Performance Analysis of Novel User Pairing-Based Hybrid NOMA System With Fixed/Optimal Power Allocation Strategy

In the recent times, non-orthogonal multiple access (NOMA) can be seen as a paradigm shift for next generations of communication networks such as the fifth generation (5G) and beyond 5G networks. Owing to its remarkable benefits, even NOMA has been employed for the sixth generation (6G) systems as well. NOMA, in spite of bestowing with an enhancement in spectral efficiency (SE) by enabling the users to exploit the same orthogonal resource blocks with the aid of power domain multiplexing, is not devoid of its drawbacks. A slight variation of the channel conditions corresponding to NOMA users and the rapid increase in the number of users results in significant degradation in the performance of NOMA system. Therefore, to circumvent such drawbacks, it is vital to employ hybrid multiple access (HMA) strategies such as the combination of both orthogonal multiple access (OMA) and NOMA. In this context, this paper proposes novel user pairing (UP) strategy for downlink power domain (PD) NOMA system over Rayleigh fading channel with perfect and imperfect successive interference cancellation (pSIC/ipSIC) comprising of even number of NOMA users. It is to be noted that these NOMA users which are served by the base station (BS) can be paired according to their channel conditions with improved sum rates (SR). If an odd number of NOMA users are in one group and one group has more users than the others, one user may be left unpaired, making pairing difficult. Considering this, we present an efficient UP method that pairs unpaired users with one of the existing pairings instead of utilizing OMA benefit. Finding an ideal (best) pair to serve the unpaired user is the major aim of our proposed method. Furthermore, we worked towards enhancing the total SR which is also known as pair sum rate (PSR) and we even provided power allocation (PA) optimization process for each pair in order to maximize the SR/PSR of our proposed UP NOMA system. In addition, we derive various key metrics such as the upper and lower bounds of PA coefficients, imperfect SIC parameter, and channel gain difference parameter for NOMA paired users. Further, we provide an elaborate analytical framework for various performance metrics such as the PSR, ergodic pair sum rate (EPSR), and outage probability (OP) expressions for the proposed downlink PD-NOMA system over Rayleigh fading channels.