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Chapter 17: Who Needs White Space?

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.

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