We consider a system with a single base station communicating with multipleusers over orthogonal channels while being assisted by multiple relays. Severalrecent works have suggested that, in such a scenario, selection, i.e., a singlerelay helping the source, is the best relaying option in terms of the resultingcomplexity and overhead. However, in a multiuser setting, optimal relayassignment is a combinatorial problem. In this paper, we formulate a relatedconvex optimization problem that provides an extremely tight upper bound onperformance and show that selection is, almost always, inherent in thesolution. We also provide a heuristic to find a close-to-optimal relayassignment and power allocation across users supported by a single relay.Simulation results using realistic channel models demonstrate the efficacy ofthe proposed schemes, but also raise the question as to whether the gains fromrelaying are worth the additional costs.
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