  This paper describes an approach for half-duplex cooperative transmission in
a classical three-node relay channel. Assuming availability of channel state
information at nodes, the approach makes use of this information to optimize
distinct flows through the direct link from the source to the destination and
the path via the relay, respectively. It is shown that such a design can
effectively harness diversity advantage of the relay channel in both high-rate
and low-rate scenarios. When the rate requirement is low, the proposed design
gives a second-order outage diversity performance approaching that of
full-duplex relaying. When the rate requirement becomes asymptotically large,
the design still gives a close-to-second-order outage diversity performance.
The design also achieves the best diversity-multiplexing tradeoff possible for
the relay channel. With optimal long-term power control over the fading relay
channel, the proposed design achieves a delay-limited rate performance that is
only 3.0dB (5.4dB) worse than the capacity performance of the additive white
Gaussian channel in low- (high-) rate scenarios.
