The increasing demand for large data downloads on cellular networks is increasing congestion which decreases end user quality of service. This work addresses the problem of offloading the cellular network while distributing common content to a group of mobile devices that cooperate during the download process by forming device-to-device communication networks. The base station unicasts different chunks of the content to selected mobile devices that multicast it to each other over local ad hoc networks using multihop cooperation while maintaining fairness constraints on the energy consumption of the mobile devices. The optimal cellular offloading problem is formulated as a mixed integer linear programming problem and the corresponding complexity is analyzed. Then, a dynamic programming approach is proposed to adapt the solution to the dynamics of the network as the mobile devices move. Cellular offloading assuming single hop cooperation among the mobile devices proves to be significantly less computationally complex than cooperation using a higher number of hops; however both problems are NP-complete. Thus, polynomial time greedy algorithms are presented to obtain computationally fast solutions with good performance. Performance results demonstrate that significant cellular offloading gains can be achieved, even if only a very small fraction of the mobile devices' battery levels can be consumed for cooperation.
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