DSL Advances

11.1 DSL Unbundling Evolution

11.1.1 Cable Modem Architecture ”DSL's Competition

11.1.2 DSL Evolution

11.1.3 The Essential Steps of DSM

11.2 Multiuser Basics

11.2.1 Multiuser Channels and Rate Regions

11.2.2 The Matrix Channel

11.2.3 GDFE Theory

11.3 Spectral Balancing (Iterative Water-filling )

11.3.1 Yu's Iterative Water-filling

11.3.2 Examples of Improvements

11.4 Vectoring

11.4.1 Upstream Multiple Access

11.4.2 Downstream Broadcast

11.4.3 Examples

11.5 MIMO Channel Identification

11.5.1 Method of Least Squares

11.5.2 Packetization

11.5.3 Blind Training

11.6 Predictions for the DSL Age

Appendix 11A Multiuser Detection

11.A.1 Basic Receiver prerequisites for MUD

11.A.2 Soft-Cancellation

11.A.3 Simplied Cancellation by Grouping of Excess Dimensions

Dynamic spectrum management ( DSM ) is the newest DSL method that enables an array of highly desirable improvements to DSL service:

1. Automatic detection and/or prevention of service faults caused by crosstalk

2. Greater mixture of symmetric and asymmetric services

3. Higher and more reliable data rates

DSM can be thought of as an adaptive form of the earlier spectrum management (see Chapter 10).

Figure 11.1 shows a reference diagram for DSM. The DSL interface is presumed to be one of the standardized DSLs in common use. As depicted in Figure 11.1 and Figure 11.2, a single service-provider's DSL maintenance center performing DSM-Data coordination may optionally accept line and crosstalk information in a specified format across an interface DSM -D . The DSL spectrum maintenance center (SMC) can process the information received and provide such processed data results for the use of DSL service providers according to specified criteria and format at interface DSM -S . The SMC performing DSM-Control coordination may also dynamically specify downstream and upstream DSL line spectra and signals of any modems to which it is attached when those modems are fully DSM capable. Thus, the SMC may be implemented within a DSLAM ( especially when used in a fiber-fed remote DSL-LT). DSM is adaptive in that the DSL-LT and DSL-NT can react to recommendations provided automatically by the SMC. The DSL service provider may also adaptively react to reported situations in the loop plant by altering loops to which specific DSL modems are attached or by taking other corrective actions. It is also possible for significant DSM improvement when the lines operate autonomously (without any directed control from the service provider or SMC other than perhaps the data rate desired). Especially in the autonomous case, DSM standards will enumerate training procedures that cause a DSM modem to use no more power nor bandwidth than it needs for reliable operation. This chapter explores the possible improvements as DSL progresses from a highly autonomous state to one in which ultimately signals are co-generated at remote DSL-LT's of the future.

DSL standards such as ADSL (Chapter 3) and VDSL (Chapter 7) have capably enabled individual transmission links to play near their individual point-to-point fundamental theoretical limits, igniting a worldwide interest in broadband access. To date, these standards normally operate autonomously without active concern for other lines, whether or not their own performance exceeds or misses the capability demanded by the individual customer associated with that line. High performance of one line could generate large crosstalk interference into other lines, perhaps preventing the desired reliable transmission of a good data rate on some of those other lines. Spectrum management in Chapter 10 is an early attempt to mitigate such "DSL binder hogging" by any one DSL line through enforced imposition of a fixed set of DSL spectrum masks on each type of DSL modem. These masks compromise different customer's interests based on presumed fixed channel conditions. The management thus enforced by early spectrum management only best balances the customers' varied performance in the one fixed situation presumed in the design of the masks. DSM attempts to balance customer interests adaptively as a function of line situation and of binder and loop topology and of crosstalk activity. Dynamic behavior may occur at several periodic intervals, as well as with several degrees of coordination from autonomous to co-signal generation. The time interval may be per-service order, per-day, per-session , or continuously adaptive.

At time of writing, DSM was in early drafting stages within the T1E1.4 DSL standardization group . This chapter attempts to illustrate the basic concepts and issues in DSM. Section 11.1 address the evolution of DSL unbundling and services, showing that unbundling is inevitably migrating to a packet or "wholesale" level, especially as fiber is increasingly deployed in the feeder portion of the telephone loop plant. Section 11.1 also outlines the fundamentals of multiuser communication theory that apply to DSL. Section 11.2 then addresses a cable characterization that can be used to enable DSM, given the evolution described in Section 11.1. Section 11.3 provides technical description of the improvements of early autonomous DSM that can dramatically improve existing DSL deployments without need for the SMC. Section 11.3 illustrates the significant benefits of a method known as "iterative water-filling" that may be implemented autonomously and that always improves DSL binder performance if used correctly no matter what the mixture of DSM-practicing or nonpracticing modems occurs. Section 11.4 then proceeds to the more sophisticated vectored DSL systems that do require DSLAM-side coordination and enable extremely high data rates and variable mixtures of symmetry in downstream/upstream data rates. Section 11.5 investigates methods for on-line in-service measurement of the critical channel and crosstalk parameters. Appendix 11A describes some mutiuser detection methods that can be used in the absence of DSM to limited benefit in all DSL systems.

This chapter should be viewed as an early snapshot of DSM activities and capabilities. The area is likely to evolve and improve considerably after the publication of this book.