ultiple uplink bands. This reduces (client) implementation complexity and cost. Physical Layer The Physical Layer is responsible for channel encoding including physical-layer HARQ; mod- ulation; multiple-input, multiple-output (MIMO) (multiantenna) processing; and mapping of the signal to the appropriate physical time-frequency resources. Channel mapping The communication path all the way from the wireless interface (the air) and RLC layer is done through a set of so-called channels or communication pipes and interfaces that function at differ- ent levels of the stack. These channels are well-defined and become the anchor point for imple- mentation and test. At the lowest level toward the air interface, a physical channel is defined by time/frequency resources in the OFDM grid that is used for transmission of a Transport channel that holds logical information coming from the MAC layer. The Physical Layer mapping function translates (maps) transport channels to physical channels. Transport channels are pre- sented by the Physical Layer to the MAC layer where each transport channel has its own charac- teristics for transmission over the air interface (say, broadcasted or not or with specific targets for reliability and latency). Logical channels are presented by the MAC layer to the RLC. They are defined by the type of information they carry. By implication, multiplexing control channels to transport channels is done by the MAC layer. Logical, transport, and physical channels in the downlink are shown in Figs. 2-26 and 2-27. Logical channels (presented by MAC layer to RLC) are: Downlink: UL Logical Channel UL-SCH Transport Channel PUCCH PUSCH PRACH UL Physical Channel FIGURE 2-27 Uplink 5G channels. Chapter 2 System components, requirements, and interfaces 71 Broadcast Control Channel (BCCH)-used in DL only to transmit Master Information Block (MIB) that includes basic information required by the client to detect the cell site and the SI blocks that contain more RAN-related information including frame and radio acc