Ronald S. Indeck

Iterative detection and decoding for separable two-dimensional intersymbol interference

We introduce two detection methods for uncoded two-dimensional (2-D) intersymbol interference (ISI) channels. The detection methods are suitable for a special case of 2-D ISI channels where the channel response is separable. In this case, the 2-D ISI is treated as the concatenation of two one-dimensional ISI channels. The first method uses equalization to reduce the ISI in one of the two dimensions followed by use of a maximum a posteriori (MAP) detector for the ISI in the other dimension. The second method employs modified MAP algorithms to reduce the ISI in each dimension. The implementation complexity of the two methods grows exponentially in the ISI length in either the row or column dimension. We develop two iterative decoding schemes based on these detection methods and low-density parity-check codes as error correction codes. Simulation results show that the bit-error-rate performance loss caused by the 2-D ISI for the separable channel response considered is less than 1 dB over a channel without ISI. This motivates equalizing a general 2-D ISI channel response to a nearby separable matrix.

The Mercury system: exploiting truly fast hardware for data search

In many data mining applications, the size of the database is not only extremely large, it is also growing rapidly. Even for relatively simple searches, the time required to move the data off magnetic media, cross the system bus into main memory, copy into processor cache, and then execute code to perform a search is prohibitive. We are building a system in which a significant portion of the data mining task (i.e., the portion that examines the bulk of the raw data) is implemented in fast hardware, close to the magnetic media on which it is stored. Furthermore, this hardware can be replicated allowing mining tasks to be performed in parallel, thus providing further speedup for the overall mining application. In this paper, we describe a general framework under which this can be accomplished and provide initial performance results for a set of applications.

Iterative decoding and equalization for 2-D recording channels

Summary form only given. As conventional magnetic recording challenges physical limits, we are motivated to consider 2-dimensional (2-D) data storage techniques. Further motivation is provided by the fact that data in 2-D recording may be arranged in bigger sectors than in 1-D and error correcting codes (ECC) such as low density parity check codes (LDPCC) get arbitrarily close to capacity for large enough block lengths. In this paper we consider the use of LDPCC as an ECC in conjunction with equalization methods, iterative and otherwise, for a 2-D recording medium with 2-D ISI. Equalization methods for 2-D recording are discussed as is iterative detection.

Spatial noise phenomena of longitudinal magnetic recording media

The authors have studied the spatial noise characteristics of uniformly magnetized media and implemented a synchronous magnetic pulse detection technique to measure waveforms from precise radial and azimuthal locations on a magnetically recorded disk. They introduce a correlation analysis which includes measuring noise waveforms from the same position on the disk under different remanent magnetic states. Results from this analysis demonstrate that at remanence only minor changes occur in the output waveform and its (presumed) corresponding magnetic structure from write to write, while at DC demagnetization large variations are seen for successive writes. This sensitive correlation technique is used to analyze the random and deterministic components of media and head noise. >

A magnetoresistive gradiometer

A novel magnetoresistive transducer, sensitive to magnetic field gradients, has been analyzed for use as a read transducer in magnetic recording systems. The device uses two thin-film magnetoresistive elements separated by a thin, nonmagnetic insulating layer. A theoretical analysis shows that the magnetoresistive elements can be biased and sensed to reduce common-mode noise while maintaining sensitivity to magnetic field gradients. An experimental transducer was fabricated, showing nonmultidomain elements that eliminated Barkhausen noise. Effects due to thermal drift were reduced by more than 10 dB, stress effects were undetectable, and uniform field rejection was almost 20 dB. Gradiometer operation of the device was confirmed using a magnetic recorded transition. >

Massively parallel data mining using reconfigurable hardware: approximate string matching

Summary form only given. Data mining is an application that is commonly executed on massively parallel systems, often using clusters with hundreds of processors. With a disk-based data store, however, the data must first be delivered to the processors before effective mining can take place. Here, we describe the prototype of an experimental system that moves processing closer to where the data resides, on the disk, and exploits massive parallelism via reconfigurable hardware to perform the computation. The performance of the prototype is also reported.

Spin-stand measurements of time and temperature dependence of magnetic recordings

Spin-stand experiments were performed to study the effect of temperature and demagnetizing fields on the stability of magnetic recordings. Tracks of varying linear bit densities were written on a low Mrδ medium that was heated in situ to different temperatures. The readback amplitude, which reflects the changes in the magnetization of the recordings, was observed from 32 ms to 12 h after the tracks were written. The readback amplitude was found to decay with time. The rate of decay increased at higher densities due to higher demagnetizing fields. A further increase in the decay rate was observed at higher temperatures. This increase in decay rate exceeded that predicted by proportional temperature scaling.

A model for predicting heating of magnetoresistive heads

A simplified geometric model is presented for analyzing the thermal conduction problem in magnetoresistive heads. The model leads to an approximate analytical solution for the thermal resistance as a function of the key geometric and material parameters. The model reveals trends that will be helpful in designing the next generation of high resolution recording heads.

Texture induced spatial variation in switching fields

A method for mapping the microscopic spatial variation of switching fields in magnetic recording media using a magnetic force microscope is described. The method is applied to samples with different substrate textures to examine the effect of texture on magnetic switching behavior. Reversals are found to occur in chains along the texture direction for fields coincident with the texturing. For fields applied transverse to the texture, no long range order is seen in the reversal process. Untextured media also show some tendency for reversal to progress along the field direction.

MFM observation of localized demagnetization in magnetic recordings

The micromagnetic details of magnetization reversal were studied using Magnetic Force Microscopy (MFM). We developed a correlation technique using MFM images to measure changes in magnetization. These correlations are related to the changes in magnetization, as measured in a Vibrating Sample Magnetometer (VSM). This technique was used to examine the onset of demagnetization near a transition.