  In this paper, we propose a systematic solution to the problem of cross-layer
optimization for delay-sensitive media transmission over time-varying wireless
channels as well as investigate the structures and properties of this solution,
such that it can be easily implemented in various multimedia systems and
applications. Specifically, we formulate this problem as a finite-horizon
Markov decision process (MDP) by explicitly considering the users'
heterogeneous multimedia traffic characteristics (e.g. delay deadlines,
distortion impacts and dependencies etc.), time-varying network conditions as
well as, importantly, their ability to adapt their cross-layer transmission
strategies in response to these dynamics. Based on the heterogeneous
characteristics of the media packets, we are able to express the transmission
priorities between packets as a new type of directed acyclic graph (DAG). This
DAG provides the necessary structure for determining the optimal cross-layer
actions in each time slot: the root packet in the DAG will always be selected
for transmission since it has the highest positive marginal utility; and the
complexity of the proposed cross-layer solution is demonstrated to linearly
increase w.r.t. the number of disconnected packet pairs in the DAG and
exponentially increase w.r.t. the number of packets on which the current
packets depend on. The simulation results demonstrate that the proposed
solution significantly outperforms existing state-of-the-art cross-layer
solutions. Moreover, we show that our solution provides the upper bound
performance for the cross-layer optimization solutions with delayed feedback
such as the well-known RaDiO framework.
