Femtocells - Turning Cellular Communications Inside Out
Simon Saunders, Chairman of The Femto Forum (www.femtoforum.org) explains the compelling reasons for the deployment of femtocells.
The founding function of cellular communications was to extend the reach of the phone outdoors into the wider world - so one could always be in touch. But over the past few years the cellular telecommunications industry has undergone a metamorphosis so that it is now most excited about a new cellular technology that turns this thinking inside out. Operators are now looking to bring their cellular networks inside the home. They plan to supplement their outdoor mobile phone masts with the polar opposite in the home - an extremely low power, tiny cellular access point - a femtocell. Femtocells not only improve mobile service indoors but also change the economics of the industry - bringing down costs for consumers and operators alike. But why have these femtocells come about and what challenges do they face?
The mobile industry has always grappled with two major issues:- coverage and capacity. This is as true today as ever with 3G networks providing poor in-building coverage and capacity concerns in the operator’s backhaul network meaning that mobile broadband services cannot support widespread usage. The reasons for this are very simple.
Mobile broadband radio networks generally use relatively high frequency signals due to the greater availability of bandwidth. However signals at these frequencies do not penetrate well through walls, thereby creating coverage issues inside buildings. Additionally, mobile users indoors require more powerful signals to penetrate walls which removes capacity and means fewer users can be accommodated in a cell. Somewhat ironically, 90% of data usage takes place indoors proving that mobile broadband isn’t all about mobility. And although 3G services have been slow to be adopted, the growing availability of usable mobile devices and flat-rate near-unlimited data plans have led to a huge leap in take-up which is in turn putting a major strain on backhaul capacity.
So mobile operators face a dilemma - mobile broadband clearly represents the future of their business, yet coverage and capacity challenges prevent them from delivering such services effectively and economically over the long term. In femtocells, operators have found a dramatically unconventional approach to resolving this situation.
Femtocells linked to a cellular network
A femtocell is a small device that plugs into a domestic broadband connection and can give perfect mobile phone coverage throughout a home or office, cheaper higher quality phone calls, faster mobile broadband and brand new services - all using existing cellular handsets and devices. In a stroke, operators eliminate both their capacity and coverage challenges. Instead of pouring more investment into their outdoor networks to provide extra capacity in the backhaul and over the air, femtocells offer a genuine alternative. Also by solving the in-building coverage issue from the inside out, the upper reaches of the spectrum which offer massive bandwidth potential are no longer out of consideration.
What are Femtocells?
But let’s take a step back - what exactly do we mean by a femtocell? For example how is it any different from a Wi-Fi access point? First and foremost, femtocells use licensed spectrum, unlike Wi-Fi, which mean they deliver assured service quality over the air and can only be offered by a mobile operator. The devices which operators are currently planning to launch will mostly support 3G networks although some 2G devices exist and WiMAX devices are planned. Femtocells can come in the form of a standalone access point or as part of a home gateway which would also typically include Wi-Fi and a DSL or cable modem. They must meet a consumer price point and must be entirely end user installed - in fact most femtocells are ‘zero-touch’ meaning they have no buttons and auto-configure once attached to power and a broadband connection.
Typically, a single femtocell will deliver voice services simultaneously to at least four users within the home, while allowing many more to be connected or ‘attached’ to the cell, accessing services such as SMS. Additionally, femtocells will deliver data services to multiple users, typically at the full peak rate supported by the relevant air interface technology, currently several megabits per second and rising to tens and hundreds of megabits per second in the future. But by removing the capacity hungry indoor mobile users from the outdoor network, femtocells also in effect improve performance for consumers outside. Indeed, for each additional indoor femtocell user, system resources are freed to serve about ten outdoor users. The femtocell behaves like a normal base station in that as users enter or leave the home their voice or data services are seamlessly handed over from or to the outdoor network as required.
Subscribers benefit from perfect cellular coverage and faster mobile broadband in the home as well as a more competitive voice and data tariff. Operators get optimum cellular coverage and more mobile usage in the home and dramatically reduced operating costs especially through backhaul - their single largest OPEX - and power savings. Equally importantly the cellular operators’ capital expenditure will significantly drop because accelerating data usage means they will inevitably have to heavily invest in their outdoor network in terms of new cell sites and backhaul to meet expected demand - something femtocells do at a fraction of the cost. In fact, Paul Jacobs, Qualcomm’s CEO, recently said that the gains in throughput available to femtocell users are “equivalent to that brought by the cell phone’s shift from analogue to digital.”
Finally, as mobile operators look beyond 3G to LTE or WiMAX, femtocells offer a new, dramatically lower-cost model for network rollout. For example, LTE femtocells could be employed using higher frequencies to deliver targeted intense high bandwidth requirements inside buildings - exactly where subscribers most demand it. Operators can then use their existing networks outdoors as demand slowly builds up and then use the scarce lower frequency spectrum to provide good quality LTE coverage across entire markets with the minimum number of outdoor network cells. As we have seen, the simple proposition of lower costs, for both operators and consumers, combined with improved coverage and services is compelling. Yet there are also challenges which must be overcome before widespread commercial deployments can become a reality.
Building standards and inter-operability
Femtocell deployments require not only a device in the home, but also equipment in the mobile core or cellular network that integrates potentially millions of femtocells into the network. Unfortunately mobile networks are designed to support thousands of base stations, not millions, so a femtocell concentrator is required. If the costs of deployments are to be kept low it is important that the concentrator is standardised thereby encouraging economies of scale.
Fortunately, not only are there a number of existing standardised network solutions that can accommodate femtocells, but the various vendors and operators have already collaborated in the Femto Forum to create a femtocell specific standard interface, which will be standardised within 3GPP. This standardisation effort will in turn have a major affect on reducing the cost of femtocell deployments due to economies of scale and competition. Crucially, unlike technological standards wars such as CDMA vs GSM and BluRay vs HD-DVD, consumers aren’t affected as their handsets will work with any femtocell.
The Interference Challenge
A major technical challenge that femtocell designers have faced comes in the form of interference. Many operators have only two or three distinct radio channels available for operating 3G networks. These operators have little or no scope for separating the spectrum used by femtocells from that used by their existing outdoor cell network. A key design goal for femtocells is then to be able to operate on the same frequency as the outdoor cells, without degrading the overall network performance with interference. This has to be achieved despite the dramatic increase in the number of cells involved in large femtocell deployments. Fortunately a strong inherent interference mitigation factor is provided by the fact that the walls of buildings that have served to keep 3G signals out also keep the signals in. However, clearly some signals emitted from the femtocell will escape the home. As such femtocells will include specific features, such as frequent monitoring of their surrounding radio environment combined with adaptive power control. This allows femtocells to realise substantial capacity gains while ensuring that the performance available to all users is at least as good as the network without femtocells.
Keeping it legal
But what if an enterprising subscriber decided to pack their femtocell in their holiday suitcase in order to enjoy discounted voice and data wherever they go? Such a situation is likely to constitute an illegal use of transmitting devices and an inappropriate use of the spectrum which is a vital business asset for operators. As a result operators have specified that femtocells are designed so that this does not happen.
Femtocells are highly intelligent devices, quite different from the illegal ‘boosters’ which are sometimes installed without an operator’s permission. Operators can identify the femtocell’s location using a variety of means, such as ‘sensing’ the surrounding network or by using the IP addresses of the DSL or cable network and restrict service appropriately.
Don’t panic!
While the analogy of the femtocell as a domestic cellular base station is instructive, it is also open to misinterpretation. While a femtocell carries out the same basic function as a mobile base station it uses tens of thousands of times less power - indeed even typically one-tenth of that of a Wi-Fi access point. Mobile phones themselves emit far less power when connected to a femtocell than in their normal environment, thereby helping to extend battery life. A key challenge for the industry is just getting this message across in an environment where some elements of the press are likely to respond without an understanding of the engineering facts.
All of these challenges - and several smaller ones - need to be addressed swiftly if femtocells are to realise their potential. However, they certainly aren’t holding many operators back.
Last year, Sprint-Nextel in the US became the world’s first operator to deploy femtocells but only for 2G voice - not broadband data where the main benefits reside. Japan’s SoftBank recently announced it is to deploy 3G femtocells in January 2009 and several other European operators are likely to follow in quick succession. It now looks like 2010 will be the year where mass uptake will occur.
Femtocells represent a huge new opportunity for the mobile industry and provide a service for customers where a pent-up demand already exists. The concept of moving the operator’s radio network into the consumer’s home turns traditional cellular thinking on its head. Yet the proposition holds so many benefits for customers and operators alike that its appeal is undeniable.
Professor Simon Saunders is one of the world’s leading authorities on femtocells and is currently chairman of the Femto Forum (www.femtoforum.org). As chairman Simon works to drive the uptake of femtocell technologies through open standards, market education and ecosystem development. He is an independent wireless communications specialist with more than 20 years industry experience. Simon has consulted for a range of companies including O2, Ofcom, NTL, BT, Motorola, BBC and many others. He is the author of books and articles and is a regular speaker at industry conferences. In May 2007 Simon was appointed to Ofcom’s Spectrum Advisory Board.
