Wim M. J. Coene

Modeling and compensation of asymmetry in optical recording

Asymmetry, also known as domain bloom, is a systematic imperfection caused by the writing process in optical or magneto-optical recording. At the reading end of the system, asymmetry causes shifts of adjacent signal transitions in opposite directions. We present a simple nonlinear model for the replay signal in the presence of asymmetry. The model is specified by a single parameter, which is proportional to asymmetry, and its accuracy is demonstrated by application to replay signals from digital video disk drives. Based on the proposed model, a maximum-likelihood sequence detector is designed for replay signals with asymmetry. Simple modifications of the proposed detector lead to significant reductions in complexity, while the attainable performance, evaluated both analytically and through error-rate simulations, is superior to that of conventional techniques.

Two-Dimensional Optical Storage

Two-dimensional optical storage aims at increasing the data capacity and data rate for a given physical read-out system. It uses parallel read-out in combination with advanced signal-processing. Experimental results results on read only memory (ROM) discs are presented that proof the concept. Laser beam recorded discs proof the concept, and electron beam recorded disc show the feasibility at real physical parameters for a density at 35 GB with ample tilt margins.

Evaluation of adaptive PRML/Viterbi bit detection for DVD and beyond

Two adaptive PRML/Viterbi bit detectors have been evaluated based on DVD. They are able to handle non-linearities. It is shown that adaptive PRML/Viterbi detection can help to improve the tangential tilt margin or to increase the possible density.

Transition detector for CD and DVD

This bit detector for d=2 modulation codes is remarkably simple, supports very high data rates, can be adaptive, and is capable of handling nonlinearities. The performance approaches that of a full-fledged maximum-likelihood sequence detector.

Sequence Detection Based on a Variable State Trellis for Multidimensional ISI Channels

Near-optimum sequence detection in channels with intersymbol interference (ISI) is based on the Viterbi algorithm. However, its complexity increases exponentially with the number of taps within the span of the ISI and with the number of symbol levels. We present a low-complexity sequence detection scheme, based on the Viterbi detection algorithm, which is generalized for multidimensional ISI channels in the presence of the predominant signal-dependent noise. The proposed variable state trellis (VST) approach detects and discards in real time states that are unlikely to correspond to the survivor path, thus reducing the complexity of the sequence detection significantly. After analyzing and exploiting a number of criteria (such as a priori reliability information, channel characteristics, and probability of error occurrences among different symbol levels), we propose three approaches to implement the VST scheme. The VST can be applied to any multidimensional ISI channel, including magnetic and optical storage. We compare the performance of the VST and conventional Viterbi algorithms for the multilevel two-dimensional optical storage channel, where media noise is predominant, and show that, with negligible performance loss, the computational complexity is reduced significantly

Experimental characterization for the binary two-dimensional optical storage channel

A new and challenging concept for optical storage is being developed, in which the information written on the disc has a two-dimensional characteristic, with the aim of realizing an increase of a factor of two in data density and a factor of ten in data rate over the current 3rd generation of optical storage (Blu-Ray Disc, BD). Experimental channel characterization provides insight into the magnitude of intersymbol interference, signal levels, modulation and noise distribution from the measured signal waveform. This information is used to evaluate the quality of the mastered discs, and provide feedback in the optimization process. It also yields essential input to the actual design of the signal processing solutions. The stages involved in characterizing the channel are downsampling and model fitting using a scalar diffraction model. The information gathered from channel characterization is demonstrated with experimental measurements from discs with an initial density of 1.4/spl times/BD using the same physical read-out system (blue laser and high-NA lens).

A simple nonlinear model for the optical recording channel

This study is concerned with the development of a simple yet sufficiently accurate model for the replay signal in optical discs. A method is proposed which models data storage (the write channel) as a nonlinear process and data retrieval (the read channel) as a linear one. The result is a low-complexity nonlinear model, of which the linear binary-PAM model is a special case. The performance of the proposed model is demonstrated by application to experimentally measured DVD data, and is found to be both accurate as well as robust to different recording media and conditions.

Two-dimensional optical storage: high-speed read-out of a 50 GByte single-layer optical disc with a 2D format using lambda=405 nm and NA=0.85

With ever increasing storage capacities, long read-out times of content are becoming a bottleneck for the convenient use of optical storage devices. Two-dimensional optical storage (TwoDOS) is a new concept that solves this data-rate problem through parallel read-out. Furthermore, an increase of at least a factor of 2 in storage capacity is achieved by exploiting a two-dimensional format. The advantages offered by TwoDOS are obtained without the need of changing the physics of the read-out system. Using Blu-ray Disc optics, single-layered 12 cm discs with capacities up to 50 GByte have been read out successfully. Signal level and error analysis show the feasibility of a further capacity increase. Also, bit-rates as high as 700 Mbit/s during read-out have been reached. All this is achieved by applying advanced signal processing and advanced disc mastering techniques and the proper design of the optical path.

Parity-check codes and post-processing for d=1 optical recording channels

We have developed an advanced detection approach with parity-check (PC) codes and post-processing for optical recording channels. In seeking the advanced detection approach, we investigated different types of PC codes, existing as well as new. We developed a novel remedy scheme to minimize the miscorrection of error events that are split across codeword boundaries. Simulation shows that the performance of the developed PC codes approaches the corresponding bit-error-rate bounds, at both nominal density and high density.