1. Seminar Presentation: Adaptive Multi-Rate Wideband Speech Codec deployment in 3G Core Network
Sergei Hyppenen
Supervisor: Professor Sven-Gustav Häggman
HELSINKI UNIVERSITY OF TECHNOLOGY
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2. 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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3. Abbreviations 3G: 3rd Generation HR: Half Rate speech codec
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 µ-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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4. Introduction Speech contains frequencies up to the 10 kHz
Current fixed and mobile telecommunication systems operate with a narrow audio bandwidth: Hz (ITU-T G.711)
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 kb/s
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5. 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, kb/s
Coding mode kb/s produces better quality than G.711 (64 kb/s)
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6. Network architectures: GSM and 3G (Release 4)
GSM: Transcoder (TC) is a part of Base Station Subsystem (BSS)
In core Network Sub-Systems (NSS) speech signals are transferred in G.711 form
3G, Release 4: Core Network (CN) is divided to Packet Switched (PS) and Circuit Switched (CS) domains
CS domain is separated to Control Plane (Signaling) and User Plane (Data)
TC moved to core network, but still, the most common scheme to transfer speech in CN is G.711
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7. Speech transmission GSM
In current telecommunication systems transcoding is performed at least twice
In core networks speech signals are transferred in narrowband G.711 form and one one-way connection requires a 64 kb/s channel
GSM
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!
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8. TrFO and TFO
Transcoder Free Operation (TrFO) transfers coded speech frames in ATM- 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 least significant 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 16th LSB
When both mobile terminals switch to a compatible codec, coded speech frames can be merged into PCM-based stream that was decoded from those coded frames
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9. 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
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10. 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 2nd LSBs) and the extension frame part (3rd and 4th LSBs).
During the transmission through the core network TFO frames should not be modified by noise suppression, level control or other enhancement algorithms
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11. 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 kb/s
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12. Lawful interception But how bad the noise really is?
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 is noisy
The solution is utilization of the passive TFO protocol
But how bad the noise really is?
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13. 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 the testing 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
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14. Test results: Noise floor values
Linear notation of the A-law is 13 bits and the µ-law is 14 bits. The first bit is the sign bit and it is not one of the effective bits in representation
In theory only half of the bits are really replaced → measured noise floor values are lower than the calculated ones
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15. 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
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16. Conclusions → the passive TFO protocol is needed indeed!
SNR values of the intercepted signals with AMR-WB-specific TFO frames were 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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