A small amount of terminology may be of use before we proceed further into the world of femtocells. A macrocell defines the cell controlled by the base station of a mobile network operator. A femtocell is the cell controlled by the femto base station. The main difference between a femtocell and a base station is the range. A base station might have a radius of 20-30 km, whereas a femtocell has a radius up to10 metres. However, a femtocell uses the same licensed frequency band as the macrocell. Femtocells can operate with various types of mobile systems such as UMTS and CDMA2000. In future, LTE and WiMax capable femtocells are expected to be deployed.
For backhaul, the femtocell requires a connection to the mobile network operator. This is typically via a fixed line, be it cable, fibre-optic or a twisted pair telephone line. This connection enables the femtocell to interface with the operator’s base station, and provides a way for the two to coordinate when (for example) to handover your mobile device to the femtocell or back to the main base station.The femtocell needs to co-exist peacefully with neighbouring users and possibly neighbouring femtocells. This is not too difficult; a femtocell can sniff the spectrum and detect other femtocells. A femtocell will only use the frequency bands it has been allotted. Its short range and inbuilt intelligence means that it can adjust transmission power up or down, according to where its users are and whether neighbouring users are using the spectrum.
Advantages and disadvantages
There are a few controversies around the deployment of femtocells. One of them was pointed out by an erstwhile colleague when he was trying to sort out his home phone issues. He got somewhat irate when the company suggested a femtocell. His gripe was that the Telco company was making him fix their black spots.
Of course, there are advantages to having a femtocell. One of these is that the coverage is likely to remain consistent wherever you are located in the office/home, due to the femtocell. This has the flow-on effect that your mobile terminal/phone won’t use as much power, hence giving more battery time. A further advantage is that a mobile phone can be used as the main phone(s). Femtocells have the capacity to limit how many people are permitted to log on. This is a mechanism to restrict coverage. It makes using a femtocell in a small office attractive.
There are drawbacks. Femtocells utilise the broadband connection, which may also be used for other applications such as video streaming. There can be problems when the provider of the broadband service differs from the mobile network provider. These relate to Quality of Service guarantees. A further issue relates to interference. Although the deployment of femtocells suggests that interference with other femtocells is not a huge issue, there is still some controversy over whether this will continue to be the case.
The use of femtocells is part of a more general trend in mobile communications toward smaller cell sizes. Whilst there are some drawbacks to femtocells, the advantages of using them could be seen in homes and in small offices or home offices.
Scott Reeves has worked for Hewlett Packard on HP-UX servers and SANs, and has worked in similar areas in the past at IBM. Currently he works as an independent IT consultant, specializing in Wi-Fi networks and SANs.