We propose a novel approach that aims at extending femtocell coverage by enabling nearby idle User Equipments (UEs) in the proximity of femtocells to serve as relays, called UE-Relays. The users that would nominally connect to macrocells are offloaded to femtocells via the coverage extension provided by the UE-Relays. The problem of determining the optimal subset of idle UEs to be enabled as UE-Relays and the user association to these relays is a non-trivial problem because it depends on the projected capacity of the relay access and backhaul.We formulate the user association as an optimization problem that maximizes the network-wide log utility of the user rates. Our simulation results corroborate our expectations: overlaying a femtocelltier with UE-Relays will increase the user throughput and the network throughput. In addition, we compare our approach with a MAX-SINR based user association, and evaluate the performance profiles at varying scenario settings. The proposed rate-based scheme outperforms the SINR-based cell selection, thus validating the idea that, in heterogeneous cellular networks, user association should be performed based on the load rather than relying merely on link quality indicators.
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