Transcript High-Speed Wireline Communication Systems Prof. Brian L. Evans Dept. of Electrical and Comp.

High-Speed Wireline
Communication 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
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
18 - 2
Quadrature Amplitude Modulation (QAM)
Q
Xi
Modulator
I
Bits
00110
Constellation
encoder
I
Q
cos(2p fc t)
Lowpass
filter
Lowpass
filter
Transmit
-
Bandpass
sin(2p fc t)
magnitude
channel
fc
frequency
– 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. 18 - 3
Multicarrier Modulation
• Divide broadband channel into narrowband subchannels
– No ISI in subchannels if constant gain in
every subchannel and if ideal sampling
– Each subchannel has a different carrier
pulse
DTFT-1
• Discrete multitone modulation
w
– Based on fast Fourier transform
-wc
– Standardized for ADSL and VDSL
– Used in Schlumberger downhole modems
magnitude
sinc
wc
n
sin w c n 
pn
channel
carrier
Subchannels are 4.3 kHz wide in ADSL and VDSL
subchannel
(QAM signal)
frequency
18 - 4
Interne
t
Digital Subscriber Line (DSL)
Broadband Access
DSLAM
high data rate
Central
Office
DSL
modem
DSL
modem
low data rate
Voice
Switch
LPF
LPF
Customer Premises
Telephone
Network
DSLAM - Digital Subscriber Line Access Multiplexer
LPF – Low Pass Filter (passes voiceband frequencies)
18 - 5
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]
18 - 6
Multichannel Discrete Multitone Transmission
Duplex Channel
NEXT
FEXT
NEXT: Near End Crosstalk
FEXT: Far End Crosstalk
• Different levels of coordination
– Multiuser detection (no coordination)
– Joint spectra optimization (coordination of transmit spectra usage)
– Vectored transmission (full signaling coordination at both ends)
18 - 7
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
18 - 8
Backup Slides
Multiuser Detection
Duplex Channel
NEXT
FEXT
NEXT: Near End Crosstalk
FEXT: Far End Crosstalk
• 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
18 - 10
Joint Spectra Optimization
Duplex Channel
NEXT
FEXT
NEXT: Near End Crosstalk
FEXT: Far End Crosstalk
• 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
18 - 11
Vectored Transmission
Duplex Channel
NEXT
FEXT
NEXT: Near End Crosstalk
FEXT: Far End Crosstalk
• Signal level coordination
– Full knowledge of downstream transmitted signal and upstream
received signal at central office
– Block transmission at both ends fully synchronized
Zi  Ti Ui  Ni
• Channel characterization
Zi : received signal Ui : transmitted signal
– Pertone basis
Ti : MIMO channelfrequency responsefor tonei
– Multi-channel
18 - 12
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
18 - 13
ADSL Equalization
Simulation Results for 17-Tap TEQs (con’t)
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]
18 - 14
Data Transmission in an ADSL Transceiver
N/2 subchannels N real samples
quadrature
amplitude
modulation
(QAM)
mapping
Bits
00110
S/P
TRANSMITTER
N/2 subchannels
QAM
decoder
add
cyclic
prefix
P/S
D/A +
transmit
filter
each block programmed in lab and
covered in one full lecture in EE 345S
each block covered in one full lecture
RECEIVER
P/S
mirror
data
and
N-IFFT
invert
channel
=
frequency
domain
equalizer
P/S parallel-to-serial
channel
N real samples
N-FFT
and
remove
mirrored
data
S/P serial-to-parallel
remove
S/P cyclic
prefix
time
domain
equalizer
(FIR
filter)
receive
filter
+
A/D
FFT fast Fourier transform
18 - 15
Introduction
Discrete Multitone (DMT) DSL Standards
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)
ADSL:cable modem market 1:2
in US & 5:1 worldwide
G.DMT Asymmetric
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
Data band
Upstream
subcarriers
Downstream
subcarriers
Target upstream rate
Target downstream rate
ADSL DMT VDSL
0.025 – 1.1
0.138 – 12
MHz
MHz
32
256
256
2048/4096
1 Mbps
3 Mbps
8 Mbps
13/22 Mbps
18 - 16
Introduction
Spectral Compatibility of xDSL
Plain Old Telephone Service
Any overlap
with the AM
radio band?
1.1 MHz
ISDN
ADSL - USA
ADSL - Europe
HDSL/SHDSL
HomePNA
Any overlap
with the FM
radio band?
VDSL
10k
100k
1M
10M
Frequency (Hz)
Upstream
Downstream
100M
12 MHz
Mixed
18 - 17
Modulation
A Digital Communications System
•
•
•
•
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
Message
Source
Decoder
Encoder
Noise
Modulator
Transmitter
Channel
Message
Sink
Demodulator
Receiver
18 - 18
Modulation
Amplitude Modulation by Cosine Function
• Example: y(t) = f(t) cos(w0 t)
F(w)
f(t) is an ideal lowpass signal
Assume w1 << w0
Y(w) is real-valued if F(w) is real-valued
1
-w1
Y w  
0
w1
w
1
1
F w  w 0   F w - w 0 
2
2
½Fw  w0
Y(w)
½Fw - w0
½
-w0 - w1
-w0
-w0 + w1
0
w0 - w1
w0
w0 + w1
w
• Demodulation is modulation then lowpass filtering
• Similar derivation for modulation with sin(w0 t)
18 - 19
Modulation
Amplitude Modulation by Sine Function
• Example: y(t) = f(t) sin(w0 t)
F(w)
f(t) is an ideal lowpass signal
Assume w1 << w0
Y(w) is imaginary-valued if
F(w) is real-valued
Y w  
1
-w1
Y(w)
j
-j ½Fw - w0
½
w0 - w1
-w0
w
j
j
F w  w 0  - F w - w 0 
2
2
j ½Fw  w0
-w0 - w1
w1
0
-w0 + w1
-j
w0
w0 + w1
w
½
• Demodulation is modulation then lowpass filtering
18 - 20
Modulation
Multicarrier Modulation by Inverse FFT
e
X1
g(t)
g(t)
Xi
e
X1
e
+
Discrete
time
X2
e
g(t) : pulse shaping filter
X N /2
1
n
N
j 2p
2
n
N
x
j 2p f N / 2 t
x
j 2p
x
I
j 2p f 2 t
x
g(t)
e
X N /2
Q
x
e
X2
j 2p f1t
j 2p
+
N /2
n
N
x
Xi : ith symbol from encoder
18 - 21
ADSL Transceivers
Multicarrier Modulation in ADSL
Q
Xi
00101
QAM
I
X0
N/2
subchannels
(carriers)
X1
X2
XN/2
Mirror complex
data (in red) and
take conjugates:
e j   e- j   2 cos()
XN/2-1*
X2*
N-point
Inverse
Fast
Fourier
Transform
(IFFT)
x0
x1
x2
xN-1
N realvalued
time
samples
forms
ADSL
symbol
X1*
18 - 22
ADSL Transceivers
Multicarrier Modulation in ADSL
Inverse FFT
ADSL
CP
N
v samples
CP
N samples
s y m b o l (i)
CP
s y m b o l ( i+1)
copy
CP: Cyclic Prefix
downstream upstream
4
32
64
512
copy
D/A + transmit filter
ADSL frame is an ADSL symbol plus cyclic prefix
18 - 23
ADSL Transceivers
Multicarrier Demodulation in ADSL
S/P
~
X0
N/2
subchannels
(carriers)
~
X N 2-1
~
XN 2
~*
X N 2-1
~*
X1
N-point
Fast
Fourier
Transform
(FFT)
~
x0
~x
1
~
x
2
N time
samples
~
xN -1
18 - 24
ADSL Transceivers
Bit Manipulations
• Serial-to-parallel
converter
• Parallel-to-serial
converter
110
00110
Bits
110
S/P
S/P
00
00
Words
Words
• Example of one input bit
stream and two output
words
00110
Bits
• Example of two input
words and one output bit
stream
18 - 25
Combating ISI
Inter-symbol Interference (ISI)
2.1
• Ideal channel
1.7
111 1
1
.7
.4
.7
.1
=
*
-1
1
Channel
impulse
response
Received
signal
– Impulse response is an impulse
– Frequency response is flat
• Non-ideal channel
causes ISI
– Channel memory
– Magnitude and phase variation
• Received symbol is weighted
sum of neighboring symbols
Threshold
at zero
11 1 1 1
Detected
signal
– Weights are determined by
channel impulse response
18 - 26
Combating ISI
Single Carrier Modulation
• 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
nk
– Continuous time: g d(t-T
Channel
Equalizer
yk
xk
rk
ek
– Discrete time: g d[k-D]
+
w
+
h
+
• Equalizer
– Shortens channel
impulse response
(time domain)
– Compensates for
frequency distortion
(frequency domain)
Ideal Channel
z-D
g
Discretized Baseband System
18 - 27
Combating ISI
Combat ISI with Equalization
• Problem: Channel frequency response is not flat
• Solution: Use equalizer to flatten channel frequency response
• Zero-forcing equalizer
– Inverts channel (impulse
response forced to impulse)
– Flattens frequency response
– Amplifies noise
• Minimum mean squared
error (MMSE) equalizer
– Optimizes trade-off between
noise amplification and ISI
Zero-forcing
Equalizer
frequency
response
MMSE
Equalizer
frequency
response
Channel
frequency
response
• Decision-feedback
equalizer
– Increases complexity
– Propagates error
18 - 28
Combating ISI
Cyclic Prefix Helps in Fighting ISI
subsymbols to
be transmitted
cyclic
prefix
mirrored
subsymbols
to be
removed
equal
18 - 29
Combating ISI
Cyclic Prefix Helps in Fighting 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
v samples
CP
N samples
s y m b o l (i)
copy
CP
s y m b o l ( i+1)
copy
18 - 30
Combating ISI
Channel Impulse Response
frequency (kHz)
18 - 31
Combating ISI
Channel Impulse Response
frequency (kHz)
18 - 32
Combating ISI
Combat ISI with Time-Domain Equalizer
• Channel length is usually longer than cyclic prefix
• Use finite impulse response (FIR) filter called a timedomain equalizer to shorten channel impulse response to
be no longer than cyclic prefix length
channel
impulse
response
shortened
channel
impulse
response
D
18 - 33
ADSL Equalization
Eliminating ISI in Discrete Multitone
Modulation
• Time domain equalizer (TEQ)
n1
channel
impulse
response
– Finite impulse response (FIR) filter
– Effective channel impulse response:
convolution of TEQ impulse response
with channel impulse response
effective
channel
impulse
response
• Frequency domain equalizer (FEQ)
– Compensates magnitude/phase distortion
of equalized channel by dividing each FFT
coefficient 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 number
of bits on each subcarrier that can be supported
D
D: transmission delay
n: cyclic prefix length
ADSL G.DMT Values
Down
Up
stream stream
4
n
32
64
N
512
18 - 34
ADSL Equalization
Time-Domain Equalizer Design
• 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]
18 - 35
ADSL Equalization
Minimum Mean Squared Error TEQ Design
Channel
xk
h
nk
yk
+
z-D
TEQ
w
b
rk
ek
- +
bk-D
• Minimize E{ek2} [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
T
T
-1
– Minimum MSE achieved when w  b R xy R yy where w  0
• Disadvantages
Amenable to real-time fixedpoint DSP implementation
– 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) 18 - 36
ADSL Equalization
Maximum Shortening SNR Solution
• Minimize energy leakage outside shortened channel length
• For each possible position of a window of n+1 samples,
 energy inside window after T EQ 

maxSSNR in dB  max10log10 
w
w
 energy outside window after T EQ
h
w
• Disadvantages
– Does not consider channel capacity
– Requires Cholesky decomposition and
eigenvector calculation
– Does not consider channel noise
D
• Amenable to real-time fixed-point DSP realization
18 - 37
ADSL Equalization
Maximum Shortening SNR Solution
• Choose w to minimize energy outside window of desired length
– Locate window to capture maximum channel impulse response energy
nk
T
T
T
xk
rk
yk
h
w
+
h wall h wall  w H wall H wall w  wT Aw
hTwinh win  wT HTwinH win w  wT Bw
hwin, hwall : equalized channel within and outside the window
• Objective function is shortening SNR (SSNR)
 wT Bw 
 subject towT Bw  1
maxSSNR   max10log10  T
 w Aw 
w
w


wopt 
Bq
T
-1
min
q min : eigenvector of min eigenvalueof C
C
 B  A
-1
B
T

-1
18 - 38
ADSL Equalization
Matlab DMT TEQ Design Toolbox 3.1
• Single-path, dual-path, per-tone & TEQ filter bank equalizers
Available at http://www.ece.utexas.edu/~bevans/projects/adsl/dmtteq/
default
parameters
from
G.DMT
ADSL
standard
23
-140
various
performance
measures
different
graphical
views
18 - 39
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
18 - 40
Notes
Notes
Applications of Broadband Access
Residential
Application
Downstream Upstream Willing to pay
rate (kb/s)
rate (kb/s)
384
9
High
Database Access
384
9
Low
On-line directory; yellow pages
1,500
1,500
High
Video Phone
1,500
64
Low
Home Shopping
1,500
1,500
Medium
Video Games
3,000
384
High
Internet
6,000
0
Low
Broadcast Video
24,000
0
High
High definition TV
Demand
Potential
Medium
High
Medium
Medium
Medium
Medium
High
Medium
Business
Application
Downstream Upstream Willing to pay
rate (kb/s)
rate (kb/s)
384
9
Medium
On-line directory; yellow pages
1,500
9
Medium
Financial news
1,500
1,500
High
Video phone
3,000
384
High
Internet
3,000
3,000
High
Video conference
6,000
1,500
High
Remote office
10,000
10,000
Medium
LAN interconnection
45,000
45,000
High
Supercomputing, CAD
Demand
Potential
High
Low
Low
High
Low
Medium
Medium
Low
18 - 42
Notes
DSL Broadband Access Standards
xDSL
ISDN
T1
HDSL
SHDSL
Splitterless
ADSL
Full-Rate
ADSL
VDSL
Meaning
Integrated Services
Digital Network
T-Carrier One
(requires two pairs)
High-Speed Digital
Subscriber Line
(requires two pairs)
Single Line HDSL
Splitterless
Asymmetric DSL
(G.Lite)
Asymmetric DSL
(G.DMT)
Very High-Speed
Digital Subscriber
Line (proposed)
Data Rate
144 kbps
Mode
Symmetric
Applications
Internet Access, Voice,
Pair Gain (2 channels)
1.544 Mbps Symmetric Business, Internet
Service
1.544 Mbps Symmetric Pair Gain (12 channels),
Internet Access, T1/E1
replacement
1.544 Mbps Symmetric Same as HDSL except
pair gain is 24 channels
Up to 1.5 Mbps Downstream Internet Access, Video
Up to 512 kbps Upstream Phone
Up to 10 Mbps Downstream Internet Access, Video
Up to 1 Mbps Upstream Conferencing, Remote
LAN Access
Up to 22 Mbps Downstream Internet Access, VideoUp to 3 Mbps Upstream on-demand, ATM,
Up to 6 Mbps Symmetric Fiber to the Hood
Courtesy of Mr. Shawn McCaslin
18 - 43
Notes
ADSL and Cable Modems
• 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-ondemand, 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.
18 - 44
Notes
ADSL Modems
• 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.
18 - 45
Notes
Discrete Multitone (DMT) Modulation
• 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.
18 - 46
Notes
Two Types of Transmission
•
Two versions of ADSL
1. Frequency Division Multiplexing: the upstream and downstream
channels do not overlap: the upstream uses channels 6-31 and the
downstream uses channels 32-255.
2. 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 < 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
18 - 47
Notes
Channel Attenuation
• 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
12 kft -90 dBm/Hz
14 kft -100 dBm/Hz
16 kft -110 dBm/Hz
18 kft -120 dBm/Hz
• Because of severe effects in the channel, the ADSL
standard defines channel coding using cyclic prefixes and
employs error correcting codes
18 - 48
Notes
Bridge Taps
• 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.
18 - 49
Notes
ADSL Modems
• ADSL modem consists of a line driver plus 3 subsystems:
1. analog front end (15 V)
2. digital interface (3 V)
3. discrete multitone processor (3 V)
• Analog front end provides the analog-to-digital and digitalto-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.
18 - 50
Notes
Motorola CopperGold ADSL Chip
•
•
•
•
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
18 - 51
Notes
Motorola Copper Gold ADSL Transceiver
• 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.
18 - 52
Notes
Minimum Mean Squared Error TEQ
nk
xk
yk
h
+
w
b
z-D
rk
ek
- +
b  [b0 b1  bNb ]T
w  [w0 w1  wNw ]T
zk
MSE  {ek2}  bT Rxxb - 2bT Rxy w  wT Ryy w
minimumMSE is achievedonlyif bT Rxy  wT Ryy

]
MSE  bT Rxx - Rxy R-yy1 Ryx b  bT Rx|yb
T
Define RD  OT Rx|yO then MSE  b RΔb
Matrix O selects the proper part out of Rx|y corresponding to the delay D
18 - 53
Notes
Simulation Results for 17-Tap TEQ
Achievable percentage of upper bound on bit rate
ADSL
Maximum Maximum
CSA Minimum Geometric Shortening Minimum Maximum
Loop
MSE
SNR
SNR
ISI Bit Rate
43%
84%
62%
99%
99%
1
70%
73%
75%
98%
99%
2
64%
94%
82%
99%
99%
3
70%
68%
61%
98%
99%
4
61%
84%
72%
98%
99%
5
62%
93%
80%
99%
99%
6
57%
78%
74%
99%
99%
7
66%
90%
71%
99%
100%
8
Cyclic prefix length
FFT size (N)
Coding gain
Margin
32
512
4.2 dB
6 dB
Input power
Noise power
Crosstalk noise
POTS splitter
Upper
Bound
(Mbps)
9.059
10.344
8.698
8.695
9.184
8.407
8.362
7.394
23 dBm
-140 dBm/Hz
8 ADSL disturbers
5th order Chebyshev
18 - 54
Notes
Simulation Results for Three-Tap TEQ
Achievable percentage of upper bound on bit rate
ADSL
Maximum Maximum
CSA Minimum Geometric Shortening Minimum Maximum
Loop
MSE
SNR
SNR
ISI Bit Rate
54%
70%
96%
97%
98%
1
47%
71%
96%
96%
97%
2
57%
69%
92%
98%
99%
3
46%
66%
97%
97%
98%
4
52%
65%
96%
97%
98%
5
60%
71%
95%
98%
99%
6
46%
63%
93%
96%
97%
7
55%
61%
94%
98%
99%
8
Cyclic prefix length
FFT size (N)
Coding gain
Margin
32
512
4.2 dB
6 dB
Input power
Noise power
Crosstalk noise
POTS splitter
Upper
Bound
(Mbps)
9.059
10.344
8.698
8.695
9.184
8.407
8.362
7.394
23 dBm
-140 dBm/Hz
8 ADSL disturbers
5th order Chebyshev
18 - 55