Piya Kovintavewat

Generalized partial-response targets for perpendicular recording with jitter noise

In this paper, we propose new generalized partial-response (GPR) targets for perpendicular recording whose transition response is modeled as an error function and compare their performance with the partial-response (PR) targets both in the presence and absence of jitter noise. Regardless of any jitter noise amount, results indicate that the GPR targets outperform the PR targets, especially at high linear-recording densities. We also determine that the dominant error sequence for this perpendicular recording is the same for all targets when jitter noise is low. Therefore, the system performance can be further improved by designing and using codes to avoid this dominant error sequence. Another significant point is the fact that the dominant error sequence of perpendicular recording is different from longitudinal recording, thus requiring design of different types of codes than the ones used for longitudinal recording. Finally, we show that the effective signal-to-noise ratio can be equivalently used instead of the bit-error-rate as a measure to compare the performance of different targets.

Per-survivor timing recovery for uncoded partial response channels

A conventional receiver performs timing recovery and equalization separately. Specifically, conventional timing recovery is based on a phase-locked loop that relies on the decision provided by its own symbol detector. We propose a new timing recovery scheme based on per-survivor processing (PSP) that jointly performs timing recovery and equalization for uncoded partial response channels. In the proposed scheme, each survivor of the Viterbi algorithm maintains its own estimate of the timing offset, and this estimate is updated according to the history data associated with the survivor path. As compared to conventional timing recovery at BER = 10/sup -4/, the proposed scheme can provide a 0.5 dB gain in SNR.

Per-survivor iterative timing recovery for coded partial response channels

We propose a new iterative timing recovery scheme based on per-survivor processing that jointly performs timing recovery and turbo equalization on partial response channels with error-correction codes. The scheme embeds the timing recovery process inside the Bahl, Cocke, Jelinek, and Raviv (BCJR) equalizer, using per-survivor processing. This per-survivor BCJR equalizer then iteratively exchanges soft information with an error-correction decoder. Results indicate that the proposed scheme yields a better performance than a conventional receiver that performs timing recovery and turbo equalization separately, especially when the channel encounters severe timing jitter noise. We also present evidence that suggests that the proposed scheme can correct a cycle slip much more efficiently than the others.

A new timing recovery architecture for fast convergence

For any given communication channel, it is desirable to recover all the initial timing information during acquisition and/or any phase and frequency changes during tracking as fast as possible. In this paper, we propose the oversampled per-survivor processing (PSP) timing recovery (TR) architecture to achieve fast convergence rate in the applications of magnetic recording channels. Its performance is compared with the symbol-rate PSP-based TR, the oversampled conventional TR (OCTR), and also with the symbol-rate conventional TR (CTR) architecture used in todays magnetic recording read-channel chip architectures. Results indicate that the oversampled PSP (OPSP) TR yields the best bit-error rate (BER) performance among other TR architectures when fast convergence is required.

Reduced-complexity per-survivor iterative timing recovery for coded partial response channels

We propose a new iterative timing recovery scheme based on per-survivor processing that jointly performs timing recovery and turbo equalization on partial response channels with error-correction codes. The scheme embeds the timing recovery process inside the Bahl, Cocke, Jelinek, and Raviv (BCJR) equalizer, using per-survivor processing. This per-survivor BCJR equalizer then iteratively exchanges soft information with an error-correction decoder. Results indicate that the proposed scheme yields a better performance than a conventional receiver that performs timing recovery and turbo equalization separately, especially when the channel encounters severe timing jitter noise. We also present evidence that suggests that the proposed scheme can correct a cycle slip much more efficiently than the others.

Robustness of per-survivor iterative timing recovery in perpendicular recording channels

Per-survivor iterative timing recovery was proposed to jointly perform timing recovery, equalization, and error-correction decoding. In this paper the robustness of per-survivor iterative timing recovery against thermal asperity (TA) and in ultra-high media noise environment in perpendicular recording channels was investigated.

Thermal asperity suppression based on least-squares fitting in perpendicular magnetic recording systems

Thermal asperity (TA) causes a major problem in data detection process. Without the TA detection and correction algorithm, the system performance (even with perfect synchronization) can be unacceptable, depending on how severe the TA effect is. This paper proposes a new method to suppress the TA effects in perpendicular magnetic recording channels. The TA detection is a threshold-based approach, while the TA correction is done by averaging the readback signal and applying a least-squares fitting technique to estimate the TA signal. Then, the corrected readback signal is obtained by subtracting the TA-affected readback signal by the reconstructed TA signal. Results indicate that the proposed method performs better than the existing one in terms of bit-error rate and is robust to changes in the peak TA amplitude.

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.