Femtocell technology tutorial

- an overview, tutorial or primer about femto cells or femtocell technology describing the essentials of what they are and how they work.

Femto cells or femtocells are small cellular telecommunications base stations that can be installed in residential or business environments either as single stand-alone items or in clusters to provide improved cellular coverage within a building. It is widely known that cellular coverage, especially for data transmission where good signal strengths are needed is not as good within buildings. By using a small internal base station - femtocell (femto cell), the cellular performance can be improved along with the possible provision of additional services.

In order to link the femtocells with the main core network, the mobile backhaul scheme uses the user's DSL or other Internet link. This provides a cost effective and widely available data link for the femtocells that can be used as a standard for all applications.

There are many advantages for the deployment of femtocells to both the user and the mobile network operator. For the user, the use of a femto cell within the home enables far better coverage to be enjoyed along with the possible provision of additional services, possible cost benefits, and the use of a single number for both home and mobile applications. For the network operator, the use of femtocells provides a very cost effective means of improving coverage, along with linking users to their network, and providing additional revenue from the provision of additional services.

Although there are advantages and disadvantages to the use of femtocells, their use has many advantages for both user and network provider.

Femtocell history

The first interest in femto cells started around 2002 when a group of engineers at Motorola were investigating possible new applications and methodologies that could be used with mobile communications. In addition to developing a mobile television scheme, they also put together a very small UMTS base station.

A couple of years later in 2004, the idea was beginning to gain some momentum and a variety of companies were looking into the idea. In particular two new companies, Ubiquisys and 3WayNetworks were formed in the UK to address the area of femtocells.

With the idea gaining momentum, and many more companies investigating femto cell technology, the Femto Forum (www.femtoforum.org) was set up in July 2007. Its aim was to promote the wide-scale adoption of femtocells. With mounting industry pressure to be able to deploy femto cell technology, the Femto Forum also played a coordinating role in ensuring that the standards were agreed and released as fast as possible.

Femto cell system basics

Femtocells are essentially a form of cellular access point. The femtocells provide connectivity between the local mobile phones and an internet router. However this is only part of the overall femto cell system required. Several items are needed:

  • Femtocell itself
  • Internet router to enable the data to be passed to and from the femto cell via the Internet
  • Internet link
  • Mobile telecommunications service provider core network gateway

The two key elements of the system are the femtocell which is installed within the user premises and also the network gateway that provides the link from the Internet into the telecommunications network.

Femtocell issues

There are a number of issues that needed to be resolved in terms of the basic system design and set-up.

  • Femto cell interference issues:   One key issue associated with femtocells is that of interference. There is only limited spectrum on which the cellular systems can run. Some 3G operators for example may only have one channel in some places. Therefore it is necessary that femtocells are able to operate within the normal spectrum shared with many other cellular base stations. There are a number of ways in which this can be achieved: the use of cognitive radio technology; the use of systems that are tolerant to interference (3G and 4G are able to tolerate interference and single channel working); spectrum planning where possible.
  • Femtocell spectrum issues:   radio spectrum is a particularly scare resource, especially when large amounts of data are required. Planning the available spectrum so that it can be used with the possible huge numbers of femtocells can require careful attention, although in some instances single channel operation with main base stations may be required.
  • Femtocell regulatory issues:   Femtocells operate in licensed or regulated spectrum. Unlike Wi-Fi which operates in unlicensed spectrum, femtocells need regulatory approval. The spectrum and radio regulations vary from one country to the next and therefore regulations may need to be changed in each country. International agreement may also be required, because private individuals may take femtocells from one country to the next.
  • Femtocells and health issues:   With a large public awareness of the possible dangers of RF radiation, one key issue has been that of health and safety. As a femtocell is a cellular base station, there could be public concern regarding the levels of RF radiation received. However the power levels emitted by femtocells are small - no greater than most Wi-Fi access points which are common in very many homes. As a result it is not believed by the industry in general that there are any health issues that should cause any concern.

LTE femtocells

Femtocells form a basic element of the business models for moving forward with many types of cellular telecommunications network. While they operate well with CDMA technology, they will also be able to be used with LTE, long term evolution systems. LTE uses OFDM as the signal format, and therefore LTE femtocells will require development to be undertaken to ensure that the optimum operation is achieved. Nevertheless LTE femtocells are on the roadmaps of many manufacturers and they will appear in due course.

Femtocells are now an integral part of the development strategy for cellular telecommunications operators. Not only do femto cells provide additional advantages for users in terms of improved performance within the home, or business office, but they also provide the possibility for additional services and the promise of lower charges. They also offer the change of convergence where a single phone can be used instead of the landline as well as for roaming. For operators they provide a cost effective manner in which they can improve their coverage and gain extra revenue by the provision of additional services. Accordingly the use of femtocells will become a mainstay in the cellular telecommunications roadmap for the future.

By Ian Poole

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