Re: Recording XM signals...an issue

Your points are well taken.....you are right about wimax...I had
forgotten that technology in my zeal to promote SS ;>). On the other
hand, true SS is literally noise....if it is cluttering the frequency
bands to other more commercial users, then it is a not the real thing.

Consider noise: FM...lightning strikes wipe out AM but FM barely
hears a click. The noise immunity in SS is even greater. The beauty of
SS is that a well-designed receiver picks the signal out of literally
the noise...very low power using optimal filtering and signal
tracking. My guess is that WiFi is a cheap imitation and therein lies
the problem. Commercial silicon is still not up to the task...but it
will be.

As for WiMax, it will not be worth the efforts until a true SS
implementation at low prices can be fielded.

As for something for nothing, you are paying for the capability
through use of very wide bandwidth and very expensive transmitters and
signal processing....though with todays production costs for
semiconductor signal processors, I can't image the cost would remain
an issue...it is still likely a performance hit due to marginal
capabilities at affordable commercial prices.

Be that as it may...this discussion is making my head hurt...XM is not
in my future.

Thanks for the dialog.

Henry


On Tue, 29 Nov 2005 01:47:47 -0000, dplatt@xxxxxxxxxxxx (Dave Platt)
wrote:

>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.
.

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