  In this paper, we study the capacity of the diamond channel. We focus on the
special case where the channel between the source node and the two relay nodes
are two separate links with finite capacities and the link from the two relay
nodes to the destination node is a Gaussian multiple access channel. We call
this model the Gaussian multiple access diamond channel. We first propose an
upper bound on the capacity. This upper bound is a single-letterization of an
$n$-letter upper bound proposed by Traskov and Kramer, and is tighter than the
cut-set bound. As for the lower bound, we propose an achievability scheme based
on sending correlated codes through the multiple access channel with
superposition structure. We then specialize this achievable rate to the
Gaussian multiple access diamond channel. Noting the similarity between the
upper and lower bounds, we provide sufficient and necessary conditions that a
Gaussian multiple access diamond channel has to satisfy such that the proposed
upper and lower bounds meet. Thus, for a Gaussian multiple access diamond
channel that satisfies these conditions, we have found its capacity.
