# OFDM and FEC **URL:** \ **Category:** GNU Radio\ **Created:** [November 11, 2014, 9:33pm UTC](https://www.ruby-forum.com/t/ofdm-and-fec/233021 "2014-11-11T21:33:15Z")\ **Posts on this page:** 3\ **Page:** 1
**Author:** ![addis\_a](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/c4cdca/32.png) [@addis\_a](https://www.ruby-forum.com/u/addis_a)\ **Post date:** [November 11, 2014, 9:33pm UTC](https://www.ruby-forum.com/t/ofdm-and-fec/233021/1 "2014-11-11T21:33:15Z")
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](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](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](http://hal.g7iii.net/GRC/OFDM/ofdm_rx_fcd.py) - Raw python Flowgraph [http://hal.g7iii.net/GRC/OFDM/ofdm\_tx\_usrp.grc](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](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](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](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](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](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](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](http://hal.g7iii.net/GRC/OFDM/ofdm_tx_usrp_fec.grc.png) [Screenshot] [http://hal.g7iii.net/GRC/OFDM/ofdm\_tx\_usrp\_fec.py](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 ---
**Author:** ![Iain\_YoungS\_G7III](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/i/3da27b/32.png) [@Iain\_YoungS\_G7III](https://www.ruby-forum.com/u/Iain_YoungS_G7III)\ **Post date:** [November 18, 2014, 2:46pm UTC](https://www.ruby-forum.com/t/ofdm-and-fec/233021/2 "2014-11-18T14:46:11Z")
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 ---
**Author:** ![Iain\_YoungS\_G7III](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/i/3da27b/32.png) [@Iain\_YoungS\_G7III](https://www.ruby-forum.com/u/Iain_YoungS_G7III)\ **Post date:** [November 19, 2014, 4:10pm UTC](https://www.ruby-forum.com/t/ofdm-and-fec/233021/3 "2014-11-19T16:10:10Z")
On Tue, Nov 18, 2014 at 8:44 AM, Martin B. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) 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