In LTE release 8, all the (Layer 1) L1 control information is transmitted on the control region of subframes (i.e., first few OFDM symbols of each subframe). As shown in Fig. 1, the PDCCHs are distributed across the full downlink bandwidth. The capacity of the control channel is limited; there is not much room for frequency domain inter-cell interference coordination.
Figure 1. Downlink resource mapping
In addition to the conventional PDCCH of Release 8, the EPDCCH is available in Release 11. The EPDCCH the PDSCH resources for control information transmission. The main reasons why we need ePDCCH are as follows.
The EPDCCH uses PDSCH resources for control information transmission. It is UE-specific, meaning that different UEs can have different EPDCCH configurations. The EPDCCH is configured via RRC signaling. Each UE can be configured with two sets of EPDCCHs. The configuration can also be different between the two sets. The following figure illustrates an example of two EPDCCH sets of an UE when the total carrier bandwidth is 20 MHz.
Figure 2. ePDCCH Configurations
Each EPDCCH set can have 2, 4, or 8 PRB pairs. For example, in Figure 2, there are 4 PRB pairs in the EPDCCH set 1 and 2 PRB pairs in set 2. One enhanced resource element group (EREG) consists of 9 resource elements (REs), and, one enhanced control channel element (ECCE) normally has 4 EREGs. But in some cases, such as extended CP or special subframe in a TDD system, one ECCE also could consist of 8 EREGs. Finally, one ePDCCH will have one or more ECCEs depending on the aggregation level.
One should note that, since EPDCCH is UE specific, only UE-specific search space will be used for the EPDCCHs. DCI formats 3/3A and 1C for multiple UEs are not supported in EPDCCHs. UE may need fast PDSCH decoder to meet HARQ timing because it needs to wait until the end of the subframe to detect the entire EPDCCH.
For resource utilization efficiency, the resource blocks which have been configured for EPDCCHs in a subframe can still be used for PDSCH transmission if they are not actually used for the EPDCCH transmissions during a given subframe.