LAA and WiFi

University of Chicago students conducted a study on and near the campus, looking at how LAA (Licensed Assisted Access) affects WiFi. Cellular carriers began using LAA technology in 2017. This technology allows a cellular carrier to snag unlicensed spectrum to create bigger data pipes than can be achieved with the traditional cellular spectrum. When cellular companies combine frequencies using LAA, they can theoretically create a data pipe as large as a gigabit while only using 20 MHz of licensed frequency. The extra bandwidth for this application comes mostly from the unlicensed 5 GHz band and can match the fastest speeds that can be delivered with home routers using 802.11AC.

There has always been an assumption that the cellular use of LAA technology would interfere to some extent with WiFi networks. But the students found a few examples where using LAA killed as much as 97% of local WiFi network signal strength. They found that when LAA kicked in that the performance on nearby WiFi networks always dropped.

This wasn’t supposed to happen. Back when the FCC approved the use of LAA, the cellular carriers all said that interference would be at a minimum because WiFi is mostly used indoors and LAA is used outdoors. But the study showed there can also be a big data drop for indoor WiFi routers if cellular users are in the vicinity. That means people on the street can interfere with the WiFi strength in a Starbucks (or your home).

The use of WiFi has also changed a lot since 2017, and during the pandemic, we have installed huge numbers of outdoor hotspots for students and the public. This new finding says that LAA usage could be killing outdoor broadband established for students to do homework. Students didn’t just use WiFi hotspots when they couldn’t attend school, but many relied on WiFi broadband in the evenings and weekends to do homework. Millions of people without home broadband also use public WiFi hotspots.

LAA usage kills WiFi usage for several reasons. WiFi is a listen-before-talk technology. This means that when a WiFi device wants to grab a connection to the router that the device gets in line with other WiFi devices and is not automatically connected immediately. LAA acts like all cellular traffic and immediately grabs bandwidth if it is available, This difference in the way of using spectrum gives LAA a priority to grab frequency first.

LAA connections also last longer. You may not realize it, but devices using WiFi devices don’t connect permanently. WiFi routers connect to devices in 4-millisecond bursts. In a home where there aren’t many devices trying to use a router, these bursts may seem continuous, but in a crowded place with a lot of WiFi users, devices have to pause between connections. LAA bursts are 10-milliseconds instead of 4-ms for WiFi. This means that LAA devices both connect immediately to unlicensed spectrum and also keep the connection longer than a WiFi device. It’s not hard for multiple LAA connections to completely swamp a WiFi network.

This is a perfect example of how hard it is to set wireless policy. The FCC solicited a lot of input when the idea of sharing unlicensed spectrum with cellular carriers was first raised. At the time, the technology being discussed was LTE-U, a precursor to LAA. The FCC heard from everybody in the industry, with the WiFi industry saying that cellular use could overwhelm WiFi networks and the cellular industry saying that concerns were overblown. The FCC always finds itself refereeing between competing concerns and has to pick winners in such arguments. The decision by the FCC to allow cellular carriers to use free public spectrum highlights another trend – the cellular companies, by and large, get what they want.

It will be interesting to see if the FCC does anything as a result of this study and other evidence that cellular companies have gone a lot further with LAA than promised. I won’t hold my breath. AT&T also announced this week that it is starting to test LAA using the unlicensed portion of the 6 GHz spectrum.

More Pressure on WiFi

As if we really needed more pressure put onto our public WiFi spectrum, both Verizon and AT&T are now launching Licensed Assisted Access (LAA) broadband for smartphones. This is the technology that allows cellular carriers to mix LTE spectrum with the unlicensed 5 GHz spectrum for providing cellular broadband. The LAA technology allows for the creation of ‘fatter’ data pipes by combining multiple frequencies, and the wider the data pipe the more data that makes it to the end-user customer.

When carriers combine frequencies using LAA they can theoretically create a data pipe as large as a gigabit while only using 20 MHz of licensed frequency. The extra bandwidth for this application comes mostly from the unlicensed 5 GHz band and is similar to the fastest speeds that we can experience at home using this same frequency with 802.11AC. However, such high-speed bandwidth is only useful for a short distance of perhaps 150 feet and the most practical use of LAA is to boost cellphone data signals for customers closest to a cell tower. That’s going to make LAA technology most beneficial in dense customer environments like busy downtown areas, stadiums, etc. LAA isn’t going to provide much benefit to rural cellphone towers or those along interstate highways.

Verizon recently did a demonstration of the LAA technology that achieved a data speed of 953 Mbps. They did this using three 5 GHz channels combined with one 20 megahertz channel of AWS spectrum. Verizon used a 4X4 MIMO (multiple input / multiple output) antenna array and 256 QAM modulation to achieve this speed. The industry has coined the new term of four-carrier aggregation for the technology since it combines 4 separate bands of bandwidth into one data pipe. A customer would need a specialized MIMO antenna to receive the signal and also would need to be close to the transmitter to receive this kind of speed.

Verizon is starting to update selected cell sites with the technology this month. AT&T has announced that they are going to start introducing LAA technology along with 4-way carrier aggregation by the end of this year. It’s important to note that there is a big difference between the Verizon test with 953 Mbps speeds and what customers will really achieve in the real world. There are numerous factors that will limit the benefits of the technology. First, there aren’t yet any handsets with the right antenna arrays and it’s going to take a while to introduce them. These antennas look like they will be big power eaters, meaning that handsets that try to use this bandwidth all of the time will have short battery lives. But there are more practical limitations. First is the distance limitation and many customers will be out of range of the strongest LAA signals. A cellular company is also not going to try to make this full data connection using all 4 channels to one customer for several reasons, the primary one being the availability of the 5 GHz frequency.

And that’s where the real rub comes in with this technology. The FCC approved the use of this new technology last year. They essentially gave the carriers access to the WiFi spectrum for free. The whole point of licensed spectrum is to provide data pipes for all of the many uses not made by licensed wireless carriers. WiFi is clearly the most successful achievement of the FCC over the last few decades and providing big data pipes for public use has spawned gigantic industries and it’s hard to find a house these days without a WiFi router.

The cellular carriers have paid billions of dollars for spectrum that only they can use. The rest of the public uses a few bands of ‘free’ spectrum, and uses it very effectively. To allow the cellular carriers to dip into the WiFi spectrum runs the risk of killing that spectrum for all of the other uses. The FCC supposedly is requiring that the cellular carriers not grab the 5 GHz spectrum when it’s already busy in use. But to anybody that understands how WiFi works that seems like an inadequate protection, because any of the use of this spectrum causes interference by definition.

In practical use if a user can see three or more WiFi networks they experience interference, meaning that more than one network is trying to use the same channel at the same time. It is the nature of this interference that causes the most problems with WiFi performance. When two signals are both trying to use the same channel, the WiFi standard causes all competing devices to go quiet for a short period of time, and then both restart and try to grab an open channel. If the two signals continue to interfere with each other, the delay time between restarts increases exponentially in a phenomenon called backoff. As there are more and more collisions between competing networks, the backoff increases and the performance of all devices trying to use the spectrum decays. In a network experiencing backoff the data is transmitted in short bursts between the times that the connection starts and stops from the interference.

And this means that when the cellular companies use the 5 GHz spectrum they will be interfering with the other users of that frequency. That’s what WiFi was designed to do and so the interference is unavoidable. This means other WiFi users in the immediate area around an LAA transmitter will experience more interference and it also means a degraded WiFi signal for the cellular users of the technology – and they reason they won’t get speeds even remotely close to Verizon’s demo speeds. But the spectrum is free for the cellular companies and they are going to use it, to the detriment of all of the other uses of the 5 GHz spectrum. With this decision the FCC might well have nullified the tremendous benefits that we’ve seen from the 5 GHz WiFi band.


Cell-TowerRecently, the NCTA asked the FCC to make sure that wireless carriers don’t interfere with WiFi spectrum. I wrote a blog a few weeks ago talking about all of the demands on WiFi, and the threat that the NCTA is warning about is another use of the already busy WiFi spectrum.

Cellular carriers are using LTE technology to deliver 4G data. Cellular carriers today deliver 4G data and voice using spectrum for which they have paid billions (at least in the US and Europe). But in urban areas the LTE spectrum is already stressed and the demand for the existing spectrum is growing far faster than the carriers can find new spectrum to offload the extra demand.

The cellular carriers have had their eye on the 5 GHz unlicensed band of spectrum that is used for WiFi. This is a big swatch of spectrum that in some markets is larger than the band that some carriers have for LTE. Recently, various carriers have been experimenting with using this public spectrum to deliver LTE. Huawei and NTT demonstrated this capability last August; Qualcomm showed this capability at the CES show earlier this year. It’s rumored that T-Mobile plans to run a trial of this technology this year.

This new technology is being called LTE-U (for Unlicensed). NCTA filed at the FCC on behalf of their cable company members who use this WiFi spectrum to deliver WiFi for various uses such as distributing data wirelessly around a home or to bring data to settop boxes. They are worried that if the cellular companies start using the spectrum that they will swamp it and make WiFi useless for everybody else, particularly in urban areas where WiFi is under the most pressure.

That certainly is a valid concern. As my recent blog noted, the list of companies and technologies that are planning on using WiFi spectrum is large and growing. And there is already notable stress on WiFi around crowded places like large hotels, convention centers, and stadiums. The fear is that if cellular carriers start using the spectrum this same crowding will spread to more places, making the spectrum useless to everyone.

The cellular carriers argue that the swath of WiFi is large enough to allow them to use it without hurting other users. They argue that nobody can use all of the 400 MHz of spectrum in that band all at once. While that is true, it doesn’t take a huge pile of LTE-U customers at one time to locally overdraw the WiFi spectrum in the same manner that they are overloading the cellular spectrum today.

Engineers tell me that LTE uses the spectrum more efficiently today than does most WiFi technologies. This is due to the fact that the LTE specifications very neatly limit the bandwidth that any one customer can draw while most WiFi applications will let a user grab all of the bandwidth if it’s available. This means you can fit a lot more LTE customers into the spectrum that might be assigned to one WiFi customer.

There is a characteristic of WiFi that makes it incompatible with the way that LTE works. WiFi has been designed to share spectrum. When one customer is using WiFi they can grab a huge swath of spectrum. But when another customer demands bandwidth the system dynamically decreases the first connected customer to make room for the second one. This is very different than how LTE works. LTE works more like a telephone network and if there is enough bandwidth available to handle a customer it will assign a band to the customer or else deliver a ‘busy signal’ (no bars) if there us not enough bandwidth. The problem with these two different operating systems is that LTE would continually grab spectrum until it’s all used and the WiFi users are shut out, much like what you might get in a busy hotel in the evening.

The LTE providers say they have handled this by introducing a new protocol called LAA (Licensed Assisted Access) which introduces the idea of coexistence into the LTE network. If it works properly, LAA ought to be able to coexist with WiFi in the same manner that multiple WiFi customers coexist. Without this change in protocol LTE would quickly gobble all of the free WiFi spectrum.

But this still doesn’t answer the concern that even with LAA there could be a lot of people trying to grab bandwidth in environments where the WiFi is already stressed. Such a network never shuts anybody out like an LTE system will, but rather will just keep subdividing the bandwidth forever until the amount each customer gets is too small to use.

It will be interesting to see what the FCC says about this. This was discussed years ago and the FCC never intended to let licensed cellular holders snatch the public WiFi spectrum. I will also be curious to see if wireless carriers try to charge customers for data usage when that data is being delivered over a free, unlicensed swath of spectrum. And how will customers even know that is where they are getting their data?

I hope the FCC doesn’t let the wireless carriers run rampant with this, because I think it’s inevitable that this is going to cause huge problems. There are already places today where WiFi is overloaded, and this new kind of data traffic could swamp the spectrum in a lot more places. The wireless carriers can make promises all day about how this won’t cause problems, but it doesn’t take a huge number of LTE-U users at a cell site to start causing problems.