Common-Mode Aided Wireline Communications
Abstract: Wireline communication is the Internet access technology with the largest number of subscribers worldwide. On physical-layer level, the traditional way to transmit signals at one end of a wire pair and to extract receive signals at the other end is to apply differential-mode signalling: transmit and receive signals correspond to voltage differences between the two wires. Differential-mode signalling over twisted-wire pairs, proposed and patented more than hundred years ago, yields a communication channel with good and stable transmission characteristics and high immunity to noise and interference. The differential mode is the only propagation mode in a two-conductor system. Taking into account earth as a third conductor yields a second independent mode, the so-called common mode, which can be used for communication. This thesis deals with information processing methods to exploit both modes jointly in order to improve transmission. The material can be divided into three parts. The first part investigates basic characteristics of the common mode and the resulting channel when taking into account the common-mode signal at the receiver. Measurements of common-mode signals and common-mode cable properties are presented and analysed. The design of receivers that use the common-mode signal for mitigation of both narrowband disturbers and broadband interference is addressed. The second part focuses on the potential gain in throughput that can be achieved with common-mode aided transmission techniques. The Shannon capacity of the common-mode aided wireline channel is derived and evaluated, and the performance under specific constraints of existing digital subscriber line techniques is addressed. A rough prospect of the Swedish copper access network used with common-mode aided methods and other advanced techniques is presented. The third part of the thesis deals with techniques to reduce unwanted spectral egress, which is a prerequisite for employing the common mode as active mode for transmission. The investigations focus on discrete Fourier-transform based multicarrier systems with a Gaussian channel known to transmitter and receiver. Information processing techniques that maximise the throughput under arbitrary power spectral density constraints are devised. In particular, transmit-windowing techniques and the so-called spectral-compensation method are investigated.
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