Femtocells

3.5.1 Presentation  

Definition: “Femtocells are low-power wireless access points that operate in a licensed spectrum to connect standard mobile devices to a mobile operator’s network using residential DSL or cable broadband connections.” [7] Femtocells are deployed in homes or company buildings where they deal with from 2 to 4 active mobile phones in private houses, and up to 16 in business offices.

Femtocells have been developed to comply with some criteria:

- Low-power access points - Using mobile technology - In licensed spectrum

- Generating coverage and capacity - Over internet-grade backhaul - At low prices

- With full operator management - Self-organizing / self-managing

Femtocells rely on small device that any mobile customer can buy from his operator. It consists in a reduced size antenna which uses operator-licensed spectrum and is connected to the mobile network through a secured link over the Internet. From the customer point of view, there is no difference from a femtocell and a normal macro cell. They both provide all the mobile services (voice call, sms, mms…), and mobile phones deal with the handover from one to another.

3.5.2 Access  management    

Femtocells use as backhaul a private link, most of the time a DSL link associated to a DSL

provide better capacity and coverage. Restriction access solutions are needed to manage access rights.

The different restricted areas may be divided into three main categories [8]

- Open Subscriber Group (OSG). This pattern allows any user who subscribed a contract with the femtocell provider (the mobile operator) to have access to the femtocell. However, this may disturb the personal DSL access because of high data consumption of neighboring handsets. It might lead to some trouble with the DSL subscriber unless the femtocell provider offers discounts to share DSL access price.

This solution is however the most effective because it allows numerous mobile users to be connected to femtocells inside buildings in case of an advanced femtocell deployment.

- Close Subscriber Group (CSG). This management allows only authorized handsets to connect to the femtocell, which is the most common access restriction.

This way, the femtocell is used as a way to improve capacity and coverage in the customer’s private area. But it remains poor in terms of traffic offloading, because only users who are at home can use it.

- The Hybrid Access Restriction uses both OSG and CSG solutions features. In this case, all the customers of the femtocell provider are allowed to connect to the femtocell but only if none of the authorized handsets want to use it. This way it gives benefits to the DSL subscribers and also enables the mobile operator to offload traffic when the femtocell is not in use.

3.5.3 Interference  management  

One of the most critical issues to deal with during femtocells deployment is interference management. Adding femtocells in the current mobile networks may lead to interference, as femtocells’ spectrum band is the same one as macro cells. Interferences commonly create coverage and capacity drops, and thus a waste of money. [11]

Here is some usual radio frequency interference issues related to bad femtocell deployment:

- Desensitized femtocell or handset: Handsets and base stations are calibrated to operate in a given dynamic range. However, if femtocells antenna and handsets are too close to each other, it creates high signal levels beyond the sensitivity range of the receiver [12]. On the download link, the handset receiver can be saturated and degrade demodulation performance. On the upload link, a high noise can be detected at the femtocell make the system unstable.

- Interference between macro and femtocells: Femtocells are deployed inside buildings, where they are likely to interfere with the macro cells outdoor signal, which goes through barriers such as walls and glass. Thus, interferences can be observed on both download and upload links. For the download link, interference may be created outside and damage outdoor signals. On the upload link, the handsets connected to the femtocell may interfere with the handsets connected to the macro network.

- Interference between femtocells is also likely to happen because of unplanned deployment. As an example, a femtocell deployed installed near a wall between to two apartments can cause significant interference to other apartments. In that case, the femtocell with the strongest radio frequency signal may not necessarily be the serving femtocell due to restricted access as explained earlier.

Many solutions have been developed to rule out those issues during femtocells deployment. One of them is the Femtocell Download  Link  TX Power Self Calibration:

“To avoid a femtocell creating coverage holes outside, a solution is to adjust signal power between the femtocell and the macro cell. The above mentioned cell uses an autonomous self-calibration. It means that femtocells make download  link  measurements from macro cells and other femtocells to calculate the required transmission power level.”

3.5.4 Advantages  and  limitations  

Femtocells allow operators to provide satisfactory coverage in indoor areas that are difficult to cover with outdoor base stations. They also allow operators to provide full services in rural areas, which would be expensive for operators to provide with macro cells. So they enable isolated areas to be as fully connected as an urban city may be.

They are able to scan surrounding signals and then to choose by themselves their band

which are sometimes limited for wide-area coverage. In addition, they can use lower frequencies bands in order to reduce power consumption.

They improve the quality of service by providing extended coverage, capacity and service to consumers. Following this idea, a more efficient network will attract more customers and will result in more competition, thus to better prices for customers, innovations and good quality services.

However, femtocells generate an additional electromagnetic radiation in a private area, generating high frequency electromagnetic pollution and interference in the home. They represent risks to the confidentiality of private communications, but still lower than Wi-Fi, as authentication is managed by the SIM card. Also, the antenna device almost cannot be controlled in any way by the user, when it is installed. Finally, even if the power consumption is quite low, yet it has to be paid by the household or company.