Antenna weight verification for closed-loop downlink eigenbeamforming

Abstract

Adaptive antennas at the base stations have a big potential to increase downlink capacity and coverage in WCDMA systems. One of the most promising techniques of adaptive antenna control is closed-loop transmit diversity. This method achieves beamforming gain as well as diversity gain by feedback of downlink fast fading characteristics from mobile station to base station. Due to the limited feedback rate, typically the best antenna weight vector chosen from a restricted set is reported. If this feedback is subject to transmission errors, two performance degrading effects occur: a suboptimal weight vector is used and channel estimation errors take place at the mobile station. The latter effect arises because the mobile station derives the channel estimate for the dedicated channel from the reported weight vector and the channel estimate per antenna based on the common pilot channel. In effect, feedback errors severely distort this estimate and lead to an error floor effect. However, channel estimation can also be based on the pilot symbols in the dedicated channel itself. This estimate has high variance, but is not distorted by feedback errors. If both estimates are combined in a process called antenna weight verification, performance can be dramatically increased. Within this paper an antenna weight verification scheme for eigenbeamformer is presented, which maximizes the estimation accuracy and eliminates the error floor. This is demonstrated by theoretical analysis and link-level simulations. It is shown that this antenna weight verification reduces the performance degradation to 0.2 dB at 1% frame error rate.