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 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$S$</tex> and two sets of users, <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathcal{M}_{\mathcal{R}}$</tex> and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathcal{M}_{\mathcal{D}}$</tex>. Every user in <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathcal{M}_{\mathcal{D}}$</tex> has a direct link (DL) to <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$S$</tex>, while for users in <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathcal{M}_{\mathcal{R}}$</tex>, 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 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathcal{M}_{\mathcal{R}}$</tex> with cooperation from users in <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathcal{M}_{\mathcal{D}}$</tex>. 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 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\vert \mathcal{M}_{\mathcal{R}}\vert$</tex> and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\vert\mathcal{M}_{\mathcal{D}}\vert$</tex>, we employ combination/one of the two schemes to provide connectivity to users in <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathcal{M}_{\mathcal{R}}$</tex>. In order to do so, under scenarios A and B, we propose user pairing schemes to pair users within <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathcal{M}_{\mathcal{D}}$</tex> and also pair users between <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathcal{M}_{\mathcal{R}}$</tex> and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathcal{M}_{\mathcal{D}}$</tex>. 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.