Equalisation Techniques For Multi-Level Digital Magnetic Recording

Abstract

There has been tremendous advancement in the field of digital magnetic storage since 1971, when Kobayashi exploited the utilisation of the Viterbi algorithm for decoding in magnetic recording. The introduction of partial response maximum likelihood (PRML) in hard disk drives led to an exponential increase in the areal densities achievable by the magnetic storage devices. A large amount of research has been put into areas of signal processing, medium design, head and servo-mechanism designs and coding. In order to overcome the effects of super-paramagnetism in the conventional longitudinal magnetic recording, perpendicular magnetic recording has also been developed. This work presents some further investigations in the signal processing and coding aspects of longitudinal and perpendicular digital magnetic recording. To overcome the effects of transition jitter in perpendicular magnetic recording, it is necessary to modify the decoder which would take into account the jitter noise. It is shown that the asymmetric decoder (AD) which is modified to take into account the jitter noise in the metric calculations, has better performance than classical BCJR decoders with the use of error correction codes (ECC). It is shown that generalised partial response (GPR) targets with arbitrary coefficients yield a better performance than a full dc response PR target with integer coefficients, even in the presence of jitter noise. The GPR target and its equaliser are designed to maximise the ratio of the minimum squared euclidean distance of the PR target to the noise penalty introduced by the PR filter. With reference to the previous work by Mackintosh and Jorgensenon multi-level record- ing, this work explains in detail the benefits of multi-level recording using different equal- isation techniques. Although, as the number of levels increase, the SNR requirement to achieve lower error rates also increases, which is reduced by using ECC. Two methods of equalisation including the industrys standard partial response (PR), and a novel soft-feedback-equalisation (SFE) have been discussed which are complimen- tary to each other. The work on SFE was derived from the problem of intersymbol interfer- ence (ISI) in the PRML scheme. The SFE scheme is not optimum since it has a drawback of residual ISI. It is shown that multi-level SFE with MAP/BCJR feedback based magnetic recording with ECC has similar performance when compared to high density binary PRML based magnetic recording with ECC, thus documenting the benefits of multi-level magnetic recording. Investigation to the problem of residual ISI leads to replacement of the threshold- ing device with PRML to obtain the best initial estimate, improving the performance of SFE based magnetic recording channel. It is shown that this combined PRML and SFE based magnetic recording with the application of maximum transition run (MTR) codes has clearly a gain of 0.5 dB over conventional PRML based magnetic recording systems.

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