ADSL System Enhancement with Multiuser Detection

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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  1014.08.52 )
 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,L1  {s1k,i1,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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