  In this paper, we adopt a multiobjective optimization approach to jointly
optimize the rate and power in OFDM-based cognitive radio (CR) systems. We
propose a novel algorithm that jointly maximizes the OFDM-based CR system
throughput and minimizes its transmit power, while guaranteeing a target bit
error rate per subcarrier and a total transmit power threshold for the
secondary user (SU), and restricting both co-channel and adjacent channel
interferences to existing primary users (PUs) in a statistical manner. Since
the interference constraints are met statistically, the SU transmitter does not
require perfect channel-state-information (CSI) feedback from the PUs
receivers. Closed-form expressions are derived for bit and power allocations
per subcarrier. Simulation results illustrate the performance of the proposed
algorithm and compare it to the case of perfect CSI. Further, the results show
that the performance of the proposed algorithm approaches that of an exhaustive
search for the discrete global optimal allocations with significantly reduced
computational complexity.
