4G Physical layer: Downlink Channels and Signals

4G Radio Channels / Signals broadly are:

  • PBCH
  • PSS and SSS
  • Cell specific RS & CSI-RS
  1. Physical data control channel (PDCCH) :- conveys Downlink Control Information (DCI). It carries information such as scheduling and resource grant to UE. It includes one or more control channel elements (CCE, consists of 9 resource element group). A PDCCH can span from symbol 1 to 3 in the time domain.
  2. Physical Broadcast Channel (PBCH) :- The PBCH carries part of the system information (MIB) required by the UE to access the network. Together with the PSS and the SSS, PBCH performs the initial access procedure. It spans across 72 subcarriers in frequency domain and 4 ofdm symbols across time domain.
  3. Primary and Secondary Synchronization Signals (PSS & SSS) :- PSS and SSS are used for cell-search procedures and cell identification.
    PSS and SSS together carry the PCI. PSS sends one of three orthogonal sequences and SSS sends one of 167 binary sequences. Therefore, there are 504 unique PCIs.
  4. Physical control format indicator channel (PCFICH):- Carries the number of OFDM symbols used for transmission of PDCCHs in a subframe
  5. Physical Hybrid Automatic request (HARQ) indicator (PHICH) :- ACK/NACK feedback to the UE for the uplink blocks received by eNB
  6. Cell specific reference signal – To estimate all the channels in the physical layer & measure signal strength and quality.
  7. Channel State Information Reference Signal (CSI RS) : It is used for RSRP measurements in advanced antenna technique and calculate CQI, PMI, Rank indicator related data.
  8. Physical data shared channel (PDSCH) :-
    PDSCH carries user data, paging info & SIB msgs. All the Resource elements left after allocation if all above messages, rest resources are allocated to PDSCH. Different formats can be used depending upon the transmission modes.

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