On Thu, Sep 12, 2013 at 11:47 PM, Matthieu Tourne <matthieu.tourne@gmail.com> wrote:
Hi!


On Thu, Sep 12, 2013 at 7:33 PM, Marc Juul <juul@labitat.dk> wrote:
Hi!

Some of us at sudo room / east bay mesh are working on a low bandwidth disaster recovery mesh project based on software defined radios.

We're prototyping with a combination of USRP1 and RTL-SDR devices and GNU Radio.

We're having some issues getting the receivers to demodulate our data streams (using DQPSK and QAM modulation).


I never thought about doing much more than tuning to FM radios with my rtl-sdr, but that's a pretty cool project!


Thanks! You should come hack with us!
 
I think that depends what's your goal, if you want to decode qpsk frame from classic radios [1] I think rtl-sdr can do it.

If you want to Tx/Rx data accurately, I think you need a digital modulation chip [2] 
(my understanding is that it works best at the hardware level if you want a decent bandwidth) 

The RTL-SDR is definitely not a high quality SDR, but we have very low bandwidth requirements. For the transmitters we're only using USRPs while prototyping. We're also working on a cheap transmit-only SDR circuit, but that is definitely a longer term project and for now we just want a full functioning prototype.
 
Something like this [3] could help, if you know someone who can program fpga cards ..

That's lower cost than the USRP, but still more expensive than we'd like.
 
But it's a pretty cool project, maybe you can use an OFDM stream instead [4][5]. 
That's the hd tv data stream the chip RTL2832U [5] in the rtl-sdr is meant to demodulate in hardware. 

I hadn't actually considered using the built-in de-modulation :-S I'll look into that.
 

.. I even found an opencore for ofdm modulation [7], which could (in theory) be loaded on an fpga ..

Interesting. Thanks for all the links!
 
Cheers,

Matthieu