People routinely share houses through Airbnb and cars through Uber. Now the sharing economy is expanding to include another scarce resource: radio spectrum.
Spectrum is scarce because we can’t make any more of it and our appetite for it just keeps growing. By 2020, 20 billion devices will be online—up from 8 billion today. To handle this explosive growth, the capacity of wireless networks must triple over the next four years.
Meanwhile there are entire frequency bands reserved for the U.S. Department of Defense, which uses them for secure military communications, including the air-traffic-control radar systems on Navy aircraft carriers and assault ships. When in use, this spectrum serves an important purpose. But outside of coastal regions this military spectrum is essentially never used.
Early this year the U.S. government will start sharing that Navy spectrum, the 3.5-gigahertz band, under a new three-tiered spectrum-sharing framework called the Citizens Broadband Radio Service. This framework is the first of its kind; in fact, when the idea was initially proposed in 2012, the technology to pull it off didn’t yet exist, and no one knew quite how to build it.
Since then, advances in machine learning algorithms and cloud computing have allowed us to create scalable software that makes real-time decisions about who gets access to what portion of the spectrum. The idea is for unlicensed, licensed, and federal users all to be able to use the 3.5-GHz band (found between 3.5 and 3.7 GHz) at the same time.
In December 2016, the FCC awarded provisional certification to seven vendors, including Virginia-based Federated Wireless, where I am chief technology officer, to operate a system, based on these advances and others, that could allow three types of users [see below] to peacefully share the same frequency band. That technology is now ready to boost the capacity of wireless networks at 3.5 GHz. And if it’s rolled out across more frequencies, it could add far more bandwidth in the future.
Historically, the U.S. government—and most other national regulators—managed spectrum by giving out free licenses that designated a specific frequency for a company to use. Then, in the 1980s, the FCC introduced the unlicensed bands now used for Wi-Fi and Bluetooth. Anyone could operate in these bands so long as they used certified radio equipment and agreed that their usage would be subject to interference.
In the mid-1990s, with wireless communications on the rise, the government began holding competitive auctions for licenses. Today, most mobile data traffic travels through networks that rely on either licensed spectrum that was purchased in an auction (such as 4G LTE) or on unlicensed spectrum (such as Wi-Fi).
However, much of the “beachfront” spectrum—that in the low and middle ranges, on which signals can travel relatively far and be reliably received—is allocated for other purposes. Overall, nearly 60 percent of the frequency bands in these ranges are reserved for federal use or managed in a manner that otherwise restricts commercial access.
Recent spectrum policy has focused on “clearing” federal users from spectrum bands and reassigning these bands to companies. Unfortunately, this process is expensive and takes a very long time. The National Telecommunications and Information Administration once estimated [PDF] that relocating federal users from a single band would cost $18 billion and take at least 10 years.
The prevailing “clear and auction” spectrum management policy has put commercial and federal users on a collision course. It was against this backdrop that policymakers began to explore other options. In 2012, a presidential advisory council said that [PDF] if the United States could find a way for different types of users to share spectrum, it could turn a scarcity into an abundance.
The 3.5-GHz band consists of 150 megahertz of spectrum that was previously reserved for naval radar and satellite services. Under the FCC’s new sharing rules, these incumbent users get prioritized access to the band whenever they need it. Because the band is underutilized by these incumbents (tier 1), up to 100 MHz of the band will be available to companies that purchase licenses to access a portion of it (tier 2), and an 80-MHz chunk of that band will now become available for general use (tier 3). Unlicensed users will have the lowest priority. The highest priority across the entire band goes to federal agencies, and licensed users are second in line.
That means someone could listen to a podcast (tier 3) while a military base operates a radar (tier 1) elsewhere in the band and T-Mobile fills another part with traffic from its LTE network (tier 2). Access to the shared spectrum will be managed nationwide to maximize efficiency and minimize interference. This arrangement should help everyone avoid awkward situations, such as sports fans knocking a military radar offline by streaming a big game.
Sharing spectrum is not a new idea. Cellular carriers frequently strike deals to allow one another to access the spectrum they’ve licensed. And Wi-Fi devices follow a “listen before talk” protocol, in which a Wi-Fi radio checks the surrounding radio environment before transmitting in order to reduce interference. But this technique doesn’t work if some users (such as the military) require a higher priority than everyone else.
The challenge for sharing the 3.5-GHz band is how to determine which users are accessing the band and assign bandwidth to them based on their priority status, all within a few milliseconds. The FCC has come up with a name for the new technology to accomplish this: a spectrum access system.
At least three of the seven FCC-certified vendors have begun over the past several years to build their systems and will soon operate them in various U.S. regions. We at Federated Wireless aim to have our system up and running nationwide within the next six months. Once these systems go live, we believe spectrum sharing will improve existing networks and bring affordable broadband coverage to more corners of the United States.
When we set out to design our spectrum access system, we realized it would require two main parts: a cloud-based database and a real-time sensing network. We began building it in 2015 and ran our first tests that same year. In September 2017, we launched our spectrum controller, a suite of spectrum-sharing technologies that includes a spectrum access system, and made it commercially available for large-scale trials prior to FCC certification.
The first component of our system is real-time sensing of naval radar in the 3.5-GHz band, something that doesn’t exist in telecommunications today. With shared spectrum, we knew we would need a secure, robust sensing network that could tell when high-priority users wanted access to the spectrum, as in the case of Navy ships that are approaching the shore.
We deployed secure, hardened radio-frequency sensors along the U.S. coasts and near federal ground-radar facilities. Within these areas, this network detects radars operating in and around the 3.5-GHz band and securely relays that information to the spectrum access system.
Based on that information, the system instructs other users to vacate a channel and directs them to an alternative channel when they are too close to federal radar systems. If necessary, it also has the power to cut off their transmissions to avoid interference to radar systems. It’s important to note that the sensing network does not store, retain, transmit, or disclose any information about the locations or movements of any radar systems or other federal operations, in order to protect their security.
Aside from radio-frequency sensors, our spectrum controller also collects information from special devices about who’s operating in the 3.5-GHz band. These devices include indoor access points deployed just like Wi-Fi routers and slightly more powerful outdoor access points similar to what today’s cable operators call strand-mounted Wi-Fi access points—so called because they can be hung from cables, such as power lines. Once installed, these devices automatically register with the spectrum controller, which can then manage power and channel assignments for all such devices within the area.
The spectrum controller takes the information it gathers from these devices and the radio frequency and adds it all to a database; the database groups all the devices within a geographic area that may try to access the 3.5-GHz band at any time. It includes each device’s priority and location, and it updates the data every few milliseconds.
Next, the Federated Wireless system assigns a frequency and power level to each user or radio in the vicinity. We automated this process by incorporating machine learning algorithms that model the radio environment. We built the database on Amazon Web Services’ cloud-based platform to let the spectrum controller harness the processing power it needs to quickly make decisions and to insulate it against outages.
To help all spectrum-access-system vendors coordinate and share data, the nonprofit Wireless Innovation Forum has specified protocols for operating within the 3.5-GHz band. Other protocols let vendors exchange information directly with individual devices or with a proxy that represents a network of devices (for example, all the devices in an area that belong to a Verizon network).
The spectrum controller must always retain full authority over the users. We learned this lesson the hard way, when earlier spectrum-sharing schemes failed because owners modified their devices, placed them poorly, or simply failed to report when they malfunctioned. When the system, instead of the device, is responsible for reducing interference, it should be easier to protect high-priority users.
This technology represents a fundamental change from past wireless networks. Early analog wireless networks avoided interference by subdividing the available spectrum into small increments. Each wireless tower would broadcast on its own dedicated frequencies, which were completely separate from those used by nearby towers. With our system, wireless networks no longer need exclusive frequency assignments—only an assurance that other signals will be dynamically managed for fairness according to rules set up by the FCC.
In the future, spectrum sharing could be applied to more bands, making networks even more adaptable. With the fine-grained control that a spectrum access system offers, networks can more easily balance traffic from connected sensors, smartphone users, and critical infrastructure. Network administrators will no longer need to manage interference by separating users. Instead, they can balance users’ needs on any swatch of available spectrum.
Telecommunications companies could use this technology to expand rural wireless coverage through relatively inexpensive 3.5-GHz licenses. Both the vendors that specialize in cellular service and those that specialize in Wi-Fi are adding 3.5-GHz radios to access points. In fact, hotels and convention centers could use small cells programmed to 3.5 GHz to deliver wireless Internet to their guests as a supplement to Wi-Fi. And businesses could use them to deploy private networks for industrial use.
After we at Federated Wireless launched our spectrum controller, we began large-scale trials with customers, including American Tower, Charter Communications, and Verizon. Most of the leading radio vendors associated with the Citizens Broadband Radio Service are involved in these trials as well, including Ericsson, Nokia, Ruckus Wireless, and Samsung, and they have already integrated their equipment with our controller. The FCC is on track to certify our spectrum access system and real-time sensing capabilities early this year. After that happens, we will begin commercial rollouts.
In the next five years, we expect to see spectrum sharing rolled out to a dozen more bands within the United States. The FCC is already looking at spectrum-sharing opportunities in additional midband spectrum, including the 3.7- to 4.2-GHz and 6-GHz bands. We’ve also started to see designs for new 5G radios for base stations that incorporate shared-spectrum protocols.
The United States has led the world in enabling federal and commercial users to share spectrum, but the concept could soon start popping up elsewhere. Countries in Europe, Asia, and the rest of the Americas have been watching U.S. progress with the technology and may develop plans to adopt it if they like what they see.
Just like Airbnb and Uber, spectrum sharing has the potential to fundamentally change the way basic services are delivered. Companies will now be able to take advantage of new spectrum without paying billion-dollar license fees. Nearly a century after the first commercial radio broadcast, this new scheme can finally make airwaves available on wireless networks as fast as the demand for them grows.
About the Author
Kurt Schaubach is the chief technology officer for Federated Wireless, a spectrum-management company based in Arlington, Va. He has spent more than 25 years in the telecommunications industry. He earned his B.S. and M.S. in electrical engineering from Virginia Tech.