Which is better: One-way or two-way relaying with an amplify-and-forward relay?

Amplify-and-forward (AF) based two-way relaying (TWR) has shown its sum rate advantage over AF based one-way relaying (OWR) in the literature. However, this advantage can not be achieved in some cases. In this paper, we evaluate the sum rates of AF based OWR/TWR and provide the relationship with respect to the relative channel gains and the relative transmit power of the three nodes, i.e., the relay node and two source nodes. Upon this, we determine the metric on choosing OWR or TWR given instantaneous channel state information and we further devise power allocation schemes. At last, the numerical results validate our conclusion and reveal some interesting facts.