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Cooperative communications can achieve spatial diversity gain and enhance coverage. However, conventional half-duplex relaying schemes suffer from the loss in spectral efficiency, because the two channel uses are required for transmission from the source to the destination. In this paper, we present spectral efficient transmissions for a user cooperative diversity system over multipath fading channels. To increase the spectral efficiency, a node, which acts as a relay, employs a superposition coding to transmit its own data block and the data block from the other node simultaneously. For superposition, we consider two different re-encoding schemes. One is based on complex field superposition coding and the other is based on Galois field superposition coding. In both re-encoding schemes, we introduce a user-specific interleaver to the data blocks for user separation. We also propose an iterative detection scheme to combat the effects of inter-symbol interference caused by multipath fading and to extract diversity gain from the superposed signals. For performance estimation, we develop a density evolution technique. It is shown that the proposed schemes can obtain diversity gain without sacrificing spectral efficiency and the predicted performances are consistent with simulated performances.