LTE - Downlink Control

In LTE, following physical channels are involved in downlink control.

• Physical downlink control channel (PDCCH) carries
  • Downlink (DL) assignment
  • Uplink (UL) grant
  • TPC commands for a group of users
  • Physical downlink shared channel (PDSCH) arrival time instance
  • Paging indication
  • SPS activation/release
  • Etc.
• Physical control format indicator channel (PCFICH) indicates the time-domain duration of the PDCCH in each subframe.
• Physical HARQ indicator channel (PHICH) indicates the acknowledgement of PUSCH.

Just as its name implies, the basic element for PDCCH is Control channel element (CCE), which is comprised of 9 resource element group (REG), and a REG consists of 4 RE's in an OFDM symbol without cell-specific reference signal (CRS), or 6 RE's in an OFDM symbol with CRS. Irrespective of the both cases, there are always 4 effective RE's/REG, which is favorable of space frequency block codes (SFBC) based transmit diversity for PDCCH.

An aggregation level number of CCE's compose one PDCCH. The value of aggregation level can be 1, 2, 4, 8.

As mentioned above, the time-domain duration of PDCCH (in number of OFDM symbols) is signaled by PCFICH, which is transmitted in 1st symbol of each downlink subframe over 4 REG's, i.e., 16 RE's in a fully distributed pattern across the whole bandwidth.

• For system bandwidth between 6 and 10 RB's, the PDCCH can occupy 2, 3, or 4 OFDM symbols.
• For system bandwidth larger than 10 RB's, the PDCCH can span 1, 2, or 3 OFDM symbols.

In LTE, synchronous HARQ is adopted in UL data transmission. Namely, after a PUSCH subframe, a UE will check a HARQ retransmission indication or a scheduling grant in a predefined downlink subframe, e.g., 4 ms later.