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Forward Error Correction

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

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  1. AG6QR

    AG6QR Premium Subscriber QRZ Page

    No disagreement, but asking for repeats is, by definition, not Forward Error Correction.

    FEC is useful for things like broadcast transmissions, where the receivers don't have the ability to send requests for repeats back to the broadcaster.

    FEC, by itself, can't guarantee that a message will get through. It can be pretty effective at assuring that an errored message won't be mistakenly presented as having been received without error, though.

    Handshaking can be combined with FEC for more robustness, in those situations where it's possible to ask for repeats.
    AI3V and NL7W like this.
  2. KX4Z

    KX4Z Ham Member QRZ Page

    Thanks! Good point. I thought when you REALLY needed things to work, you used both [in the case that the recipient did have the ability to transmit] -- example, on field day you say, "1A 1A NFl north Florida" which is sorta analagous to FEC....and you wait for the other guy to say QSL or ask for a fill. it seemed to me that people have been doing both in ham radio for a long, long time, or analogous things anyway. I just wanted to be sure that i understood what the impact of ONLY having FEC would be.....
  3. SM0AOM

    SM0AOM Ham Member QRZ Page

    The use of FEC in one-way transmission scenarios only gives you a somewhat higher probability of receiving the message correctly or to at least have a decent chance of detecting and correcting errors.

    The downsides with FEC compared to "unprotected" transmissions are a somewhat longer transmission time due to the need to send the parity bits together with the information bits, or to a somewhat higher error rate due to having to increase the data rate.

    If you have the possibility to detect errors and ask for "fills" via a return channel, the protocol is by definition ARQ. There is another scheme around, "Hybrid ARQ", where quite extensive FEC is applied to the transmission blocks,
    so that only the actual missing parts of a longer message have to be retransmitted.

    KX4O and NL7W like this.
  4. KY5U

    KY5U Subscriber QRZ Page

    Forward error correction is sending someone out with the written message to the location by courier so it can be compared with the digital message for accuracy.
    NL7W likes this.
  5. KX4Z

    KX4Z Ham Member QRZ Page

    That hybrid ARQ sounds like a good thing!
  6. W6RZ

    W6RZ Premium Subscriber QRZ Page

    Digital Television is an example of a protocol with FEC. In the US, over the air ATSC uses a 2/3 rate Viterbi code, concatenated with a 187/207 Reed-Solomon code. So 40 percent of the bits sent are error correction bits. But since it's broadcast, there can be no ARQ.

    AI3V and KX4Z like this.
  7. KX4Z

    KX4Z Ham Member QRZ Page

    Thanks! I guess that is why TV just sometimes "disappears" when the signal can't effectively work. 40% overhead! Impressive effort on their part.

    What about cell phones? do they use both FEC and ARQ or just one or the other?
  8. W0PV

    W0PV Ham Member QRZ Page

    Many FD stations, despite many fills, still log me as North or South Florida instead of West Central Florida. They hear "Florida", perhaps don't realize there is another newer section, and give up quickly. :rolleyes:

    So, a strict analogy of voice to ARQ falls apart because even if the human op requests a fill it doesn't mean it will be copied correctly. An erroneous acknowledgement, QSL, may be issued. The data may then remain faulty.

    An ARQ data mode may be the only way to ensure absolute precision xmsn (?)
  9. W6EM

    W6EM Ham Member QRZ Page

    It appears that inquiring minds fear impending loss of data compression?
  10. KA8NCR

    KA8NCR Ham Member QRZ Page

    Sometimes hams target practice with their own toes, unfortunately.

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