Hi Adrian,
for CDMA, two things need to be different:
1. you need at least one orthogonal other sequence used by another user / data stream –
that MA in CDMA is multiple access :)
2. In classical CDMA, you would need your data to be a constant that you multiply with
your sequences – if you multiply a signal that changes during the duration of your
sequence, you lose all guarantees that another signal using another orthogonal sequence is
still orthogonal.
> is there really any
> advantage to using it as opposed to standard FDMA as traditionally used on
> QO100? From an amateur radio operator perspective.
Hm, I think the point was distributing the signal energy across a larger range. That can
be done for several reasons – inhibiting detection through an adversary being one (and the
classical origin story of Frequency-Hopping Spread Spectrum), although one that doesn't
apply to amateur radio.
The other one is that in classical FDMA, you take a x kHz wide signal, and put it on one
of a defined set of x kHz wide channels. If something else disturbs that channel, bad
luck. In spread-spectrum systems, you gain robustness agains narrowband interferers.
There's a couple other "side-benefits" you might be getting from a wider bandwidth; from
the ability to more easily transport a higher-resolution time information (nice if you
want to do ranging!) to the fact that if I take some fixed transmit power and distribute
it across a wider band, I still get the same energy per bit that I transmit, but at a
lower spectral power density – and often, that's what is regulated. (that is of course
what allows to make signals harder to detect: pushing the PSD of your signal below noise
floor)
Cheers,
Marcus
