THE USE OF TENTATIVE DECISIONS TO CANCEL INTERSYMBOL INTERFERENCE ANDNONLINEAR DISTORTION (WITH APPLICATION TO MAGNETIC RECORDING CHANNELS)

In this paper we develop a theory of the error rate of receivers that use tentative decisions for adaptive cancellation of intersymbol inter ference (ISI), We formulate precise conditions under which tentative d ecisions can be effectively used to cancel linear or nonlinear ISI, an d verify the predictions of the theory by Monte Carlo simulations on f our typical channels, In the case of magnetic recording channels, we s how that the conditions for effective cancellation are satisfied by th e precursor interference generated by inductive read heads, resulting in an improvement of the noise margin of 2 dB at the output of a symbo l-by-symbol detector or 1 dB at the output of a Viterbi decoder at a b it error rate of 10(-9). This technique has been incorporated in an ex perimental VLSI partial response maximum-likelihood (PRML) receiver, a nd the results confirmed by laboratory measurements, We also show that no improvement is obtained when this technique is used to compensate nonlinear distortion, which is another common impairment of magnetic r ecording channels, and we establish fundamental limits for the improve ment achievable by any nonlinear equalization technique in magnetic re cording.