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We consider joint design of source and relay precoders in a cognitive multiuser multi-way relay system, which supports simultaneous transmission of multiple secondary users concurrently with primary network with the help of a relay station. The design criterion is minimum mean-square error (MMSE) of all secondary users under a transmit power constraint for each transmitting node and a constraint on the interference to primary network assuming complete knowledge of the channel state information is available. To solve this non-convex optimization problem, an iterative algorithm is proposed to iteratively solve the precoding matrices at secondary source nodes and relay node, and the decoding matrices at secondary nodes, where each subproblem is convex. To reduce the computational complexity, a matrix distance based non-iterative algorithm which can be implemented in a distributed manner is also proposed. Given the fact that perfect channel state information is usually non-realistic, a robust precoder design is also proposed to address this problem. Simulation results show the effectiveness of our proposed algorithms.