What are licensed and unlicensed wireless frequency bands?
Licensed frequency bands are reliable and offer better performance than unlicensed frequency bands, which are low cost and easy to deploy but interference is common.
We are surrounded by wireless technologies now more than ever.
Some wireless technologies come to mind more easily because we tend to access them for personal use, like Wi-Fi and cellular networks such as 5G. But the wireless concept encompasses an incredibly far-ranging landscape -- from Bluetooth and broadcast technologies to military radars and satellite communications -- each landing in its own slice, or slices, of wireless spectrum.
Although the terms spectrum and bands are often used interchangeably, a band starts and stops at discrete frequencies, whereas spectrum is a more generic term for the entire range of radio frequency signals. To keep it straight, a simple example is in order: Wi-Fi spectrum is made up of several discrete bands.
In the United States, the Federal Communications Commission (FCC) sets rules that govern the use of radio spectrum, and similar agencies exist in other countries around the world. The general regulatory construct for spectrum is that it is either licensed or unlicensed.
Unlicensed spectrum allows end users to set up any device they choose, without needing to ask permission as long as they operate within the regulatory limits set for that spectrum. For licensed spectrum, end users must apply to operate specific devices on tightly specified discrete frequencies.
It's helpful to understand the differences between licensed vs. unlicensed frequency ranges and how the frequency management landscape is evolving.
Benefits of licensed spectrum
Licensed spectrum ensures consistent performance and reduces the risk of interference because individual companies or users pay a licensing fee for exclusive transmitting rights on discrete frequencies in the spectrum. Licenses are limited and carefully coordinated based on location so a given band remains well-managed from the spectrum perspective, giving a signal its best chance to succeed.
To understand licensed bands further, let's go back to simpler times.
When broadcast radio and TV were new, specific controlled spectrum was allotted for stations to use. Within that spectrum, individual stations applied for licenses. The frequency channels were specified in each license agreement, along with power output and geographic coverage areas.
The license framework was, and still is, designed to make sure an FM radio station on 88.3 MHz, for example, has plenty of space between itself and other stations on the same frequency. This construct ensures a well-coordinated radio frequency landscape across the licensed frequency ranges. Use cases for licensed spectrum include cellular networks and high-speed wireless bridges used for network backhaul.
Major advantages for technologies that use licensed spectrum include the following:
- Reliability. Because there is no spectrum contention in a location, issues like interference are generally nonexistent provided everyone follows the rules.
- Performance. Each frequency is typically dedicated to a specific use, and systems can generally perform at their best if they're well maintained.
- Protection. If another entity tries to use radio equipment in the same band without authorization, legal remedies in the form of FCC enforcement and other actions are available.
Disadvantages of licensed spectrum
There is no free lunch when it comes to technology, and every paradigm is a series of tradeoffs. In the case of licensed frequencies, the benefits are balanced by the following disadvantages:
- Fees. Any licensed service has high costs of licensure compared to unlicensed frequencies.
- Bureaucracy. License applications can be complicated, with lengthy approval processes and the need to use third parties to help with paperwork.
- Coveted frequencies aren't always "safe." On occasion, licensed frequencies come under threat from other technologies that petition the FCC for the right to use specific bands, to the possible detriment of incumbent licensees.
Benefits of unlicensed spectrum
Unlicensed spectrum has a wide range of benefits, including the following:
- Low-cost access. Aside from the purchase price of equipment, frequencies are free for anyone to use, provided users follow regulatory constraints on output power.
- Fast product development. These bands give innovators and other entrepreneurs the opportunity to introduce new services and technologies quicker than if they had to apply for permission to use licensed spectrum, which has become increasingly scarce.
Disadvantages of unlicensed spectrum
Despite being easy to use, unlicensed spectrum has its own challenges, including the following:
- Contention. Reliability can be hard to guarantee when several technologies need to coexist in the small swath of spectrum. Interference is common.
- Lack of recourse. To make use of the unlicensed spectrum, device makers and end users must accept that everyone has equal rights to the spectrum. If my wireless camera knocks out your VoIP call on Wi-Fi, not much can be done except to try to reach an agreement on how not to disrupt each other.
- Somewhat wild device market. There is no clean delineation between the enterprise wireless client category and consumer-friendly wireless devices. Consumer devices aren't always carefully regulated for conformance to the FCC's rules or built for use beyond home networks. When consumer devices are brought to work, they can cause havoc for enterprise wireless systems in the same frequency bands.
Lightly licensed spectrum on the rise
Even as the FCC attempts to creatively reallocate which technologies are allowed in which band, regulators and industry alike are realizing there are only so many frequencies to use. The endless thirst for more spectrum is driving more use of so-called lightly licensed spectrum bands, such as 3.5 GHz, which is used by Citizens Broadband Radio Service (CBRS) systems.
These spectrum ranges rely on internet-based databases to control what discrete channel a given component can use based on other channels in that band that might be in use by other CBRS systems or higher-priority users, such as military radar and satellite communications. This model is becoming a popular, FCC-prescribed way to ensure coexistence with other users of the same spectrum, while slightly opening specific bands for new users. Implementing these frequency-control mechanisms can be slow and technically challenging.
Comparing licensed vs. unlicensed spectrum vs. lightly licensed spectrum
To recap, each category of spectrum has its own advantages and concerns. Licensed spectrum can be the cleanest and most reliable, but you don't get there without government approval, long lead times and potentially high costs. The device market for licensed spectrum is also fairly small and specialized.
Unlicensed spectrum makes for easy adoption of devices and quick building of relatively low-cost networks using technologies like Wi-Fi and Bluetooth. But in that spectrum, your devices get no higher priority than mine, and when we're in close physical proximity, we need to find our own path to coexistence. In the U.S., common unlicensed spectrum can be found at 900 MHz, 2.4 GHz, 5 GHz and 6 GHz.
Finally, lightly licensed spectrum in the 3.6 GHz band used by CBRS delivers some of the advantages of both licensed and unlicensed spectrum. It costs far less to use and shares spectrum with higher-priority devices by using automated channel selection mechanisms to prevent interference.
Lee Badman is a network architect specializing in wireless and cloud technologies for a large private university. He's also an author and frequent presenter at industry events.
