Re: [Discuss-gnuradio] OFDM and FEC

[Martin Braun]

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

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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