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

[Tony Langdon]

> 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?

In simple terms, an approximation is as follows:

take two digital signals:

One is your desired data (at a relatively slow rate).
The other is a high speed pseudo random bitstream.

Take both signals, exclusively OR them and then feed them into the
transmitter.

Result:  One form of DSSS - no "frequency hopping" involved.

On the receiving end, the same pseudo random sequence is exclusively OR'd
with the received (demodulated) signal, and the original data signal pops
out!

Very over simplified, but you get the idea. :-)

> > Putting a signal on picosecond pulses will _certainly_ cause its
> > spectrum to spread.  
> 
> Why? I don't see how the one implies the other.

Do a Fourier transform on a narrow pulse one day. :-)  A pulse of "infinite
height" and zero duration has a spread of energy from zero to infinite
frequency (obviously physically impossible, but that's the mathematical
result).  That implies infinite bandwidth.  A narrow pulse of finite width
contains a broad range of frequencies (again, this can be proven
mathematically).

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