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February 02, 2010

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."

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