| Most practical digital transmission channels exhibit intersymbol interference (ISI) and a number of additive noise impairments, such as thermal noise, residual echo, quantization noise, crosstalk, and impulse noise. To reliably transmit and receive the highest data rate possible through such non-ideal channels, every component of the communication system must be optimized. In this dissertation, we focus on one particular element of the overall system optimization; namely, the optimization of the system transmission bandwidth. While the classic water-pouring energy distribution is known to achieve the capacity of a channel, it is impractical to implement. We assert that by optimizing the transmission bandwidth, a flat on/off energy distribution will perform virtually as well as the water-pouring energy distribution for most practical applications. Furthermore, by examining and comparing the asymptotic performance levels of a single-carrier system employing Quadrature Amplitude Modulation (QAM) with a Minimum-Mean-Squared-Error Decision Feedback Equalizer (MMSE-DFE) receiver and a multicarrier system using Discrete Multitone (DMT) modulation, we show that multicarrier bandwidth optimization techniques can be adapted for single-carrier system use as well. We introduce the concept of adaptive transmitters for quasi-stationary duplex channels in the context of bandwidth optimization. A physical medium on which these proposed techniques can be tested is the copper twisted pairs in the current telephone network. Through computer simulation, we find that very high data rates (6.4+ Mbps) can be achieved, in the presence of additive white Gaussian noise (AWGN) and crosstalk, on many Asymmetric Digital Subscriber Line (ADSL) loops utilizing a bandwidth optimized transmission system. In some Digital Subscriber Line (DSL) services, however, large-amplitude, impulsive disturbances, known collectively as impulse noise, may prove to be a dominant source of impairment. In order to mitigate the effects of impulse noise, we propose several novel error control techniques designed specifically for a DMT system that will exploit impulse noise characteristics in both time and frequency domains. Although we concentrate our simulation efforts around DSL applications, techniques proposed and studied in this dissertation can be applied to a wide range of digital transmission systems over spectrally shaped channels with impulse noise. |
