Improved Performance of Cell-Free NOMA Networks Employing Common-User Clustering

In the existing cell-free (CF) non-orthogonal multiple access (NOMA) networks, each user equipment (UE) receives signals from a group of access points (APs) simultaneously, however, the UE belongs to only a single NOMA cluster. This prevents the exploitation of the potential benefits of having clusters with common UEs. Previous studies have somehow investigated this for cellular NOMA networks by considering that UEs at the cell borders can communicate with base stations through multiple clusters using coordinate multipoint techniques. However, this approach has not yet been investigated for the CF networks. In this paper, we consider CFNOMA networks in the downlink scenario wherein three different UE clustering strategies are applied, including singleUE OMA (simply denoted by CF-OMA), double-UE NOMA with no common UE (CF-NOMANC), and a new scheme called double-UE NOMA with common UE (CF-NOMAC) clusters. The proposed CF-NOMAC clustering improves the sum rate through having a lower number of orthogonal clusters compared to the CF-OMA, and CF-NOMANC methods. We analyze the three clustering methods and prove that the proposed CF-NOMAC clustering method outperforms others in terms of sum rate due to the incorporation of a lower number of orthogonal clusters in a specifically allocated bandwidth, provided that certain conditions hold. The analytical results are verified through simulations.