Licensed vs. Unlicensed Frequency in Street Lighting – Does it Matter? (Part 2)

Part 2: The ISM Bands In Part 1, I discussed why manufacturers of luminaires and street lighting control systems must ensure that their solutions are well suited to the environments in which they are deployed. For critical applications, this is especially important.

This section takes a deeper look at unlicensed frequencies, their typical use cases, and their potential limitations.

Unlicensed Frequencies

Today, most networked street lighting control and smart city solutions that operate on unlicensed RF spectrum utilize what are known as the Industrial, Scientific, and Medical (ISM) bands. Common ISM applications include microwave ovens and RF-based process heaters, both of which emit strong signals and may cause significant electromagnetic interference.

Despite their original designation, the fastest-growing use of these frequencies has been for short-range communications, typically indoors. Well-known examples of wireless technologies operating on ISM bands include Wi-Fi—used for indoor broadband connectivity—and Bluetooth, commonly used to pair devices such as mobile phones and wireless accessories.

To extend RF range for outdoor environments, various methods have been developed. These include wireless mesh networking, ultra-narrowband technologies, and LPWAN solutions such as LoRa®. These approaches attempt to expand ISM-band communication to support wider-area coverage. However, they were not originally designed for this purpose.

It is widely recognized within the industry that ISM bands are often referred to as “congested bands” due to the high level of interference. Devices operating in these bands must contend with signals from both true ISM applications and other communication systems using the same unlicensed spectrum. Additionally, there are no regulatory protections against interference from devices operating within permitted limits.

With the exponential growth in connected devices, spectrum congestion has increased significantly. It is estimated that there are now more than four connected devices per person globally—totaling approximately 24 billion devices—most of which operate on ISM frequencies.

Using these frequencies for non-critical applications—such as utility meters, in-home Wi-Fi networks, baby monitors, and in-vehicle connectivity—is generally effective. However, when applied to mission-critical systems such as public street lighting, the challenges associated with interference and reliability must be carefully considered.

Licensed Frequencies

Operating a wireless network on licensed RF frequencies requires obtaining regulatory approval, such as a license from the Federal Communications Commission (FCC) in the United States. While this process involves administrative steps and a fee, it provides significant operational advantages.

Once a license is granted, the system operates on a dedicated set of frequencies that are protected by regulation. In most cases, no other wireless systems are permitted to use these frequencies within the same geographic area.

This results in minimal electromagnetic interference and a more stable communication environment. If interference does occur, regulatory authorities such as the FCC are responsible for addressing the issue or reallocating frequencies.

In addition, licensed frequencies are specifically designated for communication purposes, which enables more efficient and reliable data transmission. For example, rather than being limited to short-range communication, a networked streetlight controller operating on licensed frequencies may support transmission over distances exceeding 15 miles.

This enables a single gateway to communicate directly with devices across a wide geographic area, eliminating the need for multi-hop communication or dense gateway deployment, which are often required in unlicensed networks.

In practical terms, this means that many municipalities could manage their entire street lighting infrastructure using a minimal number of gateways. As an analogy, this would be similar to maintaining Wi-Fi connectivity over a 15-mile radius—something that is not feasible with conventional consumer-grade wireless networks, but is achievable with licensed spectrum-based systems.

Even in scenarios where full coverage is limited by terrain or environmental factors, networks operating on licensed frequencies typically offer greater robustness and reliability compared to unlicensed alternatives.

Summary

The comparison between licensed and unlicensed RF spectrum highlights a key trade-off between flexibility and reliability. While unlicensed bands offer ease of deployment and broad accessibility, licensed frequencies provide a more controlled, interference-resistant communication environment—making them particularly well suited for critical infrastructure such as street lighting systems.

Closing

The comparative chart below highlights the strengths and weaknesses of deploying a star networked street lighting control system using licensed frequencies versus widely deployed mesh or star networks operating on unlicensed spectrum.