PDCCH Dimensioning

Expert Opinion

Expert Opinion
Short LTE articles and opinions by our team of Subject Matter Experts (SMEs)

PDCCH Dimensioning

  • Comments 3

We have discussed PCFICH resource mapping, PHICH resource mapping, and PDCCH construction. Now we are ready to look at PDCCH dimensioning. Since the PDCCH is constructed from Control Channel Elements (CCEs) with an aggregation level of 1, 2, 4, or 8, we'll calculate how many CCEs are available.

Let's use the table "PDCCH Dimensioning (10 MHz)" channel bandwidth as an example. We assume 1 or 2 or 3 OFDM symbols are allocated for PDCCH.

  • If the Control Format Indicator (CFI) is 1, only the first OFDM symbol in a subframe is used for the PDCCH.
  • There are a total of 50 PRBs in a 10MHz channel.
  • Each PRB has 12 subcarriers and has 12 Resource Elements (REs) in the first OFDM symbol. So 50 PRBs have 50*12=600 REs in the first OFDM symbol.
  • Each PRB has 4 REs for Reference Signals (RS), so 50 PRBs have 50*4 =200 REs reserved for RS, assuming 2 transmit antennas at the eNB.
  • A total of 16 REs is reserved for PCFICH, regardless of system bandwidth.
  • The PHICH occupies 84 Res, assuming Ng=1.
  • The number of REs available for the PDCCH = Total REs - RS REs - PCFICH REs - PHICH REs = 600 - 200 - 16 - 84 = 300 REs
  • Each CCE consists of 36 REs, so the number of CCEs is floor(300 / 36) = 8 CCEs

An exercise for you: calculate the number of available CCEs if 2 or 3 OFDM symbols are allocated for the PDCCH.

 

Similarly, we can also calculate PDCCH dimensioning for 5 MHz and 20 MHz channels.

What does the number of available CCEs indicate? It is a good indication of PDCCH capacity, such as how many UEs can be scheduled per subframe. Note that the PDCCH carries resource allocations for both user traffic and control signaling messages. The control signaling overhead includes SIBs, paging, PRACH response, and uplink Transmit Power Control (TPC) commands. The frequency of the control signaling transmission depends on the system configuration.

Knowing the PDCCH capacity, dynamic PDCCH allocation can be used to improve throughput. If the number of active users is low, fewer OFDM symbols are used; if there are more active users in the system, more OFDM symbols will be allocated for the PDCCH.

PDCCH capacity does not generally impact the throughput of data traffic that can tolerate more delay. For delay-sensitive traffic such as Voice over IP (VoIP), PDCCH capacity and utilization need to be carefully monitored for any potential negative impact.

  • What is the significance of the parameter Maximum number of OFDM symbols for PDCCH, As we see that for 2CCE(aggregation level -2) are sufficient for DCI format 2, so when do we need this parameter to incresae PDCCH capacity.

  • If it is for expansion , then how CCE of PDCCH are allocated to UE, IS it 2CCE(AL2) per user ?

  • Dear Hongyan ;

    many thanks  for this details about  PDCCH Dimensioning.

    can you please explain briefly the PDSCH Dimensioning .

    BR