How DL Channel works in LTE?
The PDCCH, Physical Downlink Control Channel, allocates the DL and UL resources.
There is one PDCCH per sub-frame.
It helps the UE retrieve the transport blocks from the PDSCH:
- Allocated RB
- Modulation and Coding Scheme (MCS)
- Multi-antenna transmission
- Shared between all UE of the cell
- Each TTI contains an aggregation of PDCCH allowing to address several users simultaneously (UL and/or DL)
- The CRC of each PDCCH is scrambled (XOR) with the UE RNTI
- Each UE tries to decode PDCCH and compares the resulting RNTI with its own identity (blind decoding)
PDCCH resource granularity = 1 CCE (Control Channel Element)
1 CCE = 36 Resource Element = 72 bits (QPSK modulation only for PDCCH)
Physical mapping is such that RE of a same CCE are scattered in time and frequency to get the best diversity
Physical mapping depends on the physical parameter ncell_id so that CCE of synchronized neigbouring cells do not collide
The reception of the PDCCH is essential for the data transmission.
Its size and location are not pre-defined since the amount of control data depends on the traffic.
The Physical Control Format Indication Channel (PCFICH):
- allows the UE to decode the PDCCH.
- is carried on pre-defined time-frequency resources.
- indicates the number of symbols occupied by the PDCCH.