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.

Another case of interference is the interference received by the mobile 2, we assume that this device is managed by the macro-cell and does not have permission to access the femto-cell 1, however, it is within the coverage area of ​​the femto-cell, this will produce interference with the mobile device, since the frequency channels for communicating with the macro-cell and femto-cell are located in the same frequency band, this type of interference is produced from femto-cell to macro-cell, it can also occur in the opposite direction from macro-cell to femto-cell.

When a mobile is managed by a macrocell and it is far from eNodeB and close to a femto-cell, it will transmit at a higher power to cover losses associated with the link, however, in doing so, generates more interference to the femto-cell. Moreover, the femto-cell will request to the managed mobiles, an increment in the transmission power to cover the interference, this in turn, will generate a higher level of interference to the mobile which is managed by the macro-cell. The network access methods will have to manage the problem of power management in an efficient way to allow coexistence and ensure quality of service.

In this context, a problem arises concerning the femto-cell coverage management, since it should minimize interference from them, and simultaneously, meet the requirements of service quality. Therefore, the service provider must have a precise control of the femto-cell in several ways:

  • It must ensure the use of the allocated spectrum and verify that the femto-cell is not transmitting out the corresponding frequency band.
  • Moving of residences must be managed properly, as this could develop some communities with a lot of interference by the accumulation of femto-cells nearby.
  • It must be noted, that traditional planning techniques such as frequency reuse have no sense because the user is who installs the femto-cells.

On the other hand, some problems were found in the femto-cell implementation in areas where there is a Wi-Fi network, in [1], the authors comment some cases where the FAP experiences difficulties in transferring data, even in voice services.

References

  1. Vikram C, Jeffrey GA, Alan G. Femtocell Networks: A Survey. IEEE Comunication Magzine (2008) 46: pp. 59-67.
  2. G DLR, A V, D L Jie Z. Access control Mechanisms for Femtocells. IEEE Communications Magazine (2010) 48: pp. 33-39.