  In this paper we study decode-and-forward multi-antenna relay systems that
achieve bi-directional communication in two time slots. We investigate
different downlink broadcast schemes which employ binary or analog network
coding at the relay. We also analyze and compare their performances in terms of
diversity order and symbol error probability. It is shown that if exact
downlink channel state information is available at the relay, using analog
network coding in the form of multi-antenna maximal-ratio transmit beamforming
to precode the information vectors at the relay gives the best performance.
Then, we propose a Max-Min antenna selection with binary network coding scheme
that can approach this performance with only partial channel state information.
