CBRS, SAS and Spectrum Sharing: The Complete Guide

Wireless spectrum is a scarce resource that has to be organized and distributed between federal and commercial use. 

When we talk about spectrum, we’re referring to the range of electromagnetic radio frequencies, from 3 kHz to 300 GHz, utilized to transmit wireless signals throughout the world. Because the spectrum is finite and can’t be produced, it must be reallocated. 

Government agencies such as the FCC (in the United States) set the rules and regulations for managing the wireless spectrum, which is a complex process that defines network transformations.

Traditionally, there were two main ways of accessing spectrum:

1. participate in an auction to buy spectrum from the regulator, which usually entailed dedicated access, free from interference.
2. use an unlicensed spectrum, which could mean dealing with unknown interferers and no protection or coordination.

But now, the Citizens Broadband Radio Service (CBRS) offers a third option: shared CBRS spectrum. 

With an increasing demand for spectrum as more users, devices, and apps require connectivity, and recognizing the challenges of scarce spectrum access, the FCC authorized the full commercial deployment of the CBRS in early 2020.

CBRS is a 150 MHz wide broadcast band located in the 3550-3700 MHz band. It’s a new approach to spectrum management introduced by the FCC. 

This band was exclusively used by the US Federal Government for Navy radar systems and aircraft communications. But since it was underutilized, and to address the shortage in high-quality spectrum, the FCC introduced 3.5 GHz CBRS band for wireless broadband commercial shared use. 

Licenses in CBRS are in three tiers:

• Tier 1: Incumbent protection – Reserved for fixed satellite stations or government agencies.
• Tier 2: Priority Access License (PAL) – Users in this tier must bid for these licenses to have priority access.
• Tier 3: General Availability Access (GAA) – GAA users do not have to bid for licenses but they are required to register.

The CBRS band makes possible a wide range of deployment options and innovative use cases, enabling new business models to efficiently deliver LTE and 5G services, including:

But operators cannot simply start using the spectrum. To regulate and manage access in this spectrum sharing model, the FCC has mandated rules that require all users to use a Spectrum Access System (SAS).

The Spectrum Access System (SAS) is a cloud-based service that manages the wireless communications of devices transmitting in the CBRS band, in order to prevent harmful interference to higher priority users.

It is an automated radio spectrum coordinator responsible for assigning spectrum channels and their associated transmit power to CBRS Service Devices (CBSDs). CBSDs need authorization from the SAS before they can start to transmit in the CBRS band. 

The SAS uses information provided by Environmental Sensing Capability (ESC) sensors to manage the spectrum and ensure that FCC rules are always enforced while giving maximum commercial access to available spectrum.

The Environmental Sensing Capability (ESC) is a network of sensors, deployed along the U.S.Coastline, used to detect use of CBRS.

ESC sensors are deployed along the U.S. coastline so that Naval radars operating in 3.5 GHz are protected from any interference by shared spectrum users. 

The ESC monitors incumbent radio activity in a particular geographic region. When an ESC sensor detects a federal transmission, it activates a protection zone and alerts the SAS of radar operation.

SAS reacts to ensure there is no interference between CBRS and radar operations by inhibiting other CBSDs from operating in that area and channel. Instead, it dynamically reallocates those Tier 2 and 3 users to other parts of the band to prevent interruption to incumbent access.

The SAS needs to dynamically manage spectrum for all three tiers – taking into account available bands at a given time in a given location – before granting access across the different classes of users.

The SAS maintains a database of all CBRS radio base stations including their tier status, geographical location, and other pertinent information. Along with real-time sensing, this information is used to automatically determine spectrum availability and coordinate frequency and transmit power assignments.

Devices that want to use the CBRS band put in requests to the SAS to reserve unused channels in a particular geographic area. If the channel is free, SAS will grant access, ensuring there’s no interruption to either incumbent or commercial services.

When CBSDs with grants to use channels are done with them, those channels are once again placed into the pool that the SAS can draw from to grant further requests.

The SAS has no way to proactively reach out to a CBSD, so the CBSD must initiate all communications.

A CBSD is served by a SAS either directly or through a Domain Proxy (DP). A DP is an aggregation point that interfaces with the SAS on behalf of all the devices behind it. 

There are six ways that a CBSD can communicate with a SAS according to the SAS-CBSD API: 

• Registration: Providing the SAS with essential details about the CBSD to register with a unique identifier and be able to access the spectrum.
• Spectrum inquiry: Finding out what spectrum is available in a given location and installation characteristics.
• Grant requests: Requesting a certain amount of spectrum to be reserved for the CBSD’s use.
• Heartbeat requests: Asking for authorization to transmit in the pre-approved reserved spectrum.
• Relinquish a grant: Leaving the granted access when the CBSD no longer wishes to use it.
• Deregistration: Removing the CBSD’s registration from the SAS, if the CBSD is decommissioned or moved.

You should choose CBRS-certified equipment when you use CBRS to ensure that your network infrastructure is high quality, interoperable and secure. BLiNQ Networks suite of OnGo certified products offers the perfect solution for any CBRS deployment. 

Because SAS is a cloud-hosted service, no extra infrastructure is needed. CBSDs installed in the operator’s network are programmed to connect securely to the SAS via the internet.

But it’s important to choose the right SAS Administrator with proven reliability and experience to guarantee the highest level of service in your network.

We’ve put together a free downloadable checklist that includes the first steps you need to take to implement CBRS in your network as well as things to consider when selecting a SAS administrator.

CBRS opens up a lot of possibilities for new innovations in wireless broadband such as 4G and 5G services, as it provides excellent range, capacity, and performance. It could become an affordable and technologically feasible solution to improving both in-building and outdoor coverage and capacity expansion as well as deploying private wireless networks.

Learn more about BLiNQ Networks range of CBRS-certified technology and equipment and find the right solution for you today.