Analysis and simulation of a baseband precoded TCM system in DSL channel with crosstalk noise and ETSI impulse noise

Decision feedback equalization (DFE) and trellis-coded modulation (TCM) are commonly used to mitigate the channel impairments in a digital subscriber loop (DSL) system. In order to combine the benefit of both DFE and TCM, a Tomlinson-Harashima (TH) precoder is introduced to form a precoded TCM system. This dissertation addresses the implementation and the performance issues of a baseband precoded TCM system in a practical DSL environment. The practical impairments studied in this dissertation include: channel inter-symbol-interference (ISI), near-end self-crosstalk (Self-NEXT) and an European Telecommunications Standards Institute (ETSI) defined impulse noise. The tradeoff between the implemented size of the DFE and the idealized DFE performance is derived in a closed-form matrix equation. It is found that the ISI is usually negligible when the size of the DFE is chosen properly. Without the presence of the impulse noise, the Self-NEXT is the limiting factor for such a DSL system. The performance of the precoded TCM system in an additive white Gaussian noise (AWGN) and a Self-NEXT environment is analyzed and simulated respectively. Depending on the size of the code used, the precoded TCM system performance is 0.2 to 0.5 dB lower in the Self-NEXT environment, where the larger codes suffer less coding gain degradation. When the ETSI impulse noise is added to the DSL system, the realizable coding gain is dropped significantly. Both analytical methods and computer simulations show that the precoded TCM system enjoys less than 2 dB of coding gain in this ETSI impulse noise.