[RFI] What's up at 5Gig?

Jim Jarvis jimjarvis at comcast.net
Sun Apr 18 08:21:39 EDT 2004

Got a note asking "what's the big deal at 5 GHz?"...

Thought I'd comment on what I see, even though I'm far
from an expert.

Right now, 802.11b and g, in their lower 6 channels, 
overlap the amateur 2.4GHz segment used for weak signal
satellite downlinks.  So do cordless phones.  The noise
floor on that band is the pits as a result.

802.11a uses 5GHz, as does 802.16.  So as WiFi and WiMAX
and the other implementations start populating 5GHz, one
would expect to see the noise floor rise.  As demand for 
space increases, I would expect to see firms lobbying for
increases in their allocation.  Note that long distance links
in the so called Uniband are license-free, and not restricted
as to emission levels.

As Tom Clark, w3iwi, explained it in an AMSAT seminar, the
good part of this is that the 5GHz wireless band overlaps our
satellite UPlink, not our downlink.  So as the noise floor 
increases, it won't kill us.  And we shouldn't kill them,
so long as we're transmitting UP to satellites.  

My point was made to heighten awareness of the future need
for vigilance about the DOWNlink band at 5GHz, which lies
about 180MHz above the uplink.  

Why would anyone CARE?  (and shoot, I operate HF, will I ever
use 5Ghz anyway?)  AMSAT is forming plans to orbit multiple high
earth orbit satellites, providing daily coverage by 2009 and
continuous coverage by 2012.  A 5Gig digital transponder, full
duplex, digital signals, such that doppler is not an issue, is
among the packages proposed.  Using software defined radio at
its core, it would enable voice/text/images.  

Ground stations using software defined radios would match up..
at relatively low cost, with small dish antennas.  Work the
world with an 18" dish on a photo tripod, or a patch array lying
on a flat surface.  No towers, restrictions, permits, covenant 
battles...wideband data speeds...

Seems to me it has great potential for space restricted applications
like apartments and controlled communities, as well as for portable
and mobile operations.  Once you get the birds up, it's great for
long range or local emergency communications.  (and, incidentally,
why not for local repeaters, as well.  A whole new ballgame!)

Back to the initial point:  I mention this only to make you aware that
there is need to protect our microwave bands, and that WiMax and Wifi,
and similar technologies from which we benefit...ALSO tend to impact
our spectrum.  

This note composed/distributed via my C band WLAN.  (card in laptop).

jimjarvis at ieee.org 

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