Existing types of Speech CodingCurrently there are two speech coding schemes used in GSM.FR - Full Rate (The original coding scheme)EFR - Enhanced Full Rate ( An enhancement of FR)EFR offers better quality speech.Most modern mobiles are EFR capable.The majority of Network Operators have EFR capable Networks.Motorolas BSS has supported EFR since GSR3 - with a GDP board fitted at the Transcoder function.

Why change from what weve got ?The traditional fixed coding schemes (FR or EFR) use a fixed amount of bandwidth - some dedicated to encode the speech and the rest used to offer data protection - Forward Error Coding, Checksums etc. When poor RF conditions exist, there may be errors in the speech data which cannot be corrected or recovered. The errors will be heard as an interruption in the speech.With EFR or FR, we can only achieve a best effort with the coding scheme and protection available - AMR offers some improvements.

AMR ConceptAMR allows multiple different coding schemes to be used. The higher speech bandwidth schemes offer good speech quality - with lower error protection. The lower speech bandwidth schemes offer slightly lower speech quality - with much higher error protection.

4.75 kbps5.15 kbps5.9 kbps6.7 kbps7.4 kbps7.95 kbps10.2 kbps12.2 kbpsBetter ProtectionBetter Speech QualityEFRFR13 kbps

Mos Scores Vs C/I (Static)

AMR Codec RatesUp to 4 codec modes are allowed in AMR at any one time - This is known as the Active Codec set.The Active codec set may contain something like :-12.2, 7.95, 5.9 and 4.75 kbps.AMR switches between the 4 rates - independently on the Uplink and the Downlink - based on the prevailing RF conditions on each link.Under good RF conditions - a Rate of 12.2 kbps may be used which is similar to EFR. Under poor conditions 4.75 kbps may be used - offering lower quality speech - but higher protection against errors.

AMR Codec Rate SwitchingAMR will switch modes within the active codec set, based on C/I and RxQual calculations.

Improvements offered by AMRImproved overall speech quality due to the adaptive algorithm switching to more robust coding schemes when RF conditions are poor.Possibility of Cell extension - Because AMR offers a more robust link - calls with adequate speech quality may be possible over a longer range Possibility to use AMR as an input to HDPC - for example to reduce MS/BTS power based on Speech Quality - as well as traditional RxLev and RxQual.

Half Rate AMRAMR allows for Half Rate speech channels.Half Rate can mean twice as many subscribers using the same amount of infrastructure (RF) equipment. This is a very attractive proposition to Network operators.The Half Rate codec modes are only suitable when the C/I is ~> 15dB.Half Rate AMR will be possible on Motorola infrastructure - new hardware is required at the BSC and Transcoding functions.Half Rate offers worse speech quality - but more traffic capacity. - Call tariffs will probably sell Half Rate.

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