High-Speed Wireline High-Speed Wireline Communication Systems: Communication Systems: Semester Wrap-up Semester Wrap-up Ian C. Wong, Daifeng Wang, and Prof. Brian L. Evans Dept. of Electrical and Comp. Eng. The University of Texas at Austin http://signal.ece.utexas.edu http://www.ece.utexas.edu/~bevans/ projects/adsl
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High-Speed Wireline Communication Systems: Semester Wrap-up Ian C. Wong, Daifeng Wang, and Prof. Brian L. Evans Dept. of Electrical and Comp. Eng. The.
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High-Speed WirelineHigh-Speed WirelineCommunication Systems: Communication Systems:
Semester Wrap-upSemester Wrap-upIan C. Wong, Daifeng Wang, and
Prof. Brian L. EvansDept. of Electrical and Comp. Eng.The University of Texas at Austin
http://signal.ece.utexas.edu
http://www.ece.utexas.edu/~bevans/projects/adsl
2
OutlineOutline
• Asymmetric Digital Subscriber Line (ADSL) Standards– Overview of ADSL2 and ADSL2+
• Mandatory support of Trellis coding (G.992.3, §8.6.2)– Block processing of Wei's [Wei87] 16-state 4-dimensional trellis code
shall be supported to improve system performance
– Note: There was a proposal in 1998 by Vocal to use a Parallel concatenated convolutional code (PCCC), but it wasn’t included in the standard (http://www.vocal.com/white_paper/ab-120.pdf)
• Data modulated on pilot tone (optional, §8.8.1.2)– During initialization, the ATU-R receiver can set a bit to tell the ATU-
C transmitter that it wants to use the pilot-tone for data
– The pilot-tone will then be treated as any other data-carrying tone
• Mandatory support for one-bit constellations (§8.6.3.2)– Allows poor subchannels to still carry some data
• Programmable number of overhead bits (§7.6)– Unlike ADSL where overhead bits are fixed and consume 32 kbps of
actual payload data
– In ADSL2, it is programmable between 4-32 kbps
– In long lines where data rate is low, e.g. 128 kbps,
• ADSL: 32/128 = 25% is overhead
• ADSL2: as low as 4/128 = 3.125% is overhead
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3. Achieved higher coding gain3. Achieved higher coding gain
• On long lines where data rates are low, higher coding gain from the Reed-Solomon (RS) code can be achieved
• Flexible framing allows RS code to have (§7.7.1.4)• 0, 2, 4, 6, 8, 10, 12, 14, or 16 redundancy octets
• 0 redundancy implies no coding at all (for very good channels)
• 16 would achieve the highest coding gain at the expense of higher overhead (for very poor channels)
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4. Loop Bonding4. Loop Bonding
• Supported through Inverse Multiplexing over ATM (IMA) standard (ftp://ftp.atmforum.com/pub/approved-specs/af-phy-0086.001.pdf)– Specifies a new sublayer (framing, protocols, management) between
• Perfect Training Sequence– All of its out-of-phase periodic autocorrelation terms are 0 [1]
• Suggested training sequences for DMT– Pseudo-random binary sequence with N samples
– Periodic by repeating N samples or adding a cyclic prefix
[1] W. H. Mow, “A new unified construction of perfect root-of-unity sequences,” in Proc. Spread-Spectrum Techniques and Applications, vol. 3, 1996, pp. 955–959.
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Training SequencesTraining Sequences
• y = S h + n– h: L-tap channel
– S: transmitted N x L Toeplitz matrix made up of N training symbols
– n: additive white Gaussian noise (AWGN)
Domain Method Minimum
MSE
Complexity Optimal Sequence*
Time Periodic (LS)[1] Yes High (2N) Yes
Aperiodic [2] No Medium (N2) YesL-Perfect (MIMO)
[3]
Almost Low (N log2N) Sometimes
Frequency Periodic [4] No Low (N log2N) Sometimes
[1] W. Chen and U. Mitra, "Frequency domain versus time domain based training sequence optimization," in Proc. IEEE Int. Conf. Comm., pp. 646-650, June 2000.
[2] C. Tellambura, Y. J. Guo, and S. K. Barton, "Channel estimation using aperiodic binary sequence," IEEE Comm. Letters, vol. 2, pp. 140-142, May 1998.
[3] C. Fragouli, N. Al-Dhahir, W. Turin, “Training-Based Channel Estimation for Multiple-Antenna Broadband Transmissions," IEEE Trans. on Wireless Comm., vol.2, No.2, pp 384-391, March 2003
[4] C. Tellambura, M. G. Parker, Y. Guo, S . Shepherd, and S . K. Barton, “Optimal sequences for channel estimation using Discrete Fourier Transform techniques,” IEEE Trunsuctions on Communicutions, vol.47, no.2, pp. 230-238, Feb. 1999
* impulse-like autocorrelation and zero crosscorrelation
MIMO is multiple-input multiple-output
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Training-Based Channel Estimation for MIMOTraining-Based Channel Estimation for MIMO
• 2 x 2 MIMO ModelDuplex Channel
TX 1
RX 2
RX 1
TX 2
h11
h21 h12
h22
1 11 121 2
2 21 22
( ) ( )y Sh z [ ( , ) ( , )] z
( ) ( )
where y and z are of dimension 2( 1) 1
( ) (0) ( 1) , or 1, 2
( 1) (0)
( ) (1)( , )
( 1) (
t t
t
T
ij ij ij
i i
i ii t
i t i
y h L h LS L N S L N
y h L h L
N L
h L h h L i j
s L s
s L sS L N
s N s N
, 1, 2
)t
i
L
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Crosstalk Estimation Crosstalk Estimation
• Noises are “unknown” crosstalkers and thermal/radio– Power spectral density N(f)
– Frequency bandwidth of measurement
– Time interval for measurement
– Requisite accuracy
• Channel ID 1– Estimate gains at several frequencies
– Estimate noise variances at same frequencies
– SNR is then gain-squared/noise estimate
• Basic MIMO crosstalk ID– Near-end crosstalk (NEXT)
– Far-end crosstalk (FEXT)
Transmitter User i
Receiver User j
NEXT FEXT
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Spectrum BalancingSpectrum Balancing
• Decides the spectral assignment for each user– Allocation is based on channel line and signal spectra
– For single-user, ‘water-filling’ is optimal
– For the multiuser case, performance evaluation and/or optimization becomes much more complex
• Methods – Distributed power control
• No coordination at run-time required
• Set of data rates must be predetermined
– Centralized power control
• Coordination at central office (CO) transmitter is required
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Distributed Multiuser Power ControlDistributed Multiuser Power Control
Downstream: MIMO Precoding Downstream: MIMO Precoding
Transmitted signal Original symbols
Channel
£
=Received signalcrosstalk-free
• We can also use Tomlinson-Harashima precoding(as used in High-speed DSL) to prevent energy increase
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CommentsComments
• Because of limited computational power at downstream Tx (reverse of that in typical DSL/Wireless systems)– Successive crosstalk cancellation at Rx makes more sense
• Do the QR decomposition also at Rx
• Don’t need to feedback channel information, since it is used at the receiver only
• Transmit optimization procedures can also be done at Rx– It is actually simpler since we can assume that the cross-talk is
cancelled out
• Just do single-user waterfilling for each separate user (loop)
– Optimal power allocation settings fed back to transmitter
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OutlineOutline
• Asymmetric Digital Subscriber Line (ADSL) Standards– Overview of ADSL2 and ADSL2+