Ankita Chauhan

In this work, a novel signal-space-diversity-based full-duplex (FD) cooperative non-orthogonal multiple access (SFD-CNOMA) system is introduced for a multi-user scenario. In the proposed system, cell-centered and cell-edge users’ information will be multiplexed in the superimposed signal’s inphase and quadrature components. This reduces successive-interference-cancellation (SIC) operations, detection delay, and complexity compared to conventional FD cooperative NOMA (CFD-CNOMA). An analytical framework is introduced to assess the average sum rate, outage probability, and diversity order for both perfect and imperfect SICs, considering order statistics. Comparative analysis demonstrates that SFD-CNOMA outperforms CFD-CNOMA under both perfect and imperfect SICs.

Multi-carrier power-domain non-orthogonal multiple access (MC-PD-NOMA) has been considered a key technology for next-generation wireless networks. In MC-PD-NOMA, the available channel bandwidth is divided into several sub-carriers in order to serve the multiple users in each sub-carrier. In this work, to reduce the number of successive interference cancellation (SIC) operations, detection delay, SIC complexity and to enhance the SIC stability of the MC-PD-NOMA, a system, namely, MC-quadrature-NOMA (MC-Q-NOMA), is proposed. Furthermore, the proposed MC-Q-NOMA follows the essential upconversion architecture to provide orthogonality in the pass-band. Moreover, a novel analytical framework is introduced to evaluate analytical expressions of average sum-rate (ASR), outage probability (OP), and diversity order for the proposed system under the generalized α - μ channel for perfect and imperfect SICs. In addition, the proposed framework is utilized to determine the analytical expression of ASR, OP, and diversity order for MC-PD-NOMA. Comparative analysis indicates that under imperfect SIC, MC-Q-NOMA outperforms the MC-PD-NOMA due to the lesser number of SICs. However, under perfect SIC, MC-Q-NOMA demonstrates comparable performance with the MC-PD-NOMA. Furthermore, the analytical expression of the average bit-error-rate is evaluated for the MC-Q-NOMA. Numerical results demonstrate that the error performance of MC-Q-NOMA is superior to MC-PD-NOMA for all signal-to-noise ratio values.

The power domain non-orthogonal multiple access (PD-NOMA) performs reasonably well under perfect successive interference cancellation (SIC) conditions. However, under imperfect SIC, the average sum-rate (ASR), outage performance, and $\epsilon -$ outage capacity of PD-NOMA diminish drastically. This work introduces a new PD-NOMA system called multidimensional signal space NOMA (MD-NOMA) to mitigate this sudden decrease in performance. Employing multidimensional signal space significantly reduces the number of SICs, leading to less stringent constraints on the power allocation, reducing the detection delay, and decoding complexity. The analytical expressions of ASR, outage-probability (OP), $\epsilon -$ outage capacity, and diversity order of MD-NOMA, Quadrature-NOMA (Q-NOMA), and PD-NOMA are evaluated under Nakagami- $m$ fading channel with the help of order statistics for a generalized- $K$ user scenario. It is noticed from the analysis that MD-NOMA, with a higher number of signal spaces, significantly outperforms all other systems under imperfect SIC and exhibits comparable performance with them under perfect SIC. Furthermore, the analytical expression for bit error rate (BER) and average-BER (ABER) is determined for MD-NOMA. It is noticed that MD-NOMA can withstand higher modulation order ( $M$ ), whereas the performance of all other systems degrades drastically as $M$ increases. Furthermore, numerical results demonstrate there is a tradeoff between ABER and spectral efficiency.

This letter determines the power allocation constraint (PAC) for a K -user power domain non-orthogonal multiple access (PD-NOMA) system, where each user employs an identical M -ary phase-shift keying modulation scheme. The angular periphery method is used to derive the PAC that determines the range of the power allocation factor to ensure reliable error performance. Further, analytical expressions for the bit error rate (BER), average BER, and diversity order of K -user PD-NOMA are derived under Nakagami- m fading channel with the help of order statistics for a generalized value of M . Numerical results demonstrate the importance of the proposed PAC and display the appropriate water-falling nature of error rate curves when following the PAC. Moreover, the analysis also gives insight about the optimal value of the power allocation factor to minimize error performance.

Reconfigurable intelligent surface (RIS)-assisted power-domain non-orthogonal multiple access (PD-NOMA) system has emerged as a revolutionary technology to enhance the spectrum efficiency for future wireless networks. This work introduces two novel RIS systems, namely, RIS partition-assisted (RISP) PD-NOMA (RISP-PD-NOMA) and RISP-Quadrature-NOMA (RISP-Q-NOMA), to maximize the diversity order and to improve the signal quality of all users by dedicating fixed RIS units to each user. The closed-form expressions of average sum-rate, outage probability, and diversity order are evaluated for both systems under the Rician fading channel for perfect and imperfect successive interference cancellation (SIC) operations. Further, the performance of both RISP-PD-NOMA and RISP-Q-NOMA systems is compared with RIS partitioned PD-NOMA, RIS-assisted PD-NOMA, and PD-NOMA systems. It is noticed from the analysis that under perfect SIC, RISP-PD-NOMA outperforms all other systems. However, under imperfect SIC, RISP-Q-NOMA demonstrates superior performance to other systems due to the lesser number of SIC operations that reduce detection delay and decoding complexity. Furthermore, the analytical expression for bit error rate (BER) of RISP-PD-NOMA and RISP-Q-NOMA is derived, and then the closed-form expression of average-BER is evaluated. Numerical results demonstrate that RISP-Q-NOMA is superior to RISP-PD-NOMA.

Reconfigurable intelligent surface (RIS)-assisted power-domain non-orthogonal multiple access (PD-NOMA) system has emerged as a revolutionary technology to enhance the spectrum efficiency for future wireless networks. This work introduces two novel RIS systems, namely, RIS partition-assisted (RISP) PD-NOMA (RISP-PD-NOMA) and RISP-Quadrature NOMA (RISP-Q-NOMA), to improve the signal quality of all users by dedicating fixed RIS units to each user for phase cancellation. The closed-form expressions of average sum-rate, outage probability, and diversity order of both systems are evaluated under the Rician fading channel for perfect and imperfect successive interference cancellation (SIC). Further, the performance of both systems is compared with RIS-division PDNOMA (RISD-PD-NOMA) system where the RIS subsurface assigned to one user is not exposed to another user. It is noticed from the analysis that under perfect SIC, RISP-PD-NOMA outperforms RISP-Q-NOMA and RISD-PD-NOMA. However, under imperfect SIC, RISP-Q-NOMA demonstrates superior performance than other systems, due to lesser number of SIC operations that lead to less stringent constraints on the power allocation, reduce detection delay, and improve SIC stability. Furthermore, the analytical expression for bit error rate (BER) of RISP-PD-NOMA and RISP-Q-NOMA is derived, and then closed-form expression of average-BER is evaluated. Numerical results demonstrate that RISP-Q-NOMA is superior to RISPPD-NOMA and RISD-PD-NOMA.