  In this letter, we propose a joint transmission mode and transmit/receive
(Tx/Rx) antenna configuration scheme referred to as X-duplex in the relay
network with one source, one amplify-and-forward (AF) relay and one
destination. The relay is equipped with two antennas, each of which is capable
of reception and transmission. In the proposed scheme, the relay adaptively
selects its Tx and Rx antenna, operating in either full-duplex (FD) or
half-duplex (HD) mode. The proposed scheme is based on minimizing the symbol
error rate (SER) of the relay system. The asymptotic expressions of the
cumulative distribution function (CDF) for the end-to-end signal to
interference plus noise ratio (SINR), average SER and diversity order are
derived and validated by simulations. Results show that the X-duplex scheme
achieves additional spatial diversity, significantly reduces the performance
floor at high SNR and improves the system performance.
