5G System Architecture Overview

Before building a private 5G network, let's understand the overall architecture and basic concepts of 5G systems. This knowledge will help you understand the role of each component and the purpose of our subsequent configurations.

5G Network Overall Architecture

A 5G network consists of three main parts:

┌─────────────────────────────────────────────────────────────────────────┐
│                        5G Network Architecture                          │
├─────────────────────────────────────────────────────────────────────────┤
│                                                                         │
│   ┌─────────┐        ┌──────────────────┐        ┌─────────────────┐   │
│   │   UE    │◄──────►│   RAN (gNB)      │◄──────►│   5G Core (5GC) │   │
│   │Terminal │ Radio  │ Radio Access Net │Backhaul│   Core Network  │   │
│   └─────────┘        └──────────────────┘        └─────────────────┘   │
│                                                           │             │
│                                                           ▼             │
│                                                    ┌─────────────┐     │
│                                                    │   Internet  │     │
│                                                    │   / DN      │     │
│                                                    └─────────────┘     │
└─────────────────────────────────────────────────────────────────────────┘

Component	Full Name	Description
UE	User Equipment	End-user devices such as phones, CPE, IoT devices
RAN	Radio Access Network	Handles wireless signal transmission and reception
gNB	gNodeB	5G base station, the core device of RAN
5GC	5G Core	5G core network for authentication, session management, etc.
DN	Data Network	Usually refers to the Internet or enterprise networks

Radio Access Network (RAN) Architecture

Traditional vs Modern Base Stations

Traditional 4G base stations (eNodeB) are integrated devices, while 5G base stations (gNB) adopt a more flexible disaggregated architecture.

gNB Functional Decomposition

A 5G base station can be decomposed into the following logical units:

┌─────────────────────────────────────────────────────────────────┐
│                     gNB Functional Split                        │
├─────────────────────────────────────────────────────────────────┤
│                                                                 │
│   ┌───────────┐     ┌───────────┐     ┌───────────┐            │
│   │    RU     │◄───►│    DU     │◄───►│    CU     │            │
│   │Radio Unit │     │Distributed│     │Centralized│            │
│   └───────────┘     │   Unit    │     │   Unit    │            │
│        │            └───────────┘     └───────────┘            │
│        ▼                 │                  │                   │
│   ┌─────────────────────────────────────────────────────────┐  │
│   │  • RF transmission     • Physical layer   • RRC protocol│  │
│   │  • ADC/DAC conversion  • MAC layer        • PDCP layer  │  │
│   │  • Power amp/filter    • Real-time sched  • UP/CP       │  │
│   └─────────────────────────────────────────────────────────┘  │
│                                                                 │
└─────────────────────────────────────────────────────────────────┘

Unit	Full Name	Function
RU	Radio Unit	Also called RRU/AAU, handles RF signal Tx/Rx, includes antennas, power amplifiers, filters
DU	Distributed Unit	Processes time-critical physical layer and MAC layer functions
CU	Centralized Unit	Handles non-real-time higher layer protocols, can be split into CU-CP (Control Plane) and CU-UP (User Plane)

Why Disaggregate?

Functional separation allows:

• Flexible deployment: CU can be centralized while DU/RU are distributed
• Resource sharing: Multiple DUs can share one CU
• Cost optimization: Choose different deployment options based on needs

Common Deployment Scenarios

Option 1: Traditional Integrated Deployment
┌─────────────────────────────────┐
│  RU + DU + CU (Integrated gNB)  │
└─────────────────────────────────┘

Option 2: RU Separation (Common for DAS)
┌──────┐      ┌──────────────┐
│  RU  │◄────►│   DU + CU    │
└──────┘      └──────────────┘

Option 3: Full Disaggregation (Large Networks)
┌──────┐      ┌──────┐      ┌──────┐
│  RU  │◄────►│  DU  │◄────►│  CU  │
└──────┘      └──────┘      └──────┘

5G Core (5GC) Architecture

The 5G core network adopts a Service-Based Architecture (SBA), where each network function provides services as microservices.

┌─────────────────────────────────────────────────────────────────────────┐
│                        5G Core Network Architecture                     │
├─────────────────────────────────────────────────────────────────────────┤
│                                                                         │
│   ┌──────┐  ┌──────┐  ┌──────┐  ┌──────┐  ┌──────┐  ┌──────┐          │
│   │ NSSF │  │ AUSF │  │ UDM  │  │ PCF  │  │ NEF  │  │ NRF  │          │
│   └──┬───┘  └──┬───┘  └──┬───┘  └──┬───┘  └──┬───┘  └──┬───┘          │
│      │         │         │         │         │         │              │
│   ───┴─────────┴─────────┴─────────┴─────────┴─────────┴───────────   │
│                        Service Bus (SBI)                               │
│   ───┬─────────────────────────────────────────────────────┬───────   │
│      │                                                     │          │
│   ┌──┴───┐                                             ┌───┴──┐       │
│   │ AMF  │◄───────────────────────────────────────────►│ SMF  │       │
│   └──┬───┘                                             └───┬──┘       │
│      │  N2                                                 │ N4       │
│      ▼                                                     ▼          │
│   ┌──────┐                                             ┌──────┐       │
│   │ gNB  │◄───────────────────N3──────────────────────►│ UPF  │──►DN  │
│   └──────┘                                             └──────┘       │
│                                                                         │
└─────────────────────────────────────────────────────────────────────────┘

Core Network Functions

NF	Full Name	Function
AMF	Access and Mobility Management Function	Access and mobility management, handles UE registration, authentication, mobility
SMF	Session Management Function	Session management, handles PDU session establishment, modification, release
UPF	User Plane Function	User plane function, responsible for packet forwarding, QoS handling
AUSF	Authentication Server Function	Authentication service, handles UE security authentication
UDM	Unified Data Management	Unified data management, stores user subscription data
PCF	Policy Control Function	Policy control, defines QoS and charging policies
NRF	Network Repository Function	Network repository function, service discovery and registration
NSSF	Network Slice Selection Function	Network slice selection
NEF	Network Exposure Function	Network capability exposure

Open5GS Implementation

In our lab, Open5GS implements most of the core network functions and can be used as a complete 5G SA core network.

5G Base Station Physical Forms

In the real world, you may have seen various forms of base station equipment. Understanding them helps you understand how 5G networks are deployed.

Macro Cell

Macro cells are the largest coverage base station type, usually installed on towers or building rooftops.

5G Macro Cell Tower - A typical 5G base station in Malta (Image: Wikimedia Commons, CC BY-SA 4.0)

Characteristics:

• Coverage radius: 1-10 km
• Transmit power: 20-40W (per antenna)
• Application: Outdoor wide-area coverage

Deutsche Telekom 5G AAU antenna close-up - Note the square Active Antenna Unit (Image: Wikimedia Commons, CC BY-SA 4.0)

Visual identification:

           ▲ Antenna
          /│\
         / │ \      ← Large antenna array (AAU)
        /  │  \
       /   │   \
      ─────┴─────
           │
           │        ← Feeder/Fiber
           │
      ┌────┴────┐
      │  BBU    │   ← Baseband Unit (usually in cabinet)
      └─────────┘

Common antenna forms:

• Traditional antenna + RRU: Separate antenna and radio unit
• AAU (Active Antenna Unit): Integrated antenna and radio unit, supports Massive MIMO

Small Cells

Small cells supplement macro cell coverage in hard-to-reach areas or add capacity in hotspot areas.

5G Small Cell antennas installed on a rooftop - Pittsburgh, USA (Image: Wikimedia Commons, CC BY 2.0)

Micro Cell

• Coverage radius: 200m - 2km
• Common in streets, commercial areas

Pico Cell

• Coverage radius: 100-200m
• Common in large indoor venues

Femto Cell

• Coverage radius: 10-50m
• For homes or small offices

Distributed Antenna System (DAS)

Large buildings often use indoor distributed antenna systems:

Distributed Antenna System (DAS) architecture diagram - Multiple antenna nodes connected to a common source (Image: Wikimedia Commons, CC BY-SA 4.0)

                    ┌─────────┐
                    │ Source  │  ← Macro signal or small cell
                    └────┬────┘
                         │
                    ┌────┴────┐
                    │ Master  │
                    │  Unit   │
                    └────┬────┘
                         │
         ┌───────────────┼───────────────┐
         │               │               │
    ┌────┴────┐    ┌────┴────┐    ┌────┴────┐
    │ Remote  │    │ Remote  │    │ Remote  │
    │  Unit   │    │  Unit   │    │  Unit   │
    └────┬────┘    └────┬────┘    └────┬────┘
         │               │               │
        ⊕               ⊕               ⊕
     Antenna         Antenna         Antenna

Indoor DAS antenna mounted on ceiling - Samsung KX, London (Image: Wikimedia Commons, CC BY 2.0)

How to Identify 5G Base Stations

Visual characteristics:

1. Antenna size: 5G antennas are usually larger than 4G due to Massive MIMO support
2. AAU form factor: 5G AAUs are typically square or rectangular integrated devices
3. Quantity: A single site may have multiple AAUs covering different sectors

Typical 5G AAU Size Comparison:

Type	Typical Size	Features
4G Antenna	Smaller/lighter	Usually passive antenna
Sub-6G 5G AAU	~50×50×15 cm	Supports 64T64R MIMO
mmWave 5G AAU	Smaller	Highly integrated, beamforming

5G Frequency Bands

Global 5G Spectrum Allocation

5G uses two main frequency ranges:

Frequency Range	Spectrum	Characteristics
FR1 (Sub-6GHz)	410 MHz - 7125 MHz	Good coverage, strong penetration, currently mainstream
FR2 (mmWave)	24.25 GHz - 52.6 GHz	Large bandwidth, high speed, limited coverage

China 5G Spectrum Allocation

The spectrum allocation for China's three major operators:

Operator	Band	Frequency Range	Bandwidth
China Mobile	n41	2515-2675 MHz	160 MHz
China Mobile	n79	4800-4900 MHz	100 MHz
China Telecom	n78	3400-3500 MHz	100 MHz
China Unicom	n78	3500-3600 MHz	100 MHz
China Broadnet	n28	703-733 / 758-788 MHz	30+30 MHz
China Broadnet	n79	4900-4960 MHz	60 MHz

Spectrum Usage Notice

These bands are allocated to operators by national radio management authorities. When conducting private 5G experiments, you must:

1. Use a shielded box or extremely low power to avoid interfering with public networks
2. Or apply for experimental frequency license
3. Understand and comply with local radio regulations

Frequency Bands for Our Lab

In this lab environment, we typically use the following configuration:

Parameter	Recommended Value	Description
Band	n78	3.5 GHz band, widely supported by devices
Center Frequency	3750 MHz	Can be adjusted based on actual conditions
Bandwidth	20 MHz	Recommended for beginners, reduces hardware requirements

5G Network Types

SA vs NSA

There are two main 5G deployment modes:

NSA (Non-Standalone)

┌──────┐        ┌─────────┐
│  UE  │◄──────►│ 4G eNB  │◄──────►┌─────────┐
└──────┘        └─────────┘        │ 4G EPC  │
    ▲                              └─────────┘
    │           ┌─────────┐             ▲
    └──────────►│ 5G gNB  │◄────────────┘
                └─────────┘

• Uses 4G core network (EPC)
• 4G handles control plane, 5G assists user plane
• Transitional solution, commonly used in early deployments

SA (Standalone)

┌──────┐        ┌─────────┐        ┌─────────┐
│  UE  │◄──────►│ 5G gNB  │◄──────►│ 5GC     │
└──────┘        └─────────┘        └─────────┘

• Uses 5G core network (5GC)
• Completely independent 5G network
• Supports full 5G features, the ultimate goal

Our Lab Environment

This project builds a 5G SA network, including a complete 5G core network (Open5GS) and 5G base station (srsRAN gNB).

Public vs Private Networks

Type	Characteristics	Use Cases
Public	Operator-deployed, public-facing	Personal communication, public Internet
Private	Enterprise/organization-built	Factories, campuses, ports
Hybrid	Public-private coordination	Scenarios requiring high security and reliability

Mapping to Our Lab

After understanding the above concepts, let's see how they map to our lab environment:

Real Component	Lab Equivalent	Description
5G UE (Phone)	5G-capable phone / srsUE	Commercial phones need 5G SA mode enabled
RU/AAU	SDR device (USRP B210)	Handles RF signal Tx/Rx
DU + CU	srsRAN gNB software	Software base station running on PC
5GC	Open5GS	Core network running on PC/server

┌─────────────────── Our Lab Environment ──────────────────┐
│                                                          │
│   ┌───────┐     ┌──────────────────────────────┐        │
│   │ Phone │◄───►│ USRP B210 ──► srsRAN gNB     │        │
│   │ srsUE │     │   (RU)         (DU+CU)       │        │
│   └───────┘     └────────────────┬─────────────┘        │
│                                  │                       │
│                                  ▼                       │
│                          ┌─────────────┐                │
│                          │  Open5GS    │                │
│                          │   (5GC)     │                │
│                          └──────┬──────┘                │
│                                 │                        │
│                                 ▼                        │
│                          ┌─────────────┐                │
│                          │  Internet   │                │
│                          └─────────────┘                │
│                                                          │
└──────────────────────────────────────────────────────────┘

Next Steps

Now that you understand the basic architecture of 5G systems, you can start building:

• Prerequisites - Install necessary software and drivers
• Open5GS Core Network - Deploy the 5G core network
• srsRAN Base Station - Configure the 5G gNB

Further Reading

If you want to dive deeper into 5G technical details, you can refer to:

• 3GPP TS 23.501 - 5G System Architecture
• 3GPP TS 38.300 - NR Overall Description
• O-RAN Alliance Specifications - Open RAN standards