Robust beamforming for FDD mobile systems over Rayleigh fading channels

A beamforming approach is developed for mobile wireless systems operated in vehicular scenarios such that the channel undergoes fast Rayleigh fading. In this approach, the downlink beamforming weights of the base antennas are generated based on the orthogonal basis of the uplink signal subspace. The downlink transmission to a particular mobile is through its effective signal subspace via multiple diversity branches. Thanks to the stability of the signal subspace, the received signal strength at the mobile is robust to fast Rayleigh fading. When the distance between the uplink and downlink subspaces is negligible, the open-loop beamforming can be applied to FDD systems. Ray tracing simulations in a three-dimensional urban physical model show that the subspace beamforming enhances the link quality of mobile systems over Rayleigh fading channels.