Simultaneous Diagonalization: On the DoF Region of the K-user MIMO   Multi-way Relay Channel
  The K-user MIMO Y-channel consisting of K users which want to exchange messages among each other via a common relay node is studied in this paper. A transmission strategy based on channel diagonalization using zero-forcing beam-forming is proposed. This strategy is then combined with signal-space alignment for network-coding, and the achievable degrees-of-freedom region is derived. A new degrees-of-freedom outer bound is also derived and it is shown that the proposed strategy achieves this outer bound if the users have more antennas than the relay. 
Anas Chaaban, Karlheinz Ochs, Aydin Sezgin
cs.IT math.IT