The increasing data rates in broadband communication systems such as digital-subscriber-lines (DSLs) are enabling quality-of-service sensitive applications such as HDTV and home-networking among others. Achieving the high data-rates for these applications requires efficient optimization of the users' transmit spectra as well as interference cancellation techniques. In addition, a stable operation of the broadband link is required to provide quality-of-service. This dissertation develops techniques to optimize the interference and outage in multi-carrier networks such as DSL under different levels of coordination between users.
At the first level, a single-user model is employed where each user's transmission is considered separately. The trade-off between throughput and stability is studied for such a model, where the stability at the physical layer of a communication system is measured in terms of the outage probability. The outage-events occur because of slow variations in the channels or noise-spectra over time. Employing a dynamic-spectrum-management framework where a management center monitors the channel and noise conditions of the users in the network, the signal-to-interference-and-noise-ratio (SINR) margins are optimized using the statistics of the variation in the noise-spectrum. The optimized SINR margins allow the users in the network to operate at the desired trade-off between stability and the single-user data-rates.
The second level of coordination is then considered by modeling the DSL system as a multicarrier interference channel, where the interference from other users is considered as noise. Using this model, the spectra and margins of the users can be jointly optimized to obtain better stability and data-rate tradeoffs compared to the single-user model. The highly-coupled margin and spectrum optimization problems are disentangled by introducing a new definition for the noise margin of the system, using which algorithms for optimizing the outage-probabilities are presented. A low-complexity, near-optimal algorithm is presented to solve the spectrum optimization problem. Numerical results show that the proposed margin and spectrum optimization algorithms provide 50-100% improvements in the data-rates and an order of magnitude improvement in the outage probability. Thus, margin and spectrum optimization help stabilize broadband links by operating at the desired trade-off point between data-rate and outage-probability.
The third level of performance improvement is achieved using signal-level coordination of different lines and by exploiting new transmission dimensions via common-mode signaling. The DSL binder's metallic sheath is used as a common reference to obtain reliable channels for the common-mode signals, which are modeled using multi-conductor transmission line theory. The lines in the binder are coordinated to cancel differential-mode crosstalk and also to avoid interference caused by radiation from the common-mode within the binder. The high level of common-mode noise is countered by exploiting the correlation between the differential-mode and common-mode noises. Numerical results based on the derived models show data-rate improvements of up to 100% compared to using only differential-mode transmission. Next, a spatial noise cancellation method is developed, which uses extra wire-pairs available in the DSL downstream to significantly improve DSL line stability and data rates when alien noise (stationary or intermittent) is dominant.
Finally, the effect of a non-zero SNR margin and coding gap on the achievable rate region of the Gaussian Multiple-Access Channel (MAC) is investigated. The Frequency-division multiple-access (FDMA) scheme is shown to be the only sum-rate-maximizing scheme for a scalar Gaussian MAC when the margin/gap is not zero. Furthermore, successive decoding is shown to be strictly sub-optimal from a sum rate perspective. The achievable rate region of the scalar Gaussian MAC with non-zero margin/gap is also characterized. Unlike the capacity region of a scalar MAC, the optimal achievable rate region with non-zero margin/gap is not a pentagon. It is also shown that when there is non-zero margin/gap, the optimality of the FDMA scheme can be generalized to a Gaussian MAC with inter-symbol interference.
|Advisor:||Cioffi, John M.|
|School Location:||United States -- California|
|Source:||DAI-B 69/05, Dissertation Abstracts International|
|Keywords:||DSL, DSM, Interference, Multicarrier, Optimization, Outage|
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