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    ön izlemeyi ön izlemeyi Presentation Transcript

    • Seminar Presentation: Adaptive Multi-Rate Wideband Speech Codec deployment in 3G Core Network Sergei Hyppenen Supervisor: Professor Sven-Gustav Häggman HELSINKI UNIVERSITY OF TECHNOLOGY 11.04.2006
    • 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
    • Abbreviations
      • 3G: 3 rd 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]
    • 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
    • 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)
    • 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
    • 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
      • 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!
    • 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 16 th 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
    • 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
    • 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 (1 st and 2 nd LSBs) and the extension frame part (3 rd and 4 th LSBs).
      • During the transmission through the core network TFO frames should not be modified by noise suppression, level control or other enhancement algorithms
    • 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
    • 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?
    • 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
    • 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
    • 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
    • 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!