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Case Study ADSL Modems

Case Study ADSL Modems. Geert Leus Fac. EEMCS, TUDelft leus@cas.et.tudelft.nl. ADSL Modems. Overview ADSL (VDSL) modems Modem technology : voice-band vs. xDSL Communication impairments (...call for DSP solutions) Equalization Echo Cancellation. ADSL (VDSL) Modems. References:

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Case Study ADSL Modems

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  1. Case Study ADSL Modems Geert Leus Fac. EEMCS, TUDelft leus@cas.et.tudelft.nl

  2. ADSL Modems Overview • ADSL (VDSL) modems Modem technology : voice-band vs. xDSL Communication impairments (...call for DSP solutions) • Equalization • Echo Cancellation

  3. ADSL (VDSL) Modems References: • J.A.C. Bingham, `The theory and practice of modem design’, Wiley, 1980 • J.A.C. Bingham, `Multicarrier modulation for data transmission, an idea whose time has come’, IEEE Communications Magazine, May 1990, pp.5-14 • W.Y. Chen, `DSL simulation techniques and standards development for digital subscriber line systems’, Macmillan, 1998 • ...

  4. ADSL (VDSL) Modems Modem Technology : • …-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 (-> … 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), ...

  5. ADSL (VDSL) Modems Performance improvement results from: • improved telephone channel quality (digital switches, etc.) • improved modulation/coding/DSP techniques (QAM, trellis coded modulation (TCM), echo cancellation (EC), adaptive equalization,…) • semiconductor technology 1972 4.8kbits/sec (V.27) 8-PSK simplex 1976 9.6kbits/sec (V.29) 16-QAM simplex 1984 9.6kbits/sec (V.32) 32-QAM/TCM EC-duplex 1989 19.2kbits/sec (V.33b) 64-QAM/TCM EC-duplex 1996 33.6kbits/sec (V.34) 128-QAM/TCM EC-duplex

  6. ADSL (VDSL) Modems PS: duplexing techniques • Frequency division duplexing (FDD) `downstream’ = central office -> remote user `upstream’ = remote user -> central office • Time division duplexing (TDD) : `ping-pong’ • Full duplex with echo-cancellation (EC) up down frequency up down time up & down time & frequency

  7. ADSL (VDSL) Modems • ADSL : `Asymmetric Digital Subscriber Line’ …-1993: ADSL spurred by interest in video-on-demand (VOD) 1995 : ADSL/VOD interest decline 1996 : ADSL technology trials prove viability. 1997-... : ADSL deployment, reoriented to data applications, as telco’s reaction to cable operators offering high- speed internet access with cable modems.

  8. Access Node Backbone Network max 3.5…5 km ADSL (VDSL) Modems • ADSL : `Asymmetric Digital Subscriber Line’ • VDSL : `Very High Speed Digital Subscriber Line’ copper wire cabinet (ONU) Access Node Backbone Network optical fiber copper wire max 0.3…1.5 km

  9. ADSL (VDSL) Modems • Distance/bitrate trade-off (upstream) bitrate (Mbits/sec) 40 VDSL 30 20 10 ADSL 1 2 3 4 distance (km)

  10. useful signal NEXT FEXT ADSL (VDSL) Modems Communication Impairments : • Frequency-dependent cannel attenuation introduces inter-symbol interference (ISI) • EM coupling between wires in same or adjacent binders introduces crosstalk Near-end Xtalk (NEXT) = US into DS, DS into US Far-end Xtalk (FEXT) = DS into DS, US into US

  11. ADSL (VDSL) Modems Communication Impairments :

  12. ADSL (VDSL) Modems Communication Impairments (continued) : • Xtalk from other systems 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 modulation, DSP,etc. !

  13. ADSL (VDSL) Modems 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’, each of them QAM-modulated, see below. • VDSL DMT or single carrier (CAP/QAM) systems ? =undecided..

  14. ADSL (VDSL) Modems 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. Modulation/demodulation by FFT/IFFT (see below)

  15. IFFT P/S S/P Discrete equivalent channel FFT FEQ r(t) ADC DAC p(t) ch(t) Tx filter Channel Rx filter Tx clock Rx clock ADSL (VDSL) Modems

  16. ADSL/VDSL 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.

  17. ADSL/VDSL Spectrum • ADSL spectrum :

  18. Equalization Problem Statement : • ISI/ICI-free transmission if prefix length >= L-1 L is channel impulse response length • In ADSL : prefix-length is 32 samples channel length is….500 samples (approx.) !!! Solution : • Insert time-domain equalizer for channel shortening

  19. Equalization ps: standard (zero-forcing) equalization means `shortening to 1 non-zero tap’ (cfr. C(f).H(f)=1) ps: complexity ! Example: 32 taps @ 2.2 MHZ -> 70 Mflops...

  20. Equalization

  21. 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

  22. Conclusion • Advanced features in ADSL/VDSL systems - DMT modulation, prefix trick & co - Equalization - Echo Cancellation - (Windowing) - etc… • Confirms importance of digital signal processing techniques in present-day high-performance communication systems

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