The present document presents the outcome of the Study Phase initiated at SMG#20 on the concept of the Adaptive Multi-Rate (AMR) codec. The AMR concept represents a new approach to achieving consistent high quality speech combined with efficient spectrum usage. It was recognized at SMG#20 that this would require novel techniques whose feasibility should first be assessed before proceeding with a full development of the AMR system and its control.
to 4 provide an overview and background to the AMR concept. Clause 5
provides a basic description of the AMR system functionality including working assumptions that have been agreed during the Study Phase. Feasibility aspects are considered in clause 6
. The requirement specification, which will form the basis for the development phase, is contained in clause 7
. Clause 8
gives an indication of the MS and network upgrades needed to support AMR. Clause 9
outlines how the AMR codec will be developed, tested and selected including the proposed time-plan. Clauses 10
and 11 conclude with a summary of the risks and recommendations.
The principal goals of the AMR codec as presented to SMG#20 (October 1996) (see SQSG report, Tdoc SMG 447/96) are to provide:
wireline quality combined with capacity advantages offered by half-rate operation;
increased robustness to high channel error rates when operating at full-rate.
This is to be achieved by controlling the channel and codec modes according to the radio channel conditions and traffic loading.
To address the needs of certain GSM markets, especially in the US, and to ensure the earliest provision and penetration of AMR handsets, it was also decided at SMG#20 to set an aggressive time-scale for the development and standardization of the AMR. This requires that the AMR specifications be ready for GSM Release 1998, i.e. end of 1998.
The terminology and acronyms used in this report are given in Annex A
Most speech codecs including the existing GSM codecs (FR, HR and EFR) operate at a fixed coding rate. Channel protection (against errors) is added also at a fixed rate. The coding rates are chosen as a compromise between best clear channel performance and robustness to channel errors. The AMR system exploits the implied performance compromises by adapting the speech and channel coding rates according to the quality of the radio channel. This gives better clear channel quality and better robustness to errors. These benefits are realized whether operating in full-rate or half-rate channels.
As well as quality improvements, the need to enhance capacity by allocating half-rate channels to some or all mobiles is also recognized. The radio resource algorithm, enhanced to support AMR operation, allocates a half-rate or full-rate channel according to channel quality and the traffic load on the cell in order to obtain the best balance between quality and capacity. It is intended that the control system should not be fixed but can be enhanced as experience of the AMR system is gained on real networks; it may also be tuned to meet particular operator's network needs.
The AMR codec concept is adaptable not only in terms of its ability to respond to radio and traffic conditions but also to be customized to the specific needs of network operators. There are three levels of adaptation of the AMR system:
handovers between half-rate and full-rate channels according to traffic demands;
variable partitioning between speech and channel coding bit-rates to adapt to channel conditions in order to obtain best speech quality;
optimization of channel and codec control algorithms to meet specific operator needs and network conditions.
This allows the codec to be applied in many ways of which three important examples are:
full-rate only for maximum robustness to channel errors but no capacity advantage. This additional robustness may be used not only to extend coverage in marginal signal conditions, but also to improve capacity through tighter frequency re-use, assuming high AMR MS penetration;
half-rate only for maximum capacity advantage; more than 100 % capacity increase achievable relative to FR or EFR (i.e. same as existing HR); significant quality improvements relative to existing HR will be given for a large proportion of mobiles as a result of codec mode adaptation to channel conditions;
mixed half/full rate operation allowing a trade-off between quality and capacity enhancements according to radio and traffic conditions and operator priorities.
This is explained further in Annex B