This article is a synopsis of the key points highlighted during the “CBRS: What you need to know” webinar with Alpha Wireless, Baicells and ISP Supplies.

In the absence of mid-band spectrum, US operators lack a critical tool to evolve 4G LTE with densification and to deploy 5G in suburban markets. This has become patently obvious across the industry as commercial deployments of low-band and mmWave spectrum accelerate.

The house of cards stands in a precarious state when a layered spectrum strategy does not encompass a combination of low, mid and high-bands. For operators, it all boils down to the harsh reality that there is no one-size-fits-all solution in the pursuit of enhanced coverage and capacity.

A conundrum of similar magnitude applies to enterprises seeking to create private cellular networks. The key difference is that enterprises, however large they may be, are newcomers to the cellular game. They demand cost-effective access to licensed spectrum, particularly mid and high-bands, within very specific geographic areas.

CBRS pioneers Mid-Band Spectrum Sharing

To address the above requirements and many more, considerable interest has been brewing in the Citizens Broadband Radio Service (CBRS) frequency band. This is a shared medium of mid-band spectrum (3550-3700 MHz) with three distinct tiers of sharing that have been developed by the CBRS Alliance and the Wireless Innovation Forum.

With the introduction of 150MHz of new spectrum for 4G LTE (3GPP Band 48) and eventually 5G NR, the Spectrum Access System (SAS) will be pivotal in the commercialisation of CBRS. Following a hierarchical structure, it coordinates access to the spectrum in real-time and enforces priorities and power levels.

For context, the dynamic nature of the SAS with CBRS is not wildly dissimilar to the concept of TV White Spaces and their databases however the SAS is more advanced. The tiers of sharing accommodate federal incumbents alongside non-federal wireless operators and private enterprises.

The top of the pyramid-like sharing framework is reserved for uses such as federal radiolocation, fixed satellite services (FSS) and the United States Armed Forces (ship-based radar). These incumbents in Tier 1 are protected from interference caused by activities in the lower tiers, thus safeguarding access for critical services.

Existing activities within Tier 1 (3650–3700 MHz) are subject to non-exclusive national (NN) grandfathering. The majority of these grandfathered licenses will expire in April of 2020 at the completion of a five-year transition period. When these license expirations occur, Non-Part 96 devices such as legacy WiMAX will be forced to cease operation.

In Tier 2 lies Priority Access Licenses (PAL). This tier enables non-federal use of the CBRS band, with county wide 10 MHz channels at the lower end which can be renewed after ten years. A competitive bidding process will be initiated to assign licenses in the tier (3550-3620 MHz), and Priority Access users will enjoy precedence over activities in Tier 3.

There is, however, a purchase limit of 70 MHz for Priority Access Licenses (PALs) in each county for Tier 2, and the limit is 40 MHz on a per-county basis for a single entity. Competition from a multitude of stakeholders, combined with a large number of counties, means that acquisition of Priority Access Licenses should be relatively cost-effective.

General Authorised Access (GAA) is the lowest tier in the spectrum access system. Unlike Tiers 1 and 2, spectrum in this tier is unlicensed (3550-3700 MHz but dedicated GAA between 3620-3700Mhz) and constitutes 50 MHz at upper range (with >30 MHz at lower range). Importantly, however, users of this portion of the CBRS band are required to register with the SAS.

The criticality of Tier 3 to the fundamental goal of CBRS – to enable flexible access to mid-band spectrum for a diverse array of applications – cannot be underestimated. General Authorised Access users can dynamically exploit any unused spectrum for free, including that held by Priority Access Licenses in Tier 2.

It is the Environmental Sensing Capability (ESC) feature of the CBRS architecture that enables the spectrum access system to dynamically enforce three-tier spectrum sharing. This functions with strategically placed sensors across predominantly coastal regions (near Naval stations) which detect incumbent activity in the spectrum.

When in operation, the ESC sensors create a feedback loop with the SAS and interact with CBRS Devices (CBSD) to facilitate maximum efficiency and minimum interference in the use of the shared spectrum. There are two categories of CBRS Devices, and each category features different size and power restrictions.

Category A CBSD conform to what the industry recognises as small cells. These lower power base stations (Maximum EIRP: 30dBm/10MHz) can be self-installed and located both indoors and outdoors (≤6 metres in height). In contrast, Category B CBSD are higher power (Maximum EIRP: 47dBm/10MHz), macrocell-like base stations that must be installed outdoors and by a Certified Professional Installer (CPI).

Potential Use Cases for CBRS: Diverse and Monetizable

Private cellular networks, in particular, will reap tremendous benefit from the ability to access spectrum in Tiers 2 and 3. Enterprises have been searching for a way to enhance on-campus and venue-specific connectivity beyond the limitations posed by WiFi and without the costs associated with Distributed Antenna Systems (DAS).

Wireless market trends in the US strongly hint to the likelihood of CBRS being exploited by cable operators and entrant MVNOs as a springboard for both wholesale cost reduction and network experience enhancement. The ability to rapidly deploy a high capacity wireless network in usage hotspots enables MVNOs to offload traffic away from the host operator, reducing long-term costs while also improving performance.

The emerging crop of neutral host operators stand to benefit from CBRS in a similar fashion. With access to CBRS and core network integration with mobile operators, they will be in a prime position to provide localised coverage and capacity across everything from event venues to hospitals, university campuses and even airports.

Of course, however, 4G LTE capacity enhancement and Fixed Wireless Access (FWA) will be the most immediate use cases for CBRS. In a market where unlimited data allowances have become the norm, there is an insatiable appetite amongst mobile operators for more licensed spectrum. The combination of favourable propagation characteristics and wide channel bandwidth renders CBRS alluring.

Conclusion: CBRS is just the beginning for Spectrum Sharing

CBRS is quickly becoming an “innovation band”, with a diverse ecosystem of use cases, chipsets and equipment poised to thrive on the fundamental benefits of spectrum sharing. The 150 MHz of spectrum up for grabs forges radically new opportunities for both long-standing operators and market entrants, all without adversely affecting incumbents in the band.

The recent intensification of interest in spectrum sharing across every layer of the telecoms industry and beyond is an indicator that spectrum policy is in the midst of a revolution.