Cooperative NOMA Aided Improved Connectivity in Downlink Through Smart User Pairing

Cooperative non-orthogonal multiple access (C-NOMA) is one of the promising techniques which improves the connectivity and transmission rates between nodes. In this paper, we consider using C-NOMA in a system model with a source $S$ and two sets of users, $\mathcal{M}_{\mathcal{R}}$ and $\mathcal{M}_{\mathcal{D}}$. Every user in $\mathcal{M}_{\mathcal{D}}$ has a direct link (DL) to $S$, while for users in $\mathcal{M}_{\mathcal{R}}$, there is no DL. We employ two C-NOMA schemes - Half-Duplex NOMA (HD-NOMA) and Improved-Virtual-Full-duplex NOMA (I-VFD-NOMA) - which serve users in $\mathcal{M}_{\mathcal{R}}$ with cooperation from users in $\mathcal{M}_{\mathcal{D}}$. We consider two scenarios, A and B, where in A, the inter-user channels are assumed unknown, while in B, they are assumed known. Based on $\vert \mathcal{M}_{\mathcal{R}}\vert$ and $\vert\mathcal{M}_{\mathcal{D}}\vert$, we employ combination/one of the two schemes to provide connectivity to users in $\mathcal{M}_{\mathcal{R}}$. In order to do so, under scenarios A and B, we propose user pairing schemes to pair users within $\mathcal{M}_{\mathcal{D}}$ and also pair users between $\mathcal{M}_{\mathcal{R}}$ and $\mathcal{M}_{\mathcal{D}}$. Considering average sum rate (ASR) for users as the metric, we perform Monte-Carlo simulations to evaluate the proposed schemes. Although HD-NOMA and I-VFD-NOMA are key enablers for the system model under study, both suffer from imperfect successive interference cancellation (I-SIC). Additionally, I-VFD-NOMA suffers from residual inter-user interference (R-IUI). Hence, through simulations, we study the impact of I-SIC and R-IUI on ASR as well.