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TCP-group 1995


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Re: Radios for higher speeds,



Apostolos K. Salkintzis wrote in a message to Mike Bilow:

 AKS> This is a great chance for me to set my point... (sorry
 AKS> Doug, this is surely not the type of reply you expected for)
 AKS> From my basic telecommunications knowledge, I'm convinced,
 AKS> that the occupied bandwidth of an FM transmiter is *only*
 AKS> dependend on frequency deviation. So, widen up a transmiter,
 AKS> is equivalent to increasing the frequency sensi- tivity.
 AKS> Isn't that so ?
 AKS> But, then, another question goes to the line...
 AKS> Why it is nessesary to have a 15KHz bandwidth for 9.6Kb,
 AKS> when in the wireline case it is sufficient a 2.4KHz for
 AKS> 28.8Kb ??

You are probably going to get a lot of responses to this, but the basic
confusion is that it is possible to send multiple bits per baud.  It requires
at least 3.2 kbaud to send 28.8 kbps on the telephone line, which works out to
9 bits per baud, and the V.34 protocol will be happier with 3.429 kbaud if it
can get it.  Actually, more than 9 bits per baud are transmitted, with the
extra bits used for error correction.

To oversimplify considerably, what you lose in sending multiple bits per baud
is the signal-to-noise ratio.  This makes intuitive sense, since the ability to
distinguish between fine details of the signal will be lost if the noise is too
high.  This is sometimes known in Information Theory as the "no free lunch"
rule.

 AKS> Also, this frequency game always confuses me...
 AKS> I thought that we have split the allocated bandwidth (say
 AKS> that of the 2m band) into a number of 25KHz channels. Wrong?

We do not consider ham bands to be channelized in the U.S.  There are band
plans which are generally followed, but some of these may allocate some space
for CW (which is pretty narrow) and other space for 100 kHz data channels.  We
usually follow either 15 khz or 20 kHz separation on the 144.5-148 Mhz band.

 AKS> How can these bandwidth (Doug talks about: 60KHz, 90KHz) be
 AKS> fitted in this picture ?

Hams tend to use simple FSK.  This is reasonable, since hams have to contend
with a lot of things that telephone modems do not, particularly multipath
distortion.  If you use phase to encode lots of information, which is how
telephone modems work, then you tend to be very susceptible to things which
distort phase.  Hams are usually able to get better results by running FSK over
wider bandwidth than trying to pack in multiple bits per baud, even if they
have to go into the UHF or microwave bands to do it.

-- Mike





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