Downlink Layer 1 / Layer 2 (L1/L2) control signaling is used for transmitting downlink scheduling assignments required for the terminal to properly receive, demodulate and decode the DL-SCH channel.
The uplink scheduling grants informing the terminal about resources and transport format for UL-SCH transmission, and hybrid-ARQ (HARQ) acknowledgements in response to UL-SCH transmission also arrive on Downlink L1/L2 control.
As illustrated in Figure below, the downlink L1/L2 control channels are mapped to the first (up to three) OFDM symbols within the sub-frame (According to 3GPP, you can use first four OFDM symbols of a subframe for control region). Thus, each sub-frame can be said to be divided into a control region, followed by a data region. The size of the control region is always equal to an integer number of OFDM symbols (1, 2,3 or 4 OFDM symbols can be used for control signalling) and can be varied per sub-frame. This maximizes the spectral efficiency as the control signalling overhead can be adjusted to match the instantaneous traffic situation.
The location of the control signalling at the beginning of the sub-frame is advantageous, as it allows the terminal to decode the downlink scheduling assignment prior to the end of the sub-frame. Processing of the DL-SCH can begin earlier compared to the case of the control signaling, spanning the full sub-frame duration. This minimizes the delay in the DL-SCH decoding and thus the overall downlink transmission delay. It can also result in lower UE power consumption as a result.
The downlink L1/L2 control signaling consists of three different physical channel types:
PCFICH (Physical Control Format Indicator Channel) – It informs the terminal about the number of OFDM symbols (1, 2, 3 or 4) used for L1/L2 control signaling in the current sub-frame. There is only one PCFICH in a cell.
PDCCH (Physical Downlink Control Channel) – It carries downlink scheduling assignments and uplink scheduling grants. Typically, there are multiple PDCCHs formats in a cell based on the type of signaling needed.
PHICH (Physical Hybrid-ARQ Indicator Channel) – It is used to transmit ACK/NAK in response to reception of UL-SCH transmissions. Typically, there are multiple PHICH in a cell.