Femtocell's access methods

05 May 2014
Published in QoS

Femtocells are low-power and low-cost base stations that provide residential cellular services, providing a coverage of roughly 10 m [1]. They can integrate with mobile operator through a broadband connection, typically ADSL. In General, femtocell makes that the traffic from the home cellular system, deviates through the broadband connection, releasing the resource consumption of the macrocell.

The 3GPP has introduced the concept of Closed Subscriber Group (CSG), which essentially identifies a group of subscribers who have access permission to one or more cells. Femtocells basically have three different operating modes: open access, closed access and a combination of both named hybrid access [2] and [3], the access methods are:

  • Open ccess: In this case, the UE can access the femtocell without any restrictions, this is seen in the Figure # 1 wherein the EU2 has unrestricted access to the femtocell in the building.
  • Closed Access: The administrator defines the only femtocell users (CSG) that can access the network. For this mode, emergency calls are exempt.
  • Hybrid Access: In this type of access, a limited amount of resources is allocated for access to users who are not part of CSG.


Figure # 1. Example of different femto-cell's access methods [2]: Open access, closed access and hybrid access.

QoS managing

An important point with regard to providing QoS is to reduce signaling and interference. With this, operators can properly manage resources without such problems, in order to improve the services provided to users.

In this new scenario, the traffic generated from the UE to the eNodeB is deflected by an IP network before reaching the mobile network. The transmission of a information packet over an IP network is susceptible to delays, so the femto-cell may experience problems for obtaining an immune base time to jitter. Operators should ensure the concept of QoS in femto-cells, the parameters of these scenarios should be comparable to those offered by the macro-cells.

Some studies show that over 50% of voice calls and 70% of the data traffic comes from inside [1], voice calls do not require high bandwidth, but certain quality parameters are require to ensure the voice recognition of users at the other end of the communication, on the contrary, the data traffic requires high transmission rates to send information with a lot of megabytes, such as multimedia applications require, to name one example. One way to ensure high transmission rates on radio systems, is to maintain high signal quality at both ends of the communication, and hence, mitigate the effects of loss and interference in the communication channel.

In this context, operators have deployed femto-cells systems in order to decrease the distance to the EU and to maintain good signal quality. However, femto-cells must operate in the designated band frequency for such communication and match to the assigned frequencies to the operator, from this perspective, femto-cell generates interference and it can be given in the following ways [2]:

  • Femto-cell to femto-cell.

  • Femto-cell to macro-cell

  • Macro-cell to femto-cell.

The interference problems between femto-cell is due the coverage that they have, this type of interference will occur primarily within the coverage limits where the femto-cells interact each other, the Figure # 1 shows this phenomenon, in which the FAP 1 and FAP 2 coverage have overlaps producing signal degradation for users of both femto-cells in that area.

Figure # 1. Interference problems between macro/femto cells.

The telecommunications network of an ISP (Internet Service Provider) is actually an interaction of various networks types, within LTE (4G) can be highlighted as an access network to the end user. The information flow reaches to the user through several infrastructures, on which the ISP carries data traffic, from this perspective, some necessary points will be discussed in order to ensure the Quality of Service (QoS) in such environments.

From the point of view of LTE management, it is permitted to define profiles and classes of services, which are key points when negotiating QoS mobile requirements during the communication establishment, the transit of packets, even in handovers. However, it is necessary to consider the access and transport network if we want to ensure end-to-end QoS.


Transport Network

A typical scenario of an Internet service provider could be described as shown in Figure # 1, it can be seen that LTE is only a part of the provider's access network. The ISP has different types of access networks depending on the provided services. All the necessary infrastructure to transport information from the access networks, must transit through the core network. The core is also connected with other different network types, where the end service is usually located (Internet, PSTN, ftp services, video streaming, voice or other), which the user wants to access.


Figure # 1. ISP's simplified topology [2].

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