1. Introduction to
AMR
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Jonathan Hopkinson 20/8/03
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2. Existing types of Speech Coding
• Currently 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.
• Motorola’s BSS has supported EFR since GSR3 - with
a GDP board fitted at the Transcoder function.

3. Why change from what we’ve 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.

4. AMR Concept
• AMR 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 5.15 5.9 6.7 7.4 7.95 10.2 12.2 13
kbps kbps kbps kbps kbps kbps kbps kbps kbps
EFR FR
Better Protection
Better Speech Quality

5. Mos Scores Vs C/I (Static)

6. AMR Codec Rates
• Up 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.

7. AMR Codec Rate Switching
• AMR will switch modes within the active codec set,
based on C/I and RxQual calculations.
C/I
12.2 kbps
Hysterisis
7.95 kbps
Hysterisis
5.9 kbps
Hysterisis
4.75 kbps

8. Improvements offered by AMR
• Improved 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.

9. Half Rate AMR
• AMR 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.