In the previous blog, we have discussed what EPDCCH is and why we need it. We have known that, by and large, EPDCCH is transmitted on RBs in PDSCH region which have been configured for EPDCCH. This blog is going to take a close look on how an EPDCCH gets transmitted on specific resource elements, specifically, localized transmission vs. distributed transmission.

An EPDCCH will have one or more ECCEs depending on the aggregation level. One ECCE normally has 4 EREGs. One EREG consists of 9 REs. There are 16 EREGs per physical resource block pair. The way how EREGs map on to the PRB pair is as follows,

  1. Numbering all resource elements, excluding REs for DMRS, in one PRB pair, from 0 to 15 in an increasing order of frequency domain first, then time domain.
  2. All resource elements with number j in that physical resource block pair constitutes EREG number j. For example, all REs in yellow marked 0, and EREG 0 consists of 9 yellow REs.

An example of this mapping is shown in Figure 1.

Figure 1. EREG to RE mapping.

The EREGs are further grouped into four groups. EREG group 0 consists of EREG with indices 0, 4, 8, and 12; group 1 with EREG 1, 5, 9, and 13; group 2 with EREG 2, 6, 10, and 14; group 3 with EREG 3, 7, 11, and 15 shown in Figure 2 respectively. In total, there are four EREG groups in a PRB pair.

Figure 2. EREG group and Localized transmission.

One EREG group forms an ECCE when an ECCE consists of four EREGs as in Figure 2. If eight EREGs are needed for an ECCE, EREG group 0 and 2 or group 1 and 3 will combine to form an ECCE. Note that all these EREG groups are in the same PRB pair. This is called localized transmission of EPDCCH. Localized transmission is usually used where reliable subband CSI feedback is available such that frequency selective and advanced antenna technologies can be explored to increase the spectral efficiency.

The other EPDCCH transmission type is called distributed transmission. The basic units in the distributed transmission are all the same as in the localized transmission type. With distributed transmission type, EREGs in an EREG group to form an ECCE are from different PRB pairs. Figure 3 illustrates an example where EREG group 0 consists of EREG 0, 4, 8, and 12 from different four different PRB pairs.

Figure 3. Distributed transmission.

Distributed transmission is less dependent on channel feedback, so it is usually used where reliable CSI feedback is not available. It uses frequency diversity by transmitting control signal over multiple PRB pairs across the full system bandwidth. The similar principle is also used in the conventional PDCCH.