"The limited number of available frequency bands is a strong barrier to the growth of new ICT services. To solve this fundamental problem of spectrum scarcity, government regulatory bodies recently started to review their spectrum allocation policies proposing opportunistic spectrum access (OSA). This project addresses the problem of wireless networks coexistence in the context of OSA which enables multiple networks to share the same radio spectrum. The priority in accessing the radio channel identifies primary and secondary networks, which have the higher and the lower priority respectively. The activity of the secondary network using OSA and the effect of the aggregate interference injected into the primary network are mutually dependent, and they are influenced by the spatial positions of both primary and secondary users. Therefore the geometry of the networks plays an important role for spectrum usage optimization. The aim of this project is to develop a theoretical framework, based on stochastic geometry, addressing the network coexistence problem with a detailed model of the aggregate interference where activity and geometry of both networks, as well as OSA protocols are considered. This model will incorporate the non-homogeneity of the spatial distribution of the nodes and will be flexible enough to accommodate a variety of realistic operating environments. The theoretical framework will be used to gain insights into the design of networks for optimal spectrum usage considering also the effect of different cooperative strategies. The project will also analyze if and how a fully distributed OSA can be beneficial for the overall spectrum utilization in a multi-tier network.The supervisor during the outgoing phase is Prof. Win from the Massachusetts Institute of Technology (MIT). The scientist in charge of this project is Prof. Chiani from the University of Bologna"