Robust Downlink Beamforming Using Covariance Channel State Information
The problem of multiuser downlink beam-forming is studied under the assumption that the transmitter has erroneous covariance-based Channel State Information (CSI). The goal is to minimize the transmit power under the worst-case Quality-of-Service (QoS) constraints. Previous convex optimization-based solutions to this problem involve several coarse approximations of the original problem. In the authors' proposed solution, such coarse approximations are avoided and an exact representation of the worst-case solution is obtained using Lagrange duality. The so-obtained problem is then converted to a convex form using Semi-Definite Relaxation (SDR). Computer simulations show that the SDR step does not involve any approximation as the resulting solution is always rank-one. Simulation results demonstrate substantial performance improvements over earlier worst-case optimization-based downlink beam-forming techniques.