  In this paper, the diamond relay channel is considered, which consists of one
source-destination pair and two relay nodes connected with rate-limited
out-of-band conferencing links. In particular, we focus on the half-duplex
alternative relaying strategy, in which the two relays operate alternatively
over time. With different amounts of delay, two conferencing strategies are
proposed, each of which can be implemented by either a general two-side
conferencing scheme (for which both of the two conferencing links are used) or
a special-case one-side conferencing scheme (for which only one of the two
conferencing links is used). Based on the most general two-side conferencing
scheme, we derive the achievable rates by using the decode-and-forward (DF) and
amplify-and-forward (AF) relaying schemes, and show that these rate
maximization problems are convex. By further exploiting the properties of the
optimal solutions, the simpler one-side conferencing is shown to be equally
good as the two-side conferencing in term of the achievable rates under
arbitrary channel conditions. Based on this, the DF rate in closed-form is
obtained, and the principle to use which one of the two conferencing links for
one-side conferencing is also established. Moreover, the DF scheme is shown to
be capacity-achieving under certain conditions with even one-side conferencing.
For the AF relaying scheme, one-side conferencing is shown to be sub-optimal in
general. Finally, numerical results are provided to validate our analysis.
