Constrained coding for binary channels with high intersymbol interference

Partial-response (PR) signaling is used to model communications channels with intersymbol interference (ISI) such as the magnetic-recording channel and the copper-wire channel for digital subscriber lines. Coding for improving noise immunity in higher order partial-response channels, such as the ``extended{''} class-4 channels denoted EPR4, E(2)PR4, E(3)PR4, has become an important subject as the linear densities in magnetic recording approach those at which these partial-response channels are the best models of real channels. In this paper, we consider partial-response channels for which ISI is so severe that the channels fail to achieve the matched-filter bound (MFB) for symbol error rate, assuming maximum-likelihood decoding. We show that their performance can be improved to the MFB by high-rate codes based on constrained systems, some of which may even simplify the Viterbi detectors relative to the uncoded channels. We present several examples of high-rate constrained codes for E(2)PR4 and E(3)PR4 channels and evaluate their performance by simulation.