CSI Report - Overview
CSI stands for Channel Status Information. As the name implies, it is a kind of indicator how good or bad the channel is at a specific time. The CSI has three major components as below. Not all of these indicator is measured for every CSI report. Depending on the situation and configuration from network, UE perform different combination of measurement.
At high level view, it would not be difficult to understand overall concept of CSI. However, getting deeper into the details.. it would become much complicated .. and tooooooooooooo confusing (at least very confusing to me). That is one of the reason why I wrote multiple pages for the same topic (CSI). Multiple pages for the same topic can be additional confusion to some readers (even to me). However, I thought the page would get too big for download if I put everything in single page and I also thought it would not be bad to provide a little bit different aspect for the same topic with multiple post. But as I add more pages (post), I thought it would be good to write a page to provide high level view and help readers combine all those multiple pages that I wrote. Followings are topics to be touch in this page.
Actually all of the three options in the question are possible. It is just a matter of configuration. For recap, let me summarize three possible configurations regarding CSI report.
< Case 1 > No CSI Report
This would be the case that are adopted for general functionality test with UE test equipment (Of course, most of the UE test equipment allow to configure CSI report if necessary). However, you wouldn't see this case in live network.
< Case 2 > Peridic CSI Report
UE report CSI in certain time interval configured by higher layer (RRC Message).
< Case 3 > Aperidic CSI Report
This is the case where UE report CSI report only when network tell it to do it. Network can tell UE to send CSI Report by setting CSI Request field in DCI Format 0.
In 3GPP, only < Case 2> and < Case 3> are specified as below (If none of < Case 2 >, < Case 3>, it automatically mean < Case 1 >)
One question we can think of would be : Would this CSI be represent the channel quality across the full spectrum (e.g 20 Mhz) assigned to the UE ? or represent only some portion of the allocated spectrum ?
In terms of implementation and complexity, it would be easier to let the single CSI report represent the channel quality of the full band. But in reality, those report would not accurately represent the channel quality for every point across the spectrum.
In terms of accuracy, it would be better to let a single CSI report represent only each and every small portions of the spectrum. However, it would be more complicated to implement and UE would need to send multiple CSI reports to notify the whole area of the spectrum.
The simplest compromise would be to provide the both ways and let network select one of the option as necessary. The case where UE report CSI that represent the full bandwidth is called 'Wideband CQI' and the case where UE report CSI that represent a small portions of bandwidth is call 'Subband CQI'.
For subband CQI feedback (It would not need any further description on Wideband CQI)
Following table from 36.213 gives you the answer. In short, UE use PUSCH to report CSI in aperiodic manner and use PUCCH or PUSCH for periodic report. (This used to be specified in 36.213 Table 7.2-1, but the table has become void since Rel 12 implying that the channel allocation for CSI report is not as clear as before.) Following is a new table based on my understanding taking into account many things in the situation in later 3GPP specification.
Just considering the description mentioned above, it may sound like 'Periodic CSI report is conveyed only by PUCCH' and 'Aperiodic CSI reporting is conveyed only by PUSCH'.. but if you read carefully the following description (green part), you would notice that periodic CSI can be reported either PUCCH or PUSCH depending on the situation.
Just to give you a little bit of details/clarification, I put the descriptions about this from 36.213 Rel 8 and Rel 11 specification.
There are several different sub types of CQI and PMI. For example, Among CQI are Wideband CQI and Subband CQI and in Subband CQI there are two types of configuration, UE Selected and Higher Layer Configured.
In PMI, there are also two types Single PMI and Multiple PMI.
Then you may ask on exactly when (at which subframe) UE has to report CSI ? Overall guideline for the report cycle/pattern is explcitely specified by Higher Layer (RRC message) as shown below and some further details are implicitely determined by various tables and formula.
The RRC message that can specify CSI report configuration can be any message that can carry following IEs. The most typical RRC messages are RRC Connection Setup and RRC Connection Reconfiguration message.
For the details of configuration for Periodic and Aperiodic Report, refer to follows pages for each topic.
In case of Periodic Report, UE can figure out exactly when it should send report from RRC message and in case of Aperiodic Report, Network notify UE to transmit the report via DCI.
In both Aperiodic and Periodic CSI, what to be included in the CSI message is configured by RRC message.. but they are configured by different IEs (Information Elements) in the RRC message.
Now with the introduction of Carrier Aggregation, UE need to know for Which cell it has to measure CSI. This is configured by the combination of RRC message and DCI (Refer to CQI, PMI, RI Reporting Configuation-Details on Aperiodic Report).