Design paradigms of logic circuits with Quantum-dot Cellular Automata (QCA)have been extensively studied in the recent past. Unfortunately, due to thelack of mature fabrication support, QCA-based circuits often suffer fromvarious types of manufacturing defects and variations, and therefore, areunreliable and error-prone. QCA-based Exclusive-OR (XOR) gates are frequentlyused in the construction of several computing subsystems such as adders, linearfeedback shift registers, parity generators and checkers. However, none of theexisting designs for QCA XOR gates have considered the issue of ensuringfault-tolerance. Simulation results also show that these designs can hardlytolerate any fault. We investigate the applicability of various existingfault-tolerant schemes such as triple modular redundancy (TMR), NANDmultiplexing, and majority multiplexing in the context of practical realizationof QCA XOR gate. Our investigations reveal that these techniques incurprohibitively large area and delay and hence, they are unsuitable for practicalscenarios. We propose here realistic designs of QCA XOR gates (in terms of areaand delay) with significantly high fault-tolerance against all types of cellmisplacement defects such as cell omission, cell displacement, cellmisalignment and extra/additional cell deposition. Furthermore, the absence ofany crossing in the proposed designs facilitates low-cost fabrication of suchsystems.
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