Modeling, equalization and detection for two-dimensional quadratic storage channels

Volume Holographic Storage (VHS) is a promising digital storage technology because of its high data storage density, high data rate and short access times. The ability to multiplex several pages into a given volume of the material leads to potentially high densities. Also, VHS uses a highly parallel two-dimensional or page like format in recording and retrieval leading to high data rates. One way to increase the data density in a volume holographic storage channel (VHSC) is to use a frequency plane aperture and thus reduce the size of the effective recording area on the holographic medium. However, such frequency plane aperturing causes severe inter-symbol interference (ISI) leading to poor bit-error rate (BER) performance. In addition to ISI, optical and electronic noise affect the BER adversely and thus limit the capacity of the VHSC. Equalization and detection methods should be employed to mitigate the effects of ISI and noise and to increase the storage capacity. However, the VHS output detector array detects the intensity of the incident light wavefront and this results in sign information loss due to the quadratic nonlinearity. This sign loss prevents the applicability of conventional equalization/detection schemes used in 1-D magnetic and optical storage channels. The goal of this thesis is to develop and evaluate equalization and detection methods for such quadratic storage channels.; In this thesis, we first address channel modeling under quadratic nonlinearity. We then design and analyze various equalization/detection methods using this quadratic channel model. We first derive a simple linear equalization technique, namely linear minimum mean square (LMMSE) equalization and then consider the applicability of more advanced methods based on decision feedback and partial response maximum likelihood. The performance of equalization/detection methods is quantified in terms of equivalent density gain offered for optical noise dominated channel as well as electronic noise dominated channel. We show that iterative magnitude square decision feed back equalization (IMSDFE) is the most promising method for high-density and high-ISI VHSC. We also evaluate the effect of modulation coding, particularly balanced codes and low pass coding, and magnification error on the equalization/detection performance. These results indicate that (LMMSE + 6:8 balanced code) and (IMSDFE + low pass coding) are good candidates for high-density, high-ISI VHSC with magnification errors.