Mission-Critical Applications over CBRS and Unlicensed Spectrum

The 3rd Generation Partnership (3GPP) started to standardize Mission-Critical Applications (MCA) in 2015 with the introduction of Priority and Preemption over LTE in Release 12. This standardization process enabled mobile broadband technologies such as 4G and 5G to have built-in critical communications mechanisms that can offer a more robust and reliable platform to end users—i.e., oil and gas companies, first responders (FirstNet)—as well as a mature and fast-growing ecosystem.

In addition, 4G and 5G allowed the introduction of new forms of information such as video-to-video communications, real-time video surveillance, and group voice calls, among others. To this end, the 3GPP standardized Critical Push to Talk (MCPTT) in R13 and Mission-Critical Data (MCDATA) and Mission-Critical Video (MCVIDEO) in R14, with additional improvements in R15. R16, frozen in July 2020, introduced improved features such as enhanced Ultra-Reliable Low Latency (eURLLC), the Future Railway Mobile Communication System (FRMCS), Time-Sensitive Networking (TSN), and the inclusion of 5G NR in Unlicensed spectrum (5G NR-U) for the use of the Industrial Internet of Things (IIoT) in order to support a wide range of verticals such as smart retail, connected healthcare, and smart automation. However, commercial implementations for most of these features are still in development and have yet to become available for the market.    

Furthermore, the performance and reliability of MCA depends not only on mobile broadband technology, but also on the type of spectrum in which the system is deployed. Today, first responders are making use of licensed spectrum for their operations. For instance, FirstNet in the United States is deploying an LTE network operating in the licensed Band 14 (700 MHz) and the Emergency Service Network (ESN) in the United Kingdom is running over a commercial LTE network with priority access. In addition, other verticals, such as transportation, have adopted licensed spectrum to support critical operations. For example, the railway service in the United Kingdom deployed the Global System for Mobile Communications-Railway (GSM-R) in the licensed 900 MHz band to dispatch and control trains, and China has refarmed the spectrum to deploy LTE for Railways (LTE-R) and also support legacy GSM-R systems. Thus, many MCA are now operating in mobile broadband technologies such as 4G and in licensed spectrum, which support the requirements of these types of applications such as reliability, mobility, low latency, high bandwidth, and Quality of Service (QoS). There are, however, additional spectrum alternatives that may be an option for MCA such as unlicensed (5G NR-U), shared (OnGO) and locally assigned.

OnGo is a three-tier shared spectrum initiative unique to the United States that allows access to a 150 MHz wide broadcast channel in the Citizens Broadband Radio Service (CBRS) band from 3550 MHz to 3700 MHz. The three spectrum tiers are known as:

• Tier 1 Incumbents, which have priority access to the 150 MHz of spectrum in the CBRS band. The incumbents are the Federal Government, Fixed Satellite Service (FSS), and Grandfathered Wireless Protection Zones (GWPZ).
• Tier 2 Priority Access License (PAL) users have access to 100 MHz of the CBRS band, from 3550 MHz to 3650 MHz. The licenses are to be bought at auction, which will provide defined interference protection from other PALs and all General Authorized Access (GAA) users.
• Tier 3 GAA have access to the 150 MHz of spectrum in the CBRS band if available. This lightly licensed tier is considered a best effort since there is no priority given in order to access the spectrum.

The Spectrum Access System (SAS) controls access to the spectrum and gives priority to Incumbents and PAL users over GAA users. The SAS also receives information about incumbent operation from the Environmental Sensing Capability (ESC), and if any Citizens Broadband Radio Service Devices (CBSD) are transmitting in the requested incumbent or PAL channel the SAS must confirm either suspension of the CBSD operation or its relocation to another unoccupied frequency. The Incumbents’ operation is expected in offshore regions and near coastal areas, meaning that the CBRS band has an operational limitation in terms of deployment geography. Thus, it is not advisable to make use of the CBRS band near the operation of the incumbents for MCA, since it is likely that the system will suffer from interference and perhaps service interruption due to inaccessibility to the spectrum.

Now, if MCA are deployed in other areas of the country with no expected incumbent operation, two different scenarios can be considered when a PAL user wants to make use of the channel:

1. If MCA are operating over GAA, the SAS will give priority access to the PAL user and will try to relocate the GAA user to any unoccupied channel. However, in highly densified areas, there might be coverage overlaps increasing the noise floor of the systems as well as interference among GAA users. 
2. If MCA are operating over a PAL license, the SAS will provide an unoccupied channel to the new PAL user. PAL users are less likely to suffer from interference issues.

In addition, 5G NR-U standardized in 3GPP Release 16 promotes the use of unlicensed spectrum for the IIOT where Synchronized Sharing and Coordinated Multi-Point (CoMP) mechanisms aim to control access to the channel, reduce resource collisions, manage interference between Access Points (AP), and offer a more predictable QoS over the network. However, due to the free nature of the unlicensed spectrum, CoMP mechanisms result in inefficiencies in the control of random interference introduced by external AP not preconfigured in the X2 interface of the network’s AP. Furthermore, CoMP mechanism are not yet commercialized and could increase the cost of the equipment. These synchronization mechanisms will also change the nature of the unlicensed use of the spectrum, which has been predominantly accessed asynchronously or in an ad-hoc fashion. Thus, it is not likely that MCA will operate over unlicensed spectrum since interference issues and congestion in the channel can negatively impact the performance of the system, which can lead to a loss of business or even lives.

ABI Research believes that mission-critical communications should be deployed in licensed or locallly assigned spectrum. This spectrum choice offers system reliability and QoS to MCA as it is interference and incumbent free since the license holder is the only one authorized to use the spectrum, as opposed to the other initiatives (such as lightly shared and unlicensed spectrum). Then, applications such as data transfer of monitoring systems to remote Supervisory Control and Data Acquisition (SCADA) for data management and processing will not be interrupted.

Regarding the deployment of MCA in the CBRS band, ABI Research expects that two characteristics need to be fulfilled. Firstly, a PAL license should be held in order to have priority over GAA users and secondly, the operation of the system needs to be far from the exclusion zones and incumbent’s operation, which is mainly offshore and near the coastal areas.

Finally, regardless of the adoptions of new techniques and mechanisms in unlicensed spectrum, the spectrum is vulnerable to interference and congestion even if deployed in greenfield and isolated areas. If 5G NR-U is to be deployed in IIoT, it is recommended to deploy it in dual connectivity with licensed LTE anchor band or carrier aggregation by leveraging the 5G NR Non-Standalone (NS) core. The table below maps different use cases that can be deployed in 5G NR and its different spectrum choices: