GSM

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

13/04/20160

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
signaling
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
kbit/s)
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