Coding and detection for multitrack systems

In traditional magnetic disk drive systems user data is recorded on concentric tracks as a sequence of changes of the magnetization of small domains. The direction of the magnetization of a domain depends on the polarity of a data-driven write current. The areal density of data stored on a disk can be increased by either reducing the size of the magnetic domains along tracks (i.e., by increasing the linear density) and/or by reducing the track pitch (i.e., by increasing the track, or radial, density). The linear density is limited by several factors, including the finite sensitivity of the read head, properties of magnetic materials, head design [47, 51], as well as the ability to detect and decode recorded data in the presence of intersymbol interference (ISI) and noise. Most research in disk drive systems has focused on increasing linear density. Extremely high densities, for example, 10 Gbits/in², and data rates approaching 1 Gbits/s have been already demonstrated in commercially available systems. Recent progress in heads and media design has opened the possibility of reaching densities of 100 Gbits/in² and perhaps even 1 Terabit/in2. However, the rate of increase of the linear density in future magnetic recording systems is not likely to be as high as in the past. This is due to the fact that as the linear density increases, the magnetic domains on the disk surface become smaller and increasingly thermally unstable. The so-called super-paramagnetic effect [6] represents a fundamental limiting factor for linear density.