Two level precoding scheme for out of band radiation reduction in multiuser OFDM based cognitive radio

In this paper, we propose an out of band radiation (OOBR) suppression scheme for multiuser cognitive radio (CR), where secondary users employ orthogonal frequency division multiplexing (OFDM) as a transmission technique. Our proposed method comprises of a spectral precoder followed by an optimization scheme for reducing the OOBR caused by secondary users. The proposed scheme can be viewed as a two level precoding in the sense that the second level precoding is designed to optimize the first one. The first level precoder for the CR user is designed by selecting the notched frequencies from the out of band region that are very close to temporarily allocated spectrum for the CR user. In the second level, an optimization onto the first level precoded data is performed by using a precoder designed with notched frequencies which are spaced far away from the CR user band. Results of extensive simulation studies demonstrate that the proposed technique can suppress the OOBR significantly at a cost of very small degradation in bit error rate (BER) performance of secondary users.