Study Finds Interference Between Femtocells and Maco Network Not a Problem

Femtocell trade group, The Femto Forum has published the results of a study which studied problems with network interference caused by the close proximity of femtocells and macrocells. The study found that femtocells have the potential to deliver an order of magnitude more capacity than the macro network alone when used in dense deployments, even when occupying the same radio channel as the macrocells. It also identified numerous technological solutions that mitigate potential interference with the macro network to remove barriers to wide-scale deployment and maximize the capacity benefits.

The research assessed the impact of femtocells on a mobile network in a wide range of deployment scenarios in order to explore co-existence issues including interference.

The study looked at both femtocells using a separate carrier to the surrounding macro network and those using the same carrier - which pose the greatest interference challenge, but also the greatest opportunity for increased spectrum efficiency.

Although femtocells using a separate carrier were demonstrated to provide a simple means to essentially eliminate interference, many 3G operators do not have enough spectrum for this to be practical. The Femto Forum therefore identified technological solutions that mitigate the potential interference where femtocells share the same carrier as the macro network. These methods are already being developed for pre-standard femtocell solutions, and Femto Forum members are working to bring them within the standards framework. The key solutions are:

• Adaptive Pilot Power Control, whereby the femtocell dynamically adjusts its transmit power in response to the current level of signals from surrounding cells and the desired coverage area.
• Extended Tests for Dynamic Range, 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 (this test has already been incorporated into the latest 3GPP Release 8 25.104 specification).
• Uplink power capping of the mobile phone when operating in the femtocell environment, ensuring that, even in difficult radio conditions, the phone hands-off to the macro network before its transmit power increases to the point where macro noise rise is a problem.
• Dynamic receiver gain management in the femtocell (Automatic Gain Control or adaptive attenuation), to ensure that femtocells can offer good service to both near and far mobile phones without unnecessarily increasing the phone transmit power, therefore keeping the noise rise to a minimum.

This particular study relates to WCDMA, but the Femto Forum represents the full range of radio standards and similar studies are underway for other air interfaces.

"At a time when mobile operators are seeing data usage rocket, femtocells offer an economic and effective way to deal with demand," said Professor Simon Saunders, the Femto Forum's Chairman. "By utilizing the identified interference management techniques it has been shown that femtocells offer operators an effective method to increase capacity and coverage within their existing networks through dense cell deployments."

The research builds upon work done within 3GPP by putting femtocells through a more extreme series of interference tests. The full results can be found in a white paper available on the Femto Forum website.

Posted to the site on 2nd December 2008