Weijun Tan

Signal-to-noise ratio mismatch for low-density parity-check coded magnetic recording channels

Signal-to-noise ratio (SNR) mismatch is found in simulations to have great influence on the performance of low-density parity-check coded magnetic recording channels. While an inappropriate SNR mismatch degrades the performance dramatically, a properly selected optimum SNR mismatch can improve it significantly. In this paper we analyze the causes of this phenomenon and find optimum SNR mismatch values for specific magnetic recording systems with physical impairments such as electronic and media noise as well as erasures, using both density evolution analysis and Monte Carlo simulations. We observed that two characteristics of the probability density function (pdf) of the channel message, namely, the Gaussianity and the variance-to-mean ratio (VMR) have a major effect on the SNR mismatch. Generally speaking, if the channel message is approximately Gaussian-distributed and the VMR is larger than two, a negative SNR mismatch substantially improves the system performance. Numerical results show that for a magnetic recording channel with additive white Gaussian noise (AWGN), the optimum SNR mismatch is about -3 to -2 dB, while for a channel with 10% AWGN and 90% media noise, is about -10 to -8 dB, whether erasures are present or not.

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.

Detection of media defects in perpendicular magnetic recording channels

Two new algorithms for detection of media defects in perpendicular magnetic recording channels are proposed. These algorithms are based on amplitude thresholding of the channel reliability information. The performance of these algorithms is evaluated for low-density parity-check coded channels using generalized partial response targets with different dc-components.

Erasure detection algorithms for magnetic recording channels

The effects of precoding on the detection of erasures based on violations of run-length-limited (RLL) constraints for low-density parity-check (LDPC) coded magnetic recording channels are investigated and an improved erasure detection algorithm is proposed. Its performance on an ME/sup 2/PR4 channel is evaluated using computer simulations.

Array codes for erasure correction in magnetic recording channels

A type of quasi-cyclic binary low-density parity-check (LDPC) codes based on array codes, which have low encoding complexity and good performance, are considered for use in magnetic recording channels. Their error correction capability, for both electronic and media noise, and more importantly, for erasures, is investigated using simulations. Compared with random binary and q-ary LDPC codes, these codes offer a good tradeoff between performance and complexity.

Analyzing low-density parity-check codes on partial response channels with erasures using density evolution

We have developed a numerical technique based on density evolution to analyze low-density parity-check (LDPC) codes on partial response channels with erasures. We computed the average bit-error rates and signal-to-noise ratio thresholds and used them as criteria to evaluate the asymptotic performance of an ensemble of LDPC codes. We defined and estimated two important erasure parameters, namely, the fading depth threshold for erasure insertion and the erasure recovery capability of LDPC codes. We found that, at least asymptotically, LDPC codes are well suited for handling erasures in magnetic recording.

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.

Bounds for low-density parity-check codes over partial-response channels

Summary form only given. The application of low-density parity-check (LDPC) codes to magnetic recording systems motivates the search for analytical ways to evaluate system performance over partial-response (PR) channels. In this paper a technique proposed by Duman and Kurtas (IEEE Trans. Inform. Theory, vol. 47, pp. 1203-1205, 2001) is extended to bound the frame error rate (FER) of high-rate LDPC codes. We circumvent the requirement that the weight distribution of the underlying code must be known by using estimates (A. Ashikhmin et al, ibid., vol. 47, pp. 1050-1061, 2001). This new bounding technique is used to compute the bounds for geometrically constructed LDPC codes over PR channels. Simulations are used to evaluate the computed bounds.

Advanced channel detection and iterative decoding for perpendicular recording

While a great deal of research on perpendicular recording physics (media and head materials and designs) has been carried out, the focus has been on building the best possible physical channel. The approach taken here is to consider the best channel that fundamental physics can deliver and ask two important questions. First what is the capacity of this channel and how can it be approached with a realizable system. The tools to accomplish this goal are signal processing and coding algorithms for read channels. This paper addresses soft-decision decoding techniques and advanced joint-track channel detection for perpendicular recording channels.

Detection of media defects in perpendicular recording

Previous studies examined the algorithms used to detect media defects (MDs) in longitudinal recordings. The basic idea was to make tentative decisions at the output of the Bahl-Cocke-Jelinek-Raviv (BCJR) channel detector based on the signs of the log-likelihood-ratios (LLR) and check if any run-length-limited (RLL) k-constraint was violated. The algorithm was analyzed by tracking the BCJR recursions. Amplitudes in addition to the sign of the LLRs were used to improve detection. This paper extends the algorithm to a perpendicular recording. This is a nontrivial extension because the partial response targets of perpendicular recording have no zeros at either zero or the Nyquist frequency, therefore a zero-output waveform sequence are invalid.