An Adaptive Precoder for Out-of-band Power Reduction in OFDM-Based Cognitive Radio System

Out-of-band (OOB) power emission is a conventional issue in orthogonal frequency division multiplexing (OFDM) system. The spectral leakage results in interference to licensed users (LU) in OFDM-based cognitive radio (CR) system. In this article, an adaptive precoding scheme based on the adaptive orthogonal projection matrix is proposed for OOB power suppression. Low complexity is the main feature of this preceding scheme. Simulation and analysis show that this method reduces the out-of-band power significantly by forcing the power spectrum density (PSD) of several frequency points in OOB to be zero. An advanced precoding scheme proposed later achieves a tradeoff between performance of OOB power suppression and bit error rate (BER) in the receiver. With those schemes, an adaptive precoder is proposed, which has three ways to adaptively adjust precoding matrix to achieve a balance among the complexity of system, the performance of out-of-band power reduction and BER in the receiver.