On the spectral efficiency of cell-free large-scale MIMO non-orthogonal multiple access systems

Abstract

The spectral efficiency (SE) of an uplink cell-free large-scale multi-input multi-output (MIMO) non-orthogonal multiple access (NOMA) system which relies on the superimposed pilot (SP) transmission strategy is investigated. Closed-form expressions of the achievable SE with the maximal ratio combining (MRC) receiver filters are derived and compared against the orthogonal multiple access (OMA) counterparts. Based on the tractable results, we quantitatively explore the impacts of the pilot training strategy, the OMA and NOMA schemes, the number of user equipments (UEs), the power balance factor between pilot and data signals, and the length of the coherent interval (CI) on SE enhancement. Several insightful conclusions are drawn by assuming that the SP interference in the data detection process can or cannot be perfectly eliminated from the data estimates. All the analytical results are verified by numerical simulation experiments.