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AMR - Adaptive Multi Rate
An overview
by Sudha Kandi T
Codec Channel coding Interleaving AMR Messages in AMR Advantages of AMR TMG mechanisms. BSS Capacity
AGENDA
A device used to transform analogue signalsamples into digital bit words and back again.
CODEC-CODER / DECODER
CHANNEL CODING IN GSM
Adding redundant bits to source information, errors in a noisy channel can be reduced without sacrificing the information trasmission rate,provided the information rate is less than the channel capacity
AMR - ADAPTIVE MULTI RATE
A Codec offering a wide range of data rates. The philosophy behind AMR is to lower the codec rate as the interference increases and thus enabling more error correction to be applied.
NORTEL-USEAGE OF AMRTo Increase voice Quality in degraded radio conditions, due to adaptation of the pair (Source , Channel) to the radio channel Quality.
QUALITY OF SPEECH
CHANNEL CODING
SPEECH CODIN
G
CHOICE: To use a high coder rate with little redundancy, or a low coder rate with moreredundancy.
Speech quality will be very good in excellent radio conditions, as long as speech frames can be decoded properly. But in bad radio conditions, a high proportion of speech frames will not be decoded.
In the second case, speech quality will be medium or low, but will resist very well to radio channel impairments, due to the high level of redundancy.
AMR techniques is Adaptive, and multirate. It means that it allows to adapt the compromise between source coder rate and channel coding/redundancy to actual radio conditions.
Adaptive Multi- Rate Codecs (AMR )Best Codec Mode for the connectionAMR is made of different modes to optimize certain C/I regions
Mode= Channel coding + Source Coding Link Adaptation between modes of the same codec rate Handover between FR and HR Based on Link Performance
AMR FR AMR HR12.210.27.95 7.957.4 7.46.7 6.75.9 5.9
5.15 5.154.75 4.75
AMR CODEC SETSIn the recommendations, the following coding modes are defined (in kbits/s) :
hardware capacity : all TRX types have to havethe same AMR capacities,
intrinsic quality of each codec mode in term ofvoice quality and functioning range,
rec. limitation to 4 active codec mode at thesame time,
only the following coding modes are loaded in theBSS :
AMRFR
AMRHR
10.26.7 6.75.9 5.94.75 4.75
CONSTRAINTS
Adaptive Multi-Rate Codecs(AMR)Capacity and Quality without Costs
Optimal quality and capacity through appropriateand efficient selection of codec modes
Increase spectral efficiency and higher fractional reuse with ROBUST AMR FRHigher erlang capacity in good radio conditions with AMR HR without additional equipmentImprove voice quality at cell edge and in building with AMR FR
MESSAGES/ DEFINITIONS
Initial codec mode : at the TCH allocation, the initial codec (ICM) mode used bythe MS and the BTS is the 5.9 kbits/s.
AMR may operate in full rate channels, or half rate channels. This is called the “channel type”
Basis of AMR is that within the channel (FR or HR), there is a set of voice coders, along with associated channel coding, among which the best combination can be selected to maximize speech quality according to conditions met on the radio link. This is “codec mode adaptation”.
For uplink adaptation, the Quality Indicator, as measured in the BTS is compared to certain thresholds and generates, also considering possible constraints from network control, a Codec Mode Command (CMC) indicating the codec mode to be used on the uplink.
MESSAGES/ DEFINITIONS
Both for uplink and downlink, the presently applied codec mode is transmitted inband as Codec Mode Indication (CMI) together with the coded speech data.
MS BTS
CMI
CMR
CMI
CMR
CMI
CMC
CMI
CMC
20ms
40ms 20ms
40ms
MS BTS
CMI
CMR
CMI
CMR
CMI
CMC
CMI
CMC
20ms
40ms40ms 20ms
40ms
MESSAGE FLOW
It can be noticed that this choice ensures
• A Good overlapping between each codec mode• An Optimal voice quality,• A Good trade-off between stability and codec mode adaptation
RATSCCH Robust AMR Traffic Synchronized Control Channel
• Frame Stealing : one speech frame is stolen for each RATSCCH message, and on TCH/AHS two speech frames are stolen
• FACCH frames have higher priority than RATSCCH frames
• The RATSCCH protocol elements consist of a number of REQuest Messages and three ACKnowledgement Messages
ACK_OK message ACK_ERR message ACK_UNKNOWN message CMI_PHASE_REQ message AMR_CONFIG_REQ message THRESH_REQ message
TMG MECHANISMS
The BSC should determine whether AMR is to be used, and select between FR or HR. This mechanism is based on proprietary algorithms, which provide to the operator a full control of the allocation.
--------------------------------------Setup[Bearer Cap] --------------------------->
< ---------------------------- Call Proceeding [Bearer Cap] -------------------- + Assignment procedure
These decisions are made based on several criteria :OAM flags which indicate if the BSC, the TCU, and the cell supportAMR, and strategy selectedMS capability, which is reported by the MSC in Assignment requestor Handover request messages2,radio context-RR Management
Principle •“(n,p)” voting
If in the last p requested codec modes, at least n are strictly less than a target codec mode, then the condition is fulfilled
HANDOVER
HANDOVER TYPES
Alarm inter-cell handoverIntra –Cell handoverHandover AMR FR -> AMR HRDirect half rate TCH allocationInter-zone handover
Pre handover BTS has to intercept :
•Assignment Command in case of intra-cell,•Handover Command in case of inter-cell handover,
and to perform up to 2 codec mode adaptations, in order to activate the initial
codec mode (5.9 kbits in all cases) and to stop the adaptative mechanism.
This induces :•an increase of around 150ms on the handover duration from the BSS
point of view, •a delay of around 150ms on the handover starting time from a MS
point of view, but no impact for the end-user in term of voice quality
(i.e. same speech gap).
In case of handover failure when the MS returns on the old channel, the
adaptation mechanism is restarted by the BTS at reception of the Start
Measurement message.
HANDOVER CONTD..
ADVANTAGES
• Good Call quality
• Better solution for long term fading
• Good Handover control
POWER CONTROL
•HRPowerControlTargetMode:7.4,6.7,5.9,4.75
•FRPowerControlTargetMode:12.2,10.2,6.7,5.9,4.75
Based on Step by step power control
• decrease power level by one step if last requested codec mode is greater than target codec mode• increase power level by one step if last requested codec mode is lesser than target codec mode
AMR Management• AMR Vocoders, • Dedicated VAD/DTX, • Link Adaptation Mechanism, • RATSCCH channel • TRAU frames
L1m MechanismAdaptation of current mechanism (power controland handover) to the intrinsic quality of AMR channel
TMG Management Allocation and management of AMR channelsand specially half rate channels
IMPACTED AREAS
BSS CAPACITYAbis PCM
AMR Half Rate allows to double the number of calls that could
be carried on a Abis PCM.
BSC e3 capacity
The introduction of AMR services on BSC induces :
handovers between half rate and full rate TCH, in order to
adapt the radio link to radio conditions,
more complex algorithm,Channel allocation),
Thus the BSC load of an AMR call is greater than the load
induced by a non AMR call.
TCU e3 capacityAMR services induce more complexity on TCU, but no capacity reduction compared with EFR or FR calls is expected.
GOOD DAY!Thank you