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Signal Processing Challenges in DSL Networks

Signal Processing Challenges in DSL Networks. Michail Tsatsanis Aktino, Inc. Overview. DSL achievements to date: Evolution of DSL technologies Past technical challenges and solutions Current status and next steps in copper networks New services and requirements Performance challenges

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Signal Processing Challenges in DSL Networks

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  1. Signal Processing Challenges in DSL Networks Michail Tsatsanis Aktino, Inc

  2. Overview • DSL achievements to date: • Evolution of DSL technologies • Past technical challenges and solutions • Current status and next steps in copper networks • New services and requirements • Performance challenges • Promising technologies (DSM, MIMO)

  3. Bandwidth enterprise Access Optical Core The Bottleneck in Access • Backbone capacity is not matched by access capacity • Similar bottleneck appears in the home network

  4. Fractional T1 / T1 T1 / nXT1 Cable: 2-3 Mbps DSL 1.5-3 Mbps 40 Kbps Dialup The Access Network Landscape (USA) 1995 2005 DS3/OCn Services DS3/OCn/10/100 Gig-E Services SONET SONET/DWDM/Metro Ethernet • Next steps for access services: • Residential: 25 Mbps asymmetric to support video services • Businesses: 10-45 Mbps symmetric (DS3 or Ethernet)

  5. Central Office DLC RT DLC RT The Copper Plant Architecture • Central Office to Customer Premises • Loop lengths: • Vary with geography and country • Get shorter with incremental build-outs: 18 -> 12 -> 6 Kft • Shorter loops offer higher bandwidth ADSL Symmetric Fiber

  6. Copper in the Evolving Network (USA) Today’s network Remote Cabinet 9 – 12 Kft 12–18 Kft Central Office OCn HDSLx/T1 SAI Cross-connect GigE GigE 5 – 7 Kft FTTN network FTTH network Remote Cabinet GigE Fiber Node • FTTN architecture has copper component • FTTH architecture has no copper

  7. Copper in the Network (Asia) • DSL for in-building connections (fiber to the basement) • Loop lengths less than 300m • Speed is everything (>50 Mbps targeting 100 Mbps)

  8. Copper in the Network (Europe) • Favorable loop lengths <2-3 Km • Good competitive landscape • Well funded competitive carriers • Little cable TV presence • TV delivery will be the driving force for higher bandwidth

  9. The Technical Challenges of Copper -AFE/ADC -Algorithms -Multichan. Transeivers Bandwidth -Spectral Issues -Legacy 100m 1000m 3000m Loop Length • spectral use and interference management

  10. The Bandwidth Question • Frequency Response of twisted pairs • No well defined band limits (different from wireless) • Bandwidth varies greatly with loop length • To use higher BW footprint size must shrink

  11. Business Access: Historic Perspective • DDS Services (1970s) • 9.6 Kbps and later 64 Kbps • AMI modulation • ISDN (late 1980s) • 144 Kbps, 4-PAM modulation • HDSL (early 1990s) • 1.5 Mbps, 4-PAM modulation • 2 pairs per connection • HDSL2/SHDSL (1998/2001) • 1.5/2.3 Mbps • 16-PAM Trellis Coded Modulation

  12. Evolution of Symmetric Technologies • Spectra of Symmetric Services • Spectral Efficiency • DDS, T1: 1 bit per Baud - AMI • ISDN, HDSL: 2 bits per Baud - 4 PAM • SHDSL: 3 bits per Baud - 16 PAM/TCM

  13. Central Office Copper Pair Binder The Copper Plant Architecture • Feeder Plant and Distribution Plant • Binder Groups • 25 or 50 pair binders • Multiple binders per feeder cable Feeder Plant FDI Distribution Plant Up to 3 Km

  14. The Pain Called Crosstalk • NEXT is the strongest type of interference • Independent of loop length • FEXT is a major bottleneck in short loops (<6000 ft) • Is attenuated by the channel response and is of less concern in longer loops Aggressor line Tx Rx NEXT Rx Tx Victim line Aggressor line Tx Rx FEXT Tx Rx Victim line

  15. Copper Pair Binder Near End Cross-talk • NEXT coupling frequency response • Characterization/modeling of cross-talk couplings • [Lin et al], [Kerpez et al] • NEXT increases 15 dB per decade • completely incapacitates the link beyond 0.5 MHz Add figure NEXT coupling

  16. Comparison of NEXT & FEXT

  17. FDM Spectra upstream downstream FDM PSD FEXT UP FEXT DOWN downstream FDM NEXT Frequency PSD upstream Frequency FDD: The Solution to the NEXT Problem Overlap Spectra • FDM takes NEXT out of band • Higher BW systems need FDD duplexing • EC: ISND -> HDSL -> HDSL2 0.1 MHz 0.2 MHz 0.4 MHz • FDD: ADSL -> VDSL -> VDSL2 1.1 MHz 12 MHz 30 MHz

  18. Residential Services • ADSL (mid-late 1990s) • Asymmetric service • 0.25-8 Mbps downstream (depending on distance) • VDSL (2000) • Asymmetric Service • Up to 46 Mbps downstream (short distances) • VDSL2 (2006) • Up to 100 Mbps

  19. Evolution of Asymmetric Technologies • Spectra of asymmetric services • Modulation: • Discrete Multi-tone (DMT) with constrained waterfilling • Duplexing: • Frequency Division Duplexing

  20. The Reality of Legacy • NEXT from legacy EC disturber into FDD system • Legacy is always a concern in access • Passive elements have a life span of 50-70 years • Electronics have a life span of 5-15 years

  21. Spectral Management Open Issues (1) FDM Symmetric • Mixtures of symmetric/ Asymmetric services upstream downstream NEXT Crosstalk FDM Asymmetric downstream upstream

  22. Spectral Management Open Issues (2) • The Near-Far Problem • Strong FEXT from unequal length loops • Affects ADSL and VDSL deployments Victim loop: 10-18Kft CO ADSL CPE ADSL RT Distance: 9Kft RT ADSL CPE ADSL

  23. Example of Band Separation Principle • Modems can avoid each other’s transmission bands in extreme interference cases • Can it be done dynamically? PSD of RT deployed ADSL PSD upstream downstream frequency PSD of CO deployed ADSL PSD down up frequency

  24. DSLAM SP#2 DSL Modems Loop Binders Dynamic Spectral Management • Centralized: • Optimal Spectral Management [Dendrillon, 2003] • Decentralized • Iterative Waterfilling [Cioffi, 2000] DSLAM SP#1 DSM software Central Office

  25. Review of Waterfilling Principle • Optimal distribution of signal power over frequencies with varying noise power • Bit allocation strategy • Rate for one user • Rate for multiple users Train 1st Train 2nd Binder Train 4th Train 3rd

  26. Iterative Waterfilling • Rate adaptive • Fixed power, • fixed margin, • maximum total bit rate • Margin adaptive • Fixed power, • fixed rate, • maximum margin • Fixed margin • Fixed bit rate, • fixed margin, • minimize power (effective in combating “near-far” effects)

  27. Fixed Margin Example [Cioffi] • The RT into CO “near far” problem (Verizon data)

  28. Fixed Margin IW Performance • Iterative waterfilling allows the co-existence of the RT line with the CO line

  29. Comparison with Fixed PBO

  30. MIMO: From Wireless to Copper Wire-line • MIMO in wireline is effective against NEXT and FEXT. • 10G Ethernet and VDSL2 are considering MIMO standardization. Wireless • MIMO is used in multiantenna wireless applications for both mobile and fixed systems. • WiFi (802.11) and WiMax (802.16) are standardizing OFDM MIMO. MIMO Tx/Rx CPE CO Copper

  31. Vectored Transmission for DSL • Vector channel formulation: • Assumptions: • Channel strongly diagonal (FEXT) • Noise strongly spatially colored (NEXT) • Channel slowly changing – known to the transmitter

  32. CPE Tx/Rx CPE Tx/Rx CO MIMO Tx/Rx CPE Tx/Rx CPE Tx/Rx CO MIMO Tx/Rx CPE MIMO Tx/Rx Loop Topology and MIMO Architectures • Point-to-Point MIMO architecture • Point-to-Multipoint MIMO architecture

  33. MIMO Capacity (FEXT)

  34. MIMO Capacity (NEXT)

  35. Receiver Architectures • Linear architectures: • Zero forcing • MMSE

  36. Decision Feedback Architectures • Zero forcing (QR) [Ginis] • Set matrices: Receiver

  37. Two Sided Architectures • SVD Solution: • Set matrices: • Channel is diagonalized Transmitter Receiver

  38. Transmitter Architectures Equalizer • Linear architecture • QR architecture • Tx energy preservation: Generalized Tomlinson-Harashima precoder [Ginis, Cioffi] Receiver 1 Pre-comp Transmitter Equalizer Receiver M

  39. Conclusions Those wires are buried but they aren’t dead • DSL transceivers have come a long way: • Bandwidth • Spectral efficiency • DMT modulation, advanced coding • Next steps: • Crosstalk mitigation • Co-existence of different services • Next DSL challenge: • Achieve the step in bit-rate needed to support video services and business connectivity

  40. Lack of Fiber Availability • “It has taken 20 years to provide fiber access to less than 10% of commercial buildings...” • Source: CurrentAnalysis, 6/2004 • “Only 8-10% of major office buildings have fiber.” • Source: IDC estimate, 2003 • “Only 5% of the commercial buildings in the US have fiber.” • Source: Spigler Group, 9/02. • “Despite aggressive deployment of fiber, today only 11% of businesses are served by fiber.” • Source: Ryan Hankin & Kent, 1/2002. • Vertical Systems Group, 5/2004

  41. Business Bandwidth Demand • Applications driving higher BW usage: • VPNs both pt-pt and mobile user • Transparent LAN • Multi-media File sharing • Remote data backup • Centralized data base access • POS access to inventory data • eCommerce websites • Digital photography Sources: Frost & Sullivan Ryan, Hankin & Kent Private consultants

  42. The Duplexing Problem: From EC to FDD • How can we transmit in both directions? • Use two pairs (e.g., DDS) • Use one pair + echo cancellation • Use one pair + FDD How to duplex on a low-pass channel? • Choice of Duplexing has important cross-talk implications • EC introduces NEXT interference

  43. MMSE Decision Feedback Solutions • Avoid explicit pre-whitening of received signal

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