Uplink to downlink spatial covariance matrix transformation concepts for downlink beamforming

We present a comparison of several uplink to downlink covariance matrix transformation methods that can be used in downlink beamforming for frequency selective channels of frequency division duplex (FDD) systems. The performance improvement obtained from the transformation methods is compared with that from the method, which directly uses the uplink covariance matrix for downlink beamforming. For this purpose, a dynamic system level simulator developed for investigating the capacity improvement of a UMTS-FDD system employing smart antennas at the base stations is used. Downlink capacity is considered as a measure of the performance of the system by finding the number of users for which a certain quality of service (QoS) requirement is fulfilled. System level simulations are carried out for antenna arrays with 2, 4 and 8 elements. Simulation results show the necessity of robust transformation methods for the improvement of the downlink capacity, when the uplink signature is not known exactly at the base station (BS).