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Femtocells - Signal improvements

Femtocells aim at extending communications link options inside business premises. E&T catches up with this emerging class of access booster.

Patchy indoor coverage has been an irksome issue for mobile phone users from consumers to the largest enterprise, but has only recently been addressed by deploying dedicated internal access points to extend signal strength inside premises. Though SMEs (small-medium enterprises) in particular have missed out, as femtocells emerged to solve residential coverage problems, while more sophisticated systems based on distributed antennae or micro cells have been available some years for large industrial or business sites. But the latter involve significant radio frequency (RF) planning, along with expensive onsite management on the part of the operator, and are not cost-effective for most SMEs.

Along came femtocells, with powerful units aimed at SMEs and smaller offices of larger enterprises. A femtocell is a small low-power version of the conventional cellular radio base station, initially designed just for indoor deployment in homes serving up to four simultaneous users, although the concept has now been expanded to include outdoor devices in thinly populated areas, as well as to commercial buildings. Unlike earlier large enterprise systems, the femtocell is plug-and-play, so that in principle it can be installed without much technical know-how. Enterprise femtocells come in two broad categories. First there are single unit systems, which may double as top-end residential units, serving up to eight simultaneous users. These have been used quite widely for small business sites where full coverage can be provided by a single cell, but cannot scale-up to cover larger buildings.

The second class, the 'true' enterprise femtocell, is not just more powerful – capable of supporting up to 16 or even 32 simultaneous users over larger distance of perhaps 200m maximum – but also capable of deployment in multiple cell configurations to cover large sites. The common factor, which is an essential property of all femtocells, is ease of installation, which must not require RF expertise.

Femtocell and SON

For femtocells in the second class capable of configuration in multiple cells, this requires the additional ability to configure themselves automatically in self-configuring networks (SONs), so that they can form a grid between them covering the whole site with a perfect '5 bar' signal. The cells communicate with each other and adjust their output power in coordinated fashion in an attempt to cover the whole designated area. 'They each recognise the other femtocells as being part of the same team, then start talking to each other over the corporate local-area network, acting like a single organism [so to speak],' says Keith Day, VP of marketing at femtocell vendor Ubiquisys.

Each of the femtocells in the grid might cover a floor of a medium-sized building, or one single open plan area of a larger one, allowing up to 16 or 32 simultaneous users depending on the unit. The total number of users per cell depends on the nature of the business, and would obviously be much larger where only a few calls are made than say for a sales team constantly on the phone. In the case of Network Norway's femtocell trial (see box-out), the largest site had about 200 users served by around eight femtocells, and bigger deployments than that are hard to find at present.

This comes to the nub of the debate over enterprise femtocells – the degree to which grids can scale up to very large industrial sites, which until now have been served by more complex pico cell approaches, or Distributed Antennae Systems (DAS), which require significant radio planning and are also more costly to set up and run.

There is near-unanimous agreement that femtocells are a great solution for SMEs and prove very popular, as was borne out by Network Norway's trial already mentioned; to be sure, a recent study by Alcatel-Lucent's Bell Labs showed that femtocell solutions cost 60 per cent less than pico cell or DAS deployments, and provide complete coverage of a site much more quickly.

Covering everything

When it comes to large sites, however, pico cell vendors, such as ip.Access that are moving down to the femtocell market, still argue that careful radio planning is needed to implement an efficient femtocell grid. 'In a large complex building, there is no substitute for doing it properly,' insists Chris Cox, GSM infrastructure solutions firm Ip.Access's director of marketing.

Cox cited two particular issues – lift shafts, which screen the radiation, blocking signals, and upper floors of tall buildings, where care has to be taken with hand off between the femtocell grid and the external macro cellular network, because many different external base stations are in range, causing potential interference problems.

Network Norway's head of strategy Tom Guldberg insists, however, that femtocells worked well in many-storied buildings, and actually improved coverage there. As for lift shafts, the problem can be solved by deploying a sufficient number of femtocells to ensure there are no coverage 'black spots', according to Ubiquisys' Day. 'Lift shafts are a problem because they're built to withstand fire, but only if you have a single femto access point trying to control the whole floor,' he explains. 'The answer is to try putting one each side of the lift and seeing if that covers everywhere. If it doesn't put a third one round the corner. This is 'coffee cup planning', and you can do it on a screen.'

It may be, though, that when the number of femtocells covering a site becomes very large, scaling-up to perhaps 100 or more, it becomes desirable to manage the grid as a separate sub-network within the larger external cellular macro network, according to Aditya Kaul, practice director for mobile networks research firm ABI Research.

'There is some debate over whether you will need a controller to connect tens, or even hundreds of femtocells, or have them going back directly to the operator's network,' he avers. 'If you have many femtocells it makes sense to have a controller, because you then have more visibility of what is going on, and local routing becomes easier to do.'

As with the beginning of client/server datacommunications networks, devices intended primarily to boost bandwidth can also double as management tools.

Independent femtocells

Yet while there is debate over the best architecture for large femto grids, the likelihood is that they will become cheaper for the operator to manage, as remote monitoring and configurability will be built-in. 'We found we do not need to go out and check femtocells once they are set up,' reports Network Norway's Guldberg. 'The femtocells are self-configuring and we can communicate with them from our offices.' This reduces ongoing costs significantly, because a relatively small centralised management team can serve a large number of enterprise customers.

There is, however, broad agreement that an enterprise femtogrid will involve more effort than installing a single domestic femtocell, if only because it has to be integrated with the enterprise's ICT infrastructure. The point, though, is that this can be done by existing IT staff according to specialist knowledge, according to Simon Saunders, chairman of the Femto Forum, the trade association for the femtocell industry.

'We see a shift in the amount of expertise needed for enterprise deployment,' Saunders says. 'The enterprise wants to know it is secure and the operator wants to know it is working properly across the whole site, so inevitably there is a greater degree of intervention than for a residential femtocell; but the precise location of each femtocell is not important as each cell can adjust its coverage. We are bringing it to the situation where people deploying those are skilled in IT, but they don't have to be RF experts.'

A quite different area of debate concerns the relationship between femtocells and Wi-Fi, which is already widely used for data communication and access to the Internet. There is no doubt that femtocells provide a better solution for voice, but there is a case for using them for data as well. Femtocells are now being used for SMS and email access from smartphones, and for this they score over Wi-Fi because 3G radio consumes much less power. 'If you have a femtocell you do not need to turn Wi-Fi on,' said ABI's Kaul.

Indeed as Guldberg noted, femtocells provide around 20Mbps – about the same bit rate as Wi-Fi – and have turned out to provide more consistent data coverage during trials.

The one clear difference is that femtocells operate in licensed spectrum, while Wi-Fi uses unlicensed, and this brings advantages for data, according to Saunders: 'Many devices share the unlicensed spectrum and it is hard to keep track of them and know where they are; so alongside Wi-Fi, lots of enterprises are looking for something with the Quality of Service guarantees that licensed spectrum can provide.'

Yet nobody believes that femtocells will oust Wi-Fi in the near future, and the FemtoForum has an initiative to integrate the two so that both can be deployed and managed via a single set of devices, and cooperate. This heralds a future where the distinction between femtocells and Wi-Fi will blur, also bringing together fixed broadband and mobile communications. *

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