  In this paper, we present nontrivial upper and lower bounds on the secrecy
capacity of the degraded Gaussian diamond-wiretap channel and identify several
ranges of channel parameters where these bounds coincide with useful
intuitions. Furthermore, we investigate the effect of the presence of an
eavesdropper on the capacity. We consider the following two scenarios regarding
the availability of randomness: 1) a common randomness is available at the
source and the two relays and 2) a randomness is available only at the source
and there is no available randomness at the relays. We obtain the upper bound
by taking into account the correlation between the two relay signals and the
availability of randomness at each encoder. For the lower bound, we propose two
types of coding schemes: 1) a decode-and-forward scheme where the relays
cooperatively transmit the message and the fictitious message and 2) a partial
DF scheme incorporated with multicoding in which each relay sends an
independent partial message and the whole or partial fictitious message using
dependent codewords.
