The main contributions of this thesis are summarized as follow:
1. System level study of future recording channels
In view of the shortcomings of the micromagnetic model discussed in Section 1.2, a novel GFP channel model was developed in this chapter in place of the micromagnetic model. The GFP model uses micromagnetic simulations to estimate the probability of grains flipping in a given circumstance. The circumstance is characterized by the grains’ relative position to the write field, the surrounding bit patterns and magnetic coercivity. The model characterizes a look-up
7
table (LUT) of grain probabilities from micromagnetic simulations. It can then be used to produce signals that accurately model the noise characteristics of granular media, the dominant noise source in ultra-high density magnetic much faster than via micromagnetic simulations.
Error performance studies would otherwise have been impossible. The ability to carry out error performance studies using an accurate model in a reasonable amount of time enables a comprehensive analysis of HAMR and SMR recording channels and the signal processing components associated with it. The GFP model was also used in the rest of the thesis to allow signal processing algorithms that are proposed to be evaluated.
The use of the GFP channel model for a system level study of the HAMR channel was presented in the following conference and published in the following journal:
S. S. Shafi’ee, E. M. Rachid, K. S. Chan, H. T. Wang and E. Kwaku, “Application of the grain flipping probability model to heat assisted magnetic recording”, 56th Conference on Magnetism and Magnetic Material, Arizona, USA, pp. CF-11, October–November 2011.
S. S. Shafi’ee, E. M. Rachid, K. S. Chan, H. T. Wang and E. Kwaku, “Application of the grain flipping probability model to heat assisted magnetic recording”, J. Appl. Phys., vol.
111, no. 7, pp. 07B714 - 07B714-3, April 2011.
2. Joint Viterbi Detector Decoder
In view of the sub-optimality of the state-of-the-art iterative detection/decoding technique, two joint detection and decoding techniques are proposed in this thesis. The first proposed technique involves the JVDD. In this technique, detection and decoding are carried out simultaneously in one step, over a channel trellis. The proposed JVDD is optimal over a coded ISI channel. Its high computational complexity is however a drawback. To manage this, a new class of codes known as the JVDD class of codes was proposed specifically for the JVDD. By employing this class of codes in this thesis, the JVDD reaped a significant reduction in complexity. The choice of the design parameters of the codes was however crucial in
8
determining the performance and complexity outcome of the JVDD. As such, extensive simulations were carried out together with theoretical analysis such as free distance computations to study the impact of the code design parameters on JVDD. The study indicated that an optimal set of design parameters of the codes exists and it gave JVDD its best performance and lowest complexity. The proposed codes were thus analytically optimized using a Response Surface Methodology (RSM). The key results for the study on JVDD codes were presented at the following conferences:
S. S. Shafi’ee, K. S. Chan and Y. L. Guan, “A Performance Study of Joint Viterbi Detector Decoder (JVDD) with GDLD codes”, International Conference on Computational Intelligence, Computing and Signal Processing (CICSP), Hong Kong, China, pp. 339 – 343, 19 - 20 October 2014.
S. S. Shafi’ee, K. S. Chan and W. L. Goh, “Optimization of Codes for the Joint Viterbi Detector Decoder”, accepted for publication in International Conference on Computing, Networking and Communications (ICNC) 2016.
Using the optimized codes, the performance of the JVDD was benchmarked against the state-of-the-art iterative detector-decoder through intensive computer simulations over a 1D ISI channel as well as a 1D PMR channel using the GFP channel model, at various channel conditions, codeword length and code rate. A modification of the JVDD algorithm was subsequently proposed for application to 2D channels. The proposed 2D scheme was coined the 2D-JVDD. Its performance and complexity was investigated over a 2D ISI channel. The study on JVDD was presented in the following conferences:
S. S. Shafi’ee, K. S. Chan, Y. L. Guan and E. M. Rachid, “Joint Viterbi Detector Decoder for Grain Flipping Probability Model”, 24th Magnetic Recording Conference (TMRC 2013), Tokyo, Japan, pp. 14, 20 -22 August 2013.
K. S. Chan, S. S. Shafi’ee and Y. L. Guan, “Optimal Joint Viterbi Detector Decoder (JVDD) over ISI Channels”, International Conference on Computing, Networking and Communications (ICNC) 2014, Hawaii, USA, pp. 282 - 286, 3 – 6 February 2014.
9 3. Joint Factor Graph Detector Decoder
The second joint detection and decoding technique proposed in this thesis involves the JFGDD.
In this technique, detection over a channel factor graph and decoding over a code factor graph are simultaneously carried out to recover transmitted bits. Despite having the advantage of performing detection and decoding jointly, the performance of the JFGDD is susceptible to the presence of short cycles in the factor graphs. Existing Error Correction Code (ECC) technique was used to remove cycles in the code factor graph. Two methods were proposed to remove the cycles in the channel factor graph. First, a non-binary transformation of JFGDD was proposed by grouping consecutive bits in a stream so as to reduce the number of edges and therefore cycles in the channel factor graph. Second, designing a constrained target using a GPR equalizer was proposed to arrive at a channel factor graph with fewer or no cycles. Simulations were extensively carried out over a 1D ISI channel as well as PMR channel to evaluate the performance of the JFGDD, non-binary JFGDD and JFGDD with GPR equalizer against the state-of-art system. The study on JFGDD was presented in the following conference and journal:
S. S. Shafi’ee, E. M. Rachid, K. S. Chan and Y. L. Guan, “Application and Optimization of Factor Graph-Based Detector on 1D ISI Magnetic Recording Channel”, Asia-Pacific Magnetic Recording Conference 2012, Singapore, 31 October – 2 November 2012.
S. S. Shafi’ee, E. M. Rachid, K. S. Chan and Y. L. Guan, “Application and Optimization of Factor Graph-Based Detector on 1D ISI Magnetic Recording Channel”, IEEE Transactions on Magnetics, vol. 49, no. 6, pp. 2500 - 2503, June 2013.
4. Equalization/Detection Schemes for 2D Channels
In this thesis, various 2D GPR equalization and detection schemes were proposed for 2D channels. The 2D equalizers which consist of an SMR equalizer, symmetric and asymmetric TDMR equalizer, and 2D TDMR equalizer were designed with the aim of either equalizing away the effect of Inter-Track Interference (ITI) and employing 1D detection for ISI cancellation or making use of 2D detection techniques to simultaneously undo the effects of ITI and ISI. In this thesis, each of the proposed equalizers was integrated with the JVDD and the conventional
10
iterative detector-decoder in different set-ups. The equalization/detection schemes were studied and their performance for a triple reader head system was analyzed over a 2D GFP channel model through a large number of simulations for various code and channel conditions.
The results for the equalization/detection schemes for 2D channels were presented in the following conference and journal:
S. S. Shafi’ee, K. S. Chan, Y. L. Guan, “Novel Joint Detection Schemes for TDMR Channels”, 25th The Magnetic Recording Conference (TMRC 2014), California, USA, 11 - 13 August 2014.
S. S. Shafi’ee, K. S. Chan, Y. L. Guan, “Novel Joint Detection Schemes for TDMR Channels”, accepted for publication in IEEE Trans. on Magn., vol. 51, no. 4, pp. 0018-9464, April 2015.