Impact of CSI Optimization and CRS Selection on Performance of LTE Release 8 Networks

A key reference signal in 3GPP Long Term Evolution (LTE) Release 8 is the cell specific reference signal (CRS). For example, the LTE transmission mode 4 (TM4) uses the CRSs to derive the channel and the interference estimates, utilized in the demodulation process and for acquisition of channel state information (CSI). LTE networks may further operate using different CRS configurations. In a non-shifted CRS configuration the same time and frequency resources are used for CRS transmissions in all cells. In a shifted CRS configuration different cells transmit CRSs on resources that are shifted in frequency. The non-shifted configuration avoids that the CRSs interfere with data transmissions, but is also associated with a systematic CSI estimation error; especially noticeable at low traffic. Using the shifted configuration the CRSs interfere with data transmissions but the CSI estimation error is smaller. In this paper, the LTE downlink performance is evaluated in a heterogeneous network deployment using shifted and non-shifted CRS configurations. The network operation is optimized by means of adjusting the transmission rank selection and channel quality estimation for each CRS configuration. The results demonstrate that under such operational conditions, a non-shifted CRS configuration delivers the highest performance.