that at 38GHz, the signal to
noise ratio (SNR) is significantly affected by rain or
in the case of paths affected by foliage, wind
conditions. So the signal to noise ratio changes with
wheather conditions. Thus plugging in measured SNR figures
in Shannon's formula would be rather meaningless.
This is unlike copper, where a very complete database
of attenuations as a function of loop lengths and
frequencies is available. Note also that within a cell,
under LOS conditions, attenuation increases as the square
of the distance to the receiver. So capacity is distance
dependent. I believe however that OFDM schemes have
been implemented for some satellites in the Ku band
(around 12-18GHz), so that OFDM may not be totally far
out for LMDS. Another idea which has been tested in Japan
consists in using adaptive modulation patterns, where
depending on the prevailing SNR conditions, the
modulation pattern is varied between QAM (requires
high SNR) and QPSK (low SNR).

The need for high frequency circuits comes in from
the fact that if you are processing a signal with
a 100MHz bandwidth, and need to perform an A/D
coversion, the A/D converter will typically need
to operate at about 1GHz to achieve a sufficient
bit accuracy. Note that several non GaAs technologies
are just around the corner: SiGe and SOI.

Best regards,

Bernard Levy
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