[By Subject] [By Date] [By Sender [By Thread]
Previous In Time

Next in Time

From: karn@qualcomm.com
Subject: Re: REALITY CHECK!
Date: Thu, 5 Dec 1996 22:27:31 -0800 (PST)
I don't really care who said what about whom.


>Mr. Karn and Dr. Clark, may we now return to the original question that
>started this whole string on this reflector: can spread-spectrum and
>weak-signal enthusiasts co-exist on the same amateur bands without a
>degradation in either's existing capabilities? Harry Brown, W3IIT, after all
>of the above had been posted here, has already attempted to bring us back to
>the original subject by publicly posting a related question which was
>addressed to Dr. Clark; I have not seen a reply as yet.

Sure, why not.


>Please keep in mind that not all other amateurs are conversant with the
>theory and applications of ss; concepts such as the various codings, etc.,
>that you, Dr. Clark, have mentioned, are not only alien to our way of
>thinking, but also of very little interest to most weak signal enthusiasts.
>All that we REALLY want to know is:

And this is the part that shows you haven't been paying attention. We are not just asking the weak signal to tolerate the use of new modulation and coding techniques on the ham bands. We are trying to point out that perhaps nowhere else are they of as potential benefit as in weak signal communications!

It is common for weak-signal enthusiasts to spend enormous time, resources and effort improving their station's capabilities by very small amounts. E.g., doubling the size of an antenna array generally buys, at most, 3dB. The coding schemes that Tom and I have outlined can provide performance gains that are dramatic in comparison.

System gains of up to 7 dB on the nonfading additive white gaussian noise (AWGN) are typical in a coded system. The gains on a fading channel (including ionospheric, tropo and EME reflection but excluding satellites, which are typically AWGN) can be considerably more, often 30-35 dB over uncoded systems. And gains against pulsed interference, such as from the radars that share all our upper bands, can be even greater. These gains are all just as real as those you get from building bigger antennas, more powerful antennas and quieter preamps.

Don't these improvements interest anyone in this group? Why are they so "alien", new and threatening to the amateur community? Actually, they're "new" only in the context of the amateur service, having long ago become standard practices in the non-amateur outside world in which we are becoming increasingly irrelevant.

Example: at the recent AMSAT meeting I met a recently licensed ham who has worked in commercial satellite communications for some time. In private, this person expressed shock at the primitive, brute-force approach that prevails in the amateur service. Slow data rates. Poor reliability. Ridiculously large antennas and RF power outputs. Little, if any, reuse of spectrum. Very little, if any, careful system design in evidence. Just what are we doing to justify our allocations, this person wondered.


>What do you, as an expert on spread-spectrum communications, predict will
>happen to our noise floor on the various V/UHF amateur bands as heard on our
>typical narrowband receiver systems when one or more ss stations operate on
>the same band utilizing whatever transmission parameters might be expected
>to be considered normal by other ss users?

This question can only be answered by analysis and experimentation using real numbers for location, propagation constants, antenna gains and patterns, power levels, receiver system temps, processing gains, etc. The same can be said for *any* amateur communications that share the same amateur band, whether these communications are narrowband or wideband.

Phil ------ Submissions: vhf@w6yx.stanford.edu Subscription/removal requests: vhf-request@w6yx.stanford.edu Human list administrator: vhf-approval@w6yx.stanford.edu