Orthogonal STBC [4], [5] has been designed to achieve the spatial diversity offered by multiple transmit and/or receive antennas. The STBC

schemes proposed in [4] and [5] are designed for flat-fading channels,

which lead to a performance degradation in time- and frequency-selective channels. Our goal is to design efficient STBC schemes to counteract the effects of doubly-selective channels. The basic idea of the

proposed STBC for a 21 system is to multiplex the data sequence

in two data subsequences and to generate two orthogonal full-diversity

subchannels over doubly-selective channels. Then, we apply a scheme

that is similar to the Alamouti code.