LTE physical layer has different channels defined on both uplink and downlink, each with a predefined purpose. These physical channels are fed using the transportation channels of higher layers. The physical layer channels are widely divided into data, control and random access channels.
The different physical layer channels on downlink are as below,
- Physical Downlink Shared Channel – PDSCH
- This channel is the main data channel on downlink, which carries the data on downlink. The name shared because, it is a shared channel between multiple users and also it is shared by many logical channels such as DL-SCH, PCH, BCCH etc.
- Physical Downlink Control Channel – PDCCH
- This is the main control channel on downlink, which carries the downlink control information in the form of DCI. The PDCCH carries information about data mapping on downlink, uplink scheduling information etc. The CCH logical channel is mapped to PDCCH
- Physical Broadcast Channel – PBCH
- This is the dedicated broadcast channel in LTE downlink, which carries the vital system parameters such as the system bandwidth, PHICH information, system frame number and also number of transmit antennas used by the eNodeB. The BCH logical channel is mapped to PBCH
- Physical Control Format Indicator Channel – PCFICH
- The PCFICH channel is a downlink channel that conveys the number of control symbols present in the current subframe. In LTE there can be a maximum of 3 control symbols. The PCFICH is always transmitted in the first symbols of the subframe
- Physical HARQ Indicator Channel – PHICH
- The PHICH is the channel used for acknowledging the uplink packet reception to the UE. The PCFICH carries the ACK/NACK information to the UE, to indicate if the eNodeB has successfully decoded the uplink packet or not
- Physical Multicast Channel – PMCH
- The PMCH is a channel that is used for multi-casting a packet to a set of UE, instead of a single UE. The channel structure is similar to that of PDSCH
Each of these downlink physical channels are mapped onto different places in the LTE frame structure.