Re: [Discuss-gnuradio] OFDM and FEC

[Tom Rondeau]

On Tue, Nov 18, 2014 at 8:44 AM, Martin Braun <address@hidden> wrote:

Oh wow, I wanted to answer this a while back. Guess I didn't.

First, have you checked out gr-mac? It looks like you're duplicating
those efforts. I know that someone (might have been Tom) has done this
(OFDM + FEC).

One thing I've noticed is that you do FEC->CRC on tx, and FRC->CRC on
rx. You might want to make sure the order is inversed on tx and rx.

Cheers,
M

Yes, you can check out my fec_gmsk on my fork of gr-mac. This was really just for my testing purposes of the new FEC API that we put into GNU Radio, so it's not very clean. But you might be able to see some of the work done there to get the FEC blocks properly integrated.

The OFDM world is a bit more difficult from what I recall. There isn't an opening in the flowgraph at the right place to put in the FEC encoder and/or decoder. But looking at what I did with the GMSK modems to fit them in might help clue you in there.

Tom

 

On 11/11/2014 09:31 PM, Iain Young, G7III wrote:
> Hi Folks,
>
> I am attempting to get some FEC working with my GRC OFDM Transmitter
> and Receiver, which I created by modifying the example versions. You
> can see the flowgraphs at:
>
> http://hal.g7iii.net/GRC/OFDM/ofdm_rx_fcd.grc - GRC FCD RX Flowgraph
> http://hal.g7iii.net/GRC/OFDM/ofdm_rx_fcd.grc.png - FCD RX Screen-shot
> http://hal.g7iii.net/GRC/OFDM/ofdm_rx_fcd.py - Raw python Flowgraph
>
> http://hal.g7iii.net/GRC/OFDM/ofdm_tx_usrp.grc - GRC USRP TX Flowgraph
> http://hal.g7iii.net/GRC/OFDM/ofdm_tx_usrp.grc.png - USRP TX Screen-shot
> http://hal.g7iii.net/GRC/OFDM/ofdm_tx_usrp.py - Raw Python Flowgraph
>
> Currently they use a UDP Port for convenience, but I will be migrating
> them to use a tuntap PDU in due course. And yes, I know by some
> standards these are very low bandwidths, but 125k is quite high for Ham
> purposes :)
>
> There is a USRP receiver as well, but I'm working with some folks that
> only have a Funcube Dongle Pro plus, so am concentrating on that receive
> flowgraph for now.
>
> These flowgraphs do work. I do get data to transfer between the two
> ends, however:
>
> I transmit a test file of 1000 lines. Not all lines make it, some get
> missed, and some get corrupt (missing characters), so having found the
> FEC examples, thought I would try and hack them into my flowgraphs.
>
> The resulting flowgraphs are here:
>
> http://hal.g7iii.net/GRC/OFDM/ofdm_rx_fcd_fec.grc [GRC FCD RX w/FEC]
> http://hal.g7iii.net/GRC/OFDM/ofdm_rx_fcd_fec.grc.png [FCD RX w/FEC s/s]
> http://hal.g7iii.net/GRC/OFDM/ofdm_rx_fcd_fec.py [Raw Python Flowgraph]
>
> http://hal.g7iii.net/GRC/OFDM/ofdm_tx_usrp_fec.grc [GRC USRP TX w/FEC]
> http://hal.g7iii.net/GRC/OFDM/ofdm_tx_usrp_fec.grc.png [Screenshot]
> http://hal.g7iii.net/GRC/OFDM/ofdm_tx_usrp_fec.py [Raw Python Flowgraph]
>
> Now, when testing with the dummy encoders, everything behaves exactly
> as if the FEC wasn't there (Okay, this was good, as it's exactly what I
> expected, but at least it proved I hadn't messed up entirely!])
>
> So I got adventurous, and started to play with some of the other
> encoders and decoders. This turned out not so good, as after a number
> of "experiments" I could not get any data transferred when using the CC
> (&CCSDS) encoder and decoder pairs, nor when using the Repetition
> encoder/decoder pair.
>
> However, when running the replicator encoder with the dummy decoder, OR
> the replicator decoder set to 1 replication, I did see 3 copies of each
> character arrive.
>
> When running the CCSDS and CC encoders with the dummy decoder, I saw
> "garbage", so I must have the code partially correct.
>
> Can anyone tell me where I might not be getting it quite so right ? Or
> even any working examples of OFDM and FEC ?
>
>
> Best Regards
>
> Iain
>
>
>
>
>
>
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