Degree Name

Doctor of Philosophy


School of Electrical, Computer & Telecommunications Engineering - Faculty of Informatics


DMT modulation is an OFDM-based modulation scheme used in ADSL and proposed for various other high-speed broadband access systems. Existing mathematical models for transmission lines make a number of simplistic assumptions about the distribution of noise, in particular, the assumption that impulsive noise originates at either end of the transmission line. It is therefore desirable to improve the accuracy of the transmission-line model to allow better prediction of broadband modem performance, and to further improve the bit-allocation algorithms and equaliser designs used in DMT-based modems. This Thesis presents a new channel model particularly well-suited for simulation of high-speed digital subscriber line systems. The model extends a commonly-used physical channel model by distributing the points of noise ingress along the physical length of the transmission line. Simulation results are presented for a highspeed multicarrier modem operating on channels modelled with both the conventional and new models. Comparison with the same modem operating over a real channel demonstrates that the new model provides a better estimate of the bit error rates and temporal error distribution expected on actual telephone lines than is possible with the conventional model. A number of improvements to modulation and coding schemes for DMT modems are also presented in this Thesis, including a robust frequency-domain decision feedback equaliser, a new algorithm for allocating bits to sub-carriers based on a measurement of per-carrier BER, and a technique for encoding data with fractional numbers of bits ii Abstract iii per symbol, thereby providing additional channel capacity.

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