This question appears in Mumbai University > Wireless Networks subject

Marks: 4 M & 5 M

Year: Dec 2013, May 2013

Adaptive Multi-rate Coding

i. The adaptive multi-rate (AMR) codec has eight source rates; 4.75, 5.15, 5.90, 6.70 (PDC-EFR), 7.40 (IS-641), 7.95 (VSELP), 10.2 and 12.2 kbps (GSM-EFR) for use in UMTS and GSM.

ii. During high cell loading, such as during busy hours, the AMR codec uses lower bit rates to offer higher capacity while providing slightly lower speech quality.

iii. Also, if a mobile is running out of the cell coverage area and using maximum transmission power, a lower AMR bit rate is used to extend the cell coverage area.

iv. The AMR uses algebraic code excited linear prediction (ACELP) coding.

v. At every 160 speech samples, the speech signal is analyzed to determine the parameters of the CELP model (i.e., LP filter coefficients, adaptive and fixed codebooks’ indices and gains).

vi. The speech parameter bits are classified into class Ia, Ib, and II bits based on their sensitivity to errors. Class Ia is the most sensitive; the strongest channel coding is applied to class Ia bits.

vii. The AMR speech codec can tolerate about 1% frame error rate (FER) (about a bit error rate of 〖10〗^(-4)) of class Ia bits without any deterioration of speech quality. For class Ib and class II bits, a higher FER is allowed.

viii.The AMR codec provide discontinuous transmission (DTx) with the help of comfort noise generation.

ix. DTx prolongs terminal battery life and reduces the average required bit rate, providing a lower interference and an increase in system capacity.

x. The AMR also deals with error concealment using frame substitution and muting.

Thus, with the AMR speech codec, it is possible to achieve a trade-off between capacity, coverage, and speech quality as per the service provider’s requirements.