  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.
