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Recently, Device-to-Device (D2D) has been brought inside mobile (cellular) networks with the introduction of the LTE-D2D standard into the 5G ecosystem. This cellular D2D operates in the same operator's frequencies used for regular communications with access points (i.e., base stations). In D2D mode, terminals can communicate directly and do not need to go through a base station. However, D2D communications are authorized and controlled by operators to implement their requirements and policies. A notable example of D2D is data offloading, which helps in reducing traffic congestion in mobile networks. In this scenario, terminals collaborate using their D2D connections to carry data, usually over multiple D2D hops, using other terminals as relays and avoiding base stations. However, the latter still must decide on routing (e.g., which devices should be part of the path) and wireless resource allocation (which frequencies to use by devices). Also, base stations must manage interferences between D2D and cellular communication since they all share the same spectrum. Besides, there is also the energy issue in employing battery-constrained terminals as relays. Another concern, in offloading designs, is how they scale when terminals density increases, such as in crowded-platform scenarios. These scenarios include mobile users in waiting halls of airports and train stations, or stadiums. In such situations, the decision problems mentioned before must be solved rapidly. Doing so avoids long delays in communications that can affect user experience or limit responsiveness. In this thesis, we address the problem of optimizing routing and …