Bit-patterned Media Recording Systems Research Articles

Design of Non-Isolated Modulation Code With Minimum Hamming Distance of 3 for Bit-Patterned Media-Recording Systems

In bit-patterned media recording (BPMR), the magnetic islands are adjusted closer together to increase the areal density. This makes the bit-error rate (BER) performance of BPMR worse by inter-symbol interference (ISI) and inter-track interference (ITI) from cross-and down-track directions, respectively. To solve the ISI and ITI issues, the data is modulated into non-isolated patterns before storing on the medium of BPMR systems. Furthermore, for improving the non-isolated patterned modulation code, we introduced a method to create the non-isolated patterned code with a minimum Hamming distance (MHD) of 3 instead of 2 as in the previous studies. In our proposed method, firstly, we considered the generator matrix of the Hamming code to create one-dimensional codewords with an MHD of 3. Further, these codewords were converted into two-dimensional codewords, mitigating the isolated pattern. Finally, to create a fast encoder and decoder, we used the multi-layer perceptron to encode the original data into the non-isolated codewords with an MHD of 3. The results show that our proposed model improved the BER performance of the BPMR system.

Track Mis-Registration Estimator Based on K-Means Algorithm for Bit-Patterned Media Recording

Bit-patterned media recording (BPMR) has been considered among the key technologies to extend recording densities to 1 Tb/in2 and beyond. However, the BPMR system encounters the challenges of two-dimensional intersymbol interference and track mis-registration (TMR). In this article, we propose a TMR estimator based on the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$K$ </tex-math></inline-formula> -means algorithm for estimating the TMR levels and preventing the performance degradation of the BPMR system. The TMR estimator calculates the distances between the centroid of the readback signal and predetermined centroids according to the TMR level. The equalizer coefficients and partial response target are adjusted to the corresponding TMR level. At a bit error rate of 10−3 and a TMR of 10%, the proposed detection scheme performs better by approximately 1 dB in comparison with the conventional detection.

Modulation Decoding Based on K-Means Algorithm for Bit-Patterned Media Recording

In bit-patterned media recording (BPMR) systems, the readback signal is affected by neighboring islands that are characterized by intersymbol interference (ISI) and intertrack interference (ITI). Since increasing the areal density encourages the influence of ISI and ITI, it is more difficult to detect the data. Modulation coding can prevent the occurrence of specific data patterns that can cause severe ISI and ITI. In this study, we propose a modulation decoding method based on the K-means algorithm for a BPMR system to improve decoding capabilities. As the K-means algorithm helps understand data patterns and characteristics, the K-means decoder shows the best performance.

Bit-Flipping Scheme Using K-Means Algorithm for Bit-Patterned Media Recording

As the need for storing significant amounts of data has increased, an increased areal density (AD) of hard disk drives is required. Bit-patterned media recording (BPMR) with the ability to increase AD is regarded as one of the future magnetic storage systems. Reducing the distance between islands storing 1 bit can provide high AD, but it creates problems of intersymbol and intertrack interferences. In this study, we propose a bit-flipping scheme using the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$K$ </tex-math></inline-formula> -means algorithm for the BPMR system. In the proposed scheme, the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$K$ </tex-math></inline-formula> -means algorithm, which is an iterative clustering method for identifying similar samples and assigning them to clusters, is used to assign the samples that consist of the main and neighboring data to clusters. The predicted index from <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$K$ </tex-math></inline-formula> -means is utilized to compare and flip the sign of the main data when the sign is different. The proposed scheme flips the sign of data that is predicted to be an error and improves the performance using the neighboring information.

One-Dimensional Detection Using Interference Estimation by Multilayered Two-Dimensional General Partial Response Targets for Bit-Patterned Media Recording Systems

In bit-patterned media recording (BPMR) systems, two-dimensional (2D) interference, which includes intersymbol and intertrack interference from down- and across-track directions, respectively, increases when the distance between the islands is reduced with increasing areal density. In recent studies, a general partial response (GPR) target was often deployed with the equalizer to estimate and reduce interference. With this combination, the BPMR system becomes a promising candidate for high-density storage systems and achieves high bit error rate (BER) performance. Therefore, in this study, we investigate the detection capability while varying the form of the GPR target and implement a multilayered 2D GPR target and a detection method for improving the accuracy in estimating the interference. The simulation results demonstrate that the proposed model improves by a gain of ~1 dB compared to the previous studies.

Soft-Output Detector Using Multi-Layer Perceptron for Bit-Patterned Media Recording

As conventional data storage systems are faced with critical problems such as the superparamagnetic limit, bit-patterned media recording (BPMR) has received significant attention as a promising next-generation magnetic data storage system. However, the reduced spacing between islands at increased areal density causes severe intersymbol and intertrack interference, which degrade BPMR system performance. In this study, we introduce a soft-output detector using multi-layer perceptron to predict reliable information. A received signal is equalized and detected by the MLP detector. The MLP detector provides a well-estimated value by using the binary-cross entropy function as a loss function and the identity function as an activation function for the output layer of the MLP detector. This study investigates the received probability distributions out of the detectors and compares the performance of various versions against a conventional detector. Compared with the conventional detection, the proposed MLP detectors provide a small variance and better BER performance than the conventional detection. Simulations of MLP designs show an advantage over conventional detection. Moreover, the proposed MLP detectors with the demodulator exhibit better BER performance than the conventional detector with the demodulator.

Soft-information flipper based on long-short term memory networks for ultra-high density magnetic recording

Currently, researchers have been developing new ultra-high density magnetic recording technologies to meet the exponential growth of data storage demand. One of the main prospective technologies is bit-patterned media recording (BPMR) technology which is expected to upgrade the areal density (AD) up to 4.0 Terabit per square inch (Tb/in2). To achieve the expected high AD, the distance between each magnetic island in BPMR medium must; however, be reduced significantly, and it will enhance the two-dimensional (2-D) interference, namely inter-symbol interference (ISI) and inter-track interference (ITI). These two effects need to be probably handled to maintain overall system performance. Therefore, we propose a soft-information flipper based on long-short term memory (LSTM) networks combined with the rate-5/6 2-D modulation code in the coded three-track/three-head BPMR systems. In the proposed system, three soft-information sequences produced by the multiple 2-D soft-output Viterbi algorithms are employed as LSTM network inputs to generate the coded data sequences. During the supervised learning process, the known values of the coded data sequences are used as the targets at the output stage of LSTM network. The simulation results indicate that, at the same user density of 2.5 Tb/in2, the proposed system can provide bit-error-rate performance over both the soft-information flipping scheme based on a priori log-likelihood ratios summation and conventional uncoded systems. Moreover, the results also reveal that the proposed system is more robust to the media noise compared to other systems.

Constrained codes in single-reader/two-track reading bit-patterned magnetic recording

A reduction in track width of ultra-high density magnetic recording such as bit-patterned media recording (BPMR) results in an increase in areal density. However, the unfortunate appearance of extreme inter-symbol interference (ISI) and inter-track interference (ITI) can severely deteriorate the system performance. To deal with these problems, we propose to use a single-reader/two-track reading (SRTR) technique, where a single reader was employed to read two parallel-track at once and the bit islands were arranged in a staggered manner. Then, a received readback signal was sampled at an over-sampling rate, where an obtained data sequence can be operated using only a one-dimensional equalization and detection scheme. Moreover, we also present the simple constrained coding schemes consisting of the rate-3/5 and -4/6 constrained codes, where both are properly designed for an SRTR BPMR system by avoiding all data patterns that are more susceptible to ISI and ITI effects. Simulation results show that the severe ISI and ITI effects can be mitigated through the use of our proposed coding schemes. Consequently, the proposed systems are superior to the conventional system (without our constrained codes) at a same user density of 3.0 Tb/in2.

Iterative Signal Detection Scheme Using Multilayer Perceptron for a Bit-Patterned Media Recording System

Because of the physical and engineering problems of conventional magnetic data storage systems, bit-patterned media recording (BPMR) is expected to be a promising technology for extending the storage density to beyond 1 Tb /in2. To increase the storage density in BPMR systems, the separation between islands in both down- and cross-track directions must be reduced; this reduction results in two-dimensional interference from neighboring symbols in those directions, which is a major performance degradation factor in BPMR. Herein, we propose an iterative signal detection scheme between a Viterbi detector and a multilayer perceptron to improve the performance of a BPMR system. In the proposed signal detection scheme, we use the modified output of a multilayer perceptron as a priori information to improve equalization and extrinsic information to decrease the effect of intertrack interference.

Design of an advanced soft‐output Viterbi algorithm detector for bit‐patterned media recording systems

An advanced soft-output Viterbi algorithm detector aided by extrinsic information resources is proposed for bit-patterned media recording (BPMR) systems. These BPMR systems offer significant advantages over conventional magnetic recording systems. However, the performance system is significantly affected by a two-dimensional interference. The proposed method improves the channel detection output under the assistance of a variety of extrinsic information sources. Especially, combining the proposed model with an adaptive scheme for extrinsic information to enhance system performance is examined. Furthermore, evaluating the worth of extrinsic information in the different signal-to-noise ratio (SNR) conditions is also considered in this work. The results show that the SNR gains from the proposed model are about 4 dB compared to conventional solutions at non-misregistration.

LDPC 부호 구조에 따른 비트 패턴드 미디어 기록장치 시스템의 성능

차세대 자기 데이터 저장장치인 비트 패턴드 미디어 기록장치의 고밀도 성능을 달성하기 위해서는 아일랜드들 간의 간격이 줄어들어야 한다. 그러나 이러한 감소는 인접 심볼간 간섭 및 인접 트랙간 간섭의 영향을 증가시키며, 데이터 검출을 어렵게 만든다. 또한 데이터 저장장치 시스템은 다른 통신 시스템과는 달리 재전송이 불가능하기 때문에 데이터 저장장치의 디코딩 실패의 확률은 매우 낮아야 한다. 본 논문에서는 비트 패턴드 미디어 기록장치에서 세가지 LDPC 부호 방식에 대한 성능을 비교한다. 오류를 정정하고 데이터 저장장치의 성능을 향상시키기 위하여 반복적 디코딩을 사용한 LDPC 부호는 중요한 기술로 사용된다. 유사한 시스템 구조와 부호율의 사용을 통해 세가지 LDPC 부호 구조의 성능을 확인한다.

One-Dimensional Serial Detection Using New Two-Dimensional Partial Response Target Modeling for Bit-Patterned Media Recording

Bit-patterned media recording (BPMR) is a candidate technology that extends the density to up to 4 Tb/in2 to handle large amounts of data. However, in BPMR systems, the major challenge is two-dimensional (2-D) interference, which includes intersymbol interference from the down-track direction and intertrack interference from the across-track direction. To combat 2-D interference, it is natural to consider a 2-D Viterbi detection scheme. Since the complexity of pure 2-D Viterbi detection is quite high, we propose a series of two 1-D Viterbi detectors, which is based on new 2-D partial response (PR) target modeling for the BPMR channel. The use of new 2-D PR target model and the two serial 1-D Viterbi detection scheme ensures that the complexity is greatly reduced compared to 2-D Viterbi detection and the bit error rate performance is improved.

Multitrack Detection With Two-Dimensional Hybrid Equalizer for High-Density Bit-Patterned Media Recording

Bit-patterned media recording (BPMR) is one of the most promising candidates to further increase recording density beyond 1 Tb/in2. For high-density BPMR systems, severe intertrack interference (ITI) and media noise caused by decreased track pitch bring severe deterioration to the readback signals. To mitigate the impact of ITI and media noise on bit error rate (BER) performance, we developed a multitrack detection scheme with a two-dimensional (2-D) hybrid equalizer, which utilizes 2-D distinct equalizer coefficients corresponding to the different estimated ITI patterns. By analyzing the probability density distribution of the log likelihood ratio (LLR) output by the decoder, a LLR threshold is implemented to evaluate the reliability of estimated ITI patterns. Then, the 2-D variable equalizer trained with different ITI patterns is implemented for the reliably estimated ITI patterns; otherwise a 2-D fixed equalizer trained with a pseudorandom bit sequence is utilized. Hence, compared to the conventional 2-D fixed equalizer, the 2-D hybrid equalizer decreases the mean squared error of equalization with reduced error propagation risk. The simulation results indicate that the 2-D hybrid equalizer provides increased signal-to-noise ratio gains compared to the 2-D fixed equalizer when the media noise and track misregistration become more severe.

Error-Correcting 5/6 Modulation Code for Staggered Bit-Patterned Media Recording Systems

Bit-patterned media recording (BPMR) is a promising candidate for application in future magnetic recording systems. To achieve a high areal density in BPMR systems, the distance between bit islands is reduced. However, two-dimensional (2-D) interference may occur, including intersymbol interference in the down-track and intertrack interference in the cross track. The overall performance of BPMR is mainly limited by the emergence of 2-D interference in the readback signal. With the staggered-array layout, islands along the neighboring track are displaced by a half period from those on the main track. An island on the main track is primarily affected by the closest islands on the neighboring tracks. To overcome this problem, we introduce an error-correcting 5/6 modulation code that matches the structure of staggered-array bit-patterned media. The simulation results show that the system with the 5/6 modulation code achieves better performance.

Signal Detection Under Multipath Intersymbol Interference in Staggered Bit-Patterned Media Recording Systems

Bit-patterned media recording was invented with the aim of achieving high areal density. However, bit-patterned media recording suffers from two-dimensional (2-D) interference problems, including intersymbol interference in the down-track direction and intertrack interference in the cross-track direction. The 2-D interference corrupts the readback signal and degrades the performance of bit-patterned media recording. Unlike the layout of regular-array bit-patterned media, in the layout of staggered-array bit-patterned media, islands along neighboring tracks are displaced by a half-period from islands on the main data track; hence, in terms of intertrack interference, the islands on the main data track are predominantly affected by the closest four islands on neighboring tracks. To address this problem, a proper signal detection scheme for staggered bit-patterned media recording should be developed. In this letter, we introduce a multipath soft-output Viterbi algorithm for reducing the 2-D interference in staggered bit-patterned media recording. We investigate different partial response targets and compare the performances of the one-dimensional, two-dimensional, and proposed three-path soft-output Viterbi algorithm.

Reduced Complexity Multi-Track Joint Detector for Sidetrack Data Estimation in High Areal Density BPMR

For the high areal density bit patterned media recording (BPMR) systems, various multi-track equalization and detection techniques were proposed to mitigate the inter-track interference (ITI) so as to recover recorded data from the detecting tracks efficiently, but most works rarely aim to recover the recorded data on the sidetracks concurrently. As anticipation of that the readback signal has the intense contributions from the sidetracks due to high areal density, in this paper, we attempt to estimate the recorded data from the sidetracks as well as the detecting tracks by a multi-track joint detector that processes multi-track readback signals jointly in a single detector. To avoid the high complexity, which is the main shortfall in realizing the multi-track joint detector, we propose to employ a multi-track joint detector with a reduced complexity trellis for multi-head multi-track systems, in particular, four-head four-track (4H4T) BPMR system. To reduce the number of states in the trellis, the signals are equalized to 2-D generalized partial response targets whose cornered ITI coefficients are set to zero. The parallel branches are also incorporated in the trellis to estimate the data on the sidetracks. Moreover, we propose a two-head four-track (2H4T) system which generates the estimates of the recorded data on the four tracks by processing the signals from only two tracks. According to the simulation results, the proposed detector performs well on the data estimation of the detecting tracks as well as the sidetracks in both 4H4T and 2H4T systems.

Twin Iterative Detection for Bit-Patterned Media Recording Systems

Bit-patterned media recording (BPMR) has been developed into a technology for the next generation of hard disc drives, which can push area density beyond 1 Tb/in2 for satisfying the ever increasing demand of storing massive amounts of data. One of the major challenges in BPMR systems is 2-D interference that is regarded as a gist of degrading the system performance. From a signal processing viewpoint, 2-D optimal detection schemes are very complicated for 2-D interference channels. In this paper, we propose a twin iterative detection scheme based on 1-D detectors and decoders in order to reduce the complexity. The performance of the proposed scheme is compared to that of a conventional simplified 2-D iterative detection scheme, and the results indicate that the proposed model is better by approximately 1 dB under the effect of 10% track misregistration.

Mitigation of TMR Using Energy Ratio and Bit-Flipping Techniques in Multitrack Multihead BPMR Systems

Track misregistration (TMR) in ultra-high density bit-patterned media recording (BPMR) is one of the crucial problems, because it can severely degrade the overall system performance. In practical, TMR can be detected and adjusted by a servo control loop system. However, this paper proposes to utilize multiple readback signals obtained from the optimized positioning of the two side read head closer to the main read head to improve the TMR prediction process in a multitrack multi-head BPMR system with position jitter noise. In addition, we also propose the soft-information exchange and the bit-flipping techniques for the multitrack data detection, so as to improve the bit-error rate (BER) performance of all three data tracks simultaneously. Simulation results indicate that the proposed system is superior to the conventional system, especially, when the amount of TMR and position jitter is high. Furthermore, we also found that the upper and lower read heads, which are moved closer to the center track by 25% of a track pitch, will provide the best BER performance with and without position jitter noise.

Single-Track equalization method with TMR correction system based on cross correlation functions for a patterned media recording system

Bit-patterned media recording (BPMR) is a promising technology for ultra-high density media, however, there are some challenges that need to be addressed including two-dimensional (2D) interference, and track mis-registration (TMR). The system can experience TMR due to misalignment of the head and the track center. Conventionally, TMR is tackled using a servo system in which the head position offset is estimated by processing the overhead servo sequences before reading the data sequences. However, TMR impairment can also occur when the head is reading data sequences that are beyond the servo mechanism. To address this problem, we proposed TMR detection and correction techniques based on our previous work involving a single-track equalization method for a BPMR system using cross-correlation functions between the single readback signal and each of the training sequences from three adjacent tracks. In this proposed technique, the presence and level of TMR is detected and estimated based on the changes in the value of mean square error (MSE) between the equalized and feedback signals from the detectors after passing through a one-dimensional (1D) target for each sequence. Then, the estimated TMR levels are used in selecting the appropriate equalizer and the generalized partial response (GPR) target pair to tackle the TMR from the readback signal. The simulation results show improvement in the data recovery of a BPMR system using the proposed method when the system is experiencing 2D interference and TMR impairment.

Elimination of two‐dimensional intersymbol interference through the use of a 9/12 two‐dimensional modulation code

This study presents a 9/12 two-dimensional (2D) modulation code as a way to overcome 2D intersymbol interference (ISI) in high-density storage systems. The next generation of data-storage systems is being continually developed to satisfy a massive demand for reliable storage regarding enormous amounts of data. Holographic data storage, bit-patterned media recording (BPMR), and 2D magnetic recording are promising candidates for the attainment of area-density increases that are beyond the capacities of conventional storage systems. One of the main challenges for these systems is a 2D ISI problem consisting of 1D ISI from neighbour bits and intertrack interference from adjacent tracks. The proposed modulation code maps every 9 bit sequence of user data into a 2D output array of a 3-by-4 size so that the fatal 2D ISI patterns are avoided in every output array. For the assessment of the quality of the proposed modulation code, a simulation model is carried out in a BPMR system. The results show that the proposed modulation code offers a gain of ∼2 dB over that of a system without encoding. In particular, a gain of ∼1 dB is obtained over that of a 6/8 modulation code regarding the same code rate.