Performance of random-access-memory-based equalizers in magnetic recording

We discuss two algorithms for mitigating the deleterious effects of nonlinear intersymbol interference in PR4- and EPR4-equalized magnetic recording channels. We call these algorithms RAM-Search algorithms because they borrow the idea of employing random-access memory (RAM) from the RAM decision-feedback equalizer (DFE) and because they involve a search. The RAM-Search algorithms have been devised to mitigate the error propagation problem that arises in making feedforward decisions (for precursor terms) in intersymbol interference (ISI) cancellers used in combination with partial-response signaling. We adopt a simple partial-erasure model for the nonlinear write process and a third-order polynomial model for the nonlinear read process, and show that improvements of about 2.5 dB are possible. We also demonstrate the efficacy of the algorithms on captured spinstand signals that suffer from a significant amount of nonlinear ISI.