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In this paper, we present an open-loop transmit scheme for MIMO communications. We first use the uplink channel correlation matrix (UCCM) to calculate the array weight vectors for downlink beamforming (DLBF). Then based on the symbol error rate (SER) upper bound as design criterion, we derive an algorithm for adaptive power allocation among multiple beams, and thus develop the transmit scheme joint adaptive beamforming (ABF) with space-time block coding (STBC). The main benefit of the scheme is that it can almost achieve the optimal performance with low complexity. Next, using the moment generation function (MGF) approach and the Gauss-Chebyshev integration, we derive a simple and accurate numerical analysis method for the proposed scheme under three widely used modulations. Finally, computer simulation results demonstrate the superiority of the open-loop transmit scheme.