Reducing out-of-band radiation of OFDM-based cognitive radios

In an OFDM-based cognitive radio system, mutual interference among the users needs to be managed in order to enable spectrum sharing while protecting the licensed users. However, a conventional OFDM waveform suffers from high out-of-band radiation (OBR) and may cause harmful interference to other users in the system. In this work, we develop a transmit-windowing-based approach that reduces OBR by allowing for a controlled amount of Inter-Carrier Interference (ICI) in the OFDM system. Unlike previous work on transmit windowing, this approach does not require any additional time-frequency resources and shapes the carrier spectrum in a limited and deliberate manner. In particular, we examine the use of Hamming and Hanning transmit-windowed OFDM. The windowing operation at the transmitter consists of simple time-domain multiplications and is implemented in combination with an efficient fast Fourier transform (FFT) algorithm. We show that transmit-windowed OFDM has the potential to reduce OBR and improve spectral efficiency, and can be considered as an attractive alternative modulation waveform for cognitive radios.