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REALITY CHECK! (Received DSSS Signal levels); DUCT: 1/r
7 Dec 1996 22:32:19 +0200
In Europe we have (had?) a so called SYLEDIS pulse transmission system for place location which is very broadband, but not SS. It has been operating in the 430 MHz and higher bands and during duct conditions it is known to blank out weak signal communications over hundreds of kilometers.

`I have been building DSSS measurement systems in the 2 GHz range, and I have a lot to say (but not this time) about the features and effects of this modulation. I am not against the spread-spectrum usage, but I would strongly recommend to initially limit it to one or two higher bands, OR allow SS-type of schemes at lower frequencies with bandwidth limitations equivalent to NBFM, say 15 kHz. Indeed very interesting things can be done with narrow-band spread-spectrum especially in the monitoring of propagation conditions.

The 15 kHz bandwidth limit is due to the wide availability of analog filters. This is important, since SS generation schemes are often by nature VERY wide-band, so a 15 kHz wide IF-filter could take the sidebands down sufficiently. However, a MAL-functioning SS-trans- mitter is able to regenerate the wide spectrum at any stage after the filtering, and this is where extreme caution is needed. Certainly there is nothing good to be said about a splattering SSB signal, but at least it is intermittent, as a DSSS-signal could be permanent all the way through one transmission.

I would go for some narrow-band-SS experiments on the lower VHF right away, and for the wide-band (but limited to part-band) stuff on the higher bands.

73, Zaba OH1ZAA / NNoY

P.S. There is a lot to be said about power control with respect to system throughput. Also frequency hopping is a completely different matter, not adressed above. In all the matter is too complex for a quick grasp, but therefore caution is needed before wide spread use on the bands.

`At 23:02 6.12.1996 -0800, wrote:
>Robert,
>
>You're quite right -- the value 4 for the distance propagation factor
>is only an empirical approximation to reality. It varies from 2 for
>very short distances where propagation is line of sight up to more
>than 4 in some cases. Lee's other book (Mobile Communications Design
>Fundamentals, ISBN 0-672-22305-8) cites a variety of experiment-based
>models giving a range of distance exponents: 3.84 for a generic
>"surburban" area, 3.68 for Philadelpha PA and 4.31 for Newark
>NJ. Japanese studies show greater attenuation (particularly in Tokyo),
>probably because of that country's denser cities. The models are quite
>detailed and include antenna height and operating frequency as
>parameters.
>
>I do know that at work everybody seems to use