CSI-RS (LTE-Advanced)

CSI-RS Channel State Information Reference Signal is the Channel State Information Reference Signal and is used by the UE User Equipment (e.g. cell phone) to estimate the channel and report channel quality information (CQI Channel Quality Indicator) to the base station.

In Release 8, Cell-specific RS (C-RS) was designed for use in channel estimation for up to 4-layer spatial multiplexing, with separate C-RS Cell-specific RS sequences for each antenna port (0-3). With the addition of up to 8-layer spatial multiplexing in Release 10 came the need for 8-layer channel estimation. However, extending C-RS to 8 layers would add more signaling overhead than was desired, so the CSI channel state information Reference Signal was added.

Since Release 8/9 UEs are not aware of CSI-RS and will see CSI-RS as interference (which can be placed in PDSCH Physical Downlink Shared Channel resource elements), the placement of CSI-RS was designed to be sparse in both the time and frequency domains to minimize the effect on these UEs. And, although the sparse placement of CSI-RS means that CQI will be reported over longer time intervals than C-RS CQI, the target UE devices for higher-layer spatial multiplexing are static or low-mobility devices, so this should not be a major issue.

CSI-RS is transmitted on different antenna ports (15-22) than C-RS (although likely sharing physical antennas with other antenna ports), and instead of using only time/frequency orthogonality like C-RS, CSI-RS uses code-domain orthogonality as well.