LTE DCI (Downlink Control Information) concept in detail

DCI (Downlink Control Information) in LTE
LTE’s Downlink Control Information (DCI) is a vital part of the resource allocation process. It’s a complex system, but it’s essential for ensuring that data is transmitted and received efficiently.
One of the challenges of resource allocation in LTE is the sheer number of possible combinations of time, frequency, and modulation schemes. Without DCI, it would be impossible for the receiver to figure out where to look for the data and how to decode it.
In LTE, DCI is more versatile and contains additional information beyond resource allocation, including Power Control Command, CSI Report Request, and CQI Report Request.
Different DCI formats exist, each carrying a specific set of information.
The choice of DCI format depends on the specific communication situation.
DCI formats are required to tell the UE how to get its data which is transmitted on PDSCH in the same subframe.
PDCCH is a control channel that carries DCI information. DCI is like a map for a UE to find and decode PDSCH from the resource grid.
The DCI format gives the UE, details such as:
Number of resource blocks
Resource allocation type
Modulation scheme
Transport block
Redundancy version
Coding rate
Each DCI format, when encoding is attached with a CRC that is scrambled with a UE-RNTI to which the PDSCH is intended to. So only that UE can decode the DCI format and hence the corresponding PDSCH.
The packed DCI information is the payload to the PDCCH encoding chain.
In other words, DCI formats are required to provide the UE with the necessary information to decode the PDSCH data that is intended for it. Without DCI, the UE would not know which resource blocks to decode, or how to decode them.
Here is an example of how DCI formats are used:
The eNB wants to send data to a UE.
The eNB selects a set of resource blocks on the PDSCH to carry the data.
The eNB encodes the data into a transport block.
The eNB encodes the DCI information for the transport block.
The eNB transmits the DCI information on the PDCCH.
The UE receives the DCI information and decodes it.
The UE uses the DCI information to decode the transport block on the PDSCH.
Once the UE has decoded the transport block, it can extract the data that was intended for it.
Types of DCIs
The UE procedure for receiving the physical downlink shared channel (PDSCH) is as follows:
The UE monitors the PDCCH (physical downlink control channel) for DCI (downlink control information) intended for it.
If the UE detects a PDCCH with DCI format 1, 1A, 1B, 1C, 1D, 2, 2A, 2B, 2C, or 2D intended for it in a subframe, it decodes the corresponding PDSCH in the same subframe.
The UE is only expected to decode a maximum number of transport blocks as defined by the higher layers.
If the UE is configured with multiple serving cells, it decodes the PDSCH on the serving cell with the different frame structure type first.
If the UE is not configured to decode PMCH (physical multi-cast channel) on the primary cell (PSCell), it does not monitor the common search space on the PSCell for PDCCH with CRC scrambled by the system information reference signal-RNTI (SI-RNTI).
The UE assumes that positioning reference signals are not present in resource blocks in which it decodes PDSCH according to a detected PDCCH with CRC scrambled by the SI-RNTI or P-RNTI (paging-RNTI) with DCI format 1A or 1C intended for the UE.
If the UE is configured with the carrier indicator field for a given serving cell, it assumes that the carrier indicator field is not present in any PDCCH of the serving cell in the common search space. Otherwise, the UE assumes that for the given serving cell the carrier indicator field is present in PDCCH/EPDCCH (enhanced PDCCH) located in the UE specific search space.
PDCCH and PDSCH configured by SI-RNTI (Table 7.1-1)
If the UE is configured to decode PDCCH with CRC scrambled by the SI-RNTI, it decodes the PDCCH and the corresponding PDSCH according to any of the combinations defined in Table 7.1-1 of 3GPP TS 36.213. The scrambling initialization of PDSCH corresponding to these PDCCHs is by SI-RNTI.
PDCCH and PDSCH configured by P-RNTI (Table 7.1-2)
If a UE is configured to receive paging messages, it will listen for a special type of PDCCH message that is scrambled using the P-RNTI. If the UE receives this type of PDCCH message, it will decode it and the corresponding PDSCH message. The UE will use the P-RNTI to unscramble the PDSCH message.
The UE is not expected to listen for this type of PDCCH message on the common search space on the PSCell. This is because paging messages are typically sent to the UE on a dedicated paging channel.
Relationship between RNTI and DCI Format
The RNTI and the DCI format are two important pieces of information that are used in LTE to identify the UE and the type of downlink transmission that is being transmitted. The image you sent shows the usage of RNTI and DCI formats in different situations.

[@shreyasrai]

How does UE decide which DCI format to use ?
Does all DCI format come in the PDCCH?

[@techLteworld]

The UE determines which DCI format to use based on the control information received on the PDCCH, identified using its RNTI and decoded according to its configuration and the DCI payload. All DCI formats are transmitted over the PDCCH.

[@shreyasrai]

If NDI is not toggled then all data of complete resource block for that Uae should be retransmitted ?

[@techLteworld]

@@shreyasrai yes if not toggled it’s the case of retransmission

[@techLteworld]

If the NDI is not toggled, it does not imply that all data of the complete resource block for that UE should be transmitted. Instead, it implies that the current transmission is a retransmission of the previously sent data due to an acknowledgment (ACK) not being received from the UE, indicating the data was not correctly decoded.