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In this paper, we propose a spectrally efficient cooperative transmission protocol for multiple access scenarios. The key feature is to utilize multi-user diversity and fully exploit the dynamic nature of radio propagation. In particular, by carefully scheduling the multiple sources and relays' transmissions, a source with a poor connection to the destination can have higher priority to obtain help from a relay with better channel condition. As a result, the full diversity gain is achievable even though only a fraction of relays is scheduled to help each user. We developed an achievable diversity-multiplexing tradeoff for the proposed transmission protocol to assist performance evaluation. When the number of relays is large, the diversity-multiplexing tradeoff achieved by the proposed scheme can approximate the optimal multiple-input single-output upper bound. Both analytical and numerical results show that the proposed protocol outperform other comparable schemes in most conditions.