Robust uplink to downlink spatial covariance matrix transformation for downlink beamforming

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 (K. Hugl et al., 1999) and (T. Aste et al., 1998) 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 outperforms both (K. Hugl et al., 1999) and (T. Aste et al., 1998) by supporting about 48% and 15% more average number of users than that of (K. Hugl et al., 1999) and (T. Aste et al., 1998) , respectively without any significant degradation in performance when compared to (T. Aste et al., 1998) with known uplink steering vector.