A study of half-duplex two-way decode-and-forward relay transmission with asymmetric traffic using superposition coding

In this paper, we study the outage performance of a two-way relay transmission, where each terminal has a single antenna and operates in a half-duplex manner, especially, a decode-and-forward (DF) relay is employed to help the bidirectional communications between two user terminals and superposition coding is applied at the relay to re-encode the successfully decoded messages. Unlike existing works, we consider a more general setup that the two user terminals have different target rates (i.e., asymmetric two-way relay transmission). First, an achievable rate region of the network scenario under consideration is characterized followed by a definition of the overall system outage probability. Then, the investigated network scenario is evaluated in terms of outage probability, after which the corresponding closed-form expressions are formulated over Rayleigh fading channels. Next, to maximize the energy efficiency, optimum power allocation between the superposed signals at the relay are examined. As a result, two different policies implementing fixed and variable scaling coefficients of superposition coding are proposed. Then, simulation experiments are performed and performance comparisons are conducted. The provided results validate the accuracy of our derived expressions and the efficiency of our propositions.