How EnodeB decides the Aggregation Level to use ?
It is decided on the basis of size of DCI and the CQI value received in uplink. The size of different DCI formats is defined in spec. After selecting the DCI format, we know the number of bits needs to send on PDCCH after applying the code rate.
As we know that in 1 CCE 72 bits can be transmitted as QPSK modulation is used in PDCCH. After considering the DCI size and the CQI value, eNodeB decides the aggregation level to use or the number of CCE’s required for the control information for this particular DCI.