Review And Improvement Of Approximate Adder In Quantum_dot cellular automata

نوع: Type: Thesis

مقطع: Segment: masters

عنوان: Title: Review And Improvement Of Approximate Adder In Quantum_dot cellular automata

ارائه دهنده: Provider: Moustafa Malmir

اساتید راهنما: Supervisors: DR.Hatam Abdoli

اساتید مشاور: Advisory Professors:

اساتید ممتحن یا داور: Examining professors or referees: DR.Abass Ramezani , DR.shakour Vakilian

زمان و تاریخ ارائه: Time and date of presentation: 2026

مکان ارائه: Place of presentation: سمینار گروه کامپیوتر

چکیده: Abstract: .As CMOS technology approaches its physical limits at nanoscale dimensions, it has prompted the adoption of other technologies. Quantum-dot cellular automata (QCA), as one of the options, enables the implementation of high-density digital circuits with ultra-low power consumption and suitable speed by exploiting Coulomb interactions instead of electric current. On the other hand, approximate computing, by accepting bounded error in the output, has created a suitable platform for improving hardware metrics such as area, delay, and energy consumption in error-tolerant applications. In this thesis, a new architecture for QCA-based approximate adders is presented that follows the carry-preserving approach. In this scheme, approximation is limited only to the generation of the Sum output in the least significant bits (LSBs), while Carry generation is kept fully exact. This prevents error propagation to the most significant bits (MSBs) and maintains computational accuracy at an acceptable level. Accordingly, an approximate full adder (AFA) cell was designed with a minimum number of cells and then used in 2-bit, 4-bit, and 8-bit ripple-carry adder (RCA) structures. For the high-order part, accurate multilayer and single-layer full adders were used so that calculations in the significant part are exact. The proposed designs were implemented in two types of architecture: coplanar and multilayer. Evaluations were performed using QCADesigner software with standard parameters. The results for the 4-bit single-layer approximate adder with two approximate bits are a compact design with 51 QCA cells, an area of 0.06 μm², a delay of 1.25 clock cycles, and a circuit cost of 0.09375, which shows a significant improvement compared with the best previous designs. Energy analysis was also investigated using QCADesigner. Error metrics, including error rate, normalized mean error distance (NMED), and maximum absolute error (MED), were calculated, showing that the error is limited to the LSBs. In the evaluation of the image-processing application—brightness enhancement and combination with the dark version in MATLAB—the peak signal-to-noise ratio (PSNR) for the one- to four-approximate-bit configurations was obtained in the range of 55.1 to 337 dB, which indicates desirable quality for error-resilient applications. The results of this research show that carry preservation in QCA-based approximate adders establishes an effective trade-off between hardware efficiency—reducing area, delay, and energy—and computational accuracy/error control in the least significant bits. The proposed architecture, with the ability to adjust the approximation level according to application requirements, is considered an efficient option for designing low-power nanoscale circuits.