  We consider a communication system with multi-access fading channel. Each
user in the system requires certain rate guarantee. Our main contribution is to
devise a scheduling scheme called "Opportunistic Super-position Coding" that
satisfies the users' rate requirements. Using mean-field analysis, i.e., when
the number of users go to infinity, we analytically show that the energy
required to guarantee the required user rate can be made as small as required
at the cost of a higher delay ("delay-energy tradeoff"). We explicitly compute
the delay under the proposed scheduling policy and discuss how delay
differentiation can be achieved. We extend the results to multi-band
multi-access channel. Finally, all the results can be generalized in a
straightforward fashion to broadcast channel due to the AWGN
multiaccess-broadcast duality.
