Reference Signal Design for Flexible MIMO Operation in LTE-Advanced Downlink

In this paper we address the issue of demodulation reference signal design for LTE-Advanced downlink. Practical systems such as LTE-Advanced should operate in highly time-frequency selective environment. Hence, they impose a very tight trade-off between the radio resources available for reference signals used for channel estimation and spectral efficiency targets to be met. As an additional constraint, LTE-Advanced brings to real deployments state-of-the-art multi-antenna technologies such as multi-user MIMO and cooperative MIMO. The difficult radio environment and the multitude of different multi-antenna techniques set very high challenges to the reference signal design. In this paper we discuss selected issues of realistic demodulation reference signal design for LTE-Advanced. We highlight the trade-offs that are needed for enabling advanced MIMO technologies, while at the same time allowing low complexity channel estimation at the receiver side without increasing related signaling overhead. Based on the analysis we propose a certain reference signal design for LTE-Advanced that fulfills all the required design criteria.