NOMA-inspired coexistence enabling method in cell-free massive MIMO using opportunistic interference alignment

Abstract

A novel scheme referred to as non-orthogonal multiple access (NOMA)-inspired coexistence enabling (NICE) is introduced for the downlink of cell-free massive MIMO (CF-mMIMO) networks. Motivated by the cognitive concept in NOMA, the NICE strategy is designed to accommodate secondary users (SUs) within the CF-mMIMO. Despite of existing works in the CF-mMIMO, this paper considers the same transmitters for both primary and secondary users. Besides, the promising opportunistic interference alignment (OIA) strategy is exploited for the first time in CF-mMIMO networks. However, CF-mMIMO deals with a large number of users and access points (APs) which may enlarge the minimum number of transmit/receive antennas. The proposed NICE scheme resolves this problem such that a large outer bound on the degrees of freedom for SUs is achieved while a small number of transmit/receive antennas is required. The effectiveness of the proposed strategy is demonstrated through simulations for both perfect and imperfect channel models.