[MLB-WIRELESS] ./: Can 802.11 Become A Viable Last-Mile Alte rnative?

[dwayne]

"Tony Langdon, VK3JED" wrote:
> 
> At 09:17 PM 14/05/2002 +1000, you wrote:
> 
> >Well, not really, spread spectrum involves rapidly hopping from one
> >frequency to another, with so little time in each frequency that there
> >is little overall interference.  UWB involves transmissions across the
> >entire RF band of picosecond duration, but with very little power being
> >transmitted.  I think.
> 
> Spread spectrum can take many forms.  802.11b, for instance, does _not_ use
> frequency hopping.  It uses direct sequence which involves modulating the
> signal with a high speed "chipping" code to spread it across a wider bandwidth.

Um?

As far as I am aware, they BOTH jump from frequency to frequency.

Direct sequence uses an unchangeable sequence hard-coded by the
manufacturer.
Frequency hopping randomly jumps between a bunch of assigned
frequencies.

But both types essentially rapidly change frequencies to avoid
interference.

What does "It uses direct sequence which involves modulating the signal
with a high speed "chipping" code to spread it across a wider bandwidth"
mean?

> >Errr, sure, but you seem to have the wrong idea of SS. It's all about
> >frequency hopping, this is all about spreading the signal across the
> >frequency domain while concentrating it in the time domain.
> 
> FH is only one form of SS, DS is another (and we should be familiar with
> both of those, as they have been or are being used for WLANs) - there are
> others.  

As far as I know there are just those two.

> Putting a signal on picosecond pulses will _certainly_ cause its
> spectrum to spread.  

Why? I don't see how the one implies the other.

> And UWB is likely to have similar characteristics to
> what we know as "spread spectrum" - high interference rejection and low
> interference to conventional (narrowband) systems.  With that kind of
> bandwidth, the amount of signal in a given MHz bandwidth is going to be
> very small.

Yup, that's the idea.
 
> As an aside, 802.11b is not much of a spread spectrum system at all - There
> are non spread modulation systems that would use 11 MHz for 11 Mbps.
> 
> Anyway, as you say, I may be wrong too. :-)

Oh boy, here we go  :)

Dwayne

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