  We consider a half-duplex diamond relay channel, which consists of one
source-destination pair and two relay nodes connected with two-way rate-limited
out-of-band conferencing links. Three basic schemes and their achievable rates
are studied: For the decode-and-forward (DF) scheme, we obtain the achievable
rate by letting the source send a common message and two private messages; for
the compress-and-forward (CF) scheme, we exploit the conferencing links to help
with the compression of the received signals, or to exchange messages intended
for the second hop to introduce certain cooperation; for the
amplify-and-forward (AF) scheme, we study the optimal combining strategy
between the received signals from the source and the conferencing link.
Moreover, we show that these schemes could achieve the capacity upper bound
under certain conditions. Finally, we evaluate the various rates for the
Gaussian case with numerical results.
