Amr Codec for Umts

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Appendix 1

AMR Codec in UMTS

Originally developed to be used in GSM by the ETSI, the Adaptive Multi-Rate

(AMR) speech codec [TS 26.071] was approved within the 3GPP forum in 1999 to

be mandatory for circuit- and packet-switched speech in UMTS networks. An AMR

speech codec adapts the error protection level to the local radio channel and traffic

conditions so that it always selects the optimum channel and codec mode to deliverthe best combination of speech quality and system capacity. AMR uses Multi-Rate

Algebraic Code Excited Linear Prediction (MR-ACELP) scheme based on two

different synthesis filters. It converts a narrowband speech signal (from 300 to 3,400

Hz) to 13-bit uniform Pulse Coded Modulated(PCM) samples with 8 kHz sample

rate. This leads to 20 ms AMR frames consisting of 160 encoded speech samples.

This means that the codec can switch mode, i.e. source bite rate, every 20 ms. AMR

has 8 coded modes in UMTS systems, whereas in GSM AMR uses either 6 or 8

modes. The eight source rates vary from 4.75 to 12.2 kbps. It also contains a low

rate encoding mode, called SIlence Descriptor(SID), which operates at 1.8 kbps toproduce background noise and a non-transmission mode.

The AMR codec dynamically adapts its error protection level to the channel

error conditions. For instance, lower speech coding bit rate and more error

protection schemes are used in bad channel conditions. This principle is illustrated

in Figure A1.1 where AMR strives to change to the best curve associated to a given

AMR mode. It has been shown that the degradation on the audio quality caused by a

lower speech coding rate is compensated by increased robustness with the channel

coding. Note, however, that this channel robustness is more beneficial in GSM thanin UMTS due to the embedded fast power control used in WCDMA systems. Using

a variable-rate transmission scheme also makes it possible to control the

transmission power of the UE, a fact that is particularly useful when the UE

U TS

Javier Sanchez Mam adou Tnioune

opyright

007 ISTE

Ltd.


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376 UMTS

suddenly attains its maximum transmit power: in CDMA: lower bit rates generally

need lower transmit power and vice versa.

Good Bad

Good

Poor

Speech quality

Channel quality

The optimum AMR codecmode is dynamically selected as

function of the channel quality

Good Bad

Good

Poor

Speech quality

Channel quality

The optimum AMR codecmode is dynamically selected as

function of the channel quality

Figure A1.1.AMR principle

A1.1.AMR frame structure and operating modes

Figure A1.2 depicts the generic structure of the AMR frame. As observed in the

figure, the frame is divided into a header, auxiliary informationand core frame. The

header contains the Frame Types and Frame Quality Indicator fields. The Frame

Type can indicate the use of one of the eight AMR codec modes for that frame, a

noise frame, or an empty frame. The Frame Quality Indicator indicates if the frame

is good or bad. The auxiliary informationpart includes the Mode Indication, Mode

Request and Codec CRC fields. The CRC field is used for the purpose of error-

detection calculated over all the Class A bits in the AMR Core frame. The Core

framepart is used to carry the encoded bits divided into A, B and C classes. In case

of a comfort noise frame, comfort noise parameters, i.e. a SID frame, replace class

A bits of the core frame while class B and class C bits are omitted.

Class C bits

Class B bits

Class A bits

Codec CRC bits (8 bits)

Mode Request (3 bits)

Mode Indication (3 bits)

Frame Quality Indicator (1 bits)

Frame Type (4 bits)Header

Auxiliary Information

(for Mode Adaptation,

and Error Detection)

Core frame(speech or comfort noise)

AMR frame

Class C bits

Class B bits

Class A bits

Codec CRC bits (8 bits)

Mode Request (3 bits)

Mode Indication (3 bits)

Frame Quality Indicator (1 bits)

Frame Type (4 bits)Header

Auxiliary Information

(for Mode Adaptation,

and Error Detection)

Core frame(speech or comfort noise)

AMR frame

Figure A1.2. Generic structure of the AMR frame


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AMR Codec in UMTS 377

Classification of the encoded bits according to their sensitivity to errors

AMR encoded bits are divided into three indicative classes according to their

importance: A, B and C. The reason for dividing the speech bits into classes is that

they can be subjected to different error protection in the network. Class A contains

the bits that are most sensitive to errors and any kind of errors in these bits typically

result in a corrupted speech frame which should not be decoded without applying

appropriate error concealment. This class is protected by the CRC in auxiliary

information field. Classes B and C contain bits where increasing error rates

gradually reduce the speech quality, but the decoding of an erroneous speech frame

is usually possible without a strongly perceptible quality degradation.

AMR operating modes

Table A1.1 depicts the 8 different modes (source bit rates) AMR can operate. It

should be noted that some of these modes are equivalent to the speech codecs

currently used in other mobile communication systems. For instance, the AMR

12.20 kbps mode is equal to the ETSI GSM called codec EFR (Enhanced Full Rate

Speech [TS 06.60]). Similarly, the AMR 7.40 kbps mode is equivalent to the

IS-641 codec used in the USA standard IS-136 (US TDMA). Finally, AMR

6.70 kbps mode is equivalent to the codec used in the PDC Japanese standard.

Frame type

index

Frame content

(AMR mode, comfort noise, or other)

ClassA

bits

Class B

bits

Class C

bits

0 AMR 4.75 kbps 42 53 0

1 AMR 5.15 kbps 49 54 0

2 AMR 5.90 kbps 55 63 0

3 AMR 6.70 kbps (PDC EFR) 58 76 0

4 AMR 7.40 kbps (IS-136 EFR). 61 87 0

5 AMR 7.95 kbps 75 84 06 AMR 10.2 kbps. 65 99 40

7 AMR 12.2 kbps (GSM EFR) 81 103 60

8 AMR SID

9 GSM EFR SID

10 TDMA EFR SID

11 PDC EFR SID

12-14 Future usage

15 No data to transmit/receive

Frame type

index

Frame content

(AMR mode, comfort noise, or other)

ClassA

bits

Class B

bits

Class C

bits

0 AMR 4.75 kbps 42 53 0

1 AMR 5.15 kbps 49 54 0

2 AMR 5.90 kbps 55 63 0

3 AMR 6.70 kbps (PDC EFR) 58 76 0

4 AMR 7.40 kbps (IS-136 EFR). 61 87 0

5 AMR 7.95 kbps 75 84 06 AMR 10.2 kbps. 65 99 40

7 AMR 12.2 kbps (GSM EFR) 81 103 60

8 AMR SID

9 GSM EFR SID

10 TDMA EFR SID

11 PDC EFR SID

12-14 Future usage

15 No data to transmit/receive

Table A1.1.AMR modes and relationship with AMR frame structure


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378 UMTS

Based on the fact that voice activity in a normal conversation is about 40%, all

AMR modes implement a Voice Activity Detection(VAD) algorithm that detects if

each 20 ms-frame contains speech or not on the transmitting side. VAD works

together with the Discontinuous Transmission (DTX) or Source Controlled Rate(SCR) [TS 26.093] techniques where RF transmission is cut during speech pauses.

When the transmission is cut, comfort noise parameters are sent at a regular rate in

AMR frames during discontinuous activity. These frames are known as SID (SIlence

Descriptor) frames. The receiver decodes these parameters and generates locally a

comfort noise. Without this background noise the participants in a conversation,

might think that their connection is broken during silence periods. The SCR

technique for AMR in UMTS is mandatory and aims at prolonging the battery life

(UE side) and reducing the interference.

A1.2.Dynamic AMR mode adaptation

The AMR mode adaptation in UMTS networks means using different AMR

coding for the data stream. Mode adaptation can independently be applied in the

uplink and the downlink. At any point in time, a different AMR mode can be used in

each direction and this can be dynamically changed during a voice conversation.

Location of the AMR speech codec in UMTS networks

The AMR speech codec is located in the Transcoder (TC) function defined to be

in the UMTS core network and as such, logically controlled byNon-Access Stratum

protocols. From the transfer point of view, this means that all AMR coded data is

going to be transmitted not only via Iub and air interface but also via Iu-interfaces.

Note, however, that the AMR mode control that generates the AMR mode command

cannot be located in the TC, since this control entity needs information from the air

interface to make a decision about valid AMR modes the AMR mode command is

used to change the current AMR mode to the new one. The only element in the

network which can provide this type of information is the UTRAN. Note that in

GSM networks the control of the codec mode is provided by the BTS. This solution

is not applicable in UTRA due to the soft-handover procedure defined for dedicated

traffic channels. Therefore, the AMR mode control function is part of the RNC, and

more precisely a part of layer 3 functionality. Within the radio interface, the rate on

the speech connection is either decreased or increased depending on the new valid

AMR mode by changing the valid Transport Format (TF) in the corresponding

MAC-d entity (see Chapter 7).

AMR mode adaptation in the downlink

As shown in Figure A1.3, the RNC generates the AMR mode adaptation

command based on existing radio conditions in the downlink as reported by the UE


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from radio quality measurements and from traffic volume measurements. The

command is sent to the encoder inside the TC via the Iu interface.

UE RNC

Request to modify

the AMR mode in DL

AMR encoded speech data in UL (ongoing call)

AMR encoded speech data in DL with new AMR mode (ongoing call)

Request to modify

the AMR mode in UL

AMR encoded speech data in UL with new AMR mode (ongoing call)

TC

AMR speech

codec

Node B

Uu Iub Iu-CS

Control

of AMR modes

Change

AMR mode

AMR speech

codec

Change

AMR mode

UE RNC

Request to modify

the AMR mode in DL

AMR encoded speech data in UL (ongoing call)

AMR encoded speech data in DL with new AMR mode (ongoing call)

Request to modify

the AMR mode in UL

AMR encoded speech data in UL with new AMR mode (ongoing call)

TC

AMR speech

codec

Node B

Uu Iub Iu-CS

Control

of AMR modes

Change

AMR mode

AMR speech

codec

Change

AMR mode

Figure A1.3. Overview of AMR codec mode control during an ongoing voice call

Uplink AMR mode adaptation

Two different alternatives for the AMR mode control in the uplink have been

proposed:

Based on the air-interface load, the RNC decides when to request the encoder

in the UE to change the valid AMR mode and a new valid AMR mode is sent to the

UE inside the AMR mode command message. When received by the UE, mode

adaptation is made accordingly (see Figure A1.3). Within this approach, the UE

does not have any rights to request the mode adaptation from the network nor tochange the used AMR mode autonomously.

In the second proposed alternative, the AMR codec control is not only included

into the RNC but also into the UE. This enables the UE to change the valid AMR

mode of the speech connection on uplink more quickly without requesting the mode

change from the RNC first. For instance, when the level of the maximum

transmission power is reached, the UE may change the valid AMR mode

independently. The new mode can, however, be selected only from the valid

Transport Format Set(TFS), which has been communicated to the UE by RRC fromthe RNC side. The changed AMR mode is discovered by the RNC from the TFCI

bits in the uplink dedicated physical data channel.


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principles as the AMR narrowband. The AMR-WB comprises nine codec modes:

6.6 kbps, 8.85 kbps, 12.65 kbps, 14.25 kbps, 15.85 kbps, 18.25 kbps, 19.85 kbps,

23.05 kbps and 23.85 kbps. The encoder of the AMR-WB is able to code an audio

signal with bandwidth between 50 and 7,000 Hz. A higher sampling rate is thusneeded compared with the narrowband approach (16 kHz instead of 8 kHz) leading

to a 14 bit samples with 16,000 samples/s. Wideband coding provides improved

voice quality especially in terms of increased voice naturalness since it covers twice

the audio bandwidth compared to the classical telephone voice bandwidth of 4 kHz

[TS 26.190,R5].

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