How is Forward Error Correction implemented in Unet audio?

Question

How is Forward Error Correction implemented in Unet audio, and what approach should be used to work on the existing algorithm or to add a new FEC technique to the Unetstack?

Answer 1

The Unet audio community version implements a ½ rate convolution FEC, which can be enabled or disabled using phy[1].fec or phy[2].fec parameters. If you disable the FEC and there are any errors in the received frame, the CRC check will likely fail, and you'll get a BadFrameNtf instead of a RxFrameNtf. The BadFrameNtf message will have the erroneous bits available as data. If you want to implement your own FEC, you can use these as hard decisions from the PHY layer and do your decoding.

So, to implement your own FEC:

1. Disable the default FEC on transmit and receive modems (phy[2].fec = 0).
2. Create your own uncoded frame by including source address, destination address, and protocol number along with your data bits. Compute a CRC and include it in the frame.
3. FEC encode your uncoded frame and send the encoded bits using a TxRawFrameReq from the transmitter. The "raw" frame request tells UnetStack not to add its default headers.
4. Receive the corresponding BadFrameNtf on the receiver and extract the received bits from data field.
5. FEC decode the received bits to yield the uncoded received frame.
6. Check CRC on the uncoded receive frame. If that passes, then extract the source address, destination address, protocol number and your data from the uncoded received frame.
7. If you want the rest of the stack to process this as if it came from PHY, you can send a RxFrameNtf with relevant fields filled in and send it on the topic(phy).