Diderot Forum 22/11/2001

1
Diderot Forum 22/11/2001
Signal Processing and Mathematical Modeling
challenges in ADSL/VDSL High-Speed Telephone
Line Modems

• Marc Moonen
• Dept. E.E./ESAT, K.U.Leuven
• marc.moonen_at_esat.kuleuven.ac.be
• www.esat.kuleuven.ac.be/sista/members/moonen.html

2
Overview

• Modem technology
• Voice-band modems versus xDSL-modems
• ADSL/VDSL
• Communication impairments
• ...call for advanced solutions
  (DSP/Comms/VLSI/)
• Transmission is quite a feat !
• Line Code
• IFFT/FFT-based modulation/demodulation
• DSP blocks equalization, echo
  cancellation, etc.
• Transmission in an unbundled environment
• Conclusions

3
Modem Technology
Modem Technology Impairments Transmission Unbundli
ng Conclusions

• Voice-band modems versus xDSL-modems
• -1970 development of efficient modulation
  techniques for
• transmission over band limited channels,
  spurred by military applications
• (-gt 9.6
  kbits/sec)
• 1970-1980 Semiconductor technology (size
  power reduction)
• 1980-1990 Introduction of coding techniques
  (TCM), DSP techniques
• (e.g. echo cancellation for full duplex
  transmission)
• (-gt 19.2
  kbits/sec)
• 1990-2000 reaching Shannon limit
• (-gt 33kbits/sec,
  asymmetric 56kbits/sec modems)
• 1995- ADSL (8Mbits/sec downstream, 60
  kbits/sec upstream)
• 2000- VDSL (52Mbits/sec downstream, 6Mbits/sec
  upstream), ...

4
Modem Technology
Modem Technology Impairments Transmission Unbundli
ng Conclusions

• Voice-band modems versus xDSL-modems
• example 10Mbyte download

Time
Modem
24 minutes
56 Kbps Voice-band
10 minutes
128 Kbps ISDN
13 seconds
6 Mbps ADSL
1.5 seconds
52 Mbps VDSL
5
Modem Technology
Modem Technology Impairments Transmission Unbundli
ng Conclusions

• ADSL Asymmetric Digital Subscriber Line
• -1993 ADSL spurred by interest in
  video-on-demand (VOD)
• 1995 ADSL/VOD interest decline
• 1996 ADSL technology trials.
• 1997-... ADSL deployment, reoriented to data
  applications,
• as telcos reaction to cable operators
  offering high-
• speed internet access with cable
  modems.

6
Modem Technology
Modem Technology Impairments Transmission Unbundli
ng Conclusions

• ADSL Asymmetric Digital Subscriber Line
• VDSL Very High Speed Digital Subscriber Line

copper wire
7
Modem Technology
Modem Technology Impairments Transmission Unbundli
ng Conclusions

• ADSL/VDSL distance/bitrate trade-off (downstream)

8
Communication Impairments .
Modem Technology Impairments Transmission Unbundli
ng Conclusions

• Frequency-dependent channel attenuation
  introduces inter-symbol interference (ISI)
• EM coupling between wires in same or adjacent
  binders introduces self crosstalk
• Near-end Xtalk (NEXT)
• Far-end Xtalk (FEXT)

9
Communication Impairments .
Modem Technology Impairments Transmission Unbundli
ng Conclusions
10
Communication Impairments .
Modem Technology Impairments Transmission Unbundli
ng Conclusions

• Xtalk from other systems (alien Xtalk)
• 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 comms techniques, DSP,
VLSI, etc.
11
Transmission is quite a feat ! .

Modem Technology Impairments Transmission Unbundli
ng Conclusions

• 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 (tones), each of
  them QAM-modulated, see below.
• VDSL
• DMT or single carrier (CAP/QAM) systems

12
Transmission is quite a feat ! .
Modem Technology Impairments Transmission Unbundli
ng Conclusions

• 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.

13
Transmission is quite a feat ! .
Modem Technology Impairments Transmission Unbundli
ng Conclusions

• ADSL spectrum

14
Transmission is quite a feat ! .
Modem Technology Impairments Transmission Unbundli
ng Conclusions

• DMT Transmitter (preview)

Cyclic Prefix
IFFT-based modulation
15
Transmission is quite a feat ! .
Modem Technology Impairments Transmission Unbundli
ng Conclusions

• The mathematics behind DMT (1)
• - Modulation/demodulation by means of
  IFFT/FFT
• In the transmitter, QAM symbols that
  modulate the
• different carriers, are frequency domain
  symbols,
• and then these are turned into a time
  domain signal
• (sum of modulated carriers) by means of an
  inverse
• discrete Fourier transform (-gtIFFT).
• In the receiver, the QAM symbols are
  reconstructed by
• means of a discrete Fourier transform
  (-gtFFT).

16
Transmission is quite a feat ! .
Modem Technology Impairments Transmission Unbundli
ng Conclusions

• The mathematics behind DMT (2)
• - The magic prefix trick
• In the transmitter, a cyclic extension
  (prefix) is added
• to each transmitted block (IFFT-output).
• In the receiver, the portion of the
  received signal
• corresponding to the prefix is removed.
• It is shown that, if the channel impulse
  response is shorter than
• the prefix, there will no
  inter-symbol/inter-carrier interference
• (ISI/ICI), and channel equalization reduces
  to a re-scaling of each
• tone separately (1-tap FEQ)
  (explanation cyclic prefix turns a
• linear convolution (channel effect) into a
  circular convolution, etc..)

17
Transmission is quite a feat ! .
Modem Technology Impairments Transmission Unbundli
ng Conclusions

• DMT Transmitter (revisited)

Cyclic Prefix
IFFT-based modulation
18
Transmission is quite a feat ! .
Modem Technology Impairments Transmission Unbundli
ng Conclusions

• DMT Receiver

19
Transmission is quite a feat ! .
Modem Technology Impairments Transmission Unbundli
ng Conclusions

• And then theres...
• - channel modeling (training based)
• - equalizer coefficient initialization
• - bit loading
• - coding
• - synchronization
• - time domain equalization (see below)
• - echo cancellation (see below)
• - RFI cancellation (VDSL)
• - Xtalk cancellation (VDSL)
• - etc...

20
Transmission is quite a feat ! .
Modem Technology Impairments Transmission Unbundli
ng Conclusions

• Example Time domain equalization
• Problem Statement ISI/ICI-free transmission if
  channel impulse response is shorter than prefix
  length
• in ADSL prefix-length32 samples
  
• channel lengthup to 500
  samples !!!
• Solution
• insert time-domain
• equalizer for
• channel shortening

21
Transmission is quite a feat ! .
Modem Technology Impairments Transmission Unbundli
ng Conclusions

• DMT Receiver
• (revisited)

PS 32 taps _at_ 2.2 MHZ -gt 70 M(fl)ops!
22
Transmission is quite a feat ! .
Modem Technology Impairments Transmission Unbundli
ng Conclusions

• Example Time domain equalization
• Optimal TEQ computed at start up
• constrained least-squares parameter
  estimation problem
• leads to eigenvalue (!) problem/procedure
  (solved iteratively)
• PS optimal synchronization delay not easily
  determined

23
Transmission is quite a feat ! .
Modem Technology Impairments Transmission Unbundli
ng Conclusions

• Example Time domain equalization
• Alternative equalization structures (ESAT/SISTA
  Alcatel)
• Per-tone equalization
• give each tone its
• own optimal equalizer
• true overall bit-rate
• optimization

6
x 10
3.5
3
2.5
2
Bitsnelheid (bits/s)
1.5
1
0.5
0
-40
-30
-20
-10
0
10
20
30
Delay d
32-taps PT-EQ
8-taps PT-EQ
32-taps TEQ
8-taps TEQ
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Transmission is quite a feat ! .
Modem Technology Impairments Transmission Unbundli
ng Conclusions

• Example Echo cancellation
• ADSL with overlapping up- and downstream
  bands requires echo cancellation

25
Transmission is quite a feat ! .
Modem Technology Impairments Transmission Unbundli
ng Conclusions

• Example Echo cancellation
• Echo cancellation based on
  optimal/adaptive filtering
• Main problem complexity
• example echo path with 500 taps _at_ 2.2MHz
  -gt ...G(fl)ops

26
Transmission is quite a feat ! .
Modem Technology Impairments Transmission Unbundli
ng Conclusions

• Time-domain echo cancellation
• Adaptive filter has complexity gt 2M
• (M is number of filter taps).
• Leads to too high computational complexity
• Time Frequency domain echo cancellation
• Part of the cancellation is brought to the
  frequency-
• domain (after the FFT).
• Leads to complexity reduction with a
  factor ..48.
• Other Joint shortening, per-tone echo, etc
• Conclusion Overlapping up- downstream bands
• difficult to implement (_at_reasonable cost)
• PS EC in FDD-system with reduced front-end
  filter specs

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Transmission is quite a feat ! .
Modem Technology Impairments Transmission Unbundli
ng Conclusions

• Some ADSL specs (ANSI standard)...
• 256 carriers
• 512-point (I)FFTs (or similar)
• prefix length 32
• carrier spacing fo 4.3215 kHz
• basic sampling rate is 2.21 MHz (with
  A/D-conv _at_ 8.84 MHz)
• ...overall computational complexity is gtgt 100
  M(fl)ops
• Some VDSL specs (proposal)...
• up to 4096 carriers
• same carrier spacing
• etc.

28
Transmission in an unbundled environment
Modem Technology Impairments Transmission Unbundli
ng Conclusions

• Unbundled ADSL (transmission layer)
• Only minor interference (cross-talk) between
  adjacent copper pairs in the same bundle.
• Cross-talk merely viewed as additional noise
  source, with no major impact on design.
• Unbundling is a valid option

29
Transmission in an unbundled environment
Modem Technology Impairments Transmission Unbundli
ng Conclusions

• Unbundled VDSL (transmission layer)
• Major interference between adjacent
• copper pairs in the same bundle NEXT/FEXT
• Cross-talk has major impact on design
• Zipper
• Windowing
• Power back-off
• (Blind) user synchronization
• Multi-user detection, etc
• . all this still very much under study
• Unbundling major challenge, viability yet to
  be proven
• e.g. allowing different line codes (DMT vs
  CAP/QAM)??

30
Conclusion
Modem Technology Impairments Transmission Unbundli
ng Conclusions

• Seen some of the advanced features in ADSL/VDSL
  systems
• - DMT modulation, prefix trick co
• - Equalization
• - Echo Cancellation
• - RFI Xtalk cancellation
• - etc
• Transmission is a quite a feat ! (clap)
• Unbundled ADSL / Unbundled VDSL ?