Bitrate Maximizing Time-Domain Equalizer Design for DMT-based Systems Koen Vanbleu Promotor: Marc Moonen Coauthors: Geert Ysebaert, Gert Cuypers, Katleen Van Acker KULeuven, ESAT SCD-SISTA, Belgium ICC 2003

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

3Introduction 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 ModemTime 56 Kbps Voice band modem 24 minutes 128 Kbps ISDN10 minutes 6 Mbps ADSL13 seconds 52 Mbps VDSL1.5 seconds ADSL Basics - Intro - DMT Transmitter - Why Equalization? - DMT Receiver ADSL Equalizer Design - Problem Description - Current Equalizers - Bitrate Maximizing Equalizers Conclusions

4Introduction 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 Upstream Downstream CustomerCentral 300 m6.4 Mbps52 MbpsVDSL 3 km640 Kbps6 MbpsADSL Line lengthUpDownFrequency band 1.1 MHz 8.8 MHz ADSL Basics - Intro - DMT Transmitter - Why Equalization? - DMT Receiver ADSL Equalizer Design - Problem Description - Current Equalizers - Bitrate Maximizing Equalizers Conclusions

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

6 Discrete Multi Tone: Transmitter Re Im 2 bits Re Im 4 bits bitsData symbols (QAM)... P/S CP Cyclic Prefix 0 IFFT N -point 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? channel noise... P/S CP IFFT N -point Transmitter Why equalization? “Invert” channel distortion while not boosting noise ADSL Basics - Intro - DMT Transmitter - Why Equalization? - DMT Receiver ADSL Equalizer Design - Problem Description - Current Equalizers - Bitrate Maximizing Equalizers Conclusions

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

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

10 Current ADSL Equalizers (1) 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] Channel h noise TEQ w delay TIR b TIR = target impulse response of (CP-length+1) MMSE criterion  ADSL bitrate maximization

11 Current ADSL Equalizers (2) 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] S/P CP... FFT FEQ D n TEQ w T taps... 1 tap/tone N -point Maximize based on SNR at FFT output residual ISI/ICInoise Approximations!

12 Approximate Bitrate Maximizing TEQs (continued) residual ISI/ICInoise Examples of approximations : Signal component in SNR does not only depend on `windowed’ (shortened) channel impulse response : do not forget DFT leakage (ICI)! ADSL Basics - Intro - DMT Transmitter - Why Equalization? - DMT Receiver ADSL Equalizer Design - Problem Description - Current Equalizers - Bitrate Maximizing Equalizers Conclusions Current ADSL Equalizers (3)

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

14 Bitrate Maximizing Equalizers (2) Exact bitrate maximizing (BM-)TEQ cost function Nonlinear cost function in w only with A n and B n tone dependent matrices function of signal statistics ADSL Basics - Intro - DMT Transmitter - Why Equalization? - DMT Receiver ADSL Equalizer Design - Problem Description - Current Equalizers - Bitrate Maximizing Equalizers Conclusions 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: N G = all used tones  nonlinear cost function “Per group” equalization: BM-TEQ w per group S G of N G tones  nonlinear cost function “Per tone” equalization: N G = 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 Bitrate Maximizing-TEQ Per-Tone Equalizer MMSE-based TEQs Approx. Bitrate Max. TEQs

17Conclusions 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