Dual Connectivity — MAC Aspects

← MAC Overview Scheduling → HARQ → DRX → CA → BFR → Sidelink → ☆ IAB MAC ☆ Cell-Free

1. DC Architecture — Two Cell Groups, Two MAC Entities

Dual Connectivity connects a UE to two nodes (MN + SN) simultaneously. Unlike CA (one MAC entity), DC creates two independent MAC entities — one for MCG, one for SCG. Data is split at PDCP, not MAC.

MCG (Master)

Anchor group. PCell carries PUCCH, SR, RA, system info. Provides mobility and security.

SCG (Secondary)

Capacity group. PSCell provides additional throughput. Can fail without affecting MCG.

PDCP Split

Data split at PDCP layer (not MAC). Split ratio controlled by threshold. Reordering at RX.

2. Bearer Types — MCG, SCG, Split

DC supports 4 bearer types depending on how data flows through MCG/SCG. The split ratio is controlled by RRC-configured thresholds.

🔀 Bearer Type Visualization — Data Flow Paths

Select a bearer type to see how data flows from PDCP through RLC/MAC/PHY on each cell group.

Bearer type: Split threshold (KB): 10

Bearer Type	RLC Path	Use Case
MCG Bearer	PDCP → RLC(MCG) only	Control plane, low-throughput
SCG Bearer	PDCP → RLC(SCG) only	High-throughput offload to SN
MCG Split	PDCP → RLC(MCG) + RLC(SCG)	Throughput aggregation
SCG Split	PDCP → RLC(SCG) + RLC(MCG)	SCG-primary aggregation
PDCP Duplication	Same PDU on both paths	URLLC reliability (99.999%)

3. Dual MAC Entities — Independent Procedures

Each cell group has its own MAC entity with independent HARQ, BSR, PHR, SR, RA, and DRX. The two MAC entities operate largely independently.

⚙️ Dual MAC Architecture — MCG MAC + SCG MAC Side by Side

Watch two MAC entities process data independently. Inject traffic on either group.

MCG DRX: SCG DRX:

Coordinated DRX (Rel-16): MCG and SCG DRX cycles can be aligned to reduce total wake time. Without coordination, UE wakes for both On Durations → higher power consumption.

4. SCG Failure and Recovery

When the SCG fails (radio link, RLC, configuration), only the SCG drops — MCG continues unaffected. UE reports failure to MN, which decides: release, change, or retry.

🔧 SCG Failure → Recovery Flow

Step through the SCG failure detection → reporting → recovery process. MCG stays active throughout.

Failure type:

Failure Type	Cause	Recovery
SCG RLF	T310 expiry, RA failure on PSCell	SCG change or release
SCG RLC failure	Max RLC retransmissions reached	SCG release
SCG config failure	UE cannot comply with SCG config	Reject config, keep old
SCG sync failure	Cannot sync to new PSCell	SCG release or retry

5. DC Variants

Variant	MN	SN	Core	Use Case
EN-DC	LTE eNB	NR gNB	EPC	NSA (Option 3x) — most common
NGEN-DC	ng-eNB	NR gNB	5GC	LTE→5GC + NR (Option 7)
NE-DC	NR gNB	LTE eNB	5GC	NR primary + LTE (Option 4)
NR-DC	NR FR1	NR FR2	5GC	Sub-6 + mmWave (Rel-17)
NTN-DC	Satellite	Terrestrial	5GC	Coverage + capacity (Rel-17/18)

EN-DC (NSA) dominance: ~90% of initial 5G deployments use EN-DC. LTE provides anchor (coverage, mobility, control plane), NR provides capacity boost. MCG MAC = LTE MAC, SCG MAC = NR MAC.

6. 6G Evolution — Multi-RAT, AI-Coordinated, Mesh

Multi-RAT DC

5G + Wi-Fi 7 + satellite simultaneously. Three+ cell groups. Unified PDCP across all RATs.

AI-Coordinated

AI predicts SCG failure proactively. Zero-interruption SCG change. Real-time split ratio adjustment.

Mesh Connectivity

UE connected to multiple SNs (macro + micro + relay). AI orchestrates across all paths.

🚀 DC Evolution — 4G to 6G

Compare DC capabilities across generations.

Generation:

Dimension	4G (LTE)	5G NR	6G (Target)
Cell groups	2	2	3+
RATs	LTE only	LTE + NR	5G + Wi-Fi + Sat
Duplication	No	Yes (Rel-16)	Multi-path
Coordination	None	Coordinated DRX	AI-orchestrated
Failure recovery	Reactive	Reactive	Predictive