Will Franks - femtocell pioneer

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17 June 2009

Enterprise Femtocells: Easy to Underestimate

Enterprise femtocells, forming a grid of self-organising coverage, are genuinely new. So it’s not surprising that they are often misunderstood.

On one hand they offer the low cost and self-install simplicity of enterprise WiFi. Yet they also offer all the benefits of 3G cellular with massive data capacity. This is a heady combination, and promises to make in-building cellular affordable for small companies because the femtocells are based on mass-produced hardware, because no radio planning is required and because they can be self installed by the IT guys. They also promise to multiply the uptake of in-building cellular in large enterprises by providing added flexibility and simplicity at a fraction of the cost of today’s manually engineered pico/DAS systems.

John Spindler, in his article for Wireless Week, makes a couple of familiar assumptions about enterprise femtocells, which I’m happy to put right.

Assumption 1: minimum coverage overlap
The first assumption is that a grid of enterprise femtocells has minimal coverage overlaps. It doesn’t. In fact the femtos deliberately organise themselves into a grid with extensive overlaps.

What femtocells do that’s different to picocells is manage interference themselves in real-time rather than rely on the complex RF engineering that would otherwise be required. The femtocells obey the policies set by the mobile operators so will never produce unmanaged interference between each other or with the macro network. For example, femtocells in the grid use algorithms to ensure maximum spatial diversity between scrambling codes.

Signal dominance
Making the wrong assumption about coverage overlap leads to worrying about phones flip-flopping in and out of the macro network. But enterprise femtocells provide a strong indoor signal and also hold onto phones using “sticky cell” technology. Together these ensure that flip-flopping doesn’t happen, and that's true of any 3G phone.

Assumption 2: underestimating femtocell capability
The second assumption is that femtocells can support only a handful of calls and have a range which is a fraction of a picocell. Not quite: enterprise femtocells can range in capacity from 8-16+ users, and each one has a range approaching today’s picocells.

Femtocells in the grid don’t need to do soft handovers because of strong cell overlap, and they don’t have to handover very often because their range is not far short of picocells. There is always a battery life penalty when handing-over, but this is more than offset by the massive reduction in handset transmit power. In any case additional battery consumption only occurs when on a call, not when the phone is moving but idle. So the net effect is a major boost in battery life over the macro network.

In a grid of enterprise femtocells every femto handles not a handful of users but 8 or 16 or more - providing robust coverage and high capacity throughout the building. Most importantly, it does so more cost-effectively than Pico/DAS solutions that require radio engineers to deploy them.

Bottom line
Ever wondered why wifi access points tend to be installed in a modular fashion, rather than in a pico/DAS-style hierarchy? The answer is because they can be – it’s simpler, easier and cheaper. And the same is true of enterprise femtocells.