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Space-time transmit diversity is mainly designed to combat channel fading in downlink transmission by combining channel coding and array diversity techniques. This expands communication system capacity and attains diversity gain and coding gain. Most modelling on space-time transmit diversity is based on the assumption that there is a single path arriving at the receive antenna from a transmit antenna. However, in practice there are multiple paths. The conventional space-time transmit diversity scheme, where the same spread code is used in the transmit antennas, has some disadvantages in the practical system and its performance is undesirable in a multipath channel. A novel space-time transmit diversity scheme, where orthogonal spread codes are used respectively at the transmit antennas is presented to solve this problem. Simulation results reveal that the space-time transmit diversity scheme presented in this paper has much better performance than the conventional one in a multipath channel. We also find that this scheme has better performance with an increase of multipath number.