Thursday, 11 July 2013

What is CQI PMI RI in LTE?

Well, we had discussed about uplink channel state information Difference between SRS and DMRS by reference signals (SRS and DMRS). Now to achieve 1Gbps or more downlink speed in LTE with effective resource utilization of full bandwidth available,  CQI PMI RI and many more parameter play very important role. So what are CQI, PMI and RI in LTE?

CQI (Channel Quality Indicator), reported by UE to eNB. UE indicates modulation scheme and coding scheme to eNB , if used I would be able to demodulate and decode the transmitted downlink  data with maximum block error rate 10%. To predict the downlink channel condition, CQI feedback by the UE is an input. CQI reporting can be based on PMI and RI. Higher the CQI value (from 0 to 15) reported by UE, higher the modulation scheme (from QPSK to 64QAM ) and higher the coding rate will be used by eNB to achieve higher efficiency. 

PMI (Precoding Matrix Indicator), UE indicates to eNB , which precoding matrix should be used for downlink transmission which is determined by RI.

RI (Rank Indicator), UE indicates to eNB, the number of layers that should be used for downlink transmission to the UE.

RI and PMI can be configured to support MIMO operation (closed loop and open loop spatial multiplexing). These both transmission modes use precoding from a well defined codebook  (the lookup table of cross coupling factors used for precoding shared between UE and eNB) to form the transmission layers. In case of transmit diversity PMI and RI need not to be reported to eNB.

In wideband CQI reporting UE report one wideband CQI for the full system bandwidth region.  However, UE can also report CQI value for sub band also.

Now, what about periodicity of CQI, PMI and RI and its values. Yes these can be periodic and aperiodic .
eNB configure type of CQI reporting by RRC signaling. Aperiodic reporting is on request based (by eNB ), which always go with PUSCH. 

Periodic CQI reporting can go on both PUCCH and PUSCH (along with data).  The minimum periodicity could be 2 ms. Periodicity are defined in 36.213 for different values of CQI-PMI-ConfigIndex (Table 7.2.2-1A for FDD). The range of CQI-PMI-ConfigIndex is 0 to 1023. Also the periodicity of RI is based on riconfig-index (Table 7.2.2-1B for FDD) and periodicity of CQI-PMI. The range of riconfig-index is 0 to 1023. 

Example: From Table 7.2.2-1A of 36.213, for the value of CQI-PMI-ConfigIndex “17” the periodicity of CQI reporting is 20 ms (say X).  From Table 7.2.2-1B of 36.213, for the value of riconfig-index “483” the Y is 8 and the periodicity of RI will be 8 times of X (20ms) =160ms.

What about if CQI/PMI/RI collides with either ACK/NACK or SR on the same subframe? If CQI/PMI/RI collides with positive SR the CQI/PMI/RI will be dropped. If CQI/PMI/RI collides with ACK/NACK and simultaneousACKNACKandCQI is false CQI/PMI/RI will be dropped otherwise CQI/PMI/RI will be multiplexed with ACK/NACK. 

It is only the eNB which decide the time and frequency on which UE can transmit the CQI, PMI and RI. 
Questions are welcome.