CSI-RS

CSI-RS is the Channel State Information Reference Signal and is used by the UE to estimate the channel and report channel quality information (CQI) 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 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 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 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.