Partial Response Maximum Likelihood Detection Research Articles

Single-head/single-track detection in interfering tracks

Off-track interference (OTI) arises as a result of head misalignment and is likely to become the dominant source of errors in future magnetic storage systems with high track density. In this paper, using a two-track and single-head model for OTI, we compare two different equalization/detection techniques aiming at recovering data from only the main track. One is a conventional approach based on suppressing OTI by a linear equalizer and then performing partial response maximum-likelihood (PRML) detection of the main track data. The other views OTI as the response of the channel to an independent data sequence, simultaneously estimates the two data sequences by joint PRML detection, and then simply discards the adjacent track data at the end. We evaluate the detection performance in terms of effective detection signal-to-noise ratio, and provide bit-error-rate simulation results. We also discuss a technique to adaptively estimate the position of the head as well as compensate for the relative phase difference in the offtrack data.

Adaptive Partial-Response Maximum-Likelihood Detection in Optical Recording Media

We developed an adaptive partial-response maximum-likelihood (PRML) detector. This detector learns the reference levels for the calculation of branch metrics from past RF samples. Therefore, it enables asymmetry and level fluctuation compensation. In this paper, we report the results of applying adaptive PRML detection to error rate measurements of 20 Gbyte optical discs.

Pattern-dependent noise prediction in signal-dependent noise

Maximum and near-maximum likelihood sequence detectors in signal-dependent noise are discussed. It is shown that the linear prediction viewpoint allows a very simple derivation of the branch metric expression that has previously been shown as optimum for signal-dependent Markov noise. The resulting detector architecture is viewed as a noise predictive maximum likelihood detector that operates on an expanded trellis and relies on computation of branch-specific, pattern-dependent noise predictor taps and predictor error variances. Comparison is made on the performance of various low-complexity structures using the positional-jitter/width-variation model for transition noise. It is shown that when medium noise dominates, a reasonably low complexity detector that incorporates pattern-dependent noise prediction achieves a significant signal-to-noise ratio gain relative to the extended class 4 partial response maximum likelihood detector. Soft-output detectors as well as the use of soft decision feedback are discussed in the context of signal-dependent noise.

A CMOS analog timing recovery circuit for PRML detectors

A fully integrated analog timing recovery circuit for partial-response maximum-likelihood (PRML) detectors for digital magnetic storage is described. The circuit uses a decision-directed minimum mean-squared error (MMSE) algorithm and achieves phase acquisition within 100-bit periods at a maximum speed of 180 Mb/s. It dissipates 76 mW from a single 3.3-V supply and has an active die area of 1.8 mm/sup 2/ in a 1.2-/spl mu/m CMOS process. At 180 Mb/s, the rms clock fitter is 15 ps and peak-to-peak jitter is 97 ps. The test results demonstrate the feasibility of an analog CMOS implementation of decision-directed MMSE timing recovery for PRML detectors.

Evaluation of partial-response maximum-likelihood detection for phase-change optical data storage

We describe the application of partial-response (PR) maximum-likelihood (ML) detection in rewritable phase-change optical data storage. The input to this detector, which is simulated in software, is the actual signal (without any equalization), reproduced from reading of the recorded sequence on an optical disk. The detection algorithm involves the extraction of the impulse response from the readout signal, PR equalization, the adjustment of gain and recovery of clock, ML sequence estimation with the Viterbi algorithm, and analysis of PRML performance. With a laser wavelength of 0.69 microm and an objective lens with a numerical aperture of 0.6, three linear densities are examined: 0.35 and 0.31 microm/bit without modulation code and 0.2 microm/bit with the (1, 7) modulation code. The equalized signal exhibits good eye patterns, especially at the densities of 0.35 and 0.31 microm/bit. Analyses of noise and bit-error rate indicate that jitter, rather than noise, is the main obstacle to realizing ultrahigh density in phase-change media with PRML detection. We also briefly discuss the problem of the inherent nonlinear effect in phase-change readout.

A PRML detector for a DVDR system

In this paper, adaptive algorithms for updating the coefficients of an equalizer and a 2-state Viterbi detector for a partial response maximum likelihood (PRML) detector in a digital versatile disc record (DVDR) system are proposed and implemented with field programmable gate array (FPGA). The conventional partial response (PR) equalization method, derived under the conventional minimum mean square error (MMSE) criterion, exhibits performance degradation due to high-frequency noise enhancement effect of the equalizer in the process of compensating the low pass characteristic of an optical channel with an eight-to-fourteen modulation-plus (EFMPlus) coded input. The proposed equalization method achieves performance improvement by effectively equalizing the channel output at the important points, i.e. zero-crossing points, where the information on actual recorded bits is stored. Considering the speed limit of the FPGA chip, the maximum likelihood (ML) detector is implemented by a 2-state Viterbi algorithm which has similar performance to the original 6-state Viterbi detector by selecting an appropriate value for threshold. Following performance analyses of the proposed algorithms for PRML detector by various computer simulation, the PRML detector is implemented by FPGA chip.

Performance comparison of a class of multi-level DFE and PRML detectors in the presence of channel nonlinearities

Nonlinear transition shift (NLTS) and partial erasure (PE) are two channel nonlinearities that degrade bit-error-rate (BER) performance of detectors at high densities. In this paper, we compare BER performances of partial response maximum-likelihood (PRML) detectors developed for (0, k) run-length-limited (RLL) coded channels with multi-level DFE (MDFE) family detectors at different levels of nonlinearity. Bit-by-bit simulations indicate that the MDFE family detectors are relatively robust to NLTS and PE compared to the PRML detectors. Results also show that the M3DFE detector, an advanced detector in the MDFE family, outperforms the other detectors at high densities under channel nonlinearities.

A compact neural network for partial-response maximum-likelihood detectors: algorithmic study

A compact neural network algorithm for partial-response maximum-likelihood (PRML) sequence detection is presented. Compact neural networks are a class of locally connected neural networks suitable for very large scale integration (VLSI) implementation. The hardware complexity for VLSI implementation of the proposed algorithm grows linearly with the level of the deliberately designed symbol interference effects of the partial-response (PR) signalling scheme. Large dedicated memory for storage of likelihood matrices in digital Viterbi-algorithm-based detectors is not needed for the proposed detector. Detailed analysis on network stability for network topology and time constant of an analog neuron is described. This detector algorithm has competitive bit-error rate performance when compared with the digital Viterbi algorithm under the noise condition for many real-world applications. The proposed algorithm is suitable for analog VLSI implementation because of its low time complexity and linear area complexity for the detection of PRML signalling schemes.

A Study of Asymmetry Compensation for Partial-Response Maximum-Likelihood Detection in Optical Recording Media

The partial-response maximum-likelihood (PRML) detection technique is effective for high-density recording media. In the field of magnetooptical recording media, it is mainly adopted in a magnetic field modulation recording system. For a light intensity modulation (LIM) recording system, however, signal asymmetry caused by the deviation from the optimum recording conditions easily deteriorates the PRML detection procedure. In this paper, we propose an “adaptive reference level technique”, which can greatly improve the write power tolerance. It is expected to suppress the influence of the signal asymmetry in the LIM recording system.

Super-Resolution Optical Readout System with Partial Response Maximum Likelihood Detection

A readout system that uses super-resolution optics and a partial response maximum likelihood (PRML) detection, which improves readout sensitivity and achieves high-density recording, has been developed. In this system, a shading band is placed in the collection path to shade the center part of the reflected beam across the track direction at the aperture plane, resulting in good optical transfer function (OTF) in the high-frequency region. The readout signal from the photo-detector is equalized for the partial responses channel using only a transversal filter and applying Viterbi detection. Experiments using a magneto-optical disk drive having a class I partial response (PR(1, 1)) and a run-length limited code (RLL(1, 7)) were performed to demonstrate the good readout sensitivity of this method. The results show that this compact system achieves 30% higher recording density than a conventional system.

Performance and implementation of adaptive partial response maximum likelihood detection

Motivated by previous comparison work, a configuration for partial response maximum likelihood detection using the Viterbi algorithm (PRML/VA) detectors with adaptive target polynomials is examined. In this configuration, a mean-squared error decision feedback equalization (MSE-DFE) is used to adapt both the forward equalizer and the target channel for the Viterbi detector. The performance of this adaptive PRMLNA is analyzed and compared with other detection techniques. The issue of convergence speed is also studied.

A comparison of detection methods in the presence of nonlinearities

In this paper we evaluate the detectability of nonlinear signals characteristic of high-density, magnetic recording channels read by a magneto-resistive (MR) head. We first introduce a signal model which performs nonlinear superposition of experimentally-collected MR-head transition responses. Feeding signals synthesized from this model through a detection simulator, we compare the performance of Decision Feedback Equalization (DFE), RAM DFE, Partial Response Maximum-likelihood detection with PR4, EPR4, and E/sup 2/PR4 targets, and Fixed Delay Tree Search with Decision Feedback (FDTS/DF) in the presence of time and amplitude nonlinearities. Signals are generated at a variety of signal-to-noise ratios (SNRs) and densities with runlength constraints of (0,3) and (1,7). We found that the most detrimental nonlinearities come from amplitude asymmetry characteristic of poorly biased MR heads, and such nonlinearities cause a loss of between 0.5 and 1 orders of magnitude in bit error rate. We found that of the schemes, FDTS/DF provided the great robustness against all nonlinearities in the presence of RLL(1,7) coding, and EPR4ML and E/sup 2/PR4ML preformed the best with RLL(0,3) coding.

Improved detection for magnetic recording systems with media noise

In this paper, data-dependent partial local feedback noise prediction schemes will be considered for magnetic recording applications where media noise is significant. These schemes are shown to provide improvement over standard partial response maximum likelihood detection as well as non-data-dependent noise prediction.

Partial-Response Maximum-Likelihood Detection Method for Magneto-Optical Recording

We detected bits sized at 0.237 µm bit length and 0.85 µm track pitch recorded by magnetic field modulation on a magneto-optical disk with a 690-nm laser diode and 0.60-numerical-aperture-objective using a partial-response maximum-likelihood detection method. We confirmed that the optimum partial-response for this high linear density had four coefficients of 1 by analyzing difference metrics of error critical paths. The recorded bits with areal density of 5.6 bit/µm2 were also detected at a low error rate, which combination is 0.237-µm bit length and 0.75-µm track pitch. Its write power tolerance is limited to ±22%.

A 160-MHz analog front-end IC for EPR-IV PRML magnetic storage read channels

A front-end IC for EPR-IV partial-response maximum likelihood (PRML) detection systems used in magnetic recording systems is developed. Reorganization of the front-end architecture reduces clock acquisition time and lowers chip complexity and power. A new six-pole 80-MHz continuous-time filter equalizes waveforms to the desired ERR-IV target. The equalizer is tuned in quality-factor and frequency to a synthesized system clock, reducing drifts due to processing and temperature variations. An on-chip timing recovery circuit, incorporating a 160-MHz sampled-analog phase detector and 200-MHz voltage-controlled oscillator (VCO) regenerates the data clock. The phase detector used is appropriate for (1, 7) code, and can be extended to operate on (0, k) codes. During head seeks, a secondary loop incorporating the VCO locks to the write clock and acquires the anticipated read-clock frequency. All signal paths are serial and fully differential. The chip is fabricated in a 1-/spl mu/m CMOS process.

PRML detection boosts hard-disk drive capacity

Over the past 10 years or so, the bit-capacity of mass-market hard-disk drives has increased 100-fold-the reward of improvements in read/write heads, magnetic materials, head-positioning systems, and much else. But as advances in those well-worn areas grow more costly and difficult, attention is turning to the read-channel electronics as a way to increase disk capacity and throughput rates in the future. More specifically, with the enormous advances made in mixed-signal semiconductor technology, it recently became practical to apply partial-response, maximum-likelihood (PRML) detection to low-cost drives. The key advantage of PRML is that it works with sequences of received data, rather than single bits, comparing data sequences and determining which one was most likely to have caused the observed signal. It is therefore more tolerant of intersymbol interference than pulse peak detection-today's most common read-channel technology.

Equalization and detection in storage channels

In this work, a model for digital magnetic recording on thin-film media is developed, and the performances of several detector/run-length-limited code combinations are compared under different densities, media noise, offtrack interference, and channel mismatch conditions. The study is based on computer simulations using an experimentally extracted transition response of a magnetoresistive read head. The detection algorithms considered include the partial response maximum likelihood detectors, decision feedback equalizer (DFE), and fixed-delay tree search with decision feedback (FDTS/DF) detector. It was found that the DFE with a (0,k) run-length-limited (RLL) code and the FDTS/DF with a (1,7) RLL code show the least bit error rate degradation, as the user density increases beyond 2.5 bits/PW50. The PRML and DFE suffer less from offtrack interference. The (0,k) coded DFE and the (1,7) coded EPRML and FDTS/DF channels are relatively robust under PW50 mismatch. At a user density of 2.5, FDTS/DF (1,7) and DFE (0,k) are found to yield the best bit error performance under varying intensities of white Gaussian noise and media noise.

Maximum transition run codes for data storage systems

A new code is presented which improves the minimum distance properties of sequence detectors operating at high linear densities. This code, which is called the maximum transition run code, eliminates data patterns producing three or more consecutive transitions while imposing the usual k-constraint necessary for timing recovery. The code possesses the similar distance-gaining property of the (1,k) code, but can be implemented with considerably higher rates. Bit error rate simulations on fixed delay tree search with decision feedback and high order partial response maximum likelihood detectors confirm large coding gains over the conventional (0,k) code.

PERD: partial error response detection

Extended Partial Response Maximum Likelihood (EPRML) detection outperforms PRML detection on very high-density magnetic recording channels but is much more complex than PRML and is difficult to implement for high data rates. PERD is an alternative to the EPRML detector which gives even better performance. It consists of a conventional PRML detector and a pattern discrimination circuit operating on the error waveform between the waveform estimated from the PRML outputs and the actual playback waveform. A bit-by-bit simulation confirms the expected performance gains over PRML detection.

Two dimensional coding for a multiple-track, maximum-likelihood digital magnetic storage system

Two dimensional run-length-limited (d, k/sub y/) modulation codes were recently introduced as a means of increasing storage capacity in multi-track recording systems. This paper furthers this coding technique by describing a reverse enumeration scheme based on the trellis description of the (d, k/sub y/) constraint for a channel employing extended class IV partial response signalling and maximum likelihood detection. >