CSI: Channel State Information

CSI is a kind of collective name of several different type of indicators that UE reports:

• Channel Quality Indicator (CQI)
• Precoding Matrix Indicator (PMI) 
• Precoding Type Indicator (PTI) 
• Rank Indication (RI)

One of the most important purpose of CSI on UE side is to estimate the channel quality and recommend a proper precoding matrix to network.

References

[1] http://www.sharetechnote.com/html/Handbook_LTE_CSI.html