  Most works on cell-free massive multiple-input multiple-output (MIMO)
consider non-cooperative precoding strategies at the base stations (BSs) to
avoid extensive channel state information (CSI) exchange via backhaul
signaling. However, considerable performance gains can be accomplished by
allowing coordination among the BSs. This paper proposes the first distributed
framework for cooperative precoding design in cell-free massive MIMO (and, more
generally, in joint transmission coordinated multi-point) systems that entirely
eliminates the need for backhaul signaling for CSI exchange. A novel
over-the-air (OTA) signaling mechanism is introduced such that each BS can
obtain the same cross-term information that is traditionally exchanged among
the BSs via backhaul signaling. The proposed distributed precoding design
enjoys desirable flexibility and scalability properties, as the amount of OTA
signaling does not scale with the number of BSs or user equipments. Numerical
results show fast convergence and remarkable performance gains as compared with
non-cooperative precoding design. The proposed scheme can also outperform the
centralized precoding design under realistic CSI acquisition.
