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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.

 

 

 

Does UE has to report CSI always or only in certain periodicity or only when network triggers case by case ?

 

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 >)

  • Periodic reporting: the UE reports CQI, PMI, and RI with reporting periods configured by the higher layer. PUCCH is used for this.
  • Aperiodic reporting: can be used to provide large and more detail reporting in a single reporting instance via PUSCH. Report Timing is triggered by DCI.

 

 

Which part of the spectrum UE has to measure ?

 

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)

  • In periodic reporting : The UE cycles through different subbands from one reporting instance to the next, to reduce overhead
  • In aperiodic reporting :
    • Higher layer configured subband reporting: UE reports the subband CQI for each band in a single feedback report.
    • UE selected subband report: UE reports the subbandCQI for the M bands with the highest CQI values

 

 

What kind of Physical Channel does a UE has to use to report CSI (CQI, RI, PMI) ?

 

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.

 

CSI Report Type

Physical Channel

Periodic

PUCCH : When no PUSCH is scheduled at CSI report subframe

PUSCH : When PUSCH is scheduled at CSI report subframe and Simultenous PUCCH and PUSCH is not enabled.

Aperiodic

PUSCH only

 

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.

 

36.213 Rel 8 - 7.2
  • A UE shall transmit periodic CQI/PMI, or RI reporting on PUCCH as defined hereafter in subframes with no PUSCH allocation. A UE shall transmit periodic CQI/PMI or RI reporting on PUSCH as defined hereafter in subframes with PUSCH allocation, where the UE shall use the same PUCCH-based periodic CQI/PMI or RI reporting format on PUSCH.
  • A UE shall transmit aperiodic CQI/PMI, and RI reporting on PUSCH if the conditions specified hereafter are met. For aperiodic CQI reporting, RI reporting is transmitted only if configured CQI/PMI/RI feedback type supports RI reporting.
  • In case both periodic and aperiodic CSI reporting would occur in the same subframe, the UE shall only transmit the aperiodic CSI report in that subframe.
36.213 Rel 11 - 7.2 : The part marked in red is the part added in Rel 10 and higher
  • If the UE is configured with more than one serving cell, it transmits CSI for activated serving cell(s) only.
  • If a UE is not configured for simultaneous PUSCH and PUCCH transmission, it shall transmit periodic CSI reporting on PUCCH as defined hereafter in subframes with no PUSCH allocation.
  • If a UE is not configured for simultaneous PUSCH and PUCCH transmission, it shall transmit periodic CSI reporting on PUSCH of the serving cell with smallest ServCellIndex as defined hereafter in subframes with a PUSCH allocation, where the UE shall use the same PUCCH-based periodic CSI reporting format on PUSCH.
  • A UE shall transmit aperiodic CSI reporting on PUSCH if the conditions specified hereafter are met. For aperiodic CQI/PMI reporting, RI reporting is transmitted only if the configured CSI feedback type supports RI reporting.
  • In case both periodic and aperiodic CSI reporting would occur in the same subframe, the UE shall only transmit the aperiodic CSI report in that subframe.

 

 

Which RRC Message (Which IE) carries CSI report configuration ?

 

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.

 

 

How UE know WHEN to send CSI Report ?

 

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.

 

 

How UE know WHAT to send in CSI Report ?

 

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.

 

 

For WHICH CELL a UE has to send CSI Report ?

 

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).