High-Speed Wireline Communication Systems

1
High-Speed WirelineCommunication Systems

• Prof. Brian L. Evans
• Dept. of Electrical and Comp. Eng.The University
  of Texas at Austin
• http//signal.ece.utexas.edu

Current graduate students Ming Ding, Zukang
Shen Ex-graduate students Güner Arslan (Silicon
Laboratories),Biao Lu (Schlumberger), Milos
Milosevic (Schlumberger) Ex-undergraduate
students Wade Berglund, Jerel Canales,David
Love, Ketan Mandke, Scott Margo, Esther Resendiz,
Jeff Wu
http//www.ece.utexas.edu/bevans/projects/adsl
2
Schlumberger Downhole Data Communications

• Downhole drilling
• Cable of several miles in length
• Power and data delivered on cables (harmonics)
• Motors downhole turning on and off (harmonics)
• Downhole borehead faces high temperatures,
  vibrations, etc.
• Need for speed
• Uplink digitized images/properties of ground
  (high data rate)
• Downlink command, control, and programs (low
  data rate)
• Need for asymmetric data communications
• High-the-better uplink data rates
• Lower-the-better bit error rates on both links
• Approaches
• Single channel, single carrier (e.g. quadrature
  amplitude modulation)
• Single channel, multiple carriers (e.g discrete
  multitone modulation)
• Multiple channels

3
Quadrature Amplitude Modulation (QAM)
Modulator
cos(2p fc t)
Lowpassfilter
Transmit
I
Bits
Constellation encoder
Bandpass
-
Q
00110
Lowpassfilter
sin(2p fc t)

• Single carrier
• Single signal, occupying entire available
  bandwidth
• Symbol rate is bandwidth of signal being centered
  on carrier frequency
• Mike Kuei-che Cheng, Improving the Performance of
  a Wireline Telemetry Receiver, MS Thesis, UT
  Austin, 1997.

4
Multicarrier Modulation

• Divide broadband channel into narrowband
  subchannels
• No ISI in subchannels if constant gain inevery
  subchannel and if ideal sampling
• Each subchannel has a different carrier
• Discrete multitone modulation
• Based on fast Fourier transform
• Standardized for ADSL and VDSL
• Used in Schlumberger downhole modems

channel
carrier
magnitude
subchannel(QAM signal)
frequency
Subchannels are 4.3 kHz wide in ADSL and VDSL
5
Digital Subscriber Line (DSL) Broadband Access
Internet
DSLAM
high data rate
Central Office
DSL modem
DSL modem
low data rate
VoiceSwitch
LPF
LPF
Customer Premises
Telephone Network
DSLAM - Digital Subscriber Line Access
Multiplexer LPF Low Pass Filter (passes
voiceband frequencies)
6
Simulation Results for 17-Tap Equalizers
Parameters Cyclic prefix length 32 FFT size
(N) 512 Coding gain (dB)
4.2 Margin (dB) 6 Input power
(dBm) 23 Noise power (dBm/Hz)
-140 Crosstalk noise 24
ISDN disturbers
High rate direction
Figure 1 in Martin, Vanbleu, Ding, Ysebaert,
Milosevic, Evans, Moonen Johnson, submitted
7
Multichannel Discrete Multitone Transmission

• Different levels of coordination
• Multiuser detection (no coordination)
• Joint spectra optimization (coordination of
  transmit spectra usage)
• Vectored transmission (full signaling
  coordination at both ends)

8
Improving Data Rates

• Per channel improvements
• Symbol synchronization (embed makers in
  transmitted data)
• Multicarrier modulation (number of channels, bit
  swapping)
• Equalization (training sequence and time)
• Error detection and correction (choice of coding
  methods)
• Multichannel improvements
• Coordination of transmit specta
• Coordination of signaling at both ends (training
  sequence and time)
• Interference cancellation

9
Backup Slides
10
Multiuser Detection

• No coordination between duplex channels
• Different service providers bundled in
  same/adjacent cable
• Must combat near-end and far-end crosstalk
• Crosstalk identification estimate crosstalk
  channel and power
• Crosstalk cancellation

11
Joint Spectra Optimization

• Coordination in joint spectra design
• Goal find multiuser power allocation to maximize
  sum of data rates
• Solution For all users, regard others as
  additional noise and perform single user
  water-filling and iterate

12
Vectored Transmission

• Signal level coordination
• Full knowledge of downstream transmitted signal
  and upstream received signal at central office
• Block transmission at both ends fully
  synchronized
• Channel characterization
• Pertone basis
• Multi-channel

13
Crosstalk Cancellation

• NEXT is suppressed by frequency division
  duplexing
• FEXT is cancelled per tone via QR decomposition
  of Ti
• Downstream
• Pertone MIMO precoding
• No crosstalk after channel
• Upstream
• QR leads to a back-substitution structure
• decode last user, decision feedback as crosstalk
• Successive crosstalk cancellation

14
Simulation Results for 17-Tap TEQs (cont)
ADSL Equalization
Parameters Cyclic prefix length 32 FFT size
(N) 512 Coding gain (dB)
4.2 Margin (dB) 6 Input power
(dBm) 23 Noise power (dBm/Hz)
-140 Crosstalk noise 24
ISDN disturbers
Downstream transmission
Figure 3 in Martin, Vanbleu, Ding, Ysebaert,
Milosevic, Evans, Moonen Johnson, submitted
15
Data Transmission in an ADSL Transceiver
N/2 subchannels
N real samples
S/P
quadrature amplitude modulation (QAM) mapping
mirror data and N-IFFT
add cyclic prefix
P/S
D/A transmit filter
Bits
00110
TRANSMITTER
each block programmed in lab and covered in one
full lecture in EE 345S
channel
each block covered in one full lecture
RECEIVER
N real samples
N/2 subchannels
P/S
time domain equalizer (FIR filter)
QAM decoder
N-FFT and remove mirrored data
S/P
remove cyclic prefix
receive filter A/D
invert channel frequency domain equalizer
P/S parallel-to-serial S/P
serial-to-parallel FFT fast Fourier transform
16
Discrete Multitone (DMT) DSL Standards
Introduction

• ADSL Asymmetric DSL
• Maximum data rates supported in G.DMT standard
  (ideal case)
• Echo cancelled 14.94 Mbps downstream, 1.56 Mbps
  upstream
• Frequency division multiplexing 13.38 Mbps
  downstream, 1.56 Mbps up
• Widespread deployment in US, Canada, Western
  Europe, Hong Kong
• Central office providers only installing
  frequency-division multiplexed (FDM)
• ADSLcable modem market 12in US 51 worldwide
• ADSL 8 Mbps downstream min.
• ADSL2 doubles analog bandwidth
• VDSL Very High Rate DSL
• Asymmetric
• Faster G.DMT FDM ADSL
• 2m subcarriers m ? 8, 12
• Symmetric 13, 9, or 6 Mbps
• Optional 12-17 MHz band

1997
2003
2003
2003
17
Spectral Compatibility of xDSL
Introduction
Any overlap with the AM radio band?
Any overlap with the FM radio band?
18
A Digital Communications System
Modulation

• Encoder maps a group of message bits to data
  symbols
• Modulator maps these symbols to analog waveforms
• Demodulator maps received waveforms back to
  symbols
• Decoder maps the symbols back to binary message
  bits

19
Amplitude Modulation by Cosine Function
Modulation

• Example y(t) f(t) cos(w0 t)
• f(t) is an ideal lowpass signal
• Assume w1 ltlt w0
• Y(w) is real-valued if F(w) is real-valued
• Demodulation is modulation then lowpass filtering
• Similar derivation for modulation with sin(w0 t)

Y(w)
w
20
Amplitude Modulation by Sine Function
Modulation

• Example y(t) f(t) sin(w0 t)
• f(t) is an ideal lowpass signal
• Assume w1 ltlt w0
• Y(w) is imaginary-valued ifF(w) is real-valued
• Demodulation is modulation then lowpass filtering

F(w)
1
w
w1
-w1
0
Y(w)
21
Multicarrier Modulation by Inverse FFT
Modulation
Q
g(t)
x
x
I
Discrete time
g(t)
x
x


g(t)
x
x
g(t) pulse shaping filter Xi ith
symbol from encoder
22
Multicarrier Modulation in ADSL
ADSL Transceivers
QAM
00101
N/2 subchannels (carriers)
N real-valuedtimesamplesformsADSLsymbol
Mirror complex data (in red) andtake conjugates
23
Multicarrier Modulation in ADSL
ADSL Transceivers
Inverse FFT
CP Cyclic Prefix
D/A transmit filter
ADSL frame is an ADSL symbol plus cyclic prefix
24
Multicarrier Demodulation in ADSL
ADSL Transceivers
S/P
N-point FastFourierTransform(FFT)
N/2 subchannels (carriers)
N time samples
25
Bit Manipulations
ADSL Transceivers

• Serial-to-parallel converter
• Example of one input bit stream and two output
  words

• Parallel-to-serialconverter
• Example of two input words and one output bit
  stream

S/P
S/P
1 1 0
1 1 0
0 0 1 1 0
0 0 1 1 0
0 0
0 0
Words
Bits
Bits
Words
26
Inter-symbol Interference (ISI)
Combating ISI
2.1

• Ideal channel
• Impulse response is an impulse
• Frequency response is flat
• Non-ideal channelcauses ISI
• Channel memory
• Magnitude and phase variation
• Received symbol is weighted sum of neighboring
  symbols
• Weights are determined by channel impulse
  response

1.7
1
1
1
1
1
.7
.1


Channelimpulseresponse
Received signal
-1
Threshold at zero
1
1
1
1
1
Detected signal
27
Single Carrier Modulation
Combating ISI

• Ideal (non-distorting) channel over transmission
  band
• Flat magnitude response
• Linear phase response delay is constant for all
  spectral components
• No intersymbol interference
• Impulse response for ideal channel over all
  frequencies
• Continuous time
• Discrete time
• Equalizer
• Shortens channelimpulse response(time domain)
• Compensates forfrequency distortion(frequency
  domain)

g d(t-T)
g dk-D
Discretized Baseband System
28
Combat ISI with Equalization
Combating ISI

• Problem Channel frequency response is not flat
• Solution Use equalizer to flatten channel
  frequency response
• Zero-forcing equalizer
• Inverts channel (impulseresponse forced to
  impulse)
• Flattens frequency response
• Amplifies noise
• Minimum mean squarederror (MMSE) equalizer
• Optimizes trade-off betweennoise amplification
  and ISI
• Decision-feedbackequalizer
• Increases complexity
• Propagates error

29
Cyclic Prefix Helps in Fighting ISI
Combating ISI
subsymbols to be transmitted
cyclic prefix
mirrored subsymbols
to be removed
equal
30
Cyclic Prefix Helps in Fighting ISI
Combating ISI

• Provide guard time between successive symbols
• No ISI if channel length is shorter than n 1
  samples
• Choose guard time samples to be a copy of the
  beginning of the symbol cyclic prefix
• Cyclic prefix converts linear convolution into
  circular convolution
• Need circular convolution so that
• symbol ? channel ? FFT(symbol) x FFT(channel)
• Then division by the FFT(channel) can undo
  channel distortion

31
Channel Impulse Response
Combating ISI
frequency (kHz)
32
Channel Impulse Response
Combating ISI
frequency (kHz)
33
Combat ISI with Time-Domain Equalizer
Combating ISI

• Channel length is usually longer than cyclic
  prefix
• Use finite impulse response (FIR) filter called a
  time-domain equalizer to shorten channel impulse
  response to be no longer than cyclic prefix length

34
Eliminating ISI in Discrete Multitone Modulation
ADSL Equalization

• Time domain equalizer (TEQ)
• Finite impulse response (FIR) filter
• Effective channel impulse responseconvolution
  of TEQ impulse responsewith channel impulse
  response
• Frequency domain equalizer (FEQ)
• Compensates magnitude/phase distortionof
  equalized channel by dividing each
  FFTcoefficient by complex number
• Generally updated during data transmission
• ADSL G.DMT equalizer training
• Reverb same symbol sent 1,024 to 1,536 times
• Medley aperiodic sequence of 16,384 symbols
• At 0.25 s after medley, receiver returns
  numberof bits on each subcarrier that can be
  supported

35
Time-Domain Equalizer Design
ADSL Equalization

• Minimizing mean squared error
• Minimize mean squared error (MMSE) method Chow
  Cioffi, 1992
• Geometric SNR method Al-Dhahir Cioffi, 1996
• Minimizing energy outside of shortened channel
  response
• Maximum Shortening SNR method Melsa, Younce
  Rohrs, 1996
• Minimum ISI method Arslan, Evans Kiaei, 2000
• Maximizing achievable bit rate
• Maximum bit rate method Arslan, Evans, Kiaei,
  2000
• Maximum data rate method Milosevic, Pessoa,
  Evans, Baldick, 2002
• Bit rate maximization Vanblue, Ysebaert,
  Cuypers, Moonen Van Acker, 2003
• Other equalizer architectures
• Dual-path (DP) design uses two TEQs Ming,
  Redfern Evans, 2002
• TEQ filter bank design Milosevic, Pessoa, Evans,
  Baldick, 2002
• Per tone equalization Acker, Leus, Moonen, van
  der Wiel, Pollet, 2001

36
Minimum Mean Squared Error TEQ Design
ADSL Equalization

• Minimize Eek2 Chow Cioffi, 1992
• Chose length of b (e.g. n in ADSL) to shorten
  length of h w
• b is eigenvector of minimum eigenvalue of
  channel-dependent matrix
• Minimum MSE achieved when
  where
• Disadvantages
• Does not consider bit rate
• Deep notches in equalizer frequency response
  (zeros out low SNR bands)
• Infinite length TEQ case zeros of b on unit
  circle (kills n subchannels)

Amenable to real-time fixed-point DSP
implementation
37
Maximum Shortening SNR Solution
ADSL Equalization

• Minimize energy leakage outside shortened channel
  length
• For each possible position of a window of ?1
  samples,
• Disadvantages
• Does not consider channel capacity
• Requires Cholesky decomposition andeigenvector
  calculation
• Does not consider channel noise
• Amenable to real-time fixed-point DSP realization

38
Maximum Shortening SNR Solution
ADSL Equalization

• Choose w to minimize energy outside window of
  desired length
• Locate window to capture maximum channel impulse
  response energy

• hwin, hwall equalized channel within and
  outside the window
• Objective function is shortening SNR (SSNR)

39
Matlab DMT TEQ Design Toolbox 3.1
ADSL Equalization

• Single-path, dual-path, per-tone TEQ filter
  bank equalizers
• Available at http//www.ece.utexas.edu/bevans/pro
  jects/adsl/dmtteq/

default parameters from G.DMT ADSL standard
23
-140
different graphical views
variousperformance measures
40
Multicarrier Modulation

• Advantages
• Efficient use of bandwidth without full channel
  equalization
• Robust against impulsive noise and narrowband
  interference
• Dynamic rate adaptation
• Disadvantages
• Transmitter High signal peak-to-average power
  ratio
• Receiver Sensitive to frequency and phase offset
  in carriers
• Open issues for point-to-point connections
• Pulse shapes of subchannels (orthogonal,
  efficient realization)
• Channel equalizer design (increase bit rate,
  reduce complexity)
• Synchronization (timing recovery, symbol
  synchronization)
• Bit loading (allocation of bits in each
  subchannel)
• Open issues for coordinating multiple connections

41
Notes
42
Applications of Broadband Access
Notes
Residential
Business
43
DSL Broadband Access Standards
Notes
Courtesy of Mr. Shawn McCaslin
44
ADSL and Cable Modems
Notes

• Need for high-speed (broadband) data access
• Voiceband data modems can yield 53 kbps (kilobits
  per second)
• Telephone voice channel capacity ois 64 kbps (the
  Central Office samples voice signals at 8 kHz
  using 8 bits/sample)
• Integrated Services Digital Network (ISDN) modems
  deliver 128 kbps
• New modem standards are necessary to meet the
  demand for higher bandwidth access for
  telecommuting, videoconferencing,
  video-on-demand, Internet service providers,
  Internet access, etc.
• Two standards tested in 1998 and now widely
  available
• Cable modems
• Asymmetric Digital Subscriber Line (ADSL) modems
• Cable Modems
• Always connected to the Internet
• Your neighbors on the same local area network
  share the bit rate
• Local area network provides either 27 or 36 Mbps
  downstream, and between 320 kbps and 10 Mbps
  upstream.

45
ADSL Modems
Notes

• ADSL modems
• Always connected to the Internet
• Call central office using a dedicated telephone
  line which also supports a conventional Plain Old
  Telephone Service (POTS) line for voice
• Connection time is 5-10 seconds
• ADSL modems are capable of delivering 1-10 Mbps
  from the central office to the customer
  (downstream) and 0.5-1 Mbps from the customer to
  the central office (upstream)
• Although ADSL lines have been available from
  Southwestern Bell since the Fall of 1997, ADSL
  modems were not commercially available until Fall
  of 1999.

46
Discrete Multitone (DMT) Modulation
Notes

• DMT uses multiple harmonically related carriers
• Implemented as inverse Fast Fourier Transform
  (FFT) in transmitter
• Implemented using forward FFT in receiver
• Transmission bandwidth
• 1.1 MHz downstream and 256 kHz upstream
• Limit of 1.1 MHz is due to power constraints
  imposed by the FCC
• For 18 kft telephone lines, the attenuation at
  1.1 MHz is -120 dBm.
• Frequency domain is divided into 256 4.3-kHz bins
• Channel 0 is dedicated to voice
• Channels 1-5 are not used due to compatibility
  with ISDN services.

47
Two Types of Transmission
Notes

• Two versions of ADSL
• Frequency Division Multiplexing the upstream and
  downstream channels do not overlap the upstream
  uses channels 6-31 and the downstream uses
  channels 32-255.
• Echo Cancelled the upstream and downstream
  channels overlap the upstream uses channels 6-31
  and the downstream uses channels 6-255.
• According to available SNR in each bin, bin
  carries
• QAM signal whose constellation varies from 2-15
  bits or
• no signal if SNR is less than 12 dB in that
  subchannel
• Constellations chosen so that overall bit error
  rate lt 10-7
• Maximum transmission rate with symbol rate of 4
  kHz
• Downstream 248 channels x 15 bits/channel x 4
  kHz 14.88 Mbps
• Upstream 24 channels x 15 bits/channel x 4 kHz
  1.440 Mbps

48
Channel Attenuation
Notes

• Reliable transmission of high-frequency
  information over a telephone line is wrought with
  several challenges.
• Telephone lines are unshielded and bundled 50
  wires to a trunk. The other lines in the bundle
  can cause severe crosstalk
• Telephone lines attenuate signals. The
  attenuation increases with increasing frequency.
  At 1.1 MHz, which is the highest transmitted
  frequency, the attenuation of a 24 gauge wire is
• 10 kft -70 dBm/Hz 16 kft
  -110 dBm/Hz
• 12 kft -90 dBm/Hz 18 kft
  -120 dBm/Hz
• 14 kft -100 dBm/Hz
• Because of severe effects in the channel, the
  ADSL standard defines channel coding using cyclic
  prefixes and employs error correcting codes

49
Bridge Taps
Notes

• Bridge Taps are unterminated lines
• During modem initialization, effect of bridge
  taps is included in channel estimate. Their
  effect would be to lower the possible channel
  capacity.
• During data transmission, bridge taps may
  saturate the front-end and at a least will be
  unpleasant for the echo canceller. The echo
  canceller should have an estimate of the echo
  channel including the bridge taps. Given that the
  reflected echo is almost instantaneous than the
  echo canceller channel estimate should capture
  them too.
• In G.lite, echo cancellation is optional
• Modems who use it can still use it
• A bigger problem in G.lite is the phone due to
  the splitterless environment
• Transmitters that do not have an echo canceller
  system can rely on their receive filters to
  reduce the echo.

50
ADSL Modems
Notes

• ADSL modem consists of a line driver plus 3
  subsystems
• analog front end (15 V)
• digital interface (3 V)
• discrete multitone processor (3 V)
• Analog front end provides the analog-to-digital
  and digital-to-analog interfaces to the telephone
  line.
• Digital interace manages the input and output
  digital message streams.
• Discrete multitone processor implements the
  digital communications and signal processing to
  support the ADSL standard. An ADSL modem requires
  much greater than 200 Digital Signal Processor
  MIPS.

51
Motorola CopperGold ADSL Chip
Notes

• Announced March 1998
• 5 million transistors, 144 pins, clocked at 55
  MHz
• 1.5 W power consumption
• DMT processor contains
• Motorola MC56300 DSP core
• Several application specific ICs
• 512-point FFT
• 17-tap FIR filter for time-domain channel
  equalization based on MMSE method (20 bits
  precision per tap)
• DSP core and memory occupies about 1/3 of chip
  area
• It gives up to 8 Mbps upstream and 1 Mbps
  downstream

52
Motorola Copper Gold ADSL Transceiver
Notes

• Contains all 3 ADSL modem subsystems on a single
  chip.
• Has programmable bit to tell it whether it is at
  customer's or central office site
• Analog front end operates at a sampling rate of
  2.208 MHz and gives 16 bits/sample of resolution.
  It uses sigma-delta modulation with an
  oversampling factor of 55 / 2.208 25.
• Discrete multitone processor consists of a
  Motorola MC56300 DSP Onyx core and several
  application-specific digital VLSI circuits to
  implement
• 256-point FFT for downstream transmission or
  512-point FFT for downstream reception if it is
  at the central office or customer's site,
  respectively
• 17-tap adaptive FIR filter for channel
  equalization (20 bits of precision per tap)
  running at 2.208 MHz
• DSP core computes the 32-point FFT for the
  downstream transmission or the 64-point FFT for
  the downstream reception.

53
Minimum Mean Squared Error TEQ
Notes

• Matrix O selects the proper part out of Rxy
  corresponding to the delay ?

54
Simulation Results for 17-Tap TEQ
Notes
Cyclic prefix length 32 FFT size (N) 512 Coding
gain 4.2 dB Margin 6 dB
Input power 23 dBm Noise power -140
dBm/Hz Crosstalk noise 8 ADSL disturbers POTS
splitter 5th order Chebyshev
55
Simulation Results for Three-Tap TEQ
Notes
Cyclic prefix length 32 FFT size (N) 512 Coding
gain 4.2 dB Margin 6 dB
Input power 23 dBm Noise power -140
dBm/Hz Crosstalk noise 8 ADSL disturbers POTS
splitter 5th order Chebyshev