Noise-predictive maximum-likelihood detection

Noise-Predictive Maximum-Likelihood (NPML) is an advanced digital signal-processing method suitable for magnetic data storage systems that operate at high linear recording densities. It is used for reliable retrieval of data recorded in the magnetic medium.

Data are read back as a weak and noisy analog signal by the read head, and NPML aims at minimizing the influence of noise in the detection process. Therefore, it allows recording data at higher areal densities than other detection schemes, such as peak detection, partial response maximum likelihood (PRML), and extended partial-response maximum likelihood (EPRML) detection.

Although advances in head and media technologies have historically been the driving forces behind the increases in the areal recording density, digital signal processing and coding established themselves as cost-efficient techniques for enabling additional substantial increases in areal density while preserving the high reliability of hard disk drive (HDD) systems. Accordingly, the deployment of sophisticated detection schemes based on the concept of noise prediction are of paramount importance in the HDD industry.

In general, NPML refers to a family of sequence-estimation data detectors, which arise by imbedding a noise prediction/whitening process into the branch metric computation of the Viterbi algorithm, which is a well known data detection technique for communication channels that exhibit intersymbol interference (ISI) with finite memory.

Reliable operation of the prediction/whitening process is in general achieved by using hypothesized decisions associated with the branches of the Trellis on which the Viterbi algorithm operates as well as tentative decisions corresponding to the path memory associated with each trellis state. The NPML detectors can thus be viewed as a family of reduced-state sequence-estimation detectors offering a range of implementation complexities. The complexity is essentially governed by the number of detector states, which is equal to 2^K, 0 ≤ K ≤ M, with M denoting the maximum number of controlled ISI terms introduced by the combination of a partial-response shaping equalizer and the noise predictor. By judiciously choosing the parameter K, practical NPML detectors can be devised for the magnetic recording channel that provide a substantial performance improvement over PRML and EPRML detectors in terms of error rate and/or linear recording density

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