Inter-track Interference Effect Research Articles

An improvement of BER using two-dimensional constrained code for array-reader-based magnetic recording

To achieve ultra-high areal densities in future magnetic recording technologies such as bit-patterned magnetic recording (BPMR) systems, each bit-island must be closely moved. However, the reduction of bit island spacing always leads to both inter-symbol interference (ISI) and inter-track interference (ITI), which can degenerate system performance. Previously, a one-dimensional (1D) run-length limited (RLL) code was employed to efficiently avoid the ISI effect, while a two-dimensional (2D) detector is widely utilized to deal with the ITI effect as well. However, its complexity is higher than the traditional 1D detector. Therefore, to avoid the fatal data patterns that lead to severe ITI effect, we first present the application of 1D RLL code to be as the 2D modulation code in the multi-head multi-track BPMR systems. Moreover, we also present the modified 2D Viterbi detector that is designed based on our proposed encoding condition. The numbers of state and branch in the traditional trellis’s structure are properly reduced according to encoding condition, thus it does not only provide a lower complexity but also operate better than the traditional 2D Viterbi detector. Results reveal that our proposed coding scheme and modified 2D Viterbi detector can efficiently protect and cope with the ITI effect.

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.

Design of 4/6 ITI-mitigating error correction modulation code for ultra-high-density bit-patterned media recording systems

A design of 4/6 intertrack-interference (ITI)-mitigating error correction modulation code for bit-patterned media recording systems is proposed in this Letter. Generally, coding schemes in data storage devices are usually accomplished using a combination of error-correction coding and modulation coding. However, an interesting approach is that the modulation code itself has the capability of correcting errors. Such a modulation code design will reduce the total size and complexity of the data storage devices. In this Letter, the authors design a new 4/6 modulation code, where the code not only mitigates the effect of ITI but also maintains minimum Hamming distance of 2 to build up a trellis structure providing error correction capability of the proposed code. Simulation results show that the proposed design outperforms prior arts. For instance, the performance of the proposed design is better than that of a conventional 4/6 modulation code about 8 dB at a bit error rate of 10 −7 under 20% read-head offtrack for the area density of 4 Tb/in 2 .

Soft-information flipping scheme based on a priori LLRs summation for ultra-high density magnetic recording

Inter-track interference (ITI) is the main problem in high areal density (AD) of magnetic recording systems such as bit-patterned magnetic recording (BPMR) technology thus the ITI avoidance scheme is the way to improve the performance of the recording system. Two-dimensional (2D) modulation codes i.e., the rate-5/6 2D modulation code, was proposed to avoid the severe ITI effect which can successfully improve the system performance when it was considered without the codeword border effect. However, when we have to consider the codeword border effect, some upper and lower tracks cannot avoid ITI because both of them still be interfered from their sidetracks. To improve this weakness, a rate-5/6 2D modulation code was combined with the bit-flipping technique based on the normalization criteria of the two-dimensional soft-output Viterbi algorithm (2D-SOVA). Although, the bit-flipping technique can improve the performance of all tracks; however, the gain optimization is difficult to describe the relationship of the soft-information flipping technique. Therefore, this research proposes a novel of soft-information flipping scheme to describe the bit-flipping technique based on a priori log-likelihood ratios of the rate-5/6 encoding constraint in BPMR system. The simulation results clearly indicate that, at the same user density, the proposed system can provide bit-error-rate performance gain over the other recording systems in both of with and without position jitter noise.

Optimal Array-Reader and Track Misregistration Mitigation Method in a Three-Reader/Four-Track Reading Bit-Patterned Magnetic Recording System

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.

A Novel ITI Suppression Technique for Coded Dual-Track Dual-Head Bit-Patterned Magnetic Recording Systems

Generally, an intertrack interference (ITI) is a critical problem in bit-patterned magnetic recording (BPMR) systems that can attain an areal density (AD) up to 4 Tb/in <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . Unavoidably, at high ADs, a very narrow track width must be employed, leading to severe ITI and unacceptable system performance. To tackle the ITI; therefore, this article introduces a novel ITI suppression technique for coded dual-track dual-head (DTDH) BPMR systems. At the first turbo iteration, the weighted readback signal of the adjacent track served as an estimated ITI signal is utilized for subtracting from the target readback signal to subside the ITI effect, before passing the refined readback signal to a turbo equalizer. Nonetheless, for the second turbo iteration onwards, the estimated ITI signal generated by the soft information obtained from a decoder at each turbo iteration will be then employed to subtract from the target readback signal during the turbo decoding process. Computer simulation results demonstrate that the proposed system can provide better performance than the DTDH system using a hard ITI suppression technique as well as the conventional system using one read head to decode one data track for all ADs, because the proposed technique can estimate the ITI signal well. Furthermore, when considering the recording system under the effects of media noise and track mis-registration, we also found that the proposed system is more robust than other systems.

A simple 2D modulation code in single-reader two-track reading BPMR systems

A reduction in the track width of magnetic recording systems results in a welcome increase in Areal Density (AD), but can severely deteriorate system performance in the unfortunate appearance of extreme Inter-Track Interference (ITI). The effect of severe ITI may be mitigated by using coding schemes. In this paper, therefore, we present a rate-5/6 2-Dimensional (2D) modulation code based on a proposed Single-Reader/Two-track Reading (SRTR) technique to cope with this serious problem in staggered Bit-Patterned Magnetic Recording (BPMR) systems. We then evaluate the Bit-Error Rate (BER) performance of the proposed system in the presence of media noises, e.g., position and size fluctuations. Our simulation results indicate that, at the same User Density (UD), the proposed system performs better than an uncoded system by about 1.0 ​dB ​at the BER of 10−5 and is also superior to the conventional recording system.

An Iterative Two-Head Two-Track Detection Method for Staggered Bit-Patterned Magnetic Recording Systems

One of the major problems in bit-patterned magnetic recording (BPMR) systems is an inter-track interference (ITI) that can significantly degenerate the system performance, especially at high areal densities (ADs). Generally, the BPMR system with staggered islands experiences less ITI effect than that with regular islands. Therefore, we propose an iterative two-head two-track detection method for a staggered BPMR system to further reduce the ITI effect, which employs two read heads to read two adjacent tracks and decodes them simultaneously. To lessen the ITI, the weighted readback signal of one track, serving as an ITI signal, is subtracted from the readback signal of the other track, and vice versa, before passing the refined readback signal to an iterative decoding process. Hence, the decoded data will be utilized to estimate the ITI signal for the next global iteration, $N_{\mathrm{ G}}$ . Supposing that the refined readback signal has only one-side ITI effect. We also present the design of the $2{\times }3$ target and its compatible equalizer that will be employed in the proposed system. Results demonstrate that the proposed system not only is superior to the conventional system but also has low complexity. Specifically, the proposed system can perform close to the conventional system whose readback signal encounters only one-side ITI effect when $N_{\mathrm{ G}} = 2$ , 3 and 5 iterations for $\text {AD} = 2$ , 2.5, and 3 Tb/in2, respectively.

Mitigating the Effects of Track Mis-Registration in Single-Reader/Two-Track Reading BPMR Systems

Bit-recording technologies that can provide areal densities (ADs) [patterned magnetic recording (BPMR) is expected to be one of the new magnetic ADs] of 1 Tera-bit per square inch or higher. To achieve these ADs, the space between tracks must be reduced, bringing them closer and closer together, leading to significant inter-track interference (ITI). Track mis-registration (TMR) or head offset can further degrade overall system performance. To mitigate the effects of ITI and TMR, we propose using a single-reader/two-track reading (SRTR) scheme together with an over-sampling technique on a staggered BPMR system. We also propose a TMR estimation technique based on a readback signal. Here, the readback signal is separated into two sequences, one odd and one even. Then, the energy ratio is calculated using these data sequences. The obtained relationship between the energy ratio and head offset can be utilized to predict the actual head offset occurrence in the reading process. Finally, a pairing of a monic constraint target and an equalizer that accordingly matches each estimated head offset level is adopted to deal with the TMR effect. The SRTR system can provide better bit-error-rate (BER) performance. Moreover, when TMR effects appear in the reading process, our proposed SRTR system with a TMR mitigation technique can yield better BER performance, especially at high head offset levels.

A Soft-5/6 Modulation Code With Iterative ITI Subtraction Scheme in Multireader TDMR Systems

The huge growth of the consumer electronic industry is driving the need for achieving ultrahigh areal storage densities. 2-D magnetic recording is a promising high-density storage technology where the shingled writing technique is employed for increasing the track per inch (TPI) and achieving higher areal densities. Nevertheless, high TPI causes a reader to cover higher number of tracks, which leads to the intertrack interference (ITI); hence the system performance is severely deteriorated. Therefore, we first propose the soft-5/6 modulation code together with iterative ITI subtraction scheme. Here, the soft information encoding and decoding schemes operate based on Boolean logic circuit for using in the iterative system. Furthermore, we also propose to move the upper and lower readers closer to the center reader to avoid the ITI effect from their outer tracks, which will then be performed together with ITI subtraction scheme. Moreover, this proposed reading technique is also applied together with the soft-4/5 modulation code. The simulation results show that the proposed systems are superior to the conventional system, especially at high user density.

Multihead Multitrack Detection for Next Generation Magnetic Recording, Part II: Complexity Reduction — Algorithms and Performance Analysis

To achieve large storage capacity on magnetic hard disk drives, very high track density is required, causing severe intertrack interference (ITI). Multihead multitrack (MHMT) detection has been proposed to better combat the effects of ITI. Such detection, however, has prohibitive implementation complexity. Reduced-state sequence estimation (RSSE) is a promising technique for significantly reducing the complexity, while retaining good performance. In this paper, several different MHMT models are considered, including symmetric and asymmetric 2H2T systems, and a symmetric 3H3T system. By carefully evaluating the effective distance between two input symbols, we propose optimized set partition trees for each channel model. Different trellis configurations for RSSE are constructed based on the desired performance/complexity tradeoff. Simulation results show that the reduced MHMT detector can achieve near maximum-likelihood (ML) performance with a small fraction of the original number of trellis states. We also use error event analysis to explain the behavior of RSSE. The proposed algorithm could be potentially applied to next generation magnetic recording systems, especially when the ML detector is infeasible due to the high computational complexity.

Concatenated Codes for Combating Effects of Intertrack Interference in Shingled Magnetic Recording

We introduce a novel concatenated code that targets errors generated by intertrack interference in shingled magnetic recording. The code has fewer overhead than the conventional product codes. We give a detailed presentation in the code construction and encoding method. We also present a novel decoding scheme for the concatenated code that has very low memory requirements.

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.

An ITI-Mitigating 5/6 Modulation Code for Bit-Patterned Media Recording

In bit-patterned media recording (BPMR), the readback signal is severely corrupted by the inter-symbol interference (ISI) and inter-track interference (ITI), especially at high recording densities, due to small bit and track pitches. One way to alleviate the ITI effect is to encode an input data sequence before recording, so as to avoid some data patterns that easily cause an error at the data detection process. This paper proposes an ITI-mitigating 5/6 modulation code for a multi-track multi-head BPMR system to eliminate the data patterns that lead to severe ITI. Specifically, each of the 5 user bits is converted into a 6-bit codeword in the form of a 3-by-2 data array, based on a look-up table. Experimental results indicate that the system with the proposed coding scheme outperforms that without coding, especially when an areal density is high and/or the position jitter is large.

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.

Influence of Writing ITI Effects in Shingled Magnetic Recording

In this paper, the influence of the intertrack interference (ITI) at writing in shingled magnetic recording (SMR) as well as at reading is examined using a read/write model based on recording on a medium composed of discrete Voronoi diagram by a triangular writer with trailing and one-side shields. Two kinds of ITI's at writing which affect the magnetic grains on the track currently being written and the previously recorded track are shown. One is that a part of signal on the previously recorded track remains on the current track due to lack of writing ability of writer. Another is that a part of signal is erased by the next-track writing. The effects are caused from the adjacent tracks at writing. Additionally the ITI at reading deteriorates the read-back waveform. Consequently these ITI's bring the decoding errors. The results by computer simulation using above R/W model show that the decoding errors occur in the low density parts of track which were originally considered to be hard to make errors.

A Parametric Study of Inter-Track Interference in Bit Patterned Media Recording

We present a parametric study of inter-track interference (ITI) in the context of bit patterned media (BPM) recording channels. Bit error rate (BER) simulation results for optimal bit detection at moderate-to-high signal-to-noise-ratio (SNR) show that, in a certain range of ITI levels, increased ITI does not necessarily degrade performance. This observation applies to channels both with and without intersymbol interference (ISI) as well as in the absence and presence of track misregistration (TMR). In the case of no ISI, an exact analysis of the BER performance of optimal bit detection provides a complete explanation of the observed effect of ITI. For channels with ISI, error event analysis of a joint-track maximum-likelihood sequence detector provides insight into the observed impact of varying levels of ITI on BER performance.

An Inter-Track Interference Mitigation Technique Using Partial ITI Estimation in Patterned Media Storage

The readback signal of patterned media storage (PMS) is corrupted by a two-dimensional interference consisting of inter-symbol interference (ISI) and inter-track interference (ITI). In this paper, we apply the turbo equalization to mitigate the ITI effects. The codeword of the outer code is divided and then written on three separate tracks. After the first iteration, the bit estimates from the outer code provide partial ITI estimates for the branch metric computation in the soft-output viterbi algorithm (SOVA) detectors. Unlike previous works, the number of states of the channel trellis is due to the ISI only since the ITI information is not embedded in the parallel branches. When compared with the modified SOVA detectors with multi-parallel branches, simulation results show that a significant performance gain is achieved by our proposed method.

Modifying Viterbi Algorithm to Mitigate Intertrack Interference in Bit-Patterned Media

Bit-patterned media are currently being investigated to obtain magnetic recording densities of 1 Tbit/in <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and more. In bit-patterned media, the adjacent tracks may be as close as adjacent bits in a track resulting in significant intertrack interference (ITI). In this work, we propose a modified trellis for the Viterbi algorithm (VA) that can mitigate the effects of ITI. Simulation results show that the modified VA improves the bit error rate. Using a simple model for track misregistration (TMR), the performance of this modified VA in the presence of TMR is also investigated. Based on the simulation results, this modified VA also appears to be more tolerant to TMR than the conventional VA

Soft-Error-Event Decoding for Multitrack Magnetic Recording Channels

Multiple-head array systems, which allow reading and writing of data simultaneously on multiple tracks, can potentially provide both high density and high speed, but the readback signal in such systems is corrupted by intertrack interference (ITI) in addition to intersymbol interference (ISI). In this paper, we report on the performance of a multitrack system and propose a low-complexity multitrack soft-error-event decoder (SEED) to counter the effects of ISI and ITI. The multitrack SEED is based on a high-rate Bose-Chaudhuri-Hocquenghem code, and is capable of correcting most frequent error events produced by the channel. Used in conjunction with a joint partial response equalizer followed by an ITI equalizer, the SEED scheme provides a gain of 0.5-2 dB at a bit-error rate of 10/sup -4/ relative to an uncoded system.