Moulay Rachid Elidrissi

Channel Models and Detectors for Two-Dimensional Magnetic Recording

Two-dimensional magnetic recording (TDMR) is a novel recording architecture intended to support densities beyond those of conventional recording systems. The gains from TDMR come primarily from more powerful coding and signal processing algorithms that allow the bits to be packed more tightly on the disk, and yet be retrieved with acceptable error rates. In this paper, we present some preliminary results for an advanced channel model based on micromagnetic simulations, coined the Grain Flipping Probability model. This model requires a one-time computationally complex characterization phase, but subsequently provides fast and accurate two-dimensional (2-D) readback waveforms that include effects captured from micromagnetic simulations and the statistical effects derived from the granularity of the recording medium. We also show the performance of several detectors over a pre-existing TDMR channel model directly as a function of channel density rather than the signal-to-noise ratio (SNR).

Modeling of 2-D Magnetic Recording and a Comparison of Data Detection Schemes

Two-dimensional magnetic recording (TDMR) together with shingled magnetic recording (SMR) are technologies proposed to extend the life of conventional granular magnetic recording. The grain flipping probability (GFP) model has been proposed to mimic the performance of micromagnetic ( μ-mag) simulations for the purpose of signal reproduction. Other work in TDMR includes the proposal of a Gaussian mixture model (GMM) that produces improved likelihood information at the output of the detector, combined with low density parity check (LDPC) codes. The contribution of this paper is threefold. First, we aim to simulate a TDMR/SMR recording system with the GFP model, both with and without the GMM detector, and with various random and structured LDPC codes, of both 4 k and 16 k block lengths, to determine areal densities that might be achieved. Second, we perform a comparison of the various model order reduced (MOR) GFP implementations to compare the effect of writing with various factors taken out of the picture. Third, we perform a validation of the GFP model and the setup as a whole, by running the system with a parameter set close to that of conventional recording. The results of these experiments give an assurance of the validity of our model, give an indication of the expected density that might be achieved in a TDMR/SMR system, and give a direction for which parameter(s) in magnetic recording systems might be optimized to yield the most gain.

A Study of SMR/TDMR With a Double/Triple Reader Head Array and Conventional Read Channels

Two-dimensional magnetic recording is a novel technology proposed to extend the life of conventional granular magnetic recording (CGMR). It is the readback counterpart technology to shingled magnetic recording (SMR) that is being pursued by the industry today. In SMR, a 2-D block of bits is written as a single unit: modifying any bit within the block requires a modification to the entire 2-D block. In TDMR, by comparison, a 2-D block is read and processed as a single unit. Latency during writing of a 2-D block is not a serious problem as the data is buffered in the RAM. Latency during readback, however, results in a performance degradation to the user who has to wait before getting his file. If there is a single reader head, 2-D readback necessitates multiple revolutions of the disk leading to a significant latency, which may not be acceptable to the user and is one of the reasons the industry is hesitating for 2-D magnetic recording (TDMR). Recent events have seen companies working on the development of multiple (two or three) reader heads that read multiple tracks simultaneously with a single pass of the head. Such double/triple reader head arrays could be used to support TDMR, obviating the need for multiple revolutions of the media and reducing the read latency. An alternative use for the multireader heads would be in an SMR scenario, where only a single track is readback at a time. In this case, the extra readers could be used to estimate and mitigate the intertrack interference coming from the adjacent tracks. The difference between these two modes is the number of tracks detected with each head pass. For TDMR there would be two or three tracks detected with a single pass while for SMR there would be just one. In this paper, we study and optimize the impact of parameters of the double/triple reader head array being used in the SMR and TDMR readback scenarios. The key difference between these scenarios is in the design of the GPR equalizer and we compare and contrast the recording system varying several key design parameters.

Analysis and design of shingled magnetic recording systems

Shingled magnetic recording (SMR) is an upcoming technology that will extend the life of conventional granular magnetic recording (CGMR). SMR differs from conventional recording in that the tracks are written in a raster scan format, in one direction only, leaving tracks that are overlapped like the shingles on a roof. This simple change means that adjacent track overwrite only occurs from one side, and tracks need to survive this overwrite only once. In contrast, conventional recording needs to survive thousands of overwrites from both sides. This work performs analysis of SMR from three perspectives. First, an analysis of how much gain one might expect for SMR based on the assumptions for the magnetic write width (MWW), magnetic read width (MRW), and erase bandwidth (EBW) is performed. Second, this analysis is corroborated via simulated 747 curves using the grain flipping probability (GFP) model. The third part validates the 747 curves from the model with results from the spinstand.

Optimal Design of MAMR and HAMR by Applying Response Surface Methodology

In this paper, we systematically perform optimal design of microwave-assisted magnetic recording (MAMR) and heat-assisted magnetic recording (HAMR) by applying response surface methodology (RSM) to optimize the signal-to-noise ratio (SNR) of the recording process. We evaluate the performance of design of MAMR and HAMR through micromagnetic modeling. Design of experiments (DoE) is conducted by using composite central design (CCD), and the influence of the design parameters on the SNR is investigated. The optimal designs of MAMR and HAMR systems are obtained by solving for the best-fit quadratic response-surface to the sampled points and solving for the parameters that maximize this cost function.

Skew angle effects in shingled magnetic recording system with double/triple reader head array

Shingled Magnetic Recording (SMR) is a scheme used to extend the life of the current perpendicular magnetic recording technology. SMR enables writing narrow tracks with a wide writer. Currently, SMR employs a single reader and will suffer inter-track interference (ITI) as the tracks become comparable in width to the reader. ITI can be mitigated by using narrower readers; however, narrower readers suffer from increased reader noise. Another approach to combat ITI is to process 2D readback and use ITI cancellation schemes to retrieve the data track. Multiple readbacks can be obtained either with a single reader and multiple revolutions or with a reader array. The former suffers from increased readback latency. In this work, we focus on the latter. When using a reader array, the skew angle poses major challenges. During writing, there is increased adjacent track erasure, and during readback the effective reader pitch varies and there is an increase in the 2D intersymbol interference caused by the rotated rea...

Channel Characterization and Performance Evaluation of Bit-Patterned Media

Bit-patterned media (BPM) is a promising approach to push back the onset of the superparamagnetic limit faced by conventional continuous granular media. Today, BPM islands can be fabricated at densities higher than can be characterized by existing methods and full bit-patterned media recording (BPMR) is still a long way off. In this work, we rely on simulations to predict how such islands would perform in a real recording scenario. The grain flipping probability (GFP) model is trained via micromagnetic simulations and reproduces the magnetic profiles used to generate readback signals for channel simulations. The geometrical parameters to the micromagnetic simulations, such as the island size variations and island position jitter are characterized from measurements of islands fabricated via e-beam at various channel densities.

Recording Performance and Comparison of Graded-

Heat-assisted magnetic recording (HAMR) signal-to-noise ratio results are presented based on Landau-Lifshitz-Bloch recording simulations using a double-layer composite Tc media. An exchanged-coupled-composite Ku media is also considered in a HAMR system. Mean Tc and σTc are varied in conditions of high and low interlayer exchange coupling, using material parameters consistent with FePt. Results suggest that using a composite Tc media may potentially regain ~1-2 dB from a single layer HAMR system with dB loss of ~3 dB, varying cure temperature distributions up to 6%. However, some of the benefits are due to creating conditions of writing above Tc. A significant performance degradation is found to occur if grains are not heated strictly above the Curie temperature, in both single and double-layer media. And use of a Ku composite media beyond deploying a composite Tc media shows insignificant gain in the recording performance.

{T_{c}}

Hard disk drive (HDD) systems today have embedded servo wedges spanning the revolution of the disk that periodically provide the head with crosstrack position information as it flies over the servo wedges. The head is subjected to forces such as from turbulence, disk-waviness, shock, and vibration that cause its position to deviate from its desired on-track position. It is the job of the servo controller to use the information in the position error signal (PES), calculated from the readback signal as the head flies over the servo wedges, to reposition the head back on-track. There are a couple of disadvantages of the embedded servo system in todays HDDs. The first is that the overhead of the servo wedges consumes some of the areal density reducing the amount of storage capacity for the users data. Today, that overhead is in the range of 3%-5% of the total disk surface. The second disadvantage is that between servo wedges, the servo control loop has no position information and the head is flying blind. In this paper, we propose an alternative method for the servo controller to obtain the PES it needs to position the head on-track without the need for servo wedges. Our method assumes the usage of a multiple-reader head that, as of today, seems to be a likely upgrade to HDD systems in view of the benefits promised by shingled magnetic recording and 2-D magnetic recording.

and -

Perpendicular magnetic recording (PMR) in the hard disk drive is approaching its physical limits. In an earlier work, the dedicated servo (DS) recording system has been proposed to provide continuous position error signal for servo, enable higher servo sampling rate, and improve the overall servo performance. A further benefit is that the DS layer results in surface area savings at the data layer. However, it was also reported that the embedded servo layer introduces baseline variation and non-linear transition shift (NLTS) to the readback signal of the data layer. In this paper, we propose novel signal processing techniques to improve the bit error rate (BER) in DS recording. The synchronous averaging technique is proposed to improve the BER in the presence of baseline variation distortions. Further, the servo and data-dependent noise prediction method is proposed to mitigate the effect of the NLTS. Through the use of these techniques the linear density loss from the conventional PMR media is reduced.