Re: Recording XM signals...an issue

In article <8l9no15p3alngp6rlcatd1sjahlb1ti7gd@xxxxxxx>,
NewGuy <newguy@xxxxxxxxxxx> wrote:

>Wow! Well, I asked for it. I use to work in the spread spectrum world
>and my hope was that technology would eventually render all this
>bandwidth angst to the junk yard. True spread spectrum, like the
>miliary use with very expensive...and in the past...GaAs based
>processors, renders bandwidth an obsolete concern....in so far as
>specrual allocations and bandwidth has the same narrow definition as
>in the past.
>
>With SS, you actually spread the signal over such a large bandwidth
>that any signal looks like noise...non-detectible noise against all
>other signals cluttering any given bandwidth allocation....with enough
>spectrum, signal reception, multipath, power, and signal fidelity and
>noise in the traditional S/N become of historical interest. Sadly, the
>technology in cheap silicon still looks deficient and too expensive.
>However, it won't be long.

T'ain't quite so simple, I'm afraid. You can't get something for
nothing.

Spread-spectrum comes at a price... it does raise the noise level (as
seen by other users of that part of the spectrum) throughout whatever
transmission bandwidth it uses. Although the increase in noise level
created by one SS transmitter may be small and difficult to detect,
put enough of 'em together in one area and you've got a large amount
of added noise energy to deal with. This added noise can degrade both
SS and non-SS uses of the frequencies in question.

The amateur radio community has been dealing with the results of this
for some time. I've been told that in many urban areas, it's no
longer possible to do weak-signal work in the 2.4 GHz ham bands, due
to the presence of many thousands of low-powered 802.11b/802.11g
direct-sequence spread spectrum radios.

> WiMax is suppose to be this technology, but I am not sure. As I look
>at the way WiMax is being implemented, I suspect they are not using
>true spread specturm.

My understanding is that WiMax uses OFDM (orthogonal frequency
division multiplexing), a system with large numbers of closely-spaced
carriers. It's similar to 802.11g in that respect.

There's some new UWB (ultra-wide-band) work being done, which shows
promise for very high bandwidths over very short distances (ideal for
in-home video and that sort of thing). One UWB group is proposing or
using OFDM, while another is using high-speed-impulse modulations
which I suspect are closer to the "true spread spectrum" stuff you
worked on in the military.

--
Dave Platt <dplatt@xxxxxxxxxxxx> AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!
.

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