Delay-Constrained Scheduling for Interference-Limited Multi-Carrier Systems

The reduction of energy consumption in digital subscriber line (DSL) networks has obtained considerable attention recently. Today's DSL is designed under an "always on" principle to keep the crosstalk noise as stable as possible. Departuring from this restriction, one approach to achieve energy savings is by "lazy scheduling" which exploits the tradeoff between energy-consumption and transmission delay inherent in many communication systems. This work extends the scope of this idea to multi-user interference limited systems employing multi-carrier modulation. Mathematical decomposition appears to be a natural approach for cross-layer optimization when the physical-layer spectrum management algorithm is already based on dual relaxation. We identify Benders decomposition as the appropriate choice of an optimization scheme for rate and delay constrained energy-minimization. Based on this we propose a cross-layer scheduler for multi-user/multi-carrier systems. By simulations of a single-hop, multi-user DSL scenario this scheduler is shown to closely approximate the optimal solution to this nonconvex problem. Furthermore, by example we demonstrate that scheduling for interference avoidance in DSL yields negligible additional performance gains over sole physical layer spectrum balancing in practice.