  A Relay Station (RS) uses a buffer to store and process the received data
packets before forwarding them. Recently, the buffer has been exploited in
one-way relaying to opportunistically schedule the two different links
according to their channel quality. The intuition is that, if the channel to
the destination is poor, then RS stores more data from the source, in order to
use it when the channel to the destination is good. We apply this intuition to
the case of half-duplex two-way relaying, where the interactions among the
buffers and the links become more complex. We investigate the sum-rate
maximization problem in the Time Division Broadcast (TDBC): the users send
signals to the RS in different time slots, the RS decodes and stores messages
in the buffers. For downlink transmission, the RS re-encodes and sends using
the optimal broadcast strategy. The operation in each time slot is not
determined in advance, but depends on the channel state information (CSI). We
derive the decision function for adaptive link selection with respect to CSI
using the Karush-Kuhn-Tucker (KKT) conditions. The thresholds of the decision
function are obtained under Rayleigh fading channel conditions. The numerical
results show that the sum-rate of the adaptive link selection protocol with
buffering is significantly larger compared to the reference protocol with fixed
transmission schedule.
