In what ways could we improve cellular-massive-MIMO based 5G? Well, to start with, this technology is already pretty good. But coverage holes, and difficulties to send multiple streams to multi-antenna users because of insufficient channel rank, remain issues.

Perhaps the ultimate solution is distributed MIMO (also known as cell-free massive MIMO). But while this is at heart a powerful technology, installing backhaul seems dreadfully expensive, and achieving accurate phase-alignment for coherent multiuser beamforming on downlink is a difficult technical problem. Another option is RIS – but they have large form factors and require a lot of training and control overhead, and probably, in practice, some form of active filtering to make them sufficiently band-selective. 

A different, radically new approach is to deploy large numbers of physically small and cheap wireless repeaters, that receive and instantaneously retransmit signals – appearing as if they were ordinary scatterers in the channel, but with amplification. Repeaters, as such, are deployed today already but only in niche use cases. Could they be deployed at scale, in swarms, within the cells? What would be required of the repeaters, and how well could a repeater-assisted cellular massive MIMO system work, compared to distributed MIMO? What are the fundamental limits of this technology? 

At last, some significant new research directions for the wireless PHY community!

Paper: https://arxiv.org/pdf/2406.00142