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Signal Processing in Communications I:
xDSL
Samuel Sheng, Ph.D.
DataPath Systems, Inc.ssheng@datapathsystems.com
408-366-339115 August 99
1800
0feet: ~ 95% of cus tomers
9000
feet: ~ 50% of cus tomers1200
0feet: ~ 80% of cus tomers
xDSL: Broadband over Twisted Pair
• xDSL: the delivery of high-speed digital data over twisted-pair local loop infrastructure
• An “Alphabet Soup” of xDSL services
HDSLHDSL2
VDSL
SDSL
MDSLADSL
CentralOffice
Twisted PairInfrastructure
Goals of xDSL Signal Processing
• As in voiceband modems, “evolution” in signal processing
• Performance
Highest data rate
Longest loop possible (reach)
• Cost
Two-pair vs. one-pair
Repeatered
Line conditioning
xTU-C
PSTN
RemoteNetwork
POTS
Splitter
xTU-R
DigitalNetwork
RemoteInstallationTwisted
Pair
CentralOffice
(ADSL)
Splitter
(ADSL)
Outline
• The Twisted-Pair Transmission Environment
• High-Bit-Rate Digital Subscriber Line (HDSL)
• Asymmetric Digital Subscriber Line (ADSL)
• Some Future xDSL Systems
• HDSL2
• Very High-Bit-Rate Digital Subscriber Line (VDSL)
• Conclusions
Line Attenuation vs. Frequency, ADSL
0.01 0.1 1 10
-140
-120
-100
-80
-60
-40
-20
0
Lin
e L
oss
, dB
5000 ft.
10000 ft.
15000 ft.
20000 ft.
MHz
Twisted Pair: Transmission Environment
• Channel Frequency Response
• Channel Background Thermal Noise: -140 dBm/Hz
Twisted Pair: Other Impairments
Bridged Taps
Near-End/Far-End Crosstalk (NEXT/FEXT)
RF Ingress/Impulse Noise
Twisted-Pair
AM Radio, HAM Radio, etc.Motors,Car Ignitions,Etc.
RX
“Other” transmitters coupling into receiver
Near-End Far-End
Twisted-Pair 1
Twisted-Pair 2
Twisted-Pair
Unterminated bridge tap
frequency
Channel Response
Spectral Nulls!
TX TX
TX
High-Speed Digital Subscriber Line (HDSL)
• “Successor” to T1/E1
• Data transmission
• DS1/T1: 1.544 Mbps
• 12 kft, 24 gauge
• 768 kbps duplex per pair, two pairs
• Repeatered beyond 12 kft
• Transceiver:
• Dual-duplex
• Echo-cancelled
• Linear 2B1Q modulation, 196 kHz bandwidth
Basic HDSL Transceiver
2B1QEncode
PulseShaping
FIRDAC
Anal
og
Front En
d +
Hyb
rid
ADC
EchoCancel.
FIR
+-
Twisted-PairLine
Forward FIR
+
Reverse FIR
2B1QSlicer
TimingRecovery
Bits In
Bits Out
AdaptiveDecision-FeedbackEqualizer
2B1QDecode
+3
+1
-1
-3
10
11
01
00
Bits QuaternarySymbol
(128 tap)
(128 tap)
(8-24 taps)
One implementation:7.1mm x 8.55mm181000 transistors
[PairGain93]
825 mW, 50 MHz
HDSL Echo Cancellation
• Fully-overlapped spectrum between TX and RX -need to cancel echo response (hybrid leakage)
• Echo cancellation FIR adapted at startup
• Echo path impulse response typically time-limited to ~280 usec
-> Typical FIR filter size: ~128 taps for baud sampling (400 kHz)
Echo lea
kage
Hybrid leakage - function of:Mismatch parasitics in hybridImpedance of loop vs. frequency
“Compromise” hybrid
Echo cancel FIR response includes:
Analog TX/RX filter responseTransformerHybrid leakage
PulseShaping
FIR
ADC
EchoCancel.
FIR
+-
Twisted-PairLine
DAC
An
alo
g Fr
on
t En
d +
Hyb
rid
• Need to minimize intersymbol interference at slicer
• Employ zero-forcing equalization:
• However, noise enhancement is a problem:use decision-feedback equalization
HDSL Equalization
Forward FIR
+
+-
Reverse FIR
2B1QSlicer
Bits Out
Precursor
Decision-Feedback(Postcursor cancellation)
ReceivedSamples
cancellation
Most ISI is postcursor:At baud rate:
8-tap forward FIR128-tap reverse FIR
ωω
Boost!TX Symbols
Channel Equalizer
ω
AggregateChannel (at Slicer)
RX Symbols
Asymmetric Digital Subscriber Line (ADSL)
• Intended for consumer deployment
• Asymmetric data transmission
Rate Adaptive: “Best effort” on a given loop:
(up to 8 Mbps downstream, 800 kbps upstream)
Single-pair, repeaterless
• Transceiver:
Full-duplex
Echo-cancelled or frequency-division
Discrete Multitone Modulation
Coexistence with POTS
• Two flavors of ADSL
g.dmt
(
g
.992.1): “full” ADSL
g.lite
(
g
.992.2): “splitterless, low-cost, rate-limited”
(1.544 Mbps peak downstream)
• Idea: separate information across narrow channels (or “bins”), with 256 (downstream) or 32 (upstream) possible bins.
• Optimize QAM constellation size for each bin based on SNR(“automatically” rate/margin adaptive)
• Symbol time is very slow: (~ 250 usec for ADSL);bits/symbol is
huge
Can theoretically deliver payload near the Shannon capacity!
Discrete Multitone Modulation (DMT)
frequency
. . .
4.3125 kHz spacing
1.104 MHzLow frequency bins unused
“Prototype” DMT Transceiver
• Leverage off of massive DSP capability
Signal synthesis is done entirely via FFT/IFFT engines
Serial-ParallelConvert
IFFT
FEQ TEQFFTParallel-Serial
Convert
To D
AC
/
Fro
m A
DC
/
Inter-polator
High-OrderFilter
High-OrderFilter
An
alo
gFr
on
t-En
d
An
alo
gFr
on
t-En
d
Group delay/amplitude equalization
Per-bin
equalization(1 complex tap per bin,
Time-domain smoothing
(time domain)256 bins)
Transmitter
Receiver
Some Key Issues....
• Timing recovery / Frequency accuracy
Intersymbol interference in the
frequency
domain
Expend downstream bin 64 (276 kHz) as pilot timing tone
• Cyclic prefix
Append L samples of the IFFT TX output
Serves as guard time against intersymbol interference
Equalization via FFT requires
circular
convolution
IFFT output
N samples
First L samples
Copy and append To DACN+L samples
“Looks” periodic!
. . .
Frequency error in receiverresults in “shift” of bins
Some Key Issues.... (cont)
• Splitterless vs. Splittered
Full rate
g.dmt
requires splitter at remote to minimize interference between ADSL and POTS bands.
g.lite
is “splitterless” for cost reasons (avoid rewiring).Proving to be difficult to deploy; customer installed “microfilters” are required.
Home/PCNetwork
POTS
Splitter
ATU-R
High-frequency ADSL signal
Low-frequency POTS signal
Without
the splitter:
ADSL intermodulates into POTS band
POTS on/off hook affects ADSL signal
An ADSL Transceiver
Serial-ParallelConvert
IFFT
UpdateEcho
FEQ
TimingRecovery
TEQ
EchoCancel
+
FFTParallel-Serial
Convert
CyclicPrefix
-
DAC
An
alo
g Fr
on
t En
d +
Hyb
rid
ADC
Twis
ted
-Pai
rLi
ne
Echo cancellationis system
•
Processing Load
([Macq98])
FFT/IFFT/TEQ: 70-90 MIPS (multiply-accumulate)
Other DSP functions: 20-50 MIPS
Total: 100-150 MIPS
Decimation/Baud Sampling
Mapper
DeMapper
ViterbiDecode
TrellisEncode
Bits In
Bits Out
Trellis Codingis systemdependent dependent
ADSL: DSP vs. ASIC?
• DSP Approach
• Extremely flexible, lower development costs
• Time to market
• Modem can evolve with standard via code change
• ASIC Approach
• Reduced flexibility, longer development cycle
• Power consumption: critical in central office
200+ MHz DSP’s vs. custom ASIC?
“Low power” DSP techniques applicable in custom
• Component may be cheaper (?)
• Both approaches seen in marketplace
ADSL: Analog Signal Processing Issues
• Goal of DMT: achieve as close to Shannon capacity as possible. Equivalently, maximize SNR in each bin
• Analog front-end performance (AFE) is critical
Operating in frequencies of the line with tremendous attenuation
Want AFE input referred noise significantly lower thanline/crosstalk noise
Intermodulation distortion in AFE will cause spillover from bin-to-bin: impacts noise floor
Analog front-end performance should not limit the system!
Dynamic Range Requirements in ADSL
• Key issue on longest lines: small in-band received signal alongwith huge out-of-band echo signal coming through the hybrid.
• Need 14-bit linear analog signal path (or better!) in theanalog front end (AFE)
DSP 14bADC
Integrated Low-Noise
Analog Front End RX Path
2-4 WireHybrid
Analog Front End TX Path
14bDAC
BIG TX SIGNAL
SMALL RX SIGNAL
-10dB
LowpassFilters
Amp
2 4 6 8 10 12 14 16 180
1
2
3
4
5
6
7
8
9
Loop Length (kFt)
Mb
ps
Ach
ieva
ble
Dat
a R
ate
Impact of the AFE on ADSL System Performance
A Few Words about SDSL...
• Symmetric (or Single-Pair) Digital Subscriber Line
Equal upstream/downstream data rates
Full duplex, single-pair
• Tends to be a single-pair variant of HDSL
• Early 1990’s:
Marginally achieved 1.544 Mbps over 6 kft of 26 AWG wire
• Late 1990’s:
Wanted: symmetric system achieving 1.544 Mbps with the same reach and robustness as HDSL.
Utilization of advanced signal processing is key!
HDSL2
• Single-pair 1.544 Mbps transmission
9000 ft. AWG 26, 12000 ft. AWG 24
Cannot adversely impact other xDSL systems present in binder
• Proposed Systems
Symmetric Echo-Cancelled Transmissions (SET)
Frequency-Division
Partial Overlap Echo-Cancelled Transmission (POET)
• End-to-end latency comparable to HDSL (< 500 usec)
• Still in committee; ratification likely later this year
• Modulation strategy:
16-level PAM transmission with Trellis-Coding
Transmit Spectra, xDSL
-100.00
-90.00
-80.00
-70.00
-60.00
-50.00
-40.00
-30.00
0
20
0
40
0
60
0
80
0
10
00
Frequency
Pow
er S
pec
tral
Den
sity
HDSL
DMT ADSL DS
DMT ADSL US
ISDN
T1
dBm/Hz
kHz
HDSL2 Spectral Compatibility
• Critical issue: NEXT interaction in binder groups
(figure cf. [Zimmer])
• Overlapped PAM Transmission with Interlocking Spectra
Slightly asymmetric upstream/downstream spectrum
Has superior spectral folding performance at baud sampling ([Zimmer])
-100.00
-90.00
-80.00
-70.00
-60.00
-50.00
-40.00
-30.000
200
400
600
800
1000
Frequency
Po
wer
Sp
ectr
al D
ensi
ty
OPTIS Upstream
OPTIS Downstream
dBm/Hz
kHz
HDSL2: OPTIS Transmit Mask
HDSL2: Signal Processing Complexity I
• Tomlinson-Harashima Precoding
Moves feedback equalizer in DFE into the transmitter
Eliminates DFE error propagation
Allows use of soft-decision error correction decoding
• Large complexity increase in transmitter (12-16 bits required)
2B1QEncode
PulseShaping
FIRForward FIR +
Reverse FIR
2B1QSlicer
Bits In Bits Out2B1Q
Decode
HDSL Transmitter HDSL Receiver
TwistedPair
Channel
TrellisEncode Forward FIR+
Reverse FIR
ModuloBits In Bits Out
ViterbiDecode
HDSL2 Transmitter HDSL2 Receiver
TwistedPair
Channel
PulseShaping
FIR
EchoCancel
FIR
Precoder
Modulo
HDSL2: Signal Processing Complexity II
• Use of Fractionally-Spaced Forward Equalizers
Doubles complexity in receiver forward FIR
• Use of Trellis Coding
Error Correction needs to provide > 4 dB (5.1 dB) SNR gain
Needs to be low latency
32 to 512 state Viterbi detectors needed: HUGE complexity!
• Estimated Processing Requirements: 200-250 MIPS
Forward FIR
Bits OutViterbiDecode
HDSL2 Receiver
Modulo
• “Next generation” in data rate to customers:
Symmetric or nonsymmetric data services
26 Mbps downstream, 3-26 Mbps upstream
Transmit bandwidth: ~ 3 - 20 MHz
• Short range: < 5000 ft. in loop
Depends having fiber-to-the-curb
Communicates to local ONU (optical network unit)
• Standard remains in question: DMT vs. QAM/CAP
Very High-Bit-Rate Digital Subscriber Line (VDSL)
ONU
< 5000 ft. oftwisted pair
FiberBackbone
VDSL: RF Ingress/Egress Issues
• RF Ingress
AM Radio: Interference PSD on line~ -80 to -120 dBm/Hz
0.56 - 1.6 MHz
HAM Radio: Interference PSD on line ~ -35 to -80 dBm/Hz
1.8 - 2.0 MHz3.5 - 4.0 MHz7.0 - 7.1 MHz10.1 - 10.15 MHz14.0 - 14.35 MHz...
• VDSL Signals
-60 dBm/Hz nominal transmit power spectrum
Typical receive power spectrum (AWG 26, 2 kft)
- 90 dBm/Hz @ 2 MHz- 140 dBm/Hz @ 10 MHz
RFI can be significantly larger than the received signal!
VDSL: RFI Cancellation (DMT systems)
• Problem: Sidelobes in FFT from RFI corrupt other bins
• Partial solution: Apply windowing before receiver FFT
• May lead to spectral broadening of received DMT tones
Intersymbol interference in frequency domain (between bins)
FEQ block now needs to consider ISI: decision feedback is needed
• Need to estimate and eliminate RFI interference
dB
Frequency Bin
--50.00
-40.00
-30.00
-20.00
-10.00
0.00
25.0 50.0
Unwindowed
Windowed
• Modified DMT receiver
• Implementation Complexity
Much higher sample rates
Interference cancellation
One recent VLSI solution [Alcatel99]:680k gates, 0.35u CMOS150 mm2, 2.7W @ 3.3V
VDSL: RFI Cancellation (cont.)
TEQ+ FFTParallel-Serial
Convert-
ADCWindow
RFIEstimation
Error SignalSynthesis
FEQ
Conclusion
• xDSL has evolved on many fronts
Data rate
Reach
One pair vs. Two pair
• With each evolution, commensurate change in signal processing technology
HDSL: Adaptive DFE, echo cancellation
ADSL: DMT
VDSL: RFI cancellation, Very high-speed DSP
HDSL2: Tomlinson-Harashima precoding, Better error correction
Future xDSL’s will continue this trend: leveraging signal processing toward higher speed and longer reach, over existing twisted pair installation!
Bibliography
[Alcatel99] D. Veithen, P. Spruyt, T. Pollet, et al. “A 70 MB/s Variable-Rate DMT-Based Modem for VDSL.” IEEE 1999 International Solid-State Circuits Conference, San Francisco, CA Feb. 1999.
[Chen98] W. Chen. DSL: Simulation Techniques and Standards Development for Digital Subscriber Line Systems. New York: Macmillan Technical Publishing, 1998.
[Chow95] P. Chow, J. Cioffi, J. Bingham. “A Practical Discrete Multitone Transceiver Loading Algorithm for Data Transmission over Spectrally Shaped Channels.” IEEE Transactions on Communications, Vol. 43, No. 2/3/4, pp. 773-775. Feb/Mar/April 1995.
[Cioffi99] J. Cioffi, V. Oksman, J. Werner, et al. “Very-High-Speed Digital Subscriber Lines.” IEEE Communications Magazine, Vol. 27, No. 4, pp. 72-79. April 1999.
[Conroy99] C. Conroy, S. Sheng, A. Feldman, et al. “A CMOS Analog Front-End IC for DMT ADSL.” IEEE 1999 International Solid-State Circuits Conference, San Francisco, CA Feb. 1999.
[Gdmt] S. Palm, ed. ITU - G.992.1, Asymmetrical Digital Subscriber Line (ADSL) Transceiver. ITU - Telecommunication Standardization Sector. Geneva, 12-23 October 1998.
[Glite] C. Hansen, ed. ITU - G.992.2, Splitterless Asymmetrical Digital Subscriber Line (ADSL) Transceiver. ITU - Telecommunication Standardization Sector. 12 October 1998.
[Ho96] M. Ho, J. Cioffi, J. Bingham. “Discrete Multitone Echo Cancellation.” IEEE Transactions on Communications, Vol. 44, No. 7, pp. 817-825. July 1996.
[Lee88] E.A. Lee and D.G. Messerschmitt. Digital Communication. New York:Kluwer Academic Publishers, 1988.
[Macq98] D. Macq. “Short Course: xDSL Broadband Interactive Communications via POTS.” IEEE 1998 International Solid-State Circuits Conference, San Francisco, CA Feb. 1998.
[PairGain93] M. Kuczynski, W. Lao, A. Dong, et al. “A 1 Mb/s Digital Subscriber Line Transceiver Signal Processor.” IEEE 1993 International Solid-State Circuits Conference, San Francisco, CA Feb. 1993.
[ProakB83] J.G. Proakis. Digital Communications. New York: McGraw-Hill Book Co., 1983
[Rausch98] D. Rauschmayer. ADSL/VDSL Principles: A Practical and Precise Study of Asymmetric Digital Subscriber Lines and Very High Speed Digital Subscriber Lines. New York: Macmillan Technical Publishing, 1999.
[Starr99] T. Starr, J. Cioffi, P. Silverman. Understanding Digital Subscriber Line Technology. New Jersey: Prentice-Hall PTR, 1999.
[Zimmer] G. Zimmerman. “HDSL2 Tutorial: Spectral Compatibility and Real-World Performance.” PairGain Technologies, June 1998.
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