Abstract Spectrum sharing has become a high priority research area over the past few years. The motivation behind this lies in the fact that the limited spectrum is currently inefficiently utilized. As recognized by the World radio communication conference (WRC)-07, the amount of identified spectrum is not large enough to support large bandwidths for a substantial number of operators. Therefore, it is paramount for future mobile cellular systems to share the frequency spectrum and coexist in a more efficient manner.

The present dissertation deals with the problem of spectrum scarcity by examining spectrum sharing paradigms where a migration from fixed to flexible resource allocation is investigated. First, a radio resource management (RRM) architecture is proposed where advanced spectrum functionalities accounting for the short-term variations of the spectrum are examined. The achievable gains are shown in a multi-cell, multi-network environment with realistic traffic patterns from a European operator, enhancing thereby spectrum utilization. Second, inter-operator resource sharing in a broadband network is considered where a packet-based cellular network is developed. It is shown that the obtained gains in terms of quality-of-service (QoS), number of operators and different data rates requirements improve the overall efficiency of the network. Besides and in order to cope with the stringent data rate requirements, direct terminal-to-terminal (T2T) communication is examined in which a realistic algorithm is proposed advocating resource reuse in a cellular system with simultaneous communications between mobiles. Numerical results confirm the advantages of resource reuse in terms of throughput, average frame delays and power consumption.

In this thesis, a proposal is made as how to enhance spectrum sharing. The concept of hierarchy is proposed in which wireless competitive operators share the same spectrum band. The decentralized hierarchical approach is shown to bridge the gap between the selfish and centralized approach.

Interference avoidance is studied for point-to-point communication in a selforganized network where different optimal power allocation strategies are examined along with the impact of frequency reuse on the ergodic capacity of the network.