In Long Term Evolution (LTE) systems, a cell specific reference signal (CRS) can be used by the evolved Node B (eNB) to enable the user equipment (UE) to perform downlink channel estimations for cell association and demodulation of channels. CRS is limited to four ports and needs to provide full coverage for the entire cell.
MIMO is a method for multiplying the capacity of a radio link between the eNB and the UE using multiple transmit and receive antennas to exploit multipath propagation. MIMO refers to a practical technique for sending and receiving more than one data signal simultaneously over the same radio channel by exploiting multipath propagation. MIMO is fundamentally different from smart antenna techniques developed to enhance the performance of a single data signal, such as beamforming and diversity.
Using MIMO techniques in LTE results in a problem with distribution of the CRS power over multiple antenna elements in order to fully utilize available power amplifier resources in the eNB. Additionally, UE channel estimation for multiple transmit antennas using CRS is not possible due to the CRS being broadcast on a large number of antennas.