Chanon Warisarn

A constructive inter-track interference coding scheme for bit-patterned media recording system

The inter-track interference (ITI) can severely degrade the system performance of bit-patterned media recording (BPMR). One way to alleviate the ITI effect is to encode an input data sequence before recording to avoid some data patterns that easily cause an error at the data detection process. This paper proposes a constructive ITI (CITI) coding scheme for a multi-track multi-head BPMR system to eliminate the data patterns that lead to severe ITI. Numerical results indicate that the system with CITI coding outperforms that without CITI coding, especially when an areal density (AD) is high and/or the position jitter is large. Specifically, for the system without position jitter at bit-error rate of 10−4, the proposed scheme can provide about 3 dB gain at the AD of 2.5 Tb/in.2 over the system without CITI coding.

An iterative inter-track interference mitigation method for two-dimensional magnetic recording systems

At high recording density, the readback signal of two-dimensional magnetic recording is inevitably corrupted by the two-dimensional (2D) interference consisting of inter-symbol interference and inter-track interference (ITI), which can significantly degrade the overall system performance. This paper proposes an iterative ITI mitigation method using three modified 2D soft-output Viterbi algorithm (2D-SOVA) detectors in conjunction with an iterative processing technique to combat the 2D interference. The codeword of the outer code is divided and then written on three separate tracks. For every iteration, all 2D-SOVA detectors exchange the soft information to improve the reliability of the a priori information and use it in the branch metric calculation, before feeding the refined soft information to the outer decoder. Simulation results show that the proposed method outperforms the conventional receiver and the existing partial ITI mitigation method.

A Rate-8/9 2-D Modulation Code for Bit-Patterned Media Recording

The 2-D interference consisting of intersymbol and intertrack interferences is a major impairment in bit-patterned media recording (BPMR), especially at high-areal density (AD). One solution to mitigate the effect of 2-D interference is to apply a 2-D coding scheme on an input data sequence before recording so as to avoid some data patterns that easily cause an error at the data detection process. Nonetheless, this method usually requires many redundant bits, thus lowering a code rate. This paper proposes a rate-8/9 modulation code for a multitrack multihead BPMR system to eliminate the data patterns that lead to severe 2-D interference. Simulation results indicate that the system with the proposed code is superior to that without coding, especially when the AD is high and/or the position jitter is large. Specifically, for the system without position jitter at bit-error rate of 10-5, the proposed system can provide 1.8 dB gain over the system without coding at the AD of 2.5 Tb/in2.

A two-dimensional coding design for staggered islands bit-patterned media recording

This paper proposes a two dimensional (2D) staggered recorded-bit patterning (SRBP) coding scheme for staggered array bit-patterned media recording channel to alleviate the severe 2D interference, which requires no redundant bits at the expense of increased an additional memories. Specifically, a data sequence is first split into three tracks. Then, each data track is circularly shifted to find the best data pattern based on a look-up table before recording such that the shifted data tracks cause the lowest 2D interference in the readback signal. Simulation results indicate that the system with our proposed SRBP scheme outperforms that without any 2D coding, especially when an areal density (AD) is high and/or the position jitter is large. Specifically, for the system without position jitter at bit-error rate of 10−4, the proposed scheme can provide about 1.8 and 2.3 dB gains at the AD of 2.5 and 3.0 Tb/in.2, respectively.

Utilization of multiple read heads for TMR prediction and correction in bit-patterned media recording

This paper proposes a utilization of multiple read heads to predict and correct a track mis-registration (TMR) in bit-patterned media recording (BPMR) based on the readback signals. We propose to use the signal energy ratio between the upper and lower tracks from multiple read heads to estimate the TMR level. Then, a pair of two-dimensional (2D) target and its corresponding 2D equalizer associated with the estimated TMR will be chosen to correct the TMR in the data detection process. Numerical results show that the proposed system can achieve a very high accuracy of TMR prediction, thus performing better than the conventional system, especially when TMR is severe.

Reduced complexity of multi-track joint 2-D Viterbi detectors for bit-patterned media recording channel

Two-dimensional (2-D) interference is one of the prominent challenges in ultra-high density recording system such as bit patterned media recording (BPMR). The multi-track joint 2-D detection technique with the help of the array-head reading can tackle this problem effectively by jointly processing the multiple readback signals from the adjacent tracks. Moreover, it can robustly alleviate the impairments due to track mis-registration (TMR) and media noise. However, the computational complexity of such detectors is normally too high and hard to implement in a reality, even for a few multiple tracks. Therefore, in this paper, we mainly focus on reducing the complexity of multi-track joint 2-D Viterbi detector without paying a large penalty in terms of the performance. We propose a simplified multi-track joint 2-D Viterbi detector with a manageable complexity level for the BPMR’s multi-track multi-head (MTMH) system. In the proposed method, the complexity of detector’s trellis is reduced with the help of the ...

Effect of hotspot position fluctuation to writing capability in heated-dot magnetic recording

This work presents the effect of hotspot position fluctuation to writing capability in heated-dot magnetic recording systems at an areal density (AD) beyond 2 Tbpsi via a micromagnetic modeling. At high ADs, the hotspot and the write field gradient may not be correctly focused on the target island because the bit islands are closely positioned to one another. This may lead to the overwriting/erasing of the previously written islands, which can severely affect the recording performance. Therefore, this work studies the 3-by-3 data patterns that easily cause an error when the hotspot and write head positions are fluctuated by various island pitches. Simulation results indicate that the data pattern that leads to the highest/lowest error occurrence frequency is the one with the first, second and fourth islands having the opposite/same magnetization direction to/as the write field, regardless of the magnetization direction of the third island. This result can, for example, be utilized to design a two-dimensional modulation code to prevent such destructive data patterns, thus helping enhance the overall system performance.

An Intertrack Interference Subtraction Scheme for a Rate-4/5 Modulation Code for Two-Dimensional Magnetic Recording

The performance of a rate-4/5 modulation code is evaluated in two-dimensional magnetic recording (TDMR) channels, where a magnetic medium is described by a discrete Voronoi model, and the two-dimensional sensitivity function of the reader is adopted to generate the TDMR readback signal. Since the read-head sensitivity function covers many tracks, it causes intertrack interference (ITI) that can deteriorate the system performance. Therefore, this letter proposes an ITI subtraction scheme in conjunction with the rate-4/5 modulation code in a coded TDMR channel to mitigate the ITI effect embedded in the readback signals before performing an iterative decoding process. We also investigate the data-dependent readback amplitude distributions and evaluate the TDMR system performance via computer simulation. Results show that the proposed scheme helps improve the TDMR system performance, especially when the areal density is high.

Spinning disk test study on erase band and write width for shingled magnetic recording

Shingled magnetic recording (SMR) has been predicted as one of the novel technologies to extend the areal density beyond 1 Tb/in2. It was widely thought by many researchers that very wide writers could be used for SMR, as it involves corner writing and thus writability would no longer be an issue in SMR. However, in our experimental study, we reveal that there exists an optimal write width for a given writer design due to the erase band limitations. Also we propose the optimal write width for a given shingle track pitch condition.

Reduced-complexity modified per-survivor iterative timing recovery using M-algorithm for magnetic recording system

A modified per-survivor iterative timing recovery (MPS-ITR), which jointly performs timing recovery, equalization, and error-correction decoding, has been proposed in [1] to deal with the problem of timing recovery operating at low signal-to-noise ratio. Practically, this scheme exploits a split-preamble strategy in conjunction with a per-survivor soft-output Viterbi algorithm equalizer to make it more robust against severe timing jitters or cycle slips. Although the MPS-ITR outperforms other iterative timing recovery schemes [1], it still has very high complexity. In this paper, we propose a reduced-complexity MPS-ITR scheme (denoted as MPS-ITR-M) to make it more implementable in real-life applications. This is achieved by applying the M-algorithm [2] to the MPS-ITR. Numerical results indicate that at low-to-moderate complexity, the MPS-ITR-M will perform better than other schemes.