RRC IE - cqi-ReportConfig

The eNB schedules the downlink well only when it knows the channel as the UE sees it. The UE reports that knowledge as CSI, and cqi-ReportConfig tells the UE what to report and when to report it.

This IEs are used to define various type of UE CSI(Channel State Information) report.

The IE sits in physicalConfigDedicated and carries three things. The cqi-ReportModeAperiodic field sets how the UE reports on PUSCH when a DCI asks for it. The cqi-ReportPeriodic field sets the regular reports on PUCCH. The nomPDSCH-RS-EPRE-Offset field tells the UE which PDSCH power to assume when it calculates the CQI. This page reads one decoded cqi-ReportConfig, compares it with 36.331, and then works out when the periodic reports go out.

What does a decoded cqi-ReportConfig contain ?

Let's start from a real configuration. The tree below is one cqi-ReportConfig as a tester decoder prints it. Each line gives the field, its ASN.1 type and, in square brackets, the value that the network sent.

Decoded RRC message, tester tree format. Field values are from a captured message, not from the specification.

| | +-cqi-ReportConfig ::= SEQUENCE [11] OPTIONAL:Exist
| | | +-cqi-ReportModeAperiodic ::= ENUMERATED [rm30] OPTIONAL:Exist
| | | +-nomPDSCH-RS-EPRE-Offset ::= INTEGER (-1..6) [-1]
| | | +-cqi-ReportPeriodic ::= CHOICE [setup] OPTIONAL:Exist
| | |   +-setup ::= SEQUENCE [1]
| | |     +-cqi-PUCCH-ResourceIndex ::= INTEGER (0..1185) [0]
| | |     +-cqi-pmi-ConfigIndex ::= INTEGER (0..1023) [5]
| | |     +-cqi-FormatIndicatorPeriodic ::= CHOICE [widebandCQI]
| | |     | +-widebandCQI ::= NULL
| | |     +-ri-ConfigIndex ::= INTEGER (0..1023) [2] OPTIONAL:Exist
| | |     +-simultaneousAckNackAndCQI ::= BOOLEAN [TRUE]

The brackets after SEQUENCE are worth a look first. The cqi-ReportConfig IE has two OPTIONAL fields, and [11] says that both are present. The setup SEQUENCE has one OPTIONAL field, ri-ConfigIndex, and [1] says that it is present too. So this network configured both aperiodic and periodic reporting, and RI reporting as well.

Now the values. The aperiodic mode is rm30, which is PUSCH reporting Mode 3-0. In that mode the UE reports a wideband CQI and a subband CQI for each subband, with no PMI. The nomPDSCH-RS-EPRE-Offset is -1, and 36.331 says that the actual value is the field value times 2 dB, so the offset is -2 dB. In the periodic part, cqi-PUCCH-ResourceIndex 0 selects the PUCCH format 2 resource. The cqi-pmi-ConfigIndex is 5, and the ri-ConfigIndex is 2. The last section of this page turns these two indexes into subframes. The report type is widebandCQI, and simultaneousAckNackAndCQI is TRUE.

simultaneousAckNackAndCQI decides what happens when a HARQ-ACK and a CQI report fall in the same subframe. With TRUE, the UE sends both together, on PUCCH format 2a or 2b with normal cyclic prefix. With FALSE, the UE drops the CQI and sends only the HARQ-ACK.

See CQI, PMI, RI Reporting Configuation section for description

See CQI/RI Feedback type section for description

[11] and [1] are presence bits : they show which OPTIONAL fields the network included.
rm30 is Mode 3-0 : wideband CQI plus subband CQI on PUSCH, without PMI.
The offset is doubled : nomPDSCH-RS-EPRE-Offset -1 means -2 dB.
TRUE keeps the CQI : simultaneousAckNackAndCQI lets the CQI travel with HARQ-ACK instead of being dropped.

How does 36.331 define cqi-ReportConfig ?

The decoded tree follows the Release 8 form of the IE. The listing below is that form in 36.331 v19.3.0, with the two types it refers to. Later releases did not change these three definitions in place. Instead, they added new versions beside them.

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

CQI-ReportConfig ::=                SEQUENCE {
    cqi-ReportModeAperiodic         CQI-ReportModeAperiodic OPTIONAL,           -- Need OR
    nomPDSCH-RS-EPRE-Offset             INTEGER (-1..6),
    cqi-ReportPeriodic              CQI-ReportPeriodic  OPTIONAL                -- Need ON
}

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
    }
}

Compare the listing with the tree. The cqi-ReportModeAperiodic field is OPTIONAL with Need OR, so leaving it out releases aperiodic reporting. The cqi-ReportPeriodic field is OPTIONAL with Need ON, so leaving it out keeps the current periodic configuration. To stop periodic reporting, the network sends release. The nomPDSCH-RS-EPRE-Offset field has no OPTIONAL, which is why the tree shows it without OPTIONAL:Exist. The subbandCQI alternative, which this capture does not use, carries the parameter K of 36.213 clause 7.2.2.

The ENUMERATED list of CQI-ReportModeAperiodic has also grown. It started with rm12, rm20, rm22, rm30 and rm31. Release 12 added rm32-v1250, and Release 13 added rm10-v1310 and rm11-v1310. Other additions come as separate IEs in physicalConfigDedicated, from CQI-ReportConfig-v920 to CQI-ReportConfig-v1530. For example, CQI-ReportConfig-v920 adds cqi-Mask-r9 and pmi-RI-Report-r9, and CQI-ReportConfig-r10 adds csi-SubframePatternConfig-r10.

Need OR and Need ON behave differently : an absent aperiodic mode is released, while an absent periodic configuration is kept.
nomPDSCH-RS-EPRE-Offset is always present : it is the only mandatory field in the IE.
New features arrive as new IEs : the Release 8 definition is unchanged, and later versions sit beside it.

In which subframes does the UE send the periodic report ?

The capture gives two index values, 5 and 2, and neither of them is a period in milliseconds. 36.213 maps each index to a period and an offset. So we need two tables and one formula to find the subframes that carry the reports.

Start with cqi-pmi-ConfigIndex, which 36.213 calls I_CQI/PMI. For FDD, Table 7.2.2-1A maps values 2 to 6 to a period N_pd of 5 subframes, with N_OFFSET,CQI = I_CQI/PMI - 2. So index 5 gives N_pd = 5 and N_OFFSET,CQI = 3. For TDD, Table 7.2.2-1C maps values 1 to 5 to N_pd = 5 with N_OFFSET,CQI = I_CQI/PMI - 1, so the offset there is 4.

Next, ri-ConfigIndex is I_RI. Table 7.2.2-1B maps values 0 to 160 to M_RI = 1, with N_OFFSET,RI = -I_RI. So index 2 gives M_RI = 1 and N_OFFSET,RI = -2.

A wideband CQI report goes out in the subframes where (10n_f + ⌊n_s/2⌋ - N_OFFSET,CQI) mod N_pd = 0. An RI report goes out where (10n_f + ⌊n_s/2⌋ - N_OFFSET,CQI - N_OFFSET,RI) mod (N_pd M_RI) = 0. Let's apply them to the FDD values. The CQI condition becomes (subframe - 3) mod 5 = 0, which gives subframes 3 and 8 of every radio frame. The RI condition becomes (subframe - 1) mod 5 = 0, which gives subframes 1 and 6. So each RI report comes 2 subframes before a CQI report, every 5 ms.

If an RI report and a CQI report ever fall in the same subframe, 36.213 drops the CQI and keeps the RI. With these values they never collide, because the negative RI offset moves every RI report ahead of the CQI report.

Indexes are not periods : cqi-pmi-ConfigIndex 5 means a 5 ms period with offset 3 on FDD.
RI shares the CQI period here : M_RI = 1, so RI is sent once every 5 ms as well.
The RI offset is negative : N_OFFSET,RI = -2 places RI 2 subframes ahead of CQI.
FDD and TDD use different tables : the same index 5 gives offset 3 on FDD and offset 4 on TDD.

Reference

3GPP TS 36.331 v19.3.0 - CQI-ReportConfig, CQI-ReportModeAperiodic, CQI-ReportPeriodic and their field descriptions
3GPP TS 36.213 v19.4.0 - clause 7.2.1, clause 7.2.2, Table 7.2.2-1A, Table 7.2.2-1B and Table 7.2.2-1C