Line Codes for Digital Subscriber Lines

1 HE SELECTION OF A LINE CODE FOR ANY TELEphone transmission system is critical to its performance. This is particularly true for the ISDN Basic Access Digital Subscriber Line (DSL). In this application, the line code affects the determinants of system performance in a crucial way, the principal reason being that the line code is instrumental in determining both the transmission characteristics of the transmitted signals and the added near-end crosstalk noise levels on other pairs in the same cable. Further, it is required that the DSL perform with a Bit Error Ratio (BER) of less than lo-' over essentially all of the non-loaded loop plant [l] in the North American wire pair plant. The complexity of this plant is exemplified in Figure 1, which gives three representative samples of the structures of loops from the 10% lossiest loops in the population. The bridged taps and gauge changes in these loops lead to substantial transmission losses because of reflected energy, intersymbol interference contributed by multiple reflections and transmission distortion, and echoes back to the transmitting end of the line. The insertion losses of these loops at frequencies of interest are shown in Figure 2. In order to economically provide Basic Access, the DSL must be implemented without conditioning the plant (e.g., by