Channel Quality Indicator (CQI) in LTE

What is CQI in LTE?

CQI is a crucial metric that a UE sends to the eNodeB to report the quality of the downlink channel. It allows the eNB to select an appropriate modulation and coding scheme (MCS) to optimize data rate, latency, and reliability.

CQI indicates the highest MCS that can be used by the UE with BLER ≤ 10%” for the current channel conditions.

This means CQI is not a raw channel measurement like RSRP or SINR, but quantized feedback of how well the UE can decode the downlink data.

CQI is an index ranging from 0 to 15

Higher CQI → Better channel → Higher MCS → Higher throughput

Why is CQI Important?

• Enables adaptive modulation and coding (AMC)
• Prevents under-utilization of resources
• Avoids link failures and retransmissions
• Essential for throughput maximization and QoS assurance

Without CQI, eNB would transmit blindly using fixed MCS—leading to poor efficiency and increased retransmissions.

CQI Reporting Types

CQI is reported in two main ways:

• Periodic- Reported at regular intervals (e.g., every 2ms/5ms)
• Aperiodic- Triggered by eNB via uplink grants (PUSCH)

Sub-types:

Wideband CQI

• One value reported for the entire bandwidth
• Suited for flat fading channels

Sub-band CQI

• Multiple CQI values reported for Resource Block Groups (RBGs)
• Useful in frequency-selective scheduling
• Used in Type 2 reports (as per TS 36.213, Section 7.2.2)

CQI to MCS Mapping Table

This table defines the maximum MCS that can be supported by the UE at a given CQI with BLER ≤ 10%.

Based on Table 7.2.3-1 of TS 36.213:

CQI	Modulation	Code Rate (x1024)	Spectral Efficiency (bps/Hz)
1	QPSK	78	0.1523
7	16QAM	602	2.4063
15	64QAM	948	5.5547

Example:

• CQI = 7 → 16QAM with Code Rate ≈ 0.6 → ~2.4 bps/Hz
• For 10 MHz BW (50 RBs), throughput ≈ 2.4 * 180 kHz * 50 ≈ 21.6 Mbps

SINR vs CQI Relationship

CQI is derived from SINR, but the mapping is not 1:1 and may vary with UE capability.

A logarithmic curve showing how CQI increases with better SINR values.

CQI = 5 → SINR ≈ 5 dB

CQI = 10 → SINR ≈ 15 dB

How is CQI Calculated?

• UE receives Cell-specific Reference Signals (CRS) in the DL.
• Estimates channel SINR for each subcarrier or RB.
• Converts SINR to CQI using internal table.
• Reports either wideband or sub-band CQI to eNB.

Let’s assume: UE measures SINR = 9 dB

Based on internal mapping, CQI = 8 is selected

CQI 8 corresponds to: Modulation = 16QAM

Code rate ≈ 0.6

MCS Index ≈ 15

CQI vs Throughput Plot

Throughput (Mbps) = CQI_Efficiency * 180kHz * No. of PRBs

A smooth curve showing throughput increasing with CQI and saturating at higher CQIs.

Relation between CQI, PMI, RI

• CQI (Channel Quality Indicator): Indicates downlink channel quality and helps select modulation & coding.
• RI (Rank Indicator): Shows how many spatial streams (layers) the UE can handle. More layers = higher parallel data flow.
• PMI (Precoding Matrix Indicator): Suggests the optimal precoding to exploit MIMO gains based on current channel conditions.

Key Differences:

Parameter	Purpose	Value Range	Affects
CQI	Indicates best MCS for DL	0–15	Throughput
RI	Indicates number of spatial layers	1–4 (typically)	MIMO layering
PMI	Selects best precoding matrix	0–N (N varies)	Beamforming gain

• As CQI improves, throughput increases.
• RI increases as CQI improves, suggesting the channel can support more MIMO layers.
• PMI adapts accordingly to optimize spatial diversity or multiplexing.

CQI in LTE is more than just a number—it drives the adaptive engine of modern wireless networks. By providing a compact but rich representation of the downlink channel, CQI enables LTE to optimize every bit transmitted. Understanding its generation, transmission, and interpretation is key to designing high-performing networks.

References

• 3GPP TS 36.213 v10.3.0
• Technical Reference: ts_136213v100300p.pdf (Section 7.2, Table 7.2.3-1)
• LTE Drive Test Logs and Vendor Documentation (Keysight, Rohde & Schwarz)