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