Dual Layer Beamforming with Reduced Channel State Information in TD-LTE System

Time division duplex long term evolution (TD-LTE) system specified by the 3rd Partnership Project (3GPP) supports beamforming which can fully exploit channel reciprocity. And spatial mulplexing such as dual layer based beamforming has been supported on release 9 of the 3GPP specification in recent years. The performance of beamforming utilizing channel reciprocity depends on the accuracy of the channel state obtained by the base station (BS). The critical factor in this respect is the density of the sounding reference signal (SRS) transmitted from the terminal. Moreover terminals are not required to support sounding from all receiving antennas so this behavior is supposed to impact the dual-layer beamforming performance heavily. In this paper we investigate the impact on the performance of average and coverage throughput from practical sounding limitations and the evaluation results show that performance losses are negligible in most cases. For the special case of dual layer transmission with single antenna sounding we propose two beamforming schemes, all of which utilize the combination of long-term beamformer and inter-stream orthogonality criteria (but different criteria for the two schemes) to maximize performance. The simulation results show that the proposed schemes could guarantee the dual-layer beamforming performance even in case of very sparse sounding resource.