Enhancing Spatially-Correlated Multi-Way Massive MIMO NOMA Relaying With Downlink Pilots

We consider a multi-way massive multi-input multi-output (mMIMO) relaying system wherein a relay aids multi-way data exchange between multiple users by employing non-orthogonal multiple access (NOMA). The relay achieves this by superposing different user signals in the power domain. Each user then sequentially decodes the data of all other users by performing successive interference cancellation (SIC). To perform SIC, a user utilizes the downlink channel information, which it estimates using the precoded pilots transmitted by the relay. We derive a closed-form spectral efficiency (SE) expression for this downlink-pilot-aided NOMA multi-way relaying system by considering spatially-correlated channels, and imperfect SIC at the user. We next design two algorithms to maximize this SE by optimally allocating the user transmit powers and the NOMA variables. Both these algorithms provide a similar SE, but the latter one, due to its closed-form power updates, has a much lesser complexity. The efficacy of downlink pilots in multi-way NOMA relaying, with practical correlated Rayleigh-faded channels, is shown by demonstrating that they vastly outperform the case when users perform SIC using channel statistics.