Improving Voice Quality in International Mobile-to-Mobile Calls

Aram Falsafi, Seattle, WA

aram@aramfalsafi.com

PIMRC 2008

18 September 2008

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Presentation Outline

Introduction & Problem Statement Current State of the Art Description of Proposed Architecture Instrumentation Methodology Results Conclusion Q & A

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Introduction & Problem Statement Current State of the Art Description of Proposed Architecture Instrumentation Methodology Results Conclusion Q & A

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Mobile-to-Mobile Call

E1-E3: Voice encoders

D1-D3: Voice decoders

Note: In the interest of simplicity, nodes that don’t affect the voice format (MSC and intermediate switches) are not shown.

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Mobile-to-Mobile call with extra vocoders

DCME: Digital Circuit Multiplication Equipment

VoIP: Voice over IP

Note: In the interest of simplicity, nodes that don’t affect the voice format (MSC and intermediate switches) are not shown.

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International Mobile-Mobile Call:Up to five tandem vocoder pairs

Voice quality degrades quickly with increased number of tandem vocoders

Increase not necessarily linear Similar to effect of retransmission of a FAXed

document (quantization error)

Solution: Minimize number of tandem vocoders Tandem Free Operation (TFO) is already part of the

3GPP and 3GPP2 standards, for 2G and 3G equipment.

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Introduction & Problem Statement Current State of the Art Description of Proposed Architecture Instrumentation Methodology Results Conclusion Q & A

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Mobile-to-Mobile Call

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Mobile-to-Mobile Call (TFO setup)

In-band TFO Negotiation

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Mobile-to-Mobile Call with TFO

Reduction from 2 vocoder stages down to 1.

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Introduction & Problem Statement Current State of the Art Description of Proposed Architecture Instrumentation Methodology Results Conclusion Q & A

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International Mobile-Mobile Call

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International Mobile-Mobile Call

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International Mobile-Mobile Call

Reduction from 3 vocoder stages down to 1!

Significantly reduced delay

Much lower-cost DCME/VoIP equipment

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Introduction & Problem Statement Current State of the Art Description of Proposed Architecture Instrumentation Methodology Results Conclusion Q & A

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Requirements

A “clean” 64 KBPS PCM file Serves as voice quality reference file Long enough to give a good estimate

Encoders and decoders for standard GSM and UMTS networks

For full rate (FR), enhanced full rate (EFR), half rate (HR), and different rates of adaptive multirate (AMR)

Must be host-based Available in open source format

Objective voice quality measurement software

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Perceptual Evaluation of Speech Quality (PESQ)

Standardized as ITU-T recommendation P.862 (*) Uses psycho-acoustic metrics – attempts to

measure distortions only in the perceptually significant aspects of the voice signal

Calculates an objective voice quality score, and maps it to “a MOS like scale, a single number in the range of –0.5 to 4.5, although for most cases the output range will be between 1.0 and 4.5, the normal range of MOS values …”(*)

(*) ITU-T Rec. P.862, “Perceptual Evaluation of Speech Quality (PESQ), an Objective Method for End-to-End Speech Quality Assessment of Narrowband Telephone Networks and Speech Codecs,” Feb 2001

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Instrumentation

3-minute sample file used PESQ software takes 2

input files & generates a single output value

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Introduction & Problem Statement Current State of the Art Description of Proposed Architecture Instrumentation Methodology Results Conclusion Q & A

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Effect of Tandem Vocoders

Applying the same vocoder multiple times Shows the effect of tandem vocoding Simple to simulate, but not common in the real world

Tandem Vocoders

2.60

2.80

3.00

3.20

3.40

3.60

3.80

4.00

1 2 3 4

# of tandem vocoders

PE

SQ

AMR 12.2

EFR

AMR 10.2

GSM-FR

GSM-HR

AMR 4.75

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Two Scenarios

Case 1: High Voice Quality Requirement E2/D2 uses AMR12.2 vocoder For example Tier-1 operator in mature market

Case 2: High Bandwidth Savings Requirement E2/D2 uses AMR4.75 vocoder For example severely congested backhaul, or satellite

links

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Case 1 – High Voice Quality

With TFO: PESQ score of 3.93 (single AMR12.2 vocoder end to end)

EFR AMR122 HR AMR475

EFR 3.49 3.51 3.24 3.19

AMR122 3.55 3.54 3.26 3.21

HR 3.32 3.31 3.08 3.05

AMR475 3.25 3.23 3.02 3.02

Second Mobile Stage (listener)

First Mobile Stage (speaker)

Without TFO:

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Case 2 – High Bandwidth Savings

EFR AMR122 HR AMR475EFR 3.20 3.26 2.98 3.03AMR122 3.23 3.25 2.98 3.02HR 3.09 3.10 2.89 2.91AMR475 3.07 3.06 2.84 2.90

Second Mobile Stage (listener)

First Mobile Stage (speaker)

With TFO: PESQ score of 3.41 (single AMR4.75 vocoder end to end)

Without TFO:

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Introduction & Problem Statement Current State of the Art Description of Proposed Architecture Instrumentation Methodology Results Conclusion Q & A

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Conclusions Tandem vocoders can significantly impact

voice quality And add to the total delay – an effect that has not

been quantified in this study

TFO has been created to address this problem in mobile to mobile calls

TFO can be applied to other scenarios where tandem vocoders (including standard VoIP and proprietary DCME) are in the transmission path

This has many advantages Measureable improvements in voice quality Lower delay Cheaper transmission equipment – fewer vocoders

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Introduction & Problem Statement Current State of the Art Description of Proposed Architecture Instrumentation Methodology Results Conclusion Q & A