  We consider a diamond-shaped dual-hop communication system consisting a
source, two parallel half-duplex relays and a destination. In a single antenna
configuration, it has been previously shown that a two-phase node-scheduling
algorithm, along with the decode and forward strategy can achieve the capacity
of the diamond channel for a certain symmetric channel gains [1]. In this
paper, we obtain a more general condition for the optimality of the scheme in
terms of power resources and channel gains. In particular, it is proved that if
the product of the capacity of the simultaneously active links are equal in
both transmission phases, the scheme achieves the capacity of the channel.
