Evolution of reference signals for LTE-advanced systems

3GPP LTE Release 10 standards (also known as LTE-Advanced) adopted some of the state-of-the-art radio access technologies that include carrier aggregation, eight-layer downlink spatial multiplexing, and four-layer uplink spatial multiplexing. For facilitating these enhancements, reference signals have significantly evolved in LTE-Advanced. This article examines underlying design principles of the LTE-Advanced reference signals. Specifically, newly introduced dedicated demodulation reference signals and channel state information reference signals for downlink and improvements of demodulation reference signals and sounding reference signals in uplink are discussed.