Voice Coding in 3G Networks

1
Voice Coding in 3G Networks

• S-38.130 Postgraduate Course in
  Telecommunications
• Spring 2001
• Tommi Koistinen
• Nokia Networks

2
Contents

• PART I
• Short introduction to 3GPP reference architecture
  models
• Media Gateway (MG)
• Multimedia Resource Functions (MRF)
• PART II
• Speech compression why ?
• Tandem avoidance
• Adaptive Multirate (AMR) speech codec
• Wideband speech coding (AMR-WB)
• Demonstrations

3
3GPP Release 99

• R99 first phase of 3G
• entities involved with speech processing are
  circled with red

4
3GPP R4

• separates MSC to MSC Server and to Media Gateway

5
3GPP R4R5

• IP Multimedia Subsystem (IMS)

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Media Gateway

• support for several interfaces (A-interface for
  2G and Iu-interface for 3G) and for several
  transmission protocols (ATM, IP, TDM)
• support for several codecs including the Adaptive
  Multirate (AMR) codec and future coming wideband
  codecs
• electric and acoustic echo cancellation
• announcement services
• DTMF and call progress tone generation and
  detection
• support for fax/modem/data protocols
• support for Tandem Free Operation (TFO) and
  Transcoder Free Operation (TrFO)
• bad frame handling
• IP protocol handling (RTP/RTCP, encryption, QoS
  support)

7
Media Resource Functions Unit

• audio/video conferencing services
• speech enhancements ?

8
Tandem Avoidance in 2G
Current status no Tandem Free Operation (TFO)
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Tandem Avoidance in 2G
Better speech quality with Tandem Free Operation
(TFO)
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Tandem Avoidance in 3G
Transcoder Free Operation (TrFO) AMR modes are
negotiated by inband procedure.
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Speech Compression Why ?

• to save transmission capacity
• to save radio resources
• to save storage capacity
• more compression (40) with voice activity
  detection (VAD) and discontinuous transmission
  (DTX)
• error robustness with bad frame handling (BFH)

12
Speech coding techniques

• Waveform coders
• correlation between adjacent samples
• G.711, G.726 ADPCM etc.
• Analysis-by-synthesis types of coders
• Code Excited Linear Prediction (CELP)
• G.723, G.729, GSM EFR, GSM AMR

13
The CELP model
vocal tract
glottis
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Adaptive Multirate (AMR) speech codec

• only mandatory codec for 3G
• improved speech quality in both half-rate and
  full-rate modes by means of codec mode adaptation
  i.e. varying the balance between speech and
  channel coding for the same gross bit-rate
• ability to trade speech quality and capacity
  smoothly and flexibly by a combination of channel
  and codec mode adaptation this can be controlled
  by the network operator on a cell by cell basis

15
AMR source rates
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Structure of AMR encoder
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Encoder output
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Structure of AMR decoder
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Demostration I Full Rate vs. AMR-NB

• Erroneous channel (C/I 264 dB)
• 1. sample FR 13 kbps
• 2. sample AMR-NB 5.9-12.2 kbps

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Wideband speech coding

• Narrowband 300 3400 Hz
• Wideband 50 7000 Hz
• Wideband AMR speech codec (3GPP R5)

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AMR-WB source rates
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EFR vs. AMR-NB vs. AMR-WB(in 16 kbps full rate
traffic channel)
AMR-WB
Excellent
AMR-NB
Very good
Subjective speech quality
EFR
Good
Poor
Unacceptable
Error-free
13
10
7
4
Carrier-to-interface ratio (dB)
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Demostration II AMR-NB vs. AMR-WB

• Clean speech (highest modes)
• 1. sample AMR-NB 12.2 kbps
• 2. sample AMR-WB 23.85 kbps

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Demostration III GSM EFR vs. AMR-WB

• Erroneous channel
• 1. sample GSM EFR 12.2 kbps
• 2. sample AMR-WB 6.6-14.25 kbps

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Demostration IV AMR-NB vs. AMR-WB

• Music (highest modes)
• 1. sample AMR-NB 12.2 kbps
• 2. sample AMR-WB 23.85 kbps