Perpendicular Recording Channels Research Articles

Reducing the Thermal Asperity Effect in Perpendicular Magnetic Recording System

In this paper, we propose a new method to estimate and reduce effectively thermal asperity (TA) in the perpendicular recording channels with state trellis. The proposed method comprises of recursively applying a previous noise estimate to a calculation of a current noise estimate to eliminate the effects of TA from the equalized signal. With partial response a maximum-likelihood detector base on partial-response (PR) target with DC-full component can improve the system performance by more than 50% when TA occurs. Unlike previous studies of TA cancellation or suppression, this method is different because we dont use PR targets with non-dc components and high pass filter.

Effect of Bandpass Filters for TA Suppression in Perpendicular Recording System

Thermal asperities (TAs) cause a crucial problem in magnetic recording systems because they can distort the readback signal to the extent of causing possible sector read failure. This problem becomes even more severe in perpendicular recording channels because these channels contain a d.c. component. This paper presents a novel TA suppression method by use of a bandpass ¯lter to mitigate the TA effect. We also investigate the effect of different bandpass filters for TA suppression. Results indicate that the proposed TA suppression method using a bandpass filter polynomial G(D) = (1+D)2(1-D) yields lower bit-error rate than the existing ones at the expense of increased complexity, and is also robust to large peak TA amplitudes.

Nonbinary LDPC Codes for 4-kB Sectors

The performance of nonbinary LDPC codes on perpendicular magnetic recording channels (PMRCs) with 4-kB sectors is investigated. Each sector is encoded by either 4- or 0.5-kB-long codes. Both quasi-cyclic (QC) and nonquasi-cyclic (non-QC) nonbinary low-density parity-check (LDPC) codes are designed, and a set of simulations in a mixture of 10% additive white Gaussian noise (AWGN) and 90% position jitter noise power, with and without turbo equalization and with a rereading technique, is presented. The results show that sector-long codes perform better than 0.5-kB codes for all decoding schemes considered. In addition, the erasure correction capabilities of these nonbinary LDPC codes are calculated under both maximum-likelihood decoding and iterative decoding, and, as expected, longer codes can correct longer erasures. QC and non-QC nonbinary LDPC codes exhibit similar performance and have almost the same erasure correction capability under iterative decoding. Because of their efficient encoding, QC nonbinary LDPC codes are a good choice for perpendicular recording channels with 4-kB sectors, and offer a tradeoff between performance and code length.

Ordered Statistics Decoding of Linear Block Codes Over Intersymbol Interference Channels

In this paper, we utilize ordered statistics decoding (OSD) techniques for intersymbol interference (ISI) channels. To achieve good efficiency and storage complexity, a combination of a modified generalized Viterbi algorithm (GVA) and Battail's algorithm is proposed to implement OSD for ISI channels. Simulation results showed significant performance improvement for the [128,64,22] eBCH code, and the [255,239,17] Reed-Solomon (RS) binary image, on the (1+D) <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> perpendicular recording channel.

The Read Channel

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> In this paper, we provide a survey of the novel read channel technologies that found their implementation in products over the past decade, and we outline possible technology directions for the future of read channels. Recently, magnetic recording read channels have undergone several changes. In addition to switching from longitudinal to perpendicular recording channels, detectors tuned to media noise sources are now readily implemented in read channel chips. Powerful numerical techniques have emerged to evaluate the capacity of the magnetic recording channel. Further, improved coding/decoding methods have surfaced both for the incumbent Reed–Solomon codes and the promising low-density parity-check codes. The paper is a tutorial-like survey of these emerging technologies with the aim to propel the reader to the forefront of research and development in the areas of signal processing and coding for magnetic recording channels. </para>

Read channel simulation based on microtrack and micromagnetic modeling for 1 Tb/in 2 magnetic recording

Channel design for ultra-high-density perpendicular magnetic recording requires fast and precise modeling to generate readback signals corrupted by media noise. In this paper, we present a geometric-dependent approach to model random readback pulses, where a three-dimensional head and media combination for 1 Tb/in 2 density design is considered in the micromagnetic simulation. A systematic approach is developed to extract media noise statistics from micromagnetic modeling and generate readback pulses based on a fast microtrack model incorporating nonlinear effects. The feasibility of the proposed model is demonstrated through bit-error-rate (BER) simulation of a turbo equalization scheme over a low-density parity-check (LDPC) coded, general-partial-response (GPR) equalized perpendicular recording channel.

Performance Evaluation of Partial Response Targets for Perpendicular Recording Using Field Programmable Gate Arrays

We present the results of performance evaluation of partial response (PR) targets for perpendicular recording by simulation using a field programmable gate array (FPGA). Previous research proposed several PR targets for perpendicular recording channels. We evaluated the bit error rate (BER) performance of those PR targets. Different PR targets led to not only different BERs for given signal-to-noise ratios (SNRs), but also different slopes of BER versus SNR curves. The result can be explained in terms of error event distributions of the corresponding PR targets. In the FPGA simulation, additive white Gaussian noise was used, but no jitter noise was introduced. The soft output Viterbi algorithm is used as the channel detector. A rate-8/9 low-density parity check code with code length 4923 and column weight 3 is used as the error correcting code. Two channel iterations are applied. We attain BERs as low as 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-12</sup> . Our results also show that the BER behavior of the PR4 target is significantly different from that of the [1 2 2 1] target

A MAP-Based Algorithm for Joint Estimation of Transition Jitter and Timing Error

To improve the robustness to timing errors in future high-density perpendicular recording channels where the dominant noise source is transition jitter, this paper proposes a scheme that jointly estimates transition jitter and timing error along with the data recovery. This estimation is based on the intuition that timing error is independent from transition jitter, while both can be quantized and characterized in an extended trellis together with the bit sequence. Simulation results demonstrate significant performance improvement using this scheme, where the cycle-slip dominant bit error rate (BER) is lowered to the same order of magnitude as the detection performance in the absence of timing errors

A New Class of Error-Pattern-Correcting Codes Capable of Handling Multiple Error Occurrences

A new class of high-rate error-pattern-correcting cyclic codes that correct most single occurrences of target error patterns and a significant portion of their multiple occurrences is proposed. This code is based on first designing a generator polynomial of the lowest possible degree that is tailored to any single occurrence of target error patterns. The generator polynomial is then multiplied by a primitive polynomial so that a cyclic code based on a higher degree generator polynomial can handle all single occurrences of the target error patterns, as well as some highly probable multiple occurrences, using the captured syndrome and reliability measure. A performance comparison is provided for a jitter-dominant perpendicular recording channel

Evaluation of Low-Density Parity-Check Codes on Perpendicular Magnetic Recording Model

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> Low-density parity-check (LDPC) codes have shown superior error-correcting performance in a variety of data storage system studies, including traditional longitudinal magnetic recording systems. However, perpendicular magnetic recording systems (of increasing interest) exhibit impairments different from longitudinal magnetic recording systems, and thus present new challenges for error-correcting codes. In this effort, we evaluate a structured LDPC code using a perpendicular magnetic recording channel model that includes impairments such as transition noise, nonlinear transition shift, transition percolation, and baseline wander (BLW). The channel model, as well as the LDPC encoder and the decoder are implemented in field-programmable gate array (FPGA) hardware. The LDPC coded system is evaluated down to bit error rate (BER) of 10<formula formulatype="inline"> <tex>$^{-12}$</tex></formula> and frame error rate (FER) of 10<formula formulatype="inline"> <tex>$^{-8}$</tex></formula>. The impact of individual impairments on coding performance is studied separately. The soft output Viterbi algorithm (SOVA) <formula formulatype="inline"><tex>$+$</tex></formula> LDPC system maintains its superior error-correcting performance under the perpendicular recording channel. </para>

On the Concatenation of Soft Inner Code With Reed–Solomon Code for Perpendicular Magnetic Recording

We evaluate the performance of coding system consisting of inner code and Reed-Solomon (RS) code. Three types of inner codes are considered: single-parity check (SPC) code, multiparity check code, and interleaved single-parity check (ISPC) code. The evaluation is performed by simulation on a perpendicular recording channel model. Evaluation methods include direct counting of bit errors and sector errors, and semianalytic method based on block multinomial model. The performance of the coding system under detectable and undetectable media defects is also studied

Optimal detection for perpendicular recording channels with transition noise

This paper proposes a jitter-sensitive-sequence-detection scheme based on a realistic transition noise model for high-density perpendicular magnetic recording channels. In this scheme, the transition jitter is assumed to have one of the Q discrete values, and the discretized jitter sequence is estimated jointly with the recorded bit sequence using maximum a posteriori criterion. The detection performance is compared with that of the pattern dependent noise predictive scheme based on an autoregressive Gaussian noise model. Simulation results show that the scheme provides a significant gain when the transition jitter is large and the resulting transition noise dominates over the electronic noise.

Tensor-product Parity codes: combination with constrained codes and application to perpendicular recording

A parity code and a distance enhancing constrained code are often concatenated with a Reed-Solomon code to form a coding system for magnetic recording. The tensor-product parity coding scheme helps to improve efficiency of the parity code while retaining the same level of performance. In this paper, we present two methods for combining a tensor-product parity code with a distance-enhancing constrained code. The first method incorporates a constrained code with unconstrained positions. The second method uses a new technique, which we call word-set partitioning, to achieve a higher code rate relative to the first method. We simulate the performance of several coding systems based upon the two combination methods on a perpendicular recording channel, and we compare their symbol error rates and sector error rates with those of a system that uses only a Reed-Solomon code.

The combined constraints for perpendicular recording channels

The maximum transition run (MTR) and running digital sum (RDS) constraints have a wide variety of applications in magnetic recording systems. The property of an MTR constraint is to eliminate long runs of transitions so that certain common error events can be eliminated. The property of an RDS constraint is desirable for achieving high-density recording for recording channels exhibiting low frequency components, such as perpendicular recording channels. It is very common to implement more than one constraint in practice to achieve the desired effect in the recording channel. In this paper, the labeled graph representations of RDS and MTR [or the time-varying MTR (TMTR)] constraints are combined to obtain new classes of constraints: RDS-MTR and RDS-TMTR constraints. The capacity of a combined constraint is slightly lower than that of its constituent constraints. The spectra of RDS-MTR and RDS-TMTR constraints show the suppression of low frequency contents compared to the conventional MTR and TMTR constraints. A method for designing block codes is proposed to implement the properties of RDS, MTR (or TMTR), the run-length-limited (RLL) (0,k), and twin constraints. The cardinalities of block codes are maximized over various situations of codeword boundaries. The code design methodology is demonstrated via a rate 100/108 RDS-TMTR block code based on a rate 20/21 RDS-TMTR mother code.

Joint design of optimum partial response target and equalizer for recording channels with jitter noise

We address joint design of optimum generalized partial response (GPR) target and equalizer for perpendicular recording channels with jitter noise. We develop a new cost function which accounts for the data-dependent nature of jitter noise based on the minimum mean square error (MMSE) criterion. Using the step-response-based channel model, we derive expressions for the statistics required to compute the optimum equalizer and target in the presence of jitter noise. We also derive a bit-response-based model for the jitter noise channel. We present an approach for doing simulations as well as analytical computations for the jitter noise channel without resorting to the widely used Taylor series approximations. Our computational and simulation results show that, while the targets designed without accounting for the jitter lead to error-floor effect in the bit-error-rate performance, the targets designed by our approach give significant performance improvement under high jitter conditions, with no sign of error-floor effect for the range of signal-to-noise ratios considered.

Thermal asperity suppression in perpendicular recording channels

In this paper, we present an approach for detection and suppression of thermal asperity (TA) induced signal distortions in perpendicular recording channels. When the MR read head comes in contact with the disk surface, the resulting thermal transient causes a change in the resistance of the MR read element. This change in the resistance manifests as an additive voltage transient in the readback signal. Typically, a TA signal has a short rise time (60-150 ns) with a longer decay time (1-5 /spl mu/s). The peak TA amplitude can be as large as 250%. If precautions are not taken, such transients could cause sector read failure. As the flying height continues to decrease and as the recording density continues to increase, the adverse effects of TA will become even more serious in future drives. Thus, it is particularly important to develop means to handle TA effectively in perpendicular recording channels. The study shows that the proposed method is robust with respect to the changes of the peak TA amplitude and to the mismatch between estimated and actual TA durations. Thus, in conclusion, we have developed an easily implementable and effective TA detection and mitigation approach for perpendicular recording channels.

Signal processing for perpendicular recording channels with intertrack interference

Signal processing techniques for perpendicular magnetic recording channels with intertrack interference (ITI) are studied in this paper. Both single-track and joint-track equalization and detection algorithms are considered. Modified minimum mean-square error equalization methods are used to select optimum generalized partial response targets for ITI channels with either dominant additive white Gaussian noise or media noise. Simulation results show that good targets for single-track detection of ITI-free channels may not be acceptable for channels with ITI. Short targets are just slightly worse than longer targets for joint-track detection, making it possible its use in practical systems with ITI.

Evaluation of detection algorithms for perpendicular recording channels with intertrack interference

Channel detection algorithms for handling intertrack interference (ITI) in perpendicular magnetic recording channels are studied in this paper. The goal is to optimize channel detection to attain the best possible performance for a practical system. Two channel detection models, namely, the single-track model and the joint-track model are evaluated using information rate analysis as well as simulations. Numerical results show that joint-track detection may be needed when ITI is severe.

Nested Codes for Perpendicular Recording Channels

The performance of nested error-correction codes for perpendicular magnetic recording channels is evaluated in the presence of burst noise using Reed-Solomon codes as well as low-density parity-check codes as component codes. Erasure detection and iterative decoding algorithms for nested codes are considered, and their performance is evaluated using computer simulations. The results illustrate the advantages of the nested schemes over noninterleaved architectures in terms of sector error rate and complexity for both current and future native block sizes.

Baseline Wander Compensation for the Perpendicular Magnetic Recording Channel

Baseline wander compensation allows the effective use of low-frequency signal components in a perpendicular recording channel. In this paper, simple baseline wander models are presented, the associated compensation algorithms are developed, and their performances are formulated. The effect of loop delay on the compensation algorithms is also addressed. Simulation results quantify the performances of the proposed compensation algorithms.