A trellis-coded 2.048 MB/s data transmission system for ISDN

The two major impairments in high rate digital subscriber loop (HDSL) transmission are severe intersymbol interference (ISI) and the near-end crosstalk (NEXT). At the primary rate (2.048 MB/s) the problem becomes even worse because the impulse response of the channel stretches over many symbol intervals and the NEXT noise increases with frequency. The authors use a double side (both pre- and postcursor) fractionally spaced (T/2) forward filter (FSFF) to suppress the NEXT noise in the detector. The double side FSFF cancels the precursor ISI as well. The postcursor part of the ISI will either be canceled by a decision feedback equalizer (DFE) or be used by a maximum likelihood detector. They propose a trellis-coded 8PAM for 2.048 Mb/s data transmission on the DSL channel using the reduced state sequence estimation (RSSE) or the sequential sequence estimation (SSE) method. This system is compared with an uncoded 4PAM (called 2B1Q) system in which the DFE is used to eliminate the IS of the channel