Re: [Discuss-gnuradio] Transmit and Received Signals are Different

[Martin Braun (CEL)]

On Thu, Jul 26, 2012 at 04:09:31PM -0700, Frederick Lee wrote:
> The channel model block contains a timing offset, and noise adder
> which I didn't use in my simulations. The only things I changed were
> the frequency offsets and the multipath variable (from what I found
> online, it's used to change amplitude and phase shift of the signal).
> The only difference that I can see between the channel model and using
> actual USRPs is that  USRPs up converts the signal into IF ( I believe
> ), converts it to analog, and the daughter board converts it to RF.
> Then the reverse happens in the other USRP. When the two USRPs are
> transmitting, any of the four things that the channel model does can
> happen, but the main ones I'm concerned about is the frequency shift.,
> and the DPSK block seemed to have taken care of that ( at least in the
> simulation ).
> I was using the */gr-uhd/examples/grc/uhd_dpsk_mod.grc and other
> similar examples as somewhat of a guide, and I don't see anything that
> looks very different from what I used. What kind of other adjustments
> must I make?


Frederick,

all kinds of stuff can go wrong.

When you say you see 'nothing' on the FFT sink, do you mean you see only
noise, or literally nothing?

Here's a typical ist of things I do when debugging such a link:

1) Use a spectral analyzer to see if the transmitter is actually
transmitting (where you want it to)
2) Connect an FFT sink directly to the UHD source to see if the signal
looks as expected

You've got a couple of things that surprise me in your FG:
- the packet decoder (I'm not 100% what that does) searches for a bit
string? If so, you have no way to guarantee you're actually receiving
that specific sequence. If you're literally rx'ing nothing, perhaps that
doesn't trigger.
- Why do you LPF bits?

MB

-- 
Karlsruhe Institute of Technology (KIT)
Communications Engineering Lab (CEL)

Dipl.-Ing. Martin Braun
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KIT -- University of the State of Baden-Württemberg and
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