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We investigate an application of distributed power control (DPC) and opportunistic power control (OPC), respectively, to cooperative relaying systems that consist of multiple sources, multiple relays, and a single destination. Two types of relaying gains are considered for half-duplex amplify-and-forward (AF) relays: fixed and variable gains, respectively. Using half-duplex AF relays, remote users benefit by making the path loss smaller by two shorter paths. However, they lose bandwidth and suffer from additional interference due to amplification of the noise at the relays and the unnecessary signal from the relays, where power control plays an important role. We begin with redefining and extending the existing power control algorithms to be fit for the relaying environments and then, using standard techniques, construct the convergence, except when the number of variable-gain relays is even. With numerical investigation, we show the advantages and disadvantages of the power-controlled systems with relays in terms of the outage probability, the average transmit power consumption, and the transmission capacity. Simulation results show that DPC with relays achieves significant improvement in the outage performance and power consumption. On the other hand, in OPC, some negative effects arise by using the relays in the capacity since the relays increase the interference that severely affects the opportunistic capacity.