Arto Lehti

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.

Introducing Dual Pilot Closed Loop Transmit Diversity for High Speed Uplink Packet Access

The alternatives for HSUPA uplink transmit diversity are being currently investigated in 3GPP. This paper addresses the uplink transmit diversity from the perspective of closed loop beamforming where NodeB determines transmit antenna weights, and additional feedback is used for signaling the optimal weights to the user equipment. More precisely, this paper introduces a novel transmit diversity technique which involves transmitting two non-beamformed DPCCH (pilot) signals from two antennas whereas the actual data part of the signal is beamformed. Additionally, the dual pilot transmit diversity technique is benchmarked on system level in various conditions against the baseline system performance without transmit diversity. The studies show cell level throughput gains up to 53 % in Pedestrian A channel and up to 26 % in Vehicular A channel comparing to the baseline system. The highest relative gains are achieved in highly loaded cells with long inter-site distances.