Forward Error Correction

Discussion in 'Ham Radio Discussions' started by KX4Z, Jul 11, 2019.

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  1. KX4Z

    KX4Z Ham Member QRZ Page

    Please help me understand how Forward Error Correction fits in -- it adds extra data bits so that errors can hopefully be recognized and perhaps corrected, I think.

    But what happens if a message is sent with Forward Error Correction and the transmission of that segment just fails? Does it just leave a gap? That's what my friends tell me happens with FEC modes they use.

    So how does this work when you are sending something IMPORTANT. Like,
    "We need 42 vials of Cephalexin 1 gram and 106 ampules of gentamycin 80 mg/ml and ....."

    if something just drops out, do you possibly receive

    "We need 42 ampules of gentamycin 80 mg/ml......"

    how does this work? it would seem to me that FEC is useful....but not sufficient if the information is of high importance. Am I missing something?

    KX4O likes this.
  2. KX4Z

    KX4Z Ham Member QRZ Page

    And by "fail" or "drop out", I include the case where the data has more errors than the FEC algorithm can it returns nothing. At least that is what my friends tell me happens with some modes with FEC.
    KX4O likes this.
  3. W4EAE

    W4EAE Premium Subscriber QRZ Page

    FEC not designed to guarantee accuracy or readability, but rather to increase them. At the very edge of RF range, digital modes can benefit dramatically from FEC.
    KX4O likes this.
  4. KX4Z

    KX4Z Ham Member QRZ Page

    Hi -- that is sorta what *I* thought -- i thought the best came (for IMPORTANT stuff) when you combined them. FEC would reduce the need for retries, but the ability to RECOGNIZE missing chunks and ask for repeats.....seems of critical importance if what you are sending is at all important. Is there any disagreement with this?
  5. AG6QR

    AG6QR Premium Subscriber QRZ Page

    FEC is a generic term for any sort of encoding that sends data in a redundant way in order that errors may be detected and corrected at the receiving end without requesting retransmissions. There are many schemes for FEC, and they don't all behave equally.

    A common aspect of the strategy is to shuffle the bits and send them out of order, so that, for example, a temporary static burst or bit of QRM would, instead of causing two entire letters to be lost, cause twenty letters to each lose a tenth of their redundantly transmitted information, so that each letter can still be reconstructed at the receiving end.

    There are also hashing and checksumming algorithms to allow errors to be detected even when not enough information made it through to correct the errors.
    K2CAJ and KA0HCP like this.
  6. KP4SX

    KP4SX Premium Subscriber QRZ Page

    Think 'checksums'. Its not as simple as that but a similar concept.
  7. W4EAE

    W4EAE Premium Subscriber QRZ Page

    Yes, but gaps, missing chucks, and obviously inhuman audio are exactly what you hear when FEC fails. If does not 'guess.'
  8. SM0AOM

    SM0AOM Ham Member QRZ Page

    FEC simply works by transmitting more information than actually needed, in the form of parity bits or at more than one time or frequency.

    Sometimes FEC is implemented as transmitting the information twice or more either at different times or on different frequencies and sometimes both, and then there is a time-diversity error-correction.
    This was popular in the 80s, the BR Communications 6028 Time-Diversity modem comes to my mind.

    Depending on the implementation, when the FEC scheme runs out of error correction it either "drops" the lost information or replaces it with one or more "error symbols" to indicate that the errors could not be corrected with the amount of parity bits available.

    If the FEC message is checksum protected, and the checksum does not match, the message will be flagged as corrupt, and depending on higher layers in the protocol stack, it will be suppressed or presented with errors as "best effort".

    In the classic implementation of FEC which is part of CCIR 476 or AMTOR "Mode B" the effects of running out of error correction looks like this:

    Original message:


    Corrupted message when fading or interference makes the parity code run out of error correction:

    THE QUI** **OWN FOX JUMPS *V** THE LAZY *OGS BA*K 12345**89*

    KA2CZU and NL7W like this.
  9. KX4Z

    KX4Z Ham Member QRZ Page

    So what I am hearing from all of you is that it simply cannot guarantee a faithful reproduction of the desired message. It can come close, and is definitely helpful....but lacking the ability to get a is doomed to eventual failure.

    So how would a Field Day work if voice operators were forced to ONLY use FEC -- they can say their exchange 6 times....but they cannot retransmit it if the receiving station misses it....doesn't seem to me that even on voice, anyone would be willing to accept only "FEC"....that since the beginning of ham radio, people asked for "fills".
  10. KX4Z

    KX4Z Ham Member QRZ Page

    Thanks! Lots of smart ways to try and get the data cross....but none of them can guarantee transmission, right?

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