nowledged mode (UM). In the case of the acknowledged mode, the errors are corrected via the ARQ (Automatic Repeat reQuest) mechanism. In order to reduce latency compared to LTE-RLC sublayer, the NR-RLC sublayer no longer performs: - data concatenation; - data reorganization, this function is delegated to the PDCP sublayer. NG-RAN and 5G-NR The MAC sublayer multiplexes the logical channels to optimize the physical resources of the radio layer and performs rapid error correction (H-ARQ: Hybrid ARQ). In the case of carrier aggregation, each H-ARQ entity is independently implemented and there is one H-ARQ acknowledged per cell. The MAC sublayer also manages the sequencing of data by allocating radio resources to the mobiles according to the QoS parameters requested at each TTI (Time Transmission Interval). The MAC sublayer manages the multiplexing of logical channels into transport channels which are then multiplexed into physical channels. Three types of channels are defined (Figure 2.3): - the logical channel defines the structure of the data at the interface between the RLC and MAC sublayers; - the transport channel defines the structure of the data at the interface between the MAC sublayer and the physical layer; - the physical channel defines the data structure between the two parts constituting the physical layer: on the one hand, coding, and, on the other hand, modulation and multiplexing. User traffic Control Flow NAS Message IP Packet RRC message SDAP layer PDCP layer RLC layer Logical hannel MAX Layer Transport Channel UL-SCH DL-SCH Physical layer Physical channel PUCCH PUSCH PRACH PDCCH PDSCH Uplink direction Downlink Direction Figure 2.3. The structure of the radio interface The physical layer (PHY) manages the encoding/decoding of data (user plane and control plane), OFDM modulation/demodulation, and the distribution of flows by antenna with digital precoding to allow different transmission modes (SISO, SIMO, MISO, MIMO). The RF radio chain transposes the baseband signal to an RF signal and vice versa. N