A Virtual User Pairing Scheme to Optimally Utilize the Spectrum of Unpaired Users in Non-orthogonal Multiple Access

This letter proposes a strategy to efficiently utilize the spectrum of unpaired users in non-orthogonal multiple access (NOMA) systems. This is done by pairing multiple similar gain users to a single distant user in a nonoverlapping frequency band pairing fashion. We consider a case where the number of far users in a cell is larger than the number of near users. Thus, some far users cannot be paired and should get data using conventional multiple access (OMA). In such case, we propose a strategy where a near user can be paired with multiple (two) far users to optimally utilize the spectrum of unpaired users in the NOMA system. Performance of the proposed pairing scheme is evaluated in terms of ergodic sum capacity over independent Rayleigh flat fading channel. Analytical and simulation results show that significant increase in the capacity gain of the system is achieved by using the proposed strategy compared to OMA and conventional NOMA.