  The time-varying feature of wireless channels usually makes the hard delay
bound for data transmissions unrealistic to guarantee. In contrast, the
statistically-bounded delay with a small violation probability has been widely
used for delay quality-of-service (QoS) characterization and evaluation. While
existing research mainly focused on the statistical-delay control in single-hop
links, in this paper we propose the QoS-driven power-allocation scheme over
two-hop wireless relay links to statistically upper-bound the end-to-end delay
under the decodeand- forward (DF) relay transmissions. Specifically, by
applying the effective capacity and effective bandwidth theories, we first
analyze the delay-bound violation probability over two tops each with
independent service processes. Then, we show that an efficient approach for
statistical-delay guarantees is to make the delay distributions of both hops
identical, which, however, needs to be obtained through asymmetric resource
allocations over the two hops. Motivated by this fact, we formulate and solve
an optimization problem aiming at minimizing the average power consumptions to
satisfy the specified end-to-end delay-bound violation probability over two-hop
relay links. Also conducted is a set of simulations results to show the impact
of the QoS requirements, traffic load, and position of the relay node on the
power allocation under our proposed optimal scheme.
