O-RAN (Open RAN) is an industry alliance that defines open, interoperable interfaces between RAN components (CU, DU, RU) from different vendors. Traditional RAN uses proprietary interfaces, locking operators to single vendors. O-RAN specifies: Open Fronthaul (7.2x split between DU and RU), O-CU and O-DU software running on commercial off-the-shelf (COTS) hardware, RAN Intelligent Controller (RIC) for AI/ML-based RAN optimization, and standardized APIs for third-party applications (xApps and rApps). Major operators (Rakuten, DISH, Deutsche Telekom) have deployed O-RAN networks, though performance and integration challenges remain versus integrated solutions.

What transport is needed for 5G RAN?

5G RAN requires three transport segments: (1) Fronthaul (RU to DU): carries time-domain IQ samples via eCPRI over 25 GbE fiber. Requires <100 microsecond latency and precise timing (IEEE 1588 PTP). Bandwidth: 25-100 Gbps per cell for massive MIMO. (2) Midhaul (DU to CU): carries F1 interface over IP/Ethernet. Latency budget: 1-10 ms. Bandwidth: 1-10 Gbps. (3) Backhaul (CU to 5GC): carries NG interface over IP/MPLS. Latency: 5-20 ms. Bandwidth: 1-10 Gbps. The fronthaul bandwidth requirement is the biggest challenge: a 64T64R massive MIMO cell at 100 MHz bandwidth generates approximately 50-100 Gbps of IQ data.

mmWave & 5G

5G RAN

Q: What is the CU/DU split?

3GPP split the gNB into two functional units: CU (Central Unit) handles upper protocol layers (RRC, PDCP) and can be centralized in a data center serving multiple cells. DU (Distributed Unit) handles lower layers (RLC, MAC, PHY-high) and is co-located near the antenna site. The RU (Radio Unit) handles PHY-low (IFFT/FFT, beamforming) and RF. CU-DU communicate via F1 interface (midhaul). DU-RU communicate via fronthaul (eCPRI or O-RAN 7.2x split). This architecture enables centralized RRM, pooling gains, and flexible deployment: CU in edge cloud, DU at cell site, RU on tower.

The Radio Access Network for 5G NR (NG-RAN), consisting of gNBs that provide the air interface to UEs and connect to the 5G Core via the NG interface. 5G RAN introduces the CU/DU/RU functional split, enabling centralized baseband processing, and is being disaggregated via O-RAN Alliance specifications for multi-vendor interoperability. The architecture supports diverse deployments from mmWave small cells to massive MIMO macrocells.

Category: mmWave & 5G

Architecture: CU / DU / RU split

Interfaces: NG, Xn, F1, E1

Understanding 5G RAN

In LTE, the eNB was a monolithic unit handling all protocol layers from RRC down to PHY. In 5G NR, 3GPP disaggregated the gNB into CU, DU, and RU to enable flexible deployment. The CU (upper layers: RRC, PDCP) can be centralized in an edge data center serving dozens of cells. The DU (lower layers: RLC, MAC, upper PHY) sits near the cell site. The RU (lower PHY, RF) is on the tower or rooftop.

This split enables centralized scheduling, coordinated multi-point (CoMP), and pooling gains. The O-RAN Alliance takes disaggregation further by defining open interfaces between CU, DU, and RU from different vendors, plus a RAN Intelligent Controller (RIC) for AI/ML-driven optimization.

5G RAN Architecture

gNB functional split:
CU (Central Unit): RRC, PDCP, SDAP
DU (Distributed Unit): RLC, MAC, PHY-high
RU (Radio Unit): PHY-low (FFT, beamforming), RF

Interfaces:
NG: gNB ↔ 5GC (N2 control, N3 user plane)
Xn: gNB ↔ gNB (mobility, DC)
F1: CU ↔ DU (F1-C control, F1-U user)
E1: CU-CP ↔ CU-UP
Fronthaul: DU ↔ RU (eCPRI, O-RAN 7.2x)

CU can serve 10-100+ DUs. DU can serve 1-3 RUs.

5G RAN Transport Requirements

Segment	Interface	Latency	Bandwidth	Medium
Fronthaul	eCPRI (DU-RU)	<100 μs	25-100 Gbps	Fiber (dark)
Midhaul	F1 (CU-DU)	1-10 ms	1-10 Gbps	Fiber/microwave
Backhaul	NG (CU-5GC)	5-20 ms	1-10 Gbps	IP/MPLS

Common Questions

Frequently Asked Questions

What is the CU/DU split?

CU handles upper layers (RRC, PDCP), centralized in edge cloud. DU handles lower layers (RLC, MAC, PHY-high) near the site. RU handles RF and lower PHY on the tower. Connected via F1 (midhaul) and eCPRI (fronthaul). Enables centralized RRM and pooling gains.

What is O-RAN?

Open RAN Alliance defining open, multi-vendor interfaces for CU, DU, RU. Includes RAN Intelligent Controller (RIC) for AI/ML optimization. Deployed by Rakuten, DISH, others. Aims to break vendor lock-in while maintaining performance.

What transport is needed?

Fronthaul: <100 μs, 25-100 Gbps (IQ data, fiber required). Midhaul: 1-10 ms, 1-10 Gbps. Backhaul: 5-20 ms, 1-10 Gbps. Fronthaul bandwidth is the biggest challenge for massive MIMO.

Related Terms

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