# 5G Deployment Options

## Summary


Though 5G has been standardized, it has a number of options. Two network operators can deploy 5G in very different ways. This choice of option depends on the spectrum licensed to an operator, the geographic area they serve (terrain and user density), capabilities of the equipment they use, and business factors (cashflow and decision making). 

3GPP has defined options covering both 4G and 5G technologies with respect to Radio Access Network (RAN) and Core Network (CN). These options can guide operators as they migrate from current 4G deployments to 5G deployments.

It's generally expected that operators would first deploy 5G NR, let 4G RAN and 5G NR coexist, and finally deploy 5G Core. This implies that 4G+5G handsets would come out first and they would connect to both 4G eNB and 5G gNB.

## Discussion

### What are the broad challenges in deploying a 5G network?

Ideally, an operator acquires 5G licenses, invests in 5G equipment for both RAN and CN, and deploys the network for full coverage. The operator then asks subscribers to switch to 5G. After a short transition period, the old 4G network is retired.

In reality, subscribers may be slow to migrate since they have to invest in 5G-capable handsets. The operator's 5G subscription plans may be costlier. The 5G network may have poor coverage in many areas where it's been deployed in only the mmWave band. 

An incumbent operator has most likely invested heavily on 4G licenses and equipment. Their current 4G licenses may be in spectrum bands not supported by 5G. Perhaps the equipment they use can't easily be upgraded to 5G.

It's also possible that the government has delayed the auctioning of 5G spectrum. Operators don't want to wait. They may want to offer 5G services on 4G licensed spectrum. Even with 5G licenses, they would want 4G to coexist with 5G and steadily migrate to 5G as more 5G subscribers are added.


### Which are the main 5G deployment options?

In LTE, both RAN and CN had to use LTE standards. 5G gives more flexibility. For example, 4G RAN can be combined with 5G Core or 5G NR can be combined with 4G EPC. This gives rise to two broad deployment scenarios:  

    + **Standalone (SA)**: Uses only one radio access technology, either LTE radio or 5G NR. Both control and user planes go through the same RAN element. Deployment and network management is perhaps simpler for operators. Inter-RAT handover is needed for service continuity. Under SA, we have option 1 (EPC + 4G eNB), option 2 (5GC + 5G gNB), and option 5 (5GC + 4G ng-eNB).
    + **Non-Standalone (NSA)**: Multiple radio access technologies are combined. Control plane goes through what's called the master node whereas data plane is split across the master node and a secondary node. There's tight interworking between 4G RAN and 5G NR. Under NSA, we have option 3 (EPC + 4G eNB master + 5G en-gNB secondary), option 4 (5GC + 5G gNB master + 4G ng-eNB secondary), and option 7 (5GC + 4g ng-eNB master + 5g gNB secondary).

### What are the differences across options 3, 3a and 3x?

In all three options, control plane is between EPC and eNB via S1-C interface, and eNB and gNB via X2-C interface. There's no direct signalling traffic between EPC and gNB. The differences are in how user plane traffic is routed. Below we describe downlink but it applies to uplink as well. 

In option 3, user plane traffic is from EPC to eNB where **PDCP sublayer splits** the traffic so that some traffic is sent to gNB across the X2-U interface. In option 3a, EPC has a direct S1-U interface to gNB. In this option **EPC splits** the traffic. Option 3x is a hybrid: EPC splits the traffic for eNB and gNB, and/or gNB PDCP sublayer sends some traffic to eNB. For example, eMBB services use 5G NR whereas VoLTE uses LTE radio. 

gNB connected to EPC via S1-U is more specifically called **en-gNB**. It's part of E-UTRA-NR Dual Connectivity (EN-DC). The interface between en-gNB and eNB is also called **Xx interface**. 

Similar variations of routing user plane traffic involving 5G Core give rise to options 4a, 7a and 7x. 


### Could you compare possible migration paths from 4G to 5G?

Since options 1 and 3 use EPC, they can't support many 5G use cases. It's been noted that, 

> There's no real 5G without 5G Core.

However, option 3 enables faster time-to-market since core network can be upgraded later. Due to 5G NR, users can experience better throughput. However, with this increased traffic, EPC may become a bottleneck. Traffic flow is split at the EPC. 

From NSA option 3, the operator can migrate to NSA option 7 or SA option 5. With both these options, 5GC enables all 5G services. eNB and en-gNB are upgraded to ng-eNB and gNB respectively to connect to 5GC. Option 3 will continue to support UEs that can't talk to 5GC. 

It's also possible to add SA option 2 to complement NSA option 3. An alternative path is to deploy SA option 2 from the outset, thus immediately enabling all 5G use cases. However, operator has to acquire and deploy 5G equipment. Ideally, NR coverage is achieved on a wide area. Otherwise, frequent inter-RAT handover to SA option 1 or NSA option 3 may occur. 


### What scenarios can benefit from 5G deployment options 4, 5 and 7?

Options 4, 5 and 7 enable operators to continue using legacy 4G equipment while connecting to 5GC. With the higher bandwidths of options 4 and 7, all 5G use cases are possible.

Migration paths 3→5, 3→7, 3→4+2, 3→4, 7→4, 5→4, 1→4, 1→7, and 4→2 have been suggested. 

For some, **option 5** is not attractive. Legacy 4G UEs have to be replaced. eNB has to be upgraded substantially. Lots of interoperability testing are needed. If UE moves out of 5G NR coverage (option 2), traditional MBB/voice use cases can be supported simply with option 1 and inter-RAT handover. In the long term, improving option 2 coverage is a better path. **Option 7** depends on option 5 and inherits the same problems.  

**Option 4** is an extension of option 2. Using dual connectivity, LTE radio is added to 5G NR anchor. This improves coverage and bandwidth. However, it requires upgrade to eNB, gNB and UE, along with necessary interoperability testing. Instead, it would be better to focus on improving option 2 coverage. 


### Could you highlight the differences among eNB, gNB, ng-eNB and en-gNB?

3G's *NodeB (NB)* has become *evolved NodeB (eNB)* in 4G. In 5G, this has evolved to *next generation NodeB (gNB)*. ng-eNB and en-gNB are variations of eNB and gNB respectively, depending on CN. 

We compare the different RAN elements: 

    + **eNB**: A 4G network element. Connects to a 4G UE and EPC. This relates to options 1 and 3.
    + **gNB**: A newly introduced 5G network element. Connects to a 5G UE and 5G Core. This relates to options 2, 4 and 7.
    + **en-gNB**: Sits in a 4G RAN and connects a 5G UE to EPC. Both 4G and 5G radio resources are active using dual connectivity. eNB is the master node while en-gNB is the secondary node. "en" refers to E-UTRA New Radio. This relates to option 3.
    + **ng-eNB**: Connects to 5G Core but serves a 5G UE over 4G radio. "ng" refers to Next Generation. This relates to options 4, 5 and 7. There's dual connectivity in option 4 (gNB is master, ng-eNB is secondary) and option 7 (ng-eNB is master, gNB is secondary).

### How do 5G deployment options map to spectrum bands?

Current 4G deployments might be in sub-1GHz and 1-3GHz bands. 5G NR is then deployed in mmWave band and possibly in mid-bands 3.5-8GHz. This brings higher throughput/capacity/density and lower latency. At the same time, 4G RAN ensures good wide-area coverage and serves subscribers who haven't migrated to 5G. With Dual Connectivity (DC), high-band NR downlink can be combined with low-band 4G uplink. More throughput can be achieved via inter-band Carrier Aggregation (CA). This is **option 3**. 

When the operator activates 5G Core, **option 2** comes into play. Initially, this will be limited to 5G NR in mmWave band and mid-bands 3.5-8GHz for Fixed Wireless Access (FWA) and industrial deployments. At a later time, when 4G spectra are re-farmed, or via spectrum sharing, option 2 can be deployed to wider areas. When a UE moves out of option 2 5G NR coverage, it triggers intersystem handover to EPC, either in option 3 or 1. 

Even when 5G is widely deployed, for Massive Machine Type Communications (mMTC), NB-IoT and LTE-M will be used in **option 1**. 


### What's a possible migration path from LTE EPC to 5G Core?

As an example, we describe Samsung's offering. They claim that their LTE EPC is already virtualized and provides Control and User Plane Separation (CUPS). For 5G NSA, EPC software is upgraded for dual connectivity. For 5G SA, EPC *Network Elements (NEs)* become 5GC *Network Functions (NFs)*. Specifically, GW-C, GW-U, HSS and PCRP are upgraded to SMF, UPF, UDM and PCF respectively. New NFs AMF, NRF, NSSF, NEF and UDF are introduced. LTE's MME functionality goes into AMF, SMF and AUMF. The final deployment is a **common core** that covers LTE, 5G and Wi-Fi. 

5G Core is a Service-Based Architecture (SBA). NFs will be **virtualized** in the cloud and implemented using microservices and containers. LTE's stateful NEs that store UE state will be replaced with **stateless** NFs. Increasingly, cloud native architecture will be used with lightweight containers. There will be centralized orchestration of containers, network slicing, centralized operation, and centralized analytics. Overall, network control, monitoring and optimization will be **automated**. This will be the bigger impact of moving from LTE EPC to 5G Core. 

## Milestones

Dec  
2017

3GPP publishes Release 15 "early drop". This includes **NSA option 3**. Corrections to this option are made in June 2018. 

Apr  
2018

GSMA's report shares Korea Telecom's 4G to 5G migration plan. Early deployments are likely to be NSA option 3 with 5G NR only in 28GHz mmWave band. As 5GC gets introduced, multi-RAT interworking would become important. 5G NR coverage would improve with 3.5GHz band. NSA option 7 would be used, with eLTE being the anchor. Finally, EPC would be retired. 

Jun  
2018

3GPP publishes Release 15 "main drop". This includes **SA option 2** and **SA option 5**. 

Sep  
2018

A white paper by Nokia recommends either option 3 or option 2 for initial 5G rollout. The report also identifies **option 3X** in which high bandwidth traffic flows are routed to 5G gNB to avoid overloading 4G eNB. LTE user plane (SGW/PGW) would require performance improvements and a distributed architecture. 

Mar  
2019

3GPP publishes Release 15 "late drop". This includes **SA option 4** and **SA option 7**.