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.
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