In this paper, we propose relay assisted orthogonal frequency-division multiple access (OFDMA) spectrum sharing system. We then design downlink radio resource allocation schemes for the proposed system with the objective of maximizing the secondary service sum rate. Two cases are considered, 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. For both cases, we further consider the availability of imperfect CSI of the channel between the secondary base station and the relay stations and the channel between the relay stations and the secondary users. The computational complexity issue is then addressed by proposing suboptimal solutions. Extensive simulation studies show that the impact of imperfectness in channel estimation on the system performance can be controlled by employing relays in the system. Simulation studies are also conducted to compare the performance of the two earlier mentioned cases, which indicates that in case of imperfect CSI estimation, it is often better to use CDI and let the collision probability be managed through corresponding optimised radio resource allocation. Using CDI then eliminates the need for feedback signalling between primary and secondary systems. Copyright (c) 2012 John Wiley & Sons, Ltd.