Yibin Ng

Modeling and Two-Dimensional Equalization for Bit-Patterned Media Channels With Media Noise

In this paper, we first propose a simple but general analytical channel model for bit-patterned media (BPM) systems. It facilitates the generation of accurate readback signals with a large amount of media noise, as well as the analytical design of equalizer and channel partial response (PR) target. We further propose an analytical approach to jointly design the two-dimensional (2-D) equalizer with one-dimensional (1-D) channel PR target for BPM systems with media noise, according to the minimum mean square error (MMSE) criterion with monic constraint. Simulation results show that the proposed 2-D equalizer with 1-D target based approach achieves significant performance gain over the system designed without considering media noise.

Picket-Shift Codes for Bit-Patterned Media Recording With Insertion/Deletion Errors

We report the results of an investigation of bit-patterned media recording (BPMR) channels containing insertion/deletion errors that may be introduced because of the write synchronization problem. We first describe a simple channel model for insertion/deletion errors in BPM, as a result of write clock frequency offset. Based on this channel model, we propose a new error correction coding (ECC) scheme to correct insertion/deletion errors-picket shift. Simulation results show that picket shift performs significantly better than a no-ECC scheme in the presence of insertion/deletion errors.

Modeling, detection, and LDPC codes for bit-patterned media recording

In this paper, we present a thorough and comprehensive study for bit-patterned media recording (BPMR), from a signal processing and coding perspective. We first propose a recording-physics-based generic channel model for BPMR, which includes all the major characteristics and impairments of the system. It also provides a fair basis for the performance comparison of different coding and detection schemes. We further propose various channel algorithms and techniques for BPMR, including a two-dimensional (2D) equalization scheme with one-dimensional (1D) generalized partial response (GPR) target to mitigate inter-track interference (ITI) and media noise, a maximum a posteriori (MAP) detector for BPMR with write errors, various low-density parity-check (LDPC) codes, as well as the iterative detection and decoding schemes. The corresponding performance gains are illustrated at 4Tb/in2.

Dedicated Servo Recording System and Performance Evaluation

The perpendicular magnetic recording (PMR) in hard disk drives is approaching its physical limitation. The emerging technologies, such as heat assisted magnetic recording and microwave assisted magnetic recording have been proposed to record on magnetic media with thermally stable smaller size grains at higher areal density (AD). However, in the media fabrication, achieving well-isolated small size of grains is more challenging than obtaining high Ku material as recording media. Reducing the number of grains per bit is a major path for keeping AD growth of PMR in recent years. To minimize the SNR penalty at a smaller grain number per bit, pushing more on track density is the right approach. With the 2-D magnetic recording (TDMR) readers for inter-track interference cancellation, the off-track read capability is improved significantly for allowing a narrower track read. In the drive working environment, when the external vibration or other mechanical disturbance happens during the writing process, it creates more track squeeze at adjacent tracks and leaves a very narrow track at some locations of the track. When the track width is narrower than the squeeze to death width in the 747 curve, it causes hard failure in the channel. To solve the track squeeze problem, this paper proposes to add an additional magnetic recording layer in between the data recording layer and the soft underlayer of conventional PMR media. This additional recording layer is used to record servo information only. The continuous positioning error signal is able to improve the servo performance and to provide the real-time monitoring of the positioning error. When it is under bad servo conditions, the writing process can be stopped to avoid nontolerable track squeeze. The continuous servo signals are designed to be of moderate intensity at very low frequency, and its impact on data signal has been minimized. The linear density gap between the dedicated servo media and the conventional PMR media is able to be controlled within 3%. As the dedicated servo system keeps only around 100 wedges of track ID and sector ID at the data layer, the surface area saving at the data layer can break even in capacity. The dedicated servo technology together with TDMR readers is the key technology to achieve ultrahigh track density during both writing and reading processes.

Channel Modeling and Equalizer Design for Staggered Islands Bit-Patterned Media Recording

In this paper, we first present a recording physics based analytical channel model for bit-patterned media recording (BPMR) systems with staggered islands configuration. We further propose an analytical approach to jointly design the equalizer with partial response (PR) target for BPMR systems, by using the minimum mean-squared error (MMSE) criterion with monic constraint, taking into account inter-track interference (ITI) and media noise. Simulation results show that the proposed approach performs better than the system designed without considering ITI and media noise. We further investigate the performance of staggered array against regular array islands with bit-aspect ratios (BAR) of 1 and 2, and with different amount of media noise, inter-symbol interference (ISI), and ITI. We found that staggered array islands with BAR of 2 offer better bit error rate (BER) performance and better tolerance to media noise.

Bi-Directional Pattern-Dependent Noise Prediction for Heat-Assisted Magnetic Recording With High Jitter Noise

In this paper, we first report the results of an investigation of perpendicular heat-assisted magnetic recording (HAMR) channels containing high jitter noise that is likely to be the case at very high areal densities (e.g., 4 Tb/in2). To model the HAMR channel, we use the thermal Williams-Comstock model and the microtrack model to derive the transition response, without large thermal spot approximation. Further, we propose a novel bi-directional pattern-dependent noise prediction (biPDNP) detector to improve the performance of the HAMR channel under high jitter noise conditions. The biPDNP detector utilizes backward linear prediction in the noise prediction process, as well as the conventional forward linear prediction. At bit error rate of 10-3, biPDNP detector offers 0.6-1.4 dB performance gain over the conventional PDNP detector.

Performance of perpendicular magnetic recording with track squeeze using bidirectional pattern-dependent noise prediction detector

In this study, we investigated the performance of perpendicular magnetic recording with squeezed tracks. Based on spinstand signals, we developed a signal generation module to generate the readback signals for tracks of various squeezed track width and bit length. To mitigate the drop in bit error rate (BER) as squeezed track width decreases, we implemented the bidirectional pattern-dependent noise prediction (BiPDNP) detector with low-density parity check codes. To improve the BiPDNP detector over earlier work, we modified the log-likelihood ratio computation. Our results show that our proposed BiPDNP detector improves the bit error rate, and allows smaller squeezed track width and smaller bit length for the same BER performance.

Signal Processing for Dedicated Servo Recording System

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.

Bidirectional Pattern-Dependent Noise Prediction With LDPC Codes for HAMR

As areal densities increase, substantial jitter noise is expected in heat-assisted magnetic recording (HAMR). To mitigate the effects of jitter noise, in an earlier work we proposed the bidirectional pattern-dependent noise prediction (BiPDNP) detector, which employs backward linear prediction as well as the conventional forward linear prediction. However, no error correction codes were used in this earlier work. In this paper, we implement BiPDNP in the Bahl-Cocke-Jelinek-Raviv (BCJR) detector, and investigate its performance with low-density parity check (LDPC) codes. For the LDPC coded channel, by combining the BCJR detector with BiPDNP, we observe that a SNR gain of 1 dB (at bit error rate 10

Iterative detection and decoding for non-binary LDPC coded partial-response channels with written-in errors

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