Transmit diversity techniques provide attractive solutions for increasingrndownlink capacity in 3G systems. Open-loop and closed-loop transmitrndiversity modes with two transmit antennas have already been included intorn3G Partner Project WCDMA (wideband code division multiplexing access),rnand schemes exploiting a large number of transmit antennas are beingrndeveloped. System capacity can be increased from that of open-loop modesrnif the transmitter is equipped with additional side information of therndownlink channel. In a frequency division duplex (FDD) system this meansrnthat the receiver has to provide the information through some feedbackrnmechanism.rnIn this work, a scheme which combines a closed-loop and open-looprntransmit diversity (adaptive space-time transmit diversity) is investigatedrnand the signal-to-noise ratio (SNR) to bit error rate (BER) performance ofrnthis adaptive space-time transmit diversity schemes are evaluated on a flatrnRayleigh fading channel.rnThe performance results obtained from the simulations show that adaptivernspace-time transmit diversity schemes have better error rate performancernthan the space-time transmit diversity. Finally, this work has shown thatrnadaptive power allocation scheme in conjunction with the Alamouti schemernhas an improved error rate than the open loop transmit diversity (Alamoutirnscheme) and also adaptive space time modulation has an improved error rate performance than the Alamouti scheme. Moreover, this thesis showedrnthe tradeoff between schemes combining both adaptive rate and power withrndifferent percentages and increasing the power at the same time reducingrnthe data rate has an improved error rate performance than reducing thernpower at the same time increasing the data rate of the system