How does the eNodeB decide that it is time to ask the UE to send an Aperiodic CQI report ?

Question

The UE might send the aperiodic cqi report on the PUSCH when the enodeB requests it. My question is, WHY does the eNB want the UE to send an aperiodic cqi report when it is already reporting a periodic cqi report. What is the trigger for the eNode to ask the UE to send an aperiodic cqi report ?

Answer 1

The periodic CQI report is carried by PUCCH but if the UE needs to send UL data in the same subframe as the scheduled periodic CQI report, the periodic CQI report will use the PUSCH, together with UL data transmission. This is because a UE can't transmit on both PUCCH and PUSCH simultaneously. In this case, the periodic PUCCH resource will be idle.

Since periodic CQI report brings in the "always on" signaling overhead, the report granularity is relatively rough. In order to get more detail CQI report, the eNB can trigger aperiodic CQI report when needed. The aperiodic CQI report is transmitted on PUSCH, together with UL data or alone.

Comments

Thanks for your answer -

But what exactly is the meaning of "In order to get more detail CQI report " -
What would be missing in a periodic cqi report which would be needed by in the aperiodic cqi report ?  Why would the eNodeB suddenly need that missing information ?

Answer 2

If periodicity is more for periodic CQI reporting then it could delay DL link adaption (if your algo is depedent on CQI reports). Hence, in order to adapt to change in radio link eNB might require Aperiodic reports to adapt.