This kind of 'trick' can also be/is employed on what
I call 'discrete frequency' systems like the TDMA
systems using a 'channel equalizer' that works with
the delays contribited by the different propagation
'paths' that otherwise can intoduce ISI (Inter-Symbol
Interference) due to this 'Time Diversity' effect.

By 'equalizing' each uniquely identified delay path (and
tracking same as the delay will change as a subscriber
moves about) and it's associated 'channel' these
'channels' may be 'equalized' before being numerically
summed to represent the true digital waveform that was
transmitted - and is now being 'demodulated'/decoded.

So, this approach isn't unique to SS/CDMA systems only.

Jim


----- Original Message -----
From: "Barry Gershenfeld" <barry@ZMICRO.COM>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Monday, April 15, 2002 8:56 PM
Subject: Re: [OT]:Spread Spectrum Efficiency for Continuous signals


> >I haven't seen anybody mention the advantage of eliminating multi path
> >fading problems with SS.
>
> I didn't want to get that deep into it :)
>
> >In an industrial environment with transmitters moving around you could
have
> >several paths from the transmitter to the receiver due to the signal
> >bouncing on the surrounding structures inside a building. When two
> >different paths differs in length with a multiple of one half wavelength,
> >the signal could effectively be blanked out at the receiver. By
> >continuously changing the channel this problem could be limited to just
> >some channels.
>
> This is called "diversity reception" and might be applicable while
> hopping, but SS is different enough that you can't just ignore
> some channels.
>
> >This requires that the system allows that data that is lost at some hops
> >could be retransmitted in another hop, just like collisions in a CSMA
> >scheme, which implies that it isn't suited for continuous transmission.
>
> As I said before, in SS each bit is spread over several hops.  And it's
> worse (or better) than that.  The hopping is so fast that the each
> "hop", as we would think of it, doesn't look like a discrete signal
> on a certain frequency.  It is a wideband signal that covers a large
> range.
>
> >I don't really know if this works in practice but it is one of the
> >advantages that is mentioned in favor of using SS methods.
>
> What they actually are able to do is see the echoed signal being
> received at a later time (remember SS clocks are _fast_) and
> so they can recognize and filter them out.
>
> When I heard this last item, I said, "If they can see them they ought
> to be able delay them and add them back into the real signal to
> recover even more of the signal."   I don't know if that's been
> done but it seems obvious so I expect they have.
>
> >>
> >> -Brandon Irwin
> >>
> >> -------------------------------------
> >> This message was written using the Dvorak keyboard layout.
>
> lgid ks mddk tsfw mo h.sipv
> or is it
> bcj. yr m..y frgw mp ekrpat
>
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