Invasion of the femtocell

We are witnessing a quiet revolution in the delivery of cellular phone service. This revolution involves a drastic reduction in the size and cost of cell sites for small indoor areas. The technology that enables this shake-up commonly is referred to as the femtocell, but the terms microcell and picocell also are used. In this article we examine why they are useful and how they work.

The problem. Traditional cell sites require thousands of pounds of equipment to be installed on rooftops or steel towers. These sites come at high cost. They require expensive broadband connections between the cell site and the cellular telephone switch, costly backup power, painful site leases, and 24/7/365 maintenance. Further complicating the cellular operator’s job is the trend toward shorter tower height to accommodate tighter frequency-reuse and greater call density (users/MHz/square kilometer). As each site’s coverage shrinks, indoor coverage suffers and the need for distributed antenna systems (DAS) rises. A DAS is justified when the customer is an airport, convention center or large office building, but a DAS is usually too expensive for a single residence or small business.

Recognizing this need, cellular operators now offer femtocells to individual customers, often for free. Unlike the traditional cell site, femtocells are the size of a Wi-Fi access point and cost the operator just a few hundred dollars. The key to success with femtocells is the broadband Internet connection provided by the customer at no cost to the cellular operator. Transmit power levels are very low, as the femtocell is designed to cover just a few hundred square feet. In fact, it is essential that the femtocell has limited coverage in order to prevent harmful interference to the macro network, because femtocells use the same radio spectrum as all cell sites. Thus, femtocells are different than Wi-Fi cordless phones, which use 2.4 GHz signals and voice over IP (VoIP), independent of the cellular network. Once installed, the femtocell is transparent to the cellular user — he uses his phone the same way he does everywhere else.

Nomenclature isn’t important. Many different terms are used to describe tiny cell sites. Some consider the maximum range of a microcell to be 2 km, a picocell 200 meters and a femtocell
10 meters. We should not dwell on the differences. Despite the fact that “femto” means 10^–15, marketers apparently are not hung up on the math because these names often are used interchangeably. For example, AT&T Mobility calls its femtocell a “microcell,” because consumers are unfamiliar with the term “femto” and customer confusion is the enemy of sales.

How does it work? Most operators offer some femtocell service and hardware is available for all common airlink protocols including GSM, UMTS, CDMA, cdma2000, EV-DO, WiMAX and LTE. The cellular subscriber connects the femtocell base station to his broadband Internet connection (e.g., cable modem or DSL) via Ethernet. A GPS receiver usually is provided and access to GPS is required to set up the femtocell. GPS allows the operator to tailor operating parameters to location and also enables the operator to disable service if the base station is moved. Presumably, knowing the base station location also helps troubleshoot interference problems should they occur.

Typical femtocell base stations transmit 20 mW, which is a thousand times weaker than an outdoor base station and about one-fifth the power of a typical Wi-Fi access point. Range is limited to about 30 meters (about 100 feet) but depends on the height of the base station and the local clutter environment. The subscriber must enter the telephone numbers of up to 10 devices that are authorized to access the base station. Typically, up to four simultaneous users are allowed in residential settings and up to 10 are allowed in enterprise settings. Handoff between the femtocell and the macro network is seamless and most handsets will indicate the current serving network on the display.

The wireless standards organizations publish specifications for femtocells. Such standards vary somewhat by airlink protocol, but all of them have three main elements:

Benefits. The femtocell user benefits from more reliable indoor service, longer handset battery life and higher mobile data rates. (However, higher data rates may be a non-issue for users who roam onto a home Wi-Fi network.) The network operator benefits from increased customer satisfaction at low cost, thereby reducing churn and increasing profit margins. The operator also sheds traffic from its expensive wireless infrastructure to a low-cost broadband connection.

Issues. When femtocells were first introduced, the biggest worry for the operator was the potential for harmful interference to the macro network. Because femtocells use self-organizing techniques to detect nearby macrocells, adjust power, change frequencies and change spreading codes, much of this concern has waned.

Femtocells, however, do create an issue for E-911 calls because they do not incorporate the same infrastructure as the macro network to pinpoint a subscriber’s location. (A similar problem occurs with DAS calls.) Most femtocells require a GPS signal to set up the base station, but do not require GPS for continued operation after setup. Another 911-related problem is hardware reliability. The cellular operator is obligated to provide an extremely reliable network, including backup power, but there is no guarantee that the subscriber’s broadband connection is as reliable.

Finally, like all voice calls placed over the Internet, femtocell calls can suffer from poor quality of service due to latency.

Jay Jacobsmeyer is president of Pericle Communications Co., a consulting engineering firm located in Colorado Springs, Colo. He holds bachelor’s and master’s degrees in electrical engineering from Virginia Tech and Cornell University, respectively, and has more than 25 years experience as a radio-frequency engineer.

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