All About PDCCH channels in LTE

Hello folks, Back after long time. Today we will discuss about one of the most important channels in LTE i.e PDCCH.

PDCCH(Physical downlink Control Channel):- The PDCCH can be called as heart of the data transmission in LTE as its carries the control information about the data being transmitted on the current subframe and the information about the resources which UE need to used for the Uplink data. Which means for any data transmission and reception UE must need to decode the PDCCH channel.

So next important questions comes in my mind is
Information carried by PDCCH?
Where it carries the information?
And for Whom?

We will try to answers these questions one by one.

Information carried by PDCCH:- PDCCH carries a message called DCI ( Downlink Control Information ) which includes resource assignments for a UE or group of UE's. EnodeB can transmit many DCI's or PDCCH's in a subframe. 


we have following DCI formats:-


Format 0 for transmission of resources to UE for sending their uplink data, i.e Uplink grant.

Format 1 for downlink allocation of resources for Single Input Multiple Output (SIMO)case.

Format 1A for downlink allocation of resources for SIMO operation or allocating a dedicated preample signature to a UE for random access.

Format 1B for transmission control information of Multiple Input Multiple Output (MIMO) rank 1 based compact resource assignment

Format 1C for very compact transmission of PDSCH assignment

Format 1D same as format1B with additional information of power offset

Format 2 and Format2A for transmission of DL-SCH allocation for closed and open loop MIMO operation, respectively

Format 3 and format3A for transmission of TPC command for an uplink channel.

Where it carries the information?- 
Allocation of resources happens in terms of CCE ( Control Channel Elements ).



1 CCE = 9 continuous REG's ( Resource element Group )

1 REG = 4 RE ( Resource Element )

So 1 CCE = 36REs.


PDCCH uses the resources present in first n OFDM symbols where

n - Value present in PCFICH ( Number of OFDM symbols )



So the number of CCE's present to transmit the control information will be variable depending on the

1.PCFICH value

2.Bandwidth of the system from 1.4 Mhz to 20 Mhz.

3.Number of antenna ports present which in turn will effect the reference signals present.

For Whom?- 
eNodeB uses the PDCCH for sending the control information for a particular UE or a group of UE's. It means eNodeB uses the PDCCH  for some broadcast information also which is common for all the UE's. So to make that process easier eNodeB divided its CCE's into two parts which we call them as search space:


Common search space :- It consists of CCE's which are used for sending the control information which is common for all the UE's .Maximum number of CCE present in common search space is 16.
For Example:- Common search space CCE's are used by eNodeB for sending the control information of SIB's which is common for all UE's.  


UE specific search space :- CCE's belonging to UE specific space are used for sending the control information for a particular UE only. That means information present on UE specific CCE's can only be decoded by a specific UE.


Note:- eNodeB can also send the control information for a specific UE on the common search space.

We will discuss next blog that how eNodeB calculate CCE for UE.