Key Technical Points of 5G Radio Group (RAN2) in R18

RAN2 is responsible for the radio interface architecture and protocols (such as MAC, RLC, PDCP, SDAP), radio resource control protocol specifications, and radio resource management procedures in the 3GPP Radio Access Network (RAN2) technical specifications. RAN2 is also responsible for developing technical specifications for 3G evolution, 5G (NR), and future radio access technologies.

I. Enhanced L1/L2 Mobility and XR Protocols
RAN2 focuses on MAC/RLC/PDCP/RRC protocols to achieve mobility, XR, and power efficiency. Key features include:

1.1 L1/L2-centric inter-cell mobility (dynamic cell handover, L1 beam management).

• Working Principle: In connected mode, the UE measures L1-RSRP via SSB/CSI-RS with no RRC gap. The gNB triggers CHO (Conditional Handover) based on the L1 threshold; the UE performs handover autonomously; L2 handover is performed via MAC CE (without RRC).
• Progress: Based on RRC, the handover interruption time is 50-100 milliseconds; the handover failure rate on high-speed railways (500 km/h) is as high as 40%.
• Implementation Results: Interruption time is less than 5 milliseconds, and the handover success rate reaches 95% at a speed of 350 km/h.

1.2 XR Enhancement (Multi-sensor Data, Dual Connectivity Activation).

• Working Principle: RRC configures XR QoS streams and performs attitude/motion reports (sending 6 degrees of freedom data every 5 milliseconds). Conditional PSCell activation activates UE measurement SCG L1-RSRP, triggered by MAC CE, without requiring RRC reconfiguration; multi-sensor tagging distinguishes video/haptic/audio streams.
• Progress: Rel-17 DC activation interruption exceeding 50 milliseconds leads to XR synchronization interruption; multi-sensor QoS cannot be distinguished.
• Implementation Results: SCG activation latency is less than 10 milliseconds, and the QoS of each sensor stream is independent (haptic priority).

1.3 Multicast Evolution (MBS in RRC_INACTIVE state, dynamic group management).

• Operating Principle: gNB configures MBS sessions via RRC; inactive UEs join via group ID, requiring no state transition.
• Dynamic Handover: Unicast to multicast handover is performed based on a UE count threshold. HARQ combines multicast and unicast reception.
• Work Progress: Rel-17 MBS requires the RRC_CONNECTED state (IoT device power consumption 70%).
• Result: Software update saves 70% energy, stadium capacity increases by 90%.

1.4 RRC State Optimization (Small data transmitted through inactive state, slice-aware reselection).

• Operating Principle: SIB carries slice-specific RACH events/PRACH masks. UEs in idle/inactive states perform slice-aware reselection (prioritizing the highest priority S-NSSAI). UEs in the RRC_CONNECTED state report allowed NSSAI changes during handover.
• Work Progress: Rel-17's lack of support for slice-aware access resulted in 25% of URLLC UEs accessing eMBB slices. Results: The initial slice access success rate reached 95%.

1.5 Energy Saving (Extended DRX, Reduced Measurement Interval).

• How it Works: Extended DRX allows User Equipment (UE) to extend its sleep time by reducing the frequency of paging and control channel listening. Reducing the measurement interval minimizes data transmission interruptions caused by measurement demands by optimizing or combining the measurement interval with other signaling events.
• Progress: Due to frequent control channel listening and measurement intervals leading to frequent radio state switching, UEs experience high power consumption. By extending the DRX cycle and reducing the measurement interval, battery life is significantly improved across all device categories, especially for IoT devices requiring long-term operation.

II. Areas of Improvement:

• High-speed rail (achieving L1/L2 handover latency <5ms through CHO/DAPS evolution).
• Cloud gaming/AR (XR QoS streaming with latency <10ms).
• Massively Multi-level Internet of Things (MBS multicast can reduce the power consumption of software updates by 70%).

III. Protocol Changes

• Protocol Stack Changes: L1 measurements now use RRC signaling (new report triggering is based on SSB/CSI-RS), and CHO uses MCG/SCG targets.
• Example: Conditional PSCell added to NR-DC; UE measurement L1-RSRP trigger activation no longer requires RRC intervals (tested in the lab using Keysight equipment, SCG setup speed improved by 50%).