OFDM and FEC

GNU Radio
1

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 :slight_smile:

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

2

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

3

On Tue, Nov 18, 2014 at 8:44 AM, Martin B. [email protected]
wrote:

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