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In this letter, a spatial-domain joint-nulling method is proposed as a new means of suppressing the self-interference (or echo) occurring in full-duplex relays. Both amplify-and-forward (AF) and decode-and-forward (DF) type relays are considered. While the conventional method searches for the relay processing matrix from a discrete set of basis vectors obtained from the singular value decomposition (SVD) of the echo channel, the proposed method directly solves the optimal relay processing matrix over a continuous domain. As a result, it is shown that the proposed approach gives better performance than the conventional one in terms of the achievable rate.