Re: Reed-Solomon Decoding (Not for Digital Television)

[Ron Economos]

Reed-Solomon predates digital television and is used in many applications. Here's a nice article about Voyager 1.

https://destevez.net/2021/12/voyager-1-and-reed-solomon/

The DVB-T Reed-Solomon blocks expose all the parameters, so you can reuse them for whatever block size you'd like. The default for DVB-T is 188 byte input packets and 204 byte output packets for the encoder and the reverse for the decoder.

Here's an explanation of each parameter.

1) p and m are not used. These are leftovers from a previous implementation and retained to keep the block API the same.

2) GF polynomial. This is the Field Generator polynomial and 285 (1 0001 1101) corresponds to x^8 + x^4 + x^3 + x^2 + 1.

3) N. This is the size of the field and is always 255.

4) K. This is the message size. 239 means that there will be 255 - 239 = 16 parity bytes (that can correct up to 8 message bytes).

5) t is not used.

6) Shortening size. This is the number of message bytes padded with zeroes. For DVB-T, 239 - 51 = 188, which is the size of a MPEG-2 Transport Stream packet.

7) Blocks. This determines the vector size of the input and output pins. 188 * 8 = 1504 for the encoder input and 204 * 8 = 1632 for the encoder output. It can be set to 1 if desired.

So you have to chose a message size that you can live with and all your messages have to be the same size (or padded to the same size). And of course, you have to use the encoder at the transmitter and the decoder at the receiver.

Ron

On 12/12/21 12:32 PM, pwhines2@verizon.net wrote:

Hello all,

 

My question is fairly simple, how would I perform Reed-Solomon error correction after demodulating my incoming signal?  I see there are a few uses of RS decoding for digital television, but my signal is just bursts of data and has nothing to do with television.  From what I’ve gathered I think I need to use the FEC Decoder block and somehow tell it to use RS decoding, but I’m not 100% sure.  I would preferably like to use blocks that come with gnuradio for this, but if that’s not possible then I’ll settle for whatever solution is available.

The end of my flowgraph is Binary Slicer > Pack K Bits > File Sink, and I would imagine the FEC would go after the binary slicer.  The packets I’m demodulating now are correct, however there are some errors in the packets while the bursts are demodulated (which is why I need FEC).  Anyone know how I would go about doing this?

 

Thanks,

Patrick