Dr Heinrich J. Stüttgen is Vice President of NEC Laboratories Europe, responsible for NEC’s network and IT related R&D activities in Europe; he joined NEC Europe Ltd. as the founding manager of NEC’s Network Laboratories in Heidelberg. Previously, Dr Stüttgen worked at IBM Germany in various R&D positions. Dr Stüttgen is an IEEE Fellow, the Director of Conference Development and a Member the Board of Governors of the IEEE Communications Society. In 2009 he was Technical Program Vice-Chair of the IEEE ICC Conference in Dresden, the largest telecommunications research conferences world-wide. Heinrich Stüttgen was a Fulbright scholar at the State University of New York at Buffalo (NY) where he earned a Master of Science degree. He obtained his Doctor of Science degree in computer science from the University of Dortmund.
The majority of data traffic in cellular networks originates indoors, where macro cellular coverage is often patchy and data rates are low; femtocells are an attractive low-cost, licensed spectrum, plug and play solution to this problem. Femtocells use the customer’s own on-premise fixed-access connections to backhaul the traffic to the mobile network and help mobile operators to provide additional capacities at low cost. Although there are commercial deployments, femtocell is still an emerging technology and research and standards are still needed.
Across the world, operators have deployed 3G mobile networks and are preparing to deploy the next-generation radio access technology, LTE. Currently, there are two important weaknesses of current 2G/3G mobile network deployments - good indoor coverage and high capacity for broadband data services over cellular. Femtocells promise to alleviate these weaknesses. Femtocells? A femtocell is a small, low power cellular base station operated by a mobile operator. Typically, it is designed for indoor use in a home or enterprise context. As illustrated in Figure 1, the femtocell is connected to the mobile operator’s network via the user’s broadband connection (such as DSL or cable). Femtocells are based on the same radio access technology as macro cellular base stations and operate in the operator’s licensed spectrum. This implies that femtocells are controlled by the mobile operator and femtocell deployments must satisfy the same regulatory requirements (e.g. support emergency calls and/or lawful interception) as the macro network. In contrast to macro base stations, femtocells often allow access only for a specific user group. For example, they would allow access only to mobile phones of family members or employees of an enterprise. Since the radio technology and the interface between the mobile terminal and femtocell are identical to that of macro cellular base stations, femtocells are accessible via “off-the-shelf” mobile terminals without the need to configure anything on the mobile terminal. For the user, the access to mobile services via a femtocell is entirely seamless. Figure 1. Typical femtocell Deployment Operator benefits The initial motivation in femtocells for mobile operators was to enhance poor indoor coverage in homes or in office buildings. Poor indoor coverage results from the shielding effects of radio waves from macro cellular base stations by the structure (walls, windows, etc.) of the building. Considering that most traffic (70-80 per cent) in mobile cellular networks is generated indoors, good indoor coverage is especially important. Providing indoor coverage by means of femtocells is particularly attractive as coverage inside buildings through additional macro cellular base stations is extremely costly. It also only partially addresses the problem of low-access rates due to poor radio performance at the cell edges. Femtocells, set up by subscribers in their homes or in enterprise environments, are a very appealing and low-cost alternative to address this problem. Besides indoor coverage enhancements, femtocells have the great advantage of being extremely spectrum efficient. The simple fact that the cell size of femtocells is small (compared to a macro cell), while providing the same bandwidth available for communication, leads to a much higher system spectral efficiency compared to macro cellular networks. A third important benefit of femtocells to operators is that they require little operation or management effort. They are designed to self organize within the radio environment and to adapt to changes without the need of human intervention. As mobile operators are starting to struggle with the high load of mobile Internet traffic stemming from new Smartphone terminals, femtocells are becoming especially attractive. They allow operators to offer their subscribers high-speed data access via their mobile phone, while at the same time, they help operators to offload this new/additional traffic from the macro cellular network (see Figure 2). Finally, femtocells also enable mobile operators to offer new services in the customer’s home. The enhanced data rates at low operational costs help operators to extend their service portfolio to high-data bit rate applications, such as IPTV or Video on Demand. Furthermore, the femtocell as a trusted point of presence in the customer’s home or at the enterprise allows mobile operators to offer new customer premises-based services, such as home automation (e.g. remote control of the heating system), mobile presence (e.g. inform parents when their children return home), etc. Consequently, femtocells open up a new dimension to service opportunities for fixed-mobile convergence. Figure 2. Data traffic routing scenarios User benefits Femtocells offer end users affordable high-speed data access to mobile services while at home or at the office. Together with fast 3G HSPA and upcoming 3.9G LTE radio technologies, femtocells improve data speed significantly. With HSPA femtocells users already experience peak data rates up to 14 Mbit/s in the downlink and 5.8 Mbit/s in the uplink. LTE femtocells will further increase the access speed with downlink peak rates in the order of 100 Mbit/s and uplink rates in the order of 50 Mbit/s. Because of the high-speed on the radio interface, in many deployments the limiting factor will be the user’s broadband connection (i.e. DSL or cable). With respect to the high indoor data speeds, femtocells naturally compete with WLAN. Yet, femtocells offer additional value to the user, such as: • Less radio interference problems as femtocells operate in licensed spectrum; • Seamless mobility and access to mobile services when entering or leaving the house/office; • Hassle-free set-up of femtocells without the need for configuration of the terminals; and • Support by the mobile operator. While femtocells, compared to WLAN, may offer more secure and dependable services, femtocells with management support provided by the mobile operator, this convenience comes at the price of higher dependency on the mobile operator and, likely, some small extra charge. However, due to the fact that many users already have access to data services (via WLAN access points) at home, which is charged based on a flat rate for the fixed access connection (e.g. Cable, DSL or FTTH), data access via femtocells must be competitive in price. It is therefore expected that femtocell deployments will result in low-cost charges for the end-user, such as for example a very low-cost flat rate for mobile data access via the home. Since femtocells are connected to the subscriber’s home network, they also enable mobile users to gain direct access to local data and services in the home while they are connected through the femtocell. This makes it for example possible to directly synchronize multimedia data (e.g. MP3 songs) between a user’s mobile terminal and a media server in the home. Finally, the fact that femtocells are part of the home networking environment will benefit end users, as they can offer a service platform for new home-based services for mobile users as indicated above. Challenges, standards and market situation Over the past years, many of the challenges of femtocell technology have been addressed. For example, the cost of femtocells is now affordable for mobile operators and end users. Issues related to the deployment and device management of femtocells, as well as access control, have been resolved through specific standards. The usability of femtocells from an end user point of view has been addressed through smart plug and play technologies. Other issues are currently being addressed in various standards and industry fora. For instance issues such as support for quality of service and traffic prioritization over the fixed access, Radio Frequency (RF) interference between femtocells and the macro network, as well as regulatory hurdles. However, femtocells are not as new as it might appear. Several years ago proprietary solutions for small, indoor base stations were developed by some companies. Still, the big boost in femtocell development started only when standardisation had been introduced. Only recently, 3GPP (The 3rd Generation Partnership Project) finalized its first standards release for femtocells as part of its Release 8 standard. Similarly, 3GPP2 has developed femtocell standards. The Femto Forum is another important player in promoting and supporting femtocell standardization. Although not producing standards on its own, it provides a platform for the industry for discussion and alignment regarding standardization and marketing of femtocells. As of today, many mobile operators have already started to deploy femtocells in their networks. Most of these deployments are in some state of trial (e.g. for consumers, enterprise or SMEs). Several operators in US, Europe and Japan have even started their first commercial Femto services. Nevertheless, the femtocell technology is still young and just entering the market. Comprehensive experience with the deployment and operation is still missing. Further research is needed to address problems and insufficiencies of today’s first-generation standalone femtocells. Future research The European Commission is sponsoring the BeFEMTO research project within its 7th R&D Framework Programme. BeFemto investigates enhancements of today’s femtocell technology based on lessons learnt from initial commercial deployments. A second focus of the project lies on femtocell enhancement based on the LTE-Advanced radio technology, which is currently being standardized as the successor of LTE. The third objective is to study new usage scenarios for femtocells, such as co-operative femtocell networks and mobile femtocells (see Figure 3. Future Research on femtocell Technologies in EU FP7 BeFEMTO). Co-operative femtocell networks aim to offer the benefits of the femtocell technology in larger indoor or outdoor areas, such as hospitals, shopping malls or airports. From a technical perspective, co-operative femtocells require architectural enhancements facilitating a tight integration and co-operation among individual cells. As well as with available infrastructure networks and novel concepts and algorithms for resource and interference management. Figure 3. Future Research on femtocell Technologies in EU FP7 BeFEMTO Mobile femtocells are to be installed in passenger vehicles such as buses and trains providing broadband communications to people on the move. Such femtocells have to cope with a wireless backhaul link and have to be tightly integrated into an overall heterogeneous network deployment without jeopardizing macro network capacity and quality. This requires special attention to, for example, radio resource and interference management, cell selection and admission control. For all those scenarios, BeFEMTO aims to address the challenge of designing autonomously self-managing and self-optimizing femtocell solutions (e.g. for automatic coverage estimation and advanced interference management) and also to reduce operational expenditures, improve performance and energy consumption. Consumer demand for mobile data is already rapidly growing and has significant future growth ahead. Mobile operators are therefore challenged to increase their radio access and network capacities. Considering that the majority of data traffic in cellular networks originates indoors, where macro cellular coverage is often patchy and data rates are low, femtocells emerge as an attractive technical solution to address this problem. Femtocells help mobile operators to provide additional capacities at low cost. The fact that femtocells are hosted at the customer’s premises and use the fixed-access connections to backhaul the traffic to the mobile network makes this a very appealing proposition. In conjunction with the support for local breakout of data traffic, femtocells further enable operators to offload the growing mobile Internet data traffic (e.g. YouTube, Google) from the operator’s core network. Femtocells are becoming an attractive, low-cost alternative to the well established WLAN technology for mobile operators to offer high-speed access to the Internet in addition to mobile services in their customers’ home or office. The benefits of licensed spectrum, plug and play setup and operator support, as well as seamless access to mobile services including mobility support make this a compelling technology. Yet, although the first commercial deployments have been launched, femtocell technology is still an emerging technology and continuous efforts are needed in research and standards to mature and advance it, so that it meet its promises.