Table of Contents
Introduction
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.
In 4G LTE, 16-bit CRC was used and the increased CRC size reduces the risk of incorrect reception at the receiver end. Different DCI formats are defined to meet needs for 5G efficient working but the number of DCI formats are less as compared to LTE. Unlike LTE DCI formats, several of NR DCI formats can share same DCI size.
There are following three Category of DCIs shown below used for Uplink Allocation DCI Format 0_x, Downlink Allocation DCI Format 1_x and UE group Common Signaling DCI Format 2_x.
Key Pointer for 5G DCI
- The no. of different DCI sizes configured to monitor is up to four for a cell
- The no. of different DCI sizes with C-RNTI configured to monitor is up to three for a cell
- DCI are two category Fallback DCI e.g. DCI Format 0_0 and 1_0 and non Fallback DCI e.g. DCI Format 1_1
- Fallback DCI formats in general avoids uncertainties during e.g. RRC reconfiguration in which case the network doesn’t know the exact time when the UE has applied the configuration
- Fallback DCI formats are usually of less size compared to non-fallback DCI formats
- The size of the non-fallback DCI format varies depending on the active configuration.
Different Types DCI
The following is the list of the DCI formats, the corresponding RNTI types and their usage for 5G NR.
- DCI Format 0_0 :It is the fallback DCI format for uplink resource allocations on the PUSCH
- DCI Format 0_1 : It is the standard DCI format for uplink resource allocations on the PUSCH
- DCI Format 1_0 : It is the fallback DCI format for downlink resource allocations on the PDSCH
- DCI Format 1_1 : It is a standard DCI format for downlink resource allocations on the PDSCH
- DCI Format 2_0 : It is provision of Slot Format Indicators (SFI)
- DCI Format 2_1 : It is used to provision of Pre-emption Indications
- DCI Format 2_2 : It is used to provision of closed loop power control commands applicable to the PUCCH and PUSCH
- DCI Format 2_3 : It is used to provision of closed loop power control commands applicable to the SRS
DCI use for Downlink Data scheduling
UE will be able to get following information after decoding DCI;
- Time and Frequency domain information for user data
- Modulation and Coding Scheme for data transmitted
- Antenna ports and no. 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.
DCI use 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 gNodeB received the scheduling request.
DCI parameters include time and frequency location, modulation and coding scheme and other information such as precoding for uplink data to be transmitted.
References:
- 3GPP 38.212 5G NR;. Multiplexing and channel coding
- 38.213, 5G NR Physical layer procedures for control
- 38.214, 5G NR Physical layer procedures for data
- 38.331 5G NR RRC Protocol