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Diderot Forum 22/11/2001

Signal Processing and Mathematical Modeling challenges in ADSL/VDSL High-Speed Telephone Line Modems. Diderot Forum 22/11/2001. Marc Moonen Dept. E.E./ESAT, K.U.Leuven marc.moonen@esat.kuleuven.ac.be www.esat.kuleuven.ac.be/sista/members/moonen.html. Overview. Modem technology

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Diderot Forum 22/11/2001

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  1. Signal Processing and Mathematical Modeling challenges in ADSL/VDSL High-Speed Telephone Line Modems Diderot Forum 22/11/2001 Marc Moonen Dept. E.E./ESAT, K.U.Leuven marc.moonen@esat.kuleuven.ac.be www.esat.kuleuven.ac.be/sista/members/moonen.html

  2. Overview • Modem technology Voice-band modems versus xDSL-modems ADSL/VDSL • Communication impairments ...call for advanced solutions (DSP/Comms/VLSI/…) • Transmission is quite a feat ! Line Code IFFT/FFT-based modulation/demodulation DSP blocks : equalization, echo cancellation, etc. • Transmission in an unbundled environment • Conclusions Marc Moonen, ESAT-SISTA, K.U.Leuven

  3. Modem Technology Modem Technology Impairments Transmission Unbundling Conclusions Voice-band modems versus xDSL-modems : • …-1970 : development of efficient modulation techniques for transmission over band limited channels, spurred by military applications (-> 9.6 kbits/sec) • 1970-1980 : Semiconductor technology (size & power reduction) • 1980-1990 : Introduction of coding techniques (TCM), DSP techniques (e.g. echo cancellation for full duplex transmission) (-> 19.2 kbits/sec) • 1990-2000 : reaching Shannon limit (-> 33kbits/sec, asymmetric 56kbits/sec modems) • 1995-… : ADSL (8Mbits/sec downstream, 60 kbits/sec upstream) • 2000-… : VDSL (52Mbits/sec downstream, 6Mbits/sec upstream), ... Marc Moonen, ESAT-SISTA, K.U.Leuven

  4. Modem Technology Modem Technology Impairments Transmission Unbundling Conclusions Voice-band modems versus xDSL-modems example : 10Mbyte download Modem Time 56 Kbps Voice-band 24 minutes 128 Kbps ISDN 10 minutes 6 Mbps ADSL 13 seconds 52 Mbps VDSL 1.5 seconds Marc Moonen, ESAT-SISTA, K.U.Leuven

  5. Modem Technology Modem Technology Impairments Transmission Unbundling Conclusions • ADSL : `Asymmetric Digital Subscriber Line’ …-1993: ADSL spurred by interest in video-on-demand (VOD) 1995 : ADSL/VOD interest decline 1996 : ADSL technology trials. 1997-... : ADSL deployment, reoriented to data applications, as telco’s reaction to cable operators offering high- speed internet access with cable modems. Marc Moonen, ESAT-SISTA, K.U.Leuven

  6. Access Node Backbone Network max 3.5…5 km cabinet (ONU) Access Node Backbone Network optical fiber copper wire max 0.3…1.5 km Modem Technology Modem Technology Impairments Transmission Unbundling Conclusions • ADSL : `Asymmetric Digital Subscriber Line’ • VDSL : `Very High Speed Digital Subscriber Line’ copper wire Marc Moonen, ESAT-SISTA, K.U.Leuven

  7. bitrate (Mbits/sec) 40 VDSL 30 20 10 ADSL 1 2 3 4 distance (km) Modem Technology Modem Technology Impairments Transmission Unbundling Conclusions • ADSL/VDSL distance/bitrate trade-off (downstream) Marc Moonen, ESAT-SISTA, K.U.Leuven

  8. useful signal NEXT FEXT Communication Impairments . Modem Technology Impairments Transmission Unbundling Conclusions • Frequency-dependent channel attenuation introduces inter-symbol interference (ISI) • EM coupling between wires in same or adjacent binders introduces `self’ crosstalk Near-end Xtalk (NEXT) Far-end Xtalk (FEXT) Marc Moonen, ESAT-SISTA, K.U.Leuven

  9. Communication Impairments . Modem Technology Impairments Transmission Unbundling Conclusions Marc Moonen, ESAT-SISTA, K.U.Leuven

  10. Communication Impairments . Modem Technology Impairments Transmission Unbundling Conclusions • Xtalk from other systems (`alien’ Xtalk) e.g. HPNA (Home Phone Network Alliance) vs. VDSL • Radio Frequency Interference (RFI) e.g. AM broadcast, amateur radio • Noise e.g. impulsive noise (=high bursts of short duration) • Echo due to hybrid impedance mismatch Conclusion: Need advanced comms techniques, DSP, VLSI, etc. Marc Moonen, ESAT-SISTA, K.U.Leuven

  11. Transmission is quite a feat ! . Modem Technology Impairments Transmission Unbundling Conclusions Line Codes (modulation technique) : • ADSL (ANSI standard) DMT (`Discrete Multi-tone Modulation’, a.k.a. multi-carrier modulation, orthogonal frequency division multiplexing (OFDM)) 256 different `carriers’ (tones), each of them QAM-modulated, see below. • VDSL DMT or single carrier (CAP/QAM) systems Marc Moonen, ESAT-SISTA, K.U.Leuven

  12. Transmission is quite a feat ! . Modem Technology Impairments Transmission Unbundling Conclusions Line Codes (modulation technique) : • DMT : Multiple carriers are QAM-modulated with a low-rate bit stream. The high-rate bit stream is thus carried by dividing it into hundreds of low-rate streams. Marc Moonen, ESAT-SISTA, K.U.Leuven

  13. Transmission is quite a feat ! . Modem Technology Impairments Transmission Unbundling Conclusions • ADSL spectrum : Marc Moonen, ESAT-SISTA, K.U.Leuven

  14. CP Im 10 00 0 Re ... ... 11 01 2 bits N-point P/S Im IFFT ... ... Re 4 bits Transmission is quite a feat ! . Modem Technology Impairments Transmission Unbundling Conclusions • DMT Transmitter (preview) Cyclic Prefix IFFT-based modulation Marc Moonen, ESAT-SISTA, K.U.Leuven

  15. Transmission is quite a feat ! . Modem Technology Impairments Transmission Unbundling Conclusions • The `mathematics’ behind DMT (1) - Modulation/demodulation by means of IFFT/FFT : In the transmitter, QAM symbols that modulate the different carriers, are `frequency domain’ symbols, and then these are turned into a `time domain’ signal (=sum of modulated carriers) by means of an inverse discrete Fourier transform (->IFFT). In the receiver, the QAM symbols are reconstructed by means of a discrete Fourier transform (->FFT). Marc Moonen, ESAT-SISTA, K.U.Leuven

  16. Transmission is quite a feat ! . Modem Technology Impairments Transmission Unbundling Conclusions • The `mathematics’ behind DMT (2) - The magic `prefix trick’: In the transmitter, a cyclic extension (prefix) is added to each transmitted block (IFFT-output). In the receiver, the portion of the received signal corresponding to the prefix is removed. It is shown that, if the channel impulse response is shorter than the prefix, there will no inter-symbol/inter-carrier interference (ISI/ICI), and channel equalization reduces to a re-scaling of each tone separately (`1-tap FEQ’) (explanation : cyclic prefix turns a linear convolution (=channel effect) into a circular convolution, etc..) Marc Moonen, ESAT-SISTA, K.U.Leuven

  17. CP Im 10 00 0 Re ... ... 11 01 2 bits N-point P/S Im IFFT ... ... Re 4 bits Transmission is quite a feat ! . Modem Technology Impairments Transmission Unbundling Conclusions • DMT Transmitter (revisited) Cyclic Prefix IFFT-based modulation Marc Moonen, ESAT-SISTA, K.U.Leuven

  18. Remove prefix Im 1 tap / toon 10 00 CP ... Re 11 01 ... ... 2 bits ... Im FEQ N-point S/P FFT Re 4 bits FFT-based demodulation Transmission is quite a feat ! . Modem Technology Impairments Transmission Unbundling Conclusions • DMT Receiver Marc Moonen, ESAT-SISTA, K.U.Leuven

  19. Transmission is quite a feat ! . Modem Technology Impairments Transmission Unbundling Conclusions • And then there’s... - channel modeling (training based) - equalizer coefficient initialization - bit loading - coding - synchronization - time domain equalization (see below) - echo cancellation (see below) - RFI cancellation (VDSL) - Xtalk cancellation (VDSL) - etc... Marc Moonen, ESAT-SISTA, K.U.Leuven

  20. Transmission is quite a feat ! . Modem Technology Impairments Transmission Unbundling Conclusions Example : Time domain equalization • Problem Statement : ISI/ICI-free transmission if channel impulse response is shorter than prefix length in ADSL : prefix-length=32 samples channel length=up to 500 samples !!! • Solution : insert time-domain equalizer for `channel shortening’ Marc Moonen, ESAT-SISTA, K.U.Leuven

  21. Remove prefix Im 1 tap / toon 10 00 CP ... Re 11 01 taps ... ... 2 bits ... Im FEQ TEQ N-point S/P FFT Re 4 bits FFT-based demodulation Transmission is quite a feat ! . Modem Technology Impairments Transmission Unbundling Conclusions • DMT Receiver (revisited) PS: 32 taps @ 2.2 MHZ -> 70 M(fl)ops…! Marc Moonen, ESAT-SISTA, K.U.Leuven

  22. time-domain equalizer channel impulse response target impulse response `synchronization delay’ Transmission is quite a feat ! . Modem Technology Impairments Transmission Unbundling Conclusions Example : Time domain equalization • `Optimal’ TEQ computed at start up: constrained least-squares parameter estimation problem leads to eigenvalue (!) problem/procedure (solved iteratively) • PS: optimal synchronization delay not easily determined Marc Moonen, ESAT-SISTA, K.U.Leuven

  23. Transmission is quite a feat ! . Modem Technology Impairments Transmission Unbundling Conclusions Example : Time domain equalization • Alternative equalization structures (ESAT/SISTA & Alcatel) `Per-tone’ equalization = give each tone its own optimal equalizer = true overall bit-rate optimization 6 x 10 3.5 3 2.5 2 Bitsnelheid (bits/s) 1.5 1 0.5 0 -40 -30 -20 -10 0 10 20 30 Delayd 32-taps PT-EQ 8-taps PT-EQ 32-taps TEQ 8-taps TEQ Marc Moonen, ESAT-SISTA, K.U.Leuven

  24. UP &DOWN Trad. tel. UP DOWN Trad. tel. DOWN 4 25 138 1104 f (kHz) 4 25 138 1104 f (kHz) Transmission is quite a feat ! . Modem Technology Impairments Transmission Unbundling Conclusions • Example: Echo cancellation ADSL with overlapping up- and downstream bands requires echo cancellation Marc Moonen, ESAT-SISTA, K.U.Leuven

  25. Transmission is quite a feat ! . Modem Technology Impairments Transmission Unbundling Conclusions • Example: Echo cancellation Echo cancellation based on `optimal’/adaptive filtering Main problem = complexity example : echo path with 500 taps @ 2.2MHz -> ...G(fl)ops Marc Moonen, ESAT-SISTA, K.U.Leuven

  26. Transmission is quite a feat ! . Modem Technology Impairments Transmission Unbundling Conclusions • Time-domain echo cancellation : Adaptive filter has complexity > 2M (M is number of filter taps). Leads to too high computational complexity • Time + Frequency domain echo cancellation : Part of the cancellation is brought to the frequency- domain (after the FFT). Leads to complexity reduction with a factor ..4…8. • Other : Joint shortening, per-tone echo, etc… • Conclusion :Overlapping up- & downstream bands difficult to implement (@reasonable cost) • PS :EC in FDD-system with reduced front-end filter specs Marc Moonen, ESAT-SISTA, K.U.Leuven

  27. Transmission is quite a feat ! . Modem Technology Impairments Transmission Unbundling Conclusions Some ADSL specs (=ANSI standard)... • 256 carriers • 512-point (I)FFT’s (or `similar’) • prefix length = 32 • carrier spacing fo= 4.3215 kHz • basic sampling rate is 2.21 MHz(with A/D-conv @ 8.84 MHz) ...overall computational complexity is >> 100 M(fl)ops Some VDSL specs (=proposal)... • up to 4096 carriers • same carrier spacing • etc. Marc Moonen, ESAT-SISTA, K.U.Leuven

  28. Transmission in an unbundled environment Modem Technology Impairments Transmission Unbundling Conclusions Unbundled ADSL (transmission layer) • Only minor interference (cross-talk) between adjacent copper pairs in the same bundle. • Cross-talk merely viewed as additional noise source, with no major impact on design. • Unbundling is a valid option Marc Moonen, ESAT-SISTA, K.U.Leuven

  29. Transmission in an unbundled environment Modem Technology Impairments Transmission Unbundling Conclusions Unbundled VDSL (transmission layer) • Major interference between adjacent copper pairs in the same bundle : NEXT/FEXT • Cross-talk has major impact on design : `Zipper’ Windowing Power back-off (Blind) user synchronization Multi-user detection, etc … …. all this still very much under study • Unbundling =`major challenge’, viability yet to be proven e.g. allowing different line codes (DMT vs CAP/QAM)…?? Marc Moonen, ESAT-SISTA, K.U.Leuven

  30. Conclusion Modem Technology Impairments Transmission Unbundling Conclusions • Seen some of the advanced features in ADSL/VDSL systems - DMT modulation, prefix trick & co - Equalization - Echo Cancellation - RFI & Xtalk cancellation - etc… • Transmission is a quite a feat ! (clap) • Unbundled ADSL / Unbundled VDSL ? Marc Moonen, ESAT-SISTA, K.U.Leuven

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