Hi,
Greg Maxwell wrote:
> > Note that for QAM modulation you need I/Q modulation and linear TX and
> > RX
> > signal paths (due to non-constant envelope). This is non-trivial to
> > implement,
>
> You can do the I/Q modulation in entirely in software. This shouldn't be
> too hard.
>
> As for the linear signal paths.. I'm assuming you mean linear power
> responce in frequenceny space. This is corrected using a FIR filter.
> Modems do this.
I assume you don�t mean generating a QAM modulated baseband signal with
the
sound card and modulating that FM onto a RF carrier? This would be a
very funny thing to do, which would be interesting to simulate, but
I would not expect any performance gain from it.
When I talked about linear signal paths I meant we would effectively
have to use SSB transceivers. By the way this is what the 56Kbps modems
do - linear transceiver with linear transverter to 430 MHz or whatever
frequency band neccessary.
Note that the worst point here is having the PA operate in class-A mode,
wasting a lot of power.
All signal amplifiers in the RX signal path need to be gain-controlled
(AGC), in FM transceivers they typically run full-throttle.
> I dont know how you would generate the EQ on a non-point to point link
> though. :(
Equilization on non point-to-point links is usually done with training
sequences (see GSM, in the middle of each packet there is a training
sequence of - hope I am correct here - 26 symbols where only the inner
16 are non-zero).
> Even if we could only use QAM over point-to-point links, I think that
> the prospect of 150Kbit/s (22050 symbols/s, 4I+3Q)
What about 5AFSK? ;)
> over a point-to-point
> radio link using nothing other then a commonly available Celeron 300a
> equipt system
> with soundcard is quite attractive.
It would, in fact I already started implementing n-FSK and FEC
(convolutional, ML (viterbi) decoder). As that worked out to gain
real performance I stopped. We should really try not to make a historic
mistake (like the self syncronizing scrambler in G3RUH modem) which
we can not correct because the system is in use in the original version
all over the world. HDLC is definetly not the way to go with this.
-- Jens
