Citizens Broadband Radio Service is one of the latest revolutionaries in the cellular world.
Named by the Federal Communications Commission (FCC), CBRS is the band of spectrum from 3.55 GHz to 3.7 GHz. The idea behind CBRS is to enable shared access to spectrum so more users can implement private cellular networks at a low cost.
While the U.S. has allotted the 3.55 GHz-3.7 GHz band for CBRS, other countries use different frequencies for shared access. For example, the European Telecommunications Standards Institute has implemented licensed shared access using the 2.3 GHz-2.4 GHz band.
How to obtain CBRS spectrum
One of the primary drivers of enterprise interest in CBRS is the level of performance and reliability that comes at a relatively low cost, said Andrew Froehlich, founder and principal analyst at InfraMomentum, during a recent webinar on CBRS deployment.
"The reason we're talking about private cellular is largely due to the fact that LTE and 5G have closed the performance gap compared to Wi-Fi," Froehlich said. "The other reason is we have the ability to tap into CBRS spectrum that doesn't require an expensive license to operate."
The CBRS band is sectioned into three tiers of shared access. Those tiers are the following:
- Incumbent Access
- Priority Access
- General Authorized Access (GAA)
Incumbent Access is reserved for the U.S. Navy and fixed satellite service stations, which receive priority and interference protection from the other tiers.
In 2020, the FCC held auctions in which ISPs and organizations could bid on Priority Access Licenses (PALs) for 10 MHz blocks of spectrum in different counties, which received an allocated number of bidding units based on population size. Interested bidders made an upfront payment estimated at $0.01 per MHz of population covered, according to the FCC. The Aug. 2020 auction raised $4,543,232,339 in net bids, with a total of 228 bidders receiving 20,625 PALs, according to a public notice from the FCC.
The final GAA tier offers free licenses to any user.
But how do enterprises obtain spectrum within the GAA tier?
Conduct spectrum availability analysis
Enterprises looking to deploy a private cellular network using CBRS should first determine what spectrum is available in their geographic area, said Marc Brumfield, mobility practice architect at Insight Enterprises, an IT services company based in Tempe, Ariz. To do this, teams can conduct a spectrum availability analysis that shows CBRS heatmaps, incumbent activity and other stats.
Verify with Spectrum Access System
Within the GAA tier, spectrum is unlicensed but managed, compared with the "Wild West" of Wi-Fi in which clients fight for spectrum, Froehlich said. So, while anybody can use the GAA tier for private cellular, they must first obtain verification to use a particular frequency.
CBRS is managed by a cloud-based service called Spectrum Access System (SAS), which acts as a centralized repository for all CBRS device operators. SAS tracks the location of every access point (AP) or base station using CBRS in specific areas. When somebody installs a new AP, the device checks with the SAS repository and asks for a grant to use a specific channel. This verification helps avoid neighbor interference.
"A lot of Wi-Fi deployments have serious interference because it is the Wild West," Froehlich said. "But CBRS has the ability to have some control over which frequencies can be used in any given location, which helps reduce the risk of interference."
CBRS deployment best practices
Once teams check spectrum availability, they can proceed to get their devices online. Brumfield advised teams to start simple and add layers of complexity as they see good results.
Teams can mount cellular APs in similar locations to traditional Wi-Fi APs. For outdoor environments, however, Brumfield said it's important to keep in mind how the APs will reach local power and where to install the antennas.
"Go to the location to see what the environment looks like, and start documenting," Brumfield said. This documentation helps provide insights to cabling teams about what will be involved during installation.
Enterprises can also overlay a private cellular network on top of a Wi-Fi network, but teams should ensure they leave 1 meter of separation between cellular and Wi-Fi APs, said Brian White, director of systems engineering at Celona Inc., a private cellular vendor.
Anybody can install a CBRS device, but a certified professional installer must register the device within SAS in order to bring it online, Brumfield said.
In indoor environments, private cellular networks use Cat6 cables plugged into usual switches.
Private cellular has the same power requirements as a regular Wi-Fi network. It can use Power over Ethernet switches, for example.
Private cellular with CBRS offers better coverage than Wi-Fi networks, requiring fewer radios and APs.
Marc BrumfieldMobility practice architect, Insight Enterprises
"If you're going for coverage, you're going to have fewer indoor radios covering an area compared to Wi-Fi," Brumfield said. "The [CBRS] spectrum definitely covers larger areas than traditional Wi-Fi."
White said he tends to shy away from considering how much coverage a device offers per square foot because environments can have different interference and propagation factors. Instead, he tends to stick to a ratio of one cellular AP for every four or five Wi-Fi APs.
"That ratio tends to hold, no matter what kind of deployment you're talking about," he said.
As teams assess what kind of devices they want to put on the private cellular network, they should look for devices labeled as Band 48, or B48, which is another name for CBRS. Most devices in client environments, starting from iPhone 11 and Samsung S20 models, already support Band 48, Brumfield said. For devices that don't support Band 48, teams can also use USB dongles, cellular gateways and adapters to onboard devices to the private cellular network.
How to decide between CBRS and Wi-Fi
Both CBRS and Wi-Fi offer the benefits of mobility and connectivity to enterprises. And there's room for both, Brumfield said.
"One's not going to replace the other. It's a different tool for a different job," Brumfield said.
Enterprises need to determine which tool best matches their use cases. With Wi-Fi, the goal is mobility over reliability, White said. Wi-Fi is a contention-based medium, with every device and radio fighting for access to spectrum.
"Picture 50 people in a room all trying to get a word in," White said. "It's chaotic. Yes, communication is happening. I might ask you to repeat yourself. I might hear you or miss a word."
Teams that implement Wi-Fi networks are willing to accept link unreliability because they get the benefit of mobility. But not all use cases can afford that unpredictability, he added.
In contrast, cellular is an infrastructure-controlled medium, which means no device transmits without the infrastructure scheduling and telling it when to transmit. So, in that room of 50 people, one person tells all the others when to speak. In the private cellular context, this adds more order, as well as the ability to prioritize certain clients and prescribe a predictable amount of latency, no matter how much traffic is on the channel, White said.
CBRS use cases and applications
Some potential CBRS use cases include the following:
- automated guided vehicles
- vehicular computing, such as forklifts
- warehousing and manufacturing
- ports and transportation hubs
Ultimately, when deciding to implement a private cellular network or a Wi-Fi network, White said enterprises should ask the following two questions:
- Does the use case require mobility?
- Does it require extreme link-layer reliability and predictable latency?
"If those are the criteria you place with these scenarios -- and the answer is yes to both questions -- then private cellular makes sense," he said.