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Advanced DAS could aid femto’s enterprise debut, predicts RFS

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The much lauded femtocell could make the transition from domestic homes to larger scale commercial in-building coverage applications, by teaming up with advanced distributed antenna systems (DAS), predicts wireless technology leader, Radio Frequency Systems (RFS). The company's Vice President Wireless Indoor Solutions, Marc Kaeumle (pictured), has noted that advanced passive and active DAS architectures are fast evolving into forms that could ultimately assist femtocell technology overcome its mid-to-large enterprise challenges, allowing femtocells to play a role in the corporate in-building world.

"The femtocell is an exciting domestic coverage technology, particularly as we start to see more affordable consumer-level units," said Kaeumle. "It is destined to play an immediate and powerful role in ensuring that the premium data throughput of HSDPA-, WiMAX- and LTE-based services needed to provide broadband wireless data applications will soon be achieved deep within our homes."

In applying it to larger premises, said Kaeumle, the femtocell potentially presents enormous ‘scaling' advantages, as each femtocell brings with it a finite unit of both coverage and network capacity. This makes it an immensely scalable RF technology.

According to Kaeumle, although the femtocell does offer such theoretical scaling advantages, multi-femtocell networks deployed over larger corporate premises currently face three practical application challenges: core network connectivity (backhaul); network operation and management; and handover and signal ‘spillage' issues.

The first of these--establishing the link between base transmitter station (BTS) and the access gateway--presents a complex challenge, particularly when contemplated over hundreds of femtocells that might be required in a corporate campus. "In conventional macro cellular systems, the BTS-to-RNC link is supported over private high-capacity links. This approach isn't commercially viable for a network comprising a multiplicity of femtocells, so less costly compromises--such as using the public Internet to realise ‘femto-to-core-network' connectivity--have to be considered," Kaeumle said.

Equally challenging are the issues of cell-to-cell handover, and network operations and management. A network comprising hundreds of active femtocells will demand elaborate active unit monitoring and alarming, plus complex handover procedures. "The challenge with femtocell handover in a multi-cell architecture is that the small cell size dramatically escalates the rate of cell handover events per cell, when compared with conventional outdoor cell systems. As a result, the handover network burden is predicted to be very high indeed in multi-cell femtocell networks," he said.  

Similarly, signal ‘spillage' outside of the premises' boundary will tend to occur, due to the femtocell's inherently fixed ‘omni' RF distribution pattern. This will inevitably result in an increase in unauthorised femtocell access attempts by external callers inadvertently passing through the ‘spillage area', resulting in a ‘ping-pong' effect (where call access is attempted and subsequently rejected) that will further burden the network. 

"Today's active DAS topologies are bringing enormous functionality and benefits to the in-building domain," said Kaeumle. "Most obvious, is the shaped and sculptured RF coverage pattern they offer, which minimises spillage and maximises in-building signal strength. In addition, the systems are driven from a central BTS or NodeB, overcoming handover issues. Also, advanced active DAS systems are capable of efficiently extending network coverage far from the BTS or NodeB, using technologies such as RF-over-fibre, and other exciting transport mechanisms."

Furthermore pairing the femtocell with an active DAS system goes a long way towards solving many of these femtocell application problems. "In high-capacity femtocell applications, the advanced active DAS system can act as a coverage booster to the core-placed femtocell base station. In such applications, the active DAS can boost or extend the coverage of the femtocell, thus meaning that fewer femtocells--in terms of total cell count--are required for any given space. This overcomes many of the femtocell's current consumer problems. It's important to note that the inherent coverage flexibility and scalability benefits of the active DAS, particularly when compared with conventional distributed base station architectures. In extreme cases, we could see the femtocell as a pure capacity provider, and the DAS as the coverage distribution medium-a perfect pair."

Combining the femtocell with an active DAS will bridge the ‘domestic-to-corporate' gap for femtocell technology, ultimately seeing it play an effective role in larger corporate wireless data applications. "Advanced active DAS technologies will help overcome the shortfalls that the femtocell currently experiences outside of the domestic environment. Quite frankly, the two technologies could pair to form powerful combinations for future in-building solutions," Kaeumle predicted.