Why do we need multiple aggregation levels?
First, to support multiple DCI (Downlink Control Information) formats to improve resource utilization. We know that the DCI size varies a lot depending on the format and the channel bandwidth. We can view PDCCHs with different aggregation levels as different sizes of container, which can increase the granularity of the resource utilization, instead of a “one size fits all” solution.
Second, to accommodate different RF conditions. The ratio between the DCI size and the PDCCH size indicates the effective coding rate. With the DCI format fixed, higher aggregation levels provide more robust coding and reliability for the UEs under poor RF conditions. For a UE in good RF conditions, lower aggregation levels can save resources.
Third, to differentiate DCIs for control messages and DCIs for UE traffic. Higher aggregation levels can be used for control message resource allocations to provide more protection. The aggregation level for control messages can be 4 or 8, while the aggregation level for UE-specific allocation can be 1 or 2 or 4 or 8.