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A Dual-Path TEQ Structure for DMT-ADSL Systems

A Dual-Path TEQ Structure for DMT-ADSL Systems. Ming Ding 1 , Arthur J. Redfern 2 and Brian L. Evans 1 1 Embedded Signal Processing Laboratory Department of Electrical and Computer Engineering The University of Texas, Austin, TX

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A Dual-Path TEQ Structure for DMT-ADSL Systems

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  1. A Dual-Path TEQ Structure for DMT-ADSL Systems Ming Ding1, Arthur J. Redfern2 and Brian L. Evans1 1 Embedded Signal Processing Laboratory Department of Electrical and Computer Engineering The University of Texas, Austin, TX 2 DSP R&D Center, Communication Systems Laboratory Texas Instruments, Dallas, TX

  2. Discrete Multitone Modulation (DMT) • Divide channel into narrowband subchannels • Multicarrier modulation computed by FFT • Standardized for Asymmetric Digital Subscriber Line (ADSL) modems channel carrier (tone) magnitude subchannel frequency Subchannels are 4.3 kHz wide in ADSL and VDSL

  3. Cyclic prefix: guardtime between symbols Time domain equalizer Conventionally a single FIR filter Shortens channel impulse response to be no longer than cyclic prefix length n copy copy s y m b o l ( i+1) CP CP s y m b o li CP: Cyclic Prefix v samples N samples Combating Intersymbol Interference shortened impulse response channel impulseresponse D is transmission delay

  4. ADSL Transceiver (ITU Structure) TRANSMITTER N/2 subchannels N real samples N + n real samples S/P quadrature amplitude modulation (QAM) encoders mirror data and N-IFFT add cyclic prefix P/S D/A + transmit filter Bits channel RECEIVER N/2 subchannels N real samples N + n real samples P/S time domain equalizer (TEQ) QAM decoders N-FFT and remove mirrored data S/P remove cyclic prefix receive filter + A/D Up to N/2 1 - tap FEQs Problem: Design TEQ to maximize bit rate

  5. nk rk yk xk channel TEQ FFT FEQ + Review of Single FIR TEQ Design • Minimizing mean squared error • Minimize mean squared error method [Chow & Cioffi, 1992] • Geometric SNR method [Al-Dhahir & Cioffi, 1996] • Minimizing energy outside of shortened (equalized) channel impulse response • Maximum Shortening SNR method [Melsa, Younce & Rohrs, 1996] • Minimum ISI method [Arslan, Evans & Kiaei, 2000] • Maximum bit rate method [Arslan, Evans & Kiaei, 2000]

  6. Maximum Bit Rate (MBR) TEQ • Models achievable bit rate at TEQ output as nonlinear function of FIR taps • Maximize nonlinear function to obtain TEQ • Performance measure for TEQ design methods • Near-optimal Minimum-ISI (Min-ISI) method

  7. Per Tone Equalizer[Acker, Leus, Moonen, van de Wiel, Pollet, 2001] • Sliding FFT produces NT product FNY • Compute equalized FFT coefficient for tone i Zi = rowi(FNY) wi Sliding N-Point FFT N+n D W1,0 W1,1 W1,2 W1,Lw-1 N+Lw-1 D N+n WN/2,0 WN/2,1 WN/2,2 WN/2,Lw-1 D y N+n Up to N/2 T-tap frequency domain equalizer (FEQ)

  8. Bit Allocation Comparison • AWG 26 Loop:12000 ft + AWGN Per Tone 5.7134 Mbps MBR: 5.4666 Mbps MIN-ISI 5.2586 Mbps MSSNR 5.2903 Mbps ARMA 4.5479 Mbps MMSE 4.4052 Mbps Per Tone achieves higher bit rate than single-FIR TEQ, but has significantly more implementation complexity to train equalizer and process data than MMSE, MSSNR, Min-ISI single-FIR TEQs

  9. Path Selection for each Subchannel TEQ 1 FFT FEQ TEQ 2 FFT Dual-Path Equalizer Structure • Find better tradeoff of complexity vs. bit rate • Dual-path time-domain equalizer • Identify higher SNR part of transmission band • Design second TEQ tailored for narrower subband • First TEQ equalizes the entire available bandwidth

  10. Dual-Path TEQ (Simulated Channel) Optimized for subchannel 2-250 Optimized for subchannel 2-30

  11. Dual-Path TEQ (Real Channel) ANSI-13 Loop • Crosstalk: 24 DSL disturbers • Additive white Gaussian noise Dual-Path TEQ • Both paths use tones 33-255 • Second path only optimizes tones 55-85 Achieved Bit Rate • Path 1: 2.5080 Mbps • Dual Path: 2.6020 Mbps • 4% improvement in bit rate

  12. Matlab DMTTEQ Toolbox 3.0 • Equalizer structures • Single path FIR • Dual-path FIR • Per-tone equalizer • Design/evaluationTEQ test platform • Easy to customize • Graphical interface • http://www.ece.utexas.edu/~bevans/projects/adsl/dmtteq/

  13. Conclusions • Single path optimizes over all subchannels • Difficult to optimize over all subchannels • Dual path optimizes over two sets of subchannels • Appropriate design technique must be chosen • 4% increase in achievable bit rate over single path MBR TEQ for real channel • Lower implementation complexity that per-tone equalizer • Future research • Frequency domain approach for sub-bandwidth optimization

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