Selecting the Best Frequency Bands for Backhaul

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 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 your car.  These bands can also reach inside office buildings as they can 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.

As you move into the mid-band things start to change.  The signals don’t propagate quite so far, and they have more trouble penetrating structures, but 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 is not their sweet spot.

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.
  • There isn’t enough spectrum in these bands to come even remotely close to matching fiber speeds.
  • The spectral reuse in these bands is sub-optimal which limits 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 technology 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 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 with Wi-Fi Mesh include:

  1. Channel width is limited to at most 160 MHz and usually far less
  2. It’s unsuitable for broadband applications
  3. Backhauling over Wi-Fi is best effort and involves listen-before-talk
  4. Spectral reuse is low at these frequencies as signals can propagate widely throughout the enterprise
  5. Beamforming is a challenge at 5 GHz as the optimum size of each antenna element is roughly ½ the wavelength of the signal that is being transmitted or about 6 cm2. This limits the size of a 5 GHz array to maybe a dozen elements, which rules out extremely narrow beams with very high gain.
  6. The signal can easily escape the building, so security is poor
  7. Even the addition of the 6 GHz band won’t help, as it 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 bands.  These bands 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, but it doesn’t propagate all that well.

Challenges with the V-band include:

  • Free Space Path Loss 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.
  • Loss 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).
  • Foliage can be nearly impenetrable at 60 GHz, especially if it’s wet and windy.
  • Diffraction is very poor at 60 GHz. The wavelength of V-band signals is just too short to bend around obstructions.
  • At 60 GHz, reflective surfaces look much rougher than they would at 5.8 GHz. This results in a much more diffused reflection as opposed to a specular (mirror like) reflection.
  • Signals have difficulty passing through walls, furniture, trucks, billboards, equipment, and especially people.
  • Basically, it’s limited distance line-of-sight backhaul technology that is ill-suited to wireless access.

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 easily negated by how incredibly efficient beamforming is in the millimeter-wave bands.  Airvine WaveTunnel™ technology can pack a 256-element array into 20 square centimeters of PCB when operating at 60 GHz.  An equivalent array at 5 GHz would be 2 square meters in size.  An array with hundreds of elements can create a very narrow beam at very high gain (30 dB as seen by the intended receiver), which helps greatly in punching through obstructions.  These beams are only a few centimeters wide at 50 meters, and when you add in 20 dB of 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 and each channel is 2.16 GHz wide.

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 and it is more than a match for fiber.

In summary the sub-6 GHz bands are well suited to access and the millimeter-wave bands are well suited in-building backbone applications.  In the right situation, each can be used to provide the other function, but with significant limitations.

For more on the capabilities of different frequency bands please visit www.airvine.com