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Hi Guys, today we are learning about PDCCH , if you missed our previous article  please click here.


What are Search Spaces?

The PDCCH is used to send control information to a group of UEs as well as to a particular UE. This means that the PDCCH is used by the eNodeB to broadcast information to a group of UEs. So, in order to simplify the process of decoding the PDCCH, the CCEs are divided into two Search Spaces:

  1. Common Search Space: It consists of the CCEs which are used for sending the control information common for all the UEs. Maximum number of CCEs present in common search space can be 16. Common search space CCEs are used by eNodeB for transmitting SIBs which are common for all UEs. 
  2. UE-specific Search Space: CCEs belonging to UE specific space are used for sending the control information for a particular UE. This means information present on UE specific CCEs can only be decoded by a specific UE.

What are Aggregation Level and PDCCH Candidate?

Aggregation level is defined as the number of consecutive CCEs used for sending control information. Its values can be 1, 2, 4 or 8.

PDCCH Candidate is the number of CCE indexes searched by a UE in a subframe for a particular search space. 

How to calculate the total number of REs available for PDCCH allocation?

No. of REs for PDCCH = Total REs – (Number of REs for PHICH + Number of REs for PCFICH + Number of REs for reference signals).

Here, total REs refers to all REs in first ‘n’ OFDM symbols, where n is the value in PCFICH.
Total REs in first ‘n’ OFDM symbols = n * total number of RBs * number of subcarriers in 1 RB.

What are DCI Formats?

The DCI format is a predefined format in which the downlink control information is packed/formed and transmitted in PDCCH. It tells the UE how to get its data transmitted on PDSCH in the same subframe. DCI format (transmitted in PDCCH) is like a map for a UE to locate and decode PDSCH from the resource grid. 

The DCI format gives the UE information such as number of resource blocks, resource allocation type, modulation scheme, transport block, redundancy version, coding rate etc. Each DCI format, when encoding, is attached with a CRC that is scrambled with the UE-RNTI to which the PDSCH is intended to. 

So, only the intended UE can decode the DCI format and hence the corresponding PDSCH. 

DL DCI Formats:

  • Format 0 – used for transmission of Uplink Shared Channel (UL-SCH) allocation i.e. for providing UL grants.
  • Format 1 – used for transmission of DL-SCH allocation for Single Input Multiple Output (SIMO) operation
  • Format 1A – used for compact transmission of DL-SCH allocation for SIMO operation or allocating a dedicated preamble signature to a UE for random access
  • Format 1B – used for transmission control information of Multiple Input Multiple Output (MIMO) rank 1 based compact resource assignment
  • Format 1C – used for very compact transmission of PDSCH assignment
  • Format 1D – used same as format1B with additional information of power offset
  • Format 2 and Format2A – used for transmission of DL-SCH allocation for closed and open loop MIMO operation, respectively
  • Format 3 and Format 3A – used for transmission of TPC command for an uplink channel

UL DCI Formats:

  • Format 0 for scheduling of PUSCH (uplink grant)
  • Format 3 for Uplink transmit power control with 2 bit power adjustment
  • Format 3A for Uplink transmit power control with 1 bit power adjustment

How does eNodeB decide which DCI Format to use? 

eNodeB makes this decision based on the transmission mode and the RNTI type used for that particular PDCCH.

RNTI types:
  • SI- RNTI: Used for transmission of system information messages.
  • RA-RNTI: Used for PRACH responses.
  • P-RNTI: Used for the paging messages
  • C-RNTI: Used for transmission to a specific UE after RACH.
  • T-CRNTI: Mainly used during RACH.

How does eNodeB decide which Aggregation Level to use?

eNodeB makes this decision based on the size of DCI and the CQI ( channel quality indicator ) value received in uplink. The size of different DCI formats is defined in specs. 

After selecting the DCI format we can find out the number of bits needed to send on PDCCH after applying the code rate.

In 1 CCE, 72 bits can be transmitted; since QPSK modulation is used in PDCCH. After considering the DCI size and the CQI value, eNodeB decides the aggregation level to use or the number of CCE's required for the control information for this particular DCI. 


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