Influence of Pit-Shape Variation on the Decoding Performance for Two-Dimensional Optical Storage (TwoDOS)

We study the influence of the variation in the shape of the pits for the two-dimensional optical storage (TwoDOS) paradigm vis-a-vis the readback signal and decoding performance. The algorithm used for decoding is a message-passing based joint equalization and decoding algorithm proposed by Singla and O'Sullivan (Intl. Symp. Info. Theory, Sept. 2005). The performance dependence of the algorithm on pit shape is studied by comparing the noise-tolerance thresholds for the algorithm. The TwoDOS model under consideration also includes two forms of media noise: pit-position noise, arising due to the deviation in the location of the pits from their intended positions and pit-size noise, arising due to variations in the radii of the pits. It is shown that as the shape of the pit changes from the ideal cylindrical shape towards a conical shape there is a sharp decrease in the range of the signal intensity. This decrease in the range leads to increased signal folding which is detrimental for the decoding algorithm. However, we show that when the ratio of the area of the pit to the area of the hexagonal bit cell is large (~1) then the extent of signal folding for a conical profile is actually less than that of a cylindrical profile. Then, in this case, the performance of the decoder for a conical profile is better than for a cylindrical profile.