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Re: [Discuss-gnuradio] RFNoC FFT Size


From: Simon Olvhammar
Subject: Re: [Discuss-gnuradio] RFNoC FFT Size
Date: Thu, 3 Mar 2016 17:48:26 +0100

Thank you for your answer Marcus. Do you know when support for fragmentation of big vectors will arrive in RFNoC, are we talking more than a year or just a couple of months/weeks?

Best regards
Simon

On Thu, Mar 3, 2016 at 5:20 PM, Marcus Müller <address@hidden> wrote:
Dear Simon,

it's actually really just a constraint of how RFNoC packages data for transport. It doesn't support fragmenting big vectors (yet), so your vector has to fit one packet. Since the maximum size a packet can carry is 9000B, and a sc16 complex is 8B long, you only get 9000/8-long FFT output, or shorter.

> Ok thanks, so it's a limit in the RFNoC software and not related to Ettus FPGA hardware or something like that?
Now, whether our FPGA code having that limitation is a software or an FPGA hardware problem is up for discussion, in my opinion :)

A possible workaround would be writing your own wrapper for the Xilinx IP core used to actually do the FFT, which would be able to fragment. But then again, the question is what you want to do with the output afterwards.

Best regards,
Marcus


On 03/03/2016 05:10 PM, Simon Olvhammar wrote:
Ok thanks, so it's a limit in the RFNoC software and not related to Ettus FPGA hardware or something like that?
I dont see that limit in regular GNURadio and 8192 channels works fine, isnt the packet size also max 9000 there?
Simon

On Thu, Mar 3, 2016 at 3:45 PM, Marcus D. Leech <address@hidden> wrote:
On 03/03/2016 05:21 AM, Simon Olvhammar wrote:
Hi,

I'm wondering what it would take to modify the current FFT RFNoC block to be able to compute a 8192 channel FFT. Someone that can guide me in the right direction? I'm a quite experienced programmer, however not in HDL.

Best regards
Simon


The main current stumbling block to large FFTs in RFNOC is that the FFT output has to be able to fit into a single transport packet, and
  that is limited to 9000 bytes, which pretty-much limits you to 2048-bin FFTs.  There will be work in the future to allow FFT outputs in
  RFNOC to span multiple packets.




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