Lately I have been reading a document by the Femto Forum published back in 2008 that presents a very interesting analysis of the interference problem in Femtocell networks. The document focuses on European 3GPP networks based on UMTS and seems to be related to the FREEDOM project I discussed in my previous post.

The study includes an extremely detailed description and analysis of all the possible cases of interference in this kind of access networks and, after a set of system level simulations and analysis, the authors propose some mitigation techniques for each case of interference.

The summary conclusion that we have reached, in common with other studies, is that in order to be successful, femtocell technology must manage three things:

  • Femtocell downlink power – if femtocells transmit inappropriately loudly, then the cell may be large, but non‐members of the closed user group will experience a loss of service close to the femtocell. On the other hand, if the femtocell transmits too softly, then non‐group members will be unaffected, but the femtocell coverage area will be too small to give benefit to its users.
  • Femtocell receiver gain – since UEs have a minimum transmit power below which they cannot operate, and since they can approach the femtocell far more closely than they can a normal macrocell, we must reduce the femtocell receiver gain, so that nearby UEs do not overload it. This must be done dynamically, so that distant UEs are not transmitting at high power, and contributing to macro network noise rise on a permanent basis.
  • UE uplink power – since UEs transmitting widely at high power can generate unacceptable noise rise interference in the macro network, we signal a maximum power to the UE (a power cap) to ensure that it hands off to the macro network in good time, rather than transmit at too high a power in clinging to the femtocell.

We have also shown that, with these issues addressed, the net effect of deploying femtocells alongside a macro network is significantly to increase its capacity. In numerical terms, and in terms of the simulated scenario, the available air interface data capacity is shown to increase by over a hundredfold by the introduction of femtocells.

A document really worth reading that will be very helpful to anyone working on interference management for Femtocell networks. Despite being based on UMTS access networks, it would have been a very useful source for my recent project on Uplink Interference Mitigation for OFDMA Femtocell Networks. The interference cases and models we used, as well as the system parameters, were the same. It seems that it is pretty much established in the field how to analyze the interference problem in Femtocell networks.