Femto cells are miniature base stations — low-power cellular access points for the home environment. Plug a femto cell into your cable modem or DSL router and you light up 5 bars of CDMA or UMTS cell coverage in your home. If you live in a small town (like I do) that has been arguing about the location of cell towers for close to eight years, you will be a happy camper. In the next 12–24 months, virtually every mobile operator in the known universe will be offering femto cells to consumers.
But femto cells are not without their technical hurdles. One of those challenges is the requirement for femto cells to behave nicely in the overall macro cell radio frequency [RF] system. In other words, when thousands or millions of femto cells interact with the larger cellular infrastructure in the RF domain, they must mitigate the potential interference with other femto cells and with the surrounding macro cell network.
Most people think of femto cells as Wi-Fi routers or access points with different radios. But femto cells are quite different from Wi-Fi access points. When Netgear (NASDAQ:NTGR), D-Link, or Buffalo ship consumer-grade Wi-Fi routers or access points, they rarely lose sleep over their products’ interaction as a larger system. Wi-Fi operates in unlicensed bands on a world-wide basis — and in the world of high-volume Wi-Fi products, each product is considered autonomous as long as it passes the test for regulatory compliance.
Some Wi-Fi products automatically select non-interfering channels and adjust their transmit power to avoid interference, and the whole next generation of Wi-Fi products based on 802.11n will use MIMO technology to punch through the noise. Even so, few consumer designs consider their behavior in a larger system of millions of chatty little radios. Since no one has responsibility for the good behavior of the overall system, Wi-Fi is your basic free for all. If your Wi-Fi router is interfering with your neighbor’s, he can tell you to do something about it…that is, if he can find you.
Back to femto cells. Unlike Wi-Fi, femto cells will interact with macro cell towers and other femto cells in close proximity. As a result, every femto cell must have a means of listening to the environment around it, and then adjusting approprately to mitigate interference. What does appropriately mean?
It turns out that each femto cell manufacturer will have its own method of mitigating RF interference. But there is, in fact, no standard means of doing so at the moment. Each femto cell producer will have an opportunity to thrive on inventions that outshine the competiton in this area, or crash and burn on something that hardly works at all. Stay tuned. The next 12–18 months will be interesting.
Full disclosure: Employed by AIRV at time of writing.