Multi-RAT / Cell-Free MAC Coordination

Scheduling MAC Overview Spatial Mux Beamforming Sidelink DC IAB MAC NTN MAC Energy-Saving MAC AI Scheduling ISAC Scheduling

1. Cell-Free Architecture — No Cell Boundaries

In cell-free massive MIMO, many distributed APs jointly serve all UEs. There is no cell boundary, no handover, and no cell-edge problem. A central CPU coordinates all APs via fronthaul links.

Cellular vs Cell-Free — Coverage Comparison

Toggle between cellular (cells with boundaries) and cell-free (distributed APs, continuous coverage). Watch UE move through both.

Mode: Cell-Free mode

Distributed APs

Simple radio units (1-4 antennas) spread across coverage area. Connected to CPU via fronthaul. Many per cell-equivalent area.

Central CPU

Baseband processing for all APs. Precoding, decoding, scheduling. Manages user-centric clusters dynamically.

Fronthaul

IQ sample transport (fiber/ethernet). Centralized: ~Gbps per AP. Functional split reduces 10-100x. Critical bottleneck.

2. User-Centric Clustering

Each UE is served by its N nearest APs (the cluster). As the UE moves, APs are added/removed from the cluster dynamically. This replaces handover entirely.

Dynamic Cluster Formation

Drag the UE position. Watch the cluster of serving APs reform in real-time based on proximity.

Cluster size (N): 4 4 APs serving

No handover: In cellular, UE moving between cells triggers handover (RRC reconfiguration, data interruption). In cell-free, the cluster simply adds new APs and drops distant ones — seamless, no signaling overhead.

3. Coherent vs Non-Coherent Joint Transmission

Coherent JT aligns phases across APs for N^2 gain. Non-coherent JT transmits independently for N gain. Trade-off: coherent needs precise sync and high fronthaul.

Signal Combining — Coherent vs Non-Coherent

Compare how signals from multiple APs combine at the UE. Coherent: phase-aligned (constructive). Non-coherent: independent (diversity).

APs in cluster: 4 Mode:

Mode	Power Gain (N=4)	Sync Required	Fronthaul
Cellular (1 AP)	1x (0 dB)	None	None
Non-coherent JT	4x (6 dB)	Loose	Low
Coherent JT	16x (12 dB)	Sub-nanosecond	Very high

4. Practical Field Considerations

Factor	Challenge	Mitigation
Fronthaul capacity	~2.4 Gbps per AP (CPRI)	eCPRI, functional split
Synchronization	Sub-nanosecond for coherent	IEEE 1588 PTP, GPSDO
AP density	More APs = higher cost	Optimize per use case
Scalability	CPU processing grows with APs	Distributed processing
Cost	Higher than cellular	Target high-value areas

When to use cell-free: Best for areas needing uniform high coverage (stadiums, airports, factories). If cell-edge performance is unacceptable with cellular, cell-free eliminates the cell-edge problem entirely.

5. 6G Evolution — True Cell-Free + Holographic MIMO

Ubiquitous APs

Every light fixture, outlet = AP. Hundreds per room. Seamless coverage everywhere. No "cells" in any sense.

AI Mesh Coordination

AI decides cluster formation in real-time. Predictive: pre-forms cluster where UE will move. Zero-touch planning.

Sub-THz Cell-Free

100-300 GHz APs. Ultra-high capacity. Very dense deployment. Beam-based cell-free (beam = virtual cell).

Holographic MIMO

Continuous aperture (metasurface). Infinite spatial DoF. Reconfigurable intelligent surfaces as APs.

Cell-Free Evolution — CoMP to Ubiquitous

Compare cell-free capabilities across generations.

Generation:

Dimension	4G (CoMP)	5G (C-RAN)	5G-Adv	6G Target
Coordination	Limited (2-3 cells)	Centralized BBU	Distributed APs	Ubiquitous mesh
Joint TX	Non-coherent	Some coherent	Coherent JT	Holographic
Cell boundary	Exists	Exists	Soft (user-centric)	None
Frequency	Sub-6 GHz	Sub-6 + mmW	Sub-6 + mmW	Sub-THz + RIS