Constrained coding for binary channels with high intersymbol interference

Partial-response (PR) signaling is used to model communications channels wi th intersymbol interference (ISI) such as the magnetic-recording channel an d the copper-wire channel for digital subscriber lines. Coding for improvin g noise immunity in higher order partial-response channels, such as the "ex tended" class-4 channels denoted EPR4, E(2)PR4, E(3)PR4, has become an impo rtant subject as the linear densities in magnetic recording approach those at which these partial-response channels are the best models of real channe ls. In this paper, we consider partial-response channels for which ISI is s o severe that the channels fail to achieve the matched-filter bound (MFB) f or symbol error rate, assuming maximum-likelihood decoding. We show that th eir performance can be improved to the MFB by high-rate codes based on cons trained systems, some of which may even simplify the Viterbi detectors rela tive to the uncoded channels. We present several examples of high-rate cons trained codes for E(2)PR4 and E(3)PR4 channels and evaluate their performan ce by simulation.