Release 15, finalized in June 2018, paved the way for the commercialization of 5G (NR) technology. R15 laid the foundation for 5G networks through Standalone (SA) and Non-Standalone (NSA) architectures, introducing a service-based virtualized core network and new physical layer technologies to enhance capacity, reduce latency, and improve flexibility. During this period, 3GPP Radio Working Groups RAN1-RAN5 made significant contributions to the standardization of 5G (NR) technology. The work and key technical points of each group are as follows:
I. RAN1 (Physical Layer Innovation) Key work areas include waveforms, parameter sets, multiple access, MIMO, and reference signals:
1. Flexible subcarrier spacing and frame structure; Introduction of scalable subcarrier spacing:
Implementation: Baseband processing dynamically adjusts the FFT size and cyclic prefix according to different subcarrier spacings.
Application Cases: Low-latency industrial control (30kHz) and high-bandwidth millimeter-wave eMBB links (120kHz).
2. Mass MIMO and Beam Forming
Example: 64T64R gNB arrays form dynamic UE-specific beams, improving spectral efficiency in dense deployments.
3. OFDM-Based Duplexing and Resource Allocation
Implementation: The gNB scheduler dynamically preempts ongoing downlink transmissions to support URLLC burst transmissions.
4. Reference Signals and Synchronization：Introduction of new signals SS/PBCH, CSI-RS, PTRS, and SRS.
5. Channel Coding Evolution: LDPC coding is used for the data channel, replacing Turbo coding to improve eMBB throughput efficiency.
Application Scenario: High-reliability control signaling in variable data rate environments.
II. RAN2 (Radio Interface) MAC, RLC, PDCP, and RRC protocols define the radio interface architecture, scheduling, RRC state, bearer establishment, and signaling optimization.
1. Dual Connectivity (DC) introduces a master-slave gNB architecture, where the UE can distribute traffic between LTE and NR (NSA mode).
Application Scenario: Improving throughput in the early 5G deployment phase before pure 5G core network (EN-DC based on EPC).
2. RRC_INACTIVE State: Introduces a new UE state to minimize signaling overhead while maintaining low-latency recovery.
Implementation: The UE stores the RRC context to enable fast connection for intermittent traffic (approximately 10 milliseconds).
Application Scenario: IoT sensors with periodic small data bursts.
3. QoS Flow-Based Architecture: PDCP is reconstructed into QoS flow IDs, consistent with the 5GC architecture.
Implementation: Each PDU session routes QoS flows to the DRB via SDAP mapping.
Use Case: Video streams with dynamic bit rate adaptation.
4. Header Compression and Security: RoHCv2 optimization and enhanced encryption are adopted to reduce control plane overhead.
5. Mobility and Handover Enhancements: Unified inter-RAT handover signaling is defined between LTE-NR (NSA) and NR-NR (SA) networks.
III. RAN3 (NG Interface and Dual Connectivity Evolution) technologies include: F1, Xn, and NG interface definitions, gNB-CU/DU management, and interoperability.
1. gNB Separated Architecture (CU/DU): Logical separation between centralized units (CU) and distributed units (DU).
Implementation: The F1-C (control) and F1-U (user) interfaces adopt a flexible fronthaul transmission design.
Application Scenarios: Cloud-RAN and multi-vendor interoperability.
2. NG and 5GC Interfaces: Introduces NG-C (control plane) and NG-U (user plane) interfaces, replacing the S1 interface in LTE. Supports service-based 5G core network functions through AMF/SMF.
3. EN-DC Architecture: Defines Xn and S1* signaling for interoperability between eNB and gNB. Supports smooth operation of LTE anchor points in the early stages of 5G deployment.
4. Session Continuity and Network Slicing: Integrates a QoS-based inter-slice mobility mechanism.
Application Example: Seamless handover between different slices based on latency requirements (eMBB→URLLC).
IV. RAN4 (Radio and Spectrum) Band Definitions, Power Levels, Spectrum Aggregation, and Coexistence.
1. New Frequency Band Ranges (FR1 and FR2)
Implementation: Modular design of the device's RF front-end supports dual-band operation using switchable low-noise amplifier (LNA) chains.
2. Bandwidth and Carrier Aggregation: Up to 400MHz of channel bandwidth is defined in FR2. Aggregated carriers combine NR and LTE for hybrid deployments.
3. Power Rating and EIRP Calibration: UE ratings are established for millimeter wave devices; stringent EVM and ACLR parameters are introduced.
Application Case: Small cell base stations and CPEs using beam control for 5G FWA.
4. Coexistence and Transmit Control: Spectrum masks are defined to ensure coexistence among multiple radio access technologies (RATs). Support for sharing NR spectrum with LTE or NR-U in unlicensed bands.
5. RF Performance and Reference Sensitivity: Enhanced sensitivity modeling for massive MIMO array base stations. Introducing beam-based power control to manage the equivalent isotropic radiated power (EIRP) of each beam.
V. RAN5 (Equipment Testing and Conformance): Conformance, signaling, and UE performance testing procedures.
1. Test Specification Alignment: Introducing TS 38.521/38.533/38.141 for RF and protocol conformance testing of NR UEs and base stations.
2. OTA (Over-The-Air) Test Framework: Introducing a millimeter-wave equipment anechoic chamber test model, considering beam control and dynamic radiation patterns.
Example: 5G smartphone characteristic analysis and phased array beam switching verification.
3. End-to-End Signaling Verification: Verifying the interoperability of the RRC/PDCP/PHY layers, which is crucial for early NSA integration.
4. Performance Benchmarking: Defining key performance indicators (KPIs) for latency, throughput, and reference sensitivity in a real-world propagation environment.
Release 15 lays the foundation for the first phase of 5G, defining the NR physical layer, new radio protocols, flexible architecture, and RF/coherence aspects. It supports key 5G services, including eMBB, URLLC, and mMTC, running on a unified architecture while simultaneously supporting both NSA and SA modes.
