In this paper we consider orthogonal frequency division multiplexing (OFDM) based cognitive radio systems that operate as secondary users (SUs) in a spectrum overlay approach and study joint spectral shaping and power control subject to specified target signal-to-interference+noise ratio (SINR) as a constrained optimization problem. We discuss necessary and sufficient conditions for the optimal solution of this problem where the spectrum overlay constraints are satisfied and the specified target SINR value is achieved with minimum transmit power. We also present an algorithm that incrementally adapts the OFDM transmitter to reach the optimal solution and study the convergence speed of the proposed algorithm through simulations.

Cognitive radio (CR) technology represents a promising solution for solving the problem of spectrum scarcity and underutilization in wireless communications. While orthogonal frequency division multiplexing (OFDM) is regarded as a suitable technology for the CR physical layer, it can suffer from out-of-band radiation, which hinders its spectrum sharing capabilities. In this paper we consider a turbo-coded OFDM based CR system, and propose three methods for shaping the spectrum of the CR signal to reduce interference affecting primary users (PUs) that operate in adjacent bands. The proposed methods use multiple choice sequences (MCS) generated at the bit level by employing different pseudo random bit sequences, by using bit interleavers with different depths, or by a combination of both, respectively. In order to reduce interference to adjacent bands, the sequence with the best spectral shape is transmitted along with its index. Performance of the proposed methods is illustrated with numerical results obtained from simulations, and the trade off between a better spectral shape and the reduction in the spectral efficiency of the systems is discussed.