CBRS SAS: Simple Explanation of the Spectrum Access System

We explain the Spectrum Access System (SAS), and how it manages the CBRS spectrum assignments between different private mobile networks.
Private Mobile Network Blog
CBRS

Curious about CBRS SAS? We explain the Spectrum Access System, and how it manages the CBRS spectrum assignments between different private mobile network deployments.


What is CBRS SAS?

The SAS (Spectrum Access System) allocates frequency channels to CBRS access points (also known as CBSDs) transmitting in the 3.5 GHz band. Spectrum sharing is organized across three tiers, with a SAS administrator providing priority to the top tier, while preventing interference to lower tiers.

Note that, in order to remove friction in enterprise deployments and eliminate one extra step in IT operations, Celona’s unique 5G LAN solution handles all the interactions with SAS providers behind the scenes. Through these interactions, it dynamically manages frequency assignments to access points within earshot of each other in the same network.

What exactly is CBRS?

Citizens Broadband Radio Service (CBRS) is a new spectrum in the 3550-3700 MHz band allocated for private use by the FCC. This band was previously only used by the United States Navy and Airforce. Due to underutilization, the FCC has opened up the 3.5 GHz band for broadband private use.

The CBRS spectrum is divided into three tiers: Incumbents, Priority Access, (PAL) and General Authorized Access (GAA).

  • Incumbents: Reversed for governmental agencies, Navy ships, and fixed satellite stations. Nothing is allowed to interfere.
  • Priority Access License: Licenses that are for commercial business use and acquired through CBRS auctions, in case priority of way is required for up to 40MHz per county, assigned on a per access point (CBSD) basis.
  • General Authorized Access: GAA is the third tier on the CBRS, and ideal for private deployments. Devices in this range can include mobile and IoT devices that are commonly used within enterprise environments.

Companies can use their CBRS access to build their own private mobile networks. These mobile networks leverage LTE and 5G capabilities to provide new business models and services previously unavailable to private operators. These services include:

  • Wide-scale Industrial IoT (IIoT)
  • Autonomous vehicle communication/control
  • Massive MIMO hotspots
  • Nationwide asset management and tracking
  • Campus-wide private LTE networks
  • … among others.

How is CBRS different from other spectrums?

The CBRS spectrum offers users more control and protection than using a commercial mobile network operator. By using the CBRS spectrum your service has better protections from interference, more options for customization, and isn’t dependent on a single mobile carrier.

In order to protect the first two tiers from interference, mobile network operators and enterprise private LTE / 5G deployments must use a Spectrum Access System (SAS) to ensure that their devices do not cause interference, and adhere to the FCC regulations. This helps protect your devices on the CBRS from unlawful interference.

What is a Spectrum Access System? (SAS)

The Spectrum Access System (SAS) is an autonomous cloud-based radio spectrum coordinator that manages wireless communications operating in the CBRS spectrum. Devices register with the SAS for spectrum assignment and moderation of their power transmit levels.

Devices operating on the CBRS spectrum are called CBRS Service Devices (CBSDs). Each CBSD needs authorization from the SAS before they begin transmitting data. Currently, there are several companies approved that provide SAS administration from which IT teams can choose from. Celona primarily partners with Google and Federated Wireless and eliminates the complexity of having to build relationships with SAS vendors in order to deploy private mobile networks.

The SAS uses Environmental Sensing Capability (ESC) sensors to enforce FCC rules, manage spectrum allocation, and prevent radio interference. These ESC sensors are located across the United States, primarily along the coastline where they detect broadcasts over the CBRS spectrum. These sensors protect the Incumbent tier from interference, specifically Navy ships located off the coast.

When the ECS detects an incoming transmission on the Incumbent tier, it automatically creates a protection zone in that area to prevent interference from any other sources. Rather than blocking all communication in that zone, the SAS dynamically reroutes PAL and GAA users to other bands of the spectrum so incumbents can communicate without interference.


How does the CBRS SAS work?

Devices using the CBRS must reach out to the SAS prior to broadcasting. Currently, the SAS  cannot proactively detect and communicate to a device, so that device must contact the SAS on its own. CBSDs can communicate with a SAS either directly or by using a Domain Proxy, which is the case for Celona’s 5G LAN solution, helping automate deployments. Domain Proxies act as an intermediary between the SAS and devices sending requests. There are currently six different ways a CBSD can communicate with a SAS.

  • Registration: The CBSD shares device information with the SAS. In exchange, the SAS provides that device with a unique identifier and access.
  • Spectrum Inquiry: The CBSD sends the SAS a spectrum inquiry request to learn which parts of the spectrum are currently available. The SAS provides the best options available.
  • Grant Request: The device will send a request to operate on a particular frequency to the SAS. The SAS will deny the request if that communication may interfere with incumbents. If approved, the SAS will grant the device a grant ID.
  • Heartbeat Request: The CBSD will send out multiple heartbeat requests to the SAS for each of their approved grants. Devices must be issued a grant ID prior to getting permission to transmit. If approved, the device can transmit until the transmitExpireTime which is typically 240 seconds after the last heartbeat.
  • De-registration: If the device is being moved or retired, it will deregister itself from the SAS.
  • Relinquish Grant: When the device no longer needs the grant to operate, it relinquishes the grant.

These requests can be sent by using a SAS portal or done automatically using an API integration with a SAS administrator.

How does CBRS SAS protect users from interference?

The SAS protects incumbents by maintaining a database of all CBRS devices, their locations, access level, and unique identifiers. Combined with real-time sensor data the SAS can make adjustments to spectrum availability and broadcast power assignments using the latest information in real time.

To ensure all SAS administrators have the latest information, each SAS performs a nightly export of all data. This process is called Coordinated Periodic Activities (CPA), and occurs synchronously for all spectrum access system servers.

What hardware do I need for SAS registration?

To communicate with the SAS you’ll need to use CBRS certified equipment, similar to Celona’s access points. In a Celona 5G LAN, all access points are configured to communicate with SAS administrators automatically through the Celona platform - removing the need to purchase and install licenses for each access point separately.  

The Celona Solution

Celona partners with enterprise organizations to provide private cellular wireless services as a seamless turnkey solution. Celona utilizes its 5G LAN solution powered by cloud networking and artificial intelligence to automate deployments. Onboarding can be done alongside existing enterprise networks and industrial IoT systems, without interrupting business operations.

Plug-and-play cellular wireless access points (CBSDs) can be quickly deployed throughout the campus, while proactive monitoring ensures network SLAs such as throughput and latency requirements are consistently being met.

If you’re building your network for the future, we can help. Check out our network planner to estimate the size of your network, or test-drive our unique 5G LAN solution for yourself with a custom demo or a free trial.

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