LTE technology works based on three channel types viz. logical channel,transport channel and physical channels. These channels are used by lower layers to provide services to the upper layers.
The access points to the Layer L2/L3 are transport channels. They get mapped to physical channels. These physical channels will have different modulation-code rate as mentioned below and are exclusively used by LTE PHYSICAL Layer to carry upper layer information.
LTE Logical,Transport and Physical channels
Following figure mentions LTE logical channels,transport channels and physical channels and mapping between them.
As shown logical channels are of two types; one carrying control information and the other carrying traffic informtion. These gets mapped to transport channels as depicted in the figure. The left side is downlink channel mapping and right side is uplink channel mapping.
Physical Channels
PDSCH - Stands for Physical Downlink Shared Channel, mainly used to carry high speed data/multimedia information. Can be either QPSK/16QAM/64QAM.
PDCCH - Stands for Physical Downlink Control Channel, mainly used to carry UE specific control information. It will have QPSK modulation used.It is mapped on resource elements(REs) in first 3 OFDM syms(symbols) in first slot of subframe.
CCPCH - Stands for Common Control Physical Channel, carries cell-wide control information. QPSK is used. CCPCH is transmitted exclusively on 72 subcarriers centered around DC carrier.
Physical Signals
Physical signals do not convey L2/L3 layer information, but mainly used for synchronization and channel estimation purpose. RS is used for estimating channel response. P-SS and S-SS synchronization signals used for determining network frame timing information i.e. start of the information.
Transport Channels
• Downlink and Uplink transport channels carry L2/L3 information.
• It also configures LTE PHY layer.
• It sends status information such as packet error and CQI to upper layers.
• Also supports peer-peer signaling between higher layers.
Based on broadcast,unicast or multicast concept different transport channels exist. Downlink channels include BCH(broadcast channel),DL-SCH(downlink shared channel, to multiple mobile subscribers or UEs),PCH(paging channel, used for UE DRX and broadcasted over entire cell ),MCH(multicast channel, transmitted over entire cell). Uplink channels include RACH(Random Access Channel), UL-SCH(Uplink Shared Channel).
Uplink PRBs(Physical Resource Blocks) are assigned to UE by eNodeB scheduler. PUSCH is used and shared by multiple UEs to carry upper layer information towards eNodeB. It will employ QPSK/16QAM/64QAM modulation types.