Multiuser Spectral Precoding for OFDM-Based Cognitive Radios

Orthogonal frequency-division multiplexing (OFDM) is a candidate transmission technique for cognitive radio (CR) because of its flexible nature to support spectrum sharing. However, the out-of-band (OOB) radiation of OFDM signal from CR users must be strictly controlled to protect licensed users in adjacent bands. In this paper, we propose a spectral precoding scheme for multiple OFDM-based CR users to reduce OOB emission and enhance spectrum compactness. By constructing individual precoders to render selected spectrum nulls, our scheme suppresses the overall OOB radiation without sacrificing the bit-error rate performance of CR users. The proposed scheme ensures user independence with low encoding and decoding complexity. We also study the selection of notched frequencies to further increase the bandwidth efficiency and implementation flexibility. Simulation results demonstrate that our spectral precoding scheme effectively limits OOB radiation and enables efficient spectrum sharing.