A femto basestation, usually referred to as a femtocell, provides mobile phone reception within the home, giving unlimited voice and data usage for a monthly fee.
While traditional basestations provide wide area coverage, a femtocell provides wireless coverage in a small area.
The femtocell routes mobile traffic to the network through the user’s broadband internet connection.
The femtocell is based on proven wireless infrastructure standards (UMTS, CDMA). Unlike a macrocell network, which aggregates tens or hundreds of basestations onto the core network, a femtocell gateway must manage thousands or even millions of femtocell nodes.
Femtocells must provide the quality of service (QoS) expected from a basestation at a cost similar to a handset, and must be small and user installed.
Transmitting at low power – on the order of 100mW – femtocells must be aware of the wireless environment to mitigate interference and meet regulatory requirements. 3G femtocells must monitor UMTS channels to detect nearby basestations, as well as GSM channels to establish cells for potential handovers.
The femtocell can be viewed as two functions: analogue front-end and baseband processor. The front-end converts the digital data stream into an RF signal and vice versa, and entails trade-offs between integration and performance.
Although discrete designs can provide optimum performance, the cost would be prohibitive. Conversely, a fully integrated approach may be cheapest, but performance may not be sufficient.
Femtocell implementation
The figure shows a femtocell for local basestation operation in UMTS band one, as well as monitor signals in the 850MHz, 900MHz, 1,800MHz, 1,900MHz and 2,100MHz bands.
On the transmit side, the digital baseband feeds a 12-bit parallel data stream to the AD9863 transceiver, which converts it to an analogue I/Q baseband signal.
The baseband signal is converted to RF (by an ADF4602) and amplified by the PA gain stages, and sent to a duplexer. A power detector monitors the RF output.
A single-pole, six-throw (SP6T) switch selects which transmit or receive monitoring chain is connected to the single antenna.
This signal chain provides 13dBm output power at the RF output connector, while meeting transmit ACLR specifications as defined in 3GPP standard TS25.104.
The receive chain includes surface acoustic wave (SAW) filters and SPDT switches for monitoring the main path. The matching blocks consist of a simple series/shunt inductor for each receive port.
The RF signal is downconverted and filtered to a baseband I/Q signal. The baseband signal is sampled by the dual on-chip ADCs and converted to dual 12-bit parallel bit streams for the digital baseband.
The femtocell requires an accurate reference clock (±0.1 ppm) to meet 3GPP specifications. To achieve this, several possibilities exist, including GSM macrocell synchronisation via the monitoring receivers, GPS synchronisation, and IEEE 1588 precision timing protocol. In some instances, a combination may be implemented.
Intelligent output power
To mitigate interference, the femtocell must set its output power intelligently where multiple femtocells are nearby (e.g., in an apartment complex). Each femtocell should transmit at lower powers to avoid same-frequency interference.
Also, the femtocell cannot cause interference to geographically neighbouring macrocell basestations operating on the adjacent channel, as this would create dead spots for nearby mobile phones connected to the macrocell network.
The femtocell will thus have an adjacent channel protection requirement, forcing it to measure the power in the adjacent downlink channel and set its own power accordingly so as not to obstruct the macrocell signal.
To keep costs low and for easy customer installation, these interference mitigation techniques should be automatically initiated when the box is first turned on by the user, and updated at regular intervals thereafter.
Thomas Cameron and Peader Forbes work for Analog Devices