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

DCI Format	Usage	Significant Components
Format 0	UL Grant	Resource allocation for UL data
Format 1	DL assignment for SISO	Resource allocation for downlink data to a single UE
Format 1A	DL assignment for SISO (compact)	More compact version of DCI format 1, used for paging and triggering RACH in connected states
Format 1B	DL assignment for MIMO with rank 1	Resource allocation for downlink data to a single UE using MIMO with rank 1
Format 1C	DL assignment for SISO (minimum size)	Minimum size version of DCI format 1, used for paging and triggering RACH in idle states
Format 1D	DL assignment for multi-user MIMO	Resource allocation for downlink data to multiple UEs using MIMO
Format 2	DL assignment for closed-loop MIMO	Resource allocation for downlink data to a single UE using closed-loop MIMO
Format 2A	DL assignment for open-loop MIMO	Resource allocation for downlink data to a single UE using open-loop MIMO
Format 2B	DL assignment for TM8 (dual layer beamforming)	Resource allocation for downlink data to a single UE using TM8 dual layer beamforming
Format 2C	DL assignment for TM9	Resource allocation for downlink data to a single UE using TM9
Format 3	TPC commands for PUCCH and PUSCH with 2-bit power adjustment	Power control commands for the PUSCH and PUCCH channels with 2-bit power adjustment
Format 3A	TPC commands for PUCCH and PUSCH with 1-bit power adjustment	Power control commands for the PUSCH and PUCCH channels with 1-bit power adjustment
Format 4	UL assignment for UL MIMO (up to 4 layers)	Resource allocation for uplink data to a single UE using MIMO with up to 4 layers

• RB assignment: Resource block assignment. Specifies the resource blocks that are allocated to the UE.
• TPC: Transmit power control. Specifies the transmit power that the UE should use.
• HARQ: Hybrid automatic repeat request. Specifies the HARQ process number for the transmission.
• PMI: Precoding matrix indicator. Specifies the precoding matrix that the eNB should use.
• TPMI: Transmit precoding matrix indicator. Specifies the transmit precoding matrix that the eNB should use.
• DL power offset: Downlink power offset. Specifies the downlink power offset that the eNB should use.
• Precoding information: Precoding information for MIMO transmissions.

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.

PDCCH and PDSCH configured by RA-RNTI (Table 7.1-3)

If a UE is configured to receive RAR messages, it will listen for a special type of PDCCH message that is scrambled using the RA-RNTI. If the UE receives this type of PDCCH message, it will decode it and the corresponding PDSCH message. The UE will use the RA-RNTI to unscramble the PDSCH message.

If the UE is also assigned a C-RNTI or SPS C-RNTI in the same subframe, it does not need to listen for PDCCH messages scrambled using the C-RNTI or SPS C-RNTI on the primary cell. This is because the RAR message will always be sent on the secondary cell.

PDCCH and PDSCH configured by C-RNTI (Table 7.1-5)

• If a UE is configured to receive PDCCH or EPDCCH messages scrambled using the C-RNTI, it will decode the messages and any corresponding PDSCH messages according to the combinations defined in 3GPP TS 36.213 version 12.4.0.
• If the UE is configured with the carrier indicator field for a given serving cell and is also configured to decode PDCCH/EPDCCH with CRC scrambled by the C-RNTI, the UE will decode the PDSCH of the serving cell indicated by the carrier indicator field value in the decoded PDCCH/EPDCCH.
• The UE does not support transmission mode 8 if extended cyclic prefix is used in the downlink.
• The UE will decode the corresponding PDSCH in the same subframe if it detects a PDCCH or EPDCCH message scrambled using the C-RNTI with DCI format 1A, 2C, or 2D intended for the UE in one of the downlinks subframes indicated by the higher layer parameter mbsfn-SubframeConfigList or by mbsfn-SubframeConfigList-v12x0 of serving cell c, except in subframes for the serving cell:
• indicated by higher layers to decode PMCH
• configured by higher layers to be part of a positioning reference signal occasion and the positioning reference signal occasion is only configured within MBSFN subframes and the cyclic prefix length used in subframe #0 is normal cyclic prefix
• A UE configured in transmission mode 10 can be configured with scrambling identities, i_nDMRS, ID, i = 1,0, by higher layers for UE-specific reference signal generation.

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.