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Amid operators' excitement about femtocells and their potential to improve coverage, and to support convergence and new generation apps, there has been one issue at the top of their list of concerns - interference. New software reference designs from picoChip, the main supplier of silicon in this nascent market, take a significant step in addressing this problem allowing the tiny base stations to 'sniff' - searching out their neighbours, and self-configuring to synchronize and hand off to other femtos.

The femtocell needs to fit in with its surrounding network without negative effects on nearby connections, and unlike larger base stations, will not necessarily stay in one place if it is in the home or office. With some carriers, like Vodafone, talking about using femtos for outdoor metrozone roll-outs based on a dense collection of small cells, they require the ability for the devices to synchronize and hand off to one another in a 3G system, and to the GSM network when a user is roaming out of the zone. So while early femto adopters, notably Sprint Nextel with Samsung's Airave product, have focused on improving indoor coverage in a very simple deployment, the next wave of take-up will require a more complex approach involving thousands of femtos. At this stage, the interference and self-configuration issues become vital if carriers' high expectations are not to be disappointed.

This is especially true in an environment where femtos may be used to create zones - potentially a cost effective way for credit-crunched operators to build out 3G or 4G in well targeted areas with high value customers. In this scenario, they may behave almost like the Wi-Fi meshes, handing off to one another to create a 'cloud' of coverage.

In this context, picoChip has released software reference designs that support sniff (integrated network listening) in W-CDMA, GSM and/or TD-SCDMA networks, based on the company's PC202 single-chip femtocell platform. Called the PC8210, PC8211 and PC8810 Radio Environment Scanners, they perform cell search and decode automatically to implement self-configuration and automate provisioning. The W-CDMA RES capability is integrated into the hardware of the PC3xx system-on-chip product.

As well has handling configuration, synchronization and hand-off - and reporting metrics on the cell to help network planning - the sniff function will support entirely self-organizing networks of the type Vodafone has outlined in recent presentations. Currently, most of the interference management these require are handled in different ways by the femtocell OEMs, but each has its own proprietary algorithms, making mixed-vendor networks difficult. The picoChip designs also allow the femto silicon to run the manufacturer specific code.

The Femto Forum, the industry body supporting unified femto specifications and feeding into standards bodies like 3GPP, is also working on standards for interference management.

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