Dissertation on Interference Coordination for Femtocell Networks

On August 25, 2015, Serkan Uygungelen successfully defended his Ph.D. thesis Interference Coordination for Femtocell Networks. The Ph.D. examination committee was formed by Prof. Arne Jacob (Institute of Microwave Engineering, Optics, and Electromagnetic Compatibility), and the two reviewers Prof. Gerhard Bauch and Prof. Harald Haas (University of Edinburgh, UK).



Data traffic has dramatically increased in wireless cellular networks in recent times, thanks to various video services and high-speed mobile access of smart phones. Countering such a dramatic traffic increase has become one of the major issues for network operators. In order to meet the boosted data demand, the spatial reuse of resources is increased through decreasing cell sizes, which is applied by operators for a long time. On the other hand, due to the high operation and infrastructure costs of base stations (BSs), operators are reluctant to dense deployment of BSs. One solution to this problem is the introduction of femto-BSs, also known as home evolved NodeB (HeNB). These are low-power and small scale BSs that are deployed within traditional macro-cellular networks. With these so-called small cells, the spatial reuse of radio resources is tremendously boosted. Additionally, femto-BSs can be deployed by end users in their own premises. By deploying such small cells indoors, wall penetration losses are largely mitigated, and users can enjoy high data rates.

In addition to its various advantages, the deployment of femtocells also comes with some problems. One of the main issues is the increase in interference as illustrated in Fig.1. Unlike macrocell networks, femtocell networks are typically randomly deployed without sophisticated planning of the network topology. This causes high co-channel interference between cells, especially in networks where the BSs are densely deployed, such as within public buildings or residential apartment complexes.

The interference mitigation in femtocell networks is the main aim of this thesis. In order to achieve this goal, the characteristics and interference environment of femtocell deployments have been investigated, and the properties of interference mitigation techniques applicable to such networks are studied. It has been shown that conventional techniques used for macrocell networks is not applicable for such uncoordinated networks. To this end, dynamic resource partitioning methods have been developed where network can update itself depending on the changes in the interference environment. Central and distributed resource assignment approaches have been investigated and compared. For this purpose, extensive simulations with different scenarios have been carried out to see the overall picture clearly. Furthermore, the application of the developed techniques in real network deployments such as Long-Term Evolution (LTE) and Long-Term Evolution-Advanced (LTE-A) have been studied. Although the focus is on femtocell networks, the developed methods in this thesis can be applied to any wireless networks with minor changes.

Fig. 1: Three types of interference introduced by femtocell networks; Interference from femto-BS to macro user (1), from macro-BS to femto user (2), and from femto-BS to femto user (3)