In this paper, we consider a bidirectional communication between K node pairs, where each node is equipped with multiple antennas and has a message to be transmitted to its communication partner. There is no direct link between the 2K nodes and a relay is employed to enable the communication. It is assumed that the relay does not have enough antennas to perform receive and transmit zero forcing. As interferences are unavoidable at each receiver, we call this channel K user pair symmetric relay interference channel. We show that the Degrees of Freedom (DoF) achievable by the multiple input multiple output (MIMO) Interference Alignment in such a K user pair symmetric relay interference channel are the same as in a K user symmetric interference channel [1] without a relay. Besides this, the presence of a relay simplifies the algorithm for computing the alignment solution. We propose a two step closed form solution for the alignment problem based on the two way relaying protocol.
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