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[aprssig] Looking for 9600 baud equipment

jimlux jimlux at earthlink.net
Tue Sep 30 13:35:44 UTC 2008


	I still think we need something MUCH faster than 1200 or 9600 -- 56k
ought to be a cinch, since we can buy telephone modems to do it, but of
course we'd need to go to a much higher frequency band as well.

	7 3
	Earl

--------

Heck, 802.11 goes 50 Mbps..

It's all a matter of how much power and bandwidth you have, both of 
which are limited in typical inexpensive ham gear. 1200 and 9600 are 
both compromises driven mostly by the desire to use an existing 2m 
radio, and, in the case of 1200 bps, the desire to use surplus modems 
based on the Bell 202 standard from the 1950s.  Bell 202, in particular, 
was really designed to accommodate typical wireline properties. 9600 is 
a bit better, being specifically designed to work with typical 1970s era 
2m radios.

The 56kbps phone line modems are totally unsuited to RF links for a 
variety of reasons, but that doesn't mean that there aren't other 
modulation schemes that can get 10-20 bps/Hz sorts of spectral 
densities.  The point to point microwave links that do GigE are an 
example (using 64QAM and lots of coding).  In practice though, I would 
question whether that much sophistication is worth it. Those modulations 
tend to be very tough to use in a varying multipath or doppler 
environment. Why not go higher in frequency where there's more 
bandwidth, and do something easy, like QPSK or 8PSK.

Jim, W6RMK




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