How Adaptive Multi-Rate Codec (AMR) work in GSM ?

  • Adaptive Multi-Rate (AMR) codec consists of a family of codecs (source and
    channel codecs with different trade-off bit-rates) operating in the GSM FR
    and HR channels modes
  • The AMR system exploits the channel performance and robustness added by
    the coding rates by adapting the speech and channel coding rates according
    to the quality of the radio channel
  • AMR adapts its error protection level (select its optimum channel mode and codec
    mode) to the local radio channel and traffic load conditions to deliver the best
    possible combination of speech quality and system capacity
  • Codec mode adaptation for AMR is based on received channel quality
    estimation in both MS and BTS, followed by a decision on the most
    appropriate speech and channel codec mode to apply at a given time
  • The basic AMR codec mode sets for MS and BTS are provided by BSC via layer 3
  • MS shall support all speech codec modes, although only a set of up to 4
    speech codec modes is used during a call
  • GSM FR/EFR channel gross bit-rate is 22.8 kbit/s in GSM FR/EFR: 13 kbit/s
    speech coding and 9.8 kbit/channel coding (HR channel gross bit rate 11.4
  • For AMR case, different codecs use different bit rate to encode speech (source
    coding). The rest of the gross bit-rate is used for channel protection

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