A downlink physical channel consists of a set of resource elements that carry data from higher layers to the physical layer or from the physical layer to higher layers. For example, a downlink physical channel can convey the modulation type, resource and power assignment, and antenna mapping from the medium access control (MAC) layer to the physical layer or convey transport block error indications from the physical layer to a higher layer.
The following downlink physical channels are defined:
Physical Downlink Shared Channel (PDSCH)
Physical Multicast Channel (PMCH)
Physical Control Format Indicator Channel (PCFICH)
Physical Hybrid ARQ Indicator Channel (PHICH)
Physical Downlink Control Channel (PDCCH)
Physical Broadcast Channel (PBCH)
The following general steps describe downlink signal generation on a physical channel. Figure 1 illustrates this process.
For details, refer to the 3GPP LTE Technical Specifications documents at http://www.3gpp.org.
Scramble the coded bits in each code word to be transmitted.
Modulate the scrambled coded bits to generate complex-valued modulation symbols.
Map the complex-valued modulation symbols onto one or more transmission layers.
Pre-code the complex-valued modulation symbols on each layer for transmission at the antenna ports.
Map the complex-valued modulation symbols for each antenna port to resource elements.
Generate a complex-valued, time-domain OFDM signal for each antenna port.
Figure 1. Downlink Signal Generation