Femtocell Fantasies

Femtocell Fantasies

ADC John Spindler

ADC John Spindler

John Spindler, vice president of product management for provider of high performance fibre connectivity and wireless coverage and capacity solutions, ADC, on the rise of the femtocell.

Everyone loves an easy solution, and whenever one comes along there is a strong tendency to apply it to a wide range of problems for which it was never designed. Femtocells are a great example.

Anyone who has been paying attention to mobile service trends over the past few years knows that network services are rapidly outpacing carriers’ ability to provide them. Traditional outdoor macro cellular networks were built for voice services, and everyone enjoys those services so much that there are now a couple of billion people who are basically joined at the hip to their mobile phones. In a saturated market, the only way for carriers to grow revenues has been to start adding data services like web surfing, email, games and video and to charge more for them.


Quality of service

So, starting with 2.5 and 3G networks a few years ago, providers began offering data services. With 4G networks on the near horizon, soon everyone will expect that they can play multiplayer games or watch reruns of ‘The Office’ on their phones with the same ease and quality of service they get when chatting to grandma.

But broadband cellular data services will require much better, more localised coverage and higher network capacity than today’s macro network architectures can provide. Simply put, high frequency cellular data services don’t penetrate buildings well. While there may be a few daredevils willing to play the networked version of Scrabble on their phones while charging down the M1, the vast majority of cellular data users will be inside buildings.

According to the femtocell fanatics, there’s nothing to worry about. Designed to bring spot coverage to isolated areas such as small stand-alone offices or apartment blocks, these small cellular base stations, about the size of a smoke detector, are now being touted as the be-all, end-all solution for all indoor coverage needs. The theory goes that we can simply stick up a handful of femtocells like raisins in a pudding to solve the indoor data service problem.


Flawed plan

Well, it just won’t work. Here are a few of the problems with this fantasy:

Multiple femtocells mean lots of interference. When more than one femtocell is used, interference among them will cause performance problems that degrade the quality of service. Making those multiple femtos play nicely with each other will take a lot of RF engineering, and it won’t be easy. Imagine a large apartment block in which every flat has a femtocell, and consider the plight of the mobile carrier tasked with gaining access to all of those flats within a reasonable time in order to tune each femtocell so it gets along with its neighbours. Even in an industrial space, the delicate interaction of multiple femtocells can be thrown off when a piece of equipment is moved and thereby changes the RF characteristics of the area.

It’s hard to create a dominant signal. A mobile phone that detects two relatively equal signal sources will hunt between the two, rather like a listener trying to monitor two conversations at once. Femtocells have very low output power (milliwatts compared with 20 watts for a macro cell), so users will find their devices hunting between equidistant femtocells.

Multiple femtocells mean lots of handoffs. With multiple femtocells in use, phones must ‘hand off’ the connection from one cell’s coverage area to the next as the user moves through the building. Handoffs reduce handset battery life, so using multiple femtocells will cause handoffs to occur much more frequently and drain batteries much more quickly than in the outdoor network.  

Over-provisioning will be inevitable. Each femtocell provides capacity for just a handful of users, so conference rooms, cafeterias, and other areas with high user density will need to be over-provisioned to provide enough network capacity for peak usage times. These investments in extra femtocells will go to waste during times of low usage.


Smart Plan

Those are some of the problems, but lest we dismiss femtocells as the latest radio technology fad, let’s consider a better way to use them as they were designed. The solution involves using just one femtocell and distributing its signal efficiently through a network of antennas called a distributed antenna system (DAS). In this scenario:

There is no interference and there are no handoffs. Since just one femtocell’s radio channels are present in the building, there is just one large cell. This eliminates multi-cell interference along with the need to hand off from one cell to the next as the user moves about.

There is one dominant signal. One signal source means one dominant signal. The DAS simply provides a uniformly strong signal throughout the interior of a building so user devices don’t hunt between interior and outdoor signal sources.

There is no need to over-provision. All antennas in the DAS have access to all of a system’s capacity, so there’s no need to add new cells for higher capacity.

Rather than causing a lot of problems by forcing femtocells into heroic roles for which they were not designed, mobile network architects should combine femtocells with a DAS that delivers strong, interference-free coverage and capacity.

ADC plays a crucial role in enabling its customers to deliver dynamic video, voice, data and wireless services that are increasingly essential to businesses and on-the-go consumers. www.adc.com

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