We'll explain the details for a 3G UMTS mobile phone system here, because this is the most common technology being used for femtocells today. Other mobile phone systems would operate in a very similar way.

The femtocell appears to the standard 3G phone as just another cellsite from the host mobile operator, and can be used by almost any phone including roamers visiting from other countries.

The mobile operators telephone switch (MSC) and data switch (SGSN) also communicate to the femtocell gateway in the same way as for other mobile calls. Therefore, all services including phone numbers, call diversion, voicemail etc. all operate in exactly the same way and appear the same to the end user.

The connection between the femtocell and the femtocell controller uses a secure IP encryption (IPsec), which avoids interception and there is also authentication of the femtocell itself to ensure it is a valid access point.

The figure below illustrates the system architecture and context for femtocell operation.

Inside the femtocell are the complete workings of a mobile phone basestation. Additional functions are also included such as some of the RNC (Radio Network Controller) processing, which would normally reside at the mobile switching centre. Some femtocells also include core network element so that data sessions can be managed locally without needing to flow back through the operators switching centres.

The key functions are integrated onto a single chip, such as the PC302 from picoChip. These and other chip manufacturers document the different components in more detail in their reference designs. In addition to these highly integrated chips, a radio frontend (such as from Bitwave) and a highly accurate frequency reference crystal oscillator are also required.

The extra capabilities of a femtocell demand it to be self-installing and self-configuring. This requires considerable extra software which scans the environment to determine the available frequencies, power level and/or scrambling codes to be used. This is a continuous process to adapt to changing radio conditions, for example if the french windows are opened in a room containing the femtocell.

Within the operators network, femtocell gateways aggregate large numbers of femtocell connections (typically 100,000 to 300,000) which are first securely through high capacity IP security firewalls.

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