Re: [Discuss-gnuradio] How to synchronize feedback between blocks?

On Wed, Apr 28, 2010 at 11:29:16PM +0000, Nick Foster wrote:
> I'm struggling with what I hope isn't a naive problem regarding feedback between flowgraph blocks. I've written a simple frequency estimator for FSK signals based on the squared FFT method. Its input is an N-point FFT, created by an N-item stream-to-vector block feeding an N-bin FFT block. The output of the frequency estimator is a float value in Hz, and it is a sync_block. Therefore, the output produces samples at the original (pre-stream-to-vector) sample rate divided by N. So if my original raw sample rate is 50000, and I use a 4096-bin FFT, the output frequency estimate arrives at ~12.2 samples per second. That's fine.
>
> The problem is that I want to use the output of the frequency estimator to center the input data at baseband. With continuous streaming data, this is easy -- just use a frequency-xlating filter, and call set_center_freq() when data comes in from the frequency estimator. However, the packets I'm interested in are short, and I'd like the frequency correction to be applied at or near the start of the packet so the preamble can be detected appropriately. In order to do this, I have to be able to determine the delay it takes for feedback to come back, and that delay has to be constant. For this reason, I can't use a frequency-xlating filter, because the delay in calling set_center_freq() from the main Python loop could take any amount of time. I could write a version of the frequency-xlating filter that accepted a second stream of filter offset data at the input sample rate -- my thinking is that would make the delay a fixed number of samples. Is that assumption valid?
>
> Before I go reinventing the wheel, I'm wondering if there is a "usual" way of solving this feedback problem within the architecture of GNU Radio, or if I'm totally out in left field.

Hi Nick,

I recommend tracing the flow graph of the OFDM system. That helped me a
lot understanding how to do synchronisation in GNU Radio.

However, I think there's no direct way to feed back data between blocks.
You need to create a feed-forward structure, i.e. have a synchronisation
block near the beginning that outputs the frequency offset to your
frequency correction.

g
mb

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