  The increasingly demanding objectives for next generation wireless
communications have spurred recent research activities on multi-antenna
transceiver hardware architectures and relevant intelligent communication
schemes. Among them belong the Full Duplex (FD) Multiple-Input Multiple-Output
(MIMO) architectures, which offer the potential for simultaneous uplink and
downlink operations in the entire frequency band. However, as the number of
antenna elements increases, the interference signal leaking from the
transmitter of the FD radio to its receiver becomes more severe. In this
article, we present a unified FD massive MIMO architecture comprising analog
and digital transmit and receive BeamForming (BF), as well as analog and
digital SI cancellation, which can be jointly optimized for various performance
objectives and complexity requirements. Performance evaluation results for
applications of the proposed architecture to fully digital and hybrid analog
and digital BF operations using recent algorithmic designs, as well as
simultaneous communication of data and control signals are presented. It is
shown that the proposed architecture, for both small and large numbers of
antennas, enables improved spectral efficiency FD communications with fewer
analog cancellation elements compared to various benchmark schemes. The article
is concluded with a list of open challenges and research directions for future
FD massive MIMO communication systems and their promising applications.
