Use of Downlink Pilots for Cache-Aided Rician-Faded Cell-Free Massive MIMO Systems: Investigation, Analysis and Optimization

We consider the downlink of a cache-aided cell-free (CF) massive multi-input multi-output (mMIMO) system, wherein user equipments (UEs) exploit cached data to cancel multi-user interference (MUI) in the downlink. This work shows the importance of downlink pilots transmitted by the APs, which are used by the UEs to first estimate the instantaneous downlink channel state information (CSI), and then perform cache-aided MUI cancellation. We derive a closed-form SE expression for this system by considering practical spatially-correlated Rician fading channels with phase shifts, pilot contamination, and hardware impairments, both at the APs and UEs. We also propose a joint cache placement and power control optimization to maximize the global energy efficiency (GEE) metric. The cache placement problem is solved via clustering, while the power is optimized by designing a novel low-complexity parallel block minorization-maximization optimization. We numerically validate the benefits of downlink CSI for cache-aided CF mMIMO systems, and also show that degradation in CSI quality dramatically negates the UEs cache-aided MUI cancellation capability. We also show that the proposed optimization yields the same GEE as existing state-of-the-art optimizations, but with a much lower complexity.