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

[Tony Langdon, VK3JED]

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.

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

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. :-)

73 de Tony, VK3JED
http://vk3jed.vk.irlp.net