[5G URLLC] CQI & MCS

3 min readAug 3, 2023

CQI (Channel Quality Indicator) & MCS (Modulation Coding Scheme)

CQI (Channel Quality Indicator): UE reports CQI to gNB to reflect how good the data channel quality is.

CQI measurement is a useful metric to reflect RF/service quality. CQI is one of the components of CSI (Channel Status Information) in NR.

2. There’re 2 types of CQI reporting: Wideband CQI & Sub-band CQI

MCS (Modulation Coding Scheme): gNB determines the MCS index based on CQI index that gNB got from UE to define the possible modulation orders and coding schemes to use for data transmission.

MCS measurement is a useful metric to reflect the efficiency for PDSCH & PUSCH Resource Blocks. (3GPP TS 28.552)
MCS measurement is helpful for operator to optimize the scheduling of physical resources to improve the network efficiency and overall service quality. (3GPP TS 28.552)

Getting a higher CQI index indicates a better signal quality, and the gNB can transmit a larger transport block size.
The higher CQI index reflects a higher MCS index.
MCS index is related to CQI index, and there’s a recommended modulation from CQI table, however, the final decision on the selection of modulation order and target code rate used in the physical downlink shared channel (PDSCH) is determined by MCS.
How CQI table and MCS table mapping depends on the vendor’s implementation.

URLLC from Data Channel State Aspect

CQI Table Enhancement:

For eMBB use cases, CQI table (4-bit CQI Table 1 & Table 2) with BLER (Block Error Rate) is 0.1.
For URLLC vertical, a new CQI table (4-bit CQI Table 3) for lower BLER (Block Error Rate), 0.00001, is introduced to support URLLC service enabling the highly reliable link adaption for DL data.
The parameter of cqi-Table in CSI-ReportConfig will configure which CQI table, table1 or table2 or table3, should be used for CQI calculation. (CSI-ReportConfig information element can refer to 3GPP TS 38.331)

(Source: CSI-ReportConfig Information Element, 3GPP TS 38.331)

MCS table enhancement:

There’re 3* MCS table, table-1 supports 64 QAM, table-2 supports 256 QAM, table-3, a.k.a 64 QAM Low SE Table, supports 64 QAM with lower spectral efficiency is introduced for URLLC service using low coding rate.
Code Rate (R) = useful bits / total transmitted bits

*** total transmitted bits = useful bits + redundant bits

A lower code rate represents more redundant bits for forward error correction (FEC), i.e., to send enough redundant data to correct errors in transmitted data and no need to retransmit data to reduce bit error rate enabling higher reliable data transmission.
The parameter of mcs-Table in PDSCH-Config will configure which MCS table, qam 64, or qam 256, or qam 64 Low SE, should be used. (PDSCH-Config information element can refer to 3GPP TS 38.331)