How Downlink Control Information or DCI in LTE?

  • The PDCCH carries Downlink Control Information or DCI to indicate the resource assignment in UL or DL for one RNTI. 
  • A DCI can conveys various pieces of information, but the useful content depends on the specific case of system deployment or operations. 
  • If the MIMO is not used during the transmission , there is no need to transmit the MIMO parameter in the DCI.
  • There are several format of DCI defined by the 3GPP for each need.
  • UE feed backs the network with CQI (Channel Quality Indication) giving an estimation of the channel quality 
The network adapts downlink transmission accordingly by changing: 
  • The coding rate (amount of information bits over number of transmitted bits)
  • The modulation (QPSK, 16QAM, 64QAM)
  • Radio resources are shared among users: a trade-off is needed between maximizing the bitrate and using
  • the smallest amount of resource blocks
  • For MIMO configuration, the RI and PMI are also reported
The main DCI formats are: 
  • Format 0
    For scheduling of the PUSCH (UL Grant)
  • Format 1A
    Used during the Random Access Procedure
  • Format 1
    For the scheduling of the PDSCH (DL Grant)
    In case of TxDiv
  • Format 2A
    For the scheduling of the PDSCH (DL Grant)
    In case of Open Loop SU- MIMO
  • Format 2
    For the scheduling of the PDSCH (DL Grant)
    In case of Closed Loop SU- MIMO

Downlink Control Information (DCI) used to grant UL/DL traffic 

  • DCI0 for uplink grant of PUSCH 
  • DCI1 and DCI1A for downlink grant of PDSCH with one single codeword 
  • DCI2 for downlink grant of PDSCH with one or two codewords (closed-loop MIMO) 
  • DCI2A for downlink grant of PDSCH with one or two codewords (open-loop MIMO) 
  • DCI carried in PDCCH transport channel

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