Femtocells

The femtocells are wireless access points that operate in low power and spectrum licensed bands that connect the mobile operator network with connections using cable broadband.

The femtocells are used to extend the reach of cellular networks, particularly 3G, inside buildings. Its function is similar to that of "gap-fillers" used in the case of DTT. It is therefore a "mini" base station radio equipment equivalent to that used in the communications tower of a mobile operator. Lately there is an increasingly growing interest in such systems, strengthened with the creation of various forums (femtoforum.org) and special lectures, although its deployment is still quite behind. Most operators are evaluating the equipment in the laboratory, others have performed some pilot testing, and very few are offering commercial products to its customers. But it is hoped that this will change in coming years because a recent study from ABI Research forecasts for 2012 of around 150 million users and 70 million access points worldwide. In this article we focus on analyzing the existing technology.


Architecture
The typical architecture of femtocells is shown in Figure 1. The radio equipment located on the customer's address is very similar to an access point Wi-Fi and is ready to be easily installed. An Ethernet connection to connect the computer with a DSL or cable modem to link with the cellular network operator via the subscriber's Internet connection. Thus, the operator gets to extend the network reach inside the building taking advantage of the customer's Internet connection, while the latter benefits from better communication quality with lower costs. This is therefore a clear example of fixed-mobile convergence, where unlike other architectures, in this case does not require any dual terminal but can use the same 3G phones available.
However, the installation of femtocells at customer's home involves a whole series of issues that can not be overemphasized. Firstly it is important to know the geographic location of the team for various reasons, including appropriate courses to emergency calls or to make an accurate planning of power and RF channels to avoid interference. In particular, an operator may have different RF channels, and even bands of the spectrum in adjacent geographical areas, so it is essential knowledge of the position of the femtocells to avoid interference with the macro cells of the same operator or others. Among the commonly used options for the location of mobile users is the GPS or base station triangulation, but obviously none of these methods works indoors. Another possibility is to use the IP address of your Internet connection, but this is not always reliable except in the case of operators providing both fixed and mobile services and can have control over it. Added to this is also the problem that customers can take equipment from one place to another, but operators insist they do not, which greatly complicates the localization process. An example would be for residential customers to deliver equipment to offices or other places where different rates apply for calls. As an extreme case, the displacement of femtocells across international borders would most likely cause a violation of licensing spectrum use.

Another aspect of concern to operators is the quality of service (QoS) experienced by users. As demonstrated in many pilot tests, the low power femtocells radiation from mobile phones causes many operating near the limit of coverage, which increases the likelihood of communication failures if the extension of the house is large or several plants. This is closely linked to the Internet connection is shared with voice calls, which require a minimum capacity of 500 kbit / s with very low latency. In fact, it appears that the calls can get to cut when a family member is connected to the Internet to download a video with high bandwidth requirement. Again, only operators that provide fixed-mobile services can be combined with mechanisms to coordinate the use of Internet with femtocells and ensure a minimum QoS. Considering that the latest 3G phones announced capacities of 2 Mbit / s, and that access to these services at home is a priority, operators must perform an important role in ensuring that the quality commitments are kept in the femtocells.


Market and devices
PWe can say that Sprint is the company that has implemented the service more successfully femtocells. His service called Airave, is being used already in many U.S. households. The device, manufactured by Samsung, it costs the order of $ 100 is paid directly affects the final, although the company offers calling plans that allow considerable savings in the monthly bill. Figure 2 shows a photograph of the device along with a scheme that allows us to appreciate the simplicity of installation by the subscriber.
In the case of multiple operators Europe continue conducting pilot tests of femtocells, with particular emphasis on Vodafone and O2. Forecasts indicate that in a short space of time announced the first products and calling plans. As an example, the British company has recently launched Ubiquisys ZoneGate product shown in Figure 3. Principal among its features, we can enumerate the following:

- Connection to the networks of mobile operators as IMS protocols (SIP) or RAN Gateway (Iu).
- Future migration to 3GPP Iu-h standard for femtocells.
- Continuous monitoring of GSM and UMTS macro cells adjacent to adapt to the environment.
- Algorithm optimization of output power, frequency and pseudorandom coding to ensure coverage and minimize interference.
- Hand-over transparently between the femtocells and the operator's network.
- Supports HSPA with lower rates of 7.2 Mbit / s up to four simultaneous connections for voice or data users.
- Traffic multiplexed voice and signaling, prioritized and encrypted by IPSec.
- Architecture of Web 2.0 services.
According to a ranking prepared by the firm ABI Research in May 2008, Ubiquisys ranked first among femtocells vendors based on criteria of product implementation and innovation. The remaining companies are listed in Table I. Specifically, it is expected that the market for femtocells have economic developments as shown in Figure 4, surpassing the 1000 million in 2012.
So far we have focused on the case of 3G femtocells, though not the only option. For example, the U.S. operator Comcast plans to deploy WiMAX femtocells in the second half of 2009. The teams will also be used to extend coverage inside buildings, while routing VoIP calls over the cable network operator. Because the service operator offers HDTV, VoIP and Internet broadband (triple-play), in this case allows us to offer mobile WiMAX femtocells in the home. The chip maker picoChip has a whole series of reference modules configurable software for the development of 3G and WiMAX femtocells. In the latter case, the PC8530 reference design provides the implementation IEEE 802.16e OFDMA PHY and lower MAC (Figure 5). This design can be updated to support PC8532 WiMAX Wave 2 and IO-MIMO.
It is clear that over the coming years 3G and WiMAX will compete strongly in the access network. It is premature to predict a denouement, but probably reach a compromise where both technologies are complementary depending on the service or application and the type of end user.

Author:

Francisco Pascual Ramos. PhD in Telecommunications.
Professor of the Polytechnic University of Valencia.
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