Modulation Codes for Optical Data Storage

Abstract

In optical storage systems, sensitive stored patterns can cause failure in data retrieval and decrease the system reliability. Modulation codes play the role of shaping the characteristics of stored data patterns in optical storage systems. Among various optical storage systems, holographic data storage is regarded as a promising candidate for next-generation optical data storage due to its extremely high capacity and ultra-fast data transfer rate. In this chapter we will cover modulation codes for optical data storage, especially on those designed for holographic data storage. In conventional optical data storage systems, information is recorded in a one-dimensional spiral stream. The major concern of modulation codes for these optical data storage systems is to separate binary ones by a number of binary zeroes, i.e., run-length-limited codes. Examples are the eight-to-fourteen modulation (EFM) for CD (Immink et al., 1985), EFMPlus for DVD (Immink, 1997), and 17 parity preserve-prohibit repeated minimum run-length transition (17PP) for Blu-ray disc (Blu-ray Disc Association, 2006). Setting constraint on minimum and maximum runs of binary zeros results in several advantages, including increased data density, improved time recovery and gain control and depressed interference between bits. In holographic data storage systems, information is stored as pixels on two-dimensional (2D) pages. Different from conventional optical data storage, the additional dimension inevitably brings new consideration to the design of modulation codes. The primary concern is that interferences between pixels are omni-directional. Besides, since pixels carry different intensities to represent different information bits, pixels with higher intensities intrinsically corrupt the signal fidelity of those with lower intensities more than the other way around, i.e., interferences among pixels are imbalanced. In addition to preventing vulnerable patterns suffering from possible interferences, some modulation codes also focus on decoder complexity, and yet others focus on achieving high code rate. It is desirable to consider all aspects but trade-off is matter-of-course. Different priorities in design consideration result in various modulation codes. In this chapter, we will first introduce several modulation code constraints. Next, onedimensional modulation codes adopted in prevalent optical data storage systems are discussed. Then we turn to the modulation codes designed for holographic data storage. These modulation codes are classified according to the coding methods, i.e., block codes vs. strip codes. For block codes, code blocks are independently produced and then tiled to form a 1