Ralf Wiedmann

Performance of downlink eigenbeamformer with realistic feedback transmission

Several closed-loop transmitter diversity concepts for multiple antennas are currently under discussion for inclusion in the 3GPP W-CDMA standard. The focus is to find a concept using 4 transmitter antennas allowing for finer beamsteering while still being compatible with the current uplink feedback rate of one bit per transmission slot. Among these concepts, the downlink eigenbeamformer effectively uses long-term as well as short-term channel properties. More specifically, it is based on an eigenanalysis of the long-term spatial covariance matrix. The eigenbeams with the largest eigenvalues (largest average SNR) are determined and fed back step by step to the base station (BS). This process takes place on the same time scale as the mobile stations (MS) physical movement. Accordingly, required operations in the MS as well as required long-term feedback bits are distributed over a very large number of transmission slots. In addition, a short term selection between the eigenbeams is carried out at the MS to account for fast fading. Performance in terms of coded block error rate for the downlink eigenbeamformer concept is analysed under a set of spatial channel models. Simulation results show the influence of important parameters like, e.g., the amount of common pilot channel power compared to another transmitter diversity proposal that does not exploit the long term channel statistics. The multiplexing formats between long-term and short-term feedback information are discussed and a detailed complexity analysis is given.