Broadband Wireless — the future is 5 GHz

and eventually, spectrum from 4 GHz to 10 GHz.  

Unfortunately, the mobile industry doesn't understand the implications of MIMO or beamforming, so they are asking for the wrong thing.  They are asking for more spectrum near their current bands (below 2.1 GHz) or lower, e.g. in former TV bands below 700 MHz.  This is all wrong.

Until recently you could say: lower frequencies "work better" meaning they go farther.  But this was a technology limitation, not something in the physics. From a physical point of view, 5 GHz photons pass through the atmosphere just as well as 700 MHz photons. Today, MIMO and beamforming are correcting some of these historic technology limitations and changing everything.

• MIMO not only delivers significant performance improvement, it makes 5 GHz spectrum as useful as lower frequency spectrum for all sorts of applications.
• Beamforming delivers the next performance increment (20x and up), and it's actually easier to implement at higher frequencies.

Addressing mobile data demand

There is no doubt data consumption is growing more rapidly than mobile operators' data capacity (see slides 10 and 11 of my presentation at 4GWE). Unfortunately, US cellular operators are pressing for more low frequency radio spectrum and they are being heard in Washington. In his keynote at this year's Consumer Electronics Show, FCC Chairman Julius Genachowski said an impending shortage of wireless spectrum in the U.S. will dampen future economic growth unless action is taken to fix the problem. 

However, the short term issue is investment dollars not spectrum and the long term solution is new technology at much higher frequencies (above 4 GHz) rather than more spectrum for existing technology near existing bands.

Why 5 GHz spectrum is more useful than TV frequencies

Everyone know TV signals go long distances but remember than TV broadcasters use from 100 Kilowatts to 5 Megawatts ERP.  That's a million to a hundred million times more signal than your mobile handset puts out.  No wonder it covers a lot of distance.

There's also an equation most wireless engineers use in one form or another to calculate free space path loss which says higher frequencies have more loss. But this equation encapsulates two factors:  the true path loss and the size of the antenna.  It assumes a 1/2 wavelength antenna. Higher frequencies have shorter wavelengths, so it assumes the antenna gets smaller as the frequency goes up. Smaller antenna, less signal. With comparable antenna apertures, path loss in the atmosphere is the same from below 50 MHz to nearly 10 GHz. Thus in open air, 5 GHz photons go just as far as 500 MHz (US channel 19) photons or indeed photons for Channel 2 or Channel 50.

Multi-path and MIMO

The reason people have had trouble with higher frequencies for the past 100+ years is "multi-path" propagation.  As a signal radiates from a source, some of it goes directly to the receiving antenna but some of it goes in other directions where it may be reflected or refracted by objects it encounters. When reflected signals also reach the receiving antenna, they are slightly delayed because they traveled a slightly longer distance.  In the days of over-the-air analog TV, we saw these delayed signals as "ghosts" or shadows around images on our TV screens.  For digital data transmission, multi-path contributes to the "noise" in the signal-to-noise ratio. The historic problem with higher frequencies is their shorter wavelengths made it easier for them to be reflected and refracted by man-made objects like buildings, window frames and even closely spaced double pane glass surfaces.

 

But with MIMO, all this changes. With MIMO's multiple antennas and multiple radio front ends, it's possible to separately decode and make use of the multi-path signals.  Now "multi-path" is not only removed as a source of "noise," it adds additional signal and helps carry more data.

Beamforming

A beamformer uses signal processing to control the phase and relative amplitude of the signal at each of a group of independent antenna elements.  Radiation from multiple antenna elements causes a pattern of constructive and destructive interference in the resulting wavefront.  This can produce a tight beam just like one from a highly directional antenna.  But with beamforming, the antenna beam can be steered in software on a packet-by-packet basis.

With eight antenna elements spaced 1/2 wavelength apart (total 3.5 wavelengths), you can create a beam like this:

 

Highly directional beams significantly extend the usable range of a wireless system. And since the beam is computed with digital signal processing software, it can be steered to different directions in microseconds. What's more, the benefits of beamforming apply both while transmitting and while receiving. Either way, the beamformer accentuates the signal in the desired direction while surpressing signals to/from other directions.

But what about wavelength?  To obtain this narrow beam, the outer antenna elements are 3.5 wavelengths apart.  At 5.8 GHz, that's less than 7.5" so the whole antenna array easily fits in a ceiling mounted access point just 8" x 3" by 2".  At 700 MHz, that degree of beamforming still requires 3.5 wavelengths, but wavelengths are longer so now we need 5 feet of separation — something that may fit on a cell tower, but is difficult for a microcell and impossible for a femtocell.  For beamforming, higher frequencies are an advantage.

More spectrum at higher frequencies

Finally, there's a lot more spectrum potentially available at higher frequencies and, today, in the 5GHz band, there is over 555 MHz of license-exempt spectrum already available for applications like Wi-Fi. That's more spectrum than Verizon Wireless, AT&T Wireless, Sprint PCS and T-Mobile USA have, combined!

Wi-Fi blazes the trail

The Wi-Fi market place is vastly more diverse than the 3G/4G mobile operator market place, which means many new technologies show up in Wi-Fi years before they are deployed in mobile networks.  That is certainly true of so-called 4G modulation (OFDM) which was deployed for Wi-Fi with 802.11a (1999) and 802.11g (2003), years ahead of WiMAX (2005) or LTE (2010).

The 802.11n specification includes MIMO and optional beamforming and silicon technology is appearing to support 4x4 MIMO with beamforming.  MIMO products (2x2) have been shipping since 2007 and 4x4 MIMO in consumer products expected in the next six months. Meanwhile, many players are scrambling to deliver 11n options, including beamforming. Early systems are already deployed.

Mobile operators, please pay attention

The technology benefits of MIMO and beamforming apply to the mobile phone industry, it will just take a few years for the deliberate pace of the industry to catch up.  Meanwhile, mobile phone operators should be tracking real world measurements of Wi-Fi performance to understand what spectrum they will really need five to eight years hence.

Comments

[Bryan]

A fascinating view. How long do you estimate it will take for the MIMO and beamforming technologies in WiMAX to equal the coverage and penetration of lower frequency technologies?

[brough]

I'm not an expert on WiMAX but my impression is, WiMAX is moving faster than LTE but not as fast as Wi-Fi. As far as I know (rumors, not facts!), Clearwire is deploying 3 sectors, 5 stations/sq. mile, 10 MHz channels, 2x2 MIMO. Most handheld devices are single radio devices (not 2x2 MIMO). That's pretty much the state-of-the-art. I don't think any WiMAX system includes beamforming (although I assume they have antenna diversity).

So compared to Wi-Fi, that's like an 11g device talking to an 11n access point except with a very narrow channel (10 MHz versus 20 MHz for 11g and up to 40 MHz for 11n).

Monica Paolini has a good (futures) discussion of what coverage WiMAX should achieve once beamforming is deployed, see:
http://senzafiliconsulting.com/blog/?p=266

[Joe]

No offense, but none of the crap you go into all these details about make any bit of difference if you can't get the signal to the customer. Sure 5ghz and up is great, but nobody really cares how well high frequency signals go through the air, to get through dense trees, the only way it will work is to break the laws of physics.

This is the whole reason lower frequencies are better. It isn't cost effective to have a tower every mile. Outside of urban environments, you quite often aren't dealing with reflections where mimo will help anyway. You're just looking to push the signal through junk that is in the way.

[Franz]

I would strongly recommend to the Author some propagation course before further analysis of this problem. Photons and radio waves don't have too much in common. The only true thing in this article is MIMO and beamforming work better in higher frequency bands. 1/2 wavelengths antennas have long history in radio communication because the pattern of the radiation is reasonable from system point of view (gain and main beam direction are easy to control). I would agree that in some cases higher frequency is better for future systems, but all this analysis above is based on false facts. Outside the cities, where is no obstacles to reflect the signal MIMO doesn't have practical advantage, so low frequency as 700MHz is best to cover such bug countries as US. In the cities it's different story.

[brough]

Joe, I am well aware that lower frequencies are useful for penetrating foliage. I fully expect mobile operators to hang onto their existing spectrum for that purpose.

However, in the US, the majority of people live and/or work in urban areas where foliage is greatly reduced. These are the areas where most people are and where most capacity is needed. For the industry to miss out on the point that higher frequencies can help with their primary problem is a crock.

[brough]

Franz, Light and radio waves are both forms of electromagnetic radiation and they both involve photons. If you are not aware of this, then you're the one that needs some further study.

As I commented to Joe, my focus is urban areas as that's where the capacity problems are most acute.

Just because 1/2 wavelength antennas are simple and have a long history doesn't mean they are the universal answer. We also have a long history with fixed directional antennas that are multiple wavelengths across (dishes for example). What's new now is the ability to focus beams in real time using on-silicon signal processing software to produce adaptive beams. This will have a dramatic impact.

You also miss the extent to which 2x2 MIMO is possible in rural line-of-sight situations, i.e. without reflections. In this case, one uses two antenna elements with different polarizations (e.g. vertical and horizontal). There are many commercial examples already, for example the Motorola PTP 600 (formerly Orthogon) system.

Finally, you say my analysis is based on "false" facts, but you don't point out anything specific that is false. Are you just upset by my focus on lurban environments?

[Joe]

Ok, I'll give you the urban and line of sight argument. I agree that the higher frequencies are great for those environments, assuming you're talking about using outdoor antennas. If you're talking about indoor connectivity, then you still have to deal with penetration issues. A 5+ ghz signal simply won't go through a wall as well as a 700mhz signal will. But yes, the high frequencies do have their purpose and will serve the areas where "the majority" of people live.

But on the flip-side, let's not forget that the rural people are the ones in most need to begin with. Almost everyone in an urban area is going to have 10+ mbps service available already. Where I live, the average MAX broadband speed available is between 512k and 1.5mbps. Let's not forget the areas where there is only dial-up available. These are the situations where low frequencies are needed, and high frequencies just won't cut it.

So at any rate, yes, high frequencies are great for all the stuff you talked about, but your article didn't start off saying you were talking about urban areas, nor did it mention it anywhere else that I noticed. You made broad statements saying that "the mobile industry" is doing everything all wrong, and didn't mention that they are doing everything "all right" for the huge areas of the country where they need the low frequencies to penetrate.

[brough]

Joe, we're probably in agreement on the major points.

I'm just concerned that mobile operators want to tie up more lower frequency spectrum across the whole country to solve a problem that is mostly urban. If there is additional low frequency spectrum being made (e.g. former TV spectrum) it could better be used by independent rural wireless ISPs than by the oligopoly mobile operators.

When I say mobile operators have it all wrong, that's because their capacity problem is overwhelmingly urban, where higher frequencies could work to their advantage.

By the way, while masonry absorbs more at higher frequencies, it is a strong absorber everywhere. A typical brick-faced concrete wall represents 24 dB attenuation at 850 MHz, 34 dB at 1.9 GHz and 66 dB at 5 GHz, but through glass it's only about 1.5 dB even at 5 GHz. There's quite a bit of literature on cellular signals inside buildings that suggests signal strength variations come from interference between signals coming through adjacent windows, rather than anything to do with signals coming through masonry. However, I haven't had the time to look into this in detail...

[Joe]

Yep, I'd say we are in agreement. I actually do run a rural WISP, and would drool to have access to some good low frequency spectrum that could expand coverage dramatically. I also agree that the low frequencies are a waste in urban areas.

I'm sure there are cost issues that they face operating multiple frequencies and supporting them on handsets, but long term, it does make sense to use the higher frequencies in urban areas where they need the capacity and use a "fallback" in rural areas where all that matters is getting a signal at all :)

[Franz]

Please... bring me here one RF engineer which will agree with this whole statement: lower frequencies "work better" meaning they go farther(Franz:This is true). But this was a technology limitation, not something in the physics. From a physical point of view, 5 GHz photons pass through the atmosphere just as well as 700 MHz photons. Today, MIMO and beamforming are correcting some of these historic technology limitations and changing everything.

THIS IS NOT TRUE. It is a physics limitation.
Free space loss = ((4 x Pi x distance x frequency)/light speed)square
Is the frequency correlation clear here? Is it a "technology" formula or formula from physics?
It's a formula for free space loss, in atmosphere loss will be even more dependant on frequency.

However I didn't say that photons and radio waves have nothing in common, did I? I just said they don't have too much in common.

Photon and radio wave are not the same. If you don't agree with this fact, I think we can finish our dispute.

Operators all around the globe hire RF engineers, where at least half of them have some background in this topic. Trust me, low frequency is better, but more expensive from many point of views.

[brough]

Franz,

Photons and waves are two different descriptions of electromagnetic phenomenon. Light and radio waves are both electromagnetic phenomenon. For any electromagnetic energy transfer, either light or radio waves, the photon is the particle that carries the energy. This topic is covered in the electromagnetism section of most college physics texts. I can also recommend the free online MIT Courseware for undergraduate physic course 8.02, Electricity and Magnetism.

To understand the derivation of the path loss equation, i.e. how you get from Maxwell's equations of electromagnetism to the equation used by most RF engineers, look here:
http://en.wikipedia.org/wiki/Free-space_path_loss
and you will see that the common equation encapsulates two effects - path loss and antenna dimensions. It is the assumptions about antenna size that cause the frequency dependence, not the actual path loss.

I know a number of RF engineers who understand physics, for example:
http://twitter.com/wa8dzp/status/8546870294
To the extent they are a minority, that's a problem with RF engineers, not with physicists or physics.

[Franz]

Mate,

You are not deleting my comments, are you? Thanks for this recommendation, but I can guarantee to you, I did my electricity and magnetism course and a few more as well, however they were not free online courses.

You are writing again about photons when talking about radio communication, that's fine let's leave this for now. Your assumption about antenna size is totally wrong, especially when you want to analyze a problem in academic way. Antenna gain, size or whatever parameter you want to use cannot be taken into account when talking about free space loss. If you want to have good reference, take isotropic antenna, then you can compare which frequency will go further. I don't want to even try to explain to you antenna theory, but the rule is simple as that higher gain - narrower antenna pattern. So when your higher band, higher gain antenna is providing more gain, the cell size is smaller, until you will reach point-to-point communication.

After 15 years in mobile industry I can say, there is some future in higher bands, but with current state of technology, they cannot replace 700, 850, or 2100 MHz bands, because these bands are perfect for cellular networks. Even 2600MHz for LTE in Europe is already to high, and operator are looking for some legacy 900MHz to refarm.

And obviously there is a bright future for 10GHz band - airport body scanners :)

[Franz]

Btw,

Do yourself a favor and stop talking about photons when you are talking about radiocommunication. It might help you in your business or career.

[stephan]

This is really great (even if some of your readers appear unable to follow or understand what you link to). Are you familiar with the literature on outdoor to indoor radio wave propagation? Most is academic and thus behind journal pay walls but, for instance, see: http://www.realwireless.biz/publications/papers/ICAP%20pdfs/142.pdf

Those who have carefully measured real world signal strength within buildings come up with an amazing range of dependancies - the angle of incidence, the location of windows, the size of the Fresnel zone with respect to windows, but also w.r.t. floor-to-ceiling spacing when people are further back in a building (and we're considering mobile frequencies). There is no doubt that MIMO is a major step forward for radio performance inside buildings and it makes sense it will do more for higher frequencies. Thank you!

[Jesse Collins]

This is true. Check out Rocket M5 base station. It use the MIMO technology. Check out wicTEK broadband that offer a turnkey WISP program - low startup cost.

[Ajay]

Franz, sorry to break it to you, but Brough knows of what he speaks. Read any description of photons and you'll see that all electromagnetic radiation consists of photons, including radio waves. I think what you may be getting hung up on is that historically photons were named in respect to light, but they were then extended to all electromagnetic radiation, including radio waves, as light is simply radio waves at a different frequency. Great piece, Brough, the kind of analysis I'd pay to read. :)

[Lubo]

Sorry brough...

In a post you wrote:

"By the way, while masonry absorbs more at higher frequencies, it is a strong absorber everywhere. A typical brick-faced concrete wall represents 24 dB attenuation at 850 MHz, 34 dB at 1.9 GHz and 66 dB at 5 GHz, but through glass it's only about 1.5 dB even at 5 GHz."

Can i ask you, where do you find this information?

I need some attenuation data about 2.4 GHz and 5 GHz for different materials.
Can you help me?

Thanks,

my regards.

[brough]

Hi Lubo, There are a variety of papers with bits and pieces, but the most complete set of measurement data I am aware of is entitled Electromagnetic Signal Attenuation in Construction Materials by the US National Institute of Standards and Technology (NIST) done as part of their Construction Automation Program and published in October 1997 as "NISTIR 6055."

You can download a PDF of this report at:
http://fire.nist.gov/bfrlpubs/build97/PDF/b97123.pdf

Here is the abstract for the report:
Laboratory studies of electromagnetic (EM) signal propagation through construction materials were carried out as part of the NIST initiative in Non-Line-of-Sight surveying technology. From these data it is possible to determine several important material-specific characteristics needed for the design of engineering systems which make use of EM signal propagation through matter: 1) the power attenuation as a function of the material thickness and 2) the values of the electrical permittivity and dielectric constants for a particular material as a function of frequency. The latter can be used to calculate the propagation delay time associated with an EM pulse penetrating through a specified thickness of a given material. This information is essential for error compensation for time-of-flight metrology instrumentation systems. In this report, only the power attenuation aspects are discussed; dielectric and permittivity constants will be discussed in a future volume. The materials investigated included brick, masonry block, eight different concrete mixes, glass, plywood, lumber (spruce-pine-fir), drywall, reinforced concrete, steel reinforcing bar grids, variations of the plywood and lumber tests in which the specimens were soaked with water, and composite specimens involving brick-faced masonry block and brick-faced concrete. For each material, varying thickness specimens were fabricated in order to measure attenuation as a function of penetration distance. Each specimen was placed in a special test range consisting of spread spectrum transmission and reception horns spaced 2 meters apart with a metal RF isolation barrier located midway between the antennas to eliminate multipath signals. The isolation barrier contained a window at its center against which the specimens were fit. Measurements of power loss were taken at 2 MHz intervals from 0.5 to 2 GHz and from 3 to 8 GHz. Frequency power spectra were discretely generated for each material as a function of thickness and fit with closed-form predictor equations. Coefficients for the predictor equations are provided.

[Lubo]

Thanks a lot brough....

That report is the best!!

You are been very useful for me!

My regards.

[brough]

Just saw some interesting history and references to original papers by H. T. Friis in this thread at DSL Reports:
http://www.dslreports.com/forum/remark,24210307
entitled "Reconciling the Friis Equation."