We study the degrees of freedom (DoF) of a distributed multi-input multi-output (MIMO) multiway relay channel (mRC) where two pairs of users, each equipped with M antennas, exchange messages in a pairwise manner with the help of two separated relay nodes, each with N antennas. We establish a general framework to derive achievable DoF based on linear signal processing. We show that, to achieve a certain DoF, a joint design of the user transmit beamforming, relay precoding, and user receive beamforming is required. We propose novel signal alignment techniques to reduce the number of linearly independent equations required for interference neutralization. Then, the original joint transceiver and relay design problem boils down to a problem solely on the design of the relay precoders. Based on recent developments on solving linear matrix systems, we determine the solution to the relay design problem and derive the corresponding achievable DoF. Our analysis reveals that the DoF of the considered network is achieved for equation, which is broader than the existing results by covering an extra range of M over N ∈ (1, 2).
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