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804 IEEE SIGNAL PROCESSING LETTERS, VOL. 14, NO. 11, NOVEMBER 2007 A Simple Alamouti Space–Time Transmission Scheme for Asynchronous Cooperative Systems Zheng Li and Xiang-Gen Xia Abstract— In this letter, we propose a simple orthogonal frequency-division multiplexing (OFDM) scheme for an asyn- chronous cooperative system, where OFDM is implemented at the source node, and time-reversion and complex conjugation are im- plemented at the relay nodes. The cyclic prefix (CP) at the source node is used for combating the timing errors from the relay nodes. In this scheme, the received signals at the destination node have Fig. 1. Cooperative system architecture. the Alamouti code structure on each subcarrier, and thus, it has the fast symbol-wise ML decoding. It should be emphasized that nodes. In our proposed scheme, the relay nodes only implement the relay nodes only need to implement the time-reversion, some sign changes from plus to minus, and/or the complex conjugation very simple operations on their received signals from the source to the received signals, and no IDFT or DFT operation is needed. node: time-reversion, some sign changes from plus to minus, It is shown that this simple scheme achieves second-order diversity and/or the complex conjugation to the received signals. gain without the synchronization requirement at the relay nodes. Index Terms—Alamouti code, asynchronous cooperative diver- II. NEW SPACE–TIME TRANSMISSION SCHEME sity, orthogonal frequency-division multiplexing (OFDM). For simplicity, let us consider a cooperative system with