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In this paper, we propose a simple orthogonal space time transmission scheme for asynchronous cooperative systems. In the proposed scheme, OFDM is implemented at the source node, some very simple operations, namely time reversion and complex conjugation, are implemented at the relay nodes, and a two-step of cyclic prefix (CP) removal is performed at the destination. The CP at the source node is used for combating the frequency selective fading channels and the timing errors. In this scheme, the received signals at the destination node have the orthogonal code structure on each subcarrier and thus it has the fast symbol-wise ML decoding and can achieve full spatial diversity when SNR is large without the requirement of symbol level synchronization. It should be emphasized that since no Add/Remove CP or IFFT/FFT operation is needed at the relay nodes, the relay nodes do not have to know any information about the channels and the timing errors, and the complexity of the relay nodes is very low.