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Design paradigms of logic circuits with Quantum-dot Cellular Automata (QCA) have been extensively studied in the recent past. Unfortunately, due to the lack of mature fabrication support, QCA-based circuits often suffer from various types of manufacturing defects and variations, and therefore, are unreliable and error-prone. QCA-based Exclusive-OR (XOR) gates are frequently used in the construction of several computing subsystems such as adders, linear feedback shift registers, parity generators and checkers. However, none of the existing designs for QCA XOR gates have considered the issue of ensuring fault-tolerance. Simulation results also show that these designs can hardly tolerate any fault. We investigate the applicability of various existing fault-tolerant schemes such as triple modular redundancy (TMR), NAND multiplexing, and majority multiplexing in the context of practical realization of QCA XOR gate. Our investigations reveal that these techniques incur prohibitively large area and delay and hence, they are unsuitable for practical scenarios. We propose here realistic designs of QCA XOR gates (in terms of area and delay) with significantly high fault-tolerance against all types of cell misplacement defects such as cell omission, cell displacement, cell misalignment and extra/additional cell deposition. Furthermore, the absence of any crossing in the proposed designs facilitates low-cost fabrication of such systems.

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