In a 4G LTE subframe for downlink within a Frequency Division Duplexing (FDD) configuration, the components are structured within each downlink subframe to efficiently manage communication between the base station and UEs (User Equipments). This includes transmitting user data, control information, and system messages, all while adapting to varying channel conditions and network demands.
The main components include:
- Two Slots: Each 1 ms subframe is divided into two slots, each lasting 0.5 ms. This structure is fundamental to the LTE frame and subframe organization.
- Resource Blocks (RBs): The basic unit of resource allocation, covering 12 subcarriers in frequency over one slot in time. The number of RBs in a subframe depends on the system's bandwidth.
- Control Channels: Essential for managing the network's operation and UE communication, including:
- Physical Control Format Indicator Channel (PCFICH): Indicates the number of OFDM symbols used by the Physical Downlink Control Channel (PDCCH) in each subframe.
- Physical Downlink Control Channel (PDCCH): Carries downlink scheduling assignments and uplink scheduling grants.
- Physical Hybrid ARQ Indicator Channel (PHICH): Carries acknowledgements (ACKs) or negative acknowledgements (NACKs) for uplink transmissions.
- Data Channels: Primarily the Physical Downlink Shared Channel (PDSCH), which carries the user data and broadcast information not transmitted on the BCH.
- Reference Signals (RS) or Pilots: These are used for channel estimation to adapt the receiver's processing to the current channel conditions. Downlink reference signals are transmitted to help the UE estimate the channel quality.
- Broadcast Channel (BCH): Carries system information critical for the UE to access and operate in the network. It is transmitted over the PDSCH.