Downlink Pilots for Rician-Faded NOMA Cell-Free Massive MIMO Systems: Criticality, Analysis, and Optimization

We consider the downlink of a cell-free (CF) massive multi-input multi-output (mMIMO) system, wherein multiple access points serve clustered users by employing non-orthogonal multiple access (NOMA) technology. This work shows the importance of transmitting downlink pilots for a NOMA CF mMIMO system, and derives a closed-form spectral efficiency (SE) expression with spatially-correlated Rician channels by addressing the complexity due to i) dynamic-resolution analog to digital converters; and ii) channel estimation errors. We also design a low-complexity parallel block minorization-maximization (MM) algorithm to optimize the non-convex global energy efficiency (GEE) metric. The MM algorithm requires a surrogate function for the GEE metric, which we construct in this work. We show that the NOMA CF mMIMO system, wherein channels estimated using downlink pilots are used for performing successive interference cancellation (SIC), significantly outperforms its counterpart which uses channel statistics to perform SIC. We also show that our parallel block MM optimization yields the same GEE as an existing optimization, but with a much lower complexity.