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Module-3 : Transmission Lecture-8 (11/5/00)

Module-3 : Transmission Lecture-8 (11/5/00). Marc Moonen Dept. E.E./ESAT, K.U.Leuven marc.moonen@esat.kuleuven.ac.be www.esat.kuleuven.ac.be/sista/~moonen/. Lecture 8 : Overview. ADSL/VDSL revisited Specs, spectra, FDM vs. echo, bit loading, P-to-A,… Equalization

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Module-3 : Transmission Lecture-8 (11/5/00)

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  1. Module-3 : TransmissionLecture-8 (11/5/00) Marc Moonen Dept. E.E./ESAT, K.U.Leuven marc.moonen@esat.kuleuven.ac.be www.esat.kuleuven.ac.be/sista/~moonen/ Module-3 Transmission Marc Moonen Lecture-8 Multi-tone Modulation K.U.Leuven/ESAT-SISTA

  2. Lecture 8 : Overview • ADSL/VDSL revisited Specs, spectra, FDM vs. echo, bit loading, P-to-A,… • Equalization Time-domain equalization Frequency-domain equalization • Windowing Receiver windowing, Transmitter windowing • Echo Cancellation Time-domain echo cancellation Time+frequency-domain echo cancellation `Joint shortening’ Module-3 Transmission Marc Moonen Lecture-8 Multi-tone Modulation K.U.Leuven-ESAT/SISTA

  3. Acknowledgement -Lecture partly adopted from Module T2 `Digital Communication Principles’ M.Engels, M. Moeneclaey, G. Van der Plas 1998 Postgraduate Course on Telecommunications Special thanks to G. Van der Plas (Alcatel) -Lecture partly co-authored by Thierry Pollet, Alcatel Telecom, CRC, Antwerp (B) Module-3 Transmission Marc Moonen Lecture-8 Multi-tone Modulation K.U.Leuven-ESAT/SISTA

  4. ADSL/VDSL Revisited • ADSL-DMT Transmission block scheme (see Lecture-7) Module-3 Transmission Marc Moonen Lecture-8 Multi-tone Modulation K.U.Leuven-ESAT/SISTA

  5. ADSL/VDSL Revisited : Specs 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(=512*4.3215k) VDSL specs (=proposal) • up to 4096 carriers • same carrier spacing • etc. Module-3 Transmission Marc Moonen Lecture-8 Multi-tone Modulation K.U.Leuven-ESAT/SISTA

  6. ADSL/VDSL Revisited : Spectrum • ADSL spectrum : Module-3 Transmission Marc Moonen Lecture-8 Multi-tone Modulation K.U.Leuven-ESAT/SISTA

  7. ADSL/VDSL Revisited : Spectrum • ADSL spectrum : FDD-ADSL (=frequency division duplex) EC-ADSL (=overlapping up- and downstream band) Module-3 Transmission Marc Moonen Lecture-8 Multi-tone Modulation K.U.Leuven-ESAT/SISTA

  8. ADSL/VDSL Revisited : Spectrum • VDSL spectrum : unused tones due to line conditions Power spectrum Up Down POTS/ ISDN Frequency 17.6 MHz 1.1 MHz tone spacing(4.3 kHz) Module-3 Transmission Marc Moonen Lecture-8 Multi-tone Modulation K.U.Leuven-ESAT/SISTA

  9. ADSL/VDSL Revisited : Spectrum • VDSL spectrum : flexible setting up- & downstream rates Upstream Downstream Frequency Long line Medium line Short line Module-3 Transmission Marc Moonen Lecture-8 Multi-tone Modulation K.U.Leuven-ESAT/SISTA

  10. ADSL/VDSL Revisited : Channel • ADSL channel attenuation, crosstalk, noise Module-3 Transmission Marc Moonen Lecture-8 Multi-tone Modulation K.U.Leuven-ESAT/SISTA

  11. ADSL/VDSL Revisited : Channel • VDSL channel attenuation, crosstalk, noise, RFI: Next dominated environment + several RF interferers Module-3 Transmission Marc Moonen Lecture-8 Multi-tone Modulation K.U.Leuven-ESAT/SISTA

  12. ADSL/VDSL Revisited: Bit Loading Bit loading and gain settings : • QAM constellation size for each tone depends on SNR measured at the receiver Guarantees optimal capacity in all operation conditions (loops, noise). Module-3 Transmission Marc Moonen Lecture-8 Multi-tone Modulation K.U.Leuven-ESAT/SISTA

  13. ADSL/VDSL Revisited: Bit Loading Bit loading and gain settings • Transmit power optimization to obtain BER=10^-7 on each tone Module-3 Transmission Marc Moonen Lecture-8 Multi-tone Modulation K.U.Leuven-ESAT/SISTA

  14. ADSL/VDSL Revisited: Bit Loading Bit loading and gain settings • Transmit power optimization to obtain BER=10^-7 on each tone (continued) 5 bits 4 bits SNR 3 bits 2 bits 1 bits gain Module-3 Transmission Marc Moonen Lecture-8 Multi-tone Modulation K.U.Leuven-ESAT/SISTA

  15. ADSL/VDSL Revisited: P-to-A Peak-to-average problem • DMT signals have large dynamic range : Module-3 Transmission Marc Moonen Lecture-8 Multi-tone Modulation K.U.Leuven-ESAT/SISTA

  16. ADSL/VDSL Revisited: P-to-A Peak-to-average problem (continued) • large dynamic range implies large quantization steps in A/D & D/A, etc. Remedies • tone structure : redundant tones, modulated with linear combination of other tones, such that time-domain signals have lower dynamic range • receiver re-design : receiver compensates for clipping non-linearity (part of `channel model’) • ... Module-3 Transmission Marc Moonen Lecture-8 Multi-tone Modulation K.U.Leuven-ESAT/SISTA

  17. Equalization Problem Statement : • ISI/ICI-free transmission if L is channel impulse response length is prefix length • In ADSL : prefix-length is 32 samples channel length is….500 samples (approx.) !!! Solution : • Insert time-domain equalizer for channel shortening Module-3 Transmission Marc Moonen Lecture-8 Multi-tone Modulation K.U.Leuven-ESAT/SISTA

  18. Equalization Time-domain equalization : ps: in Lecture 5, (zero-forcing) equalization means `shortening to 1 non-zero tap’ (cfr. H(z).C(z)=1) ps: complexity ! Example: 32 taps @ 2.2 MHZ -> 70 Mflops... Module-3 Transmission Marc Moonen Lecture-8 Multi-tone Modulation K.U.Leuven-ESAT/SISTA

  19. Equalization Time-domain equalization : Module-3 Transmission Marc Moonen Lecture-8 Multi-tone Modulation K.U.Leuven-ESAT/SISTA

  20. Equalization Time-domain equalization • additional design parameter : synchronization delay (=`positioning’ of shortened channel impulse response) Module-3 Transmission Marc Moonen Lecture-8 Multi-tone Modulation K.U.Leuven-ESAT/SISTA

  21. Time-Domain Equalization (TEQ) • Equalizer design : - Possible optimization criterion is overall capacity, i.e. sum of transmitted bits (with BER 10^-7) on all carriers together. - Number of bits loaded onto 1 carrier is a function of the received SNR for that carrier. - SNR for 1 carrier is function of SNR at TEQ-output. - Leads to highly non-linear optimization criterion. Iterative non-linear optimization procedures usually lead to (sub-optimal) local minima. - Need simpler optimization criterion. Module-3 Transmission Marc Moonen Lecture-8 Multi-tone Modulation K.U.Leuven-ESAT/SISTA

  22. Time-Domain Equalization (TEQ) • Equalizer design (continued): - Alternative optimization criterion is time-domain minimum mean squared error criterion based on…. channelimpulse response time-domain equalizer synchronization delay (p20) targetimpulse response Module-3 Transmission Marc Moonen Lecture-8 Multi-tone Modulation K.U.Leuven-ESAT/SISTA

  23. Time-Domain Equalization (TEQ) • Equalizer design (continued): - minimum mean squared error criterion is... i.e., for a given TEQ-length (e.g. 16 taps), find TEQ-coefficients such that cascade (`convolution’) of CIR and TEQ is as close as possible to a (delayed) TIR with non-zero coefficients ( = prefix length) Module-3 Transmission Marc Moonen Lecture-8 Multi-tone Modulation K.U.Leuven-ESAT/SISTA

  24. Time-Domain Equalization (TEQ) • Equalizer design (continued): -This leads to a least squares problem a la the least squares problems for equalizer design in Lecture-6. - Have to include a constraint to avoid trivial (TEQ=TIR=0) solution, e.g. impose `normalized’ TIR or `normalized’ TEQ - Constrained least squares problem equivalent to eigenvalue problem (solved iteratively) Module-3 Transmission Marc Moonen Lecture-8 Multi-tone Modulation K.U.Leuven-ESAT/SISTA

  25. Time-Domain Equalization (TEQ) • Resulting structure (TEQ + 1-taps FEQ) example: ADSL-TEQs with 16…64 taps Module-3 Transmission Marc Moonen Lecture-8 Multi-tone Modulation K.U.Leuven-ESAT/SISTA

  26. Time-Domain Equalization (TEQ) • Problem with TEQ-receiver : - MMSE criterion does correspond to optimizing overall bit-rate. Leads to unpredictable behavior (sometimes lower bit-rate with longer TEQ…) - Obtained bit-rate heavily (and unpredictably) depends on chosen synchronization delay - In practice : modem start-up procedure does not allow for searching for optimal delay within a range. Hence can pick only one delay, and then have to be lucky.. Module-3 Transmission Marc Moonen Lecture-8 Multi-tone Modulation K.U.Leuven-ESAT/SISTA

  27. Frequency-Domain Equalization (FEQ) • Alternative structure based on `per-tone equalization’ : - each tone had its own TEQ, but implemented after the FFT (and combined with 1-taps FEQ) - overall `FEQ’ (=TEQ + 1-taps FEQ) can be designed to optimize SNR for each tone separately. Hence true overall bit-rate optimization. - Smooth behavior (as a function of synchronization delay) - Computational complexity = comparable - Implementation based on sliding FFT properties Module-3 Transmission Marc Moonen Lecture-8 Multi-tone Modulation K.U.Leuven-ESAT/SISTA

  28. Frequency-Domain Equalization (FEQ) • `Per-tone’ equalization structure : `difference terms’ cfr. sliding FFT properties Module-3 Transmission Marc Moonen Lecture-8 Multi-tone Modulation K.U.Leuven-ESAT/SISTA

  29. Windowing (VDSL) Problem Statement : Transmit spectrumfor 1 tone has high side-lobes, that generate interference (e.g.NEXT) in other systems. Receiver DFTmay be viewed as filter bank, each filter has high side-lobes, hence picks up interference in a broad frequency range Solution : Reduce transmit/receive side-lobes by (time-domain) windowing Module-3 Transmission Marc Moonen Lecture-8 Multi-tone Modulation K.U.Leuven-ESAT/SISTA

  30. Windowing (VDSL) Windowing basic principle = extend symbol length + apply `symmetric’ weighting around original symbol edges Module-3 Transmission Marc Moonen Lecture-8 Multi-tone Modulation K.U.Leuven-ESAT/SISTA

  31. Windowing (VDSL) Windowing at the transmitter: Reduces frequency domain side-lobes Reduces the out of band power of the NEXT signal Allows for digital duplexing Guarantees spectral compatibility with ADSL , HPNA,… Symbolbody CP CS DMT symbol CP: cyclic prefix CS: cyclic suffix Module-3 Transmission Marc Moonen Lecture-8 Multi-tone Modulation K.U.Leuven-ESAT/SISTA

  32. Windowing (VDSL) Windowing at the receiver : Reduces Inter Symbol Interference (ISI) Reduces susceptibility to RFI ps: This is equivalent to per-tone equalization (=per-tone optimized window). Symbol body DMT symbol Module-3 Transmission Marc Moonen Lecture-8 Multi-tone Modulation K.U.Leuven-ESAT/SISTA

  33. Windowing (VDSL) DMT-based VDSL-transceiver : additional features : analog & digital RFI-cancellation, egress spectrum control, ... Module-3 Transmission Marc Moonen Lecture-8 Multi-tone Modulation K.U.Leuven-ESAT/SISTA

  34. Echo Cancellation • Problem Statement : ADSL with overlapping up- and downstream bands requires echo cancellation (adaptive filter) Main problem = complexity example : echo path with 500 taps @ 2.2 MHz -> ...Gflops Module-3 Transmission Marc Moonen Lecture-8 Multi-tone Modulation K.U.Leuven-ESAT/SISTA

  35. Echo Cancellation • Time-domain echo cancellation : Adaptive filter (see Lecture 6) 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. • Conclusion : Overlapping up- & downstream bands difficult to implement (@reasonable cost) • PS : EC in FDD-system with reduced front-end filter specs Module-3 Transmission Marc Moonen Lecture-8 Multi-tone Modulation K.U.Leuven-ESAT/SISTA

  36. Echo Cancellation • PS: alternative solutions - `Joint Shortening’ : Design TEQ such that echo channel impulse response length is shortened, together with transmission channel impulse response length. Allows for shorter (cheaper) echo cancellation filter. - `Per-tone’ echo cancellation : optimize TEQ + echo filter for each tone separately (=true SNR optimization!). Allows for shorter (cheaper) echo cancellation filter Module-3 Transmission Marc Moonen Lecture-8 Multi-tone Modulation K.U.Leuven-ESAT/SISTA

  37. Conclusion • Investigated additional features in ADSL/VDSL systems - Equalization - Windowing - Echo Cancellation - … • Confirms importance of digital signal processing techniques in present-day high-performance communication systems (`box full of maths/DSP’,cfr. Lecture-1) Module-3 Transmission Marc Moonen Lecture-8 Multi-tone Modulation K.U.Leuven-ESAT/SISTA

  38. Assignment 4.1 • Recommended (optional) reading: J.A.C. Bingham, `Multicarrier modulation for data transmission, an idea whose time has come’, IEEE Communications Magazine, May 1990, pp.5-14 Module-3 Transmission Marc Moonen Lecture-8 Multi-tone Modulation K.U.Leuven-ESAT/SISTA

  39. Assignment 4.2 Echo Cancellation in an FDD/ADSL-system ? • Assume an FDD/ADSL transceiver sending an upstream signal (lower tones) and receiving a downstream signal (higher tones) • Assume the downstream path as well as the echo path is shorter than the prefix length, and the up- & downstream signals being perfectly (symbol-)synchronized. Investigate the effect of the echo... • Assume the echo path is longer than the prefix. Investigate the effect of the echo... • Assume the echo path is shorter than the prefix, but that the two signals are not (symbol-)synchronized. Investigate the effect of the echo… Module-3 Transmission Marc Moonen Lecture-8 Multi-tone Modulation K.U.Leuven-ESAT/SISTA

  40. Assignment 4.2 Echo Cancellation in an FDD/ADSL-system ? (continued) • Is it possible to have (symbol-)synchronization at the two ends of the line (i.e. in the remote terminal as well as in central office) ? • Conclusion: which extra features to be introduced in FDD modems. Echo cancellation or… ? • To be solved by using your brains and/or pen&paper (i.e. write down a few formulas) and/or your favorite simulation tool…. Module-3 Transmission Marc Moonen Lecture-8 Multi-tone Modulation K.U.Leuven-ESAT/SISTA

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