Optimization of xDSL transmission systems

xDSL (digital subscriber line) is a group of technologies for transmitting high-speed digital signals over existing metallic cables. xDSL was originally studied in the USA. These technologies have spread to other areas of the world, including Europe and Asia. In this process of introducing this technology to the different areas, several new local but critical problems have emerged, due to the local system and cable situations. In Japan, TCM (time compressed multiplex)-ISDN is widely used. This ISDN causes periodic crosstalk noise of far end cross-talk (FEXT) and near end cross-talk (NEXT) to xDSL. For the optimization of the transmission performance to this type of noise, it is efficient to use different bit assignments corresponding to the FEXT period and NEXT period. NEC has proposed a solution of the optimization for ADSL (asymmetric DSL) transmission under the TCM-ISDN crosstalk noise environment. We have compared the following transmission methods, the existing ANSI ADSL, dual bitmap (DBM)-ADSL, single bitmap (SBM)-ADSL and TCM-ADSL, with some simulation results. This shows that the performance of ANSI ADSL is greatly degraded by the ISDN crosstalk disturbance, and DBM-ADSL improves the performance significantly. SBM and TCM-ADSL also have better performances in a long reach with small modifications. The study results are explained