  We consider a bidirectional network in which two users exchange information
with the help of a buffer-aided relay. In such a network without direct link
between user 1 and user 2, there exist six possible transmission modes, i.e.,
four point-to-point modes (user 1-to-relay, user 2-to-relay, relay-to-user 1,
relay-to-user 2), a multiple access mode (both users to the relay), and a
broadcast mode (the relay to both users). Because of the buffering capability
at the relay, the transmissions in the network are not restricted to adhere to
a predefined schedule, and therefore, all the transmission modes in the
bidirectional relay network can be used adaptively based on the instantaneous
channel state information (CSI) of the involved links. For the considered
network, assuming fixed transmit powers for both the users and the relay, we
derive the optimal transmission mode selection policy which maximizes the sum
rate. The proposed policy selects one out of the six possible transmission
modes in each time slot based on the instantaneous CSI. Simulation results
confirm the effectiveness of the proposed protocol compared to existing
protocols.
