Closed-loop transmit diversity (transmit beamforming) for mitigation of interference and multipath fading in wireless communication systems

Abstract

The wireless communication channel suffers from many impairments such as the thermal noise often modeled as Additive White Gaussian Noise (AWGN), the path loss in power as the radio signal propagates, the shadowing due to the presence of fixed obstacles in the radio path, and the fading which combines the effects of multiple propagation paths and the rapid movement of mobile units reflectors. Deploying multiple antennas at the transmitter has been shown  to increase diversity and therefore improve signal quality with increased throughput. This paper proposes a transmit diversity scheme, where multiple transmit antennas are used at the transmitter. A feedback path is provided from the receiver to communicate the channel seen by the receiver to the transmitter (closed-loop). When closed-loop transmit diversity is applied, the symbol from each transmit antenna is multiplied with a complex number corresponding to the inverse of the phase of the channel so as to ensure that the signals add constructively at the receiver. From this research it was found that sending the same information on multiple transmit antenna does not always provide diversity gain. However if the transmitted symbols are multiplied by a complex phase to ensure that the phases align at the receiver, there is diversity gain though the bit error rate performance seems to be slightly poorer than the maximal ratio combining case.