Skip to main content

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.
Labels:

Popular posts from this blog

Difference between SRS and DMRS

There are two types of reference signals used in LTE uplink, to estimate uplink channel quality. Which allow eNB to take smart decisions for resource allocation for uplink transmission, link adaptation and to decode transmitted data from UE . So to take first smart decision by eNB Sounding Reference Signal  (SRS) is being used. SRS is being transmitted by UE on the last symbol of subframe (in which subframe will come to know later). This SRS report the channel quality of over all bandwidth and using this information eNB assign the resource (to UE for uplink transmission )has better channel quality comparing to  other bandwidth  region. So is SRS optional in LTE? Yes. SRS is configurable and infact we do not need SRS at all in case eNB assign all resource block or full bandwidth or have no choice. Now on the basis of configuration and node wise there are two types of SRS (refer 36.211), cell specific (Common SRS) and UE specific (Dedicated SRS).  eNB...
20 comments
Read more

What is Rank Indicaton in LTE

Rank Indication is one of the important input to eNB , in selection of the transmission layer in downlink data transmission. Even though the system is configured in transmission mode 3 (or open loop spatial multiplexing) for a particular UE and if the same UE report the Rank Indication value 1 to eNB, eNB will start sending the data in Tx diversity mode to UE . If UE report Rank Indication 2 , eNB will start sending the downlink data in MIMO mode (Transmission Mode 3). Why we need this RI in LTE concept? When UE experience bad SNR and it would be difficult (error prone) to decode transmitted downlink data  it gives early warning to eNB by stating Rank Indication value as 1. When UE experience good SNR it pass this information to eNB by indicating rank value as 2. Because of this reason, you might have observed that some time data transmitted by eNB is in Tx diversity mode, though MIMO was configured and hence you may have observed less downlink throughput than expected ...
15 comments
Read more

Positioning Reference Signal PRS LTE

Positioning Reference Signal (PRS) is taken into consideration in one of the LTE release 9 features to determine the location of User Equipment (UE) based on radio access network information. Now you might be thinking that what is the necessity of PRS,  if we have a GPS technology already built in smartphones and in other cellular equipment. Just think of it  J (GPS may not be accurate always and GPS services may not be available all around the geographical areas , also the accuracy of functioning GPS depends on money you have paid for the services and the quality of GPS device).  The end user application of this PRS feature could be supporting location based services which can be navigation (direction to hotel etc.), emergency call etc.   Process of finding UE location using PRS: The overall process of finding UE locations are based on three major steps. Step 1. UE receive PRS from cells (Reference cell and Neighbor cells)   Step 2. Based on received PRS...
2 comments
Read more