Why mid-band spectrum suits 5G

By Rex Chen, director of strategic business development for 5G, LitePoint

The 5G mid-band spectrum offers a compelling balance of capacity and coverage necessary for new applications and use cases.

5G network deployments are underway and should bring significant economic impacts to many industry sectors. The technology will provide a critical boost for remote work and distance learning applications, accelerated by the pandemic and enable new use cases such as virtual reality gaming, smart city and industry 4.0 factory automation, among others. Providing the bandwidth for these new applications requires new spectrum to offer higher capacity and lower latency.

Mid-band works for 5G
The 3GPP standard for 5G has set aside two frequency ranges. Frequency range 1 (FR1) covers from low-band starting at 600 MHz to mid-band, up to 7 GHz.

Within FR1, the mid-band has the best suited frequencies for 5G: 1 GHz to 7 GHz. Fundamentally, the mid-band has better spectrum efficiency compared to low-band, resulting in better downlink throughput. Mid-band also has a greater transmission range and so needs a smaller number of base stations compared to high band, which results in lower capital investment for mobile carriers and internet service. Additionally, massive MIMO in the mid-band contributes to higher throughput.

One use case for 5G mid-band is fixed wireless access, suitable for last-mile connectivity for broadband internet at home. In addition, 5G mid-band has better spectrum efficiency compared to low band (600 MHz).

Mid-band auctions, military and automotive spectrum sharing
The 3.7 GHz to 4.2 GHz mid-band frequencies are being considered and re-purposed for 5G usage. Mid-band is also part of U.S. FCC 5G FAST Plan to accelerate the adoption of 5G deployments. The FCC has plans to auction more than 600 MHz for 5G deployments. The first auction in the U.S. took place in July 2020 for the 3.55 GHz to 3.65 GHz band. Subsequent auctions in December for the 3.7 GHz to 4.0 GHz band include an additional 280 MHz or so called “C-band” and broke new record highs in spectrum auction prices at more than $80 billion.

The White House and Department of Defense have identified the 3.45 GHz to 3.55 GHz frequency bands for 5G use as part of a spectrum-sharing plan with the U.S. military. This spectrum band will continue to be available for military operations, who currently use it for radar operations supporting missile defense, weapons control, air traffic control, and more.

Within the 5.9 GHz band, the FCC has adopted a new model where spectrum is shared between unlicensed Wi-Fi services and automotive applications. For example, the lower 45 MHz of the 5.9 GHz band (5.850 GHz to 5.895 GHz) is repurposed for unlicensed use while the upper 30 MHz of that band (5.895 GHz to 5.925 GHz) is used for cellular vehicle-to-everything (C-V2X) based technology.

CBRS and smart factory
Another part of mid-band is the Citizens Broadband Radio Service (CBRS), which refers to 3.5 GHz spectrum that is available to individuals or companies to purchase to build out their own private 5G networks. These networks can be used in smart factories and in industry 4.0 use cases or to build out campus-wide wireless networks. In industrial applications, spectrum ownership lets enterprises establish flexible connections between machines that need a lot of machine-to-machine communication, or because in some cases wiring is not possible.

Having mid-band spectrum for these private 5G networks offers better spectrum efficiency compared to low-band and at the same time provides better propagation characteristics compared to millimeter wave (mmWave) and hence wider coverage.

Carriers and 5G spectrum
Mobile carriers that have made 5G announcements offering nationwide 5G coverage are reusing their low-band frequency (600 MHz) currently in use for 4G. For example, T-Mobile owns much of the low-band spectrum, making T-Mobile the predominate 5G carrier in the low-band. This puts AT&T and Verizon at a disadvantage who have been re-farming their existing 800 MHz and 1900 MHz in 3G for 5G mid-band.

Some carriers are focused on both mid-band and mmWave, but it will take a few years to really build out nationwide mmWave coverage. The mid-band provides a place for the carriers to improve their coverage for 5G in the intermediary while waiting for the full deployment of mmWave. It is true that mmWave provides a higher data rate, but the low-band or mid-band provides significantly wider coverage and lower deployment costs.

Even cable operators are getting into the mid-band spectrum because fiber to the home has had limited success in the U.S. and is not deployed nationwide. This mid-band spectrum allows residential homes to experience fiber-like speed without the need for underground cable construction at the last mile.

Global Deployments
5G mid-band global deployments are underway starting in the 3 GHz to 5 GHz bands with commercial products such as smartphone devices, followed by CPE and IoT devices. Figure 1 shows more than a dozen countries have already completed auctions or assigned 5G mid-band spectrum including U.S., China, Korea, Japan, Australia, Germany, UK and Italy, among others. By the end of 2022, the additional upper end of the 5G mid-band spectrum (5 GHz to 7 GHz) should be licensed as well.

The spectral efficiency for 5G low-band and mid-band will be at least 30 percent (< 1 GHz and 1.7-2.5 GHz) and 65 percent (2.5-3.5 GHz) better than 4G. One important attribute for better efficiency is the massive MIMO characteristics with more device antennas in addition to 5G enhancements.

Test Considerations
If you design customer premise equipment (CPE) or 5G mobile hotspots, you must take several considerations into account that affect testing in the mid-band spectrum.

The number of antennas on a device is dependent on the number of bands supported by the device (low-band, mid-band) and whether the device is designed for multiple band configurations to share the same antenna (for example, Wi-Fi and cellular). In the case of multiple bands on the same antenna, this could bring some technical challenges to the device as well as some stability challenges, making proper testing important. High-end phones will have a larger number of antennas to account for multiple bands that allow a single device to be used worldwide. Low-end devices, however, may be targeted only for regional use. CPE or non-mobile products might have few antennas and less band support requirements.

Devices with more than eight antennas raise interesting test challenges. Most wireless testing equipment supports up to eight ports, so you might need an external switch or splitter to accommodate additional antennas.

When testing mid-band devices, can you test cellular and Wi-Fi in the same insertion or need to be in separate station? Both scenarios are valid and may be dependent on design complexity along with factory design layout.

A third test consideration is that the low-band and mid-band frequency can be shared between 4G LTE and 5G FR1 through what is called dynamic spectrum sharing (DSS). This is done through E-UTRA NR dual connectivity (EN-DC) which provides dual connectivity for 4G LTE and 5G FR1 at the same time to increase the data rate on devices that support both 5G low-band and mid-band spectrum. Dynamic spectrum sharing (DSS) is an important evolution for 5G by enabling mobile operators to allocate spectrum across low, mid and high-band frequencies while dynamically switching between 4G LTE and 5GNR depending on network traffic usages. It offers high quality performance in 5G and broader network coverage with 4G.

Conclusion
In summary, the 5G mid-band spectrum delivers much faster speeds and greater capacity than low-band (600 MHz) while able to penetrate walls and provide a much larger coverage than high-band mmWave spectrum. 5G mid-band can handle multiple 5G application use cases including:

• enhanced mobile broadband (eMBB)
• critical IoT use cases with ultra-reliable low-latency communication (URLLC)
• massive machine-type communication (mMTC)

The balance of speed and capacity, coverage and penetration, make 5G mid-band spectrum perhaps one of the most valuable digital assets in the future of communication.

Rex Chen is the director of strategic business development for 5G at LitePoint.