  Correlation across transmit antennas, in multiple antenna systems (MIMO), has
been studied in various scenarios and has been shown to be detrimental or
provide benefits depending on the particular system and underlying assumptions.
In this paper, we investigate the effect of transmit correlation on the
capacity of the Gaussian MIMO broadcast channel (BC), with a particular
interest in the large-scale array (or massive MIMO) regime. To this end, we
introduce a new type of diversity, referred to as transmit correlation
diversity, which captures the fact that the channel vectors of different users
may have different, and often nearly mutually orthogonal, large-scale channel
eigen-directions. In particular, when taking the cost of downlink training
properly into account, transmit correlation diversity can yield significant
capacity gains in all regimes of interest. Our analysis shows that the system
multiplexing gain can be increased by a factor up to $\lfloor{M}/{r}\rfloor$,
where $M$ is the number of antennas and $r\le M$ is the common rank of the
users transmit correlation matrices, with respect to standard schemes that are
agnostic of the transmit correlation and treat the channels as if they were
isotropically distributed. Thus, this new form of diversity reveals itself as a
valuable "new resource" in multiuser communications.
