  We develop a general approach for designing scheduling policies for real-time
traffic over wireless channels. We extend prior work, which characterizes a
real-time flow by its traffic pattern, delay bound, timely-throughput
requirement, and channel reliability, to allow time-varying channels, allow
clients to have different deadlines, and allow for the optional employment of
rate adaptation. Thus, our model allow the treatment of more realistic fading
channels as well as scenarios with mobile nodes, and the usage of more general
transmission strategies.
  We derive a sufficient condition for a scheduling policy to be feasibility
optimal, and thereby establish a class of feasibility optimal policies. We
demonstrate the utility of the identified class by deriving a feasibility
optimal policy for the scenario with rate adaptation, time-varying channels,
and heterogeneous delay bounds. When rate adaptation is not available, we also
derive a feasibility optimal policy for time-varying channels. For the scenario
where rate adaptation is not available but clients have different delay bounds,
we describe a heuristic. Simulation results are also presented which indicate
the usefulness of the scheduling policies for more realistic and complex
scenarios.
