A New Hybrid OMA/NOMA Scheme for Dynamic Resource Allocation and Pilot Contamination Reduction in Distributed Massive MIMO

Pilot contamination reduction and efficient resource allocation are key points in the development of distributed massive multiple-input multiple-output (MIMO) networks, also known as cell-free massive MIMO networks. In this context, this study proposes a simple adaptive hybrid orthogonal multiple access (OMA)/non-orthogonal multiple access (NOMA) strategy to be used in those innovative networks, capable of achieving efficient resource allocation and pilot contamination mitigation. The proposed strategy exploits the advantages of both the OMA and NOMA schemes by adaptively switching between them for uplink channel estimation and downlink transmission, that is, the switching process is gradual and relies on the number of users in the network as well as the number of available pilot sequences. The decision policy is designed in a simple manner to obtain a low-complexity but powerful method that is attractive in terms of practical aspects. This simultaneous hybrid approach reduces pilot contamination between users, leads to flexible and more efficient resource allocation, improves spectral efficiency (SE), increases the network's achievable sum rates, and offers a good compromise between the number of served users and achievable rates. The results are obtained using a flexible model for imperfect successive interference cancellation (ISIC) and indicate that the proposed strategy can achieve better performance than entirely OMA and NOMA-based systems.