IEEE TRANSACTIONS ON MAGNETICS, VOL. 43, NO. 10, OCTOBER 2007 3925
Improved Data Recovery from Patterned Media With Inherent Jitter Noise
Using Low-Density Parity-Check Codes
Ioannis T. Ntokas1, Paul W. Nutter1, Cen J. Tjhai2, and Mohammed Z. Ahmed2
School of Computer Science, The University of Manchester, Manchester M13 9PL, U.K.
School of Computing, Communications and Electronics, University of Plymouth, Plymouth PL4 8AA, U.K.
Patterned magnetic media promises areal densities in excess of 1 Tbit/in2 for data storage. However, current imperfect patterning
techniques result in a variation in the dimensions and distribution of the fabricated islands. As a result, this variation introduces jitter in
the replay waveform that makes data recovery difficult. In this paper, we investigate the use of low-density parity-check (LDPC) codes
and iterative decoding for mitigating the effects of lithography jitter and improving the read channel performance in patterned media
storage systems. In addition, we show that the adoption of LDPC coding techniques permits an increase in the data storage capability
of the medium to approximately 1.6 Tbit/in2 with acceptable bit-error-rate performance.
Index Terms—Lithography jitter, low-density parity-check (LDPC) codes, perpendicular patterned media, read channel performance.
I. INTRODUCTION
THE general view is that the use of a continuous thin-filmstorage medium in current magnetic data storage systems
will be unsuitable for attaining storage densities in excess of
1 Tbit/in , due to the onset of thermal stability issues at ultra-
high storage densities [1], [2]. As such, new storage technolo-
gies, such as the use of a patterned magnetic storage medium,
must be explored [1]. However, the development of patterned
magnetic media as a viable storage solution is currently lim-
ited by the availability of efficient and cost-effective fabrication
techniques, capable of producing uniform, nanometer-sized is-
lands, regularly spaced over large areas. As a result of the lim-
itations of existing fabrication techniques, there is an inherent
variation in the size and position of the fabricated islands. If
the islands are sufficiently small to exhibit single domain be-
havior then, as a consequence of the variation in island geometry
media, noise in patterned media is dominated by lithography
jitter [1], [3]–[6], unlike perpendicular continuous media where
transition noise is dominant [7]. The effect of lithography jitter,
which can be shown to be Gaussian in nature [8], is to degrade
the replay waveform and make the reliable recovery of stored
data more difficult [9].
There have been a number of published works addressing the
issue of data recovery and channel designs for patterned media
[2], [9]–[13]. Early studies by Hughes have investigated read
channel designs for patterned media [2], [10]. A study by Nair
and New [11] investigated how island edge imperfections were
a cause of noise in patterned media recording. Nutter et al. in-
vestigated how the island geometry affected the replay wave-
form and data recovery process [9], [13]. Recent work by Hu et
al., makes a comparison between various coding and iterative
decoding schemes in patterned media storage systems [12], in-
cluding simple convolutional codes, turbo codes, and low-den-
sity parity-check (LDPC) codes. This work demonstrated that
the incorporation of coding and iterative decoding techniques
leads to a significant coding gain. However, this work failed to
take into account the significant signal degradations introduced
as a result of the presence of lithography jitter.
Digital Object Identifier 10.1109/TMAG.2007.903349
In this paper, we concentrate on the application of LDPC
codes and iterative decoding techniques to magnetic storage
systems incorporating a patterned magnetic storage medium,
for the more realistic case where lithography jitter is present.
We demonstrate that the adoption of LDPC codes and itera-
tive decoding offers a means of improving the read channel
bit-error-rate (BER) performance in the presence of lithog-
raphy jitter. In addition, an acceptable BER performance can
be observed even when the storage density of the medium
is increased to 1.6 Tbit/in (reduced island periodicity/track
period of 20 nm) for the same giant magnetoresistive (GMR)
read head configuration.
This paper is organized as follows. Section II introduces the
simulations that have been developed in order to model the re-
play and data recovery processes in a patterned media storage
system, as well as the application of LDPC coding techniques.
In Section III, the effect of lithography jitter on the BER per-
formance of the read channel is investigated, with and without
the use of LDPC codes. Finally, Section IV summarizes conclu-
sions arising from the work undertaken.
II. READ CHANNEL SIMULATION
The performance of the read channel in a storage system
incorporating a patterned magnetic storage medium has been
evaluated using MATLAB simulations of the replay and data
recovery processes. Fig. 1 illustrates the structure of the com-
plete read channel simulated, which consists of three functional
blocks (illustrated as shaded boxes): the replay model, the data
recovery channel, and the LDPC encoder/decoder. The structure
and simulation of each of these functional blocks are described
below.
A. Replay Model
The replay model predicts the signal waveform that would be
observed from a GMR read sensor as it scans along a track of
islands in a magnetic storage system incorporating a patterned
perpendicular magnetic storage medium. The replay model is
based on an extension of the standard two-dimensional (2-D)
reciprocity approach to three-dimensional (3-D) space and
takes into account the geometrical aspects of both the patterned
recording medium and the GMR read sensor used. The com-
plete modeling process is outlined in more detail in [13] and is
covered briefly below.
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