Secondary Cell Group

Description

The Secondary Cell Group (SCG) is a fundamental concept in 3GPP's dual connectivity (DC) and multi-connectivity frameworks, introduced from Release 12 onward. In a DC scenario, a user equipment (UE) is simultaneously connected to two nodes: a Master Node (MN) managing the Master Cell Group (MCG) and a Secondary Node (SN) managing the SCG. The SCG comprises one or more secondary cells (SCells) provided by the SN, which can be of the same or a different radio access technology (RAT) as the MCG—for example, LTE MCG with NR SCG in EN-DC (E-UTRA-NR Dual Connectivity). The SCG adds extra radio resources, increasing overall bandwidth and improving data throughput, reliability, and mobility robustness.

Architecturally, the SCG is controlled by the SN, which handles radio resource management (RRM) for its cells, including scheduling, bearer split configuration, and mobility within the SCG. The MN retains control of the MCG and coordinates overall UE connectivity, managing signaling like RRC connection and handover. Data flows can be split at various points: bearers may be terminated at the MN (MCG bearers), at the SN (SCG bearers), or split across both (split bearers). The SCG uses interfaces such as X2 (between eNBs in LTE) or Xn (between gNBs in NR) for coordination with the MCG. Key procedures involve SCG addition, modification, and release, triggered based on measurement reports and network policies to optimize performance.

The SCG operates with specific physical and protocol layer aspects: SCells within the SCG can be activated/deactivated dynamically to save power, and they support carrier aggregation (CA) principles. In NR-based SCGs, features like bandwidth parts (BWP) and flexible numerology are applicable. The SCG enhances network performance by enabling load balancing, reducing interruption times during handovers, and supporting high-demand use cases like enhanced mobile broadband (eMBB). It is integral to 5G non-standalone (NSA) deployments, where LTE anchors control while NR SCG provides high-speed data. As multi-connectivity evolves, the SCG concept extends to multi-RAT scenarios, forming the basis for advanced aggregation techniques in 5G-Advanced and beyond.

Purpose & Motivation

The SCG was created to address the growing demand for higher data rates and more reliable connections, which single connectivity or carrier aggregation within one node could not fully meet. Prior to Release 12, LTE Advanced relied on carrier aggregation (CA) within a single eNB, limited by available spectrum and site constraints. Dual connectivity with SCG allows aggregation of resources from geographically separate base stations, increasing total bandwidth and providing macro-diversity gains. This solved issues like cell-edge performance degradation and capacity bottlenecks, especially in heterogeneous networks with small cells.

With the transition to 5G, the SCG became crucial for smooth migration, enabling LTE-NR interworking in non-standalone mode. It allowed operators to leverage existing LTE infrastructure for coverage and control while adding NR SCGs for enhanced data capabilities, addressing the challenge of deploying 5G without a full core network overhaul. The SCG also supports service continuity and ultra-reliable low-latency communication (URLLC) by enabling redundant paths. Its development was motivated by the need for flexible, efficient multi-connectivity solutions to support diverse 5G use cases and network evolution.