Multilevel range/NEXT performance in digital subscriber loops

Presents some results on the range performance of multilevel signalling schemes in the presence of near-end crosstalk (NEXT) in the digital subscriber loop. With a data rate of 160 kbit/s at the U interface, multilevel line codes can be used to reduce the symbol rate and hence the bandwidth of the transmitted signal to yield improved performance in the crosstalk dominated environment, as this impairment has a power transfer which increases with frequency. By varying the number of transmitted signal levels over the range 2 to 128, subject to a peak power constraint, the authors obtain results for the range of the optimum linear and decision feedback equalisers for a probability of error of 10/sup -6/. These results indicate that the optimum number of levels lies between 4 and 7. In addition, the optimum 3-level PAM is compared with some commonly used 3-level line codes. It is found that the performance degradation of these suboptimal codes is less than 10%.<>