Partial Response Maximum Likelihood Channel Research Articles

A Study of Three-Dimensional Equalization for Reproducing a Double-Layer Magnetic Recording Medium

To increase the recording density of hard disk drives (HDD), three-dimensional (3-D) magnetic recording with multiple recording layers is attracting much attention as a next-generation recording method. We have reported that the two-dimensional (2-D) partial response maximum likelihood (PRML) can achieve a good BER performance using the PR channel obtained from the differential-phase bit response in the 3-D magnetic recording system with a double recording layer. In this study, we realize the 3-D equalization by the 2-D finite impulse response (FIR) (2-D FIR) filter and 2-D PRML system over two layers and evaluate the 3-D equalization to further increase the recording density. The transfer function of the 2-D PR channels is determined by the response for the in-phase or differential-phase bits in a double-layer magnetic recording medium. The results show that the 2-D PRML channels with 3-D equalization based on the differential-phase transfer function achieve a good bit error rate (BER) performance for system noise.

A Study of Channel Time-Domain Response on Equalization for Reproducing a Double-Layer Magnetic Recording Medium

To increase the recording density of hard disk drives (HDD), 3-D magnetic recording using microwave-assisted magnetic recording (MAMR) with multiple recording layers is attracting much attention as a next-generation recording method. Developing a signal processing method for 3-D magnetic recording using the multiple conventional recording layers and an magnetoresistive (MR) head reading system is desired. In this article, we evaluate 2-D partial response maximum likelihood (PRML) channels by the bit error rate (BER) performance, where the transfer function of the PR channel is determined by the response of the in-phase or differential-phase bits between layers in a double-layer magnetic recording medium. The results show that a BER of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$10^{-4}$ </tex-math></inline-formula> can be achieved by the PRML channel determined by the differential-phase response with the signal-to-noise ratio (SNR) greater than 26.0 dB for system noise.

ＰＲＭＬ方式に対する消失誤り訂正方式の一検討

In a hard disk drive (HDD), a postprocessor is used for correcting the random error in a PRML (Partial Response Maximum Likelihood) channel. However, the postprocessor needs an error-detecting code, such as a single parity check code, and the code rate decreases. This paper proposes an erasure correction by Reed-Solomon (RS) code using Squared-Distance-Increase (SDI) instead of a postprocessor. This system does not need any error detecting code. The performance is obtained by computer simulation and is compared with that of the error correction system using a postprocessor. The results show that the proposed system provides an SNR improvement of about 2.6dB compared with the system using a postprocessor in a perpendicular magnetic recording channel with random and burst errors.

Evaluation of read channel performance for perpendicular patterned media

The use of patterned media is one of the proposed approaches for extending magnetic storage densities and delaying the problems imposed by the superparamagnetic limit. In this paper, we investigate the reliable recovery of data from patterned media with perpendicular anisotropy using a software tool developed to simulate the read channel. The bit error rate against signal-to-noise ratio (SNR) performance of various partial-response maximum-likelihood (PRML) channels is evaluated. It is shown that the PRML channels for perpendicular recording offer better performance than the conventional channels for longitudinal recording. In addition, simulations reveal that the use of run-length limited encoders and the existence of a soft magnetic underlayer contribute to an improved performance in terms of SNR.

Performance comparison of post-processor for PRML channel in perpendicular magnetic recording

In this paper, post-processors for the partial-response maximum-likelihood (PRML) channel combined with the Reed–Solomon (RS) encoder/decoder are studied in perpendicular magnetic recording. The bit error rate (BER) performance is obtained by read/write experiments using a spin-stand with a perpendicular medium and single-pole-type/giant magnetoresistive (SPT/GMR) head. The experimental results show that post-processors improve linear densities of about 10–20kBPI in perpendicular magnetic recording.

A Study of Interactive Processing Between PRML Detection and Erasure Error Correction in Perpendicular Recording

In this paper, the interactive processing between a partial-response maximum-likelihood (PRML) channel and a decoder for the Reed-Solomon (RS) code is studied in perpendicular magnetic recording channel. Whereas in conventional systems a run-length-limited (RLL) encoder/decoder is insides the RS encoder/decoder, in this paper, we permute their positions so that the PRML channel and RS decoder can interact. Accordingly, the RS decoder can acquire reliability information from a PRML channel without interruption by the RLL decoder. It is demonstrated by computer simulation that the proposed system provides an SNR improvement of 0.8-1.1 dB compared with a conventional system at a linear density normalized by user bit interval, K, of 1.5.

PRML channel performance under the influence of medium noise in tape recording systems

Previous studies have shown that tape medium noise is dominated by surface roughness. Here, we use error rate analysis to evaluate the effects of surface roughness as well as particle size distributions in a partial-response maximum-likelihood (PRML) channel. We use: 1) a modified self-consistent model, incorporating a log-normal particle size distribution and a random packing of the particles to simulate the recording process and 2) a predeveloped error rate model to evaluate the system performance with fixed head/medium combinations. Surface roughness is introduced as a varying random head/tape spacing during both the recording and the playback processes. The impact of mean particle length, roughness variance, and roughness correlation length on system performance is examined for a partial response (PR4) channel. A surface roughness variance changing from 5 to 10 nm yields, at 200 kfci with a particle length of 60 nm, a change of error rate from 10/sup -10/ to 10/sup -5/. Because of the finite head read track width, roughness with shorter correlation lengths can be cross-track-averaged more readily and yields better system performance. We also found that larger particle size media have higher DC noise but lower roughness-induced transition noise. The shorter-particle-size media give better system performance, and the advantage increases with reducing roughness variance.

Performance analysis of modified PRML channels for perpendicular recording systems

We designed an optimal partial-response maximum-likelihood (PRML) channel for a high-density perpendicular magnetic recording system using a single-pole-type (SPT) recording head and a double-layered medium. The first key feature is a generalized PR (GPR) channel to achieve nearly optimal noise-whitening filtering for the desired recording channel. The second key feature is a class-III-based GPR (GPR3) channel with suppressed low-frequency components to provide a better match with a DC-distorted channel. We confirmed that for the media-noise-dominant channel, the new GPR3 channel improved the SNR by more than 1.8 dB compared to other types of PRML channels.

Partial Response Maximum Likelihood for Optical Data Storage Using Simple Optical Equalizers

The effects of introducing shading bands into the collector path of optical storage devices to equalize optically the signal to a partial response target have been investigated. A software simulation of the optical readout process and the partial-response maximum-likelihood (PRML) read channel have been developed and used to evaluate different channel configurations. It is shown that optical filtering can be used to eliminate the requirement for digital filtering in the PRML channel.

Off-track bit error rate modeling

Traditionally, channel generated bit error rates (BERs) at a certain signal to noise ratio (SNR) have been modeled by a Gaussian distribution function. This approach has been suitable for a peak detection channel as the analysis is performed in only one bit. In the case of a partial response maximum likelihood (PRML) channel, one data bit is determined by multiple code bits. There is a burst of errors detected if any one of the involved code bits gives an error. The chi-square distribution function, which is a sum of several independent Gaussian-squared distributions, is another method to model BERs of a PRML channel. Particularly, different PRML codes could be modeled by a chi-square distribution with different degrees of freedom. In this paper, the relationship of SNR to BER is generated using the chi-square method. The measured on-track rms SNR, read sensitivity function, and media magnetization are the input parameters. Old data and adjacent track interference are considered as random variables. At specified track pitch and read write misregistration, the probability distributions of these random variables are defined by the read sensitivity function and media magnetization. For a given interference, the off-track SNR is calculated. Error rate at a given interference is then derived from off-track SNR by using the SNR to BER relationship defined by a chi-square distribution. The BER at a specific track pitch and read–write misregistration is computed by integrating the error rate at a given interference with the probability distribution of the interference. This model describes the statistical nature of the BER in magnetoresistance head applications and provides very good correlation to experimental data.

10 Gbit/in.2 longitudinal media on a glass substrate (invited)

This article reports on the properties of the media prepared on glass substrates which were used in IBM’s 10 Gbit/in.2 demonstration. In order to support a linear density of 315 kbpi and a track density of 33 ktpi, the remanant coercivity Hcr and remanant moment thickness product Mrt of the magnetic layer were 3450 Oe and 0.37 memu/cm2, respectively. The media used a NiAl seed layer, a CrV underlayer, a Co alloy magnetic layer, and a carbon overcoat protection layer. The magnetic film had a grain size of 12 nm as observed by transmission electron microscopy. The preferred orientation (PO) of the magnetic layer was (101̄0). This PO enables one to sustain high coercivities at low values of Mrt. It is observed that the c-axis in-plane texture of the magnetic layer is critical to achieve a low noise medium. Using a focused-ion-beam (FIB) trimmed giant magnetoresistance head and conventional partial response maximum likelihood channel, the on-track-error rates were measured at the 10−10 level.

Rate 16/17 maximum transition run (3;11) code on an EEPRML channel with an error-correcting postprocessor

A rate 16/17 maximum transition run (MTR) (3;11) code on an EEPRML channel with a postprocessor (turbo-EEPRML) has been developed. The postprocessor compensates for the degradation in performance due to noise correlation. The proposed method has high performance and simple circuitry. Simulation results showed that the rate 16/17 MTR (3;11) code on a turbo-EEPRML channel has a performance gain of 1.0-4.0 dB over a conventional extended partial response maximum likelihood channel with the rate 16/17 run-length-limited code.

A 3-V, 10-100-MHz continuous-time seventh-order 0.05° equiripple linear phase filter

A 3-V continuous-time Gm-C filter based on a seventh-order 0.05/spl deg/ equiripple response is presented for read channels. The design features a low-voltage, wide-dynamic-range transconductor, a stable biasing scheme, and a power-efficient filter architecture. The filter is tunable for cutoff frequencies from 10 to 100 MHz and provides up to 13 dB of magnitude boost and up to /spl plusmn/30% of group delay adjustment for use in high-data-rate, extended partial-response maximum-likelihood channels. The filter is implemented in 0.29-/spl mu/m BiCMOS technology, occupies 0.5 mm/sup 2/, and dissipates 120 mW.

Sensitivity study of partial response maximum likelihood and peak detection channels to contamination buildup on air bearing surface of proximity recording sliders

This work investigates experimentally the sensitivity of partial response maximum likelihood (PRML) and peak detection channels on the increased head-disk spacing caused by the contamination buildup on the slider’s air bearing surfaces (ABS). Results obtained indicate that the peak detection channel is sensitive to both the Transition Effect (writing at increased head-disk spacing) and the Readout Resolution Effect (reading at increased head-disk spacing). The increased error rate detected in peak detection channel at increased head-disk spacing can be reduced significantly if the spacing is resumed to normal. In fact, the influence of the transition effect on the peak detection channel is more significant than the readout resolution effect. The PRML channel, however, is mostly sensitive to the transition effect only. The resolution effect, conversely, has rather limited impact on the PRML channel. Off-track sensitivity studies shows that peak shift in the peak detection channel becomes more sensitive to off-track positions when the head-disk spacing is increased. The nonlinear transition shift in the PRML channel, on the contrary, is far less sensitive to the off-track of magnetic head, even when the spacing is increased by the contamination buildup.

High-density recording performance of media with CoCrPtTa/Cr/NiP on glass ceramic substrate

The comparison of magnetic recording performance at high density between two kinds of CoCrPtTa/Cr/NiP thin films, one with Co(112̄0)//Cr(200) and another with Co(101̄1)//Cr(110) texture, on glass ceramic substrate is reported. The testing results have shown that the former films have lower transition noise, smaller partial erasure effect, and better offtrack capability in partial response maximum likelihood channel which also enhances the estimated areal density. The better performance of the films with Co(112̄0)//Cr(200) texture may be attributed to microstructure of Cr underlayer which results in bicrystal structure of magnetic layer and smaller Cr grain size.

A Study on Neural Network Equalization of PRML Channel with Nonlinear Distortion in MR head and Partial Erasure

An application of the neural network equalization to the partial response maximum-likelihood(PRML) channel with nonlinear distortion in magnetoresistive(MR) head and partial erasure is studied. First, a model of the recording/reproducing channel with the nonlinear distortion in MR head and partial erasure and the neural network equalizer are described. Then, the bit error rate(BER) performance for PRML channel is obtained by computer simulation and is compared with that of the conventional transversal equalizer. The results show that the BER performance of the neural network equalizer is better than that of the conventional transversal equalizer.

Distance spectra for PRML channels

Distance spectra for partial response maximum likelihood (PRML) channels are computed by finding generating functions for their error state diagrams. A matrix oriented technique for calculating the generating function is discussed that is reasonable for systems with a relatively small number of states. For those cases where the number of states is larger, a computer search algorithm is described which yields the first few terms of the generating function. Results are presented for channels that are of a form suitable for digital magnetic and digital optical recording. The effect of preceding is also considered.

1/4 " digital VCR PRML channel and servo processing

An analog partial response maximum likelihood (PRML) channel solution for 1/4 " helical scan VTR is presented. The main building blocks are the head amplifier, the channel IC containing an equalizer, an analog Viterbi detector, clock recovery, automatic track following (ATF) processing, and the corresponding servo and control system.

High areal density recording using an MR/inductive head and pre-embossed rigid magnetic disk

To determine whether a newly developed PERM (Pre-Embossed Rigid Magnetic) disk can be used with an MB/inductive head and a PRML (Partial-Response Maximum-Likelihood) channel, an experimental digital magnetic recording system was developed. This system used an external clocking scheme instead of a conventional self-clocking scheme. The track pitch was 5.2 /spl mu/m, and the track width was 3.6 /spl mu/m. Basic read-write characteristics, such as signal-to-noise ratio, frequency response, positive/negative symmetry or the readback pulse, were measured. An areal density of 605 kbit/mm/sup 2/ (390 Mbit/inch/sup 2/) was obtained at a bit error rate of 10/sup -6/. >

Advanced read channels for magnetic disk drives

Advanced read channels which sample the read signal and process those samples using algorithms borrowed from the communications industry allow us to reach higher storage densities and faster data rates than ever before. In this paper we describe some of the design issues involved in using a PRML (partial-response, maximum-likelihood) channel in a disk drive employing thin-film heads, zoned-recording, and embedded servo sectors. We discuss the effect of high transition densities required for a PRML channel on the design of heads and media. Finally, we survey some additional signal processing methods which might lead to even higher performance in the future. >