PDCCH and PUCCH

Physical Downlink Control Channel (PDCCH)

The downlink control signalling (PDCCH) is located in the first n OFDM symbols where n ≤ 4 and consists of:

- Transport format and resource allocation related to DL-SCH and PCH, and hybrid ARQ information related to DL-SCH;

- Transport format, resource allocation, and hybrid-ARQ information related to UL-SCH;

Transmission of control signalling from these groups is mutually independent. Multiple physical downlink control channels are supported and a UE monitors a set of control channels. Control channels are formed by aggregation of control channel elements, each control channel element consisting of a set of resource elements. Different code rates for the control channels are realized by aggregating different numbers of control channel elements.

QPSK modulation is used for all control channels. Each separate control channel has its own set of x-RNTI. There is an implicit relation between the uplink resources used for dynamically scheduled data transmission, or the DL control channel used for assignment, and the downlink ACK/NAK resource used for feedback

Physical Uplink Control Channel (PUCCH)

The PUCCH shall be mapped to a control channel resource in the uplink. A control channel resource is defined by a code and two resource blocks, consecutive in time, with hopping at the slot boundary. 

Depending on presence or absence of uplink timing synchronization, the uplink physical control signalling can differ. In the case of time synchronization being present, the outband control signalling consists of:

- CQI;

- ACK/NAK;

- Scheduling Request (SR).

The CQI informs the scheduler about the current channel conditions as seen by the UE. If MIMO transmission is used, the CQI includes necessary MIMO-related feedback.

The HARQ feedback in response to downlink data transmission consists of a single ACK/NAK bit per HARQ process. PUCCH resources for SR and CQI reporting are assigned and can be revoked through RRC signalling. An SR is not necessarily assigned to UEs acquiring synchronization through the RACH (i.e. synchronised UEs may or may not have a dedicated SR channel). PUCCH resources for SR and CQI are lost when the UE is no longer synchronized.

Ref. 36.300