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1 © 2006 Nokia AMRWB_depl.ppt / 2006-04-11 / SHy
Seminar Presentation: Adaptive Multi-Rate Wideband Speech Codec deployment in 3G Core Network
Sergei Hyppenen
Supervisor: Professor Sven-Gustav Häggman
HELSINKI UNIVERSITY OF TECHNOLOGY11.04.2006
2 © 2006 Nokia AMRWB_depl.ppt / 2006-04-11 / SHy
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
3 © 2006 Nokia AMRWB_depl.ppt / 2006-04-11 / SHy
Abbreviations
• 3G: 3rd Generation
• 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]
4 © 2006 Nokia AMRWB_depl.ppt / 2006-04-11 / SHy
Introduction
•Speech contains frequencies up to the 10 kHz
•Current fixed and mobile telecommunication systems operate with a narrow 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
5 © 2006 Nokia AMRWB_depl.ppt / 2006-04-11 / SHy
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
6 © 2006 Nokia AMRWB_depl.ppt / 2006-04-11 / SHy
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 formUm
BSS
Abis
A
TC
OtherPLMN
PSTN/ISDN
NSS
MS
SIM
BSC MSC GMSC
O&M
NMS
BTS
ME
HLR
+BTS
BTS
AuC EIR
VLR VLR
BSS
Ater TDM
GGSNSGSN
BSC
Network Management NMS
OtherPLMN
PSTN/ISDN
Internet
BTS
CN CS Domain
CN PS Domain
RNC
Node-B
UE
MSGERAN
UTRAN
Abis
Ater/Iu
Iub
Iu
Iu
Gb
Um
Uu
H.248Mc
BICC CS-2, SIP-T, ISUP
TDM/IP/ATM
MSCServer
H.248Mc
MGW
Nb
MSS/GCS
MGW
TC
• 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
7 © 2006 Nokia AMRWB_depl.ppt / 2006-04-11 / SHy
Speech transmission
• 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
BTS
BSC
A
TDM MSC
64kb/s
MSC
64kb/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!
GSM
8 © 2006 Nokia AMRWB_depl.ppt / 2006-04-11 / SHy
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
9 © 2006 Nokia AMRWB_depl.ppt / 2006-04-11 / SHy
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
ß Bearer establishment
Nb UP Initialization à
Bearer establishment à
Iu UP Initialization à
ß Bearer establishment
ß 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-O RNC-O
SETUPIAM
Paging
SETUP
Bearer establishment à
Nb UP Initialization à
Bearer establishment à
Iu UP Initialization à
ß Bearer establishment
ß Iu UP Initialization
UE UE
ALERTING
CONNECT
Bearer Information
{
Late assignment of a radio traffic channel with forward bearer establishment in CN
10 © 2006 Nokia AMRWB_depl.ppt / 2006-04-11 / SHy
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
... ... ... ... ...... ... ...
1 2 3 158 159 160 1 3 5
2 4 6315 317 319
316 318 320
2 4 6
2 4 6
1 3 5
1 3 5
}} main
frame part
extensionframe part
315 317 319
316 318 320
315 317 319
316 318 320
8k TFO frame 16k TFO frame 32k TFO frame
} unaltered sample bits
87654321
Bits
160 samplesTFO frame length=160bits
160 samplesTFO frame length=320bits
160 samplesTFO frame length=640bits
11 © 2006 Nokia AMRWB_depl.ppt / 2006-04-11 / SHy
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
12 © 2006 Nokia AMRWB_depl.ppt / 2006-04-11 / SHy
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 is noisy
• The solution is utilization of the passive TFO protocol
•But how bad the noise really is?
13 © 2006 Nokia AMRWB_depl.ppt / 2006-04-11 / SHy
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
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 signal2. Once transcoded wideband signal3. Pure narrowband G.711 signal4. Narrowband G.711 signal with possible embedded TFO frames
3
4
2b
2a
14 © 2006 Nokia AMRWB_depl.ppt / 2006-04-11 / SHy
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
Noise floor in dBov Corrupted bits
in G.711 sample
Corrupted bits
in linear values
Effective bits in
linear level representations
Unaltered
bits in linear values
Calculated (approx)
Measured (exact)
0 0 12 -72 -72.26
every 16th LSB -71.21
1 2 10 -60 -64.77
2 3 9 -54 -59.47
A –law
4 5
12
7 -42 -47.59
0 0 13 -78 -78.26
every 16th LSB -76.47
1 2 11 -66 -74.74
2 3 10 -60 -66.44
µ –law
4 5
13
8 -48 -51.42
15 © 2006 Nokia AMRWB_depl.ppt / 2006-04-11 / SHy
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
16 © 2006 Nokia AMRWB_depl.ppt / 2006-04-11 / SHy
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!