1 / 22

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:

angie
Download Presentation

Case Study ADSL Modems

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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

More Related