Robust uplink to downlink spatial covariance matrix transformation for downlink beam-forming

We propose a new uplink to downlink spatial covariance matrix transformation technique that provides robustness against uplink steering vector mismatches for Frequency Duplex Systems (FDD) in a frequency selective fading environment. Our approach leads to a matrix Robust Least Squares (RLS) problem that can be formulated in a convex form as a Second Order Cone (SOC) program and solved efficiently using the well-established interior point methods. The performance of the proposed method is compared with that of [3] and [4] using a dynamic system level simulator developed for investigating the capacity improvement of a UMTS-FDD system with smart antennas at the base stations. Downlink capacity is used as a measure of system performance by finding the number of users for which a certain quality of service (QoS) requirement is fulfilled. System level simulations are carried out for 4-element Uniform Circular Antenna (UCA) array. Simulation results show that our method outperfoms both [3] and [4] by supporting about 48% and 15% more average number of users than that of [3] and [4], respectively without any significant degradation in performance when compared to [4] with known uplink steering vector.