Georgios Ginis

Dynamic spectrum management for next-generation DSL systems

The performance of DSL systems is severely constrained by crosstalk due to the electromagnetic coupling among the multiple twisted pairs making up a phone cable. In order to reduce performance loss arising from crosstalk, DSL systems are currently designed under the assumption of worst-case crosstalk scenarios leading to overly conservative DSL deployments. This article presents a new paradigm for DSL system design, which takes into account the multi-user aspects of the DSL transmission environment. Dynamic spectrum management (DSM) departs from the current design philosophy by enabling transceivers to autonomously and dynamically optimize their communication settings with respect to both the channel and the transmissions of neighboring systems. Along with this distributed optimization, when an additional degree of coordination becomes available for future DSL deployment, DSM will allow even greater improvement in DSL performance. Implementations are readily applicable without causing any performance degradation to the existing DSLs under static spectrum management. After providing an overview of the DSM concept, this article reviews two practical DSM methods: iterative water-filling, an autonomous distributed power control method enabling great improvement in performance, which can be implemented through software options in some existing ADSL and VDSL systems; and vectored-DMT, a coordinated transmission/reception technique achieving crosstalk-free communication for DSL systems, which brings within reach the dream of providing universal Internet access at speeds close to 100 Mb/s to 500 m on 1-2 lines and beyond 1 km on 2-4 lines. DSM-capable DSL thus enables the broadband age.

Transmit pre-emphasis for high-speed time-division-multiplexed serial-link transceiver

Time division multiplexing (TDM) must be employed in multi-Gb/s transceivers in order to overcome onchip clock frequency limitations. This paper describes a transmit pre-emphasis filter for a multi-level transceiver making use of TDM. The possible applications of such a transceiver include serial links and chip-to-chip communication. The requirement of very low probability of error in the absence of coding, and the need for an adaptive solution impose a peak transmit power constraint. The TDM system is mapped to a multiple-input-multiple-output (MIMO) system, and the noise sources are analyzed. The design of the pre-emphasis filter is shown to be a non-convex optimization problem, whose optimal solution is very difficult to obtain. Still, sub-optimal solutions are derived in closed form and adaptive implementations are described. Simulation results using parameters obtained from an experimental testbed indicate that these sub-optimal solutions actually achieve very good performance.

Dynamic spectrum management for mixtures of vectored and non-vectored DSL systems

A number of Vectored DSL system prototypes have confirmed the substantial performance gains from Far-End Crosstalk (FEXT) cancellation. With field trials and initial deployments expected to materialize in the next two years, a very important question is how such Vectored DSL systems can coexist with non-vectored DSL systems sharing the same cable. This paper shows that the performance gains for Vectored DSL systems can be maintained in a mixture of vectored and non-vectored lines if a Spectrum Management Center (SMC) is assigned to control the impact of crosstalk from the non-vectored to the vectored lines. Simulation results are presented for certain scenarios, first, to illustrate that an Iterative Waterfilling strategy can achieve performance near the optimal, and second, to explore the trade-off between limiting the rates of the non-vectored lines and boosting the rates of the Vectored DSL systems.

DYNAMIC UPSTREAM POWER BACK-OFF FOR MIXTURES OF VECTORED AND NON-VECTORED VDSL

Far-end crosstalk severely degrades upstream rates in mixtures of vectored and non-vectored Very high-speed Digital Subscriber Loops (VDSL). As replacement of non-vectored VDSL systems by vectored VDSL systems is expected to be gradual, a crucial problem is the upstream rate optimization of vectored lines while maintaining the rate targets of non-vectored lines. To address this problem, this paper describes an algorithm that selects suitable Upstream Power Back-Off parameters for the vectored and the non-vectored lines. The algorithm has much lower computation requirements compared to Optimal Spectrum Balancing (OSB), yet simulation results show that the achievable vectored rates are very close to the OSB theoretical limits.