10.703GPPGSM Adaptive Multi-Rate Speech Codec (AMR)Project schedule and open issues for AMRTS
The AMR speech codec includes a set of fixed rate speech codecs modes for half rate and full rate operation, with the possibility to switch between the different modes as a function of the propagation error conditions. Each codec mode provides a different level of error protection through a dedicated distribution of the available gross bit rate (22.8 kbps in Full Rate and 11.4 kbps in Half rate) between source coding and channel coding.
The actual speech rate used for each speech frame depends on the existing radio channel conditions. A codec adaptation algorithm selects the optimized speech rate (or codec mode) as a function of the channel quality. The most robust codec mode is selected in bad propagation conditions. The codec mode providing the best quality is selected in good propagation conditions. The codec adaptation relies on channel quality measurements performed in the MS and the network and on in band information sent over the Air Interface together with the speech data.
The following diagram shows the main information flows over the key system interfaces:
In both directions, the speech data frames are associated with a Codec Mode Indication used by the receiving end to select the correct channel and source decoders. In the network, the Codec Mode Indication must also be sent to the Transcoder Units so that the correct source decoding is selected.
For the adaptation of the uplink codec mode, the network must estimate the channel quality, identify the best codec for the existing propagation conditions and send this information to the MS over the Air Interface (Codec Mode Command Data field).
For the downlink codec adaptation, the MS must estimate the downlink channel quality and send to the network a quality information, which can be mapped in the network to a ‘suggested’ codec mode.
In theory, the codec mode can be changed every speech frame. In practice, because of the propagation delays and necessary filtering in the codec adaptation functions, the codec mode should be adapted at a lower rate.
Each link may use a different codec mode but it is mandatory for both links to use the same channel mode (either full rate or half rate).
The channel mode is selected by the Radio Resource management function in the network. It is done at call set up or after a handover. The channel type can further be changed during a call as a function of the channel conditions.
The key characteristics of the selected AMR solution are:
– 8 codec modes in Full Rate mode including the GSM EFR and IS136 EFR
– 6 codec modes in Half Rate mode (also supported in Full Rate), including the IS136 EFR
– Possibility to operate on a set of up to 4 codec modes selected at call set up or handover
– Codec Mode Indications multiplexed with the Uplink Codec Mode Command and Suggested Downlink Codec Mode every other frame
– In band signaling based on a 2 bits information field sent every other block coded over the Air Interface
The full set of codec modes is listed in the following table: