A Frequency-Domain Sounding Scheme for LTE TDD Beamforming Systems

In the 3rd generation partnership project (3GPP) long term evolution (LTE) time division duplex (TDD) systems, base stations use uplink sounding reference signals (SRSs) to estimate downlink channel state information (CSI) for downlink beamforming transmissions. Because SRSs sent by up to 8 users are multiplexed on the same time-frequency resources, there exist mutual interferences among these SRSs over fading channels. Thus the current scheme inevitably impacts the estimation accuracy of CSI and thereby degrades the beamforming performance. Based on the characteristics of TDD systems, we propose a novel sounding scheme to improve the quality of CSI, in which according to the CQIs (channel quality indications) periodically reported by each user and the bandwidth requirement of each user, BS schedules each user to send SRSs only on the requisite bandwidth with best CQIs instead of on full bandwidth or specified sub-bandwidth as in the current LTE TDD systems. Simulations show that the proposed scheme outperforms the current scheme in estimation accuracy for various frequency-selective and/or time-selective fading channels, and is especially robust to frequency selective channels. The proposed scheme has been adopted as a candidate scheme for 3GPP LTE-Advanced.