Femtocells were developed to aid 3G networks, but their real impact may be much greater on the successors: the 4G networks, WiMax and LTE.
Many of the issues facing femtos in the 3G world were addressed at last month's Femtocells Europe 2008, but the conference also considered how femtos could shape the delivery of fourth generation services.
Indoor base stations have obvious benefits - they improve coverage for users and provide the operator with cheap backhaul over the user's own broadband. But operator trials on 3G networks have been slow to emerge, for reasons we've recently discussed.
The case for femtos is much clearer on the data-centric 4G networks which are due to succeed today's 3G networks over the next few years. Femtocells have to be bolted onto today's 3G networks, but can be planned into 4G networks. It's widely expected that femtos will be a fundamental part of LTE networks. The Next Generation Mobile Network Alliance has publicly promised to work with the Femto Forum, for instance.
Meanwhile, there have been suggestions that early WiMax networks might do away with macro cells at the outset, and launch with indoor cells only (a model that looks strangely like public Wi-Fi).
But the picture will be more complicated because none of these developments is happening in isolation.
Better by design
Femtos' fitness for 4G is not just a matter of timing. They will suit the technology well, and iron out difficulties with speed-boosting technologies added into 4G, points out Simon Saunders, chair of the Femto Forum. LTE includes MIMO, which uses reflections of radio signals to set up multiple paths between sending and receiving antennas - but this is much more of a benefit indoors, where femtos live, he points out.
"LTE and WiMax have a large number of modes," says Saunders, and the top data rates may only be achieved with femtocells in buildings.
This could mean that LTE will actually be led by femtocells before it is deployed on the wider network. Users could be sold a 4G handset and a femtocell, so they can use the new technology at home, and fall back to 3G outside while LTE is being implemented: "LTE won't necessarily need coverage everywhere," says Saunders.
LTE with a 3G femto?
To confuse things, though, there's an opposite possibility. With all this tricky MIMO ability, won't 4G femtos be costly to develop, and therefore unattractive? Saunders doesn't think so: "The thing about LTE or other 4G networks, is they will be building on the experience of the 3G femtocell. There are technical challenges, but the challenges of getting cellular infrastructure into consumer volumes will already have been done - 3G femto cells have cleaned the pipes for future femto cells."
The Femto Forum and the Next Generation Mobile Network Alliancewill be specifying 4G femtos, so they shouldn't suffer a delay or a price hike, says Saunders.