说明：  In this letter, we consider a half-duplex amplify-andforward two-way relaying network consisting of two sources with each having a single antenna and one relay having two antennas. For such a system, a distributed orthogonal space-time block code (STBC) is proposed by making use of the Alamouti coding scheme and jointly processing the signals the two antennas at the relay node. Such a code turns out to make the equivalent channel at each source node be a product of the two Alamouti channels and thus, is called distributed concatenated Alamouti STBC.