New Approach to Dynamic Spectrum Management for DSL Environments

Currently, the telecommunications market has brought changes to the design of the old model of the telecommunications network. The emergence of new technologies for higher speed access was inevitable in order to meet the requirements of the appearance of the multimedia services (VoD, online gaming etc.). The latest technologies for broadband access over telephone pairs are Digital Subscriber Lines or DSL. This set of xDSL technologies allow the transfer of binary high speed over telephone twisted pair by using a suitable type of line codes. They allow a flow of high-speed information both asymmetrical and symmetrical over the telephone loop. This thesis presents the state of the art of Dynamic Spectrum Management (DSM) technologies suggested to improve the performance of DSL systems and proposes a new approach to this issue. The main contributions of this thesis includes extended bandwidth channel characterization and modeling of the copper cables, modeling and measurements of the wire-shield modes of the copper cables, new technique for optimization of the upstream power back-off (PBO) parameters unique for a cable bundle and development of new bit-loading and power minimization algorithm based on mercury/waterfilling (MWF) power allocation policy. For very short loops (a few hundred meters it is sensible both from technological and from economic point of view to go beyond the current limit of 30 MHz. Thus, there is still potential for increasing the speed of Internet access over very short loops by using frequencies above this limit. Moreover, laying fiber closer to the customer a whole new generation of DSL systems can be developed. Therefore, we concentrated our attention on the measurements of the copper cables up to 200 MHz, and verified the the existing models for extrpolated for those high frequencies shows a good match. Nevertheless, in order to obtain better models we developed new models by using LMS fitting. These modeling includes the phase for the crosstalk channels (NEXT and FEXT). Lately the view of the copper cable as MIMO channel attracted a lot of attention. Therefore, the alternative modes that exist in the copper cables can be used for communications. Thus, we focus on shielded twisted pair (STP) cables and investigate alternative mode propagation in the configuration with the shield used as a common reference to other wires in the copper cable. This topic establishes valid models for common mode transmission systems and evaluates the potential benefits in the term of channel throughput. The main contribution is in the cable characterization. The number of modes in such a configuration doubles compared to the differential mode configuration. More importantly, the shield substantially reduces the level of extrinsic noise entering the cable and relaxes the electromagnetic compatibility issue. Considering these aspects, exploiting the wire-shield modes in STP cables has great potential. New technique for optimization of the upstream power back-off (PBO) parameters unique for a cable bundle gives an operator the opportunity to optimize VDSL performance according to an actual network situation. With these optimized PBO parameters we achieve a significant performance increase for actual deployed cables compared to the worst case design currently in use. Taking into consideration that this approach performs optimization according to the current situation in the network, it presents DSM with already standardized parameters. Therefore, its implementation is simpler than other approaches and can be incorporated as a pacth to the firmware of already deployed DSLAMs. Allocating power to different sub-carriers in multi-carrier system by waterfilling policy is not optimal because the distribution of the input symbols is assumed to follow the Gaussian distribution. This is not the case for practical systems where inputs are obtained by different modulation techniques. Nevertheless, the bit loading algorithms proposed for practical systems, although incorporates discrete nature of different modulations, depend on the capacity equation for Gaussian distribution corrected with Shanon gap approximation. These algorithms claim the optimality but it was recently demonstrated that mercury/waterfilling (MWF performs optimal power allocation knowing the input distribution “a priori”. In this thesis we demonstrated that using the same bit distribution as obtained by bit loading algorithms based on capacity equation and applying mercury/waterfilling better performance in bit error rate (BER) can be achieved. This can be used for reducing the noise margin or making the system more reliable. Using this bit distribution as starting point and applying mercury/waterfilling policy we developed new bit loading algorithm, termed Mercury Bit Loading (MBL) algorithm, that improves the system throughput by searching for a solution of a corresponding combinatorial optimization problem, constrained by the same power and BER restrictions as the previously developed. This optimization problem is a very complex and hard to solve, therefore, we decided to use the greedy approach. This new algorithm is not restricted to the knowledge of the bit distribution in advance but rather uses the mercury/waterfilling with bit distribution obtained in each iteration. Energy consumption accounts for a big deal of operating expenses, so any solution that improves energy-efficiency is not only good for the environment, but they also make commercial sense for operators and support sustainable, profitable business. Therefore we developed an algorithm based on mercury/waterfilling for power minimization while keeping the same throughput and bit error rate (BER) that can be achieved by standard algorithms. We termed this algorithm Mercury Power Minimization (BMPM) and demontrsted that more than 10% of the power is wasted in current DSL systems.