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Case Study ADSL Modems: 

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

ADSL Modems: 

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

ADSL (VDSL) Modems: 

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 ...

ADSL (VDSL) Modems: 

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), ...

ADSL (VDSL) Modems: 

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 1976 1984 1989 1996 4.8kbits/sec (V.27) 9.6kbits/sec (V.29) 9.6kbits/sec (V.32) 19.2kbits/sec (V.33b) 33.6kbits/sec (V.34) 8-PSK 16-QAM 32-QAM/TCM 64-QAM/TCM 128-QAM/TCM simplex simplex EC-duplex EC-duplex EC-duplex

ADSL (VDSL) Modems: 

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

ADSL (VDSL) Modems: 

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.

ADSL (VDSL) Modems: 

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

ADSL (VDSL) Modems: 

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

ADSL (VDSL) Modems: 

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

ADSL (VDSL) Modems: 

ADSL (VDSL) Modems Communication Impairments :

ADSL (VDSL) Modems: 

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. !

ADSL (VDSL) Modems: 

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..

ADSL (VDSL) Modems: 

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)

ADSL (VDSL) Modems: 

ADSL (VDSL) Modems

ADSL/VDSL Specs: 

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.

ADSL/VDSL Spectrum: 

ADSL/VDSL Spectrum ADSL spectrum :

Equalization: 

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

Equalization: 

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...

Equalization: 

Equalization

Echo Cancellation: 

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

Conclusion: 

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