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.