Enabling massive MIMO systems in the FDD mode thanks to D2D communications

We propose novel approaches to design feedback in FDD massive MIMO systems. We exploit synergies between massive MIMO systems and inter-user communications based on D2D. The exchange of local CSI among users, enabled by D2D communications, makes available global CSI at the terminals. Thus, we can construct more informative forms of feedback based on this shared knowledge. Two feedback variants are highlighted: 1) cooperative CSI feedback, and 2) cooperative precoder index feedback. For a given feedback overhead, the sum-rate performance is assessed and the gains compared with a conventional massive MIMO setup without D2D are shown.