mmWave, Low-Band, and Mid-Band Explained

    A hand holds an iPhone with a hologram that says "5G "floating out of the phone.
    Marko Aliaksandr/Shutterstock

    You’ve probably heard that 5G uses the mmWave (millimeter wave) spectrum to reach its 10 Gbps speeds. But it also uses the low- and mid-band spectrums, just like 4G. Without all three spectrums, 5G wouldn’t be reliable.

    So, what’s the difference between these spectrums? Why do they transfer data at different speeds, and why are they all critical to 5G’s success?

    How Do Electromagnetic Frequencies Transfer Data?

    Before we get too deep into low-band, mid-band, and mmWave, we need to understand how wireless data transmission works. Otherwise, we’ll have trouble wrapping our heads around the differences between these three spectrums.

    Radio waves and microwaves are invisible to the naked eye, but they look and behave like waves in a pool of water. As a wave’s frequency increases, the distance between each wave (the wavelength) gets shorter. Your phone measures wavelength to identify frequencies and to “hear” the data that a frequency is trying to transmit.

    Visual example of a modulating wave. As frequency increases, the wavelength (the distance between each wave) decreases.

    But a stable, unchanging frequency can’t “talk” to your phone. It needs to be modulated by subtly increasing and decreasing the frequency rate. Your phone observes these tiny modulations by measuring changes in wavelength and then translates those measurements into data.

    If it helps, think of this as binary and Morse code combined. If you’re trying to transmit Morse code with a flashlight, you can’t just leave the flashlight on. You have to “modulate” it in a way that can be interpreted as language.

    5G Works Best with All Three Spectrums

    Wireless data transfer has a serious limitation: frequency is tied too closely to bandwidth.

    Waves that operate at a low frequency have long wavelengths, so modulations happen at a snail’s pace. In other words, they “talk” slow, which leads to a low bandwidth (slow Internet).

    As you’d expect, waves that operate at a high frequency “talk” really fast. But they’re prone to distortion. If something gets in their way (walls, atmosphere, rain) your phone can lose track of changes in wavelength, which is akin to missing a chunk of Morse code or binary. For this reason, an unreliable connection to a high-frequency band can sometimes be slower than a good connection to a low-frequency band

    In the past, carriers avoided the high-frequency mmWave spectrum in favor of mid-band spectrums, which “talk” at a medium pace. But we need 5G to be faster and more stable than 4G, which is why 5G devices use something called adaptive beam switching to jump between frequency bands quickly.

    Adaptive beam switching is what makes 5G a reliable replacement for 4G. Essentially, a 5G phone continuously monitors its signal quality when connected to a high frequency (mmWave) band, and keeps an eye out for other reliable signals. If the phone detects its signal quality is about to become unreliable, it seamlessly jumps over to a new frequency band until a faster, more reliable connection is available. This prevents any hiccups while watching videos, downloading apps, or making video calls—and it’s what makes 5G more reliable than 4G without sacrificing speed.

    mmWave: Fast, New, and Short-Range

    5G is the first wireless standard to take advantage of the mmWave (millimeter wave) spectrum. The mmWave spectrum operates above the 24 GHz band, and, as you’d expect, it’s great for superfast data transmission. But, as we mentioned earlier, the millimeter wave spectrum is prone to distortion.

    Think of the mmWave spectrum like a laser beam: it’s precise and dense, but it’s only capable of covering a small area. Plus, it can’t handle much interference. Even a minor obstacle, like the roof of your car or a raincloud, can obstruct millimeter wave transmissions.

    Man "driving" on a computer mouse through a fast Internet connection.

    Again, this is why adaptive beam switching is so crucial. In a perfect world, your 5G-ready phone will always be connected to a mmWave spectrum. But this ideal world would need a ton of mmWave towers to compensate for millimeter wave’s shoddy coverage. Carriers might never shell out the money to install mmWave towers on every street corner, so adaptive beam switching ensures your phone doesn’t hiccup every time it jumps from a mmWave connection to a mid-band connection.

    Initially, only the 24 and 28 GHz bands are licensed for 5G use. In 2020, the FCC completed auctioning off the 37, 39, and 47 GHz bands for 5G use (these three bands are higher in the spectrum, so they offer faster connections). Now that high-frequency millimeter waves are licensed for 5G, the technology is becoming a lot more ubiquitous in the USA.

    Mid-Band (Sub-6): Decent Speed and Coverage

    Mid-band (also called Sub-6) is the most practical spectrum for wireless data transmission. It operates between the 1 and 6 GHz frequencies (2.5, 3.5, and 3.7-4.2 GHz). If the mmWave spectrum is like a laser, then the mid-band spectrum is like a flashlight. It’s capable of covering a decent amount of space with reasonable Internet speeds. Additionally, it can move through most walls and obstructions.

    Most of the mid-band spectrum is already licensed for wireless data transmission and, naturally, 5G will take advantage of those bands. But 5G will also use the 2.5 GHz band, which used to be reserved for educational broadcasts.

    The 2.5 GHz band is at the lower end of the mid-band spectrum, which means it has wider coverage (and slower speeds) than the mid-range bands we’re already using for 4G. It sounds counter-intuitive, but the industry wants the 2.5 GHz band to ensure remote areas notice the upgrade to 5G and that extremely high-traffic areas don’t end up on super-slow, low-band spectrums.

    Low-Band: Slower Spectrum for Remote Areas

    We’ve been using the low-band spectrum to transfer data since 2G launched in 1991. These are low-frequency radio waves that operate below the 1 GHz threshold (namely, the 600, 800, and 900 MHZ bands).

    Tero Vesalainen/Shutterstock

    Because the low-band spectrum is comprised of low-frequency waves, it’s practically impervious to distortion—it has great range and can move through walls. But, as we mentioned earlier, slow frequencies lead to slow data transfer rates.

    Ideally, your phone will never end up on a low-band connection. But there are some connected devices, like smart bulbs, that don’t need to transfer data at gigabit rates. If a manufacturer decides to make 5G smart bulbs (useful if your Wi-Fi cuts out), there’s a good chance they’ll operate on the low-band spectrum.

    Sources: FCC, RCR Wireless News, SIGNIANT

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