A parallel two-dimensional memory (i.e., one that stores and retrieves 2D pages (arrays) of digital data) represents a fundamentally different memory architecture. This 2D parallel architecture has the potential for capacity and speed performance which is orders of magnitude beyond the expected physical limits of traditional lD storage architectures. A volume optical memory is a system which stores data pages within a three dimensional optical medium. These systems have the potential combination of large storage capacity (> 10^l2 bits/cm^3), high speed random access via massless scanning (<10 ns), and very large data transfer rates (> 10^10 bits/sec) owing to the 2D storage/retrieval of data.. Direct application of conventional communication theoretic techniques to 2D parallel memories is complicated by the truly 2D nature of the data format. In addition, a volume optical memory implementation typically results in a storage channel with very complex noise and interference. Factors contributing to the operating environment include (I) the intensity transformation and subsequent spatial averaging of the coherent optical signal as a result of optoelectronic conversion, (ii) non Gaussian noise sources such as shot and speckle, and (iii) complex data dependent interference sources such as inter-page crosstalk and space variant intersymbol interference within a given page. This research program is concerned with signaling, equalization, and decision schemes as they apply to parallel 2D data channels, with specific consideration given to coherent (holographic) volume optical memory systems. One objective of this research program is to develop the theoretical foundation for signaling and detection in 2D digital communication systems. Another objective is the application of this theory to develop robust, implementable signaling and detection techniques for volume optical memory systems. The plan for achieving these objectives is based on the development of relatively simple models of the volume optical memory system to be used for development and analysis of specific signaling and detection algorithms. Performance bounds for the optimal detection strategies, as well as efficient algorithms for their realization, will be sought in the data detection phase of the research. Simple, suboptimal data detectors which exploit the 2D nature of the data format will also be developed and their performance characterized relative to that of the optimal detection rule. The signal design component will be concerned with error correction coding for use with 2D asymmetric channels, precoding techniques for combating data-dependent interference within and between pages, pixel profile optimization using simple optical masks, and novel signaling strategies for non-Gaussian channels. The third component involves the creation of a sophisticated simulation engine for the evaluation and refinement of new signaling and detection methods. This simulation engine will capture the detailed physical characteristics of volume optical memory systems and will facilitate measuring the performance and robustness of these novel 2D approaches in a real-world environment.
