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In this paper, we investigate the downlink radio resource allocation in underlay relay assisted OFDMA spectrum sharing system. Our main objective is to maximize the secondary service sum rate. Two cases are presented to manage the impact of secondary service activity on the primary network through radio resource allocation. In the first (second) case we assume that the channel distribution information (CDI) (the channel side information (CSI)) for the channel between the secondary transmitter and primary receiver is available at the secondary base station (BS). Simulation studies are also conducted to compare the performance of the two above mentioned cases which indicate that in case of uncertain CSI, it is more efficient to use CDI and let the collision probability be managed through corresponding optimized radio resource allocation. An interesting advantage for this approach is eliminating the need to have feedback signalling between primary and secondary systems.