This blog will look at the RF bands that are in common usage today to get a better idea of which services work best at which frequencies. We’ll focus on access technologies which connect users to Wi-Fi AP’s (or cellular networks) and backbone technologies that connect these AP’s back to the wiring closet and from there to the Internet.
RF spectrum can be divided into low bands, mid-bands, and high bands (aka millimeter-wave).
The low bands are typically defined as below 2 GHz, and they propagate extremely well. If you ever wondered why your cellphone works in an underground parking garage, it is because the network is probably using the 850 MHz band. At these frequencies the signal will bounce down concrete stairwells and eventually find their way to you and/or your car. These bands can also reach inside office buildings as they easily pass through most types of exterior and interior walls. The low bands are great if wide area coverage is your goal, but there isn’t much of this spectrum, and what there is has already been claimed by a host of different organizations both public and private. Availability of spectrum is of course what defines the capacities a given wireless system can support. More Hz, more bits/second.
As you move into the mid-band, 2GHz to roughly 6GHz, things start to change. The signals don’t propagate quite so far, and while they have more trouble penetrating structures, they still provide very good local area coverage. One technology that has taken root in the mid-bands is Wi-Fi. It is a perfect place for Wi-Fi because these bands have good but not great propagation characteristics which allows for better spectral reuse than the low bands. The latter is important in the unlicensed bands as it helps reduce interference.
Bottom line: the bands below 6 GHz are ideally suited to supporting access technologies. Wi-Fi and cellular technologies dominate this space and do a wonderful job of providing coverage. The downside of the low and mid-bands is reduced spectral reuse. Propagation isn’t always your friend. While these bands are primarily for access, it’s possible to also use them for backhaul, but this application is not their sweet spot. Remember backhauling by its very nature is a high capacity use case. The backhaul network element must carry traffic from multiple access nodes be they cellular or WiFi.
Challenges in using the low and mid-bands for backhaul:
- Spectrum is precious in these bands as indicated by the money raised @ FCC auctions and the near constant demand for more Wi-Fi capacity.
- As a result there isn’t enough spectrum in these bands to come even remotely close to meeting the throughput demands for a backhaul application.
- The spectral reuse in these bands is sub-optimal which limits overall network capacity.
The sub-6 GHz band that sees the most use in backhaul applications is 5.8 GHz, largely because it is a high-power unlicensed band (30 dBm in the U.S.). A host of companies operate here including Tarana, Cambium, Ubiquiti, and Radwin with technologies that are primarily focused on fixed wireless access (FWA) in rural areas. The base station mounts on a tower or grain silo and can connect users that are several kilometers away, but spectrum is limited (125 MHz in the U-NII-3 band) and as a result so is the throughput.
Wi-Fi mesh technology can also be used to provide backhaul in the sub-6 GHz bands. In this application a Wi-Fi AP is used to provide both access and backhaul. This has the effect of cutting the capacity of the AP in half, which can be a problem depending on the application.
Other challenges using Wi-Fi for backhaul include:
- Practical channel widths are usually limited to at most 160 MHz and usually far less, meaning lower throughputs.
- Backhauling over Wi-Fi is best effort.
- Spectral reuse is low at these frequencies as signals can propagate widely throughout the enterprise.
- The signal can easily escape the building, so security can be an issue.
- The 6GHz band which newer WiFi 7 equipment can operate in, still lacks the spectrum needed for backhauling and is needed for access.
The upside of this approach is that it is an inexpensive way to provide coverage over a wide area, such as a park, but that coverage comes with a significant loss in capacity that occurs with each hop. This makes it unsuitable for most broadband enterprise backbone applications.
This brings us to the millimeter-wave or high bands. These frequencies have a tremendous amount of spectrum, but the propagation characteristics are much more limited. The part of the millimeter-wave band that has gotten the most interest is the unlicensed V-band up at 60 GHz (57-71 GHz in the U.S.). This band has the spectrum to easily match fiber speeds, but it doesn’t propagate all that well.
Challenges with the V-band include:
- Free Space Path Loss (FSPL) as defined by FRIIS = 32.4 + 20Log10(d) + 20Log10(f) where d is in kilometers and f is in MHz. At 100 meters, you’ll see close to 110 dB of path loss @ 60 GHz.
- This band suffers from an effect called oxygen absorption. Losses from oxygen absorption can approach 16 dB per kilometer (FCC paper referenced below).
- Loss from heavy rain can add another 21 dB per kilometer (FCC paper referenced below).
- Diffraction is very poor at 60 GHz. The wavelength of V-band signals is just too short to bend around obstructions.
- Signals have difficulty passing through walls, furniture, trucks, billboards, equipment, and especially people.
The net result? Radios using this band have limited distance, line-of-sight restrictions.
FCC Bulletin #70 from 1997 is still the single best treatise on the physics of the millimeter-wave propagation that I’ve ever come across.
https://transition.fcc.gov/Bureaus/Engineering_Technology/Documents/bulletins/oet70/oet70a.pdf
This might all sound bad, but it depends on what you are trying to do. If the focus is in-building enterprise backbones these negatives either become positives or they simply don’t apply. The FSPL is partially compensated with efficient beamforming that is a hallmark of high frequency bands. Airvine WaveTunnel™ uses a very small antenna operating at 60 GHz. These beams are only a few centimeters wide at 50 meters, and when you add in side lobe suppression you get a system that supports very high spectral reuse. In a large warehouse you’d be able to use the same V-band channel dozens of times without any noticeable co-channel interference.
The V-band can deliver an enormous amount of network capacity for in-building backbone applications. Orders of magnitude more than is possible in the sub-6 GHz band.
In summary the sub-6 GHz bands are well suited to access. The millimeter-wave bands can support the capacities needed for an indoor backhaul solution. But unless your solution can overcome line of site restrictions it is of little practical use. The WaveTunnel has solved this problem and punches through walls. The world’s only Non Line of Site 60GHz radio on the market.
For more on the capabilities of different frequency bands please visit www.airvine.com
Originally Posted on August 20, 2021 by Steve Hratko