Improving Voice Quality in International Mobile-to-Mobile Calls

1. Improving Voice Quality in International Mobile-to-Mobile Calls Aram Falsafi, Seattle, WA aram@aramfalsafi.com PIMRC 2008 18 September 2008

2. Presentation Outline • Introduction & Problem Statement • Current State of the Art • Description of Proposed Architecture • Instrumentation Methodology • Results • Conclusion • Q & A

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

4. 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.

5. 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.

6. 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.

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

8. Mobile-to-Mobile Call

9. Mobile-to-Mobile Call (TFO setup) In-band TFO Negotiation

10. Mobile-to-Mobile Call with TFO Reduction from 2 vocoder stages down to 1.

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

12. International Mobile-Mobile Call

13. International Mobile-Mobile Call

14. International Mobile-Mobile Call Reduction from 3 vocoder stages down to 1! Significantly reduced delay Much lower-cost DCME/VoIP equipment

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

16. 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

17. 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

18. 3-minute sample file used PESQ software takes 2 input files & generates a single output value Instrumentation

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

20. 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

21. 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

22. Case 1 – High Voice Quality • Without TFO: • With TFO: • PESQ score of 3.93 (single AMR12.2 vocoder end to end)

23. Case 2 – High Bandwidth Savings • Without TFO: • With TFO: • PESQ score of 3.41 (single AMR4.75 vocoder end to end)

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

25. 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

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