When can tentative decisions be used to cancel (linear or nonlinear) intersymbol interference? (With application to magnetic recording channels)

Adaptive cancellation of intersymbol interference using tentative decisions has been proposed by several researchers. These authors report satisfactory results in voiceband data transmission and satellite communications. However there are other applications where this technique is completely ineffective. One such case is cancellation of nonlinear distortion in magnetic recording channels. We formulate precise conditions under which tentative decisions can be effectively used to cancel linear or nonlinear intersymbol interference. In the case of magnetic recording channels, we show that these conditions are satisfied by the precursor interference generated by inductive read heads, resulting in an improvement of the noise margin, of 2 dB at the output of a symbol-by-symbol detector or 1 dB at the output of a Viterbi decoder at a bit error rate of 10^-9. This technique has been incorporated in an experimental VLSI partial response maximum likelihood (PRML) receiver and 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 recording channels, and we establish fundamental limits for the improvement achievable by any nonlinear equalization technique in magnetic recording