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Bitrate Maximizing Equalizers for ADSL Modems

Bitrate Maximizing Equalizers for ADSL Modems. Koen Vanbleu Promotor: Marc Moonen Collaborators: Geert Ysebaert, Gert Cuypers KULeuven, ESAT SCD-SISTA, Belgium. February 6, 2003. Overview. ADSL Basics What? Transmitter/Receiver ADSL Equalizer Design Problem Description

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Bitrate Maximizing Equalizers for ADSL Modems

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  1. Bitrate Maximizing Equalizers for ADSL Modems Koen Vanbleu Promotor: Marc Moonen Collaborators: Geert Ysebaert, Gert Cuypers KULeuven, ESAT SCD-SISTA, Belgium February 6, 2003

  2. Overview • ADSL Basics • What? • Transmitter/Receiver • ADSL Equalizer Design • Problem Description • Current Equalizers • Bitrate Maximizing Equalizers • Conclusions

  3. Introduction • ADSL Basics • - Intro • - DMT Transmitter • - Why Equalization? • - DMT Receiver • ADSL Equalizer Design • - Problem Description • - Current Equalizers • - Bitrate Maximizing • Equalizers • Conclusions • Communication at high rates towards customer • telephone wire, cable, fiber, wireless • Communication over telephone wire • Evolution: ever increasing bitrates • E.g. Time to download 10 Mbyte file

  4. Introduction Down Up Line length ADSL 6 Mbps 640 Kbps 3 km VDSL 52 Mbps 6.4 Mbps 300 m Downstream Central Customer Upstream • ADSL Basics • - Intro • - DMT Transmitter • - Why Equalization? • - DMT Receiver • ADSL Equalizer Design • - Problem Description • - Current Equalizers • - Bitrate Maximizing • Equalizers • Conclusions • Broadband communication over telephone line • ADSL (Asymmetric Digital Subscriber Line) • VDSL (Very high bit rate Digital Subscriber Line) • Bitrate is function of the line length

  5. Modulation and Duplexing • Multicarrier modulation scheme: Discrete Multitone (DMT) e.g. ADSL UP &DOWN POTS UP DOWN POTS DOWN 4 25 138 1104 f (kHz) 4 25 138 1104 f (kHz) • ADSL Basics • - Intro • - DMT Transmitter • - Why Equalization? • - DMT Receiver • ADSL Equalizer Design • - Problem Description • - Current Equalizers • - Bitrate Maximizing • Equalizers • Conclusions • Assign different frequency bins to up- and downstream directions • Frequency Division Duplexing (FDD) • Overlap: Echo Cancellation (EC) • Traditional telephony (POTS) still available over the same wire.

  6. Discrete Multi Tone: Transmitter bits Data symbols (QAM) Cyclic Prefix CP Im 0 10 00 ... ... Re 11 01 2 bits N-point P/S Im IFFT Re ... ... 4 bits IFFT modulation (Inverse Fast Fourier Transform) • ADSL Basics • - Intro • - DMT Transmitter • - Why Equalization? • - DMT Receiver • ADSL Equalizer Design • - Problem Description • - Current Equalizers • - Bitrate Maximizing • Equalizers • Conclusions

  7. Why Equalization? CP noise ... ... channel N-point P/S IFFT ... ... Transmitter • ADSL Basics • - Intro • - DMT Transmitter • - Why Equalization? • - DMT Receiver • ADSL Equalizer Design • - Problem Description • - Current Equalizers • - Bitrate Maximizing • Equalizers • Conclusions Why equalization? “Invert” channel distortion while not boosting noise

  8. ADSL Basics • - Intro • - DMT Transmitter • - Why Equalization? • - DMT Receiver • ADSL Equalizer Design • - Problem Description • - Current Equalizers • - Bitrate Maximizing • Equalizers • Conclusions Discrete Multi Tone: Receiver Unbiased Frequency Domain Equalizer 1 tap / tone Data symbols Im bits 10 00 CP ... noise Re 11 01 ... 2 bits ... ... Im FEQ channel h TEQ w N-point S/P FFT Re Time Domain Equalizer taps 4 bits FFT demodulation CP length + 1

  9. DMT Equalization: Problem Description TEQ w CP where is hard with time-domain equalizer w • ADSL Basics • - Intro • - DMT Transmitter • - Why Equalization? • - DMT Receiver • ADSL Equalizer Design • - Problem Description • - Current Equalizers • - Bitrate Maximizing • Equalizers • Conclusions Im{X} 10 00 Re{X} T taps 1 tap/tone N-point 11 01 ... ... ... S/P FEQ Dn FEQ FFT 2 bits To maximize bitrate:

  10. Current ADSL Equalizers (1) noise Channel h TEQ w delay TIR b • ADSL Basics • - Intro • - DMT Transmitter • - Why Equalization? • - DMT Receiver • ADSL Equalizer Design • - Problem Description • - Current Equalizers • - Bitrate Maximizing • Equalizers • Conclusions • Channel shorteners, e.g. MMSE-based TEQ [Al-Dhahir, Cioffi] TIR = target impulse response of (CP-length+1) MMSE criterion  ADSL bitrate maximization

  11. Current ADSL Equalizers (2) T taps 1 tap/tone TEQ w N-point ... ... ... S/P FEQ Dn FFT CP Approximations! based on SNR at FFT output residual ISI/ICI noise • ADSL Basics • - Intro • - DMT Transmitter • - Why Equalization? • - DMT Receiver • ADSL Equalizer Design • - Problem Description • - Current Equalizers • - Bitrate Maximizing • Equalizers • Conclusions • Approximate Bitrate Maximizing TEQs [Al-Dhahir], [Evans] Maximize

  12. Current ADSL Equalizers (3) • not only depending on walled shortened impulse response • : do not forget DFT leakage! • Approximate Bitrate Maximizing TEQs (continued) residual ISI/ICI noise Examples of approximations :

  13. Bitrate Maximizing Equalizers (1) TEQ w CP AND then = residual ISI/ICI+noise sources (XT, RFI, …) • ADSL Basics • - Intro • - DMT Transmitter • - Why Equalization? • - DMT Receiver • ADSL Equalizer Design • - Problem Description • - Current Equalizers • - Bitrate Maximizing • Equalizers • Conclusions T taps N-point ... ... ... S/P FEQ Dn FFT Maximize where

  14. Bitrate Maximizing Equalizers (2) • ADSL Basics • - Intro • - DMT Transmitter • - Why Equalization? • - DMT Receiver • ADSL Equalizer Design • - Problem Description • - Current Equalizers • - Bitrate Maximizing • Equalizers • Conclusions Exact bitrate maximizing (BM-)TEQ cost function • Nonlinear cost function in wonly with Anand Bn • tone dependent matrices • function of signal statistics • Recursive Gauss-Newton updating algorithm: • attains good local optimum • adaptivity (to track channel/noise changes) • however: high complexity

  15. Bitrate Maximizing Equalizers (3) • ADSL Basics • - Intro • - DMT Transmitter • - Why Equalization? • - DMT Receiver • ADSL Equalizer Design • - Problem Description • - Current Equalizers • - Bitrate Maximizing • Equalizers • Conclusions • Bitrate maximizing equalizers: • Bitrate maximizing (single) TEQ: NG = all used tones  nonlinear cost function • “Per group” equalization: BM-TEQ w per group SG of NG tones  nonlinear cost function • “Per tone” equalization: NG= 1 tone •  (advantageous) linear MMSE problem [Vanacker, Leus, Moonen] HIGHER BITRATE

  16. Bitrate Maximizing Equalizers (4) • ADSL Basics • - Intro • - DMT Transmitter • - Why Equalization? • - DMT Receiver • ADSL Equalizer Design • - Problem Description • - Current Equalizers • - Bitrate Maximizing • Equalizers • Conclusions Simulations Per-Tone Equalizer Bitrate Maximizing-TEQ MMSE-based TEQs Approx. Bitrate Max. TEQs

  17. Conclusions • ADSL Basics • - Intro • - DMT Transmitter • - Why Equalization? • - DMT Receiver • ADSL Equalizer Design • - Problem Description • - Current Equalizers • - Bitrate Maximizing • Equalizers • Conclusions • ADSL Equalizer Design • Truly Bitrate Maximizing Per-Group Equalizer • Time-Domain Equalizer (1 group of tones) • Per-Tone Equalizer (groups of 1 tone) • Recursive Gauss-Newton algorithm • Good local optimum • Complex

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