Flip-and-forward achieves the optimal diversity-multiplexing tradeoff for the two-hop MIMO relay channel, with two relay antennas

The flip-and-forward scheme introduced in (Yang-Belfiore, 2007) for handling communication in multi-hop MIMO relay networks was shown to achieve a better diversity-multiplexing tradeoff than pure amplify-and-forward schemes. In the present paper, we show that the flip-and-forward scheme actually achieves the optimal diversity-multiplexing tradeoff for a two-hop relay network, when the relay is equipped with two antennas.