The physical control format indicator channel (PCFICH) carries information about the number of OFDM symbol used for transmission of PDCCHs in a subframe. PCFICH uses cell specific scrambling code. 2 bit CFI is block coded into 32 bits and mapped into 16 sub carriers with QPSK Modulation.
PCFICH is located at OFDM symbol #0 of every sub frame and the assignment to the sub carriers is determined by Cell Id Information.The PCFICH is mapped in terms of Resource Element Groups (REGs) and is always mapped onto the first OFDM symbol. The number of REGs allocated to the PCFICH transmission is fixed to 4 i.e. 16 Resource Elements (REs).
A PCFICH is only transmitted when the number of OFDM symbols for PDCCH is greater than zero. The 32 bit coded CFI block undergoes a bit-wise XOR operation with a cell specific scrambling sequence. The scrambling sequence is a pseudo-random sequence created using a length-31 Gold sequence generator. Scrambling with a cell specific sequence serves the purpose of inter-cell interference rejection.
When a UE descrambles a received bit stream with a known cell specific scrambling sequence, interference from other cells will be descrambled incorrectly and will only appear as uncorrelated noise. The scrambled bits are then QPSK modulated to create a block of complex-valued modulation symbols.
|Subframe||Number of OFDM symbols for PDCCH when||Number of OFDM symbols for PDCCH when|
|Subframe 1 and 6 for frame structure type 2||1, 2||2|
|MBSFN subframes on a carrier supporting both PMCH and PDSCH for 1 or 2 cell specificc antenna ports||1, 2||2|
|MBSFN subframes on a carrier supporting both PMCH and PDSCH for 4 cell specific antenna ports||2||2|
|MBSFN subframes on a carrier not supporting PDSCH||0||0|
|All other cases||1, 2, 3||2, 3, 4|