Multi-user Signal and Spectra Co-Ordination for digital subscriber lines
The appetite amongst consumers for ever higher data-rates seems insatiable. This booming market presents a huge opportunity for telephone and cable operators. It also presents a challenge: the delivery of broadband services to millions of customers across sparsely populated areas. Fully bre-based networks, whilst technically the most advanced solution, are prohibitively expensive to deploy. Digital subscriber lines (DSL) provide an alternative solution. Seen as a stepping-stone to a fully bre-based network, DSL operates over telephone lines that are already in place, minimizing the cost of deployment. The basic principle behind DSL technology is to increase data-rate by widening the transmission bandwidth. Unfortunately, operating at high frequencies, in a medium originally designed for voice-band transmission, leads to crosstalk between the di erent DSLs. Crosstalk is typically 10-15 dB larger than the background noise and is the dominant source of performance degradation in DSL. This thesis develops practical multi-user techniques for mitigating crosstalk in DSL. The techniques proposed have low complexity, low latency, and are compatible with existing customer premises equipment (CPE). In addition to being practical, the techniques also yield near-optimal performance, operating close to the theoretical multi-user channel capacity. Multi-user techniques are based on the coordination of the di erent users in a network, and this can be done on either a spectral or signal level. Spectra coordination, also known as dynamic spectrum management (DSM minimizes crosstalk by intelligently setting the transmit spectra of the modems within the network. Each modem must achieve a trade-o between maximizing its own data-rate and minimizing the crosstalk it causes to other modems within the network. The goal is to achieve a fair trade-o between the rates of the di erent users in the network. The rst part of this thesis investigates the optimal design of transmit spectra for a network of crosstalking DSLs. This problem was previously considered intractable since it requires the solution of a high-dimensional, non-convex optimization. This thesis uses a dual-decomposition to solve the optimization in an efficient, tractable way. The resulting algorithm, optimal spectrum balancing, achieves signi cant gains over existing spectra coordination algorithms, typically doubling or tripling the achievable data-rate. The second part of this thesis investigates multi-user signal coordination. In the upstream, reception is done in a joint fashion; the signals received on each line are combined to cancel crosstalk whilst preserving the signal of interest. Existing crosstalk cancelers are based on decision feedback, which leads to problems with error propagation, high complexity, and a long latency. To address this problem, this thesis presents a simple linear canceler based on the well known zero-forcing criterion. This technique has a low complexity, short latency, and operates close to the theoretical channel capacity. In the downstream, transmission is done in a joint fashion; predistortion is introduced into the signal of each user prior to transmission. This predistortion is chosen such that it annihilates with the crosstalk introduced in the channel. As a result the customer premises (CP) modems receive a signal that is crosstalk free. Existing precoder designs either give poor performance or require the replacement of CP modems, which raises a huge legacy issue. To address this problem, this thesis presents a simple linear precoder based on a channel diagonalizing criterion. This technique has a low complexity, does not require the replacement of CP modems, and operates close the the theoretical channel capacity. Despite the low complexity of the techniques described, signal coordination is still too complex for current implementation. This problem is addressed in this thesis through a technique known as partial cancellation. It is well known that the majority of crosstalk experienced on a line comes from the 3 to 4 surrounding pairs in the binder. Furthermore, since crosstalk coupling varies dramatically with frequency, the worst effects of crosstalk are limited to a small selection of tones. Partial cancelers exploit these facts to achieve the majority of the performance of full cancellation at a fraction of the complexity. Partial canceler and precoder design is discussed and shown to be equivalent to a resource allocation problem. Given a limited amount of available run-time complexity, a modem must distribute this across lines and tones such that the data-rate is maximized. This thesis presents the optimal algorithm for partial canceler design and several simpler, sub-optimal algorithms. These algorithms are shown to achieve 90% of the data-rate of full cancellation at less than 30% of the complexity.