  In this paper, we study a X-duplex relay system with one source, one
amplify-and-forward (AF) relay and one destination, where the relay is equipped
with a shared antenna and two radio frequency (RF) chains used for transmission
or reception. X-duplex relay can adaptively configure the connection between
its RF chains and antenna to operate in either HD or FD mode, according to the
instantaneous channel conditions. We first derive the distribution of the
signal to interference plus noise ratio (SINR), based on which we then analyze
the outage probability, average symbol error rate (SER), and average sum rate.
We also investigate the X-duplex relay with power allocation and derive the
lower bound and upper bound of the corresponding outage probability. Both
analytical and simulated results show that the X-duplex relay achieves a better
performance over pure FD and HD schemes in terms of SER, outage probability and
average sum rate, and the performance floor caused by the residual self
interference can be eliminated using flexible RF chain configurations.
