PDCCH Part-I

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What is PDCCH and what information does it carry?

Physical Downlink Control Channel (PDCCH) is a physical channel that contains control information for the downlink data (DCI) to be sent to UE in the current subframe and UL grants for the UEs that want to send uplink data.

In simple terms, PDCCH carries the Downlink Control Information (DCI) message which indicates resource assignment for single or multiple UEs.

Where is PDCCH located in a subframe?

PDCCH occupies the first n (n = the value of PCFICH) OFDM symbols of a subframe (except if 1.4MHz bandwidth are supported; then PDCCH occupies 4 OFDM symbols). The exact number of OFDM symbols occupied by PDCCH is given in PCFICH channel which occupies the first symbol of a subframe.

One advantage of allocating PDCCH at the start of a subframe is that if a UE does not get schedule in any subframe that time UE can switch off its receiver circuitry for that subframe.

Is it mandatory to read PDCCH for a UE to Upload and Download data?

Yes, UE needs to decode PDCCH successfully if UE wants to upload/download some data.

How does eNodeB identify any information (regarding downlink or uplink) to transmit to single UE or Multiple UE?

During Attach process UE transmits UE capability info, using which eNodeB can transmit supported DCI for single as well as multiple UEs.

Example: If UE A supports TM7 and UE B supports TM3/TM4/TM2 then EnodeB doesn’t transmit supported parameter of B to A as that will increase the signaling overhead for UE A.

     

What is a CCE?

To carry a PDCCH, multiple consecutive CCEs (Control Channel Elements) are allocated (allocation of resource happens in terms of CCE, it would vary with different conditions for Downlink or Uplink in good or poor coverage, and the bandwidth available with the eNodeB).

One CCE = 9 consecutive REGs.

One REG (Resource Element Group) = 4 REs

So 1 CCE = 36 REs.

What are the factors on which the number of CCEs required to transmit the control information depend?

• PCFICH value
• Bandwidth of the system (1.4 MHz - 20 MHz)
• Number of antenna ports which in turn will affect the reference signals present as well as network condition.

How is the number of CCEs in a PDCCH transmission calculated?

The number of CCEs in a PDCCH transmission depends on the PDCCH format which is a term that represents the number of bits to be transmitted. 

PDCCH format         No. of CCEs       No. of bits

0                                       1                               72

1                                       2                              144

2                                       4                              288

3                                       8                              576

How the number of bits to be transmitted is calculated from PDCCH format?

No. of bits = No. of CCEs * no. of REG in a CCE * No. of RE in a REG * No. of bits in a RE.

Example:

1) No. of bits = 1*9*4*2 (eNodeB uses QPSK modulation) = 72 bits.

2) If No. of CCEs = 8 and QPST modulation (=2 bits) used

No. of bits = 8*9*4*2 = 576 

   

Similarly, number of bits can be found for other formats.