  In this paper, we analyze the 2-relay multiple-input multiple-output (MIMO)
Gaussian diamond channel. We show that a multihopping decode-and-forward with
multiple access (MDF-MAC) protocol achieves rates within a constant gap from
capacity when a channel parameter $\Delta$ is greater than zero. We also
identify the transmit covariance matrices to be used by each relay in the
multiple-access (MAC) state of the MDF-MAC protocol. As done for the
single-antenna 2-relay Gaussian diamond channel, the channel parameter $\Delta$
is defined to be the difference between the product of the capacities of the
links from the source to the two relays and the product of the capacities of
the links from the two relays to the destination.
