Antenna weight verification for closed loop transmit diversity

Abstract

Closed loop transmit diversity (CLTD) for FDD WCDMA relies on low-rate feedback to achieve both beamforming and diversity gain. Since the feedback channel is not immune to errors, occasionally base station (BS) uses different antenna weight vector from the one requested by mobile station (MS). Surprisingly, most of the resulting performance degradation is caused not by reduced power of the Rx signal, but by erroneous dedicated channel estimation at the MS relying on knowledge of used weight vector. In this paper we introduce a general trellis-based antenna weight verification algorithm, which attempts to detect feedback errors and determine the most likely weight vector. By exploiting the structure of CLTD schemes proposed for WCDMA with 2 and 4 BS-antennas we show methods to reduce the complexity of our algorithm. Our comparative simulations show that antenna weight verification reduces the performance degradation due to feedback errors less than 0.5 dB at 1% frame error rate for considered 4 antenna schemes.