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.
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-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 |
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.
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