Femtocell Interference

- an overview addressing the interference issues associated with the femtocells.

Interference is a key issue associated with femtocell development. There are a number of issues associated with interference all of which have needed to be investigated and solutions found to ensure that the deployment of any femtocells will take place successfully.

The issue arises from the fact that femtocells will utilise the spectrum already allocated for cellular telecommunications. The femtocells will be deployed in what may be termed an ad-hoc fashion, without the network planning that is normally undertaken for the deployment of cellular telecommunications base stations. As a result there is very real the possibility that interference will arise. This could cause problems to the main network resulting in poor levels of performance being achieved, not only by those using the femtocell, but other users who may be communicating via the main cellular network.

Clearly, as interference could arise and reduce the levels of performance this is a real issue for operators. While femtocells can give major advantages in terms of coverage improvements for a comparatively low cost, these benefits could be negated if the overall performance of the network was reduced.

As a result a considerable amount of work has been undertaken to ensure that femtocell interference issues do not arise and prevent their widespread deployment. There are a number of methods that have been developed to ensure the easy minimisation of interference so that femtocells can be installed by users without the need to worry about any technical issues.

Femtocell interference scenarios

There are fewer femtocell interference problems when the femtocells use a separate carrier frequency to the surrounding macro network for the cellular telecommunications system. The more complicated scenarios arise when the femtocells use the same carrier frequency. This scenario also provides the greatest level of spectrum efficiency, but also the greatest challenges to the interference management systems.

The main interference scenarios are detailed below..

  • Femtocells interfering with base stations on the same frequency:   When a number of femtocells are operating on the same channel as a macro base station, some level of interference may be caused by the femtocells, reducing the performance of the overall network.
  • Base stations interfering with femtocells on the same frequency:   Again when a the macro network is operating on the same channel as the femtocell or femtocells, interference may be cause between the two.
  • Closely spaced femtocells interfering with each other:   If a number of femtocells are installed close to each other they will have a level of background noise that will reduce the sensitivity of each femtocell.
  • User equipment transmitting with a sufficiently high power such that it is received by more than one base station:   Even though the coverage within buildings is poor and by inference the level of signal from the mobiles reaching the base stations from inside buildings will be attenuated, there will still be signal from mobiles communicating with a femtocell that may reach the base station. This will increase the overall level of noise received by the base station.

All these scenarios result in interference that can cause problems which can result in a degradation of the level of service. While interference can be a problem within normal macro cellular networks, the situation is more acute when using femtocells because of the fact that they can be deployed in an ad-hoc fashion without the network planning that would normally be undertaken.

Solutions to interference problems

In view of the fact that there are a variety of forms of femtocell interference that arise, a number of different solutions are required. Studies that have been undertaken have devised a number of methods to reduce the performance degradation to below acceptable levels. These are already being introduced into the current femtocells being manufactured

  • Adaptive Pilot Power Control:   Using this scheme the femtocell detects the signals from surrounding cells and dynamically adjusts its own transmitter power while still aiming to maintain its own coverage area.
  • Dynamic femtocell receiver gain management:   In order that femtocells can satisfactorily operate with mobiles, or user equipments that are near toth e femtocell or at a greater distance, a satisfactory form of automatic gain or attenuation much be installed. This will enable the mobiles to operate without increasing their transmitted output power any more than is absolutely necessary. This will keep any increase in noise and interference to a minimum.
  • Mobile phone uplink power capping:   It is possible to cap or limit the maximum power output of a mobile when operating in the femto environment. This ensures that the phone will hand off to the macro network before its transmitter power rises to a point where it adds noise to the macro network.
  • Extended femtocell receiver dynamic range:   In order to ensure that femtocell designs are able to operate reliably even in the presence of nearby high power mobile phones connected to the macro network, it is necessary to ensure that their receivers are able to operate in the presence of other very strong signals. To achieve this they need to have a very high dynamic range. To ensure they meet the requried standard the relevant test has been incorporated into Release 8 of the 3GPP standards as 25.104.

Even though femtocell interference is a problem that needs to be addressed and kept in mind, there are strategies that have been developed to overcome it. The femtocells being developed and deployed have several methods of reducing and overcoming the femtocell interference problems that are likely to occur. As a result it is anticipated that it is unlikely to be the problem many thought it could be.

By Ian Poole

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