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Error propagation of source-relay (S-R) link limits the performance of decode-and-forward (DF) relay system and prohibits cooperative DF relay system from achieving full diversity gain. In order to resolve these problems, we first derive the criterion to achieve full diversity. It explains why the SNR of S-R link would be better than other links. Using the criterion, the proposed strategy of positioning the relay achieves full diversity and minimizes the error probability of cooperative DF relay system.