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Deep Neural Network Detection With an ITI Subtraction for Non-Uniform Track-Width Two-Dimensional Magnetic Recording
(2024-04-01)
Buajong, Chaiwat
;
Lee, Jaejin
;
Warisarn, Chanon
Continuously expanding the magnetic recording density results in unavoidable interferences including intersymbol interference (ISI) and intertrack interference (ITI) that both critically degrade the system performance. Even with advanced signal processing tools, two-dimensional magnetic recording (TDMR) still struggles to provide satisfactory performance. Thus, this article proposes the deep neural network (DNN)-based detection deployed in conjunction with an equalizer for the TDMR system. The coding scheme uses a low-density parity-check (LDPC) code, enabling the information exchange or the turbo decoding. The retrieval of data occurs within a group of three adjacent tracks. We also explore two different track configurations: uniform and non-uniform tracks that involve doubling the width of the middle track among the three adjacent tracks. The utilization of the highly reliable signal obtained from the double-width track enables the application of ITI subtraction technique, enhancing the information exchange. This technique can mitigate the ITI effect by subtracting the target signal with the imitated ITI signal. In addition, we investigate two different DNN architectures including the multilayer perceptron (MLP) and convolutional neural network (CNN), along with two scenarios for the detections in different passes of turbo decoding. The simulation results conducted on the Voronoi media model, with realistic grains and non-magnetic grain boundaries, show that the proposed detection systems with the non-uniform track configuration offer a performance gain up to 5.3 dB over the system with the uniform track configuration. Moreover, iteration for the turbo decoding passes incrementally improves the system performance in the proposed systems with the non-uniform track while the systems with the uniform track no longer provide performance gain as the number of iterations goes on.
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Recording media configurations for double-layered bit-patterned magnetic recording
(2024-01-01)
Buajong, Chaiwat
;
Warisarn, Chanon
The current magnetic recording technology is approaching its recording density limit. Recently, concepts focused on vertically stacking multiple recording media have gained momentum, driven by the potential to significantly expand recording density. The key challenge for such a system involves methods capable of efficiently separating data originating from the upper and lower layers within a single readback signal. To address this challenge, we first propose two distinct configurations of recording media in bit-patterned magnetic recording (BPMR) system with double recording layers. In both configurations, the centers of islands on the lower layer are positioned at the gaps between magnetic islands on the upper layer. This achieves the separation of the individual layer signals since the centers of islands in both layers are deliberately misaligned. Moreover, the equalizer design that takes the input data from both layers into consideration is also proposed. Simulation results demonstrate that when employing the same detector, the proposed configurations of the double-layered BPMR systems achieve better performance than the single-layered BPMR system.
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Multilayer perceptron-based detection for a coded two-dimensional magnetic recording
(2023-02-01)
Buajong, Chaiwat
;
Warisarn, Chanon
Recently, magnetic recording technology has been on the verge of reaching its recording density limit. Two-dimensional (2-D) magnetic recording (TDMR) is expected to be a candidate for next-generation magnetic recording because it provides powerful 2-D signal processing tools that are seamlessly compatible with the current technology. However, the degradation in signal quality due to 2-D interference when increasing the recording density far beyond the limit is inevitable even advanced signal processing tools cannot thoroughly handle such interference. In this work, we introduce two architectures of the multilayer perceptron (MLP)-based detection capable of producing soft information in the form of a log-likelihood ratio (LLR) for a coded TDMR system. This also enables turbo decoding capability to take place in the system, which further enhances the system performance. Such an architecture has two structures explicitly designed to execute before and during turbo decoding with a low-density parity-check code. In the first architecture, we employ triple MLP-based detection to process three readback sequences individually. For another architecture, it is designed to collectively process three readback sequences using just single MLP-based detection. The results show that both architectures of proposed detections outperform the systems using the conventional detection based on the Viterbi algorithm and single MLP-based detection achieves slightly better performance than triple MLP-based detection despite having much less complexity.
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Multilayer Perceptron-based Detector for a Coded Two-Dimensional Magnetic Recording
(2022-01-01)
Buajong, Chaiwat
;
Warisam, Chanon
Recently, magnetic recording technology is on the verge of reaching its recording density limit. Two-dimensional (2- D) magnetic recording (TDMR) becomes a candidate for next- generation magnetic recording because of its compatibility with the current technology and powerful 2-D signal processing tools. However, the degradation in signal quality due to 2-D interference is inevitable. In this paper, we introduce the multilayer perceptron (MLP)-based detector that can produce soft-output information in the form of a log-likelihood ratio (LLR). Such a detector can also execute turbo decoding by exchanging information with a low- density parity-check (LDPC) code. The simulation result shows that the proposed system outperforms the systems that use the conventional detector based on the Viterbi algorithm.
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An Intertrack Interference (ITI) subtraction scheme for bit-patterned media recording
(2021-05-19)
Buajong, Chaiwat
;
Warisarn, Chanon
;
Koonkarnkhai, Santi
;
Kovintavewat, Piya
Currently, hard disk drive has been struggled to overcome the super-paramagnetic limit that restricts the density increment. Bit-patterned magnetic recording (BPMR) is a candidate that can increase an areal density (AD) up to 4 Tb/in2. However, inter-track interference (ITI) arising from a narrow track width at high AD severely degrades the system performance. This study proposes the ITI subtraction technique with turbo iteration in a coded BPMR system. This method refines the equalized sequence by subtracting the ITI using an imitated ITI sequence that is generated by the soft information obtained from turbo iteration and ITI coefficients. The refined sequence that contains the partial ITI is sent to the turbo iteration as many rounds as needed. Simulation results indicate that the proposed system outperforms the conventional system whether or not media noise is considered.
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Optimal Array-Reader and Track Misregistration Mitigation Method in a Three-Reader/Four-Track Reading Bit-Patterned Magnetic Recording System
(2020-01-01)
Kankhunthod, Kittipon
;
Buajong, Chaiwat
;
Warisarn, Chanon
Track misregistration (TMR) is one of several problems in ultrahigh areal density bit-patterned magnetic recording (BPMR) systems, in which the distances between bit-islands are very narrow in both cross-track and down-track directions.Unfortunately,distance reduction leads to intersymbol interference and intertrack interference (ITI) effects,which cause system performance degradation. To deal with these effects, we develop the optimal position of an array-reader based on a three-reader/four-track reading BPMR system that is combined with a simple two-dimensional rate-3/4 modulation code and ITI subtraction technique,which delivers the best avoidance of the ITI effect from sidetracks.We model TMR estimation and mitigation methods using readback signals that are obtained from the optimal array-readers to improve bit error rate (BER) performance.The variance-ratio of these readback signals is used to estimating the TMR levels,whereas a pair of the target’s and equalizer’s coefficients,accordingly designed to the estimated TMR levels,is adopted for mitigating the TMR effect. Simulation results show that our methods provide a high TMR estimation accuracy and gain a superior BER performance compared with the recording system without these techniques under the same user density.
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Long-Short Term Memory-Based Application on Adaptive Cross-Platform Decoder for Bit Patterned Magnetic Recording
(2020-01-01)
Chantakit, Teanchai
;
Buajong, Chaiwat
;
Warisarn, Chanon
Dynamic bit encoding and decoding of the magnetic recording process remain a challenge in that the process is restrained by the balance between reading and writing performance of the decoder's bit error rate (BER). Sequential neural networks offer data streamflow for processes to reproduce recoded bits from signal distribution, overcoming the limitation of codeword mapping designed for each specific bit-patterned magnetic recording (BPMR) channel. Here, we implement the vanilla long short-term memory (LSTM) for adaptive modulation decoders in various BPMR channel designs within a single network, which benefits multi-channel decoder calibration tools with the same standardization. Signal information from media readback, a two-dimensional (2D) equalizer, 2D Viterbi, and a 2D soft-output Viterbi algorithm (SOVA) detector is arranged as a tensor that enables sequence-to-sequence bit prediction even with a highly complex data arrangement. Our adaptive model can predict recorded bits from readback with accuracies of approximately 97% for rate 4/5 decoding and 75% for crossing platforms, using a recently proposed single-reader/two-track reading (SRTR) system at an areal density of 4 Tb/in2 in a signal-to-noise ratio range of 1 to 8 dB. We conducted a BER simulation with the relevant results from conventional decoders and the LSTM model. Ultimately, our approach may demonstrate the limitation of supervised learning designed for BPMR systems and reveal a sequence data focus on LSTM that paves the way for sequential-type, unsupervised, mechanism-based, next-generation magnetic recordings.
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Modified Multitrack Joint Two-Dimensional Viterbi Detectors for Rate-4/5 and Rate-5/6 Modulation Codes in Bit-Patterned Magnetic Recording Systems
(2020-01-01)
Buajong, Chaiwat
;
Warisarn, Chanon
For an ultrahigh areal-density bit-patterned magnetic recording, system performance is severely degraded by intertrack interference (ITI). Rate-4/5 and rate-5/6 modulation codes were introduced to avoid data patterns that can cause severe ITI. However, their decoders might not be able to correctly perform for data patterns that do not match with their look-up tables during the decoding process. To further improve system performance, we use a multitrack joint two-dimensional (2-D) Viterbi detector for the recommended systems. We also use a modified trellis structure with lower complexity when compared to a conventional joint 2-D detector, which is designed based on their encoding conditions. Simulation results show that our systems outperform the conventional system, especially when they are investigated in a media-noise environment.
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Improvement in Bit Error Rate with a Combination of a Rate-3/4 Modulation Code and Intertrack Interference Subtraction for Array-Reader-Based Magnetic Recording
(2019-01-01)
Buajong, Chaiwat
;
Warisarn, Chanon
To improve the bit-error-rate (BER) performance of an ultrahigh-density bit-patterned magnetic recording (BPMR) system, the intersymbol interference and intertrack interference (ITI) effects resulting from the reduction of spacing between bit islands in both along-track and across-track directions must be efficiently handled. Array-reader-based BPMR is considered a key technology for future magnetic recording due to its advantages, such as its diversity gain against noise and ITI cancellation. This letter proposes a three-reader, four-track system where a simple rate-3/4 two-dimensional (2-D) modulation code and a new ITI subtraction technique are used to improve recording system performance. Moreover, a 2-D Viterbi detector is modified in accordance with the ITI subtraction scheme, which leads to lower complexity and better BER performance. Simulations demonstrate improvement in performance gains under position and size fluctuation effects.
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A simple inter-track interference subtraction technique in Bit-Patterned Media Recording (BPMR) systems
(2018-05-01)
Buajong, Chaiwat
;
Warisarn, Chanon
In this paper, we demonstrate how to subtract the intertrack interference (ITI) before the decoding process in multi-track multihead bit-patterned media recording (BPMR) system, which can obtain a better bit error rate (BER) performance. We focus on the three-track/threehead BPMR channel and propose the ITI subtraction technique that performs together with a rate-5/6 two dimensional (2D) modulation code. Since the coded system can provide the estimated recorded bit sequence with a high reliability rate for the center track. However, the upper and lower data sequences still be interfered with their sidetracks, which results to have a low reliability rate. Therefore, we propose to feedback the data from the center and upper tracks for subtracting the ITI effect of the lower track. Meanwhile, the feedback data from the center and lower tracks will be also used to subtract the ITI effect of the upper track. The use of our proposed technique can effectively reduce the severity of ITI effect which caused from the two sidetracks. The computer simulation results in the presence of position and size fluctuations show that the proposed system yields better BER performance than a conventional coded system, especially when an areal density (AD) is ultra high.

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