Aggregation Level = 6 REG and REG = 12 RE, so one aggregation level means (72 RE)
|Number of CCE/ Aggregation Level||Number of Resource-Element Groups (REGS)|
The PDCCH is transmitted on a specific set of REs in the frequency domain. This set of REs is called the PDCCH carrier. The PDCCH carrier can be made up of 1 or more CCEs.
A CCE is a group of 6 REGs. A REG is equal to 1 resource block during 1 OFDM symbol. A resource block is a group of 12 REs.
The Aggregation Level (AL) defines the number of CCEs which will be used for DCI transmission. The AL can be 1, 2, 4, or 8.
For example, an AL of 2 means that the PDCCH carrier will be made up of 2 CCEs. Each CCE will consist of 6 REGs, which is equal to 72 REs. Therefore, the PDCCH carrier will be made up of 144 REs.
The AL is configured by the network and communicated to the UE via RRC signaling messages. The UE then monitors the PDCCH carrier for DCI transmissions.
A higher AL keeps the PDCCH transmission at a high robustness level, making it suitable for cells where coverage has the highest priority. However, a higher AL also consumes more CCEs, which could be used by other UEs in this slot, reducing the cell capacity.