Carrier Aggregation — MAC Aspects

← MAC Overview Scheduling → HARQ → PHR → DC → BFR → Sidelink → ☆ IAB MAC ☆ Cell-Free

1. MAC Architecture — One Entity, Multiple Cells

CA uses one MAC entity managing up to 31 SCells plus 1 PCell. Each cell has independent HARQ processes (0-15), but LCP, DRX, and multiplexing are shared across all cells.

🏗️ Multi-Cell HARQ Architecture

Add or remove SCells to see independent HARQ process banks. Each cell handles its own ACK/NACK independently.

Active SCells: 2 DL traffic:

PCell

Always active. Carries PUCCH, SR, RA. Schedules itself and optionally other cells.

SCell

Activated/deactivated by MAC CE. Carries PDSCH/PUSCH only. Independent HARQ per cell.

Shared MAC

LCP, DRX, multiplexing/demultiplexing operate across ALL cells from one logical channel set.

2. SCell Activation / Deactivation

The gNB activates SCells via Activation/Deactivation MAC CE (Ci bitmap: 1=activate, 0=deactivate). After activation, the UE starts PDCCH monitoring, CSI reporting, and the sCellDeactivationTimer.

⚡ SCell Activation Timeline

Click Activate to see the MAC CE → activation delay → SCell ready flow. Toggle deactivation timer expiry.

Deactivation timer:

MAC CE	Size	Max SCells
Activation/Deactivation (1B)	1 byte	7
Activation/Deactivation (4B)	4 bytes	31

3. Cross-Carrier Scheduling (CCS)

Instead of monitoring PDCCH on every SCell, the UE can receive scheduling for SCells from PCell's PDCCH using a CIF (Carrier Indicator Field, 3 bits) in the DCI.

📡 Cross-Carrier Scheduling — PCell PDCCH → SCell PDSCH

Watch DCI with CIF values schedule resources on different SCells. Toggle self-scheduling vs CCS.

Scheduling mode:

CCS benefit: UE monitors PDCCH on fewer cells → power saving. Especially useful for FR2 SCells where PDCCH reception is less reliable — schedule them from FR1 PCell.

4. SCell Dormancy (Rel-16)

A dormant SCell stops PDCCH monitoring but continues CSI reporting. This saves ~70% power vs active while keeping channel info fresh for fast reactivation (~1-3 ms vs ~3-8 ms from deactivated).

SCell State	PDCCH	CSI	Reactivation	Power
Active	ON	ON	Immediate	High
Dormant	OFF	ON	~1-3 ms	Low
Deactivated	OFF	OFF	~3-8 ms	Minimal

5. Practical Field Considerations

Scenario	CA Config	Key Consideration
Sub-6 GHz only	PCell + 1-3 SCells (same band)	All co-located → same TAG
Sub-6 + mmWave	PCell (3.5G) + SCells (28G)	Separate TAGs, CCS from PCell
FR2 only	3-7 SCells (28/39 GHz)	Many narrow carriers, beam management
SUL	DL 3.5G + UL 1.8G SUL	Coverage extension on UL

Common pitfall: sCellDeactivationTimer too short → SCell deactivates during burst pause → reactivation overhead → "activation ping-pong" wastes MAC CEs and adds latency.

LTE vs 5G NR CA

Aspect	LTE	5G NR
Max SCells	5 (32 for LAA)	31
BWP per SCell	No	Yes
Dormancy	No	Yes (Rel-16)
FR2 (mmWave)	N/A	Up to 400 MHz/CC
SUL	No	Yes

6. 6G Evolution — Massive CA, AI-Managed

Massive CA (64+ CCs)

Beyond 32 SCells. Sub-THz bands with 1-4 GHz per carrier. Compressed bitmap activation for 128 cells.

AI-Managed Activation

AI predicts bursts → pre-activates SCells. Zero activation delay. 40-60% less signaling overhead.

Spectrum Sharing CA

CA across licensed + shared + unlicensed. AI selects carriers based on interference prediction.

🚀 CA Evolution — 4G to 6G

Compare CA capabilities across generations.

Generation:

Dimension	4G (LTE)	5G NR	6G (Target)
Max SCells	5	31	64-128
BWP per cell	No	Yes	AI-managed
Dormancy	No	Rel-16	AI-predicted
Bands	Sub-6	Sub-6 + mmWave	+ Sub-THz
Activation	MAC CE	MAC CE + DCI	AI pre-activate