When Wireless Broadband was on everyone’s lips 8 years ago, we all thought we would be able to use our laptops everywhere. About the same time that Earthlink and Metro-Fi realized you can’t make “Free” pay off for their investors, 3G started moving in to fill the void. Then YouTube came along, pushed 3G to the ground and said, “I spit in the face of your puny bandwidth (insert Austrian Accent Here).” 3G then said, “Oh yea, my big brother, 4G, is coming and he will take care of you. You will be sorry.” YouTube said, “BTW, meet my friends, Hulu and NetFlix”. 4G took one look at these guys and said, “I’ll fight you guys but you have to have one hand tied behind your back, both legs tied together, and we are only going to fight for 10 minutes. After that I win and you have to go home”.
In the meantime, the Big Land Barons who wanted to expand their holdings went to the Lords of the FCC and whispered in their ears, “Our lands aren’t big enough and we have rocks and trees in our way. If you kick out the squatters that are on some of your prime land, we will buy it from you and enrich your coffers.” So it was that the Lords of the FCC created The Great Plan. They took The Great Plan to the Council of Kings and said, “We need you to pass a law to kick out the old squatters. We will tell them they are doing the right thing for the country and then resell their land for a fat purse. We will then give some of the land to new squatters for free who will also develop the land and make our Kingdom better.”
When the Sheriffs heard of The Great Plan, they said unto the Lords of the FCC, “We have worked our lands to death and need new lands. The Bandits are smarter, faster, and meaner. We need new lands all across the Kingdom and in every village so Sheriffs are closer and can work together to stop the bandits.” In the Great Plan, the Lords of the FCC decided to appease the Council of Kings and not ask them to use the coin from the Great Sale to pay for the Huts and Barns and lookout towers with 360 degree coverage and an 8” spyglass mounted in the middle. Oh no, they were more clever than that. The thought they could fool the Land Barons into paying for the not only the land, but for the huts and barns and lookout towers with 360 degree coverage and an 8” spyglass mounted in the middle (I’m a huge Arlo Guthrie fan). The Lords of the FCC said unto the Sheriffs, “we will grant you new lands in order for you to protect the peasants from the bandits.” Thus, the Lords of the FCC were happy with their new plan and went forth to preach it. And so it was written, and so it was done.
To the peasants, the Lords of the FCC preached, “We are confiscating ill-gotten lands given away by the former Lords of the FCC in order to give you better services and enhance your cities and huts”. Prosperity will grow across thy land and all will be connected to the Great Message.” And the peasants and other squatters did not object for it was not their land being confiscated.
The FCC executed The Great Plan. They confiscated the lands and sold them off to the new Land Barons. The new Land Barons offered the Lords of the FCC a great amount of coin, thus ensuring that smaller Land Barons weren’t going to be able to buy the new prime land. At the same time, the FCC told the Sheriff’s, “We have decided that your land is too valuable to just give to you so we are going to sell it to the Land Barons. However, we are going to make the Land Barons build you houses and barns and lookout towers with 360 degree coverage and an 8” spyglass mounted in the middle.”
When the Land Barons read the scrolls written by the Lords of the FCC on how to use the new lands designated for the Sheriffs, they went to the Lords of the FCC and said, “We do not want that land. It is too costly and cumbersome to build the houses and barns and lookout towers with 360 degree coverage and an 8” spyglass mounted in the middle.” They then paid the Lords of the FCC a great amount of coin for the land they had already bought and left. The Lords of the FCC quickly left the Great Hall so as to not have to explain to the Sheriffs why their plan had failed and the Sheriffs still had no new land.
In the meantime, many Sheriffs sent many riders to the Lords of the FCC requesting the new land. The Lords of the FCC spent long seasons pontificating on what to do about the land they confiscated for the Sheriffs. After a very long time, they came up with The Little Plan. The Lords of the FCC decided in the Little Plan that they would grant the land directly to the Sheriffs. They said, “We will give you the lands, but we will tell you what you may build on the land. You may only build huts and barns and lookout towers with 360 degree coverage and an 8” spyglass mounted in the middle as to our specification.” The Sheriffs said, “We have no coin to build huts and barns and lookout towers with 360 degree coverage and an 8” spyglass mounted in the middle. Where is the coin from the Land Barons for sale of the other Land?” The Lords of the FCC said, “It has been confiscated by the Council of Kings and redistributed to friends of the Great Messiah and the Council of Kings. There is no coin for you. You have to find your coin from somewhere else to follow the Llittle Plan.” And that my dear friend, brings us to today.
Basically State and Municipal Law Enforcement sort of got left out in the cold when it came to the new 700MHz frequencies. It’s not that they didn’t eventually get the bandwidth they needed; they were told that they must deploy the most expensive technology available to actually use it, LTE. This edict was given at a time when government budgets are getting hammered from top to bottom. Since public safety has no money to deploy even two tin cans and a kite string, I don’t think we will be seeing high-speed bandwidth for mobile public safety in the near future in most major cities.
We started discussing a project last month where I needed to move 1Gbps or more of bandwidth 20 miles. There is new wideband equipment being released as we speak and I haven’t finished my homework so I’m going to table that phase of the project for a while. Since the project has several components, I thought we might jump over to the third tier, the last mile, first. The last tier of that project actually ties into the story you just read.
As many of you live in areas where your vegetation grows over 8’ and doesn’t make you look like a porcupine when you bump into it, I thought tackling a project like that would be a good idea. Living in the desert has made me oblivious to the fact that there are cities across the country where houses are surrounded by trees that could double as space elevators and have lights on top to warn airplanes away. Throw in vegetation that comes with its own zip codes, and there are places where the term Non-Line-of-Sight (NLOS) takes on a whole new meaning. Every cell tower that I saw in the area was so tall I figured I was coming down with magic beans if I ever had to climb one.
However, as high as all the cellular towers were around the county, there is no chance that 80% of the houses are ever going to see one from their front window. That means we are left to figure out how to get through the trees. In Chapter 17, we talked about how 900MHz can punch through vegetation, houses, and really mean neighbors. Since that article was written, some of the data I’m reading on current 900MHz deployments is pretty promising. However, deploying 900MHz can be kind of like trying to give a cat a bath, you get a trip to the emergency room and the cat just gets really ticked.
A quick review of 902-928MHz band means that we only have 26MHz of bandwidth to work with. By comparison, 5.8GHz has about 100MHz of spectrum to work with and 2.4GHz has approximately 60MHz. In practical application, a 5.8GHz system can deploy 4-5 APs with non-overlapping 20Mhz channels and a 2.4GHz tower can have 3 non-overlapping 20MHz channels. The 902-928MHz band is a little tight meaning that we only have room for one channel if we are using a standard 20MHz channel.
That means we need a different strategy. Assume that our coverage area is 360 degrees. Most wireless products in 902-928MHz band use down-converted WiFi chipsets. That means the throughput will be the same as 802.11a and 802.11g radios with the same channel widths. Motorola uses an FSK scheme instead of an OFDM modulation scheme that trades off a lower throughput for a better s/n ratio.
Both systems have a bandwidth rate in Mbps that is numerically about ½ the spectrum size. For example, Motorola uses an 8MHz channel and has a maximum capacity of 3.3Mbps total aggregate. WiFi down-converted systems with a 10MHz wide channel will have an aggregate throughput of about 5Mbps. Some Wifi systems have a little higher throughput but all of this assumes a connection at the highest level modulation rate. The other side of this equation is the whereas Motorola will work with a s/n ratio of 3dB, a WiFi system needs a minimum of 10dB.
802.11N 2×2 MIMO has a different formula. Not only is the efficiency of the protocol better than 802.11a/g, the processors are typically faster, more efficient, and have better sensitivity ratings than older a/g chipsets. From there, throw in the fact you are transmitting two signals in the frequency spectrum. The end result is such that the rule of thumb of throughput is approximately quadruple the channel frequency width. A 10MHz channel width should have a theoretical throughput of about 40Mbps.
The lack of channel bandwidth in the 900Mhz band is one reason that it never caught on very well. The other reason is that the noise floor is significantly higher in the 900MHz band than it is for other unlicensed bands. Because of the NLOS nature of the band, it’s heavily used for SCADA, baby monitors, cordless phones, ham operators, and many other things. In any major city or suburb, you probably have noise levels in the 50-60dBi range which makes it very difficult for most WiFi based chipset radios to operate.
The second problem with the 900MHz band is that laying out a network design for a large area is significantly more difficult. In 5.8GHz, the antenna beam patterns are pretty well defined, the signal can’t penetrate lace curtains, and it has limited reflection characteristics compared to the 900MHz. For those reasons, it’s relatively easy to define optimal tower locations. 2.4GHz sort of falls in between but the noise figures for 2.4GHz will still be lower than 900MHz at ground level. At the tower locations, I can imagine it’s a toss-up as to which frequency band will have a higher noise level between 900MHz and 2.4GHz depending on the population density around the tower. 900MHz on the other hand, bounces around like a Superball in bathroom, needs a Fresnel zone the size of Delaware, and just feels a little tickle when busting through a brick wall.
Verizon is probably about halfway to deploying their 700MHz LTE system across the country. The difference between them and us is years of experience in 800MHz and software that costs more than my last car. In addition, the antennas they use can be remotely adjusted on the fly in micro increments to fine-tune coverage zones. Since I haven’t heard anything about WISP operators learning to levitate, that means lifts or tower climbs, both of which are significantly more difficult than moving a mouse.
With these issues, why would anyone want to use 900MHz band? Well, that is because we really don’t have much of an option. It’s our only option through Sherwood Forest and we need to figure out how to make that work. Let’s first define the environment as being rural which reduces the variables and takes the noise figure off the table. Then let’s assume we are covering about 2 miles in every direction and we have trees all around. Throw in the decision to limit users to a maximum download speed of 4Mbps and upload of 1Mbps for Round 1 and the system design gets a little easier.
The first issue is the AP configuration. Since 802.11n 2×2 MIMO is pretty much standard in the WiFi industry today, the use of it in 900MHz is unique. The Ubiquiti Rocket M900 with a dual-polarity 13dBi 120 degree sector antenna makes an attractive option. Assuming a 20MHz wide channel, the AP should be able to support a theoretical maximum of 60-100+Mbps of aggregate bandwidth. However, we have to put 3 of them on the tower to cover 360 degrees and I guarantee we aren’t the only squatter on that land. That means that we are going to have to limit the channel size to 5Mbps to avoid overlapping channels and minimize interference. It also means that the maximum theoretical throughput is about 20MHz and from my experience, I would assume 10Mbps per AP.
If we use the 10-1 client/bandwidth formula, then we should be able to support 20 users per AP with a 4Mbps down and 1Mbps up scheme. That’s also 60Mbps of total capacity per tower. Those aren’t bad numbers if you are charging $40 per month per client. If you are using Water Tanks for example, where you can isolate the antennas from each other, you may have the option of using 10MHz channels. There are also antenna shields from firstname.lastname@example.org that promise that you can use 10MHz channels and radios on the same channel won’t interfere with each other. However, this could also be accomplished with GPS synchronization with close to similar results if the Rockets every support this feature.
900MHz in rural areas can also be a poor man’s Public Safety System. With the right RF engineering and other design features, the same system could deploy 2-10Mbps or more to a police vehicle. A system like this should cost about 1/20th to 1/100th the cost of an equivalent LTE system. I’m not suggesting that this type of system be deployed in any cities or suburban that have a high noise issue in that band or that plan on deploying a 900MHz smart grid system, but since over 90% of this country is still rural, this capability can be added for at a cost of under $3000 per AP with each AP covering 4-36 sq miles. The cost for the cars is less than $400, worst case. Compare that with a typical LTE deployment that the police clearly have no money for, and it’s a pretty good alternative for small towns and rural counties.
900MHz is both magical and a pain to work with. It opens up new opportunities in rural areas where wired fears to tread for financial reasons. At the same time, it’s also a little more expensive than traditional 5.8GHz PTP systems but a lot less expensive than most street level 2.4GHz muni systems. However, when compared to 3G speeds, 5GB caps, spotty coverage, or even worse, no coverage, it’s still a far better option. The application of 802.11n 2×2 MIMO technology now makes it a more viable technology for rural areas.