On Channel Estimation for Multi-User MIMO in LTE-A Uplink

In 3GPP Long Term Evolution-Advanced (LTE-A), uplink multi-input-multi-output(MIMO) transmission have been introduced as a key technology to improve the spectrum efficiency. Both single-user MIMO (SU-MIMO) and multi-user MIMO (MU-MIMO) modes are supported. With UL-MIMO, multiple layers of data are mapped on the same set of recourses and transmitted simultaneously. Accordingly, the demodulation reference signal (DMRS) in LTE-A applies both frequency domain code division multiplexing and time domain code division multiplexing to support channel estimation for each multiplexed layer. Discrete Fourier transform (DFT)-based channel estimation schemes and the applications for LTE UL-MIMO have been widely studied. However, the orthogonality among DMRS of different layers could be reduced due to the channel conditions. The effect is significant when the arrival times of the paired user equipment (UE) at the receiver are not strictly aligned if MU-MIMO is applied. Consequently, the performance of DFT-based channel estimation schemes is affected. In this paper, we demonstrate a novel DFT based channel estimation scheme that can restore the inter-layer orthogonality. Simulation results show that the proposed method provides significant improvement over conventional DFT-based channel estimation methods.