Energy efficiency in multiaccess fading channels under QoS constraints

In this paper, transmission over multiaccess fading channels under QoS constraints is studied in the low power regime. QoS constraints are imposed as limitations on the buffer violation probability. The effective capacity, which characterizes the maximum constant arrival rates in the presence of such statistical QoS constraints, is employed as the performance metric. The minimum received bit energy levels and wideband slope regions are characterized for different transmission and reception strategies, namely time-division multiple-access (TDMA), superposition coding with fixed decoding order, and superposition coding with variable decoding order. It is shown that the minimum received bit energies achieved by these different strategies are the same and independent of the QoS constraints. When wideband slope regions are considered, the suboptimality of TDMA with respect to superposition schemes is shown. For the case of superposition coding, it is proven that varying the decoding order at the receiver with the fading realizations does not enlarge the wideband slope region.