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ADSL System Enhancement with Multiuser Detection Liang C. Chu School of Electrical Engineering Georgia Institute of Technology Atlanta, GA 30332 Table of Contents Introduction Background: History of the Problem. Crosstalk ADSL and SDSL in a binder. DMT-ADSL Channel Characteristics DMT DMT-ADSL Standard Multiuser Transmission Telephone Channel Multiuser Transmission Systems ADSL System Enhancement Multiuser Detection on DMT-ADSL Channel Capacity Studies Joint MLSE Performance Studies Low Complexity Enhancement on ADSL Receiver Tone-zeroing Multi-stage JMLSE Simulation Studies and Results Conclusions Recommendations Introduction An enhancement approach on the DMT-ADSL system. Goal: spectral compatibility; better capacity utilization; support fast Internet services. Core method: either increasing signal constellation sizes / per sub- channel, or extending the deployment ranges with a fixed transmission rate, or compensating on a poor BER channel in achieving better throughput. ADSL service Telephone channel, high-bandwidth services. New infrastructure for multimedia service. Economical and less time to launch service. Physical channel medium: unshielded twisted pair line. Co-channel interference (crosstalk). TPC model and proposed multiuser model. Sub-optimal approach on receiver enhancement. Background Problems Major threat: spectral compatibility. Signals coupling in same binder crosstalk NEXT Near-end crosstalk FEXT Far-end crosstalk Crosstalk Comes From Environmental Physical media: unshielded twisted pair. Bandwidth-efficient digital transmission system. Different kinds of DSL services in same binder. Near-End Crosstalk Same Binder Group Transmit NEXT Receive Far-End Crossatlk Same Binder Group Transmit FEXT Receive Crosstalk Characteristics NEXT: dependent on frequency. H NEXT n f 3/ 2 FEXT: dependent on frequency, but attenuated by twisted cable length. H FEXT |H channel( f ) |2 k l f 2 Example on NEXT and FEXT Results:maximum theoretical data rate. NEXT and FEXT limited operation on ADSL. ANSI ADSL, 256 channels from DC to 1.104MHz Tones #7 to #255 for data transmission. Each tone: QAM at 0 to 15 bits/Hz based on SNR AWGN at –140dbm/Hz, no ISI assumed NEXT is the dominated crosstalk. NEXT Coupling Characteristics NEXT POWER SUM LOSS(dB) 1000 FT, 24 AWG PIC 70 60 50 40 30 20 10 0 0.1 1 10 100 FREQUENCY(MHz) 1% Ca se Discussions NEXT increases as f1.5 with frequency, but with significant variation in coupling function. Any given frequency, only few other pairs may contribute significantly to crosstalk, but over all frequencies, many wire lines contribute randomly. Challenge: hard to detect in single-user detection. Solution: modify receiver. Current Crosstalk Distribution Gaussian Distribution. Random interferes, central limit theorem. Practical interests and only accurate on single type of crosstalk. Drawback: dependent on error size of Gaussian and true distribution. Pessimistic on channel capacity especially on multiple DSL services. New area on multiple DSL services crosstalk models. SDSL to ADSL (Multiple DSLs) SDSL: symmetric DSL 2B1Q modulation - 4-level baseband pulse amplitude modulation signals same data rate in the upstream and downstream directions same transmit PSD in the upstream and downstream directions Focus studies on SDSL crosstalk to ADSL SDSL services in high demand, together exiting with ADSL service. PSD of 2B1Q SDSL Spectral compatibility problem with ADSL overlap in psd 1168, 1552 and 2320 kbps SDSL -30 1168 kbps -40 1552 kbps -50 2320 kbps PSD (dBm/Hz) -60 -70 -80 -90 -100 -110 0 400000 800000 1200000 1600000 2000000 Frequency (Hz) SDSL with T1.413 ADSL Results are calculated for same-binder NEXT with the standard Unger 1% NEXT model. T1.413 full-rate DMT ADSL in the presence of NEXT from SDSL (1552 kbps and 2320 kbps). DMT tones separated by 4.3125 kHz. each tone carries with a 6dB SNR margin. Downstream ADSL transmits from 160 kHz to 1104 kHz. SDSL Crosstalk to ADSL DMT-ADSL System with 24-SDSL Crosstalk Downstream Bit Rate in kbps 8000 Simulation based on ANSI T1E1.4/99-261 7000 6000 1552 kbps SDSL crosstalk 5000 SDSL self-NEXT loop limits: 4000 1552 kbps to 7.5 kft 2320 kbps to 6 kft 3000 2000 2320 kbps SDSL Crosstalk 1000 6 8 10 12 14 16 18 26-AWG Loop Length in kft Current Mitigation Plan Loop plan Testing & estimating deployment loops. Limiting coverage area and customers. Limiting on deployed data rate. Drawback: Inconvenience. Capacity waste. Observation and Plan Crosstalk channel characteristics change very slowly over the time. Modeled as static and time invariant. Types of crosstalk on practical loops does not change. Normally fixed DSL services in the same binder from the CO to CPE sides. Plan on mitigate crosstalk Enhance the ADSL receiver, “filters” the crosstalk noise. Multiaccess ADSL channel model Multiaccess ADSL Channel Model K hk is the channel impulse response when r (i ) hk (i ) xk (i ) n(i ) k=1, and sum together with crosstalk k 1 coupling function when k>1 x Noise, 2 Transmit1 (x1) r ADSL Channel + ADSL Receiver y Transmit2 (x2) Crosstalk Filtering + Transmitk (xk) Crosstalk Filtering Discussions Background noise is Gaussian. DSL: Gaussian channel Crosstalk is not Gaussian distribution. Sum of several filtered discrete data signals: ADSL (desired) and SDSL (crosstalk). Channel model: multiple input and single (vector) output. Brief on DMT Basic Principle: Split available BW into a large number of subchannels. Motivation: Make BW of each the sub-channel sufficiently narrow, then no ISI occurs on any sub-channel. Technique method: Transmits many parallel data-streams concurrently over the transmission channel. DMT-ADSL (ANSI) Two traffic channels downstream transmission: sampling rate of 2.208 MHz, a block size of 512 (FFT), meaning 256 tones from 0 to 1.104MHz. symbol rate is 4 kHz and the width of a tone is 4.3125 kHz. Average downstream PSD is –40 dBm/Hz. Upstream transmission: sampling rate of 276 kHz, a block size 64, meaning 32 tones from 0 to 138 kHz. symbol rate is 4 kHz and the width of the tone remains 4.3125 kHz. Average upstream PSD is –38 dBm/Hz DMT-ADSL Spectrum # of Bits Upstream Channel Downstream Channel 14 POTS Frequency in kHz 0 4 30 138 240 1104 Loading Algorithm Bits/channel Attenuation AM Crosstalk Frequency Frequency Frequency Physical Channel Unshielded twisted pairs does not change its physical behavior significantly with time and considered a stationary channel. The transfer function: att H (d , f ) 10 10 e RCf d The sources of noise in the telephone channel: digital quantization noise, thermal noise in detectors, impulse noise and crosstalk. Telephone channel is normally treated as a Gaussian channel. Multiuser Transmissions The fundamental limit of multiuser detection: mitigate the interference among different modulated signals. Basic model: Y HX N (4.2.1.1) x1 x2 X . multiuser . Y xL . channel Multiuser channel is described by the conditional probability distribution : pX Y Normally, many channels fit in the linear AWGN model, shown in Eq. (4.2.1.1). Optimum multiuser detection: a generalization form of the optimum single-user channel detector - maximum likelihood multiuser detector. Linear multiuser detection in AWGN channel As Eq. (4.2.1.1) detection of desired input user xl, it may be that the overall minimum distance is too small: a single fixed value for xl may corresponding to the two multiuser codewords that determine the overall dmin. defined as : d min,l min H ( X X ' ) (4.2.2.1.1) X X xl xl ' ' Results: d m in,l d m in (4.2.2.1.2) it is possible for a detector extracting a single user to have better performance on one that extracts all other users. Channel Capacity Conventional single-user ADSL receiver Sum all the crosstalk signals and background noise together as AWGN. | H c ( f ) |2 Pdesired ( f ) Csin gle user sup log 2 [1 ]df N o ( f ) | H NEXT ( f ) |2 Pint erference( f ) Pdesired, Pint erference 0 (5.1.2.5) Enhanced multiuser ADSL receiver JMLSE – selects all possible inputs, min. distance on output. | H c ( f ) |2 Pdesired ( f ) (5.1.2.8) C sup log [1 multiuser Pdesired 0 2 ]df No ( f ) Two Users Consider the two user case: Y X1 X 2 N where, N is AWGN, X 1 , the desired signal and X 2 ,an interfered signal. Capacity for user 1: é P1 ù (5.1.2.10) C B log ê1 * ú P2 nB û 1 ë Capacity for user 2: é P2 ù (5.1.2.11) C B log ê1 * ú P1 nB û 2 ë Jointly detect, then the achievable capacity: é Pi ù é P P2 ù Ri £ B log ê1 ú R1 R2 £ B log ê1 1 ë nB û ë nB ú û (5.1.2.12) Considerable capacity improvement when the interference structure is taken into account. (5.1.2.13) é Pi ù Ci B log ê1 ë nB ú û Single vs. Multiuser Channels Rate (User 1) C1 Multiuser C1* Single User Rate C* C2 (User 2) 2 Alternative Viewpoint Multiple input: x. Mutual information: I(x,y), and I(r,y). bi Data rate: Ri individual input. Ti k Aggregate data rate: Rsum Ri . i 1 Shannon theorem: R sum upbounded by I(r,y). Achievable data rate on desired channel: k Rdesired I (r , y ) Ri (5.1.2.15) i 2 Discussion: Limit on (5.1.2.15) can be much larger than the data rate based on Gaussian crosstalk assumptions. The sum on right can be much smaller number, due to frequency-selective crosstalk coupling function. Analysis and Examples SDSL Crosstalk ADSL Channel Example#1 Example#2 Example #1:crosstalk mutual information 1552 kbps SDSL coupling to ADSL Unger 1% model, 10 9 f 1.5 4.3125 kHz log 2 (1 1014.08.52 ) Mutual information of crosstalk on each DMT-ADSL tone I ( xsdsl , yadsl ) 78 .5kbps. If silence near 20 78.5kbps 20 1552kbps tones, fully detected Example #2:Throughput Comparison SDSL crosstalk Theoretic ADSL capacity: I ( xadsl , yadsl ) 21Mbps. ADSL Channel Gaussian model: Radsl 330 kbps. Conclusion: still having enough room for ADSL. too pessimistic on current model. Joint MLSE Principle search all possible transmitted signals, find a best match signal set on the received signal. The best detector, with upper bound on multiuser system. Drawback: large computational complexity. Details on Receiver Viterbi decoding: engine for MLSE receiver. Select the state having the smallest accumulated error metric and save the state number of that state. Iteratively perform the following step until the beginning of the trellis is reached: working backward through the state history table, for the selected state, select a new state, which is listed in the state history table as being the predecessor to that state. Save the state number of each selected state. This second step is called traceback. work forward through the list of selected states saved in the previous steps. Look up what best estimated input bit corresponds to a transition from each predecessor state to its successor state. Use T/2-spaced MLSE Receiver eliminate implementation for whitening matched filters - with fixed analog filters, not depend on unknown channel (pulse shaping filter). nearly insensitive to sampling time off-set, capable of recoving non synchronized cochannel signals more easily. JMLSE ADSL Receiver (Optimal) Multiple input and single output model. Detect desired ADSL and filtered coupling crosstalk signals. JMLSE ADSL Receiver extension of the single channel MLSE. assume Gaussian channel. Ex: co-channel pairs case:JMLSE selects the ith joint symbol sequence { xik,1 , xik, 2} that maximizes the metric p(r k | x k , xk ) p(r k | x k , x k ) (5.2.6.1) 1,i 2,i 1, j 2, j meaning: select a signal set with minimized distance from the received signals. Method:Joint Viterbi algorithm. Joint Viterbi Algorithm Objective: determine the pair of sequence {xik,1, xk, 2}that minimizes the sum of squared errors j k defined by the error sequence: ei , j . rk xik,1 Primary Channel Estimate f1 (k) k ˆ rk i, j e i, j + + xk 2, j - Secondary Channel Estimate f2 (k) Joint VA (JVA) for JMLSE is very similar to the standard VA. Joint state: Sik 1,L1 {s1k,i1,L 1, s2,1,L 1} 1 2 k i number of states required to implement JVA: M L1 L2 Each joint state at time k-1: Transition to M 2 states at time k. Be reached by same number of states from time k-2. Prototype on Modification of Receiver Noise, 2 Transmit1 (x1) r + + LE JMLSD Transmit2 (x2) + H Feedback Transmitk (xk) Section Performance Study (Optimal) -2 Bit Error Rate for ADSL 10 single-user detector one SDSL disturber -4 10 NEXT into one T1.413 full rate DMT-ADSL system -6 10 -8 10 BER multiuser via JMLSE -10 10 -12 10 -14 10 gap of 4 dB ,FIR channel with 256 memory states -16 10 17 17.5 18 18.5 19 19.5 20 20.5 21 21.5 22 SNR in dB 20 15 10 JMLSD 5 Margins in dB 0 -5 -10 -15 -20 -25 Single-user Detector with SDSL Crosstalk -30 4 6 8 10 12 14 16 18 ADSL Service Length in kft Low Complexity Enhancement JMLSE is an optimal solution. drawback: high computational complexity. Goal: Reduce computational complexity Multistage JMLSE: multiple MLSE “like”. Tone zeroing: use DMT loading algorithm, and “adaptive decision feedback” or “echo cancellation like”. Tone Zeroing Method Principle: Use loading algorithm to silence some selected BW tones with low SNR, then building a adaptive cancellation table. Y + DMT - Decoder Ci y FFT Crosstalk Crosstalk Detector Table SDSL Coupling to ADSL Example Adjacent pairs: SDSL to ADSL. assume ADSL channel is static. relative constant on crosstalk profile table using LMS algorithm. zeroing about 20 tones to build up a NEXT cancellation table. Result: up to 6 dB in margin. Discussions: advantage of mitigate the NEXT and complexity reduction (comparing with JMLSE) with asymmetric and symmetric services coexist. key issue for the tone zeroing is necessity of accurate modeling of noise (crosstalk). feedback section is using some kind of adaptive filter technique, and adaptive filter coefficient is largely depends on frequency components with high power. If a frequency band making NEXT noise has small power, it can not be modeled correctly due to high power frequency component until sufficient number of coefficient are used. tone zeroing modeling works well for high frequency power noise component. telephone channel, many kinds of random noises often occur in any selected frequency band. may make an error decision on the cancellation table and induce error propagation. Proposed multi-stage joint MLSE for ADSL receiver (applied to both DMT and non-DMT DSL solutions). Same Example w/Tone-Zeroing 20 15 10 JMLSE 5 Margins in dB 0 -5 -10 Tone-zeroing -15 -20 -25 Single-user Detector with SDSL Crosstalk -30 4 6 8 10 12 14 16 18 ADSL Service Length in kft Complexity Reduced JMLSE Multi-stage JVA very similar to conventional VA receiver. having multi-stage inputs and outputs. Method as adjacent pair-wise case: the primary (strong) signal r1(k) is estimated using low delay decisions from a single-channel VA, and r (k ) r1 (k ) is ˆ forwarded to the second VA section to estimate the co-channel signal. Advantage: this structure is largely reducing the complexity on optimal JVA (JMLSE). Complexity as a similar range of a conventional VA, with just a scale-increasing factor by N. N Co-channel Binder Ratio: M L1 M L2 ... M LN Multi-stage JMLSE R M L1 L2 ... LN JMLSE Assume equal lengths, L, N M L R N M L (1 N ) M N L obvious to us R is always (much) < 1. Two Methods (Pair-wise) ˆ d 1 ( k L1 ) ˆ d 2 ( k L2 ) + r (k ) rˆ (k ) Two-stage JVA ,without VA 1 1 VA r(k) M L1 states + M L2 states Feedback Section _ ˆ d1 (k ) L1 only an additional L tap ˆ d 1 ( k L1 ) filter computational ˆ d 2 (k L2 ) increasing s1 (k ) + s (k ) + VA VA r(k) + M L1 states + 2 L2 M states Two-stage JVA ,with _ _ Feedback Section ˆ ˆ r (k ) ˆ k ,L d1 1 (k ) f 1 ( k 1) 1 ˆ k ,L d2 2 (k) r2 (k 1) ˆ ˆ f 2 (k ) Make Decision 0 10 -1 10 Example on PAM Symbol Error Rate channel, signal- corsstalk-ratio=10 -2 10 dB + : MS-JMLSE-WO/FB -3 10 * : MS-JMLSE-W/FB o : Ideal-JMLSE T/2-spaced MS- JMLSE-W/FB -4 10 4 6 8 10 12 14 16 18 20 22 Signal-to-Noise Ratio Performance Simulations Test Environment SDSL and other DSLs NEXT to ADSL. Loop Characteristics 1500 ft 1500 ft 1500 ft 26 AWG 26 AWG 26 AWG Test Loop #1 3000 ft 6000 ft 1500 ft ATU- ATU - C ATU -R 26 AWG 26 AWG 26 AWG 1500 ft 1500 ft 26 AWG 26 AWG Test Loop 9000 ft 2000 ft 500 ft 500 ft ATU-C - C ATU ATU -R #2 26 AWG 24 AWG 24 AWG 24 AWG 18000 ft Test Loop ATU - C ATU-C ATU -R #3 26 AWG Test Loop #1 4 Achievable Downstream Data Rate in Mbps square : ideal JMLSE 3.5 x : multi-stage JMLSE o : conventional ADSL receiver 3 2.5 2 1.5 1 0.5 0 9 10 11 12 13 14 15 16 17 18 ADSL Service Length in kft Test Loop #2 3.5 Achievable Downstream Data Rate in Mbps 3 square : ideal JMLSE x : multi-stage JMLSE 2.5 o : conventional ADSL receiver 2 1.5 1 0.5 0 9 10 11 12 13 14 15 16 17 18 ADSL Service Length in kft Test Loop #3 10 Achievable Downstream Data Rate in Mbps 9 square : ideal JMLSE 8 x : multi-stage JMLSE 7 o : conventional ADSL receiver 6 5 4 3 extension prediction 2 1 0 4 6 8 10 12 14 16 18 20 ADSL Service Length in kft Other works on xDSL Crosstalk Crosstalk with Gaussian Distribution for DSL: (1) cook,1999; (2) zimmerman, 1998; (3) kerpez, 1995; (4) kerpez, 1993. Multiuser Detection, but for wireless communications: (5) Verdu, 1998. Multiuser detection in VDSL study: (6)Cioffi, 1998. (1) “The noise and crosstalk environment for ADSL and VDSL systems “,Cook, J.W.; Kirkby, R.H.; Booth, M.G.; Foster, K.T.; Clarke, D.E.A.; Young, G.,IEEE Communications Magazine , Volume: 37 Issue: 5 , May 1999. (2) “Achievable rates vs. operating characteristics of local loop transmission: HDSL, HDSL2, ADSL and VDSL “, Zimmerman, G.A. , Conference Record of the Thirty-First Asilomar Conference on , Volume: 1 , 1998. (3) “High bit rate asymmetric digital communications over telephone loops “, Kerpez, K.J.; Sistanizadeh, K.,Communications, IEEE Transactions on , Volume: 43 Issue: 6 , June 1995. (4)“Near End Crosstalk is Almost Gaussian”, K. J. Kerpez, IEEE Transactions on Communications, Vol. 41, No. 5, May 1993. (5) “Multiuser Detection,” S. Verdu , Cambridge Press, 1998. (6)“Mitigation of DSL Crosstalk via Multiuser Detection and CDMA”, J. Cioffi , ANSI, T1E1.4/98-253, August 1998. Related DSL Publications “An Enhancement Study on the SDSL Upstream Receiver”, 2001 IEEE International Symposium on , Volume: 4 , 6-9 May 2001, Page(s): 442 –445. “Mitigation of Crosstalk on the SDSL Upstream Transmission with Vector Equalization”, IEEE International Conference on Communications, Session AN5: Transmission Systems, Helsinki, Finland, June 11-14, 2001 . “A Study on Multiuser DSL Channel Capacity with Crosstalk Environment”, 2001 IEEE Pacific Rim Conference on Communications, Computers, and Signal Processing, Session MP4: DSP for Communications, Victoria, BC, Canada, August 24-28, 2001. “Performance Enhancement on a Multiuser Detection ADSL Modem”, In preparation to IEEE Transitions on Consumer Electronics. “Complexity Reduced ADSL System with Multiuser Detection ”, Submitted to 2002 IEEE International Conference on Communications. Conclusions Overview the problem on xDSL spectral compatibility problems. Traditional Gaussian crosstalk under-project ADSL achievable capacity. ADSL system enhancement with multiuser detection. a core method on improvements of either increasing transmission data rate, or extending deployment areas, or compensating in poor BER DSL channels, based on different requirements. Enhanced ADSL receiver has acceptable computational complexity for a chip realization. Benefit on QoS for last-mile fats Internet transmission. Recommendations This approach can apply to DMT and non-DMT ADSL, HDSL, SDSL and future VDSL studies. may extensible to fiber and wireless. Other complexity reduction methods for JVA decoding can be further studied (this thesis gives a kind of beginning point). Possible dual-mode DSL transceivers.

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