We discussed how PDCCHs are constructed from the CCEs in the
control region of a subframe. Next, let's look at the PDCCH detection procedure;
in other words, how a UE searches for the DCIs intended for it. The UE searches
for possible resource allocations in the related search space.
The search space indicates the set of CCE locations where the UE
may find its PDCCHs. Each PDCCH carries one DCI and is identified by a Radio
Network Temporary ID, or RNTI. The RNTI is implicitly encoded in the CRC
attachment of the DCI. There are two types of search space: the common search space
and the UE-specific search space. A UE is required to monitor both common and
UE-specific search space; note that there might be overlap between common &
UE-specific search spaces for a UE.
The common search space includes the DCIs for system information (using
the SI-RNTI), paging (P-RNTI), PRACH responses (RA-RNTI), or UL TPC commands
(TPC-PUCCH/PUSCH-RNTI). The UE monitors the common search space using
aggregation level 4 and 8. The UE-specific search space can carry DCIs for
UE-specific allocations using the UE's assigned C-RNTI, semi-persistent
scheduling (SPS C-RNTI), or initial allocation (temporary C-RNTI). The UE
monitors the UE-specific search space at all aggregation levels (1, 2, 4, and 8).
The starting CCE index of a PDCCH can be obtained using a formula
based on the RNTI, the slot number within a radio frame, the total number of
CCEs in the control region of a subframe, and the aggregation level.
are the steps for PDCCH blind detection.
After each blind detection, the UE checks the
CRC with the corresponding RNTI. If CRC succeeds, the UE can derive the exact
DCI format of the detected PDCCH from the payload size and RNTI. Knowing the
DCI format, the UE can go ahead and parse the payload.
For calculating REs for PDCCH how does the UE know how many first OFDM symbols it should look for (1,2 or 3)?