In the third session, I have explained the Scheduling from the perspective of the Control Channel and how it differs from the Data Channel. It also explains why Control Channel Scheduling is different and the impact of Control vs Data failures. The Control Channel is referring to the PDCCH and the PDCCH is essentially made up of CCEs (Control Channel Elements). However, the allocation of PDCCH is done in group of CCEs and these groups are called PDCCH Aggregation Levels. Typically, in 5G, there are 5 PDCCH Aggregation Levels as given below while in LTE, there are 4 PDCCH Aggregation Levels!
- PDCCH Aggregation Level-1 contains 1 CCE
- PDCCH Aggregation Level-2 contains 2 CCEs
- PDCCH Aggregation Level-4 contains 4 CCEs
- PDCCH Aggregation Level-8 contains 8 CCEs
- PDCCH Aggregation Level-16 contains 16 CCEs - This aggregation level is not used in LTE
Once the eNB/gNB get the CQI, it is converted to a certain PDCCH Aggregation Level. Low PDCCH Aggregation has lesser overhead (low CCE count) and it is used for UEs in good channel conditions while Higher PDCCH Aggregation has higher overhead (high CCE count) and it is used for UEs in bad radio conditions
Basically, higher the PDCCH Aggregation Level, higher the robustness and decoding capability of the PDCCH but higher the overhead too!