This paper proposes an orthogonal frequency division multiple access-based resource allocation algorithm for downlink cognitive radio systems

which targets to maximize the data transmission rate under the constraints of total transmit power

bit error rate and interference to the primary users. It includes two steps: firstly

the subcarriers are allocated according to the signal to interference plus noise ratio of each user over each subcarrier; secondly

the optimal bit and power solution is derived in the nonnegative real field and then adjusted into the bit and power allocation results suitable for the practical systems. Simulation results show that the proposed algorithm can provide significant data transmission rate gain comparing with the conventional resource allocation algorithms based on spectrum hole.