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

Next in Time

From: karn@unix.ka9q.ampr.org
Subject: W3OTC's quote from the W3IWI/KA9Q EME paper
Date: Tue, 3 Dec 1996 02:37:44 -0800 (PST)
As usual, Bob Carpenter quotes very selectively and out of context to try to give the impression that our own paper argues against the use of spread spectrum on EME.

As I'm sure my co-author will agree, nothing could be further from the truth. Those quotations about the physical limits of the EME channel (coherence times, delay spreads) all underscore the potential utility of spread spectrum. The fading multipath channel is where spread spectrum really shines. We said so numerous times in both our written paper and in our presentations.

The specific purpose of the CSVHFS paper was to outline an approach to a short-term demonstration of DSP-aided QRP EME that could work even with an existing narrowband transceiver, i.e., nothing wider than SSB. We did not imply, nor did we intend to imply, that it was the best possible signal design for the EME channel. The use of a narrowband transceiver severely constrained the coding and modulation methods that could be used; the whole EME problem would be *far* easier to solve were we able to use more than 2.7 KHz of bandwidth.

In fact, Bob completely ignores the later part of the presentation where I show that the link budget for a pair of "regular" EME stations could support a ~300 bps user data rate, vs ~1bps for the Oscar-class station pair. However, simply scaling up the proposed 1bps modulation scheme (which alone would take ~64 KHz of spectrum) would probably not work because of the EME channel delay spread limits. Even more bandwidth (either to increase M in the modulation, or to perform true PN spreading) would be required to mitigate the intersymbol interference that the delay spread would otherwise cause.

Phil

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