Basics of Downlink Control Information in 5G NR

How does a UE decode Downlink Control Information (DCI) in 5G NR?

Fig 1. DCI-to-PDCCH-to-CORESET mapping. Remember that PDCCH’s basic unit is CCE. CORESET’s basic unit is REG. Hence, PDCCH-to-CORESET mapping is actually CCE-to-REG mapping.

Downlink Control Information (DCI) carries information from gNB-to-UE such as info DL/UL resource scheduling, power control, modulation and coding schemes to decode a message, HARQ related aspects etc.

A DCI is carried as a payload within Physical Downlink Control Channel (PDCCH). PDCCH is itself wrapped inside something known as Control Set (CORESET). This article assumes that the reader has a basic understanding of the 5G NR channels. Otherwise, click here to learn more.

CORESETs are time-frequency resources where DCI is transmitted.

The actual DCI message passes through the chain as presented in Fig. 1 above before riding over the PDCCH over the air.

UE’s DCI Decoding Procedure

At the UE end, the following procedure takes place to read the control message.

1. The UE scans the CORESET to get PDCCH locations.
2. Within PDCCH UE can find DCI.
3. The UE performs RNTI unmasking and CRC decoding.
4. Successful CRC decoding means a UE knows if the message is intended for it or if there is an error in the DCI. Both purposes are achieved simultaneously via CRC decoding. If decoding is unsuccessful for a particular UE, even if the message is intact, it means the message is intended for a different user.
5. UE reads DCI to know the content of the message (e.g. does gNB wants to send a paging message, DL data, or UL grant etc). Resource Scheduling e.g. transmitting time and number of RBs for PDSCH or PUSCH etc. are all sent via DCI

Now, the problem is that UE cannot scan the whole CORESET(s) as it is both power and time consuming plus highly inefficient. Instead within a CORESET there are Search Spaces (SS) where PDCCH is located. Hence, UE scans these SSs blindly for PDCCH. This is called blind decoding. There are multiple copies of PDCCH within these SSs and they are instead called PDCCH candidates (or candidate PDCCH). A UE may be able to blindly decode any of the PDCCH among these candidates. The number of candidate-PDCCHs depends on something known as…