10.703GPPGSM Adaptive Multi-Rate Speech Codec (AMR)Project schedule and open issues for AMRTS
Unlike previous GSM speech codecs (FR, EFR, and HR) which operate at a fixed rate and constant error protection level, the AMR speech codec adapts its error protection level to the local radio channel and traffic conditions. AMR selects the optimum channel (half or full rate) and codec mode (speech and channel bit rates) to deliver the best combination of speech quality and system capacity. This flexibility provides a number of important benefits:
– Improved speech quality in both half-rate and full-rate modes by means of codec mode adaptation i.e. by varying the balance between speech and channel coding for the same gross bit-rate;
– The ability to trade speech quality and capacity smoothly and flexibly by a combination of channel and codec mode adaptation; this can be controlled by the network operator on a cell by cell basis;
– Improved robustness to channel errors under marginal radio signal conditions in full-rate mode. This increased robustness to errors and hence to interference may be used to increase capacity by operating a tighter frequency re-use pattern;
– Ability to tailor AMR operation to meet the different needs of operators;
– Potential for improved handover and power control resulting from additional signaling transmitted rapidly in-band.
The AMR codec concept is adaptable not only in terms of its ability to respond to changing 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 the channel conditions and always provide the best speech quality;
– Possibility of channel and codec control algorithms optimization to meet specific operator needs and network conditions.
This allows the codec to be operated in many ways of which three important examples are:
– Full-rate only for maximum robustness to channel errors. This additional robustness may be used to extend the coverage in marginal signal conditions, or to improve the capacity by using a 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 the existing HR will be given for a large proportion of mobiles as a result of the codec mode adaptation to the channel conditions and excellent (wireline like) speech quality in half rate mode for low error conditions.
– Mixed half/full rate operation allowing a trade-off between quality and capacity enhancements according to the radio and traffic conditions and operator priorities.
In the AMR standardization, the complete AMR speech and channel codecs will be defined, in addition to the codec mode adaptation control process, the new link performance metrics and their transmission in-band on the traffic channel. The AMR codec proposals submitted as codec candidates to SMG11 contained all these components. Related features such as VAD, DTX, CNG/Comfort Noise Generation and TFO will also be developed.
The AMR channel mode adaptation will rely on the existing intra-cell handover methods in terms of signaling procedures. However, the channel mode adaptation decision algorithm could be extended and use all available metrics; the existing RxQual and RxLev and possibly the newly defined metrics. This algorithm will be left open to manufacturers to develop and improve with time. The repacking of half-rate radio channels required by the AMR operation will also rely on the existing signaling procedures.
A more complicated AMR standardization phase could be launched later in order to introduce a new mode adaptation control algorithm (Half Rate vs Full Rate) using a new link quality metrics, more advanced handover or enhanced repacking algorithms, etc. The need for the AMR phase 2 will be determined after the completion of the first phase of the AMR standardization.
SMG11 was also mandated to complete the feasibility study of a complementary and optional AMR wideband mode. This AMR wideband coding mode will be developed and selected independently of the narrowband AMR at a later phase, if found feasible. The wideband option would extend the audio bandwidth from the current [300 Hz-4 kHz], to [50 Hz -5 kHz] or [50 Hz 7 kHz].
Finally, SMG#26 approved a new work item for the definition, selection and standardization of a default and optional noise suppresser for the AMR speech codec. This activity will take place once the AMR selection phase has been completed.