Dynamic Spectrum Management Algorithms for Interference Mitigation in DSL Networks

This thesis considers digital subscriber line (DSL) networks that provide fixed broadband access over copper twisted-pair lines. In order to cost-effectively achieve ever higher data rates on these twisted-pair lines, the adopted strategy over the last two decades has been to gradually replace them with fiber optic cables, thereby reducing the copper loop length and enabling signal transmission over a higher frequency bandwidth. An important disadvantage of using a higher bandwidth however, is the resulting increase in interference between the different signals in the DSL network. This thesis studies novel and advanced strategies to reduce the impact of interference in DSL networks. Two general strategies will be pursued, each corresponding to a different Part of this thesis. The strategy in Part I consists of dynamically changing the interference mitigation configuration—also referred to as the dynamic spectrum management (DSM) configuration—in real time. Such a real-time adaptive DSM design, which dynamically adapts the DSM configuration to user requirements in real time, mitigates competition between different users in the network by capitalizing on the elastic, time dependent nature of the traffic the network is carrying. The two keys to implementing real-time adaptive DSM are 1) extending the DSM design to a cross-layer design, thereby allowing users or applications to express their requirements regarding the DSM configuration, and 2) fast optimization algorithms able to adapt the DSM configuration to user or application requirements in real time. The first contribution of this thesis is a new real-time DSM algorithm which, contrary to the state-of-the-art algorithms, provably converges to a stationary point of the considered optimization problem. Simulation results additionally demonstrate that the proposed algorithm outperforms state-of-the-art real-time DSM algorithms in practice as well. To enable users or applications to express their requirements regarding the DSM configuration, cross-layer DSM design by means of network utility maximization (NUM) is considered. A fast DSM algorithm is proposed, which finds a local solution to the considered NUM problem. The proposed algorithm is applicable in many DSL deployment scenarios, and regardless of the utility function's properties. Empirical evidence shows that the proposed algorithm needs only few iterations to find a satisfactory solution. The strategy in Part II consists of reducing the impact of interference in DSL networks by accounting for the interplay between the DSM design and new features of the upcoming DSL standard—G.mgfast. These new features affect the optimality of the state-of-the-art DSM approaches and may—if they are accounted for in the DSM design rather than be treated as separate features— yield increased performance by adding new degrees of freedom to the DSM design space. Three new physical layer features are considered in this thesis, namely QoS classes, full duplex transmission, and channel shortening. In order to support QoS classes, this thesis considers providing for unequal error protection (UEP). Four new algorithms are presented for joint DSM and UEP design: two optimal spectrum balancing (OSB) algorithms—one for upstream and one for downstream transmission—and two low-complexity adaptations of the OSB algorithms, referred to as distributed spectrum balancing (DSB) algorithms. In addition, an algorithm is presented which selects an optimal modulation and coding scheme to be used for each QoS class. Results show the benefit of UEP, as it is able to achieve modest performance gains. DSM design for DSL networks implementing full duplex transmission is considered as well. A new MAC-BC duality theory for multi-user FDX networks is developed, from which an OSB algorithm is derived that determines the globally optimal DSM design. The developed OSB algorithm is—to the best of the authors' knowledge—the first to establish the globally optimal solution to the considered full-duplex DSM problem. In addition to the OSB algorithm, two low-complexity DSB algorithms are proposed. Simulations show that FDX transmission indeed leads to significant performance gains in MU DSL networks. As a final feature, channel shortening algorithms are considered, which apply an FIR filter to the transmitted or received signals to reduce the apparent channel impulse response length, thereby increasing the achievable data rates. New algorithms for joint channel shortening filter and DSM design are presented. Moreover, a duality result is presented, demonstrating a theoretical equivalence between DSL systems with either transmitter and or receiver-side channel shortening filters. Finally, it is shown that the received signals in systems using channel shortening filters can be improper. A new signal constellation design is proposed that takes this impropriety explicitly into account. Simulations demonstrate that the proposed methods and designs can achieve significant performance gains on highly dispersive long twisted-pair lines.

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