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1 2006 Nokia AMRWB_depl.ppt / 2006-04-11 / SHy

Seminar Presentation: Adaptive Multi-

Rate Wideband Speech Codecdeployment in 3G Core Network

Sergei Hyppenen

Supervisor: Professor Sven-Gustav Hggman

HELSINKI UNIVERSITY OF TECHNOLOGY11.04.2006


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Contents of the presentation

Abbreviations

Introduction AMR-WB speech codec

Network architectures: GSM and 3G (Release 4)

Speech transmission

TrFO and TFO

Out-of-Band Transcoder Control in TrFO

TFO frames

Lawful interception

Signal interception simulation

Test results: Noise floor values

Test results: MOS quality values

Conclusions


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Abbreviations

3G: 3rdGeneration

ACELP: Algebraic Code-Excited Linear Prediction

AMR-WB: Adaptive Multi-Rate Wideband speech

codec

ATM: Asynchronous Transfer Mode

BSS: Base Station Subsystem

CN: Core network

dB: decibel dBov: dB relative to the overload point of the digital

system

DTX: Discontinuous Transmission

EDGE: Enhanced Data rates for Global Evolution

G.711: PCM-based coding method with 8 kHz

sampling frequency and 8-bit A- or -law weighting GSM: Global System for Mobile Communications

HR: Half Rate speech codec

IP: Internet Protocol

LSB: Least Significant Bit

MOS: Mean Opinion Score rated 1-5

NSS: Network Sub-System

OoBTC: Out-of-Band Transcoder Control

TC: Transcoder

TDM: Time Division Multiplexing

TFO: Tandem Free Operation

TrFO: Transcoder Free Operation

UMTS: Universal Mobile

Telecommunications System

VAD: Voice Activity Detection

WB-PESQ: a tool for quality evaluation [ITU-

T: P.862]


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Introduction

Speech contains frequencies up to the 10 kHz

Current fixed and mobile telecommunication systems operate with anarrow audio bandwidth: 300-3400 Hz (ITU-T G.711)

500-3000 Hz is sufficient for understanding

The sampling frequency used in digital core networks is 8000 Hz in theory

enables transmitting signals up to 4000 Hz

Codecs utilized in mobile systems lower the quality of narrowband

speech even more than the G.711

AMR-WB speech codec improves the quality and especially the

naturalness of speech

In EDGE and UMTS all coding modes of the AMR-WB will be used,

in GSM only coding modes till 12.65 kb/s


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AMR-WB speech codec

Process 50-7000 Hz

Sampling: 16 kHz Precision: 14-bit

Coding model: ACELP

VAD and DTX

Bad frame handler

Bit rates: 6.60, 8.85,

12.65, 14.25, 15.85,

18.25, 19.85, 23.05,

23.85 kb/s

Coding mode 12.65 kb/s

produces better quality

than G.711 (64 kb/s)

A-law coded speech

AMR-WB coded speech

Original speech

HR coded speech

time time

time time


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Speech transmission

In current telecommunication systems transcoding is performed at least twice

In core networks speech signals are transferred in narrowband G.711 form andone one-way connection requires a 64 kb/s channel

BTS

BSC

A

TDM MSC64

kb/s

MSC64

kb/s

Ater

TC

G.711 G.711

TC

MSA Ater Abis

Decoding EncodingDecoding

EFR / FR / HR

Abis

Encoding

MS

BSC16 kb/s

BTS

16 kb/s

CODED

SIGNAL

22.8

kb/s

Uplink direction Downlink direction

Wideband speech cannot be transferred using the same technique Requires 16 kHz * 14 bit connection speeds, which are UNAXEPTABLY HIGH!

wideband speech should be transferred only in CODED FORM!

GS

M


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TrFO and TFO

Transcoder Free Operation (TrFO)

transfers coded speech frames inATM- and IP-based networks as

such

Transcoder-free means that the

same codec is used on the both

sides of a connection Out-of-

Band Transcoder Control (OoBTC)is needed

OoBTC requires the late

assignment of a radio traffic channel

with forward bearer establishment in

CN (see the next slide for details)

In Tandem Free Operation (TFO)

coded frames are merged into leastsignificant bits (LSB) of PCM-based

signals

The TFO is utilized in TDM networks

TFO protocol negotiates with the

distant partner a common codec to be

used by sending messages in-band

Message bits replace every 16thLSB

When both mobile terminals switch to

a compatible codec, coded speech

frames can be merged into PCM-

based stream that was decoded fromthose coded frames


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Out-of-Band Transcoder Control in TrFO

In TrFO negotiation of the codec to be used during the call has to

be performed before the bearer establishment procedures

RNC-T

MSC-S-T

MGW-T

MSC-SO -

MGW-ORNC-O

SETUP

IAM + Bearer Information

Paging

SETUP

Bearer establishmentNb UP Initialization

Bearer establishment

Iu UP Initialization



UE UE

ALERTING

CONNECT

Early assignment of a radio traffic channel with

backward bearer establishment in CN

RNC-T

MSC-S-T

MGW-T

MSC-SO -

MGW-ORNC-O

SETUPIAM

Paging

SETUP


Nb UP Initialization





UE UE

ALERTING

CONNECT

Bearer Information

{

Late assignment of a radio traffic channel with

forward bearer establishment in CN


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TFO frames 1

When TFO is operational 1, 2 or 4 LSBs of every 8-bit PCM sample are replaced

by TFO frames TFO frames requiring replacement of 4 LSBs consist of the main frame part (1st

and 2ndLSBs) and the extension frame part (3rdand 4thLSBs).

During the transmission through the core network TFO frames should not be

modified by noise suppression, level control or other enhancement algorithms

... ... ... ... ...... ... ...

1 2 3 158 159 160 1 3 52 4 6

315 317 319316 318 320

2 4 6

2 4 6

1 3 5

1 3 5

}

} mainframe part

extension

frame part

315 317 319316 318 320

315 317 319

316 318 320

8k TFO frame 16k TFO frame 32k TFO frame

} unalteredsample bits

8

7

6

5

4

3

21

Bits

160 samples

TFO frame length=160bits

160 samples


160 samples



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TFO frames 2

TFO frames are different for each codec

and each coding mode, if a multi-rate codec

is in question

TFO frames contain synchronization bits,

control and error correction bits, time

alignment bits, spare bits and actual data

bits

Synchronization and control bits are used

only in the main part

On the right is an example of the TFO

frames specified for the AMR-WB, the

coding mode is 23.85 kb/s


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Lawful interception

Before an operator may launch a commercial telecommunication network, it has

to provide the lawful interception service.

The quality provided for the authorities has to be the same or better than the quality

provided for the monitored target

PCM-based intercepted signals are directed to the authorities as such

Coded signals are converted into PCM form

What to do if the intercepted signal contains TFO frames? After all, the signal isnoisy

The solution is utilization of the passive TFO protocol

But how bad the noise really is?


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Signal interception simulation

Theoretical noise floor

values were calculated with

the assumption that every

bit in signal representation

raises the dynamics of the

signal 6 dB

The results were verified by

sending silence through thetesting system

Also the MOS quality values

of the speech signals were

evaluated using the WB-

PESQ tool In tests the scheme

presented on the right was

simulated

Input

Output

Input

Output

&

or

or

G.711

G.711

coded

coded

G.711 (+TFO)

G.711 (+TFO)

Local TFO Distant TFO

Passive TFO

Transit network

Encoder

Radio

interface

Decoder

Down-

sampler

G.711

converter

1

Decoder

G.711

coded

3

4

wideband

speech

Down-

sampler

G.711

converter

or

Interface

towards

authorities

1. Original wideband signal

2. Once transcoded wideband

signal

3. Pure narrowband G.711 signa

4. Narrowband G.711 signal with

possible embedded TFO frames

3

4

2b

2a


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Test results: MOS quality values

The level of the original signals was -26 dBov and SNR 45 dB

Decoded from TFO frames signals (2b) are slightly different than the originally

decoded ones (2a), as TFO protocol needs approx 1 second time to establish a

connection. During that time no coded speech frames are sent

Signal files Decoded (2a) G.711 (3) G.711+TFO (4) Decoded TFO (2b)

T04 3.9 3.1 1.7 3.6

T05 4.1 3.9 1.8 3.8

T14 3.7 3.4 1.8 3.6

T18 3.7 2.9 2.1 3.6

Average 3.9 3.3 1.9 3.7


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Conclusions

SNR values of the intercepted signals with AMR-WB-specific TFO frames were

15-25 dB (original signals -26 dBov) and MOS grades below two.

If the original signals would have contained noise from the beginning, as it is

usually in real phone-calls, the quality would have been lower

Using in the tests signals with lower levels, -30 and -36 dBov, which corresponds

to intensive whispering in real-world calls, the results would have been even

worse

authorities will not be satisfied with the quality of the intercepted signal

the passive TFO protocol is needed indeed!

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