Impact of Practical Codebook Limitations on HSUPA Closed Loop Transmit Diversity

Different alternatives for uplink transmit diversity for HSUPA are being investigated in 3GPP. This paper compares the performance of two-antenna beamforming using closed loop feedback information against the baseline single antenna transmission. With beamforming, multiple transmit antennas are utilized and the UE transmitter applies a weight vector to the transmit antennas in order to amplify the received signal. In this paper, the impact of different codebook sizes, delays and update intervals related to antenna weights are studied utilizing a comprehensive system level simulator. The studies show that increasing codebook size provides relatively higher performance gain over the baseline. However, increasing the number of available beamforming antenna weights will also increase signaling overhead. In terms of weight update interval the best performance is achieved with one slot interval and if the interval is prolonged to the length of Transmission Time Interval (TTI), namely 3 slots, then the performance with higher mobile velocity can be compromised. Similarly, the results with different feedback delays show that additional delays in fast fading channel should be avoided.