Physical Control Format Indicator Channel (PCFICH)
- As the name suggests, this channel communicates the number of OFDMA symbols that the PDCCH will be transmitted on, to the UE.
- The PDCCH can take either {1,2,3} OFDMA symbols for BW > 1.4 MHz and values {2,3,4} for BW 1.4 Mhz.
- The two bits required to carry one of the 4 values is encoded into 32 bits. QPSK modulation is used which yields 16 bits to be transmitted.
- The 16 bits for the PCFICH are spread over 16 REs in 4 groups of 4 REs each. This provides added protesction from frequency selective fading.
- The PCFICH shall be transmitted on the same set of antenna ports as the PBCH.
- The PCFICH is spread over the whole range of sub-carriers which provides it with some imunity to the frequency selective fading
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| PCFICH and PDCCH Channel |
Physical downlink control channel(PDCCH)
- The PDCCH is the control channel which is specified by the PCFICH, as shown in the figure above
- The channel carries downlink assignments for data on the Physical Downlink Shared Chaneel (PDSCH)
- The modulation used in QPSK
- The number of OFDMA symobols occupied by PDCCH in the first slot of each sub frame is given by PCFICH (in absence of PCFICH, the PDCCH is also absent). Let the number of OFDMA symbols be L.
PDCCH Resource Element Mapping
The block of quadruplets to be transmitted on the PDCCH are mapped to Resource Elements (k',l').
(k',l') where k' = 0 ... num-dl-rb*12 and l' = 0 ... L
The algorithm for the mapping is:
1) Initialize m'=0 (resource-element
group number)
2) Initialize k' =0
3) Initialize l' = 0
4) If the
resource element m' represents a
resource-element group and the resource-element group is not assigned to PCFICH
or PHICH then perform step 5 and 6, else go to step 7
5) Map
symbol-quadruplet w(m') to the
resource-element group represented by (k',l') for each antenna port p
6) Increase m' by 1
7) Increase l' by 1
8) Repeat from step 4 if , where l' < L, where L corresponds to the number
of OFDM symbols used for PDCCH transmission as indicated by the sequence
transmitted on the PCFICH
9) Increase k' by 1
10) Repeat from step 3 if k' < num-dl-rb*12