Valuing the white spaces above 3 GHz

Today, significant money and political capital are being expended to obtain and hold onto usable license-exempt access in the TV white spaces. These efforts are important for applications today but there’s a follow-on spectrum initiative that, if successful, would yield much greater benefits in the long term. We should be seeking similar access to as much as possible of the spectrum above 3 GHz, almost all of which is dramatically under-utilized today.

Throughout the 20th century and right up to today, it's been the case that higher frequencies "don't go as far." But this is the result of technology limits, not ultimate physical limits, and these technology limits are now being overcome.

Within ten years it will be widely apparent that higher frequencies go just as far through the atmosphere, they do just as well at penetrating buildings, and they have other extremely important benefits that lower frequencies lack.

Among their many advantages, directional antennas are smaller and more economical at higher frequencies. Directional antennas reduce received interference and facilitate spatial reuse, thus vastly increasing the utility of higher frequency spectrum.

What’s more, it's easier to send high-speed data because there is more spectrum available at higher frequencies. We'll never be able to send 1 Gbps over any real-world 6 MHz TV channel but, above 3 GHz, we can easily find 200 MHz of spectrum that's temporarily vacant and that's enough to carry more than 1 Gbps of data even with today's technology.

For the next decade or two, TV white spaces will continue to be important for penetrating foliage, but even with foliage, the physics of what is possible differs from 20th century experience. In the future, the real action will be above 3 GHz.

Finally, while it's never easy to persuade existing licensees to accept secondary users in “their spectrum” even while it’s idle and they are non-interfering, it should be easier to fight the political battles now, when most people don't realize the long term value of spectrum above 3 GHz.  Now is the time we should be seeking license-exempt access to as much as possible of the white spaces above 3 GHz.

 

Details for the technically inclined

All photons (light or radio waves of any frequency) go at the same speed (the "speed of light"). In our atmosphere, photons at frequencies above 10 GHz are subject absorption because they excite resonances in atmospheric molecules like water vapor or oxygen. But the atmosphere is transparent to radio signals between 30 MHz and 10 GHz so, with a clear line-of-sight, radio signals at 8 GHz go just as far as signals at 700 MHz or 50 MHz [2].

This physical fact is sometimes missed because the Free Space Path-Loss (FSPL) equation (see: http://en.wikipedia.org/wiki/Free-space_path_loss) commonly used to calculate radio frequency (RF) transmission losses actually encapsulates two effects. These are 1) the actual path loss (which is independent of frequency) and 2) the receiving antenna aperture which is based on wavelength. Thus the FSPL equation assumes smaller antennas for higher frequencies and of course, smaller antennas collect less energy. With equal antenna apertures, unobstructed line-of-sight radio transmissions are frequency independent, even with 20th century technology.

The problems that have favored lower frequencies are reflection, refraction, polarization and diffraction. Higher frequencies have shorter wavelengths and shorter wavelengths signals are more easily scattered. Scattered signals that reach the receiver have taken a longer path and thus arrive a little later. With 20th century technology, these delayed signals (called "multi-path" signals) were just part of the noise degrading the primary signal. Now with Multiple Input Multiple Output (MIMO, see: http://en.wikipedia.org/wiki/MIMO), it's possible to decode multi-path signals, remove them from the noise, align them in time and add them to the primary signal - multi-path signals are no longer a deficit but actually improve system performance!

MIMO technology only began to emerge in the mid-1990s but it is now an option in the latest Wi-Fi, WiMAX and LTE specifications. MIMO uses multiple radios and higher order MIMO requires increasingly sophisticated calculations, but early (2x2) MIMO systems are already widely deployed in 802.11n consumer WiFi products and continued semi-conductor progress (following Moore's Law) will make MIMO calculations and additional radios ever lower cost.

Also inherent in higher order MIMO is beamforming and beamsteering.  As the number of radios and antennas in a MIMO system increases, the system is able to simulate tighter and tighter beams, providing ever more spatial reuse of spectrum and more range or capacity for individual connections. However, tighter beams require more wavelengths of separation between the outer most antenna elements.  Again higher frequencies have shorter wavelengths, so antennas that support tighter beams require less space at higher frequencies. For example, a 10 degree beam at 700 MHz requires an antenna ~10 feet across.  To do the same at 8 GHz, the antenna need only be ~10 inches across.

Long term, the spectrum above 3 GHz will be more valuable than the spectrum below 3 GHz.  Let’s get license-exempt access to these white spaces now, while the political stakes are still (relatively) low.

Some useful references

[1] Spectrum occupancy measurements for frequencies below 3 GHz, see: http://www.sharedspectrum.com/papers/spectrum-reports/.  Fewer measurements have been made above 3 GHz but those done show negligible occupancy (certainly when compared with bands below 3 GHz), for example see:  http://www.sharedspectrum.com/papers/spectrum-reports/

[2] This graphic http://en.wikipedia.org/wiki/File:Atmospheric_electromagnetic_opacity.svg shows the transparency / opacity of our atmosphere at different wavelengths.  Note that 10 meters is 30 MHz and 3 cm is 10 GHz.

[3] While the total atmosphere (as seen from space) is highly absorbing above 10 GHz, shorter range point-to-point connections are still possible as this more detailed spectrum shows for the range 10 GHz - 1000 GHz: http://www.omlinc.com/library/other-references/atmospheric-absorption-of-millimeter-waves.html

[4] NIST, Electromagnetic Signal Attenuation in Construction Materials, NISTIR 6055, http://fire.nist.gov/bfrlpubs/build97/PDF/b97123.pdf  Note that ordinary window glass is essentially transparent to RF at the frequencies tested (500 MHz to 8 GHz) while most other building materials provide substantial attenuation.  One caveat: as this study was done to help design RF-based measurement device for the construction industry, they post-processed their data to remove delayed signals.  In other words, MIMO communications systems will do considerably better than these measurements suggest.

[5]  Okamoto, Kitao & Ichitsubo, Outdoor to Indoor Propagation Loss Prediction in 800 Mhz to 8 GHz Band for an Urban Area, March 2009, http://ds.lib.kyutech.ac.jp/dspace/bitstream/10228/2399/1/IEEE.pdf  Detailed measurements demonstrating frequency independence for signals penetrating real life buildings.

[6] Perras & Bouchard, Fading Characteristics of RF Signals due to Foliage in Frequency Bands from 2 to 60 GHz, http://horwitzinternational.com/PDF%20Files/Trees%20and%20801_11.pdf  Careful measurements show signal attenuation peaks when RF wavelengths equal leaf size and then falls off at even higher frequencies.

[7] RF Engineering for Wireless Networks: Hardware, Antennas and Propagation, by Daniel M. Dobkin, PhD, ISBN 0750678739 has useful chapters on signal propagation in the atmosphere, in the environment and in buildings.

 

Comments

[Vance Shipley]

Great point about MIMO as a game changer. I don't see how you've backed up the statement "just as well at penetrating buildings" though. I looked at the NIST report and it shows a definite decline from 0.5Ghz to 2Ghz and although it's flatter in the 3-8Ghz graphs it's down markedly from the other. Are you suggesting that most of this is the result of dispersion and not absorption? Isn't reflection a large issue here as well which MIMO wouldn't handle?

[brough]

Thanks Vance. Read section 2.6 on pages 33-35 of the NIST report to see the "post-processing" they apply. Figure 2.6.1 shows actual spectral measurements (before their post processing) for a brick wall. The spectral response is complicated - at one point transmissivity is greater than 1.0! -- but it's relatively independent of frequency. In figure 2.6.2 they show what's happening in the time domain (which they happen to calibrate in meters, based on the speed of light). Figure 2.6.2 shows a primary peak and several additional peaks that result from scattering within the brick, i.e. multi-path propagation.

Figure 2.6.3 shows the post processing NIST applies (in the time domain). This post processing suppresses all delayed, i.e. multi-path, signals. When the post processed signal is then converted back to the spectral domain, suddenly we see what 20th century (non-MIMO) communications receivers see, i.e. RF transmissivity goes down as frequency goes up. However, this is clearly the effect of the post processing. In the raw data, there is no evidence that any of the RF has been converted to heat. It's merely been scattered, thus creating multi-path signals. With MIMO, multi-path becomes a benefit.

As you suggest, MIMO is a game changer. I'm claiming we haven't begun to see how much of a game changer MIMO will be!

[brough]

Vance, I haven't found good data to show exactly what the scattering looks like, i.e. how much of the signal is reflected back towards the transmitter and how much is merely scattered within the building. I understand all the NIST raw data could be made available if a researcher wanted to do something with it. Unfortunately, I'm fully engaged starting netBlazr Inc., so I have find other researchers' works or wait for someone to look into this.

The one thing that's seems clear from the data I have seen is that masonry does not significantly absorb RF, i.e. convert it to heat. It mostly scatters RF.

[Henning]

Good post about the value of MIMO and how it can make digital radio usage much more efficient, especially for the high frequencies. Just as a comment, both beamforming and beamstearing have been used for military radar for quite some time, https://secure.wikimedia.org/wikipedia/de/wiki/Mammut_%28Radar%29 . Its just that its getting cheap enough for consumer hardware now.

But I am not sure if its possible to get "secondary/whitespace" usage before the primary usage is defined. For getting a "whitespace licence" you have to show that you do not interfere with the primary user, which sounds impossible with a frequency not allocated.

[brough]

Thanks Henning. Indeed phased array concepts go way back! Nobel Laureate Karl Ferdinand Braun demonstrated the concept in 1905 and I understood AM broadcasters were using phased arrays in the 1930s (although I can't find the reference right now). But you are correct that it was radar and WWII that drove electronic beam steering. What's exciting is that the MIMO specs, e.g. 802.11n, 802.11ac and similarly in WiMAX & LTE, provide for transmit beamforming/beamsteering while the MIMO calculations inherently provide the equivalent of receive beamforming/beamsteering. The 802.11n & 802.11ac specs combined with Moore's law pretty much insure we will get consumer priced beamforming/beamsteering within a few years.

"Secondary" access is already well established. Many of the amateur radio bands have worked on that basis for decades. In the US, the UNII-2 (5.25-5.35 GHz) and UNII-extended (5.47-5.725 GHz) bands are secondary use, based on "DFS" which is a radar sensing scheme. TV white spaces introduce database lookup as another way to determine when a secondary user can access a band without interfering with the primary licensee. There's also the 3650-3700 MHz band in the US. While the 802.11y specification was originally developed for Wi-Fi in the 3650 MHz band, the 11y committee produced a very general scheme that allows for sensing, data base look up and for master & slave devices so the extra cost of sensing and/or database look up can be separated, thus reducing the average cost of a system of devices.

[Carlson Wireless]

As the spectrum database management model matures, perhaps it will become easier to convince those license holders that it's safe to share. Thanks for this article, Brough.

[Steve Crowley]

Hi Brough. I think you're on the right track here. I agree we're in the early days of MIMO and adaptive technologies. The article implies that MIMO was the first to constructively use multipath interference, but OFDM and adaptive equalization do that also. MIMO adds significant additional gain.

Regarding the loss mechanism with water vapor, in reference 2 I see the peak of attenuation for water just above 20 GHz. Is that due to resonance? Elsewhere, I assume attenuation from water vapor would be by the same mechanism as from other materials, dielectric heating caused by molecule dipole rotation.

The FCC has a proceeding underway on dynamic spectrum access. Maybe there will be some more flexibility from that. We do need more flexible use of spectrum.

Many higher frequencies are in government bands, many of which I suspect are underutilized. NTIA is looking at repurposing some of those. Unfortunately, a GAO report earlier this year concluded, basically, that NTIA's databases are a mess, and no one really knows what's going on in the government bands. We need a good government database, informed by an inventory of spectrum assignments and usage, in my opinion.

Given a lack of appropriate regulation, if one wanted to do more now, one could try for a waiver of the rules, or try to operate under experimental authority until permanent rules are in place, not that either of those are easy.

[brough]

Thanks for the comment Steve. I agree OFDM with cyclic extension and channel adaption makes constructive use of the multi-path energy, as does a CDMA system with a rake receiver. But somehow, these systems strike me as doing the best possible mitigation whereas MIMO does all this while also leveraging multi-path to increase capacity. But yes, you are correct.

Yes, the first water peak is at 22 GHz. Water vapor has a complex spectrum with over 64,000 spectral lines listed in the 2007 HITRAN database. They are all the result of the incoming electromagnetic energy exciting different vibrational modes of the molecular bonds. It's H2O, so there are two bonds, each of which can stretch, rotate, bend, etc., either symmetrically or anti-symmetrically with the other bond.

There's a good graph of atmospheric absorption by water vapor and oxygen here:
http://www.rfcafe.com/references/electrical/atm-absorption.htm

One possible way to flesh out who claims what spectrum might be to declare it all will be made available three years from today for secondary use subject to database lookup and power limits. Then give federal agencies and others two years to object and up to three years to submit receiver location information to the database contractors. At a minimum, such a proposal might help NTIA get their databases in order. :)