Private networks, whether operated by users or by wireless carriers, requires radios, addressing, timing, and automation to make them run.

3GPP Release 16 introduced enhancements to 5G new radio (NR) and 5G core for private networks. With these additions to 5G, enterprises can now use private 5G networks. You may wonder what’s special about private 5G when enterprises already use Wi-Fi-based LANs and LTE (4G) on specific bands such as b48 (CBRS band). Here’s how 3GPP-based private 5G works and how it provides alternative network connectivity.

A private wireless network is a wireless network operated for the sole purpose of connecting devices that belong to a private entity such as an enterprise. Private entities or service providers can operate private networks. A private entity can get connectivity as a service from the service provider. From 3GPP TS 22.261, the following are the definitions of private 5G networks:

Private communication: A communication between two or more UEs belonging to a restricted set of UEs.
Private network: An isolated network deployment that does not interact with a public network.
Private slice: A dedicated network slice deployment for the sole use by a specific third-party.

Not all use cases require a 5G NR based network. Use cases include:

• Connectivity of devices (laptops, PDAs, mobile phones) within an enterprise for communication and for use of general-purpose software applications.
• AR/VR applications in enterprise.
• Connectivity of sensors, robots, actuators and the applications controlling them in a factory.
• Connectivity of equipment and their controlling applications in a particular industry vertical (shipping, mining, logistics).
• Public safety networks.

3GPP wireless technologies and use cases
Table 1 shows how different 3GPP wireless technologies fit different use cases. 3GPP defines 5G NR, NB-IoT, LTE and LTE-M for private wireless, covering a range of use cases from high-speed broadband to low latency and high device densities.

Use case	Applicable 3GPP wireless technologies	Unique offerings
Enterprise communication and general-purpose application usage (multimedia, email, document sharing, messaging, online meetings).	Private LTE (Band 48/CBRS band)	Lightly licensed spectrum, SIM-based security, QoS-based scheduler.
AR/VR application	5G NR	Works on licensed, lightly licensed as well as unlicensed spectrum, SIM-based security, tailored QoS characteristics (5QI) for AR/VR applications, low latency scheduling.
Factory equipment, sensors, robots, actuators, precision equipment and their control	5G NR	Works on licensed, lightly licensed as well as unlicensed spectrum, SIM-based security, low-latency scheduling (URLLC), deterministic QoS and latency.
Low-power, battery-operated IoT devices	NB-IoT, LTE-M with EPC core or 5G core	Licensed spectrum, low power modes, extended coverage and SIM-based security.
Audio/video production, 4K video	NR (with 5G Core)	Licensed spectrum, low power modes, extended coverage and SIM-based security.
Public safety	LTE or NR	Works on licensed, lightly licensed as well as unlicensed spectrum, SIM-based security, end-to-end system for MCPTT, MCVideo and public safety / mission critical applications.

Table 1. Use cases and applicable wireless technologies.

Traditional cellular network technologies used a concept called Public Land Mobile Number (PLMN) ID, which they broadcast over the air. Each device had a SIM card with its home network (home PLMN) credentials burnt in. The PLMN ID of a network consisted of mobile country code (MCC) and mobile network code (MNC). When a device latches on to a cell, it goes through a cell selection and PLMN selection process whereby it prefers to latch on to the same PLMN as its SIM card (if the cell broadcasts the same or equivalent PLMN.

Cellular operators had to register and get their PLMN ID (MNC) from a national assignment authority. PLMN ID was required because private networks deploy without dependence on getting a number from a national assignment body.

3GPP introduced Standalone Non-Public Network (SNPN) ID that private 5G networks can broadcast to users. The devices that belong to such a private network can latch on to the cells broadcasting this identifier. Thus, a private 5G network can operate standalone without relying on any service provider or cellular operator’s network. Figure 1 shows the structure of SNPN ID.

Here, and NID Private Enterprise Number (PEN) is the same as the IANA-assigned enterprise ID typically used in IP communications.

Public Network Integrated non-public networks (NPNs) are NPNs made available through cellular operator PLMNs by one of the following means:

• Providing a dedicated data network (DNN) behind the 3GPP user plane gateways for the enterprise devices. In this method, the user plane forwarding (UPF), the IP data networks behind the UPF and the Session Management Function (SMF) controlling the UPF can be assigned separately for the private network DNN. The networking backbone (IP routes, shown as black lines) may, however, be shared with the public network provider and other private networks.
• Providing a dedicated network slice within the 3GPP network for the enterprise devices. In this method (Figure 3), the public network provider can assign dedicated instances of gNB-CU-UP, UPF, SMF and IP data networks for the private network. These dedicated instances (shown as red and green lines) may use a dedicated and isolated IP route.

 

Network slicing
Network slicing in 5G lets a user equipment (UE) device connect through a cellular network that provides traffic isolation to the application servers, except for shared radio layers. This capability came in 3GPP Release 15. The following are some of the key network slicing capabilities in 5G.

• Assigning a network-slice identifier and providing rules for the UE to select the right network-slice identifier.
• Assigning dedicated instances of user-plane functions and session-management functions for the slice, allowing for isolation of traffic within the network.
• Mapping of slices to transport layer technologies (MPLS, SRv6, L2VPN, L3VPN, and so on).

In a PNI-NPN, network slicing provides private access but does not prevent UEs from trying to access the network in areas where the UE lacks permission to use the network slice allocated for the NPN. 3GPP introduced closed access groups (CAGs) in 3GPP Release 16, whereby public networks offer private network connectivity and broadcast the CAG ID. Only devices that have access credentials for that CAG ID can latch on to such cells, thus providing access restriction.

Private LAN and time-sensitive communications
Private LAN networks over a 5G wireless network became available with 3GPP Release-16. An Ethernet LAN network can be created behind the UPF and UEs may join those LAN networks through the 5G NR radio network. This is useful for low latency Ethernet-based LAN applications such as connecting factory machines and robots. Figure 4 shows how networks can use time-sensitive networking (TSN) where a grand master clock distributes timing and synchronization from the network to the UEs.

NR in unlicensed bands
3GPP has supported LTE in unlicensed spectrum since Release 13. LTE supported unlicensed access only as a supplementary access while the anchor carrier always remained as a licensed carrier. Hence it was called “license-assisted access.” From Release 16, 5G NR supports the use of unlicensed spectrum both in assisted mode as well as standalone mode. This allows deployment of NR radio in standalone mode (NR-U) in unlicensed bands. NR-U is supported in unlicensed bands up to 5 GHz (e.g., 5150 to 5925 MHz) as well as in the 6 GHz band (5925 to 7125 MHz). At frequencies below 5GHz, fair co-existence with existing unlicensed technologies such as Wi-Fi and LTE LAA are necessary. Bands above 6 GHz are a greenfield without co-existence issues. Regulatory aspects for this band are not yet fully established in many countries. These bands also support energy detection-based channel access.

Local area data networking
5G lets UEs run over specific data networks called local area data networks (LADN) at specific location areas. The 5G core network can advertise the availability of a particular data network to the UE when it moves into a specific location area. This feature allows deploying private networks that are purpose built for specific location-centric applications and advertise to UEs wherever they are available.

Low-latency scheduling
5G NR provides flexible frame structure and numerologies (denoted by μ) at the physical layer to enable frame scheduling with variable latencies based on frequency range. This allows for different scheduling intervals (Transmit Time Interval – TTI) ranging from 120 μsec for 120 kHz sub-carrier spacing numerology to 1 msec for 15 kHz sub-carrier spacing. Table 2 provides the TTIs available for different numerologies. In addition, 5G NR supports mini-slot scheduling, whereby scheduling takes place at sub-slot level. It also supports configured grants, flexible physical downlink control channel (PDCCH) to physical downlink shared channel (PDSCH) gaps and flexible PDCCH to PUSCH gaps, providing complete control on the uplink and downlink packet transfer latencies. These features are useful for private 5G use cases such as industrial IoT and robotics that require precision and low latency.

Numerology/Sub-carrier spacing	TTI	Applicable Frequency Range
μ=0, 15 kHz sub-carrier spacing	1 msec	FR1 (< 7.125 GHz)
μ=1, 30 kHz sub-carrier spacing	500 μsec	FR1 (< 7.125 GHz)
μ=2, 60 kHz sub-carrier spacing	250 μsec	FR1 (< 7.125 GHz) and FR2 (> 24.25 GHz)
μ=3, 120 kHz sub-carrier spacing	125 μsec	FR2 (> 24.25 GHz)
μ=4, 240 kHz sub-carrier spacing	62.5 μsec	FR2 (> 24.25 GHz) (not currently used)

Table 2. Transmit time intervals for subcarrier spacing.

Link reliability features
Link reliability features (Table 3) that began with 3GPP Release 15 provide the robustness and reliability required for ultra-reliable low latency communications (URLLC). These link reliability features are available currently up to 3GPP Release 17.

Feature	Available since
PDCP duplication through SCells and SCG cells	Release 15 and enhanced in Release 16 to support up to 4 logical channels
Conditional handovers	Release 16
Dual active protocol stack	Release 16
T312 based fast failure recovery for PCell	Release 16
Multi TRP MIMO	Release 16
Fast recovery of MCG link when the SCG link is still operational	Release 16
PDCCH enhancements – configurable field sizes for DCI for improved reliability	Release 16

Table 3. Link reliability features found in 3GPP Releases 15 and 16.

Spectrum
Different regions of the world are looking at using different bands for 5G NR in private networks. Table 4 provides a high-level view of the spectrum scenario.

In Table 4, some parts of the world are assigning dedicated licensed spectrum for the sole use of private 5G. In these bands, 3GPP NR-based private 5G network deployments are a natural choice. The n96 band co-exists with the 6 GHz band used by Wi-Fi 6E.

Region	Band/spectrum
USA	n48 (3550 MHz – 3700 MHz) – CBRS n96 (5925 MHz – 7125 MHz) – unlicensed band
Germany	n77/n78 (3700 MHz – 3800 MHz) – licensed for private use
France	n38 (2570 MHz – 2620 MHz) – licensed for private use
UK	n77 (3800 MHz – 4200 MHz) – licensed for private use
European Union	n96 (5900 MHz – 6400 MHz) – unlicensed band
Japan and China	n79 (4400 MHz – 5000 MHz) – licensed for private use
South Korea	n96 (5925 MHz – 7125 MHz) – unlicensed band

Table 4. Spectrum allocations for several countries and regions.

Private 5G end-to-end architecture
Figures 5 and 6 depict the end-to-end architecture options for deploying private 5G networks. Figure 5 shows a private network that broadcasts a SNPN NID and the RAN baseband and 5G core network functions are hosted on premises.

Figure 6 shows a deployment where a public network provides services for private networks (PNI-NPN) by broadcasting CAG and provides isolated network functions for each private network by means of network slices and dedicated DNN.

Automated network deployment
For private enterprises the deployment of wireless networks should be a single-touch operation. After doing radio planning, the only human intervention should be to physically deploy the radio units. Once the radio unit is powered, its configuration and connectivity to the baseband and onboarding into the network should be fully automated. Open RAN- based open radio units (O-RU) support automatic DHCP-based discovery of the network management system’s (called service and management orchestrator – SMO) address and subsequent NETCONF configuration protocols to call the SMO. The SMO can then push the configuration of the gNB-DU’s address to the O-RU. The O-RU then connects to the gNB-DU, which can then push the carrier configuration to the O-RU. If the network functions as shown in Figures 5 and 6 are deployed as cloud native functions, deployment can be automated by a cloud infrastructure manager such as Kubernetes.

Conclusion
Private 5G using NR as radio technology and 3GPP defined 5G core finds its niche in many use cases. The flexible design of physical-layer features such as multiple numerologies, flexible slot formats, mini-slot scheduling and flexible uplink/downlink configuration provides a rich set of features that you can use in many applications.