The PDCCH (Physical Downlink Control Channel) in LTE carries UE-specific scheduling assignments for Downlink (DL) resource allocation, Uplink (UL) grants, PRACH (Physical Random Access Channel) responses, UL power control commands, and common scheduling assignments for signaling messages (such as system information, paging, etc.).

The PDCCH occupies the first 1 or 2 or 3 OFDM symbols at the beginning of each subframe (except in the case of a 1.4 MHz channel, where it's the first 2 or 3 or 4 OFDM symbols). The actual number of OFDM symbols occupied in any given subframe is indicated in the PCFICH (Physical Control Format Indicator Channel), which is located in the first OFDM symbol of each subframe. An interesting question would be: how many OFDM symbols are needed for the PDCCH? Well, we have to look at how the PDCCH is constructed before we can answer.

Let's look at the building blocks of the PDCCH.

- Resource Element (RE): the minimum resource unit in LTE, indicated by one OFDM symbol in time domain and one subcarrier in frequency domain.
- Resource Element Group (REG): Each REG contains 4 consecutive REs (or 4 REs separated by a cell-specific Reference Signal (RS)) within the same OFDM symbol and the same resource block. Here is an example of the REGs in a RB (Resource Block) with two cell specific RSs and normal CP (Cyclic Prefix). We can see that there are 2 REGs in the OFDM symbol 0 in the first slot of a subframe, and there are 3 REGs in the OFDM symbol 1, symbol 2, and symbol 3 (if needed for a 1.4MHz channel). Knowing this will help us with PDCCH dimensioning later.

Figure 1 Resource Mapping of Resource Element (RE) and Resource Element Group (REG)

- Control Channel Element (CCE): Each CCE contains 9 REGs, which are distributed across the first 1/2/3 (/4 if needed for a 1.4MHz channel) OFDM symbols and the system bandwidth through interleaving to enable diversity and to mitigate interference.

Now, we are ready to build the PDCCH using these blocks. The number of CCEs in a PDCCH is called its CCE aggregation level, and may be 1, 2, 4, or 8 consecutive CCEs (logical sequence). The total number of available CCEs is determined by the PCFICH configuration and the system bandwidth. Each PDCCH contains exactly one DCI (Downlink Control Information).

Different PDCCHs in a subframe may use different aggregation levels. For example in Figure 2, PDCCH #0 may use aggregation level 1, PDCCH #1 and #4 may use aggregation level 2, PDCCH #2 and #5 may use aggregation level 4, and PDCCH #3 may use aggregation level 8. Note that a PDCCH with aggregation level n can only start on (CCE index mod n) = 0. For example, a PDCCH with aggregation level 4 can only start on CCE index 0, 4, 8, 12, 16, etc. This helps the UE perform blind searches.

Let's zoom in on PDCCH #3, which consists of 8 CCEs (since its aggregation level is 8). Each of the CCEs consists of 9 REGs which are spread across OFDM symbols and the system bandwidth through interleaving. Each REG includes 4 REs. The total number of REs needed for PDCCH #3 is therefore 8*9*4 = 288. Similarly, the total number of REs needed for PDCCH #0 is 1*9*4 = 36. It is obvious that different aggregation levels require different amount of REs. Why do we need multiple aggregation levels? (Please pause, think about your answer, then continue.)

Figure 2 PDCCH Construction Map

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.