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From: rcarpen@lan2wan.com
Subject: Re: My Spread Spectrum Letter to FCC
Date: Fri, 29 Nov 1996 22:50:29 -0800
Greg Jones, WD5IVD wrote:

> I think that what people call 'SS' on 902-928 has many definitions. While
> it might be spread, many a Part 15 device does not really have the
> necessary processing gain or even use any type of spreading codes to make
> them really interference resistance and non-interferring. One reason I am
> already using Horz antennas on the 902-928 band was to drop these signals I
> was seeing on the 17db vertical I was first listening on. Thet are now
> down 20db using the horz beam, which makes them disappear.

I'm sure that K5LLL is using horizontal, and he talks about 5 S units increase in noise floor, if I remember correctly.

> As to weak signal operations -- I'll just point to the paper that Phil Karn
> and Tom Clark wrote for the last Central States VHF/FM convention -- you
> want to operate weak signals...try SS with some signal processing
> techniques that Phil has been working on at Qualcomm and you can get down
> to almost theortcial limits.

I was there and Tom gave me a copy before the talk. It was very interesting, but I really hestitate to call anything less than 1 kHz wide "spread spectrum". A carefully designed signal, yes.

> Want to work EME on an AMSAT satellite
> station -- it could be possible. No need for great antennas,
> high-precision radios, and large amounts of power. SS with a little
> computer power. You can read the pdf file at
> ftp://ftp.tapr.org/tapr/ss/eme-2000.pdf Why keep using technology from
> the 50's when new technology that outperforms it a great deal more with
> little or no additional invesment in equipment -- since that equipment is
> probably already setting in your shack or at least being used by you now to
> access the Internet to read this e-mail -- is available.

When you read their numbers, you'll find that people are doing EME with not-too-different power, with human-ear signal processing, TODAY. And with a much less complex system. One of the problems with many "advanced systems" generally is that they have an extremely pronouced threshold; when they're good they are very good, but with tenths of a dB less signal, they're horrid. In commercial service, where the system is designed to ensure at least the required minimum signal, they're wonderful. Weak-sig hams always want to work a station that's just a little weaker.

> How people state or define 'Noise floor' seems to be very relative as well.

No. For weak sig people, it's the limit for the reception of weak signals, using their present receiving equipment. Not at all a complex concept. Either you can hear a weak signal of a certain power density, ON YOUR PRESENT SYSTEM, or the noise floor has been degraded by interference to the point that a stronger signal is needed.

> As the conversation you and Phil Karn had several months ago that went into
> great detail on the subject. What you should do is get Phil on this list
> and you guys can discuss the pros and cons of the various aspects of Spread
> Spectrum and Signal Floor levels in detail for everyone to read. I did
> enjoy the comments and learned a lot. But there is a lot of untold story
> there, when someone just says 'noise floor increase'.

I think that you are missing the point. What the weak sig people are talking about is interference that obscures weak signals with a noise-like SIGNAL. They are NOT talking about how close to random background noise they can go and still recover information; they know that. They don't want other hams to increase the "background noise". The weak sig people don't want their type of operation to be essentially destroyed by noise-like signals used for local communication. Ham bands are shared and newcomer techniques shouldn't render very effect existing techniques useless. The SS designed for local communication will be far different from any spread signals intended for weak-sig application.

The various designed-signal weak-sig schemes, such as the KA9Q / W3IWI talk at CSVHFS - use techniques, primarily redundancy, to overcome weak sig limitations. Wouldn't they have done nearly as well with multiple repetitions of the very constrained transmission content using almost any 500 baud transmission method? Followed (as they proposed) by a non-coherent, but time synchronized, summing technique with a one-minute or more summation time? I don't see how this 1 kHz wide signal compares, in impact on other users of a band, with a frequency hopping or DS SS signal many tens or hundreds of kHz wide. Apples vs. oranges.

73 de Bob w3otc ------ Submissions: vhf@w6yx.stanford.edu Subscription/removal requests: vhf-request@w6yx.stanford.edu Human list administrator: vhf-approval@w6yx.stanford.edu