Distributed spectrum coordination for DSL broadband access networks

Digital subscriber line (DSL) technology is the dominating broadband internet access technology with a market share of over 66% of all broadband subscribers worldwide. The main performance bottleneck of modern DSL networks is the crosstalk interference generated among different lines (i.e. twisted pair cables) inside the same cable bundle. Multi-user spectrum coordination is recognized as a key technology to tackle this crosstalk problem. It consists of optimizing the users' transmit spectra so as to mitigate the destructive impact of crosstalk, leading to spectacular data rate performance gains as well as power savings. The corresponding optimization problems are however large-scale nonconvex problems. In this paper we provide a survey of recent developments on spectrum coordination techniques where the main focus is on distributed spectrum coordination. Here, individual users optimize their transmit spectra based on local measures while they are steered by a centralized controller (spectrum management center) so as to obtain a good global network performance. It is shown how state-of-the-art techniques from mathematical programming are used to obtain distributed algorithms with very low complexity and yet good performance. Finally simulation results are shown that demonstrate the impact of spectrum coordination for DSL broadband access.