When assessing different wireless solutions for your IoT deployment, you may be surprised to discover that most technologies are adopting license-free Industrial, Scientific and Medical (ISM) frequency bands for their operations. Without the complex licensing processes and hefty fees of the licensed bands, the license-free spectrum has been a preferred choice for many developers. While there are various ISM bands available today, the decision often boils down to sub-GHz vs 2.4 GHz frequencies.

If you think the difference between these two groups of ISM frequency bands only matters to wireless system developers, think again. For all IoT adopters, being able to distinguish between sub-GHz and 2.4 GHz ISM bands is a major help in your wireless technology decision. This is because the operating frequency impacts the ways in which a radio system performs in key network criteria.

In this blog, we outline 5 major distinctions between sub-GHz and 2.4 GHz to be measured against your network priorities.

1. Sub-GHz ISM Bands = Much Longer Range

As the wavelength is inversely proportional to frequency, sub-GHz waves are much longer than the 2.4 GHz ones. A major advantage of the longer wavelength is that signals can better penetrate through walls, trees, buildings and other structures along the propagation path. On top of that, longer waves are less susceptible to reflection and can bend farther around solid obstacles (i.e. diffraction). Wavelength is also inversely proportional to free space path loss, meaning longer radio waves can travel farther in open areas. Simply put, radio signals in sub-GHz ISM bands offer a better range and can operate more reliably in structurally dense environments.

2. Sub-GHz Signals Experience Less External Radio Interference

Due to its free access, the unlicensed spectrum is filled with multiple co-existing radio technologies. In comparison, the 2.4 GHz ISM band is much more crowded because of legacy use. Wi-Fi hubs, Bluetooth-enabled devices, cordless phones, welding equipment, RF lighting and microwave ovens are just a few examples. The high radio traffic and electromagnetic noise in the 2.4 GHz airways can greatly dampen the reliability and scalability of the IoT network. Saturated 2.4 GHz channels in your facility are not recommended for your IoT deployment.

3. Systems Using Sub-GHz ISM Bands are Often Designed for Lower-Bandwidth Transmissions

Bandwidth represents the range between the upper and lower frequency limit used for transmitting a signal. The more information a signal carries, the more bandwidth it requires. In radio communications, high bandwidth is associated with a high data rate, as you can send more data per time unit. Generally, higher carrier frequencies allow for more bandwidth usage due to larger available spectrum resources, and vice versa. As a result, radio systems in the 2.4 GHz ISM band are often designed for higher-throughput data communication compared to sub-GHz systems.

4. Technologies Using Sub-GHz ISM Bands Are More Power Efficient

Because they use less bandwidth and lower data rates, systems operating in sub-GHz ISM bands offer the benefit of lower power consumption. Signals requiring less bandwidth result in lower thermal noise which in turn, provides better receiver sensitivity (the minimum power level at which a receiver can detect a signal). All things being equal, systems with higher receiver sensitivity require less transmission output, resulting in less power consumption.

Another aspect that favors the power efficiency in sub-GHz solutions is network topology. Thanks to the better range performance (i.e. many kilometers), a sub-GHz transmitter node can send a message directly to a remote receiver using the one-hop star topology. On the other hand, systems in the 2.4 GHz ISM band often resort to a relaying mesh topology to compensate for their short physical range (i.e. few to hundred meters) and extend the data communication distance. The fact that a node must actively stay awake to relay messages through them greatly increases power consumption and thus reduces battery life in 2.4 GHz mesh networks.

5. Systems Using Sub-GHz ISM Bands Are Likely to Entail Less Infrastructure Cost

In terms of infrastructure cost, both options have their benefits, but sub-GHz networks might be the winner here. As antenna length is a function of wavelength, 2.4 GHz solutions require smaller and less expensive antennas for their operations. However, in long-range applications, 2.4 GHz mesh systems often demand extra repeaters to ensure sufficient coverage. A scalable sub-GHz network, on the contrary, requires as few as one base station to cover the same area and number of sensing devices.

Key Takeaways

To wrap it up, both 2.4 GHz and sub-GHz ISM frequency bands have their own pros and cons and are geared for different applications. 2.4 GHz RF is generally a better fit for use cases requiring high data rates within a smaller network environment, such as camera surveillance, personal health and fitness, and some consumer applications. On the other hand, in low-throughput, latency-tolerant scenarios where priorities are placed on range, power efficiency and scalability, sub-GHz RF would be more feasible. This refers to the majority of batter-powered, remote monitoring use cases in industrial and commercial marketplaces.