[Discuss-gnuradio] OFDM and FEC

[Iain Young, G7III]

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