In this letter, we propose a simple orthogonal

frequency-division multiplexing (OFDM) scheme for an asynchronous cooperative system, where OFDM is implemented at the source node, and time-reversion and complex conjugation are implemented at the relay nodes. The cyclic prefix (CP) at the source node is used for combating the timing errors the relay nodes.

In this scheme, the received signals at the destination node have the Alamouti code structure on each subcarrier, and thus, it has

the fast symbol-wise ML decoding. It should be emphasized that the relay nodes only need to implement the time-reversion, some sign changes plus to minus, and/or the complex conjugation to the received signals, and no IDFT or DFT operation is needed.It is shown that this simple scheme achieves second-order diversity

gain without the synchronization requirement at the relay nodes.

Index Terms—Alamouti code, asynchronous cooperative diversity,

orthogonal frequency-division multiplexing (OFDM).-In this letter, we propose a simple orthogonal

frequency-division multiplexing (OFDM) scheme for an asynchronous cooperative system, where OFDM is implemented at the source node, and time-reversion and complex conjugation are implemented at the relay nodes. The cyclic prefix (CP) at the source node is used for combating the timing errors　the relay nodes.

In this scheme, the received signals at the destination node have the Alamouti code structure on each subcarrier, and thus, it has

the fast symbol-wise ML decoding. It should be emphasized that the relay nodes only need to implement the time-reversion, some sign changes　plus to minus, and/or the complex conjugation to the received signals, and no IDFT or DFT operation is needed.It is shown that this simple scheme achieves second-order diversity

gain without the synchronization requirement at the relay nodes.

Index Terms—Alamouti code, asynchronous cooperative diversity,

orthogonal frequency-division multiplexing (OFDM).