NOTE : 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. Refer to CSI Overview page if you are not familiar with big picture of the CSI report.

CQI has sub-types and PMI has sub-types, and one RRC field picks a combination of the two. That combination is what a reporting mode is. The rest of this page is a set of tables. They turn a mode number into what the UE actually sends, first for PUSCH and then for PUCCH.

• Reporting Modes in the Time Domain
• What kind of Physical Channel does a UE has to use to report CSI ?
• CQI and PMI Feedback Types for PUSCH
• CQI and PMI Feedback Types for PUCCH
• Which CSI Report Applies to Which Transmission Mode
• Reference

Reporting Modes in the Time Domain

Two questions decide a CSI report, and neither one answers the other. The first is when the report goes out. The second is how much of the band it covers. The two lists below take them in that order.

There are two reporting modes in the time domain

• 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.

For subbandCQI feedback

• In periodic reporting : The UE cycles through different subbandsfrom one reporting instance to the next, to reduce overhead
• In aperiodic reporting :
• Higher layer configured subbandreporting: UE reports the subbandCQI 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

The two axes stay independent because they answer different constraints. The time axis follows from how much room the uplink has at that instant. The frequency axis follows from how much the scheduler needs to know about the band.

That independence is why a subband report reads differently on each channel. A periodic report cannot fit every subband into one PUCCH instance, so it spreads them over several. An aperiodic report is carried on PUSCH and can send every subband at once.

• The time axis decides the channel : a periodic report has PUCCH available to it, and an aperiodic report does not.
• The frequency axis decides the size : a wideband report is one value for the whole carrier. A subband report is one value per band.
• Periodic reporting spreads the cost over time : it cycles through the subbands one reporting instance at a time.
• Aperiodic reporting sends every subband in one instance : that is what the higher layer configured subband report does.
• UE selected and higher layer configured differ in who chooses : the UE picks the best M bands in one case. The network fixes the set in the other.
• Both axes reappear in every table on this page : a mode number is the pair of answers written as one value.

What kind of Physical Channel does a UE has to use to report CSI ?

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.

• The two quotations differ in three places : the Rel 11 text adds a sentence about multiple serving cells. It names the serving cell with the smallest ServCellIndex. It also says CSI where Rel 8 said CQI/PMI.
• The condition on both PUCCH sentences is the same : it applies to a UE not configured for simultaneous PUSCH and PUCCH transmission.
• One rule settles a clash : when a periodic and an aperiodic report fall in the same subframe, only the aperiodic one is sent.
• v19.4.0 still carries all of it : the wording of the Rel 11 quotation survives into the current clause 7.2.
• Two things were added after Rel 11 : dormant serving cells join the multi-cell sentence. An LAA SCell condition joins the clash rule.
• Table 7.2-1 really is void : v19.4.0 prints the heading and nothing under it.

CQI and PMI Feedback Types for PUSCH

A mode number says nothing on its own. It is a cell reference into a small matrix. The matrix is what gives it meaning. The capture below shows the field that carries the number. The tables after it show the matrix it points into.

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 the question is exactly which type of CQI and RI a UE should report. Overall decision is made by the parameter in higher layer (RRC message) as shown below and the details of configuration is determined by various tables as described in this page.

The picture is a decoder view on the left and the answer on the right. The line between them starts at one field and lands in one cell of one matrix.

• The tree on the left is physicalConfigDedicated : cqi-ReportConfig is the branch highlighted in blue.
• The field highlighted in red is cqi-ReportModeAperiodic : its value in this capture is rm12.
• rm12 is Mode 1-2 : the line runs to the cell at wideband CQI and multiple PMI in the upper matrix.
• The green panel holds two matrices rather than one : the upper one is for PUSCH and the lower one for PUCCH.
• The periodic side is configured in the same branch : cqi-ReportPeriodic is set up, and cqi-FormatIndicatorPeriodic reads widebandCQI.
• widebandCQI picks the top row of the lower matrix : Mode 1-0 or Mode 1-1, and the transmission mode decides which.
• Two more fields set timing rather than content : cqi-pmi-ConfigIndex and ri-ConfigIndex are both 0 in this capture.
• simultaneousAckNackAndCQI is FALSE : this UE is not set up to send CQI and an ACK together on PUCCH.

Both fields in that tree are small, and their types say more than the tree can. One is an enumeration of mode names. The other is a choice with the subband case carrying a parameter of its own.

Following is based on 36.331 v19.3.0 (Release 19)

CQI-ReportModeAperiodic ::=      ENUMERATED {
    rm12, rm20, rm22, rm30, rm31,
    rm32-v1250, rm10-v1310, rm11-v1310
}

CQI-ReportPeriodic ::=           CHOICE {
    release                         NULL,
    setup                           SEQUENCE {
        cqi-PUCCH-ResourceIndex         INTEGER (0..1185),
        cqi-pmi-ConfigIndex             INTEGER (0..1023),
        cqi-FormatIndicatorPeriodic     CHOICE {
            widebandCQI                     NULL,
            subbandCQI                      SEQUENCE {
                k                               INTEGER (1..4)
            }
        },
        ri-ConfigIndex                  INTEGER (0..1023)   OPTIONAL,   -- Need OR
        simultaneousAckNackAndCQI       BOOLEAN
    }
}

• The enumeration names the modes directly : rm12 is Mode 1-2, rm20 is Mode 2-0, and the pattern holds for the rest.
• Three of the eight names carry a version tag : rm32-v1250 arrived in Release 12, and rm10-v1310 and rm11-v1310 in Release 13.
• Those three are the modes the screenshot below does not show : Mode 3-2, Mode 1-0 and Mode 1-1.
• The periodic field is a choice, not an enumeration : widebandCQI carries nothing and subbandCQI carries k.
• k is the subband size : 36.213 clause 7.2.2 splits the system bandwidth into N subbands of size k. The ASN.1 range here is 1 to 4.
• The capture above matches this definition field for field : resource index, config index, format, RI index and the boolean, in that order.

CQI/PMI feedback type for Aperiodic report is defined by the following table (36.213) and cqi-ReportModeAperiodic in RRC message specifies exactly which type in this table should be used.

< 36.213 - Table 7.2.1-1: CQI and PMI Feedback Types for PUSCH CSI reporting Modes>

The matrix is three CQI feedback types by three PMI feedback types. Nine cells exist and this version of the table fills five of them.

• The empty cells say as much as the filled ones : this version has no PUSCH mode for wideband CQI without PMI. It has none for higher layer configured subband CQI with multiple PMI.
• The list under the matrix maps transmission modes onto modes : transmission modes 1, 2, 3 and 7 get Modes 2-0 and 3-0.
• Transmission modes 4 and 6 get the PMI modes : Modes 1-2, 2-2 and 3-1.
• Transmission mode 5 has a line to itself : Mode 3-1 and nothing else.
• Transmission mode 8 splits on one condition : with PMI/RI reporting it gets 1-2, 2-2 and 3-1, and without it 2-0 and 3-0.
• Transmission modes 9 and 10 add a second condition : the number of CSI-RS ports has to be more than one for the PMI modes.
• The table below it rearranges the same content : it reads mode by mode where the screenshot reads type by type.

Somehow this table didn't look clear to me at the biggining. Following is my version. (Same story.. just different way of expression)

Modes Added Since the Screenshot

The screenshot above is a copy of one release. 36.213 v19.4.0 fills three more cells of the same matrix. The transmission mode list under it grew to match. Both are set out below.

The matrix below is Table 7.2.1-1 as v19.4.0 prints it. Three cells that are blank in the screenshot now carry a mode.

The list under the matrix grew with it. The table below is what v19.4.0 supports on PUSCH, transmission mode by transmission mode.

• Three cells are new : Mode 1-0 and Mode 1-1 on the wideband row, and Mode 3-2 on the higher layer configured row.
• One cell of the nine is still empty : UE selected subband CQI with a single PMI.
• Mode 1-0 and Mode 1-1 make PUSCH match PUCCH : the same two wideband modes now exist on both channels.
• Mode 3-2 is higher layer configured subband CQI with multiple PMI : its position in the matrix is what names it.
• Every transmission mode gained something : the no-PMI list gained Mode 1-0. The PMI list gained Modes 3-2 and 1-1.
• Transmission modes 9 and 10 gained a third branch : with advancedCodebookEnabled set, Mode 1-1 is not offered and the other four stay.
• The table above is a faithful copy of its screenshot : the difference is the release, not the reading.

CQI and PMI Feedback Types for PUCCH

The PUCCH side answers the same question with fewer choices. PUCCH carries a small payload, so the matrix behind it is smaller. Two of the three PUSCH feedback types have no place in it.

CQI/PMI feedback type for Periodic report is defined by the following table (36.213). cqi-FormatIndicatorPeriodic in RRC message specifies exactly which type in this table should be used. (Regarding the timing of the report(period, offset), refer to here).

< 36.213 - Table 7.2.2-1: CQI and PMI Feedback Types for PUCCH CSI reporting Modes >

This matrix is two feedback types by two PMI types, and all four cells are filled. Nothing is left blank because nothing was left out.

• There is no Multiple PMI column : no PUCCH mode reports more than one PMI.
• There is no higher layer configured row either : a subband report on PUCCH is always UE selected.
• The list splits the same way as the PUSCH one : modes 1, 2, 3 and 7 get 1-0 and 2-0. Modes 4, 5 and 6 get 1-1 and 2-1.
• Transmission mode 5 sits with 4 and 6 here : on the PUSCH side it had Mode 3-1 to itself.
• v19.4.0 still lists the same four modes : what grew is the condition on modes 9 and 10, where advancedCodebookEnabled and semiOpenLoop now appear.
• The periodic field is the one from the tile above : cqi-FormatIndicatorPeriodic chooses the row, and the transmission mode chooses the column.

Somehow this table didn't look clear to me at the biggining. Following is my version. (Same story.. just different way of expression)

Which CSI Report Applies to Which Transmission Mode

A reporting mode is not free to appear under any transmission mode. The transmission mode settles first whether RI and PMI are reported at all. Only then does the mode table apply. The table below is that first filter.

Following is the table showing which CSI report can be configured for which transmission mode. (Green color indicates that the specified CSI can be configured)

• The table has two rows because CQI needs none : every reporting mode on this page carries CQI, so only RI and PMI vary.
• The RI row matches the specification : 36.213 reports RI for transmission modes 3 and 4. It adds 8, 9 and 10 with PMI/RI reporting, and 9 and 10 without PMI reporting.
• The PMI row leaves out TM3 and TM7 : both take the no-PMI modes in every table on this page.
• TM5 has PMI and no RI : its rank is fixed, so there is no rank left to report.
• TM6 reads the same way : closed loop precoding at a single layer needs a PMI and never a rank.
• TM1 and TM2 are not in the table at all : neither reports RI or PMI, so there is nothing to colour.
• Read this table before the mode tables : it decides which column of the matrix is reachable at all.

Reference

The two specifications below are the ones this page was checked against, in the versions named. The screenshots keep the release they were taken from.

• 36.213 : 3GPP - E-UTRA; Physical layer procedures, v19.4.0. Clause 7.2 holds the CSI reporting rules, Table 7.2.1-1 the PUSCH modes, Table 7.2.2-1 the PUCCH modes, and Table 7.2-1 is void.
• 36.331 : 3GPP - E-UTRA; Radio Resource Control (RRC); Protocol specification, v19.3.0. The source of CQI-ReportModeAperiodic and CQI-ReportPeriodic.