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LTE CQI vs PMI vs RI-Difference between LTE CQI,PMI,RI

This page on LTE CQI vs PMI vs RI describes difference between LTE CQI,PMI and RI.

LTE CQI

The LTE CQI stands for Channel Quality Information. It basically includes CQI, PMI,RI components. The requirement for each of these components depend on transmission mode. All transmission modes need UE to provide CQI feedback.

LTE CQI reporting types

As mentioned in the figure, LTE CQI reports can be aperiodic or periodic. Aperiodic reports are transmitted using PUSCH. Periodic reports are transmitted using PUCCH unless any reports coincides with the PUSCH channel transmission.

Aperiodic CQI reporting is triggered when CQI request field is set to value 1 within PDCCH DCI-0 or Random Access Response Grant on PDSCH. In contrast to aperiodic reporting, the 'type of reporting' is signalled i instead of 'reporting mode' . The LTE UE uses combination of 'type of reporting' and 'transmission mode' to derive the reporting mode.

LTE PMI

LTE PMI stands for Precoding Matrix Indicators. They are applicable to closed loop transmission modes:
•  Transmission Mode-4:Closed Loop Spatial Multiplexing
•  Transmission Mode-5:Multi-User MIMO
•  Transmission Mode-6:Closed Loop Spatial Multiplexing using a single layer

The LTE UE use PMI information to signal preferred set of weights to be applied during the precoding process. UE does this in order to maximize the downlink S/N ratio.

Table-1 mentions complex weights. Based on following configurations one out of 4 is used by LTE UE.
Antenna ports-2, RI =1 , PMI = {0,1,2,3}


Codebook Index 0 1 2 3
Weight for antenna-1 0.7071 0.7071 0.7071 0.7071
Weight for antenna-2 0.7071 -0.7071 j*0.7071 -j*0.7071
Table-1: Closed Loop Spatial Multiplexing Weights(single layer & 2 Antenna Ports)

Table-2 mentions complex weights for following configurations. This configured is used by UE to select 1 of 2 sets of complex weights.
Antenna ports=2, RI=2, PMI={0,1}.


Antenna-1 Antenna-2
Codebook Index Codeword-1 Codeword-2 Codeword-1 Codeword-2
1 0.5 0.5 0.5 -0.5
2 0.5 0.5 j*0.5 -j*0.5www
Table-2 Closed Loop Spatial Multiplexing weights(2 layers & 2 antenna ports)

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

LTE PMI can be transmitted using PUSCH or PUCCH channel.

LTE RI

LTE RI stands for Rank Indicator.

RIs are applicable for open loop transmission and closed loop transmission modes. These modes use more than a single layer between layer mapping and precoding modules.

• Transmission mode-3 indicates open loop spatial multiplexing
• Transmission mode-4 indicates closed loop spatial multiplexing

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

LTE RI can be transmitted using PUSCH or PUCCH.

Number of layers = Number of codewords (for Antenna elements=2),
Here UE can signal RI equal to 0/1 to indicate 1 or 2 layers as preferred one

Number of layers >= Number of codewords (for Antenna elements=4),
Here UE can signal RI equal to {0,1,2 or 3} to indicate 1,2,3 or 4 layers as preferred one.

Scheduling Request

Scheduling request is a single bit flag which is used to request PUSCH resources from eNodeB.

In LTE, SR is transmitted using PUCCH channel.

HARQ ACK and NACK

HARQ stands for Hybrid Automatic Repeat Request. Uplink HARQ ACK/NACK are used to acknowledge downlink data transmitted on PDSCH. Either one or two ACKs can be reported based on no. of codewords transmitted during corresponding PDSCH subframe.

Downlink data is received during subframe "N" is ACKed during Subframe "N+4". HARQ ACK /NACK can be transmitted using PUSCH or PUCCH.

Refer ARQ vs HARQ and Chase combining vs Incremental Redundancy HARQ for more information.


LTE RELATED LINKS

Difference between SC-FDMA and OFDMA
LTE Cyclic Delay Diversity
LTE eNodeB Physical Layer Measurements
LTE EPC Network Inerfaces
LTE UE Physical Layer Measurements
LTE Bearer types
LTE UE Event Measurement Reporting
LTE RSRP vs RSRQ
LTE Signalling radio bearers
LTE QoS
LTE EPS mobility management states
LTE EPS connection management states
LTE UE cell selection
LTE UE PLMN selection
LTE UE Paging Procedure

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