DCI

DCI - Downlink Control Information

DCI is transmitted in the PDCCH. It tells the UE how to decode the PDSCH for getting the DL data transmitted to it in the same subframe.

There are many DCI formats which tells specific predefined downlink data formats to be used by UE.

So without decoding the DCI, UE will never come to know about the DL data sent to it by the eNodeB.

What DCI informs to UE?

UE will get the following informations from DCI after successful decoding of the DCI in PDCCH.

1. Number of RB's allocated (This will differ with channel Bandwidth)
2. Modulation & Coding scheme (MCS) used in PDSCH for the DL data.(QPSK or 16QAM or 64QAM)
3. Redundancy version - RV
4. New Data Indicator
5. HARQ process number
6. TPC command for PUCCH

Note: 
Incase of MIMO DCI formats like DCI 2 & 2A, 3 Transport blocks informations will be present in DCI for TB1 & 2. Each contails MCS, RV & NDI informations in it.

CRC added to diff CDI formats is of 16 bits & its scrambled with UE-RNTI.
After that 1/3 Rate matching will happen.

So DCI carries all those above data in it. One quick question which comes to the mind now is? How UE's will come to know whether the DCI is intended for a particular UE?

While the process of encoding, DCI will be padded with the CRC bits scrambled with the UE-RNTI. So, once the UE, descrambles the CRC of DCI, it will come to know whether the intended UE is that or not. If the UE-RNTI passed in DCI CRC not matches with the decoded UE, then UE will simply ignore it.

Different DCI formats:

There are as many as 9 DCI formats as of now in Rel8 3GPP. They are:

DCI - 0
DCI - 1
DCI - 1A
DCI - 1B
DCI - 1C
DCI - 1D
DCI - 2
DCI - 2A
DCI - 3
DCI - 3A

In those formats, DCI 0 is always used to indicate Uplink Data Transmission.

DCI formats 1 to 2A are used to indicate DL data transmission format in PDSCH

DCI formats 3 & 3A are used for Transmit Power Control information for PUCCH & PUSCH.