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Wi-Fi on Crutches

September 8th, 2010 by | 30 Comments »

The FCC recently announced that this month (September 2010) it will address the next step in its plans for unlicensed use of the TV whitespace (the portions of the TV band that are not used in a particular location to carry TV signals). Proponents of this policy claim it will bring WiFi on steroids. (A Google search for “WiFi on steroids” gave 117,000 hits with the first hit listed being Google’s own policy blog.)

I doubt that we will see anything like WiFi on steroids in the TV white space. First, modern WiFi (802.11n) is already WiFi on steroids! Computer networking in the TV whitespace is more likely to be WiFi on crutches. (TV white space might be good for some dynamite, low-cost cordless phones.)

Why do I say that performance will be so poor? Well, it’s a combination of physics, FCC regulations, and economics. Because the TV band is divided into 6 MHz channels and is heavily occupied in some urban areas, a white space device will never be guaranteed more than 6 MHz of available spectrum. So, it will be natural to build TV white space devices to operate in 6 MHz channels. Indeed, I read the current rules as limiting operation to 6 MHz channels. (See 47 CFR 15.709(a) which refers to ‘the TV channel of operation’. Given the need to protect adjacent channels, the real useful bandwidth will be more like 5 MHz.

WiFi operates today in the 2.4 and 5-GHz bands—using bandwidths three to six times greater than the 6 MHz of TV whitespace devices. Wi-Fi traditionally used a 20-MHz channel and now has an option for using a 40-MHz channel. At short ranges, such as inside the house, signals do not attenuate much between the transmitter and the receiver, and the received power is relatively high. In such high power situations, the capacity of a wireless link is determined more by bandwidth than by power. Thus, a 20 MHz channel yields 3 times the capacity of a 6 MHz channel.

There are some propagation advantages in the TV band compared to the current WiFi bands—it is probably a naïve consideration of those advantages that led some to speak of “WiFi on steroids.” However when operating at short ranges (as LANs usually do) the additional bandwidth at 2.4 GHz more than makes up for the propagation advantages in the TV band. Moreover, multiple-input, multiple-output (MIMO) technology works better 2.4 GHz and 5 GHz than at the TV band. Better MIMO performance probably gives WiFi operation in the existing bands another factor of two capacity advantage over WiFi operation in the TV white space. The general proposition is undeniable—at short ranges, WiFi operation in the existing WiFi bands permits far higher data rates than are possible in the TV white space under the current FCC rules.

Given that WiFi in the TV white space can only have a third to a tenth of the data rate of today’s WiFi in most applications, I doubt if it will sell very well.

Now there may be a variety of other uses—cordless phones, baby monitors, security cameras, TV remote controls, video game controllers—for which the TV white space will work well. But, they sure aren’t WiFi on steroids.

The TV white space is well suited for long-range services, such as wireless Internet access. However, licensed operation in the white space would be more likely than unlicensed operation to best meet this need. On balance, our nation’s TV white space policy is likely to lead to decades of waste of a valuable resource—with the TV white space delivering far fewer benefits to consumers that it could. It’s a shame.

Disclosure. I have authored or coauthored several studies on the TV white space that were supported by QUALCOMM, a manufacturer of radio equipment for use in both licensed and unlicensed bands. Because they were written in a universe with the same FCC rules and laws of physics, those studies came to the same conclusion as this blog post.

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Reader Comments

  1. Interesting post Charles.

    I discuss the Claudville whitespaces experiment here with a bit of detail.
    http://www.digitalsociety.org/2010/03/i-was-wrong-about-claudvilles-white-spaces-implementation/

  2. Hi Charles

    Great post; it’s especially interesting to see some of the technical details explained. I posted a blog on this subject earlier today: http://blog.advaoptical.com/white-space-key-greater-broadband-access.

    I’d be interested to know more about your thoughts on how Claudville made a white-space network work. If we set aside the regulatory and financial wrangling, is it not possible to replicate what Claudville achieved in other rural areas?

    Thanks,
    Gareth

  3. @Gareth

    Claudville is working because they had a temporary experimental license that effectively gave them exclusive usage of that spectrum. Now that the license is opened up for unlicensed use, they’ll start having problems once other people start using the same spectrum.

    • Thanks, George. Do you plan to conduct any follow up to your initial article, perhaps in 12-months time to see how the network has developed and what obstacles are now being faced?

      • That’s a good idea. I’m not sure about 12-month cycle, probably a little later. I’ll revisit them when there are common whitespace products available because that’s when they’ll have to start dealing with interference.

    • Hi George,

      Do you me now people can do experiments on idle TV spectrum without a temporary license from FCC? Do you mean they can just check the database by eyes and then use the TV spectrum?

      Thanks!

  4. A major limit to FCC TV whitespace rules is that the criteria for allowing them to use a channel includes whether they are inside the “Grade B contour” of a station. This contour is drawn based on a 1966 radio propagation model that works great in Kansas, but does not use complete terrain data in more rugged areas. Thus, Monterrey, CA can not use these devices under present and anticipated rules even though FCC’s own data shows that only 4 TV stations can be received there! If you don’t consider terrain, all the San Francisco area stations nominally have grade B contours that cover Monterrey.

    • Thanks, Michael. I’m fascinated by the technical details involved here. I’ll be subscribing to your posts to see how this develops.

    • While the use of “contours” for protection of television stations can be very conservative in some areas, there have been no satisfactory alternatives proposed. First, most (or all) wide-area (i.e., broadcast) propagation models analyze the problem in two dimensions only, which itself leads to a large uncertainty in the result. Second, RSSI can vary wildly with small changes in location. It’s possible to have a TV Band Device at a location with no reception, yet have a nearby TV receiver that works fine (for example, the TV Band device could be in a basement, while the TV antenna is on a roof.) Propagation models have too much uncertainty to these short-range variations, and cannot even accurately account for attenuation between floors in buildings. So, the present conservative approach seems to be the best answer.

  5. Sorry Charles, we will see on the 23rd how the problem of interference will be solved. Its not a Qualcomm solution.

  6. I have read and understood the technical reasons which prevent the use of TV white space for better (faster) WiFi connections. However, as you mentioned in your post, there are propagation advantages to the TV white space frequencies over the current WiFi frequencies, which could be useful for other applications.

    Could it not be used, therefore, to provide WiFi connections over considerably larger distances? I mean, one of the main problems with WiFi today isn’t just the capacity, but for many less demanding uses it is the distance that is generally restricted to around 100 meters. If possible, it could be incredibly useful.

    • Propagation advantages are also a major liability when you’re talking about unlicensed spectrum. Wi-Fi interference is bad enough when you can see 5 of your neighbors access points, it becomes much worse when you can see 30 of your neighbors.

  7. WiMAX is “Wi-Fi on Steroids” also. I’m getting confused.

    Chuck, thanks for a good reality-check on the limitations of whitespace devices. I’d just like to reiterate and add a few points.

    Before seeing the FCC’s rules, I see whitespace mainly as another spectrum option for wireless internet service providers (WISPs) outside the major markets, and not as a short-range Wi-Fi substitute. The 802.11 Working Group continues a whitespace project and we’ll see where it goes, but 802.22 will be more appropriate for the WISPs. Some US cellular operators would like the option to use whitespace for rural backhaul, which the WISPs don’t care for.

    It is hard to plan a whitespace business when the FCC’s National Broadband Plan calls for taking 120 MHz of TV spectrum.

    Regarding hardware, wideband amplifiers and wideband antennas will be a challenge. The emission mask will be tough to meet at a reasonable cost.

    A better analogy to “Wi-Fi on Steroids” would be a form of high-power Wi-Fi that was standardized in IEEE 802.11y two years ago. I haven’t seen much commercial activity with it.

  8. I think you have this backward. Contrary to some media reports, this isn’t “Wi-Fi on steroids”, but rather a longer range *supplement* to existing short range Wi-Fi.

    As far as I can see it will work similar to 3G, EDGE, and 2G cell coverage. If you’re near a 802.11n transmitter, your Wi-Fi device will use that single. If you’re not near such a transmitter, your device will look for a WiMAX signal. Then, finally, if nothing else is available, it will look for a low bandwidth whitespace signal.

    In other words, exactly what mobile phones do today.

    So, what exactly is your problem with this proposed system, which will be a gigantic upgrade to the usability of Wi-Fi devices (to the point of replacing mobile phones)?

  9. “Propagation advantages are also a major liability when you’re talking about unlicensed spectrum. Wi-Fi interference is bad enough when you can see 5 of your neighbors access points, it becomes much worse when you can see 30 of your neighbors”

    Have you ever heard of masking your ssid? White space will be a great thing for rural america. Sure its not going to be a giglink fiber at your door step but for those paying 179.00 for 1.5mb sat internet it will be great. There are interference probelms to deal with but I have faith in some of the greatest software engineers on the planet to figure that out.

    • “Have you ever heard of masking your ssid?”

      You think turning off SSID beacons fixes the interference problems?

  10. Once again, and it´s probably not any of your faults, the problems you have touched on are solved. http://www.xgtechnology.com

    1.5 to 7 miles no interference, unlicensed spectrum. You will find out on the 23rd. Cognitive radio, detect and avoid.

    • @John

      Interference problems can be coped with, but they’re certainly not “solved”.

  11. This is from 2005. The link is below. All great things take time and truly great things alter rules and regulations.

    xMax uses existing TV or pager frequencies and transmits data signals with extreme low power. This way the signal is not interfering with the other signals. It can only be received or heard with special antennas. The big thing is that xMax can reuse the already sparse radio spectrums, enabling a low cost infrastructure.

    http://www.i4u.com/8170/new-xmax-wireless-technology

  12. xMax? Debunked by Karn.

    http://www.ka9q.net/xmax.html

    • Debunked? The whole world knows xMax is the end all be all of cognitive radio…Wonder why a Qualcomm mid level engineer and ham radio enthusiast would feel the need to debunk xMax.

      • Phil Karn is not a mid-level engineer. He’s one of the people who was instrumental in the development of the Internet and is very well known and highly regarded in the communications industry.

        Karn’s analysis of xMax is very good.

  13. Phil Karn…development of the internet??? Is that you Mrs. Karn?
    Yeah, Phil and Al Gore got together in the basement…
    Phil Karns analysis of xMax is AWFUL, AMATEURISH and WITHOUT ANY MERIT…

    Lets see Mrs. Karn…how much credit does Karn get for “being instrumental in the development of the Internet”:

    https://www.google.com/search?q=phil+karn+instrumental+in+the+development+of+the+internet

    NOTHING! Only you!…meanwhile xG is being hailed as the greatest radio company in the history of communications…
    I think we know who wins this, Dick.

    • I just got some sort of pingback indicating that there had been comments on my post. Well, the most recent comments seem to be about Phil Karn and not TV White Space.

      I’ve never met Mr. Karn but I’ve used his software and enjoyed reading his web page from time to time.

      A quick web search (“Phil Karn” RFC) immediately reveals that he wrote at least 6 RFCs. I teach a graduate computer science course on Computer Networking. The textbook we use (Peterson and Davie) mentions Karn’s work several times.

      I know enough communications engineering to recognize that Karn’s analysis of the xMax system is correct. (I didn’t check the numbers with a calculator but I’ve done similar calculations enough times to see that the numbers look right.)

      I would be interested to know if John Wireless has any connection with Xmax or its backers.

  14. John Wireless constructed a Google search of News that didn’t find anything about Karn. I’ve edited his comment to search the web, where it finds 298,000 hits. The Wikipedia page for “Karn’s Algorithm” is a good place to start:

    http://en.wikipedia.org/wiki/Karn%27s_Algorithm

    “John Wireless” is actually Marc Dannenberg, a notorious stock pumper. See:

    http://xgtechnologyscam.blogspot.com/2009/03/xg-laundering-money.html

    I can confirm that the IP address associated with his comments comes from Marabella, Spain, where Dannenberg is known to live.

  15. Great post; it’s especially interesting to see some of the technical details explained.