LTE CQI, PMI, and RI: Key Differences Explained

This article explains the differences between LTE CQI, PMI, and RI, all crucial components in LTE (Long-Term Evolution) communication. CQI, PMI, and RI are feedback mechanisms that help the eNodeB (base station) optimize data transmission to the User Equipment (UE).

LTE CQI (Channel Quality Information)

LTE CQI, or Channel Quality Information, is a comprehensive metric including CQI, PMI, and RI components. The specific components required depend on the transmission mode being used.

All transmission modes require the UE to provide CQI feedback.

LTE CQI reporting types

As illustrated in the figure above, LTE CQI reports can be either aperiodic or periodic:

• Aperiodic Reports: Transmitted using the PUSCH (Physical Uplink Shared Channel).
• Periodic Reports: Transmitted using the PUCCH (Physical Uplink Control Channel), unless a report coincides with a PUSCH transmission.

Aperiodic CQI reporting is triggered when the CQI request field is set to ‘1’ within the PDCCH DCI-0 (Physical Downlink Control Channel Downlink Control Information format 0) or the Random Access Response Grant on the PDSCH (Physical Downlink Shared Channel).

In contrast to aperiodic reporting, the ‘type of reporting’ is signaled instead of the ‘reporting mode’. The LTE UE combines the ‘type of reporting’ and the ‘transmission mode’ to determine the actual reporting mode.

LTE PMI (Precoding Matrix Indicator)

LTE PMI stands for Precoding Matrix Indicator. PMIs are used in closed-loop transmission modes:

• Transmission Mode-4: Closed-Loop Spatial Multiplexing
• Transmission Mode-5: Multi-User MIMO (Multiple-Input Multiple-Output)
• Transmission Mode-6: Closed-Loop Spatial Multiplexing using a single layer

The LTE UE uses PMI information to signal its preferred set of weights to be applied during the precoding process. This helps maximize the downlink Signal-to-Noise Ratio (SNR).

Table 1 shows example complex weights used for precoding. Based on the configurations, the LTE UE selects one out of four options. These configurations are for 2 antenna ports, RI=1, and PMI = {0, 1, 2, 3}.

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Table 1: Closed-Loop Spatial Multiplexing Weights (Single Layer & 2 Antenna Ports)

Table 2 shows example complex weights for the following configurations. This configuration is used by the UE to select one of two sets of complex weights: 2 antenna ports, RI=2, PMI={0,1}.

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Table 2: Closed-Loop Spatial Multiplexing Weights (2 Layers & 2 Antenna Ports)

For antenna ports of 4, PMI values of {0, 1, …, 14, 15} can be used to indicate one of 16 sets of complex weights.

LTE PMI can be transmitted using either the PUSCH or PUCCH channel.

LTE RI (Rank Indicator)

LTE RI stands for Rank Indicator. RIs are used in both open-loop and closed-loop transmission modes. These modes use more than a single layer between the layer mapping and precoding modules.

• Transmission Mode-3: Open-Loop Spatial Multiplexing
• Transmission Mode-4: Closed-Loop Spatial Multiplexing

In LTE, the UE uses RI to inform the eNodeB about the number of layers required during layer mapping.

LTE RI can be transmitted using either the PUSCH or PUCCH.

• Number of layers = Number of codewords (for Antenna elements=2): Here, the UE can signal RI equal to 1 or 2 to indicate a preference for 1 or 2 layers.
• Number of layers >= Number of codewords (for Antenna elements=4): Here, the UE can signal RI equal to {1, 2, 3, or 4} to indicate a preference for 1, 2, 3, or 4 layers.

Additional LTE Parameters

Scheduling Request (SR)

A Scheduling Request is a single-bit flag used to request PUSCH resources from the eNodeB. In LTE, SR is transmitted using the PUCCH channel.

HARQ ACK and NACK

HARQ stands for Hybrid Automatic Repeat Request. Uplink HARQ ACK/NACK signals are used to acknowledge downlink data transmitted on the PDSCH. Either one or two ACKs can be reported, depending on the number of codewords transmitted during the corresponding PDSCH subframe. Downlink data received during subframe “N” is ACKed during subframe “N+4”. HARQ ACK/NACK can be transmitted using either the PUSCH or PUCCH.