# 3 GPP Evolution from 5G to 6G: A 10-Year Retrospective
## 2 3 GPP Release 15: Birth of 5G
Downlink channels include the physical downlink shared channel (PDSCH), the physical downlink control channel (PDCCH), and the physical broadcast channel (PBCH). Uplink channels consist of the physical uplink shared channel (PUSCH), the physical uplink control channel (PUCCH), and the physical random access channel (PRACH). These channels are accompanied by reference signals and synchronization signals, ensuring robust connectivity and control within the network. For the downlink, the system supports a multitude of quadrature amplitude modulation (QAM) schemes, including quadrature phase shift keying (QPSK), 16-QAM, 64-QAM, and 256-QAM [23]. For the uplink, in addition to the schemes available for the downlink, the system supports \(\pi/2\)-binary phase shift keying (\(\pi/2\)-BPSK) for DFT-s-OFDM, providing enhanced power efficiency. To ensure robust and reliable communication, NR employs advanced channel coding techniques [24]. Transport blocks are encoded using low-density parity-check (LDPC) codes, known for their high efficiency and error-correction capabilities. Figure 2: An illustration of 5G NR protocol architecture. Polar codes are utilized for control channels and the PBCH, leveraging their suitability for short block lengths and latency-sensitive applications. Key physical layer procedures include cell search, power control, uplink synchronization, and hybrid automatic repeat request (HARQ) [25]. These procedures ensure seamless operation in diverse network scenarios, from initial access to maintaining connectivity during mobility. Additionally, channel state information (CSI) reporting and beam management are critical for advanced antenna systems (e.g., massive MIMO) and millimeter wave (mmWave) communications in 5G [25]. Physical layer measurements are integral for ensuring optimal network performance. These include intra- and inter-frequency measurements for handovers, inter-RAT measurements, and timing synchronization. Such measurements provide the basis for advanced radio resource management (RRM) and seamless mobility [26]. On top of the physical layer, the NG-RAN leverages a multi-layered protocol architecture to manage data transmission efficiently. The medium access control (MAC) layer handles logical-to-physical channel mapping and the prioritization of traffic, ensuring smooth data flow [27]. The radio link control (RLC) layer offers different transmission modes: transparent, unacknowledged, and acknowledged, catering to diverse reliability requirements [28]. The packet data convergence protocol (PDCP) layer adds security features, robust header compression (ROHC), and a sequence to optimize data delivery [29]. The service data adaption protocol (SDAP) layer aligns quality-of-service (QoS) flows with radio bearers, providing an essential link between application requirements and network resource management [30]. The radio resource control (RRC) layer governs the establishment and maintenance of RRC connections, configuring data and signaling bearers while managing mobility and QoS [31]. It defines RRC_IDLE, RRC_INAACTIVE, and RRC_CONNECTED states, allowing devices to operate with varying levels of activity and energy efficiency. These states ensure that devices remain reachable while optimizing power consumption and resource usage. RRC also supports essential mobility features such as handovers and cell reselection, ensuring seamless transitions across cells. Mobility management is a cornerstone of NG-RAN functionality, enabling devices to maintain continuous connectivity while on the move [31]. Intra-NR mobility supports handovers and beam failure recovery, while inter-RAT mobility facilitates transitions between NR and legacy LTE networks. Paging mechanisms and the RAN-based notification area (RNA) updates enhance device reachability [32]. These features collectively ensure that mobility is seamless and user experiences remain uninterrupted. The QoS framework in NG-RAN is designed to deliver consistent performance across diverse applications [33].