We consider spectral and energy efficiency achieved by a novel relay communication in a cellular system, in which the so-called relay-beneficial mobile nodes communicate with base stations through appropriate relay nodes. The relay node uses the network coding to mix up- and downlink traffic; moreover, the mobile nodes and base stations adopt Maximal Ratio Combining (MRC) scheme to take advantage of cooperative transmission. For this system, we derive an information-theoretical condition, under which the cooperative transmission (combined with network coding) is more beneficial than the direct transmission. Using that condition, we suggest a two-phase algorithm to select the relay nodes (Phase 1) and to determine the transmission powers of those relay nodes (Phase 2). The numerical examples using this algorithm show improved energy efficiency compared to a plain system.