A simple Space Time Code, suggested by Mr. Siavash M Alamouti in his landmark October 1998 paper - A Simple Transmit Diversity Technique for Wireless Communication, offers a simple method for achieving spatial diversity with two transmit antennas.
Since the estimate of the transmitted symbol with the Alamouti STBC scheme is identical to that obtained from MRC, the BER should be same as that for MRC. However, there is a small catch. With Alamouti STBC, we are transmitting from two antenna’s. Hence the total transmit power in the Alamouti scheme is twice that of that used in MRC. To make the comparison fair, we need to make the total trannsmit power from two antennas in STBC case to be equal to that of power transmitted from a single antenna in the MRC case.
With this scaling, we can see that BER performance of 2Tx, 1Rx Alamouti STBC case has a roughly 3dB poorer performance that 1Tx, 2Rx MRC case.
For theoretical details on 2 transmit, 1 receive Alamouti STBC coding and its equalization, please check