Tales from the Towers, Chapter 17: Who Needs White Space?

This article explains why the “White Space” (300 to 400 MHz frequencies) may not be ideal for wireless ISPs (WISPs) operating in cities. It focuses on how WISPs can build a network using the 900 MHz frequency band instead with the new beam-forming equipment.

It’s time to step up our game.  There is no problem generating massive bandwidth from an access point (AP) location.  We have done that.  What we haven’t figured out yet is how to leap over tall trees in a single bound or walk through brick walls.  If you are willing to add in another 20Mhz of super-powerful, wall-penetrating, obstruction busting, tree smashing signal, then we have solved the problem.

You are thinking I’m going to jump on the White Space bandwagon.  I’m sure I will someday but much has to happen before that option is available.  However, the infrastructure we have designed is pretty flexible and inexpensive.  When White Space becomes feasible and cheap (my favorite word), we can add it.  In the meantime, we have two other options.  I will leave the best for last.

White Space is the current hype for two reasons: extended range and building penetration. But it has one big disadvantage.  In major cities, there may not be a lot of channels in the White Space that would be available (according to Spectrum Bridge) for wireless providers.  In addition, limited power output from clients is still going to limit high-bandwidth range back to the AP.  I also see the 6MHz channels being an issue with bonding being the same problem as trying to run 40MHz outdoors.

Before we jump into this, let me note that Ubiquiti has just released a stack of new products that are game changers in Wi-Fi, indoor and outdoor, Video Surveillance, and cellular service.  These technologies cover everything from Beam-Forming to GPS sync to dual-polarity omnidirectional antennas.  I’ve had to hold back telling you more due to non-disclosure agreements but we will now start covering how these technologies can be integrated into our Guerrilla Wi-Fi design to take the systems to a whole new level.  Most of these products are several weeks away from shipping so we have time to develop our system. I do a have a pair of the 900MHz M series 802.11N 2×2 MIMO APs in my hands and that is the topic of this article.

Option one is 900MHz.  Yes, it’s crowded, noisy, and seriously overused.  So is a Japanese subway but people still use it because it’s the best option. Until now though, the best WISPs systems limited users to about 3Mbps under ideal conditions with APs limited to about 7Mbps. There really wasn’t a lot of development in that frequency band due to the interference and reduced band size as compared to all the other unlicensed options.

900MHz has close to the same broadcast properties as White Space except for the vastly higher interference.   In a municipal system with 16 APs per square mile, APs are within approximately 600 feet (182 metres) range of everybody.  Unfortunately, 2.4GHz can’t penetrate obstructions very well.  Brick or stucco buildings that will suck the life out of 2.4GHz are merely a few dB of loss to a 900MHz signal.  At that range, trees effectively disappear.  Junior’s frequency hopping baby monitor and the SCADA transmitter hanging on your water meter is now more of a problem than obstructions.  In 900Mhz, with 802.11N 2×2 MIMO now being applied, bandwidth isn’t the issue any longer.  The biggest problem is interference.

Everything has a threshold though and you just have to find it.  With parents, it’s how low your grades can go before you become the prisoner of the bedroom Alcatraz and your friends start filing missing person’s reports.  With RF, it’s the difference between the signal and the noise and how efficiently we use the bandwidth. Now toss in a very narrow band, 26MHz, and the total bandwidth throughput from any AP is going to be limited.  Oh, yes, did I mention that we only have to go 600 feet (182 metres)?

802.11N isn’t just for 2.4GHz and 5.8GHz.  It just hadn’t been applied before in the 900MHz band.  Ubiquiti just released several new 900MHz products with 802.11N 2×2 MIMO protocol.  Using a 5MHz channel will allow up to 4 APs to operate on one pole, which each one providing up to 20Mbps.  However, using buildings and some shielding, they might allow up to 4 channels of 10-20MHz in more remote areas to allow up to 300Mbps.  I’m  guessing here but when I get my hands on one of the base station sector antennas, I will do some testing to determine what would be needed for isolation.

Keep in mind through all of this that we are still dealing with an AP/sector antenna combination that costs less than $500.  If budget is an issue and the city thinks that a several 4 foot (1.2 metre) antennas on a pole aren’t their idea of aesthetically pleasing, then look at using the Nanostation M900 Locos. They are very small, and although rated at a 60 degree beam pattern, they can easily cover 90 degrees with a small drop off in antenna gain.  In fact, the beam pattern for these radios is way over 90 degrees at 7.5dBi of gain.

The only problem here is that you need one of these radios in or on the house since there is no portable device that can support 900MHz.  That starts getting expensive at $200 for the radio and another $50-$150 for an indoor Wi-Fi device for wireless coverage.  However, the problem of building penetration is completely solved.

If I haven’t mentioned it before, we only have to go 600 feet (182 metres) from an AP location if we have 16 poles per square mile.  Realistically though, if we have 16 APs per square mile, I would probably only use a maximum of four AP locations with the 900MHz radios for budget reasons.  That means we might have to go 1300 feet (396 metres).  Of course, we are using a proprietary polling scheme and a dual-polarity signal to go that far.  If the noise floor starts at -65, the signal level needs to be at -55 or better.  At 1300 feet (396 metres), even with obstructions, our signal level should easily exceed that.

Trilliant and other Smart Grid companies are releasing MOAB (Mother of All Bombs) 900MHz radios that are up to 1W for residential installations.  If you think that a few towers can cause interference, try fighting tens of thousands of radios dropped into the middle of your coverage zones.  Motorola 900MHz WISPS from here to Canada are getting hammered and there isn’t a lot they can do about it. There are going to be cities where running 900MHz WiFi may not be feasible.  Don’t panic yet, we will take our 2.4GHz game up a notch also.

There are two advantages to the Ubiquiti 900MHz radios to fight interference.  One is the dual-polarity MIMO design.  In the city though, most radios are using antennas with such low gain, polarity isn’t going to make a lot of difference.  However, the M900 product line came out simultaneously with the ability to frequency hop.  That means you have four 5MHz channels to jump around with at 300ms rates to avoid noise try and punch a signal through.  If the Smart Grid density is too high, then even that isn’t going to matter but right now it’s the best option available.

The second advantage is AirMax.  AirMax will simply ignore other packets in the band and also eliminate the hidden node problem.  Although interference is interference, AirMax APs won’t slow down acknowledging other APs in the band.

Ahh, but what works for city folk works even better for country folk.  The dreaded trees of death for 2.4GHz and 5.8GHz are merely pin pricks to 900MHz.  Toss in the dual-polarity 2×2 MIMO design and now signal will punch through the forests like Ray Lewis through an NFL helmet.  Expand out the channel to 10 or even 20MHz, and throughput for a single AP could go as high as 80Mbps.

On the muni-wireless issue, we can assume that designating four AP sites per square mile will add approximately $1000 per AP, or about $4000 per square mile.  It also adds 320Mbps of total capacity per square mile.  Add in the CPE Capex of $200 per client, assume 25 clients need this radio to avoid a truck roll, and you have an additional $5000.  If you truck roll, add another $3750 in the Capex column for those of you keeping track.

Based on those numbers with a $30 per month fee and a free install, it will take 17 months to recoup the Capex.  Of course, we want to charge $100-$200 for an install to offset some of those costs.  In areas where municipal staff thinks antennas are cool and interference is minimal, we could even use four 900MHz dual-polarity sector antennas with 13dBi of gain.  That will provide over 6 times the coverage area which might reduce the AP locations from 4 to 2 but it will more than double the cost per AP, which is a wash.  There will be scenarios where either option will be more appropriate.

Our second option is the new beam-forming radios.  I’ll cover that in more detail in the future. Since the 2.4GHz versions of these units won’t be out for another 4 months or so, there is no hurry.  However, they add another 4-6dBi of gain over a sector antenna and 8-9dB of antenna gain over any other beam-forming AP other than Vivato.  Add in dual-polarity, which my field testing shows to be worth up to 3dBi more usable gain, and a 16 degree beam pattern to reduce noise (I’m extrapolating from the 5.8GHz beam-forming unit that was announced.  The final specs may vary.) and that’s enough gain to penetrate an extra wall or almost quadruple the coverage distance to a client.  I’ll go into the difference between between all the beam-forming units on the market in my next article.

Now we have even more tools to play with for Guerrilla WiFi.  Theoretically, it wouldn’t be hard to build a 1Gbps AP to work across multiple frequencies with beam-forming and GPs for less than $4000.  Taking this concept even further, it also wouldn’t be hard to create a load-balanced, business quality, multi-frequency design that could bond these frequencies together for very high-capacity throughput.  There are other variations of this for backhaul, redundancy, and uptime.  It’s possible, with a little networking work, to create a mission critical design capable of delivering tens or hundreds of megabytes to a CPE for less than $400 on the CPE side.  This type of system could easily deliver 99.999% uptime using unlicensed frequencies, even with scheduled maintenance.   And don’t get me started on 900MHz mobile options.  The hits just keep on coming.

PREVIOUS CHAPTER (16): Setting up large scale wireless video surveillance

NEXT CHAPTER (18): More Details on Deploying Large Scale Wi-Fi Networks

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About the Author

Rory Conaway is president and CEO of Triad Wireless, an engineering and design firm in Phoenix. Triad Wireless specializes in unique RF data and network designs for municipalities, public safety and educational campuses. E-mail comments to rconaway at triadwireless.net. Rory writes regularly for MuniWireless.com.

About Rory Conaway

Rory Conaway has been in the IT and Wireless Industries for the past 25 years as an author and consultant. He currently operates a growing WISP operation in Southern Arizona. He consults with investors, manufacturers, and WISPs, and develops financial business models for startups. In addition to writing articles in industry publications such as Mission Critical Magazine, Mr. Conaway also writes the series “Tales from the Towers” that can be found on various such as www.triadwireless.net and www.muniwireless.com. He has also engineered several wireless designs such as S.P.I.R.I.T. and Guerilla Wireless as well as building integrated wireless and video surveillance for airport security, municipal and critical infrastructure, SCADA systems, and hotel/MDU deployments.


  1. Hi there, I run a very small WISP in rural Alabama. Whitespace radios look very attractive to an outfit like ours, where we are limited with our tower placement (there are only so many places you can put base stations in an extremely rural environment like ours). We need as much tree penetration and range as possible, and we can’t have a ton of interference because often, even with 900, the signals are in the -80s.

    We started with 900Mhz, then added 5.8 Ghz (low cost option for customers with perfect LOS, rarely used), and most recently added 3.65Ghz. 3.65Ghz has been a nice compromise between 5.8 and 900 because of the high sensitivity and no noise.

    Our biggest problem lately is that we still have customers that we cannot reach with anything other than 900 – and 900 becomes more and more unusable every day because of interference. We are pretty sure the wireless cameras used by hunters are the main culprit – I think there is a lot of that gear sold outside of regulations (way too much power).

    My dream is to get rid of 900 because it is a daily headache, or come up with a solution that could mitigate interference (we are using WaveIP btw) – would love an interim solution until white space radios or something similar becomes available.

    Capacity is not a huge issue – we are talking fairly low density (30 customers right now on the 900 system, rest of our customers are on 5.8 and 3.65). We haven’t even sectorized our 900 yet – I’m not sure that sectorization would help because most of our noise seems to be localized (not at the AP). We use a MTI Wireless Edge HPOL Omni and one AP for now. Another reason we haven’t sectorized – I hate putting any more money into 900mhz infrastructure.

  2. 900MHz noise simply not going to get better going forward. However, these radios give you your best chance of getting through the trees and minimizing the noise. Dual-Polarity is worth another 3dB or more depending on the trees. AirMax works very well in high-noise environments. Sectorizing the antennas will reduce noise even more. And keep in mind that these radios will also have frequency hopping as the beta firmware is already released.

    With this combination, more than likely you will make their life more miserable than they will yours.

  3. Rory,

    You mention beamforming 900Mhz in the header of your article: “It focuses on how WISPs can build a network using the 900 MHz frequency band instead with the new beam-forming equipment”… that wasn’t announced yet was it? (I thought it was just 5.8 and 2.4) while a beamforming 900Mhz AP would be awesome, I’d imagine the panel to be gigantic…just look at the size difference between the 17db 5.8ghz sector and the 13db 900mhz sector for instance.


    Rory is spot on here, you’ll get a huge improvement if you upgrade to dual polarity mimo with frequency hopping, GPS sync, and a decent polling protocol (AirMAX). Sectoring will help even if interference is localized on the CPE end because you’ll be getting more gain coming into the CPE (over-riding noise). Sectors are a far cleaner implementation than amplifying an omni any day.

  4. TB,
    Sorry, I think faster than I type. I didn’t mean to imply beam-forming in 900MHz. I was suggesting that with beam-forming in 2.4GHz, which I touch on at the end of the article, is where we are going next. There is no beam-forming in 900MHz as far as I know.

  5. “There is no beam-forming in 900MHz as far as I know.”….or that you’re allowed to talk about 😉

    Either way, it would be cool to see, but I’m afraid the implementation would be an absurdly large panel, so until we can break the laws of physics and shrink radio wavelengths without changing their frequency….I think beamforming at 900mhz is a pipe dream…..I hope UBNT proves me wrong.

  6. Although I don’t think we will see beam-forming, we may see another feature in 900MHz, we may see the GPS option which will help.

  7. Agreed, I don’t see why they wouldn’t release GPS sync for 900mhz where channel re-use is essential.

  8. I’m pretty sure it’s being worked on. There are so many new products coming out, it’s simply a matter of priority and staff bandwidth. I just got a pair of the 900MHz Locos in today so I’ll start testing those over the weekend

  9. I’m interested in seeing how many buildings/trees they will go through compared to the GE-MDS Mercury radios. Basically it’s a test of power (30dBm on the Mercury’s) versus 802.11N 2×2 MIMO.

  10. Henry Cohen says

    Just a quick correction to an otherwise well articulated and salient article: The “white spaces” in discussion are the unused Hi-VHF and UHF-TV channels 7-51, or 174-216MHz and 470-698MHz.

    One other issue making these VHF and UHF frequencies less favorable is a larger freznel zone required compared to 900MHz, and significantly larger compared to >2.4GHz.

  11. Thank you Henry. No argument on the fresnel zone theory but the idea is that this technology gets deployed for shorter ranges in metropolitan environments and urban environments. Fresnel zone doesn’t come into play there if you are shooting for indoor deployments.

    I agree in rural it has more value for longer ranges where other licensed frequencies also have less interference. The only question to me goes back to how far will a $149 Ubiquiti radio go through trees. If it’s less than 1 mile, then we need White Space simply for tree penetration. Then it becomes a cost/benefit issue. The size of the market in the U.S. is pretty limited. Compare that with the size of the market in the rest of the world where until it’s approves there, limits the volume manufacturing, making CPE devices pretty expensive. I don’t think you will see a White Space CPE stand alone device less than $200 and an outdoor rated AP for less than $1000 for 24 months. You can barely buy mesh WiFi radios with any advanced technologies beyond the simply omni-directional antenna concept for less than $1000 and those have been out for years.

  12. Henry Cohen says

    No question that with short haul muni-wireless type deployments freznel zone issues are virtually non-existent. I mentioned it only because many of the conversations I’ve been reading over the last couple of years (including those on the Ubiquity forums as you’re well aware) were by those WISPs looking to white space frequenccies for their longer range back hauls.

    Be that as it may, I do also agree the business model and pricing structure are still quite unclear and excitement over this band may be premature.

  13. Tomas Pratt says

    The UBNT 900 Mhz radio appears to have major power problem on the station side.
    The spec reads plus or minus 26 db at the port ( so it could be 24 dBbi plus only 7.5 dB station side means it is likely between 31.5 and 33.5…which means it is likely underpowered by one half of anything else on the market.

  14. Tomas Pratt says

    I thought I would mention, further research on the UBNT 900 MIMO station reveals it may have only a single antenna port for external connection via sma cable. This will increase my costs of deployment and be a little unsightly for many homes. Shall I gather this negates the potential of MIMO for throughput but may allow me to use an external yagi or panel to make up for the lost power? I note there is no adaptive modulation in the OFDM so I would likely have to set system for lowest possible performance in order to maximize number of user stations. This would significantly reduce my throughput , likely to 16 QAM or a few Mbps. Why can’t they make a dual polarity or active diversity antenna that has 10 or 12 dbi so that I can use the full power potential of the 900 Mhz band and not have to roll a truck to change polarities for future interference issues?
    If I am mistaken in any of this I would appreciate corrections. Thanks.

  15. The Loco has a single connector which makes no sense to me. I wouldn’t have put any connector on it. The Rocket has dual connectors.

    There is also supposed to be a reflectorized model of the Loco coming out.

  16. I agree with Thomas. 31 EIRP just doesn’t cut it.