Outage and Ergodic Capacity Analysis in UAV-RIS Assisted NOMA-D2D Network

Device-to-Device (D2D) communications enable an alternate access to cellular network by utilizing unmanned aerial vehicles (UAV) as aerial relays, which extend the range of the networks and provide communication services to non-functional area (NFA) under disaster. Further, reconfigurable intelligent surface (RIS) and non-orthogonal multiple access (NOMA) are utilized to enhance UAV communication. In this paper, we investigate the performance of a D2D network assisted by a single UAV with RIS mounted on it and using NOMA-based transmission between a temporary base station (T-BS) and two destination (ground) users. More precisely, we evaluate the end-to-end outage probability and ergodic capacity of such a network in the presence of a Nakagami-m fading channel. Further, selection combining (SC) and maximal ratio combining (MRC) on the signals received via UAV and RIS are applied at the end users. The impact of diversity combining on the outage and ergodic capacity is indicated. We derive analytical expressions of outage probability and ergodic capacity based on SC and MRC. NOMA-based transmission is compared with conventional OMA. Furthermore, we assess the effect of splitting the elements of RIS for two users and indicate an optimal split factor.