DCI stands for Downlink Control Information.
It is used to carry downlink control information for one or more cells with one RNTI.
It also provides the UE with necessary information such as physical layer resource allocation, power control commands, HARQ information for both uplink and downlink.
DCI is transmitted on PDCCH with 24-bit CRC attachment.
Below table shows the different types of DCI formats
Format 0 is for uplink grant, it has the info related to UE is about to transmit in uplink.
Format 1 is for downlink, it has the info related to the data that will be sent to UE.
For both uplink and downlink there are 2 formats one with underscore 0 (_0) and one with underscore 1 (_1).
The format with _0 is called as fallback format that is compact than the full format with _1.
Format 2 address the information related to UE and TPC commands.
Downlink control information uses polar code for error protection
After encoding downlink control information is scrambled with QPSK modulated and mapped to resource blocks with a very specific pattern.
UE must look for PDCCH and decode the PDCCH to get the required DCI information for further processing.
Decode PDCCH -> Parse DCI -> Decode PDSCH.
How DCI will be used for downlink data scheduling ?
By decoding DCI, UE will be able to get the answers for below questions:
1. Where is the data for the user in time and frequency domain ?
2. What modulation and coding scheme ?
3. Antenna ports and number of layers.
When UE looking for PDCCH and if a match is found by decoding with a CRC that matches the RNTI of UE.
It will extract all the information of DCI in time and frequency domain.
Then it will performs channel estimation qualization inverse rate matching and decoding to retrieve the downlink data packet.
How DCI will be used for Uplink data scheduling ?
For uplink transmission DCI will carry uplink grant.
It will be sent as a response to a scheduling request from the UE when the gNB received the scheduling request.
DCI parameters include time and frequency location, modulation and coding scheme and other information such as precoding.
Source TS 38.212
PDCCH
A user-specific physical downlink control channel (Physical Downlink Control Channel, PDCCH) is used to schedule downlink PDSCH transmission and uplink PUSCH transmission. The information transmitted on the PDCCH is called DCI (Downlink Control Information), including 8 DCI formats including Format 0_0, Format 0_1, Format 1_0, Format 1_1, Format 2_0, Format 2_1, Format 2_2 and Format 2_3.
1. Format0_0 is used for PUSCH scheduling in the same cell
2. Format0_1 is used for PUSCH scheduling in the same cell
3. Format1_0 is used for PDSCH scheduling in the same cell
4. Format1_1 is used for PDSCH scheduling in the same cell
5. Format2_0 is used to indicate the Slot format
6. Format2_1 is used to indicate to the UE those PRB(s) and OFDM symbols that it considers no data (to prevent the UE from ignoring)
7. Format2_2 is used to transmit TPC (Transmission Power Control) commands to PUCCH and PUSCH
8. Format2_3 is used for the TPC transmitted to the SRS signal, and can carry the SRS request at the same time
The differences and usage scenarios between various DCI formats will be discussed in detail later.
The PDCCH channel adopts the Polar code channel coding mode, and the modulation mode is QPSK.