Tushar Shivam Pathak

This paper explores the limitations of a conventional reconfigurable intelligent surface-assisted spatial modulation (RIS-SM) system by analyzing the signal-spatial constellation diagram. These limitations include closely packed signalspatial points and a lack of flexibility in adjusting their positions, negatively affecting system performance. To address these challenges, this work proposes a unified RIS-SM (U-RIS-SM) system that utilizes optimal RIS reflector phases and a modified modulation scheme with a tuning parameter. The proposed U-RIS- SM system facilitates the constellation redesigning of signalspatial points by adjusting the tuning parameter. Additionally, the optimal tuning parameter value is determined to achieve the best possible performance of U-RIS-SM. Closed-form expressions for the symbol error rate (SER), average SER (ASER), and average mutual information (MI) of the proposed U-RIS-SM system under the Nakagami-m fading channel are derived and validated through Monte-Carlo simulations. Moreover, since the proposed U-RIS-SM system can be transformed into existing RIS-SM and RIS-assisted space shift keying (RIS-SSK) systems by varying the tuning parameters and RIS reflector phases, the performance metrics for existing RIS-SM/SSK systems are derived from expressions for U-RIS-SM. The U-RIS-SM system is compared with existing RIS-SSK and RIS-SM systems across various parameters, demonstrating its superior performance. Furthermore, to assess its effectiveness under practical conditions, its performance is examined using a low-complexity two-stage maximum likelihood detector, as well as under RIS phase errors and discrete RIS phase shifts.

This work inspects the channel signature (CS) constellations of reconfigurable intelligent surface-assisted space shift keying (RIS-SSK) systems. Investigations show that the optimum CS constellation occurs when the distinctness among CSs is maximized. This can be accomplished by directing all CSs in different directions with equal phase angles between consecutive CSs. Moreover, to operate with the optimal CS constellation, a novel optimum RIS-SSK (O-RIS-SSK) is proposed for an arbitrary number of transmit antennas. Furthermore, analytical expressions for symbol error rate (SER) and average SER for the O-RIS-SSK system are determined and verified by Monte-Carlo simulations. Comparative analysis shows that the proposed O-RIS-SSK system significantly outperforms existing RIS-SSK systems. Moreover, the proposed system is extended to multiple RISs and spatial modulation systems.

Quantum Dot Cellular Automata or QCA is a promising transistor less nano-technology that is growing in popularity and it has the capability to replace the ubiquitous CMOS technology in the VLSI domain. The paper discussed the simple design of single bit comparator circuit using QCA. A single-bit comparator circuit compares its two inputs and indicates which one is larger or are they both equal. This paper has focused on creating an area efficient QCA comparator circuit and a comparative study of area consumption with the previously made designs. The designed comparator circuit is the most area-efficient design as it is made up of minimum possible number of cells. A Comparator is used in equality testers and many other digital communications The circuit proposed in this paper is a three layered circuit which can alternatively be used to realize the basic logic gates. The circuit can also be used as an alternative to the majority and universal gates in QCA.