Near-end crosstalk modelling in the digital subscriber loop with application to line code comparison

The importance of near-end cross talk in digital subscriber loops is reviewed. Three statistical models are examined for predicting the mean and standard deviation of the NEXT interference power from n disturbers. The Bradley model is selected as being most accurate with moderate computational complexity. A a set of measurements on multipair cable to obtain model parameters is described. Crosstalk coupling on 26 gauge cable is found to vary with f^1.4, have mean at 80 kHz equal to -93.7 dB and standard deviation of 11.5 dB. The distribution of the loss in dB across pairs of disturbers is Gaussian. The model is applied to compare line codes for the digital subscriber loop at both the ISDN basic rate and at 800 kBit/s. The well known advantage of the 2B1Q code for basic rate ISDN is substantiated, lending confidence to the model. The 2B1Q code is also shown to have the greatest noise margin of common codes at 800 kBit/s.